J Mammal Evol DOI 10.1007/s10914-012-9198-x

ORIGINAL PAPER

Postcranial Analysis of a Carnivoran-Like Archaic Ungulate: The Case of Arctocyon primaevus (Arctocyonidae, Mammalia) from the Late Paleocene of France

Christine Argot

# Springer Science+Business Media, LLC 2012

Abstract The postcranial skeleton of the late Paleocene cheek teeth compatible with an omnivorous diet, and which Arctocyon primaevus is described based on a sub-complete show a postcranium that is morphologically more similar to associated specimen. A comparison with arboreal or scan- carnivorans than to ungulates, represent a mosaic of features sorial and fossorial extant taxa shows that on the forelimb, that is of particular interest in the evolution of mammals. several features suggest arboreal capabilities, including the development of abductors and adductors, the development Keywords Arctocyonidae . Borhyaenoids . Carnivora . of digital flexors, which allows grasping/manipulative abil- Functional morphology . Paleocene . Postcranium ity, as well as the highly mobile articulations, the convex ulna, and the pentadactyl, plantigrade foot. In contrast with the highly mobile joints of the limbs, Arctocyon had a rigid Introduction posterior thoracic area, characterized by revolute zygapoph- yses unknown in extant mammals. The morphology of the First described by Blainville in his osteography of 1841, most anterior caudal vertebra indicates that the tail was long, Arctocyon primaevus, “the bear-dog of the dawn,” belongs powerful, muscular, and rigid at its base, and that it played to Arctocyonidae and is known from the Paleocene of Europe an important role in locomotion. The morphology of the and North America, where it is probably synonymous with hind limb is congruent with that of the forelimb, the devel- Claenodon (Rose 2006). Historically, arctocyonids have been opment of the adductors, flexors, and rotators of the mobile recognized first – and for 100 years – as basal carnivorans, as hip joint being emphasized. Although the femoral trochlea is recalled by Van Valen (1966, 1969a, 1978). This author rec- longer and better defined than in highly arboreal taxa, Arc- ognized them as basal ungulates, based on their dentition, but tocyon probably moved in a controlled fashion. A compar- he acknowledged similarities between the molars of arctocyo- ison with South American borhyaenoids shows that nids and miacids. Prothero et al. (1988) also reminded us that Arctocyon is morphologically more similar to some they have long been included with creodonts and carnivorans predator-like Miocene metatherians than to any living mam- on the basis of their bear-like dentition and a skull “like some mal. It represents an interesting mix between Prothylacinus primitive carnivores” (see also Matthew 1937, and the concept and Borhyaena in overall size and proportions, and shows a of Ferungulata in Simpson 1945). This suggests that for a development of crests and processes of the humerus similar while, carnivorans were thought to have evolved from arcto- to those of Prothylacinus. Arctocyonidae, which evolved cyonids. For forty years now, arctocyonids have also been towards incipient saber-toothed canines combined with viewed as the stock from which other archaic ungulates arose (Van Valen 1978;Archibald1998), although they show no modification of the limbs toward the usual idea of what an C. Argot (*) ungulate is (as noted by Archibald 1998): no distal limb Département Histoire de la Terre, CR2P, elongation, no reduction of side digits, no fusion of limb bones ’ Muséum national d Histoire naturelle, UMR 7207 du MNHN-CNRS, or lengthening of distal parts, no development of hoofed 8 rue Buffon, CP 38, 75005 Paris, France unguals, and no deeply grooved astragalus. This is also true e-mail: [email protected] for the North American early Eocene oxyclaenid Chriacus J Mammal Evol

(Rose 1987). Although a complete phylogeny of basal ungu- including Plesiadapis tricuspidens, Pleuraspidotherium lates including several taxa of oxyclaenids and arctocyonids aumonieri, Dissacus europaeus, Tricuspiodon rutimeyeri, remains to be done, the published literature always puts arcto- and T. magistrae. Based on dental evidence, Pleuraspido- cyonids in a basal position, i.e., as a sister taxon of all the other therium is one of the most common taxa of the locality and ungulates except artiodactyls, which does not really clarify the many postcranial elements, unassociated with dental identity of arctocyonids but at least emphasizes the meaning- remains, have been identified as belonging to this taxon, less term of “Condylarthra” that appears as a highly diverse based on their size (Thewissen 1991). Thewissen concluded and paraphyletic group of early eutherians (see, e.g., Prothero that Pleuraspidotherium was a generalist terrestrial taxon et al. 1988; Archibald 1998; Ladevèze et al. 2010). with plantigrade feet, lacking the supination ability and the Arctocyon primaevus is a relatively common fossil taxon large mobility at the ankle joint described in Chriacus.By known in the late Paleocene (Thanetian) strata of Cernay and contrast, Ladevèze et al. (2010) noted a number of similarities Berru (France), constituting an important record of cranial, between the tarsus of Pleuraspidotheriidae and Chriacus. Dis- dental, and postcranial elements. Although it does not reach sacus and Tricuspiodon are known only by unassociated the abundance of Plesiadapis and Pleuraspidotherium,thisis fragmentary forelimb elements, larger than those referred to the most common arctocyonid of the fauna, much more com- Pleuraspidotherium. Dissacus, approximately the size of a mon than the smaller Arctocyonides. Most of the elements fox, is said to be terrestrial and digitigrade, sharing some known were found isolated, except for a nearly complete locomotor adaptations with cursorial carnivorans, like a medi- specimen found associated and described in Russell’smemoir olaterally compressed humeral head and a deep olecranon (1964). However, this work, published in French decades ago fossa (Thewissen 1991). However, this author also mentions and with only a few line drawings illustrating the description, some features that do not seem compatible with this conclu- does not offer a comprehensive description and analysis of the sion, such as greater supination ability than in Pleuraspido- functional morphology of this skeleton, which is the purpose therium. From the humerus and fragmentary radius and ulna, of the present paper. Nearly complete Paleocene skeletons are Thewissen (1991) concluded that Tricuspiodon (the largest of rare, and this skeleton (Fig. 1) provides a wonderful example the three taxa) had fossorial capabilities, in relation to devel- of the arctocyonid morphology, with most of its articulations oped abduction and flexion of the shoulder, and absence of preserved, as well as muscular attachments, allowing an in- supination ability. The features discussed for the humerus vestigation of the locomotor range of this species, alternative- could fit well with arboreal ability, and it does not seem ly described as aquatic (Blainville 1841), terrestrial to possible to give a definitive conclusion because of the frag- scansorial without particular adaptations (Russell 1964), or mentary nature of the material. However, if Tricuspiodon truly with fossorial adaptations (Kondrashov 2009). lacked ability for supination, then arboreal habits are improb- Arctocyon primaevus comes from a Paleocene locality able. Analyses of Plesiadapis tricuspidens, one of the largest that provided other taxa also known by postcranial elements, plesiadapiforms (estimated body weight around 2 kg), have

Fig. 1 General view of the sub-complete specimen of Arctocyon primaevus from the Paleocene of Cernay-lès-Reims (France), described and analyzed in the text. Scale bar equals 10 cm J Mammal Evol suggested diverse locomotor behaviors, but a recent multivar- the elements described do not belong to the same specimen. iate analysis (Youlatos and Godinot 2004, and see references The isolated elements studied are: scapula (MNHN.F.967, therein) placed Plesiadapis within arboreal quadrupedal, BR13919, R754); humerus (MNHN.F.968, 969, BR10101, clambering, and claw-climbing squirrels, based on the analy- R747, BR1070, BR1080, BR13903); ulna (MNHN.F.R24, sis of the olecranon and ulnar lengths, shape of the radial head, R25, R735, BR12549); radius (MNHN.F.R720, R721, femoral trochlea, and ungual phalanges. Therefore, it appears R722, R727, R33, BR12566, BR10660, BR11174); scaphoid that locomotor specializations leading to the occupation of (MNHN.F.699, 755, CR17474); triquetrum (MNHN.F.R59, various ecological niches already occurred at that time. The 1746); lunate (MNHN.F.603, 606, CR57); trapezium detailed functional analysis of a sub-complete specimen of (MNHN.F.CR823); magnum (MNHN.F.1307); unciform Arctocyon thus aims at investigating the place of this late (MNHN.F.BR10666); Mc V (MNHN.F.BR11378, Paleocene mid-sized mammal (but the largest of its fauna) in BR11879); Mc I (MNHN.F.1302, CR17473); ungual phalan- its environment. ges (MNHN.F.CR4, CR486); pelvis (MNHN.F.R77, BR12710, BR12711, BR10141, BR12557, BR11858, BR17472); femur (MNHN.F.R739, R742, BR12550, Material and Methods BR12551, BR12544); tibia (MNHN.F.R75, R76, BR12555, BR11857, BR13902); fibula (MNHN.F.R78, BR1073, This description and functional analysis are based on a BR1077, BR10883, BR12565, BR14519); calcaneum nearly complete specimen of Arctocyon primaevus found (MNHN.F.975, 979, R749, CR750, CR65, BR10670, in the Mouras quarry, eastern side of the Mont de Berru BR10671, CR12160); astragalus (MNHN.F.976, 980, 1756, (see a thorough review of the geological data in Russell 1757, 1758, CR62, CR752, BR11875, BR10667, BR12657); 1964). This specimen is housed in the Muséum national cuboid (MNHN.F.1755, CR17475); navicular d’Histoire naturelle (MNHN) of Paris, France. The individ- (MNHN.F.1747, BR11413, BR12647); ectocuneiform ual bones have unfortunately been curated under different (MNHN.F.CR58); Mt I (MNHN.F.CR335); Mt IV numbers despite the fact that they belong to the same spec- (MNHN.F.CR50); and Mt V (MNHN.F.CR332, BR11862). imen, which is represented by: the skull (CR700) and the CR means Cernay-lès-Reims, BR means Berru, and R means mandible (CR1, CR2), the axis (CR86), three posterior both. cervical vertebrae (CR87-CR89), nine thoracic vertebrae No phylogenetically closely related taxa exist for analyz- (CR90-CR98), two lumbar vertebrae (CR100, CR101), ing the adaptations of arctocyonids. For the comparative two caudal vertebrae (CR99, CR102), ten ribs (CR113- analysis, the skeletons of arboreal and fossorial extant taxa CR122), the right scapula (CR15), the two humeri (CR16, have been used, especially: the binturong Arctictis bintur- CR17), ulnae (CR18, CR19), radii (CR20, CR21), the right ong, the badger Meles meles, the pangolins Manis javanica pisiform (CR34), right cuneiform (0 triquetrum) (CR38), and M. temminckii, the anteater Myrmecophaga tridactyla, right scaphoid (CR39), right trapezium (CR35), right and and the giant armadillo, Priodontes maximus. Although left unciforms (CR36, CR37), the right Mc V (CR40), the obviously none of them is a close modern analogue, they left and right parts of the innominate, without pubis (CR69, provided useful comparisons in various anatomical features. CR70), the two femora (CR71, CR72), a patella (CR68), the Osteological and myological postcranial features function- left tibia (CR73), the left fibula (CR74), the right Mt III ally associated with arboreal-scansorial or terrestrial loco- (CR42), the right Mt II (CR41) and the right Mt I (CR67). motion have already been inferred in a series of papers Three sesamoids (CR47-CR49), two proximal phalanges analyzing the locomotion of medium-sized placental and (CR43, CR44), a single intermediate phalanx (CR66), and metatherian fossil and extant carnivorous taxa (e.g., Argot two ungual phalanges (CR45, CR46) are also associated 2001, 2003a, b , 2004a). Several of the osteo-myological with this specimen. associations found in these papers are used here as a basis Several other postcranial elements referred to A. primae- for the present work in order to infer the function of some vus are also known from the same Paleocene localities postcranial features observed in A. primaevus. Also, a com- (Cernay, Berru) and have been studied here, in order to parison of the skeleton of Arctocyon (in overall size, pro- check potential intraspecific variability and individual char- portion, shape of forelimb long bones) with those of South acteristics. Considering that it is more relevant to perform a American Neogene metatherians has revealed interesting functional analysis in studying associated bones, as far as functional convergences. possible, these other elements are mentioned in this paper Institutional Abbreviations MNHN, Muséum national only when they provide some information not available in d’Histoire naturelle, Paris, France; YPM-PU, Peabody the sub-complete skeleton, as is the case especially for the Museum of Yale University, New Haven, Connecticut, tarsal bones. It is clearly stated in the text and figures when USA J Mammal Evol

