Skull Morphology and Phylogenetic Relationships of a New Diminutive Balaenid from the Lower Pliocene of Belgium

[Palaeontology, Vol. 48, Part 4, 2005, pp. 793–816]

SKULL MORPHOLOGY AND PHYLOGENETIC RELATIONSHIPS OF A NEW DIMINUTIVE BALAENID FROM THE LOWER PLIOCENE OF BELGIUM

by MICHELANGELO BISCONTI Dipartimento di Scienze della Terra, Universita` di Pisa, via Santa Maria 53, 56126 Pisa, Italy; e-mail: [email protected]

Typescript received 26 November 2003; accepted in revised form 10 May 2004

Abstract: A new small balaenid is described and compared ment of 81 morphological character states scored for 10 to all fossil and living balaenid taxa. The specimen represents balaenids and nine non-balaenid cetaceans revealed that the a new genus and species and is named Balaenella brachyrhy- other small balaenids generally included in the genus Balaen- nus. It was discovered in the Lower Pliocene of Kallo (north- ula (including Balaenula astensis, B. balaenopsis and a Plio- west Antwerp, Belgium) and adds new information on the cene Balaenula sp. from Japan) are closer to the living genus diversity and evolution of Balaenidae. Based on both com- Eubalaena (the Right whale). As the new skull is so different parative morphology and phylogenetic analysis, Balaenella from the nominal Balaenula species, and as it is more closely brachyrhynus is morphologically closer to the genus Balaena, related to Balaena than to Eubalaena, it is concluded that a including the living Greenland Bowhead whale (B. mystice- small body size was a common condition in different Bala- tus), and two Pliocene species (B. montalionis and B. ricei) enidae clades. from central Italy and the eastern USA. Balaenella brachyrhynus has very short nasals, a short rostrum relative to the total Key words: Balaenula, Balaenidae, Belgium, Cetacea, Mysti- skull length and a horizontal supraoccipital. A cladistic treat- ceti, Phylogeny, Pliocene.

In zoological textbooks, balaenids are usually regarded as based on a fragmentary skull and associated skeleton gigantic, slow-swimming Mysticeti (Mammalia, Cetacea) but unfortunately an unambiguous character-based diag- with an arched rostrum and long baleen (Ridgway and nosis of the genus has been lacking until recently (Bis- Harrison 1985; Berta and Sumich 1999), but it is often conti 2003a). Abel (1941) re-assessed some Belgian forgotten that small and more streamlined forms have material pertaining to Balaenula and provided a recon- flourished during their evolutionary history. Small balae- struction of the type skull (Van Beneden did not desig- nids are represented by a conspicuous and widespread nate any holotype material). However, some fossil record. They occur in the Upper Miocene–Lower morphological details (such as the dorsal surface of the Pliocene of California (eastern Pacific; Barnes 1977), and supraoccipital, the relationships of maxilla and frontal, in Pliocene sediments of Belgium (north-east Atlantic; the morphology of the distal portion of the rostrum, Van Beneden 1880, 1878), Italy (central Mediterranean; and part of the architecture of the temporal fossa) are Trevisan 1941; Pilleri 1987; Bisconti 2000), Japan (western definitively lost and their reconstruction is based on Pacific; Excavation Research Group for the Fukagawa Fos- inference only. sil Whale 1982), and eastern USA (western Atlantic; In his original descriptions of the Balaenula type skull Whitmore 1994). Despite their worldwide occurrence, (namely Balaenula balaenopsis), Van Beneden (1880, only a few species have been described and figured in 1878) also discussed the morphology of periotic and detail; these are: the Japanese Balaenula sp. (Excavation tympanic bones. Unfortunately, it is not certain that Research Group for the Fukagawa Fossil Whale 1982), the these bones belong to the type skull because they were Italian Balaenula astensis (Trevisan 1941; Pilleri 1987; Bis- found disassociated from it (Van Beneden 1880, 1878), conti 2000) and, to some extent, the Belgian Balaenula and the features of the petrosal are so different from balaenopsis (Van Beneden 1878, 1880). what is observed in other balaenids that it is likely that The small balaenids are usually grouped within it does not belong to a balaenid at all (but see Bisconti the genus Balaenula Van Beneden, 1880. Balaenula is 2003a).