Fig. 2 a-c, Skull MNHN.F.CR700 of Arctocyon primaevus (sub-complete specimen). a, lateral view, articulated with the mandible, MNHN.F.CR2; b, dorsal view; c, posterior view; d, axis, MNHN.F.CR86, belonging to the same specimen, in anterior view. Scale bars equal 5 cm

Description The neural process of the axis (Figs. 2d, 3a) is extremely developed, anteroposteriorly long, and is characterized by Axial Skeleton the dorsal protrusion of the posterior apex. This morphology would not preclude, then, the development of the neural Cervical Vertebrae The axis (CR86) and three cervical ver- process of the third cervical. Unfortunately, the neural pro- tebrae (CR87-89) are associated with the sub-complete cess of the vertebra CR87 is broken. The vertebral body of specimen. From the same specimen, nine thoracic vertebrae the axis exhibits a sharp ventral crest that presents anteriorly (CR90-98), two lumbars (CR100 and 101), a proximal a prominence, located below the atlanto-axial articulation. caudal vertebra (CR99), and a distal caudal vertebra The transverse processes are not preserved. No atlas with a (CR102) are known. Russell (1964) described three lumbars preserved transverse process (i.e., atlantal wing) is known. and a distal caudal vertebra, but CR99 is a proximal caudal On the skull, the sagittal crest is extremely developed, vertebra, as described below. Several other isolated verte- prominent dorsally (Fig. 2a). The occipital area is narrow brae are known from the same locality, many being extreme- and the occipital (nuchal) crest is not particularly developed. ly damaged, reduced to the vertebral body. A few well- The occipital condyles, most of the occipital area, and the preserved caudal vertebrae, referred to Arctocyon according basicranium are not preserved (Fig. 2c). to their size, will be mentioned below as they provide On CR88, the neural process stands vertically whereas on additional information. CR89, it is inclined anteriorly (Fig. 3c). The dorsal arch of the J Mammal Evol

Fig. 3 Cervical vertebrae of Arctocyon primaevus (sub-complete specimen). a, axis, MNHN.F.CR86, in lateral view; b, posterior cervical vertebrae, from left to right MNHN.F.CR87, CR88, and CR89, supposedly C?3, C5, and C6, in dorsal view; c, the same posterior cervical vertebrae in lateral view. Scale bar equals 3 cm

cervicals usually presents a foramen between the articular crest where the scalenus muscles usually attach. The neural facets of the anterior and posterior zygapophyses (Fig. 3b). processes of these five vertebrae are long and inclined The foramen’s size is variable, as it appears larger in CR87 posteriorly (Fig. 4b). Their apex is robust on T1-T2 and a and 89 than in CR88. The transverse processes of CR87 are longitudinal fossa is visible on the lateral sides. The apex of not well preserved but apparently protruded only posteriorly, the neural process may be extended anteroposteriorly, as in the apex being divided into a dorsal and a ventral protrusion CR91-92 (T2) and especially CR94 (T4). The dorsal arch of (Fig. 3c). In CR88, the transverse process, also not preserved T1 shows a dorsal pit just anterior to the postzygapophysis, in full, extends ventrally in a blade-like apophysis, well ex- although less robust than that observed on C6. T3 (CR93) tended anteroposteriorly: this allows identification of this presents a muscular scar at the same level. vertebra as C5. In CR89, this ventral apophysis, not complete, Four posterior thoracic vertebrae are known, which artic- is also blade-like and more prominent ventrally than on the ulate one to the other (Fig. 4d, f). They are numbered CR96 previous vertebra. With this prominent inferior lamina and to CR98, CR96 representing the two first vertebrae, not anterior inclination of the neural process, CR89 is, then, separated. According to the size and inclination of the neural certainly the sixth cervical vertebra (although it was not iden- process of CR95 (T5) in comparison to that of CR96, it is tified as such by Russell 1964). The dorsal arch of this suggested that about four vertebrae are missing in between vertebra is also characterized by a dorsal pit that does not them. This means that the four posterior thoracic vertebrae appear on the other cervical vertebrae (Fig. 3b–c). The verte- could be T10 to T13. The first one, supposedly T10, is the bral bodies of these three cervicals are extremely oblique in diaphragmatic vertebra: the articular facets of the prezyga- lateral view, the most ventral part being also the most poste- pophyses are small and oriented dorsally as in the anterior rior. They show a ventral crest, which is not very salient and thoracic vertebrae (0 tangential articular mode), whereas the presents a bifid posterior extremity. postzygagpophyses face laterally, showing the radial articu- lar mode typical of lumbar vertebrae. The neural process is Thoracic Vertebrae Five anterior thoracic vertebrae are preserved in all these vertebrae except CR98 (T?12). In the known, and they articulate one to the other. They are num- diaphragmatic vertebra, this process stands vertically but the bered CR90 to CR95, but CR91 and CR92 are two elements anterior border is still inclined posteriorly. On the following of the same vertebra that was broken before this study. The vertebra, the anterior border is vertical, whereas in the last first thoracic (CR90) is wide in anterior view (Fig. 4a) vertebra of the series, the neural process is inclined anteri- because of the development and robustness of the transverse orly. This indicates that the anticlinal vertebra was T11 or processes. These five vertebrae exhibit two facets articulat- T12 (i.e., the second vertebra of CR96, or CR98). These ing with ribs, and the size of the facets on T1 is consistent four posterior thoracic vertebrae exhibit anapophyses, incip- with a robust first pair of ribs. About ten ribs have been ient in the first of the series, very robust in the last one. found associated with this specimen (Fig. 1), and the short- Transverse processes are still developed on T?10. The three est ones, interpreted as the first pair, show a sharp lateral last vertebrae of the series are characterized by the J Mammal Evol

Fig. 4 Thoracic vertebrae of Arctocyon primaevus (sub-complete specimen). a, T1, MNHN.F.CR90, in anterior view; the shape of the neural process is either pathological or related to local pressure during fossilization; b, the five first anterior thoracic vertebrae, MNHN.F.CR90 to CR95, in lateral view; c, the last thoracic known, T?13, MNHN.F.CR97, in posterior view; d, the four most posterior thoracic vertebrae, MNHN.F.CR96, CR98, and CR97, from left to right, supposedly T?10 to T?13, in lateral view; e, MNHN.F.CR97 in anterior view; f, the four most posterior thoracic vertebrae in dorsal view. Scale bar equals 5 cm

robustness of the mammillary processes and the shape of the or in between the two lumbars preserved. The neural process prezygapophyses (Fig. 4c, e). In CR98 (T?12) the mammil- of CR100 is short, vertical, with a robust apex. It is higher lary processes are extremely wide and robust, with a prom- and inclined anteriorly in CR101 (Fig. 5a). Anapophyses are inent apex, much more developed than in any extant species. present in the two lumbars but they are reduced in compar- The articular facets of the zygapophyses of the last thoracic ison to those of the last thoracics CR97 and 98, especially vertebrae are characteristic, concavo-convex, sigmoid in the anapophyses of the more posterior lumbar vertebra anterior view, interlocking adjacent vertebrae tightly and (judging from their bases, because they are broken). Mam- preventing any lateral flexion in between them. millary processes are also present, but, although still robust in CR100, they do not protrude as they do in CR97. The Lumbar Vertebrae Two lumbar vertebrae are known, one articular facets of the zygapophyses present the same charac- anterior (CR100) and the other (CR101) more posterior teristic revolute shape as those of the posterior thoracic verte- (Fig. 5a–c). It is not possible to determine the number of brae, also precluding any lateral movement (Fig. 5b). lumbar vertebrae that were present in Arctocyon. Moreover, However, in the more posterior lumbar vertebra, the posterior CR100 does not articulate with the last posterior thoracic articular facets are reduced, without the sigmoid dorsal part. vertebrae known, and it is not possible to determine how The left transverse processes are present in the two lumbars: many vertebrae (one, two?) were present in between them, in CR100, it protrudes laterally and anteriorly. The anterior J Mammal Evol

Fig. 5 Lumbar and caudal vertebrae of Arctocyon primaevus. a, from MNHN.F.CR102, from the sub-complete specimen, in dorsal view; left to right: two lumbars, MNHN.F.CR100 and CR101, and the most on the right, other caudal vertebrae referred to A. primaevus: anterior known caudal vertebra, MNHN.F.CR99, from the sub- MNHN.F.CR790, CR785, CR760, and CR107, showing different complete specimen, in lateral view; b, the same in posterior view; c, degrees of development of the transverse processes. Scale bar the same in dorsal view; d, on the left, the only known posterior caudal, equals 3 cm apex also protrudes ventrally. In CR101, the two processes isolated lumbar vertebra is known with better-preserved trans- are present but not fully preserved. They protrude anteriorly verse processes. The sacrum is unknown. but their exact length cannot be determined. In anterior view, they extend horizontally. They are located dorsally, at the Caudal Vertebrae The vertebra CR99 is a proximal caudal junction of the vertebral body and lateral pedicles. No vertebra (not described as such by Russell 1964), with a J Mammal Evol short, rounded neural process (Fig. 5a). The vertebra itself is caudal angle seems to define an anterodorsal area (visible in as long and robust as the most anterior lumbar vertebra. It CR15 but not preserved in 967 because the crest is very exhibits, like the posterior thoracic vertebrae, a radial artic- close to the damaged border) that suggests a well-developed ular mode with articular facets tightly interlocked, and the serratus anterior. posterior articular facets present a sigmoid dorsal part, in The glenoid cavity is more rounded than ovoid and is contrast to the most posterior lumbar vertebra (Fig. 5b). shallow (Fig. 6c). The supraglenoid tubercle does not appear Moreover, the mammillary processes and anapophyses are particularly massive, and the coracoid process is short. more robust than in the more posterior lumbar vertebrae. Medially, it exhibits a muscular attachment area for the The anapophysis also presents a sharp lateral crest and these coracobrachialis and biceps brachii. features emphasize the attachment of the extensors of the tail (see Discussion). The vertebra CR99 is also characterized by Humerus Several humeri of A. primaevus are known, all of having transverse processes that protrude posterolaterally, in them being damaged. The description is mainly based on contrast to those of the lumbar vertebrae (Fig. 5c). The the left humerus (CR16) of the sub-complete specimen anterior border of this process presents a well-developed (Fig. 7), and is completed by observation of some other tuberosity for the attachment of the abductor caudae dorsalis bones available (e.g., Fig. 6e). (Fig. 5c). The humeral head is rounded in proximal and posterior The vertebra CR102 is a distal caudal vertebra showing views. It is particularly not prominent posteriorly in CR16 the usual morphology (Fig. 5d). A lot of other caudal ver- (Fig. 7d). The greater tubercle is massive, higher than the tebrae are known from the same locality, but their identifi- head (Fig. 6d). The fossa for insertion of the infraspinatus is cation as arctocyonid vertebrae is based only on their size. lateral and deep. Just below, a bulge on the tricipital line Three of them illustrated here show the morphological tran- (visible, e.g., on BR10101: Fig. 7e) suggests the insertion of sition between the proximal and most distal part of a long a well-developed teres minor. The bicipital groove is deep tail, with a long vertebral body and anteroposteriorly long and narrow proximally. transverse process that divides into two parts. No other The deltopectoral and tricipital crests are extremely well proximal caudal vertebra is well-enough preserved to show developed in Arctocyon and they extend along the proximal the morphology of the transverse process. two-thirds of the diaphysis (Fig. 7a, d). The distal extremity is prominent anteriorly. Because of the development of the Pectoral Girdle and Forelimb deltopectoral crest, the proximal part of the humerus is massive, deeper anteroposteriorly than wide. The deltopec- Scapula The scapula of the sub-complete specimen (CR15) toral area is V-shaped, wide, and oriented anteriorly. On the is well preserved although the cranio-dorsal part is damaged two humeri of the same specimen (CR16 and CR17), this area (Fig. 6b). The area of the supraspinatus fossa is wider than is divided into two unequal parts by a third additional crest that of the infraspinatus and the steep cranial border, pre- (Fig. 7a). The most lateral and narrowest part was probably for served near the glenoid cavity, indicates a cranial triangular the deltoideus pars acromialis, whereas the most anterior and expansion of this fossa. The infraspinatus fossa is triangular widest part was for the pectoralis. There is no well-defined in shape, with a posteriorly prominent caudal border. The tuberosity on the tricipital line for the deltoid pars spinalis, but caudal angle is massive, the origin of the teres major being this crest is rarely well preserved. On the lateral side of the expanded. The scapular spine is high, especially near the humerus, below the head, the presence of scars in most of the glenoid cavity, which is consistent with well-developed specimens emphasizes the origin of the triceps brachii caput spinati muscles. The restoration does not allow one to as- laterale. The scapular fossa for the triceps caput longum is not certain its posterior inclination. The acromion is ovoid in particularly emphasized on the caudal border of the scapula, shape. In lateral view, the anterior expansion of the acro- dorsal to the glenoid cavity. mion does not completely overhang the supraglenoid tuber- Medially, the lesser tubercle of the humerus is appressed cle although it is more prominent anteriorly in the scapula against the head and shows a long, crescent-shaped insertion MNHN 967 (Fig. 6a) than in CR15. No clavicle has been of the subscapularis. Below, a scar that stands above the found associated with the sub-complete specimen of A. midshaft limit, and whose development varies among speci- primaevus. The scapular neck is short and wide, especially mens (and with their age at death), indicates the insertion of in the scapula 967. the teres major (and not the triceps as stated by Russell The medial side of the scapula is concave anteriorly and 1964). The slightly convex deltopectoral crest usually convex posteriorly, below the infraspinatus fossa. The later- presents a medial prominence at the same level, although ally prominent posterior border of the scapula emphasizes its development also varies among specimens. this convexity (Fig. 6b, bottom). On the medial surface, a The lateral epicondylar crest extends along the distal crest running from the apex of the anterior border to the third of the diaphysis; its lateral extension varies among J Mammal Evol

Fig. 6 Scapulae and proximal humeri of Arctocyon primaevus. a, right scapula, MNHN.F.967, in lateral view; b, right scapula, MNHN.F.CR15, in lateral, medial, and posterior view (from top to bottom; it is reversed in the medial view); c, glenoid cavity of MNHN.F.CR15; d, right humerus, MNHN.F.CR17, in proximal view; e, right humerus, MNHN.F.CR747, in proximal and lateral views. The scapula CR15 and humerus CR17 belong to the same sub- complete specimen. Scale bars equal 5 cm for a-b, 3 cm for c–e

specimens: it is particularly well developed proximally, medial crest is more prominent distally. According to the where the brachioradialis originates, on CR16 and CR17 specimen observed, the most lateral part of the posterior artic- (Fig. 7a). Medially, the entepicondyle is massive, promi- ular facet may extend proximally into the olecranon fossa, or nent, and extended proximodistally. It enlarges the distal not. This is the case in CR16 and CR17, but not in BR10101. extremity of the humerus. An entepicondylar foramen is The capitulum is rounded, slightly convex seen in distal present in all the specimens observed. It is wide, and the view (Fig. 7b), and it exhibits a small lateral extension. Just entepicondylar crest is thin. above this extension, a prominence emphasizes the origin of The trochlea is asymmetrical. The articular area is narrow the supinator brevis. On the entepicondyle, three deep in anterior view and is oriented more laterally than anteriorly notches indicate the origin of the digit flexors. Another notch because of the protrusion of the medial crest. There is no is present distally between the articular area of the capitulum coronoid fossa above this facet. In posterior view, the articular and the lateral posterior crest of the trochlea (Fig. 7b), lim- facet, as well as the olecranon fossa, is wide and shallow. The iting the range of extension of the joint. On the posterior side lateral crest of the trochlea protrudes posteriorly, whereas the of the entepicondyle, a deep fossa is present beside the J Mammal Evol

Fig. 7 Humeri of Arctocyon primaevus. a, the right and left humeri, respectively MNHN.F.CR17, CR16, of the sub-complete specimen in anterior view; b, the same humeri in distal view; the three stars underline the fossas where the different heads of the flexor digitorum profundus originate; c, the same humeri in posterior view; d, left humerus, MNHN.F.CR16, in medial and lateral views; e, left humerus, MNHN.F.BR10101, in medial and lateral views. Scale bar equals 5 cm

medial crest of the trochlea (see, e.g., on CR16, Fig. 7c), and flexor digitorum profundus. The posterior border of the suggests a well-developed flexor carpi ulnaris. diaphysis is convex, the apex of this convexity being located below the coronoid processs. The interosseous crest is Ulna The two ulnae of the sub-complete specimen (CR18 particularly prominent along the distal half of the diaphysis, and CR19) are longer than the humeri CR16 and CR17. and the medial extension of the pronator quadratus crest is They are massive, wide anteroposteriorly but compressed striking. mediolaterally (Fig. 8a–c). In anterior view, they appear Laterally, the ulna exhibits a deep proximal fossa, limited slightly concave medially, especially because of the medial posteriorly by a prominent crest running from the olecranon protrusion of the olecranon (proximally) and crest for the (Fig. 8a). The anconeus probably inserted in the fossa, pronator quadratus (distally). The olecranon is massive, whereas the extensor indicis et pollicis longus originated wide mediolaterally, and it protrudes medially, at the level on the crest. The lateral side of the ulna is also characterized of attachment of the triceps caput mediale, epitrochleo- by a prominent crest that extends along the distal two-thirds anconeus, and flexor carpi ulnaris. The medial side of the of the diaphysis, separating it into two subequal parts. It ulna exhibits a deep fossa all along its length (Fig. 8b), suggests an extremely well-developed abductor pollicis lon- suggesting a particularly well-developed ulnar head of the gus. Distally, the styloid process is reduced in comparison to J Mammal Evol

Fig. 8 The left and right ulnae, respectively MNHN.F.CR19, CR18, of Arctocyon primaevus (sub-complete specimen) in a, lateral view; b, medial view; c, left ulna in anterior view; d, focus on the proximal articular area of the left and right ulnae in anterior view; e, radial heads of Arctocyon primaevus (sub-complete specimen) in proximal view; from left to right: MNHN.F.CR21 (right radius), MNHN.F.CR20 (left radius). Scale bars equal 3 cm

the enlarged diaphysis, and suggests a loose articulation and pronator quadratus crests. Along the interosseous crest, with the cuneiform and pisiform. a rough scar suggests the development of a strong inteross- The trochlear notch is not deep and is characterized by an eous ligament or membrane (Fig. 9c). asymmetrical anconeal process, more prominent laterally The bicipital tuberosity is weak and located proximally. than medially (Fig. 8c, d). The coronoid process is The radial head is ovoid, transversely wide, and the central developed proximodistally but it is narrow and not very concavity is shallow (Fig. 8e). The most lateral part of the prominent anteriorly, consistent with the reduced anterior head (articulating with the lateral extension of the capitu- development of the humeral trochlea and the absence of lum) is more prominent proximally than the medial part. a coronoid fossa on the humerus. The radial notch is The articular facet for the ulna extends along the poster- reduced and faces anterolaterally, also suggesting a loose omedial part of the circumference and is slightly convex. joint with the radius. The asymmetry of the radial head, compared to the relative development and orientation of the lateral and medial parts, Radius The two radii of the sub-complete specimen (CR20 varies among specimens (several radii of Arctocyon are and CR21) are thin bones, strikingly less massive than the known). The anterior side of the radial diaphysis shows a corresponding ulnae (Fig. 9). The diaphysis is subcircular crest for insertion of the pronator teres, with an apex located proximally, below the bicipital tuberosity, but is triangular at midshaft. This crest is not particularly prominent or distally, in relation to the development of the interosseous convex (Fig. 9a). J Mammal Evol

Fig. 9 The right and left radii, respectively MNHN.F.CR21, CR20, of Arctocyon primaevus (sub-complete specimen) in a, anterior view; b, medial view; c, posterior view. Scale bar equals 3 cm

The distal epiphysis is massive, much more than the depression is variable, present especially in CR39. Laterally, radial head; it provides a wide articular area with the the facet articulating with the lunate is small and rounded, scaphoid and lunate, and it is more concave medially than flat to slightly convex according to specimens. Distally, laterally. The distal articulation with the ulna is reduced to a another facet, concave and elongate anteroposteriorly, is small, rounded facet. A deep groove runs anteriorly on the present, articulating with the magnum. The palmar process distal epiphysis, for the tendon of the abductor pollicis protrudes posteriorly, not laterally; it is robust but short. longus. Laterally, it exhibits an articular facet that could have con- tacted a sesamoid, possibly located in the tendon of the Carpal Bones, Metacarpals, and Phalanges The scaphoid, abductor pollicis longus (a muscle that usually inserts on cuneiform, pisiform, trapezium, and unciforms are known the trapezium or proximal part of Mc I). Distally, the most from the sub-complete specimen (Fig. 10). The fifth meta- medial facet, articulating with the trapezium, is concave, carpal (Mc V) is the only element of the hand to have been transversely wide but constricted anteroposteriorly. The lat- found associated with the skeleton (Fig. 11a). Russell (1964) eral facet, articulating with the trapezoid, is convex ante- also described a lunate and a trapezium found in the same roposteriorly but concave in the most anterior part because locality, but not associated with this specimen. Three axes of a protrusion that stabilizes dorsiflexion. This facet indi- are used for describing the carpals, the same as for the long cates that anteroposterior gliding was more restricted for the bones of the forelimb and the metacarpals: proximodistal, scaphoid-trapezoid articulation than for the scaphoid- lateromedial, and anterodorsal. The anterodorsal axis also trapezium one. In lateral view, the proximal facet for the corresponds to a dorsopalmar axis if the manus and wrist are radius and the distal facet for the trapezium form an acute considered touching the ground. angle and are separated by a sharp crest in CR39, by two or The right scaphoid (CR39) is subquadrangular in proxi- three millimeters in the two other scaphoids known. mal view (Fig. 10c), slightly deeper anteroposteriorly than The right cuneiform or triquetrum (CR38) is a big bone, wide (18 mm versus 15.5). Several other right scaphoids are kidney-shaped (Fig. 10b), wider transversely than long or known; one is illustrated in Fig. 10, subequal in size to deep. The mediolateral length is 21 mm, i.e., it is transversely CR39 and better preserved. The proximal facet articulating wider than the scaphoid. The posteroproximal facet articu- with the radius is roughly triangular in proximal view, the lating with the pisiform is also wider than deep, and flat. The lateral part being much broader than the medial one. It is stability of the pisiform-cuneiform articulation is provided convex anteroposteriorly. The anterior and posterior borders by a lateral hook on the pisiform (see description below). of the facet are characterized by depressions that stabilize The anteroproximal facet articulating with the ulna is slightly the flexion of the hand. The occurrence of the anterior broader than that articulating with the pisiform, but is more J Mammal Evol

Fig. 10 Carpal elements of Arctocyon primaevus. a, right pisiform, view; e, left lunate, MNHN.F.CR57, in lateral view; f, right magnum, MNHN.F.CR34, in proximal (dorsal) and distal (ventral) views; b, MNHN.F.1307, in anterior view; g, right trapeziums, MNHN.F.CR35 right cuneiform, MNHN.F.CR38, in proximal and distal view; c, right (left) and CR823 (right), in anterior view. All elements belong to the scaphoids in proximal, distal, and anterolateral views (from left to sub-complete specimen except the scaphoid, CR17474, the lunate, right). The upper one is MNHN.F.CR39, the lower one is CR57, the magnum, 1307, and the trapezium, CR823. Scale bar equals MNHN.F.CR17474; d, left unciform, MNHN.F.CR36, in anterior 3cm concave, except the most lateral part, which is slightly proximodistally. The distal facet articulating with the unci- convex. Its anterior border protrudes proximally, which form is kidney-shaped and concave. Medially, a crescent- helps to stabilize the manus during dorsiflexion. The lateral shaped facet articulates with the lunate. Several other cunei- side of the cuneiform is rounded and robust. It is very forms are known, e.g., a left one (CR59) and a right one smooth, which suggests a tight contact with a ligament or (CR1746). They show some variability, e.g., in the size of the tendon of a muscle. The cuneiform is compressed the pisiform facet relative to the ulnar one, the space J Mammal Evol