ª The Palaeontological Association 793 794 PALAEONTOLOGY, VOLUME 48

New discoveries made during the twentieth century provided materials for the detailed descriptions of two well-preserved skulls and associated petrotympanic complexes of small balaenids from Italy and Japan (Trevisan 1941; Excavation Research Group for the Fukagawa Fossil Whale 1982). The new specimens were assigned to the genus Balaenula based on the small size of the crania. Bis- conti (2000) assessed the phylogenetic relationships of the Italian specimen (Balaenula astensis) on cladistic grounds and found that it was close to the Belgian Balaenula balaenopsis; however, the Japanese Balaenula sp. was not included in the ingroup and its phylogenetic position is still unclear. Bisconti (2000) found differences between TEXT-FIG. 1. Location of Antwerp and some of the principal the Italian and the Belgian species in the architecture of towns of Belgium (west of the River Mose), southern Holland (north of the River Mose) and north-west Germany (south and the temporal fossa and the brain size, and found a east of the River Rhine); the black rectangle indicates the weakly supported relationship between Balaenula astensis, discovery area. B. balaenopsis and the living Right whales (Eubalaena)to the exclusion of the extant and fossil species of Balaena that were known at that time. However, the low resolu- The new balaenid was found by E. T. H. Van Tuiyll tion of his cladograms demands a new and comprehen- in 1974 at Kallo, north-west Antwerp (Belgium; Text- sive study in order to ﬁx systematic relationships among ﬁg. 1), during the construction of the First Channel dock the Balaenidae. (Eerste Kanaaldok), 18 m below sea level. The First Chan- In a review of the evolutionary history of balaenids, nel dock is one of a series of channels connecting Ant- Bisconti (2003a) provided redescriptions of several fossil werp with the Schelde River (and indirectly with the taxa from Europe and emended some morphological North Sea). The specimen was displayed in Van Tuiyll’s diagnoses including that of Balaenula. He also furnished palaeontological collection until March 2001, when the a redescription of some type materials belonging to the whole collection was donated to the Natuurmuseum Belgian Balaenula balaenopsis. Furthermore, he updated Brabant in the town of Tilburg, Holland. the systematics of fossil Balaenidae and stated that, currently, three species belong to Balaenula: the Italian B. astensis, the Belgian B. balaenopsis and an unnamed MATERIAL AND METHODS Balaenula sp. from Japan. The diagnosis of Balaenula is now unambiguous and mainly based on the orientation Institutional abbreviations. MSNT, Museo di Storia Naturale of the squamosal (in Balaenula it is directed anteriorly), e del Territorio, Universita` di Pisa, Pisa, Italy; MGP, Museo the position of the cranio-mandibular joint (which, in Geopaleontologico G. Capellini, Universita` di Bologna, Bologna, Italy; IRSN, Institute Royal des Sciences Naturelles, Bruxelles, Balaenula, is located under the orbit), and the height Belgium; NMB, Natuurmuseum Brabant, Tilburg, The Nether- of the exoccipital relative to the orbit (which, in lands; SMNS, Staatliches Museum fu¨r Naturkunde, Stuttgart, Balaenula, is very low). Germany; USNM, United States National Museum of Natural A new small balaenid is described in this paper based on History, Smithsonian Institution, Washington, DC, USA; ZMA, a fairly well-preserved skull. The specimen is described Instituut voor Systematiek en Populatiebiologie ⁄ Zoo¨logisch and compared to the other giant and small, living and fos- Museum, Amsterdam, The Netherlands; ZML, Zoo¨logisch sil balaenids. The phylogenetic position of the new skull is Museum, Leiden, The Netherlands. explored by means of a cladistic study of the best-preserved fossil balaenids, together with representatives of the Anatomical terms and abbreviations. The anatomical termin- living taxa. Based on the phylogenetic results and the com- ology describing the general features of the cetacean skull fol- parative analysis, it is concluded that the skull shows mor- lows Kellogg (1965, 1968a). Petrosal terminology is in accord phological features that distinguish it from the Italian, with Geisler and Luo (1996, 1998) and Luo and Gingerich (1999); tympanic terms are from Oishi and Hasegawa (1994), Belgian and Japanese small balaenids at generic level. Luo (1998) and Luo and Gingerich (1999). Abbreviations as Indeed, it represents a lineage unrelated to Balaenula, follows: an, antorbital notch; ap, anterior process of petrosal; being closer to the giant Balaena (the Greenland Bowhead apat, anterior pedicle for the articulation with the tympanic; whale). The skull documents an unsuspected diversity exocc, exoccipital; fm, foramen magnum; ip, infraorbital pro- among the small balaenids and, based on its phylogenetic cess of the maxilla; irfr, interorbital region of the frontal; lp, lat- position, suggests that in the past few million years a small eral protrusion of the lambdoidal crest; lpap, lateral projection size was a common condition in different balaenid clades. of the anterior process of the petrosal; lsc, lateral squamosal BISCONTI: DIMINUTIVE PLIOCENE WHALE 795 crest; mx, maxilla; mxf, maxillary foramen; n, nasal; nf, narial 257513: skull. Descriptions and illustrations can be found in fossa; oc, occipital condyle; p, parietal; pbsf, posterior border Cuvier (1823), Van Beneden and Gervais (1880), Reeves and of the stylomastoid fossa; pc, pars cochlearis; pgl, postglenoid Leatherwood (1985) and Burns et al. (1993). process; plc, posterolateral corner of the pars cochlearis; pmx, premaxilla; ppp, posterior process of the petrosal; ow, oval Balaena montalionis. MSNM MC CF31 holotype skull (see window; si, sutura interfrontalis; soc, supraoccipital; sop, supra- Capellini 1904; Pilleri 1987; Bisconti 2000, 2003a for descriptions orbital process of frontal; ttg, tensor tympani groove; zp, of the specimen). zygomatic process of squamosal; VII-g, groove for the facial nerve. Balaena ricei. Westgate and Whitmore (2002).