Fig. 11 Metacarpals and phalanges of Arctocyon primaevus. a, right ungual phalanges in dorsal view. From left to right, MNHN.F.CR45, Mc V, MNHN.F.CR40, in anterior and medial views; b, right Mc I in CR46, CR486, and CR54; the two last ones are not associated with the anterior and posterior views, MNHN.F.CR17473; it is not associated sub-complete specimen; e, the same phalanges in lateral view, same with the sub-complete specimen; c, intermediate and proximal phalan- order. It is not established that the phalanges belong to the hand. Scale ges, from left to right, MNHN.F.CR66, CR44, and CR43, in anterior bars equal 3 cm (dorsal) view, the left one showing a pathological ossification; d, between these facets and the lateral protrusion, and the relief narrow transversely (Fig. 10a). The apex, where the flexor of the unciform facet. In contrast to Russell’s statement carpi ulnaris attaches, is robust. The facet articulating with (1964), the cuneiform does not articulate with Mc V. the ulna faces proximally. It is roughly triangular in outline, The right pisiform (CR34) is 28 mm long, representing and concave; it provides a loose articulation with the ulna. 83% of the length of Mc V. It is deep proximodistally, but The facet articulating with the cuneiform is broader and J Mammal Evol extends ventrally. It is also flatter than that articulating of the bone consists of two articular facets, for the unciform with the ulna. The two facets form an acute angle and and lunate (laterally) and scaphoid (medially). are separated by a sharp crest. The medial extremity of The right Mc V (CR40) from the sub-complete specimen the two facets forms a robust process, whereas the is 33 mm long and the diaphysis is 6 mm wide. The lateral lateral one is hook-shaped. At least three other isolated side of the diaphysis presents proximally a tuberosity for the pisiforms of similar size and shape are known from the attachment of the extensor carpi ulnaris (Fig. 11a). The same locality. proximal articular facet is broad, broader dorsally than ven- The left and right unciforms (CR36 and CR37) are trally, and slightly depressed medially. On the medial side of smaller than the three proximal carpal bones coming from the diaphysis, a dorsoventrally long proximal facet articu- thesamespecimen(Fig.10d). Another right unciform lates with Mc IV. The distal epiphysis is slightly asymmet- (BR10666), equal in size, is known. The proximal articular rical, the lateral condyle beingmoredepressedthanthe facet faces proximolaterally and is mainly convex, except at medial one. The transverse axis of the epiphysis is not its lateral extremity where it is slightly concave. It articulates twisted in relation to the diaphysis. At least two isolated left mainly with the cuneiform, except for the most anteroprox- Mc V are known (BR11378 and BR11879). imal part that contacts the lunate. As in the scaphoid, the Two isolated Mc I are known, a left one, numbered 1302 palmar process is robust but short. Medially, the main axis that looks pathological, and a right one, smaller, illustrated of the facet articulating with the magnum is vertical. This here (CR17473, Fig. 11b). The left one implies a trapezium facet is elongate proximodistally but short anteroposteriorly. slightly bigger than CR35, but the size of the right Mc I, Distally, the facet articulating with Mc V is roughly trian- illustrated, corresponds to that of the known trapezium. It is gular in outline, broader anteriorly than posteriorly. The shorter but as robust as the known Mc V CR40. The prox- most medial part of the distal facet, a narrow strip extended imal articular facet is mostly convex and quadrangular in anteroposteriorly, articulates with Mc IV, but no crest sepa- proximal view. It is asymmetrical, showing a posterolateral rates it from the Mc V articulation. This narrowness indi- prominence and an anterolateral deflection. Moreover, the cates that the Mc IV mainly articulates with the magnum. lateral part of the facet is more elongate anteroposteriorly This distal metacarpal articulation provides an important than the medial part. The proximal facet exhibits also a range of anteroposterior gliding movements. dorsal indentation, corresponding to that observed on the The right trapezium (CR35) is hardly smaller than the trapezium. Medially (towards the inner side of the hand), a unciform, indicating that the pollex was not reduced proximal fossa suggests a strong ligamentous attachment (Fig. 10g). The facet articulating with the scaphoid, trian- between the trapezium and Mc I. The distal epiphysis is gular in shape, is oriented proximomedially and is convex. very asymmetrical, much more than that of Mc V, with a It allows some rotational and transverse gliding move- lateral condyle that protrudes distally and is much more ments between the two bones. Proximolaterally, it is con- developed than the medial one. The three phalanges associ- tinuous with the facet articulating with the trapezoid. The ated with the sub-complete specimen have the usual shape anterior side presents a medial prominence possibly for the of mammalian phalanges (Fig. 11c). attachment of the abductor pollicis longus. The posterior Two ungual phalanges (CR45 and CR46) are known and side is reduced and does not exhibit any palmar process. figured in Russell (1964); it is not known if they belong to This is related to the fact that when articulated, the sca- the fore or hind foot. They are 18–19 mm long, and CR46 is phoid projects far more posteriorly. The distal articular deeper proximodistally than CR45 (Fig. 11d–e). The prox- facet with Mc I is almost divided into two parts, parallel imal facet articulating with the intermediate phalanx is as one to the other, with an anteroposterior longitudinal axis. high as wide. They both exhibit a prominent flexor tubercle. The most lateral part is more concave than the medial one. They are not compressed transversely, and the proximal Another isolated right trapezium (CR823) is known and articulation is smooth, without a sharp crest. The distal illustrated here. extremity is blunt and cleft. Other isolated ungual phalanges Three isolated left lunates are also known (CR57, are known (CR53, CR54, CR486), two of them being illus- CR603, and CR606), as well as an isolated magnum (col- trated here (Fig. 11d–e), and all show a developed ventral lection number: 1307), but their attribution to Arctocyon tubercle and a cleft tip. cannot be ascertained and their size in comparison to the other carpal bones described cannot be discussed. The lu- Pelvic Girdle and Hind Limb nate (Fig. 10e) is a small bone, quadrangular in proximal view. The distal side is deeply concave, and shows two Innominate Several innominates are known, although in- facets articulating with the magnum and unciform. The complete, and the pubis is lacking in all of them. The ilium magnum (Fig. 10f) is also small and roughly triangular in is blade-like and approximately quadrangular in outline anterior view. The anterior side is reduced and the main part (Fig. 12a–b). It is characterized by the outward deflection J Mammal Evol

Fig. 12 Innominates of Arctocyon primaevus. a, MNHN.F.CR70 in left, MNHN.F.BR12711 on right, in lateral view, showing a well- lateral view; b, MNHN.F.CR69 in lateral (above) and ventrolateral preserved ischiatic tuberosity. The elements MNHN.F.CR69 and (below) views; c, MNHN.F.CR69 (right) and CR70 (left) in dorsal CR70 (a, b, c) belong to the sub-complete specimen. Scale bar equals view; d, two left partial innominates, MNHN.F.BR17472, reversed, on 5cm of the anteroventral iliac crest, where the sartorius originates full but the ischiatic tuberosity can be observed in two (Fig. 12c). The gluteal fossa represents the broadest part of isolated right innominates (Fig. 12d): it is prominent dorsal- the ilium, whereas the iliacus fossa is narrow and occupies ly but not laterally, in contrast to the anterior iliac crest. The the ventral side of the iliac blade. In between them, the main axes of the ilium and ischium form an obtuse angle, of acetabular border is sharp. The iliac neck is long, increasing approximately 150 ° in CR70 (Fig. 12a, innominate of the the distance between the acetabulum and the sacrum. The sub-complete specimen). The dorsal branch of the ischium acetabulum is widely open; the dorsal border is not promi- is characterized by a particularly large prominence (the nent laterally, and no constriction is observed at the ilio- ischiatic spine) where the ischio-caudalis and gemelli ischiatic suture. The articular facet presents the same width originate (Argot 2003a, b). all along its length, except for the most anterior part, which is slightly wider. Just anteriorly is located the rectus femoris Femur The two femora of the sub-complete specimen tuberosity. Ventrally, the tubercle where the psoas minor (CR71 and CR72) are known (Fig. 13), as well as half a inserts is small. The exact orientation of the acetabular facet dozen other femora, more or less well preserved. The diaph- is impossible to assess because of the incompleteness of the ysis is not twisted. The head is ball-shaped in all directions and innominates available. The ischium is rarely preserved in the posteromedial fovea capitis (ligamentum teres insertion) is J Mammal Evol

Fig. 13 The left and right femora, respectively MNHN.F.CR71, CR72, of Arctocyon primaevus (sub-complete specimen). a, anterior view; b, medial view; c, proximal view; d, distal view; e, posterior view; f, lateral view. Scale bars equal 5 cm

wide and deep. The posterior part of the articular facet extends trochanters, emphasizes the attachment of the quadratus slightly towards the greater trochanter (Fig. 13e). The femoral femoris (Argot 2002). More distally, longitudinal scars neck is short and thick. The greater trochanter, where the deep extending along the diaphysis from the lesser and third glutei insert, remains lower than the head. The femur of trochanters to the distal condyles (0 linea aspera) emphasize Arctocyon is characterized by the presence of a third trochan- the attachment of the adductors. ter, located on the lateral crest running from the greater Distally, the femoral trochlea exhibits sharp and symmet- trochanter, and located at the level of the lesser trochanter. rical crests (Fig. 13d). An additional facet, located proximal The lesser trochanter is prominent medially and blade-like, to the trochlea, can be observed on the femora of the sub- extended proximodistally (Fig. 13a, e). In posterior view, a complete specimen (Fig. 13a). In other specimens, this facet crescent-shaped scar, running between the lesser and the third is less individualized, and extends the articular area of the J Mammal Evol trochlea proximally. This peculiar feature (possibly anoma- does not contact the femur. The diaphysis is straight, ovoid lous or related to the age of the animal at death) seems to to triangular proximally. The proximal triangular section is emphasize the distal contact between the tendon of the related to a sharp lateral crest that shares the origin of the quadriceps femoris and the femoral diaphysis, as if in old digital extensors anteriorly (extensor hallucis longus in par- individuals, an ossification would occur in the tendon, in ticular) from that of the peronei posteriorly. At the proximal addition to the patella. A patella is known for the sub- extremity of this crest, the tubercle where the extensor complete specimen. Seen in lateral view, the trochlea extends digitorum longus attaches is prominent. Medially, the inter- farther proximally than the posterior condyles (Fig. 13b, f). osseous crest is sharp proximally; it is not prominent on the The distal extremity of the femur is anteroposteriorly tibia. A large space remains between the tibia and fibula deep and the condyles are asymmetrical. Although the two when articulated (Fig. 14b). The distal epiphysis is more condyles exhibit approximately the same width, the medial massive than the fibular head and presents two crescent- condyle is more convex transversely and protrudes distally shaped facets, the medial one for the astragalus, the lateral and posteriorly, whereas the lateral condyle is more flat, and one for the calcaneum. The facet articulating with the tibia, protrudes laterally. These differences are more or less em- in continuity with that for the astragalus, is not well phasized according to the specimen observed. It is notewor- delimited. The anterior extremity of the epiphysis protrudes thy that the main longitudinal axis of these condyles is not distally. Laterally, the groove for the tendons of the peronei vertical but slightly oblique (proximolateral/distomedial). In is shallow but is limited anteriorly by a prominent tubercle. medial view, the tuberosity where the medial ligament and The tibia and fibula both show distally an anterior sharp gastrocnemius internus originate and where the adductor crest (anterolateral on the tibia) suggesting the insertion of a magnus inserts is prominent. strong interosseous ligament, analogous to the pronator quadratus in the antebrachium. Tibia The left tibia is known from the sub-complete speci- men (CR73) as well as half a dozen isolated tibiae. The Tarsal Bones, Metatarsals, and Phalanges According to diaphysis is straight in anterior and lateral views (Fig. 14). Russell (1964), only the first, second, and third left meta- The anterior crest is not sharp but becomes sharper at the tarsals have been found associated with the sub-complete level of two scars for the insertion of gracilis and semite- specimen. Several isolated calcanei, astragali, cuboids, ndinosus caput ventrale, located above the midshaft level naviculars, and cuneiforms have been found in Cernay- (Argot 2002). Proximally, the notch for the insertion of the Berru and many of them have been allocated to Arctocyon, quadriceps tendon is very well emphasized in CR73, much based on their size. The other large genera in the same fauna less in the other tibiae. The medial side shows, proximally, a are Dissacus and Tricuspiodon, but they usually remain scar for the insertion of the medial ligament coming from smaller than Arctocyon and differ markedly in anatomy. the femur. Laterally, the fossa where the tibialis anterior The left calcaneum CR65 and the left astragalus CR62 originates is deep proximally. articulate well one to the other as well as to the tibia of the The two proximal facets articulating with the femoral con- sub-complete specimen. The most important characters of dyles are subequal in size, the medial one being concave where- the tarsals identified as belonging to Arctocyon are de- as the lateral one is flat, consistent with the morphology of the scribed (the convention used for describing tarsals is that femoral condyles. The lateral articular facet also shows a pos- anterior is towards the toes): on the calcaneum (CR750 is terior extension (Fig. 14c), suggesting, like the proximal exten- illustrated in Fig. 15a), the sustentacular and ectal facets sion of the trochlear facet, a narrow contact with the tendon of are widely separated. The sustentacular facet is slightly the gastrocnemius externus and possibly with a sesamoid inside. concave, and extends to the cuboid facet via a small Beside this posterior extension is located the facet articulating accessory facet. The ectal facet is convex and oblique with the fibula. The distal facet articulating with the fibula is with a longitudinal axis oriented posteromedially/antero- small, in continuity with the astragalar facet. The astragalar facet laterally. Laterally, it exhibits a convex area that articulates presents two concavities, whose main longitudinal axis is ante- with the fibula. In anterior view, the proximal part of the roposterior. The malleolus is not very prominent distally. ectal facet is oriented more medially than dorsally, where- as the distal part is oriented anteriorly. The cuboid facet is Fibula The left fibula is also known from the sub-complete slightly concave. The peroneal process is not very prom- specimen (CR74: Fig. 14) as well as several isolated fibulae. inent laterally. The head is small, showing a broad articular tibial facet. On the astragalus (several isolated astragali are known; Posteriorly, it shows another, much smaller, facet, possibly CR752 is illustrated in Fig. 15b), the trochlea is shallow, for a contact with the tendon of the gastrocnemius externus, without sharp crests. Anteriorly, it extends onto the astraga- and/or the parafibula inside (Fig. 14e). The head is not lar neck, which is short and thick. The posterior part extends extended anteroposteriorly, or prominent proximally, and only laterally. The posterior side is perforated by a broad J Mammal Evol

Fig. 14 The left tibia, MNHN.F.CR73, fibula, MNHN.F.CR74, and view; c, tibia and fibula in posterior view; d, tibia in lateral view; e, patella, MNHN.F.CR68, of Arctocyon primaevus (sub-complete spec- fibula in medial view. Scale bars equal 3 cm imen). a, tibia in proximal view; b, tibia, fibula, and patella in anterior foramen. Distal to this foramen, a deep groove emphasizes calcaneum is slightly convex, providing for a loose artic- the development of the powerful tendons of the flexor ulation. In dorsal view, it faces more laterally than prox- digitorum fibularis. The medial crest of this groove is more imally. Medial to this facet another facet is present, prominent than the lateral one. On the lateral side, the facet concave and oriented proximally, articulating with the articulating with the fibula is crescent-shaped, more extend- lateral part of the astragalar head. On the medial side of ed anteriorly. Medially, the articular area for the tibial mal- the cuboid, two other articular facets are present, the more leolus is reduced, consistent with the weakly developed proximal one for the navicular (it is as extended dorso- tibial malleolus. The astragalar head is oriented anterome- plantarly as the astragalar facet above), the more distal dially, very broad and robust. The medial extension of the one, smaller, for the ectocuneiform. The cuboid 1755 articular facet for the navicular is developed. At the lateral presents a second facet articulating with the ectocunei- extremity, anterodistally, a facet articulates with the cuboid. form, but this facet is not present on the other cuboids Its separation from the articular facet for the navicular is observed. The plantar tuberosity is robust and overhangs a more evident in some specimens than in others. deep groove for the tendon of the peroneus longus. The The cuboid (one among several known is illustrated in facet that articulates with the lateral metatarsals is broad Fig. 15c) is massive. The facet articulating with the and triangular in outline. J Mammal Evol

Fig. 15 Tarsus of Arctocyon primaevus. No element belongs to the MNHN.F.CR17475, in dorsal and medial view; d, left navicular, sub-complete specimen. a, left calcaneum, MNHN.F.CR750, in dorsal, MNHN.F.1747, in proximal and distal views; e, right ectocuneiform, lateral, and anterior views; b, right astragalus, MNHN.F.CR752, in MNHN.F.CR58, in proximal and medial views. Scale bars equal 3 cm dorsal, ventral, anterior, and posterior views; c, left cuboid, for a–b, 2 cm for c–e

The navicular (Fig. 15d) is flattened proximodistally in continuous, convex surface. The articular facet for the ento- dorsal view. The astragalar facet is broad and its concavity is cuneiform appears as the broadest on 1747, and apparently increased by a medial prominence. The plantar tuberosity is could allow gliding and rotational movements. This facet is massive. Laterally, the articular facet with the cuboid is slightly concave and clearly separated from the most lateral extended dorsoplantarly, slightly convex dorsally but con- articular area on another navicular referred to Arctocyon, cave plantarly. The articulation with the cuneiforms forms a numbered BR12647. J Mammal Evol

On the ectocuneiform (CR58 is illustrated in Fig. 15e), both metatarsals; it is grooved longitudinally in Mt II where- the articular facet with the navicular is rectangular in out- as the facet of Mt III is flat and faces proximomedially. The line, extended dorsoplantarly, and slightly concave. The dorsal side of these two metatarsals also exhibits proximally plantar tuberosity is massive. Laterally, a single, proximal, a rough area suggesting the insertion of tarso-metatarsal articular facet with the cuboid can be observed. Medially, a ligaments. The condyles of the distal epiphyses are not proximal, small facet articulates with the mesocuneiform. asymmetrical. The Mt I (Fig. 16a, c–e) is 28 mm long. It Two distal facets articulate with Mt II. is longer, more slender, and more curved dorsoplantarly The right Mt III (CR42), Mt II (CR41), and Mt I (CR67) than the Mc I described above. The diaphysis is as robust have been found associated with the sub-complete specimen as that of the other metatarsals, and more concave ventrally. (Fig. 16a). The Mt III is 43 mm long, representing 28% of The distal epiphysis is asymmetrical, as in Mc I, the lateral the tibial length. The Mt II is 37.5 mm long. The diaphyses condyle being more developed than the medial one, and of the metatarsals are robust, with wide distal epiphyses. more prominent distally. The proximal articular facet is The proximal articular facet is subrectangular in outline in subtriangular in outline, wider plantarly than dorsally. The

Fig. 16 Elements of the pes of Arctocyon primaevus. a, articulated primaevus, in lateral, dorsal, and proximal views; c, two Mt I in dorsal right Mt III, MNHN.F.CR42, Mt II, MNHN.F.CR41, and Mt I, view, MNHN.F.CR67 on the left, MNHN.F.CR335 on the right; d, the MNHN.F.CR67, belonging to the sub-complete specimen; b, left Mt same Mt I in lateral view; e, Mt I, MNHN.F.CR335, in proximal view. V, MNHN.F.BR11862, belonging to another specimen of A. Scale bar equals 3 cm J Mammal Evol lateroplantar extremity of this facet protrudes proximally, The complexus action in particular is similar to that of the whereas the dorsal part is depressed on the diaphysis, as in biventer cervicis, and these two muscles are thick and pow- Mc I. Laterally, the proximal epiphysis does not exhibit any erful in active predatory taxa like the lion (Barone 1967). articular facet with Mt II (whereas Mt II and III are articu- The very oblique vertebral bodies emphasize the neck cur- lated together). Medially, a proximal fossa indicates the vature until the C6 level. The development of the neck attachment of the peroneus longus or tibialis anterior. When musculature may be partly due to the size of the skull, which articulated, the Mt I diverges from the other metatarsals, and is large in comparison to the size of the cervical vertebrae. the dorsal side faces dorsomedially. Another left Mt I The long neural processes of the anterior thoracic verte- (CR335) is also known, showing the same length, shape, brae are consistent with a well-developed extensor muscu- and robustness. lature (multifidus dorsi, semispinalis dorsi, and splenius), Several other isolated metatarsals are known from attaching on the apex of the most anterior process. Their Cernay-Berru. Among them is a Mt IV (CR50) that exhibits slenderness and posterior inclination contrast with the short, a proximal facet that is horizontal in dorsal view and not vertical, and anteroposteriorly wide neural processes of oblique as in Mt III, but also depressed dorsally. The dorsal Manis and Myrmecophaga (no anticlinal vertebra exists in side of CR50 also shows proximally a rough ligamentous these taxa in contrast to Orycteropus). The vertebral neural insertion. The distal epiphysis is not asymmetrical. At least processes are particularly robust in Myrmecophaga,where two isolated Mt V are known, CR332 (a left Mt V) and they seem to form an uninterrupted wall from C7 to the sacrum BR11862 (a right Mt V, Fig. 16b). They are equivalent in (Fig. 17b). Whereas in Myrmecophaga and Priodontes the ribs length and the diaphyses show the same robustness, but the are extremely robust and wide anteroposteriorly (with only proximal epiphysis is more robust in CR332, especially the two pre-sacral vertebrae without ribs: Fig. 18b), in Arctocyon development of the peroneal tuberosity (for the peroneus the ribs are much more slender but the first pair exhibits a brevis). The proximal facet articulating with the cuboid is sharp lateral crest underlining the attachment of the scalenus, a flat, and a medial facet articulating with Mt IV can be flexor of the neck. observed, exhibiting in the middle a prominence indicating The posterior thoracic vertebrae of Arctocyon are charac- a tight interlocking between the two metatarsals. The distal terized by the revolute and tightly interlocked articular fac- epiphysis is slightly asymmetrical, the lateral part being ets that preclude any rotation and lateral flexion of the wider transversely, and the medial part more developed vertebral column. Such a specialized morphology has not distally. been observed in extant taxa but it exists in Pachyaena (Zhou et al. 1992: fig. 4A–B). In modern ungulates and especially artiodactyls, the rigidity of the vertebral column Discussion is promoted by the short but anteroposteriorly wide spinous processes linked by a poorly elastic supraspinous ligament, Vertebrae by transverse processes situated in the horizontal plane and linked by intertransversal ligaments, and by the formation of The length and posterodorsal expansion of the neural pro- hinges in the articular processes of the lumbar vertebrae cess of the axis, not observed in extant taxa but observed in (Slijper 1946;Gambaryan1974). These structures make the mesonychid Pachyaena (Zhou et al. 1992:fig.2A), the whole column much more rigid than in Arctocyon.In underlines the development of the obliquus capitis caudalis, artiodactyls, the prezygapophyses embrace the postzyga- a powerful rotator of the head, and of the spinalis capitis, an pophyses of the previous vertebra not only from the side extensor of the head that attaches on the posterior tip, which but also from above, which restrict both lateral flexion and is prominent dorsally here (Argot 2003a). The obliquus vertical flexure of the spine (Slijper 1946;Gambaryan capitis caudalis is particularly developed in extant carni- 1974). This can also be seen in Pholidota (e.g., Manis: voran taxa with active predatory habits, but it is worth Fig. 18a). However, the shape of the articular facet is not noting that a posteriorly prominent axial neural process is reallysigmoidasitisinArctocyon.Consideringthata found more usually in felids and mustelids than in canids dorsostable locomotion is energetically efficient and may and hyaenids (Argot 2003a, b and references therein). Zhou be employed by grazers, scavengers, and long-distance et al. (1992) interpreted it as able to resist the powerful pull pursuit-hunters (Hildebrand 1959; Gambaryan 1974), it is exerted by the epaxial muscles and ligamentum nuchae in impossible to conclude about the mode of life of Arctocyon relation to the size of the skull and feeding behavior. In C6 without taking into account the information provided by the (CR89) and T1 (CR90), the characteristic protrusion of the rest of the skeleton. Robust anapophyses and mammillary dorsal arch, not observed in extant taxa, underlines the processes exist both in highly arboreal taxa like the bintur- attachment of the complexus or of the multifidus and ong Arctictis binturong (Fig. 17a) or the pangolin Manis semispinalis cervicis, which are all extensors of the head. javanica and in fossorial ones (Myrmecophaga, J Mammal Evol

Fig. 17 Drawings of a, the binturong Arctitis binturong and b, the an arboreal taxon (above) and fossorial one (below). Adapted giant anteater, Myrmecophaga tridactyla emphasizing the overall pro- from Blainville (1841). Scale bars equal 10 cm portions and general morphology of vertebrae and limb bones in