Balaena primigenius. Type material described by Van Beneden Sources of comparative data (1880, 1878), held by IRSN and represented by Ct.M. 887 (left petrosal), Ct.M. 886, Ct.M. 884 (both right tympanic Eubalaena glacialis. MSNT 264: skeleton and left petrotym- bullae). Bisconti (2003a) re-assessed the type materials, conclu- panic; ZML without inventory number (abbreviated w.i.n.): ding that the taxon is too poorly known to receive a scientific skeleton and both petrotympanics; USNM 267612, 3339990, name; therefore, Balaena primigenius is a Balaenidae gen. 23077, 301637: skulls and petrotympanics. Based on molecular indet. analyses, Rosenbaum et al. (2000) and Malik et al. (2000) proposed that Eubalaena should comprise at least three differ- Balaenula balaenopsis. Type material described by Van Beneden ent living species. However, morphological features useful to (1880, 1878), held by IRSN and represented by Ct.M. 858a–b distinguish these three supposed species are not currently (right petrosal), Ct.M. 853d (posterior process of petrosal), available (Bisconti 2002), and for that reason here I Ct.M. 859–863 (tympanic bullae), Ct.M. 865a–b, 867a–c, 868, accept the traditional, morphological definition of the genus 869a–b (cervical vertebrae). Additional material held in MGP is Eubalaena (Cummings 1985; Mead and Brownell 1993), and represented by a skeleton from Poggio alle Talpe including both consider that at present the genus includes two geographically petrotympanics (central Italy; 1CMC29 ⁄ 8929; Capellini 1877; distinct species, Eubalaena australis and E. glacialis. General Portis 1883). descriptions and illustrations can be found in Cuvier (1823), Van Beneden and Gervais (1880), True (1904) and Cummings Balaenula astensis. MSNM MC CF35 holotype skull and associ- (1985). ated petrotympanics (see Trevisan 1941; Pilleri 1987; Bisconti 2000, 2003a for descriptions of the specimen). Eubalaena belgica. Type material described by Abel (1941) and Plisnier-Ladame and Quinet (1969) as Balaena belgica. The Japanese Balaenula sp. Excavation Research Group for the taxon is represented by the IRSN holotype skull, Ct.M. 879a–f Fukagawa Fossil Whale (1982). (including portions of squamosals, maxilla and frontal). McLeod et al. (1993) suggested that this form is so similar to Balaenotus insignis. Type material described by Van Beneden Balaenula balaenopsis in the morphology of the zygomatic (1880, 1878), held by IRSN and represented by Ct.M. 832, process of the squamosal that it should be referred to Balaen- 833a–b (both specimens are right petrosals), Ct.M. 834, 835 ula as the new combination Balaenula belgica. I examined the (both specimens are tympanic bullae), Ct.M. 836a–c (a is a type skull and confirmed this observation, but it should be distal fragment of right supraorbital process of frontal, b and noted that the type skull of this taxon is very large and c are rostral elements: a fragment of premaxilla and a prox- heavy, differing from the small, slender forms belonging to imal portion of maxilla), Ct.M. 837 (fragment of left squamo- Balaenula. Moreover, in this large Belgian species, the lateral sal), Ct.M. 838 (fragmentary supraorbital process of the borders of the supraoccipital are parallel whereas they are frontal), Ct.M. 840, 841a–b, 842a–b, 844a–b (cervical verteb- anteriorly convergent in Balaenula astensis and the Japanese rae), Ct.M. 850 (left scapula) and Ct.M. 856 (left fragment of Balaenula sp.; therefore, the morphology observed in the Bel- mandibular condyle). All these materials have been redescribed gian species resembles Eubalaena glacialis and not Balaenula. by Bisconti (2003a). Additional material is held in MGP A slender zygomatic process of the squamosal was described (Capellini 1877; Portis 1883): the Poggiarone skeleton by Nishiwaki and Hasegawa (1969) in a Pleistocene Eubalaena (1CMC23 ⁄ 8923, 26 ⁄ 8926, 22 ⁄ 8922), the cast of a right petro- glacialis from Japan; therefore this feature does not appear to sal from Pieve di Santa Luce (near Pisa, central Italy, be useful for generic diagnosis. Bisconti (2000, 2003a) sugges- 1CMC87 ⁄ 8987), the cast of a right petrosal from Monte Aper- ted that Balaena belgica should belong to Eubalaena based on to (near Siena, central Italy, 1CMC ⁄ 8987). Balaenotus is a the morphology of the supraoccipital, a strong temporal problematic genus. It was based on a very incomplete skull, ascending crest on the supraorbital process of the frontal, and petrotympanics and some postcranial bones by Van Beneden the angle between rostrum and frontal. (1880, 1878); however, this poor material is diagnostic at the genus level (see Bisconti 2003a); therefore, in this work the Balaena mysticetus. IRSN 1532: skeleton; ZML 1680, 3997, name Balaenotus insignis is retained. 2563, 2001, two ZML w.i.n. (both petrotympanics labelled ‘Bal- aena japonica’), and ZML w.i.n. (right petrotympanics); USNM Morenocetus parvus. Cabrera (1926). 796 PALAEONTOLOGY, VOLUME 48

SYSTEMATIC PALEONTOLOGY Type species. Balaenella brachyrhynus sp. nov.

Class MAMMALIA Linnaeus, 1758 Diagnosis. Balaenella brachyrhynus belongs to the family Order CETACEA Brisson, 1762 Balaenidae owing to the following features: maxilla trans- Suborder MYSTICETI Cope, 1891 versely compressed in its anterior three-quarters, rostrum Family BALAENIDAE Gray, 1825 highly arched, supraoccipital extending anteriorly to the orbit, supraorbital process of the frontal gently descending Genus BALAENELLA gen. nov. from the interorbital region, parietal not exposed on the Text-ﬁgures 2–7 cranial vertex, squamosal mainly developed along the dorsoventral axis, lateral squamosal crest forming a strong Derivation of name. Balaena, Latin name for the Greenland anterior convexity, triangular-shaped lateral projection of Bowhead Whale; -ella, Latin, diminutive sufﬁx indicating a form the anterior process of the petrosal, posterodorsal corner of smaller than Balaena. the stylomastoid fossa rounded in dorsal view and far from

A B

C D

TEXT-FIG. 2. Balaenella brachyrhynus holotype skull (NMB 42001). A, right lateral view. B, right premaxilla. C, left premaxilla. D, left lateral view. E, anterior view. Scale bar represents 200 mm. BISCONTI: DIMINUTIVE PLIOCENE WHALE 797 the posterior wall of the pars cochlearis, long and shallow Kallo (north-west Antwerp, Belgium; Text-Fig. 1), 18 m below stylomastoid fossa, tympanic bulla dorsoventrally com- sea level in the Zanden van Kattendijk (Kattendijk Sand) For- pressed, tympanic cavity shallow. It shares the following mation. Janssen (1974) and Nuyts (1990) provided data on features with Balaena: supraorbital process of the frontal the fossil content, biostratigraphy and lithostratigraphy of the and lateral process of the maxilla posteriorly orientated so deposits near Kallo. The specimen was found in what Nuyts (1990) described as clay, bioturbated at the top, infilled with that a continuous arch is developed by rostrum and frontal, greyish green sands. The clay belongs to the Kattendijk For- squamosal posteriorly orientated, posterior wall of tem- mation and is Early Pliocene in age based on its foraminiferal poral fossa observed when the skull is in lateral view, infra- content. The Kattendijk Formation contains a foraminiferal orbital plate absent laterally (feature figured by Cuvier association that is typical of the BFN 4 (Florilus boueanus- 1823, pl. 25). The following features are exclusively diag- Monspeliensina pseudotepida Assemblage Zone; Nuyts 1990). nostic of Balaenella brachyrhynus: skull length slightly more Janssen (1974) stated that the Kattendijk Formation is Scaldi- than 1000 mm, length of rostrum approximately 50 per sian or ‘Kattendijkien’ in age; Hoedemakers and Marquet cent of the total skull length, nasal bones about 60 per cent (1992) correlated the ‘Kattendijkien’ to the Mediterranean shorter than nasals of giant balaenids (Balaena and Tabianien. The latter is now completely incorporated in the Eubalaena) and about 75 per cent shorter than nasals of Zanclean (Monegatti and Raffi 1996), i.e. Early Pliocene Balaenula astensis, nasal bones lacking the anterior notch; (2Æ5–5Æ3 Ma; Sprovieri 1992; Rio et al. 1994), and this is the age proposed for NMB 42001. nasal bones forming a triangular complex displaying an anterior apex, anterior two-thirds of supraoccipital hori- Diagnosis. As for genus. zontal, rostrum triangular in dorsal view, lateral process of the maxilla strong. Description

The skull is substantially complete; it lacks dentaries, the right Balaenella brachyrhynus sp. nov. petrotympanic complex, the anterior end of maxillae, right orbit, jugals, lacrimals and some portions of the squamosals. The pre- Derivation of name. Greek, brachy, little; rhynus, nose, denoting maxillae are largely preserved but are detached from their nat- a small Balaena with a little nose, and the marked reduction of ural position by the collector. Taken as a whole, this skull is one the nasal bones of the holotype skull. of the best-preserved balaenid specimens in the world. Measure- ments of skull and petrotympanic complex are given in Tables 1, Holotype. Specimen 42001, Natuurmuseum Brabant, Tilburg, 2, respectively. The skull as preserved is illustrated in Text- Holland. The specimen will be referred to as NMB 42001 or the ﬁgures 2, 3; a reconstruction of the skull is presented in Text- Tilburg skull in the text. ﬁgure 4.

Type locality and horizon. The specimen was found during the Rostrum: maxilla, premaxilla, nasal. The left maxilla is almost construction of the First Channel dock (Eerste Kanaaldok) at complete, lacking only the anterior apex. Only the posterior half of the right maxilla is preserved. The maxillae are transversely compressed for the main part of their length but become wider approaching the frontal. Anteriorly, the lateral borders of the maxillae converge toward the long axis of the skull (Text- Fig. 4B); in fact, the transverse diameter of maxillae is 140 mm at the anterior end of the rostrum (as inferred by duplicating the distance between the lateral border of the left maxilla and the longitudinal axis) and an increment of 10 mm is observed 200 mm posteriorly to the apex. The width of the rostrum increases to 220 mm at 400 mm behind the apex and reaches 670 mm between the lateral apices of the infraorbital processes of maxillae at c. 560 mm behind the anterior end of the left maxilla. In dorsal view, the rostrum appears triangular (Text- Fig. 4B). In lateral view, the maxilla forms a complete arc anterior and under the supraorbital process of the frontal (Text-ﬁgs 2, 3A). Differing from living balaenids (Balaena mysticetus and Eubalaena), the maxillae are depressed in front of the frontal, TEXT-FIG. 3. A, right side of the holotype skull (NMB 42001) and the posterior three-quarters of their dorsal border are as preserved. B, left side of the holotype skull as preserved. The ventrally directed. The lateroventral margins of both maxillae scale bar represents 200 mm. See text for explanation of form a dorsoventrally and laterally concave border running labelling. onwards, and slightly converging towards the long axis of the 798 PALAEONTOLOGY, VOLUME 48

TABLE 1. Skull measurements of Balaenella brachyrhynus. TABLE 2. Periotic and tympanic measurements of Balaenella NMB 42001, holotype; measurements in mm. brachyrhynus, NMB 42001, holotype; measurements in mm.

Condylobasal length 1080 Length of posterior process of petrosal 96 Maximum width of skull (between postorbital 800 Width of posterior process of petrosal at midlength 25 processes of supraorbital processes of frontal) Length of anterior process of petrosal 42 Anteroposterior diameter of supraorbital process 130 Width of anterior process of petrosal excluding 28 of frontal lateral projection Laterolateral diameter of supraorbital process 550 Width of anterior process of petrosal including 46 of frontal lateral projection Nasal length along medial border 43 Anteroposterior diameter of pars cochlearis 26 Maximum width of both nasals at anterior end 25 Lateromedial diameter of pars cochlearis 18.5 Maximum width of both nasals at posterior end 58.5 (maximum protrusion of promontorium) Maximum width of narial opening anterior 140 Length of tympanic bulla 84 to nasal end Anterior width of tympanic bulla 60 Length of supraoccipital 310 Height of tympanic bulla (from convex face to 38 Linear length of left maxilla 560 dorsal rim of tympanic medial prominence) Length of left maxilla along external curvature 560 Height of tympanic bulla (from lateroventral 62 Posterior width of left maxilla (posterior border 375 keel to dorsal rim of tympanic medial prominence) of infraorbital process) Maximum height of tympanic cavity 30 Linear length of right maxilla 750 Maximum length of tympanic cavity 63 Length of right maxilla along external curvature 770 Maximum width of tympanic cavity 20 Posterior width of right maxilla (posterior border 345 of infraorbital process) Maximum width of supraoccipital between 460 exoccipitals Length of left premaxillary fragment 650 Anterior width of left premaxillary fragment 23 Posterior width of left premaxillary fragment 40 Length of anterior fragment of right premaxilla 330 Length of posterior fragment of right premaxilla 410 Width of supraoccipital at midlength 330 Anteroposterior diameter of temporal fossa 90 Laterolateral diameter of temporal fossa c. 250 Laterolateral diameter of left occipital condyle 75 Anteroposterior (dorsoventral) diameter of 100 left occipital condyle Estimated laterolateral diameter of foramen 64 magnum Estimated anteroposterior (dorsoventral) 80 diameter of foramen magnum Distance between lateral border of left 150 occipital condyle and lateral border of exoccipital Distance between antorbital and postorbital 105 processes of supraorbital process of frontal

skull. The anteriormost quarter of the rostrum abruptly pro- TEXT-FIG. 4. Reconstruction of the Tilburg skull (NMB jects downward over the last 57 mm. 42001). A, left lateral view. B, dorsal view. The scale bar Both maxillae bear four maxillary foramina. The infraorbital represents 200 mm. See text for explanation of labelling. processes are very robust and triangular in dorsal view. There is a slight depression 110 mm in length running above the ventrolateral border of the infraorbital process that corres- lateral to the nasals, and it is round in posterior outline. The ponds to the antorbital notch. Rare vascular sulci related to infraorbital process of the maxilla and the supraorbital process the blood supply for the baleen are present on the ventral of the frontal are divided by a space that widens distally. The surface of the maxillae. infraorbital plate is developed medially under the supraorbital The maxilla articulates with the frontal through a wide, ﬂat process of the frontal, but it is absent more laterally so that process that is formed by its posteromedial corner. This pro- it cannot be observed when the skull is examined in lateral cess superimposes on the anteromedial portion of the frontal view. BISCONTI: DIMINUTIVE PLIOCENE WHALE 799