Priodontes). However, if in Myrmecophaga jubata and lumborum, as well as a poor fusion of the muscular mass in Manis javanica the mammillary processes are dorsally arctocyonids. The development of this muscular mass is prominent, they do not reach the same robustness as in consistent with the outward deflection of the anteroventral Arctocyon, and they are associated with low and anteropos- part of the ilium. The robust anapophyses and their lateral teriorly wide neural processes, which is not the case in the crest present in the most posterior thoracic vertebrae indicate fossil taxon. The binturong exhibits short and broad neural an anterior attachment of the longissimus caudae, indicative processes in the posterior thoracic area whereas in the aard- of a long tail. The exact position of the diaphragmatic and vark the neural processes are long and slender. The anterior anticlinal vertebrae cannot be assessed, but they are present inclination of the lumbar neural processes and the anterior in the posterior part of the thoracic area; the diaphragmatic protrusion of the lumbar transverse processes in Oryctero- and anticlinal vertebra is T10 in Pachyaena, a taxon that pus afer indicate powerful tractions exerted from the ante- presents twelve thoracics and seven lumbars (Zhou et al. rior part of the body. 1992). This position is found in many predators (e.g., felids) The prominent mammillary processes emphasize a pow- and indicates a sagittal flexibility of the vertebral column, erful epaxial musculature (longissimus dorsi in particular, although the neural processes of the lumbars of Arctocyon plus iliocostalis, multifidus, and semispinalis dorsi). In (not very inclined anteriorly) and their transverse processes living didelphids, this muscular mass is not fused, and the (not particularly prominent ventrally or anteriorly) indicate most powerful muscle of the mass, the longissimus dorsi reduced flexion in comparison to fast and agile predators lumborum, attaches on the mammillary processes instead of like the wolf. It is worth noting that the less sigmoid artic- at the apex of the neural processes (Argot 2003a). The ular facets of the most posterior lumbar vertebra indicate development of the mammillary processes of Arctocyon that the flexibility of the vertebral column increased towards suggests, then, powerful tractions by the longissimus dorsi the sacrum. J Mammal Evol

Fig. 18 Dorsal view of posterior thoracic and lumbar vertebrae of a, Manis javanica, MNHN.A3122, with an enlarge- ment of the articulation of lumbar vertebrae; b, Myrmecophaga tridactyla, MNHN.A13806; c, Arctictis binturong, MNHN.A3494. Scale bars equal 5 cm

The morphology of the most proximal caudal is particu- as seen in Arctocyon. This protrusion, present on the anterior larly striking. Because it exhibits similar robustness and border of the transverse process in Arctocyon, can also be development of anapophyses and mammillary processes to observed in Arctictis binturong (Argot 2003b: text-fig. the last thoracics, Russell (1964) thought that this vertebra 12B). In Myrmecophaga, the first caudals appear compara- was a first lumbar. However, the reduced neural process and tively less massive, but the lumbar area is so modified that it the size and orientation of the transverse processes, com- does not appear relevant to make a comparison. In Orycter- pared to those of the anterior lumbar vertebra, preclude this opus the first caudal vertebrae are extremely robust, with identification. In Manis javanica, the transitional vertebra transverse processes protruding posteriorly. The morphology between the thoracic and lumbar parts (corresponding to a of the vertebra CR99 indicates that the anterior caudal twelfth thoracic vertebra) bears laterally prominent trans- vertebrae of Arctocyon were tightly interlocked like the verse processes (Fig. 18a). Nevertheless, in this case, the vertebrae anterior to the sacrum. The robust mammillary transverse processes of the following lumbar vertebrae are processes, anapophyses, and transverse processes indicate horizontal and laterally prominent, in contrast to what is that the main muscles moving the tail (longissimus caudae, observed in Arctocyon. Moreover, in Manis javanica the ischio-caudalis, abductor caudae dorsalis) were extremely transverse processes of the first caudals are extremely de- developed. However, the vertebral body does not exhibit veloped (Fig. 19a), much longer and more massive than ventrally articular facets for hemapophyses, whereas these those of the lumbars. The transverse processes of the first elements are extremely developed in taxa with a robust tail caudals may exhibit a variably developed dorsal protrusion, like the Javan pangolin, the anteater, and the aardvark. The J Mammal Evol

Fig. 19 Dorsal view of anterior caudal vertebrae of a, Manis javanica, MNHN.A3122; b, Myrmecophaga tridactyla, MNHN.A13806; c, Arctictis binturong, MNHN.A3494. Scale bars equal 10 cm

tail of Arctocyon clearly played an important role in its acromion is extremely well developed, extending anteriorly locomotion. over the shoulder joint. In Orycteropus, there is no post- scapular fossa but the acromion is also well developed Forelimb although not to the extent of the two previous taxa. In Arctocyon, the general shape of the scapula is more similar Arctocyon exhibits several features also found in arboreal to that of a more generalist taxon like Meles, Neofelis,or taxa with manipulative behavior (felids, viverrids, and Gulo, and the acromion is not particularly wide or promi- ursids among placental carnivorans, borhyaenoids among nent anteriorly, similar to that of Arctictis binturong (Argot metatherians): a scapula roughly triangular in outline, a very 2003b: text-fig. 13A). A part of the deltoid musculature long and robust deltopectoral crest, a long lateral epicondy- originates from it and, on the humerus, the deltoid insertion lar crest, and a robust ulnar diaphysis that is convex poste- area appears to be very long but narrower than the pectoralis riorly. Moreover, the shallow glenoid cavity and ball-shaped insertion area. The long and prominent deltopectoral crest is humeral head that barely projects posteriorly both indicate much better developed in Arctocyon than in the extant taxa that the humero-scapular joint is highly mobile, apparently observed and is more similar to that of fossil metatherians of stabilized mainly by the two well-developed spinati muscles similar size like Prothylacinus (see below). The robust ver- as well as the teres minor (laterally) and the subscapularis tebral border and the additional crest visible on the medial (medially). side of the scapula CR15 suggest a well-developed serratus The slender, triangular scapula is very different from that anterior, a muscle that, like the pectoralis, plays a role in of extant taxa with fossorial forelimb capabilities, such as load transmission from trunk to limb and in climbing (the Myrmecophaga and Priodontes, where it is particularly serratus for rotating the scapula, the pectoralis for adducting broad, with a wide, massive scapular neck, a secondary and medially rotating the humerus, see Argot 2001). The scapular spine, and an extended triangular postscapular fos- caudal angle of the scapula and the distinct medial scar on sa (Figs. 17b, 20b).Moreover,inthesetwotaxathe the humerus both indicate a well-developed teres major, J Mammal Evol

Fig. 20 Right forelimb in ante- rolateral view of a, Manis javanica, MNHN.A3122; b, Myrmecophaga tridactyla, MNHN.A13806; c, Arctictis binturong, MNHN.A3494. Scale bars equal 10 cm

which confirms that the adductors of the arm were devel- The massive entepicondyle and the deep fossa on the oped, more than in the badger but not as much as in spe- medial side of the ulna indicate powerful digital flexors cialized fossorial taxa such as Myrmecophaga and (especially the flexor digitorum profundus), whereas the Priodontes. The medial prominence of the deltopectoral sharp crest and fossae of the lateral side indicate well- crest suggests an adductive traction, as if the latissimus dorsi developed extensors and abductor of the most inner digits, inserted here instead of inserting in the bicipital groove or especially the pollex. The humero-radial joint is stabilized medial to it as is usual. by the contact between the flat lateral part of the radial head The short coracoid process and weak bicipital tuberosity and the lateral extension of the capitulum. Therefore, the do not suggest a strongly developed biceps brachii. The oval radial head with a relatively flat contact with the ulna, development of the triceps, especially the medial head that the right angle between the coronoid process and radial extends the elbow joint, is suggested by the medial promi- notch, and the weakly convex pronator crest suggest a nence of the olecranon. However, the length of the olecra- limited range of pronation-supination. However, similar non in Arctocyon is much less than in specialized fossorial features may be observed in the badger, which is able to taxa (see Priodontes, but also Rose and Emry 1983 and dig and handle objects, and the supination range is also MacPhee 1994). On the scapula, the origin of the triceps related to the orientation of the radial head relative to the caput longum is less developed than that of the teres major. distal end (Argot 2003b). In Arctocyon, the transverse axes The distal part of the humerus indicates well-developed of the proximal and distal ends of the radius form an angle extensors and flexors of the elbow (brachioradialis and of approximately 30°, as in Arctictis binturong and Meles supinator brevis) and of the wrist joints (extensor carpi meles, whereas this angle is close to 0° in cursorial taxa. In radialis and flexor carpi ulnaris in particular). The articular comparison to the ulna, the radius is more slender in Arcto- facets indicate that the elbow joint was not well stabilized, cyon than in fossorial extant taxa like Myrmecophaga and except by ligaments and muscles including the anconeus Priodontes. In these extant taxa, the straight to slightly and epitrochleo-anconeus. The asymmetrical trochlea indi- concave ulnar diaphysis also contrasts strongly with that of cates a forelimb usually slightly abducted. The elbow joint Arctocyon, whereas in Orycteropus and Meles the ulna is morphology of Arctocyon contrasts deeply, then, with the straight, and convex posteriorly in Arctictis (Argot 2003b: narrow distal humerus, narrow and concave trochlea, and text-fig. 15b). In Arctocyon, despite the robustness of the perforate olecranon fossa observed in terrestrial, cursorial ulna, it is worth noting that the radial head is broader than taxa like extant canids. It is much more similar to that the coronoid process, as is usual in extant felids and ursids, observed in the badger. and increases the role of the radius in the transfer of the J Mammal Evol weight from the arm to the forearm. The robustness of the the prairie dog (Cynomys ludovicianus), armadillos (Dasypus, ulna appears, then, more related to the development of the Euphractus)…etc., present ungual phalanges much narrower muscles than to load transfer. Although the distal radio-ulnar and less curved in lateral view. joint is quite loose, the development of the interosseous and pronator quadratus crests suggests, as for the shoulder and Hind Limb elbow joints, stabilization by ligaments and muscles. The pronator quadratus crest is also prominent on the ulna of Both the acetabulum and femoral head of Arctocyon indicate Meles. Considering that the movements were restricted a highly mobile hip joint, stabilized by ligaments (ligamen- more by muscles and ligaments than by the structure of tum teres) and muscles, like the gemelli. Although the broad articulations itself, it is obvious that Arctocyon had high area of the iliac blade and the presence of a third trochanter manipulative capabilities, with an emphasis put on the suggest well-developed glutei, the greater trochanter is low- abductors and adductors of the arm and forearm, and on er than the femoral head in all the specimens observed, as is the flexors and extensors of the wrist and digits. usual in arboreal and scansorial taxa. The outward deflec- The lateral carpal elements are as developed as the medial tion of the iliac crest may be related both to the development ones, and the hand was wide, with five digits equally devel- of the epaxial lumbar musculature and to that of the sarto- oped. The robust and long pisiform represents a long lever- rius, a flexor and adductor of the hip. This iliac deflection is arm for the flexor carpi ulnaris. The development of this also present in, e.g., bears, badgers, and aardvarks; i.e., it is bone also suggests very well-developed abductor and flexor usually related to a short lumbar area and taxa that are digiti minimi brevis. All the articulations (between the car- lumbosacrally strongly built. Although the origin of the pal elements themselves, as well as between the radius and adductors is not preserved because of the missing pubis, ulna) are quite loose, with movements limited by ligamen- rough scars emphasize their insertion on the femora. There- tous and muscular attachment, but also with dorsiflexion fore, as in the forelimb, the adductors of the hind limb were limited by anterior osseous stops, especially on the scaphoid powerful, as well as the rotators like the quadratus femoris. and cuneiform. In the scaphoid, the reduced anterior surface The robustness and extension of the lesser trochanter also between the proximal radial articular facet and the distal underline the development of the iliopsoas, which provides trapezium facet indicates that the metapodials formed an powerful flexion of the hip joint. This trochanter is much obtuse angle with the long bones of the forearm when more reduced in the extant taxa observed, including Manis articulated and seen in lateral view, instead of being in line javanica and Myrmecophaga, although it is large in the as in extant digitigrade taxa. The size of the trapezium and armadillo Euphractus.InOrycteropus, the third trochanter the poorly stabilized scaphoid-trapezium articulation indi- is more prominent and much more distal than in Arctocyon. cate that the pollex was mobile. The known Mc I also shows The angle (150°) existing between the ilium and ischium, a very asymmetrical distal end, which indicates that the not very different from that of Ailuropoda, remains difficult flexion-extension of the phalanges did not take place in to explain functionally. The ischium represents 42% of the the same plane as that of the other digits, providing some total pelvic length, as is usual in arboreal-scansorial taxa grasping ability to the hand. The ungual phalanges, relatively (Argot 2003b). Its exact orientation is impossible to assess short and without sharp dorsal crests, are not those of a because of the missing pubis (an attempt is made in specialized fossorial or arboreal taxon. Their dorsoventral Fig. 12c), but it was facing apparently more laterally than height is variable (compare CR45 and CR486 in Fig. 11e), ventrally, which provides for a greater range of abduction of the deepest ones being probably associated with the fore- the femur (Jenkins and Camazine 1977). The dorsal protru- limb. The very robust flexor tubercle confirms the develop- sion of the ischiatic tuberosity increases the lever arm of the ment of the flexor digitorum profundus, the most powerful hamstring muscles (biceps femoris and semitendinosus), flexor of the digits. In an eigenshape analysis of ungual which are powerful propulsors. It is also the place where the phalanges, MacLeod and Rose (1993) did not observe Arc- ligamentum sacrotuberosum attaches, a ligament that provides tocyon but did include Arctocyonides and reached no con- stability of the sacral joint. This tuberosity does not reach in clusion about the main locomotor mode of this taxon. Arctocyon the development observed in Orycteropus. However, according to the silhouettes of ungual phalanges The innominate of Arctocyon is especially characterized illustrated in their appendix 1, the unguals of Arctocyon by the development of the ischiatic spine, from which orig- exhibit a much less developed extensor tubercle than that inates the ischiocaudalis, an abductor of the tail. This feature of most carnivorans except Arctogalidia, but the dorsoven- is consistent with the presence of a long and muscular tail, tral depth and curvature are more similar to that of arboreal- and then, with the morphology of the proximal caudal scansorial than to terrestrial-cursorial taxa. The fossorial vertebra described. The ischiatic spine is much more devel- taxa illustrated, the badger (Meles meles), the mountain oped than the tuberosity of the rectus femoris, anterior to the beaver (Aplodontia sp.), the pocket gopher (Geomys bursarius), acetabulum. J Mammal Evol