The premaxillae display a strong transverse compression and a A round dorsal proﬁle. The ventral border is broken along the entire length of the bones. The nasal cavity is located between the posterior portions of the premaxillae, and its anteroventral edge is represented by an anterodorsally ascending relief of the medial wall of the premaxillae. The nasal bones display a very characteristic morphology observed in no other living and fossil balaenid (Text-ﬁg. 4). They are very short and their lateral borders converge anteriorly, forming a triangular complex slightly raised relative to the maxillae. The anterior edges of the nasals are transversely and dorsoventrally round, lacking the typical notch of the other balaenids. Posteriorly, the nasals are divided by a triangular, forward-growing portion of the frontal. The rostral length (560 mm between anterior and posterior end of maxilla) approaches 50 per cent of the condylobasal length (1080 mm). This situation is observed in newborn living B balaenid species, but the holotype of Balaenella brachyrhynus is not a juvenile individual because all of the sutures of the skull are fused. Balaenella brachyrhynus shares two features with juve- niles and newborns of living balaenids, namely the horizontal development of the supraoccipital (see above) and the ratio of rostral length to total skull length.

Frontal. The frontal includes two supraorbital processes and a small interorbital region (Text-figs 2–3, 5). The right supraorbital process is broken at 410 mm from the sagittal axis of the skull but the left one is complete. The supraorbital processes are developed along an oblique plane descending from the higher interorbital portion. They strongly project rearward and out- ward; the ventral angle between them is largely obtuse, approaching 170 degrees. The dorsal surface of the supraorbital process is distally flat, but medially displays a slight temporal TEXT-FIG. 5. A, nasal bones of the holotype skull (NMB ascending crest that makes the dorsomedial portion of the bone 42001) as preserved. B, vertex structure of Balaenella prismatic and more robust. brachyrhynus. Scale bars represents 85 mm. See text for The antorbital process is more robust and higher than the explanation of labelling. postorbital process. The sulcus for the optic nerve is shallow and narrow, and it develops on the posteroventral surface of the of the parietal forms the temporal crest overhanging the tem- supraorbital process. The frontal is exposed dorsally and forms a poral fossa (i.e. the lateral walls of the skull behind the supraor- short interorbital region, which is 32 mm in length. The anterior bital processes of the frontal are not seen in dorsal view); the process of the supraoccipital is superimposed on the dorsal sur- temporal crests follow the dorsal borders of the supraoccipital face of the posterior portion of the interorbital region of the and converge anteriorly towards the sagittal line. frontal. The frontal is anteriorly divided by an unfused sutura The squamosal is developed along the dorsoventral axis. It interfrontalis, and posteriorly bounded by the coronal suture projects downward and posteriorly, but it is less posteriorly ori- between frontals and the parietals. entated than the supraorbital process of the frontal. The squamosal forms the posterior wall of the temporal fossa, which is Temporal fossa: parietal, squamosal. The coronal suture is evi- strongly concave in this skull. This wall is laterally bound by an dent in lateral view but not in dorsal view owing to the super- undulating lateral squamosal crest (lsc). The lsc originates dor- imposition of the anterior process of the supraoccipital on the sally from the lambdoidal crest and projects forward; then it parietal and the frontal. In lateral view, the squamosal suture (of continues ventrally generating a convex curve (anterior convex- parietal and squamosal) begins from the lambdoidal suture (of ity); finally, the crest projects downward and posteriorly. The squamosal, parietal and supraoccipital) and develops ventrally. lateral surface of the squamosal is flat. The posterodorsal profile The lambdoidal suture is marked by a robust lateral tubercle of the bone is rounded. Both the zygomatic and the postglenoid protruding from the posterior temporal crest. processes are missing; thus, the morphology of the cranioman- Anteriorly, the parietal is superimposed upon the lateral and dibular joint is unknown. medial portions of the frontal covering the posterodorsal region The tubercle located on the lambdoidal suture and the con- of emergence of the supraorbital processes. Presumably, the vex curve on the lsc limit the space occupied by the lambdoi- supraoccipital superimposes on the parietal. The dorsal border dal crest. That crest is very round, differing from the anterior 800 PALAEONTOLOGY, VOLUME 48 temporal crest and the ventrolateral lsc; in fact, it is not a true the holotype skull is on a mobile box and leans on a gypsum crest but only a round convexity of the dorsal borders of the pedestal so that its basicranium is largely inaccessible. Thus, it is squamosal. The lambdoidal crest represents the connection possible to report only significant observations of those portions between the temporal crest and the lsc, and its posterior apex of the skull that are free of gypsum (Text-figs 6–7). is behind the level of the lsc but anterior to the occipital The vomer is observed in the posterior portion of the skull condyles. where it divides the descending processes of the basioccipital. There, it is flat and covers the suture between basisphenoid and Occipital region: supraoccipital and exoccipital. The supraoccipi- basioccipital, reaching a point very close to the posterior end of tal is large and wide (Text-fig. 4B); it does not display a dome the skull. The vomer is also observed in the rostrum where it is (like Eubalaena), and its anterior end is slightly raised. The lat- found between the maxillae. There, it displays a strong ventral eral borders are posteriorly convex, but lateral to the anterior convexity. process they are concave (i.e. the anterior process of the supra- The palatines are not visible. The presence of the alisphenoid occipital is laterally compressed). The anterior process is small in the ventral side of the temporal fossa is uncertain, as it is dif- and narrow, and its rostral border is linear. The anterior process ficult to observe the sutural morphology owing to the gypsum is more advanced than the supraorbital processes of the frontal pedestal. (i.e. in dorsal view, the anterior process ends more anteriorly The pterygoids are developed along the dorsoventral axis, and than the antorbital corner of the frontal). they are damaged along the posteroventral border. They display A wide area of low relief developed along the sagittal axis of both the medial and the lateral lamina, and a deep pterygoid the supraoccipital divides two anterior nuchal fossae. The main fossa. The fossa is a concavity entering the pterygoid so that it is portion of the supraoccipital is almost horizontal but the poster- ventrally bordered by a pterygoid ventral lamina. The lateral ior part, including the foramen magnum and exoccipitals, is diameter of the pterygoid decreases along the dorsoventral axis. more vertical. The pterygoid is laterally bordered by the falciform process of The exoccipitals are laterally round; in dorsal view they are the squamosal, which does not display a complete infundibulum slightly posterior to the deduced position of the foramen mag- for the external opening of the foramen pseudovale. num. The descending processes of the basioccipital are wide and Only the ventral surface of the petrosal is discussed here triangular, but their ventral corners are badly eroded. Posteriorly because the bone is articulated with the skull and its dissection they are transversely orientated, but anteriorly they parallel the for a detailed description would be too invasive for the general longitudinal axis of the skull. The vomer divides them on the preservation of the entire skull (Text-fig. 6). Measurements of ventral surface of the skull. The occipital condyles are wide and petrosal and tympanic bullae are given in Table 2. The posterior form the ventrolateral sides of the foramen magnum; ventrally, process of the left petrosal is articulated between exoccipital and they are largely separated by a deep notch. A condylar neck is squamosal and emerges in the posterolateral corner of the skull. lacking. The foramen magnum is destroyed. The posterior process is long and narrow (the lateral diameter of the pars cochlearis is c. 27 per cent of the length of the posterior Basicranial observations: pterygoid, petrosal, tympanic. As cur- process); its ventral surface is flat, and there is no sulcus or rently displayed for exhibition in the Natuurmuseum Brabant, depression for the posterior exit of the facial nerve (VII).

TEXT-FIG. 6. Periotic of Balaenella brachyrhynus holotype (NMB 42001). A, periotic as preserved in the skull. B, interpretative drawing of periotic structures. Scale bar represents 50 mm. See text for explanation of labelling. BISCONTI: DIMINUTIVE PLIOCENE WHALE 801

TEXT-FIG. 7. Left tympanic bulla of the holotype skull NMB 42001. A, medial view. B, ventrolateral view. Scale bar represents 60 mm.

Approximately 57 mm anterior to the posterior end of the pos- Balaena montalionis, B. mysticetus and Morenocetus par- terior process, a long and presumably low stylomastoid fossa vus.InEubalaena (including E. belgica), Balaenula astensis begins to develop onward; its posterior border is a depression and B. balaenopsis the lateral process of the maxilla and on the medial side of the posterior process. The posterior pro- the anterior portion of the rostrum form a right angle cess forms a right angle with the anterior process. The latter is (skull in lateral view). squared and short (the lateral diameter of the pars cochlearis is As in Balaena mysticetus and B. montalionis, in the c. 61 per cent of the length of the anterior process). A triangular process 18 mm in length projects from the posterior side of the Tilburg skull the supraorbital process of the frontal is dis- anterior process at an acute angle. The pars cochlearis is broadly tally flat; this feature is also shared with the fragment of round and does not protrude in a cranial direction. It was the frontal of the type material of Balaenotus insignis as impossible to observe the cranial foramina of the pars cochlearis. figured by Van Beneden (1878, pl. 27; IRSN Ct.M. 836a). The oval window is wide and approximately elliptical; the In Eubalaena, Balaenula astensis and Balaenula balaenopsis groove for the facial nerve is short. the supraorbital process is prismatic because the ascend- Only the left tympanic bulla is preserved (Text-fig. 7). Its lat- ing temporal crest is developed along its whole dorsal eral wall is destroyed, including the sigmoid process, but the surface forming a sharp edge. conical process is preserved. The tympanic thickness is very Balaena mysticetus and B. ricei have a supraoccipital heavy; there is a high thickening along the posterior two-thirds with a wide and round anterior border (which is narrower of the bulla, but the anterior third is narrower. The posterior than that of Balaenula, Eubalaena and Morenocetus), tympanic thickening is bisected by a transverse pair of sulci developed only along the dorsal surface. The peduncles for the whereas in Balaenella brachyrhynus and Balaena montali- articulation with the petrosal are broken; the posterior peduncle onis the anterior portion of the supraoccipital is trans- is close to the posteromedial corner of the bulla and developed versely compressed and the anterior border of the along the anteroposterior axis. The tympanic cavity is relatively supraoccipital is transversely straight. shallow and extends behind the tympanic thickening. The ven- The arrangement of the nuchal fossae of the Tilburg tromedial keel is pronounced but eroded; the ventral side of the skull is similar to that observed in Balaenula astensis in bulla is strongly concave. which a pair of anterior fossae are displayed as slight depressions on the dorsal surface of the supraoccipital. There is no anterior dome followed by two serial couples COMPARISONS of nuchal fossae as in Eubalaena. Balaena ricei has a laterally sloping median ridge, which is not observed in any Synapomorphic features other balaenid taxon. In the same region, Balaenula astensis and the Japanese Balaenula sp. display a median con- Balaenella brachyrhynus has Balaena-like characteristics in vexity that is absent in the Tilburg skull. the rostrum, in the sutural pattern of the neurocranial Resembling Balaenula astensis, the supraoccipital of bones and in the temporal fossa. The most striking fea- Balaenella brachyrhynus is developed along two planes: ture shared by Balaenella and Balaena is the lateral pro- the first plane is approximately vertical and comprises the cess of the maxilla located under the supraorbital process foramen magnum and the exoccipitals; the second is of the frontal (skull in lateral view). The supraorbital pro- approximately horizontal, and comprises that surface of cess is orientated posteriorly in Balenella brachyrhynus, 802 PALAEONTOLOGY, VOLUME 48 the supraoccipital which is anterior to the foramen mag- Autapomorphic features num. In Balaenula balaenopsis and B. astensis the supraoccipital is obliquely bent with an obtuse angle between The most striking autapomorphy of the Tilburg skull is the plane across the foramen magnum and exoccipitals, the strong reduction of the nasal bones. The nasals are and the plane of the anterior surface. The supraoccipital extremely short and their morphology is completely is horizontal in newborns and calves of living giant balae- divergent from that of the other balaenid whales. In nids (Van Beneden and Gervais 1880, 1868–79), so the Balaenula astensis, B. balaenopsis and Balaena mysticetus, pattern observed in Balaenella brachyrhynus and Morenoc- the nasals are rectangular, their long axis being parallel to etus parvus suggests that the verticality or horizontality of the longitudinal axis of the skull; in Balaena montalionis the supraoccipital would depend on the size (and the age) and Eubalaena glacialis the nasals are also rectangular but of the individuals. This hypothesis predicts that small their long axis is transverse to the longitudinal axis of the balaenid taxa have a supraoccipital developed along two skull. Moreover, the nasals of all balaenids but Balaenella planes, and the anterior portion of the supraoccipital brachyrhynus display a notch in their anterior wall that is should be approximately horizontal in all the small forms also observed in dorsal view. The nasals of the Tilburg