At the knee joint, the asymmetrical condyles suggest the articular facet, more extended on the metacarpal. Consider- possibility of slight rotations of the leg relatively to the thigh ing that the known Mc I and Mt I do not belong to the same (consistent with the development of the medial ligament), specimen, it is not relevant to discuss their respective length although, as mentioned in the description, this feature is and robustness. variable. The relatively long femoral trochlea is not a feature found in typically arboreal taxa like the binturong where the Comparison with South American Borhyaenoids femoral trochlea is very shallow and the distal end of the femur is wider than high in distal view. The femoral trochlea The arboreal adaptations suggested by the analysis of the is also very shallow in Manis javanica and Priodontes, postcranium could seem unexpected in a relatively abundant deeper in Orycteropus. The morphology observed in Arcto- medium-sized taxon. Nevertheless, the specimen of A. cyon is more similar to that found in felids and mustelids. primaevus analyzed here (its measurements are published The leg of Arctocyon, with a well-developed fibula and a in Russell 1964) is the same size as the early Miocene wide space present between the tibia and fibula when artic- (Santacruzian) South American borhyaenoids Prothylacinus ulated, is not that of a runner. The loose distal tibio-fibular patagonicus (specimen YPM-PU 15700) and Borhyaena articulation (constrained only by a strong interosseous liga- tuberata (specimen YPM-PU 15701): in particular, the ment) indicates some gliding capabilities in relation to the humerus of Prothylacinus shows the same length and ro- movements of the ankle, suggesting an adaptation to move bustness as the arctocyonid humerus (e.g., BR10101: on uneven surfaces. The emphasis is put on the development Fig. 21a–b), with a comparable development of the delto- of the main flexors and extensors of the foot (tibialis anterior pectoral and lateral epicondylar crests, and of the medial and posterior, gastrocnemius externus, and flexor digitorum epicondyle. The other long bones of the forelimb also show fibularis). a similar length in Arctocyon and Prothylacinus (for the The shallow astragalar trochlea does not provide for a ulna, 16.5 cm in both taxa; for the radius, 12.3 cm in high astragalo-tibial stability or a wide range of flexion- Arctocyon, where the radius is also less massive, versus extension. This morphology, together with an ectal facet 13 cm in Prothylacinus). On the hind limb, the femur and that is oriented more medially than dorsally (seen in anterior tibia are 87% shorter in the sub-complete specimen of view), is usually found in arboreal and scansorial taxa. The Arctocyon than in Prothylacinus, but the proportions remain two main mid-tarsal articulations (calcaneo-cuboid and the same and the crural index (tibia/femur) is similar: 0.87 astragalo-navicular) are also poorly stabilized, especially (it is 0.9 in Arctictis and Hyaena, higher in extant cursorial the calcaneo-cuboid one. It is noteworthy that the particu- taxa, see Argot 2003b). In contrast, Mt III is longer in larly wide astragalar head, reflected in the deeply concave Arctocyon: 4.15 cm versus 3.7 in Prothylacinus. Therefore, navicular showing an articular facet extended medially by a the ratio Mt III/femur is higher in Arctocyon than in Prothy- prominence and laterally by a cuboid-astragalar contact, lacinus (0.24 versus 0.19; Arctictis binturong being in be- provides for medial deviations and possibly limited supina- tween: 0.22, whereas this ratio is 0.33 in Thylacinus). tion. Arctocyon had a pentadatyl foot, with metatarsals Prothylacinus has been analyzed as being well adapted for widely spread in the sole. The Mt III length represents climbing, the capacities of flexion, abduction-adduction, 25% of the femoral length while this ratio is 22% in the and pronation-supination being particularly emphasized in binturong specimen observed, and it is higher in most of the forelimb, and with proportions similar to those of extant other carnivoran taxa, except bears (Argot 2003b: table 4). arboreal-scansorial taxa (Argot 2003b). In the analyzed The proximal lateral prominence on the fifth metatarsal specimen of A. primaevus, the neck was about 12 cm long, emphasizes the development of the peroneus brevis, an the trunk length was about 40 cm long and the shoulder abductor and evertor of the foot. According to the reduced height was 30–35 cm., i.e., lower than in Borhyaena (Argot fibular head and small peroneal process of the calcaneum, 2003b)butArctocyon was plantigrade like Prothylacinus, the peroneus longus was less developed than in highly whereas Borhyaena was probably not. Moreover, with a arboreal extant taxa with an opposable hallux. The first skull of approximately 23 cm long (but the occipital con- metatarsal is not reduced and it was mobile, considering dyles are lacking) and femurs that are 17 cm long, the sub- the almost non-existent Mt I-Mt II articulation. It was diver- complete specimen of Arctocyon presents proportions sim- gent from the other metatarsals, with a dorsal side facing ilar to those of Borhyaena tuberata, specimen YPM-PU slightly medially. The asymmetrical distal epiphysis indi- 15701 (Argot 2003b), except that the femurs are slightly cates, as in the hand, that flexion-extension of the hallux did more robust in Arctocyon, with a mean midshaft diameter of not take place in the same plane as that of the other digits, 18 mm versus 15 in Borhyaena. This means that the pro- providing some grasping ability to the foot. Actually, the portions of Arctocyon were much more similar to those of first metacarpal and metatarsal are very similar in shape, predator-like Neogene metatherians than to those of extant except in the development of the medial part of the proximal carnivorans, with a long skull compared to relatively short J Mammal Evol

Fig. 21 Comparison of forelimb long bones between Prothylacinus patagonicus, YPM-PU15700, on left and Arctocyon primaevus on right (humerus, MNHN.F.BR10101, ulna, MNHN.F.R735, and radius, MNHN.F.R727). a, right humerus in anterior view (left humerus reversed in Arctocyon); b, humerus in lateral view (left humerus reversed in Arctocyon); c, right ulna in lateral view; d, right radius in medio- posterior view. Scale bars equal 2 cm