independent of their phylogenetic affinities. skull are delicate and lack the notch. They form a trian- Balaenella brachyrhynus shares with Balaena mysticetus gular complex that interdigitates with the frontal. and Balaenotus insignis (Ct.M. 832, 833) the flatness and The ratios between the length of the nasals (along their general proportions of the posterior process of the petro- medial border) and the condylobasal length of skulls sal. Balaenula astensis also has a flat posterior process, but belonging to several fossil and living Balaenidae are as fol- in that species the process is markedly shortened (Bisconti lows: Balaenella brachyrhynus:2Æ77; Balaenula astensis: 2003a). In Eubalaena glacialis the posterior process 10Æ71 (data determined by Bisconti 2000); B. balaenopsis: appears different, displaying a complex, somewhat crest- 8Æ71 (estimated from Van Beneden 1878); Eubalaena gla- like dorsal projection from the horizontal ventral portion cialis:8Æ6, 7Æ3, 6Æ8 (mean, 7Æ56; data from Tomilin 1967); of the process (cfr. MSNT 264, USNM 23077). The mor- Balaena mysticetus:8Æ9, 8Æ91 (Tomilin 1967; Van Beneden phology of the anterior process is shared by the Tilburg and Gervais 1868–79); B. montalionis:7Æ42 (total length skull, Balaena mysticetus, Eubalaena glacialis and Balaeno- of the reconstructed skull estimated as 2Æ2 m; nasal bone tus insignis (Bisconti 2003a). Major differences exist length from Bisconti 2000). between the anterior process of Balaenella brachyrhynus The nasals of the Tilburg skull are about 60 per cent and those of Balaenula balaenopsis and B. astensis. In gen- shorter than those of the giant balaenids Balaena and eral, the differences between the petrosal of B. balaenopsis Eubalaena, and about 75 per cent shorter than those of and all of the other balaenids are so marked that a mor- Balaenula astensis. This suggests that Balaenella brachyrhy- phological analysis should be made to test the hypothesis nus approaches the most advanced stage of nasal reduc- that the petrosal does not belong to a balaenid. In fact, tion in balaenids and possibly in the entire Mysticeti. the petrosal of B. balaenopsis (IRSN Ct.M. 858a) was not The Tilburg skull bears autapomorphies related to the found in connection to the type skull (Van Beneden feeding apparatus. In particular, the robustness of the 1880, 1878), and it appears so different that it could eas- posterolateral portion of the maxilla is remarkable. It is ily belong to a completely different taxon, perhaps to a possible that the strong infraorbital process of the maxilla cetothere (but see Bisconti 2003a). The differences are in is related to the feeding behaviour, but the lack of a com- the morphology of the stylomastoid fossa, which is shar- plete dentary and postcranial bones prevent a clear inter- ply defined in Balaenula balaenopsis but is not in all of pretation of function in that taxon. the other balaenids; in the triangular-shaped cranial opening of the facial canal, which is round in other balaenids; and in the long and low anterior process, which is usually PHYLOGENETIC ANALYSIS short and high in the other balaenids. The anterior process of the petrosal in Balaenula Introduction and discussion of previous work astensis is quite delicate. Differing from the other balaenids, it is narrow, and relatively long compared with the The phylogenetic relationships of Balaenella brachyrhynus length of the pars cochlearis, but it is high like other were investigated through a cladistic analysis of living and balaenids. In this respect the petrosal of Balaenula asten- extinct members of the family Balaenidae using an exten- sis is easily identifiable, being characterized by a short, ded outgroup formed by representatives of the main mys- wide posterior process, and a long, transversely com- ticete radiations. Previous studies on Balaenidae failed in pressed anterior process. In both those features, the pet- providing a clear picture of the phylogenetic history of rosal of Balaenula astensis differs consistently from these whales. McLeod et al. (1993), for instance, did not Balaenella brachyrhynus. include fossil taxa in their analysis and this prevented an BISCONTI: DIMINUTIVE PLIOCENE WHALE 803 unambiguous interpretation of the evolutionary radiations parvus). The length ⁄ width ratio of the nasal bones could of the family. A cladistic study of Balaenidae was made be considered as another supporting character for the by Bisconti (2000), who included fossil taxa in the Morenocetus + Balaenula clade because it is similar in ingroup. He discovered a clade of small balaenids compri- Balaenula astensis and B. balaenopsis, but unfortunately sing Balaenula astensis, B. balaenopsis and the primitive the nasals are not preserved in Morenocetus parvus, Eubal- Morenocetus parvus based on the shared features listed in aena belgica and Balaena ricei so that ambiguity persists. Table 3. A reinterpretation of these features (presented in The petrosal does not provide supporting evidence for the right-hand column of Table 3) shows that only three this clade because of the peculiar morphology of that characters proposed by Bisconti (2000) are useful in bone in Balaenula balaenopsis. Moreover, the petrosal of supporting a Morenocetus + Balaenula clade, namely: flat Balaenula astensis is particularly difficult to interpret, the posterior wall of temporal fossa (shared by Balaenula posterior process being very short and the anterior pro- astensis and B. balaenopsis); supraoccipital bending less cess long. In general terms, the character states used by than 60 degrees (shared by all the small balaenids plus Bisconti (2000) need to be updated with further morpho- Caperea marginata); convex lateral supraoccipital borders logical work (see Table 3 and Bisconti 2003a for a discus- (shared by B. astensis, B. balaenopsis and Morenocetus sion of Bisconti’s 2000 dataset).