legs. This also means that the body mass of Arctocyon South America a highly specialized carnivorous saber- would be the same as that of Borhyaena when using the tooth taxon appeared during the Pliocene (Thylacosmilus femur length (i.e., 18 kgs, see equations used in Argot atrox known from northern Argentina, for a functional 2003b) but higher when using the femur midshaft diameter analysis see Argot 2004b) while another artocyonid, (44 versus 29 kgs). The hind limb of Arctocyon was rela- Anacodon from North America, developed an incipient tively shorter than in Prothylacinus in comparison to the saber-toothdentition(VanValen1969b). However, Ana- forelimbs, but the foot was longer. In the Eocene borhyae- codon is associated with cheek teeth that are flat and noid Callistoe vincei, considered as terrestrial with incipient crenulated, which suggests a trend away from carnivory digging capacities, the ulna is straighter and the anterior and that the saber-like canines had a non-feeding func- ungual phalanges are thinner and much longer than those tion (Rose personal communication). Like borhyaenoids, of the hind foot and those of Arctocyon but the scapula, arctocyonids shared their environment with large reptiles, humerus, and olecranon show no fossorial specialization e.g., Simoedosaurus lemoinei (Sigogneau-Russell 1981, (Argot and Babot 2011). An interesting point is that in 1985). Russell (1964) mentions also crocodiles, turtles, J Mammal Evol lizards, and very large birds (e.g., Gastornis)unableto lower back (Zhou et al. 1992). The dorsal protrusion of the fly and recalling the North American Diatryma (see also neural process of the axis is also found both in Arctocyon Witmer and Rose 1991;Mayr2009). This recalls the and Pachyaena, as well as an extremely robust first caudal, coexistence between Phorusracidae, Borhyaenoidea, and with a massive centrum and transverse processes expanded reptiles during the Tertiary of South America (Marshall into broad wings with distinct tubercles for muscle or liga- 1978; Argot 2004c and references herein; Argot and ment attachments (Zhou et al. 1992). These features are not Babot 2011) but in Cernay, crocodiles and birds were primitive and are related to limb bone adaptations that are thetruecarnivores. completely different in the two taxa. They illustrate the emergence of a special shape in a specific lineage, which might confirm that Mesonychidae are closely related to Conclusions Arctocyonidae (O’Leary and Rose 1995). Except for the sigmoid shape of the articulation between vertebrae, the The skeleton of Arctocyon exhibits a combination of osteo- vertebral column of Arctocyon (especially the shape of the logical features found in extant arboreal and scansorial taxa. neural and transverse processes) is more similar to that of These characters include: a roughly triangular scapula, a extant carnivorans than ungulates. mobile shoulder joint, a well-developed teres major and Although its dentition is not specialized towards cutting triceps caput longum, a long and strong deltopectoral crest, or slicing meat, meaning that Arctocyon was not a special- a long lateral epicondylar crest (i.e., development of the ized predator (but mesonychids, although considered as brachioradialis and digital extensors), a prominent, massive carnivorous, also lack carnassials or shearing teeth, see entepicondyle (i.e., development of the digital flexors), an O’Leary and Rose 1995), it is possible that it included elbow joint not stabilized, a posteriorly convex ulna, an animal prey in its diet. On the skull, the development of olecranon projected anteromedially, a low greater trochan- the sagittal crest suggests well-developed temporalis ter, a mobile hip joint, asymmetry of the femoral condyles, a muscles, while the development of the neural process of large space between the tibia and fibula, five digits on the the axis also indicates movements of the head compatible fore- and hind foot with the first slightly divergent, a shal- with an active predation (Zhou et al. 1992; Argot 2003a, b). low astragalar trochlea, loose midcarpal and midtarsal The body size reached that of several extant canids and joints, ungual phalanges curved and dorsoventrally deep, felids, with much more robust bones; the powerful limb and a long, robust tail. Because of these features and al- musculature, long canines, and five clawed digits suggest though Arctocyon is obviously not specialized toward arbor- that it was able to catch prey. Although the body size eality, characterizing it as having a “primitive” morphology suggests that it moved mainly on the ground, the morphol- (as can be read throughout the literature) seems inaccurate. ogy of the bones indicates that it was perfectly able to climb The characteristics of Arctocyon recall those of the North and handle things. American early Eocene oxyclaenid Chriacus, which was a Some features of the forelimb may appear convergently scansorial mammal with a powerful limb musculature, a in arboreal and fossorial taxa not highly specialized because large acromion, a long deltopectoral crest, a prominent of the need for powerful adductors, abductors, and flexors in entepicondyle, supination ability, a mobile hip joint, a long, both. It is worth noting that some extant taxa combine the robust, possibly semi-prehensile tail, and plantigrade, pen- two adaptations like Manis javanica and Tamandua tetra- tadactyl clawed feet with a slightly divergent hallux: an dactyla (Simpson 1931). When the specialization increases, animal probably equally adept in the trees as on the ground however, the forelimb morphology diverges (e.g., in the according to Rose (1987). Chriacus, which was the size of a extraordinary enlargement of the acromion, postscapular coati, was clearly smaller than Arctocyon. However, their fossa, olecranon, and anterior unguals in highly specialized postcranial adaptations seem very similar and an omnivo- fossorial taxa, see Rose and Emry 1983). If the teres major rous diet has been suggested for the two (Russell 1964; Rose and triceps are also well developed in both adaptations, it 1987). Considering that the earliest eutherians, Eomaia and seems that they become particularly developed in fossorial Juramaia, show scansorial features (Ji et al. 2002; Luo et al. taxa through the extension of the secondary spine and post- 2011), it is interesting to observe that both arctocyonids and scapular fossa. The hand is usually shortened, with enlarged oxyclaenids also suggest a basically scansorial heritage. ungual phalanges, and the powerful digital flexor may in- This contrasts clearly with the more terrestrial adaptations corporate in its tendon a large carpal sesamoid in some of the contemporaneous Pleuraspidotherium and Dissacus fossorial taxa, as in Manis and Notoryctes (Rose and Emry (Thewissen 1991). 1983). Simpson (1931) deals with two types of hand mor- Of particular interest is the fact that Arctocyon, like the phology according to fossorial habits, but also according to mesonychid Pachyaena, shows lumbar vertebrae with inter- the phylogenetic heritage. Nevertheless, it appears that the locking revolute zygapophyses, a feature that stiffens the scapula, humerus, ulna, and anterior ungual shape diverge J Mammal Evol more from the basic mammalian model when highly adap- South American metatherians, which suggests that a partic- ted to dig than for any other terrestrial adaptation. The joints ular morphotype may have appeared several times, in dif- tend also to be stabilized in fossorial taxa (Rose 1999), ferent lineages. mobile in arboreal ones. In Arctocyon, the lack of speciali- zation of the scapula, the mobile joints, the rather slender Acknowledgments I thank the editor John R. Wible, Kenneth D. radius, the posteriorly convex ulna with a short olecranon, Rose, and an anonymous reviewer for providing numerous comments and the pentadactyl foot with short ungual phalanges do not that improved the text. I also thank Philippe Loubry for all the photo- appear specialized for digging. The hand is not similar to graphs illustrating this paper, Charlène Letenneur for Figs. 18, 19 and that of expert diggers, in contrast to what was stated by 20 and the prothylacinid drawings in Fig. 21, Sophie Fernandez for arctocyonid drawings in Fig. 21, Cécile Colin-Fromont and Luc Vivès Kondrashov (2009), nor are the ulna or the vertebrae. How- for access to extant specimens, and Vincent Pernègre for restoration of ever, the deltopectoral crest is longer and more prominent the axis, the second thoracic vertebra, and a humerus of the sub- than in digging extant taxa like Orycteropus. complete specimen of A. primaevus. Moreover, the rest of the skeleton must be taken into account in the analysis and it is clear that arboreal adapta- tions in the hind limb are not found in strictly fossorial taxa. References Especially the mobility at the hip and ankle joints (crurota- lar, subtalar, and midtarsal joints) that may lead to the Archibald JD (1998) Archaic ungulates (“Condylarthra”). In: Janis reversal of the foot is characteristic of arboreal taxa only. CM, Scott KM, Jacobs LL (eds) Evolution of Tertiary Mammals of North America. Volume 1: Terrestrial Carnivores, Ungulates, Although full hind foot reversal is not indicated in Arcto- and Ungulatelike Mammals. Cambridge University Press, cyon, a limited supination of the foot seems possible, espe- Cambridge, pp 292−331 cially in relation to the range of movements allowed at the Argot C (2001) Functional-adaptive anatomy of the forelimb in midtarsal joints. Moreover, the posterolateral extension of the Didelphidae, and the paleobiology of the Paleocene mar- supials Mayulestes ferox and Pucadelphys andinus.JMor- the femoral head articular surface implies abduction capa- phol 247:51−79 bility and is characteristic of some arboreal and scansorial Argot C (2002) Functional-adaptive analysis of the hindlimb anatomy mammals (Jenkins and Camazine 1977). Although hind of extant marsupials and the paleobiology of the Paleocene mar- limb abduction may also be important in mammals that supials Mayulestes ferox and Pucadelphys andinus. J Morphol 253:76−108 use it to brace themselves while digging, this occurs only Argot C (2003a) Functional-adaptive anatomy of the axial skeleton of in highly specialized digging taxa like moles, not in an some extant marsupials, and the paleobiology of the Palaeocene occasional digger like the badger. Stabilization of the ankle marsupials Mayulestes ferox and Pucadelphys andinus. J Morphol joint via tibiofibular fusion is also associated with digging 255:279−300 Argot C (2003b) Functional adaptations of the postcranial skeleton (and leaping) (Barnett and Napier 1953). In Arctocyon, even of two Miocene borhyaenoids (Mammalia, Metatheria), the cruroastragalar joint is not stabilized. Rose (1987) also Borhyaena and Prothylacinus, from South America. Palaeontology emphasized for arboreal and scansorial taxa the combination 46:1213−1267 of plantigrady, convergent hallux, claw-bearing unguals Argot C (2004a) Functional-adaptive analysis of the postcranial skeleton of a Laventan borhyaenoid, Lycopsis longirostris (posterior unguals similar to those of the manus), and a (Marsupialia, Mammalia). J Vertebr Paleontol 24:689−708 long, robust tail used for balance, all features found in Argot C (2004b) Functional-adaptive features and palaeobiologic Arctocyon. Arctocyon was certainly able to dig through soil implications of the postcranial skeleton of the late Miocene occasionally, like most carnivorans do nowadays, but the sabretooth borhyaenoid Thylacosmilus atrox (Metatheria). Alcheringa 28:229−266 whole postcranial morphology suggests more adaptations Argot C (2004c) Evolution of South American mammalian predators towards arboreal than fossorial activities. It is interesting to (Borhyaenoidea): anatomical and palaeobiological implications. note that a cursorial taxon like the early Eocene Pachyaena Zool J Linn Soc 140:487–521 presents in contrast a restricted rotatory and mediolateral Argot C, Babot J (2011) Postcranial morphology, functional adapta- tions, and palaeobiology of Calllistoe vincei, a predaceous meta- mobility at most joints, providing for predominantly sagittal therian from the Eocene of Salta, northwestern Argentina. movements (O’Leary and Rose 1995), but these authors Palaeontology 54:447−480 suggested that cursorial specializations in mesonychids Barnett CH, Napier JR (1953a) The rotatory mobility of the fibula in evolved independently from those that evolved in modern eutherian mammals. J Anat 87:11−21 Barone R (1967) La myologie du lion (Panthera leo). Mammalia ungulates and cursorial carnivorans. Despite its relative 31:459−514 abundance in the fauna of its time, A. primaevus and arcto- Blainville H Ducrotay de (1841) Ostéographie ou description iconog- cyonids represent an evolutionary dead end as the family raphique comparée du squelette et du système dentaire de cinq disappears during the early Eocene. However, as the first classes d’animaux vertébrés récents et fossiles pour servir de base à la zoologie et à la géologie. Volume 3: Carnassiers: Vespertilio, wolf-sized predator-like mammal, it displays an overall size, Talpa, Sorex, Erinaceus, Phoca, Ursus, Subursus. Paris, France proportions, and general shape of the humerus and ulna Gambaryan PP (1974) How Mammals Run. John Wiley & Sons, similar to those found in non-related mammals like fossil Halsted Press, New York J Mammal Evol

Hildebrand M (1959) Motions of the running cheetah and horse. J Rose KD (2006) The Beginning of the Age of Mammals. The Johns Mammal 40:481−495 Hopkins University Press, Baltimore Jenkins FA Jr, Camazine SM (1977) Hip structure and locomo- Rose KD, Emry RJ (1983) Extraordinary fossorial adaptations in the tion in ambulatory and cursorial carnivores. J Zool Lond Oligocene palaeanodonts Epoicotherium and Xenocranium 181:351−370 (Mammalia). J Morphol 175:33−56 Ji Q, Luo ZX, Yuan CX, Wible JR, Zhang JP, Georgi JA (2002) The Russell DE (1964) Les mammifères paléocènes d’Europe. Mém Mus earliest known eutherian mammal. Nature 416:816−822 Natl Hist Nat, New Ser C 13:1−324 Kondrashov P (2009) Postcranial adaptations of European arctocyo- Sigogneau-Russell D (1981) Etude ostéologique du reptile Simoedo- nids (Mammalia, Arctocyonidae). J Vertebr Paleontol 29:128A saurus (Choristodera). IIe partie: squelette postcrânien. Ann Ladevèze S, Missiaen P, Smith T (2010) First skull of Orthaspidothe- Paleontol Vertebr 67:61−140 rium edwardsi (Mammalia, “Condylarthra”) from the late Sigogneau-Russell D (1985) Definition of the type-species of Simoe- Paleocene of Berru (France) and phylogenetic affinities of the dosaurus, S. lemoinei Gervais 1877 (Choristodera, Reptilia). J enigmatic European family Pleuraspidotheriidae. J Vertebr Paleontol 59:766−767 Paleontol 30:1559−1578 Simpson GG (1931) Metacheiromys and the Edentata. Bull Am Mus Luo ZX, Yuan CX, Meng QJ, Ji Q (2011) A Jurassic eutherian Nat Hist 59:295–381 mammal and divergence of marsupials and placentals. Nature Simpson GG (1945) The principles of classification and a classification 476:442−445 of mammals. Bull Am Mus Nat Hist 85:1−350 MacLeod N, Rose KD (1993) Inferring locomotor behavior in Paleo- Slijper EJ (1946) Comparative biologic-anatomical investigations on gene mammals via eigenshape analysis. Am J Sci 293A:300−355 the vertebral column and spinal musculature of mammals. Verh MacPhee RDE (1994) Morphology, adaptations, and relationships of Konink Nederland Akad Wetenschap, Tweede Sectie 42:1−128 Plesiorycteropus, and a diagnosis of a new order of eutherian Thewissen JGM (1991) Limb osteology and function of the primitive mammals. Bull Am Mus Nat Hist 220:1−214 Paleocene ungulate Pleuraspidotherium with notes on Tricuspio- Marshall LG (1978) Evolution of the Borhyaenidae, extinct South don and Dissacus (Mammalia). Geobios 24:483−495 American predaceous marsupials. Univ Calif Publ Geol Sci Van Valen L (1966) Deltatheridia, a new order of mammals. Bull Am 117:1−89 Mus Nat Hist 132:1−126 Matthew WD (1937) Paleocene faunas of the San Juan Basin, New Van Valen L (1969a) The multiple origins of the placental carnivores. Mexico. Trans Amer Phil Soc 30:1−510 Evolution 23:118−130 Mayr G (2009) Paleogene Fossil Birds. Springer Verlag, Berlin, Van Valen L (1969b) Evolution of dental growth and adaptation in Heidelberg mammalian carnivores. Evolution 23:96−117 O’Leary MA, Rose KD (1995) Postcranial skeleton of the early Eocene Van Valen L (1978) The beginning of the age of mammals. Evol mesonychid Pachyaena (Mammalia: Mesonychia). J Vertebr Theory 4:45−80 Paleontol 15:401−430 Witmer LM, Rose KD (1991) Biomechanics of the jaw apparatus of the Prothero DR, Manning EM, Fischer M (1988) The phylogeny of the gigantic Eocene bird Diatryma: implications for diet and mode of ungulates. In: Benton MJ (ed) The Phylogeny and Classification life. Paleobiology 17:95−120 of the Tetrapods, Volume 2: Mammals. Clarendon Press, Oxford, Youlatos D, Godinot M (2004) Locomotor adaptations of Plesiadapis pp 201−234 tricuspidens and Plesiadapis n. sp. (Mammalia, Plesiadapiformes) Rose KD (1987) Climbing adaptations in the early Eocene mammal as reflected on selected parts of the postcranium. J Anthropol Sci Chriacus and the origin of Artiodactyla. Science 236:314−316 82:103−118 Rose KD (1999) Postcranial skeleton of Eocene Leptictidae (Mammalia), Zhou X, Sanders WJ, Gingerich PD (1992) Functional and behavioral and its implications for behavior and relationships. J Vertebr implications of vertebral structure in Pachyaena ossifraga (Mammalia, Paleontol 19:355−372 Mesonychia). Contrib Mus Paleontol Univ Mich 28:289−319