TABLE 3. Discussion of potential synapomorphic characters proposed by Bisconti (2000) to support a clade including Balaenula astensis, B. balaenopsis and Morenocetus parvus.

Character Discussion

Linear mandibular rami Seems a primitive feature for mysticetes in general; many cetotheres and Pliocene rorquals share this character with small balaenids. Temporal fossa reduced along This state can be demonstrated in Balaenula astensis only; the poor preservation of anteroposterior and lateromedial Balaenula balaenopsis prevents observation of its status; the Japanese Balaenula sp. axes has a wide temporal fossa. Low parasagittal crest By ’parasagittal crest’, Bisconti (2000) meant the anterior part of the lambdoidal crest; the state can be demonstrated for Balaenula astensis and Morenocetus parvus only; the Japanese Balaenula sp. has high crests while it is impossible to observe the feature in Balaenula balaenopsis. Flat posterior wall of temporal fossa Character shared by Balaenula astensis, Balaenula balaenopsis and Morenocetus parvus. Lateral squamosal crest slightly This feature is an autapomorphy of Balaenula astensis, its presence is doubtful in Balaenula convex anteriorly balaenopsis and is impossible to demonstrate in Morenocetus parvus. In the Japanese Balaenula sp. the squamosal crest is strongly convex as in giant balaenids and the Tilburg skull. Rectangular and anteriorly directed Poor preservation of the cranio-mandibular joint makes it impossible to be sure of temporal region this condition in several taxa. The postglenoid process is not preserved in Balaenula astensis, B. balaenopsis and M. parvus. Supraoccipital bending The feature is observed in Balaenula astensis, Morenocetus parvus and the Tilburg skull. In < 60 degrees relative to the the lst of these the bending is approximately 0 degrees, approaching the condition of horizontal axis newborns of living species. Lateral borders of supraoccipital The feature is not observed in the Tilburg skull owing to transverse compression of the convex anterior process of the supraoccipital. Two longitudinal pairs of nuchal The feature is observed in Balaenula astensis only. Eubalaena glacialis has two pairs of fossae nuchal fossae but their arrangement is different from Balaenula astensis. All of the other small balaenids have one pair of anterior nuchal fossae. Vertical paroccipital process Autapomorphy of Balaenula astensis. with posterior concavity Tuberosity absent from the posterior Difﬁcult to assess without studying individual variation. However, it might be an surface of the paroccipital process autapomorphy of Balaenula astensis. Lateral diameter of foramen Possible autapomorphy of Balaenula astensis. magnum wide with respect to the width of the skull at the level of exoccipitals Ventral angle between the frontal The ‘frontal wings’ correspond to the supraorbital processes of the frontal. The character is wings about 90 degrees shared by Balaenula astensis and Morenocetus parvus. Poor preservation does not permit observation of this feature in Balaenula balaenopsis and the Japanese Balaenula 804 PALAEONTOLOGY, VOLUME 48

Apart from the failure of previous morphological stud- closely related to the Balaenopteridae than the Balaenidae. ies in depicting a clear interpretation of the phylogeny of These results conflicted with those of previous analyses in Balaenidae, molecular studies have complicated the situ- also suggesting that the divergence of Balaenidae was a ation further in that they have found very few genetic dif- basal event in the evolution of baleen-bearing mysticetes. ferences between the living genera. This led A´ rnason and Given this uncertainty about the phylogenetic position of Gullberg (1994) to propose that living balaenids all balaenids among the mysticetes, an obvious outgroup to belong to the same genus, i.e. Balaena. This conclusion this family with which to root the cladograms was not was also shared by Gatesy (1998) but is in conflict with available. Therefore, the outgroup was arranged in such a the traditional morphological view in which Balaenidae way that the major radiations of non-balaenid mysticetes includes two living genera (Mead and Brownell 1993). were included. The following taxa were included within Westgate and Whitmore (2002) hypothesized that Balaena the outgroup: the ‘cetothere’ Parietobalaena palmeri from included at least five species during the Pliocene and this the Lower Miocene of the Calvert Formation of Maryland implicitly suggests that the genetic differences found in (‘cetotheres’ are extinct rorqual-like mysticetes; there is no the living taxa may not completely represent their phylo- general agreement about the systematics of this group; in genetic history. This problem is investigated and resolved fact some authors have claimed that they are para- or po- through the cladistic analysis of living and fossil Balaeni- lyphyletic whereas others have supported their monophyly: dae reported here. for further information, see McLeod et al. 1993; Fordyce and Barnes 1994; Fordyce and De Muizon 1999; Sanders and Barnes 2002a, b), the Grey whale Eschrichtius robustus Material and methods (Eschrichtiidae), the Pygmy Right whale Caperea marginata (Neobalaenidae), the Humpback whale Megaptera no- Eighty-one morphological characters were scored for 19 vaeangliae (Balaenopteridae, Megapterinae), and the Fin taxa including 10 balaenids in the ingroup, and seven whale Balaenoptera physalus (Balaenopteridae, Balaeno- non-balaenid mysticetes plus two archaeocete cetaceans in pterinae). Toothed mysticetes were represented by the aeti- the outgroup. An annotated character list and the charac- ocetid Aetiocetus polydentatus from the Upper Oligocene ter · taxon matrix used in the cladistic analysis are pre- of Kyushu Island (Japan). The phylogenetic analysis used sented in the Appendix (sections 1 and 2, respectively). two archaeocete taxa as the most external outgroups: the The ingroup included the following taxa: Balaena monta- middle Eocene Protocetus atavus and the early Middle–Late lionis, B. mysticetus, B. ricei, Eubalaena belgica, E. glacialis, Eocene Zygorhiza kochii. A list of non-balaenid specimens Balaenula astensis, B. balaenopsis, the Japanese B. sp., examined in this work is provided in Table 4, together Balaenella brachyrhynus and Morenocetus parvus. Poorly with their repositories and the references used to known taxa, such as Balaenotus insignis,‘Balaena’ etrusca supplement my personal observations. and ‘Balaena primigenius’ were excluded from the analysis Character states were treated as unordered and because they are largely incomplete (Bisconti 2003a). unweighted by PAUP 4.0b10 (Swofford 2002) under the However, despite the exclusion of these fragmentary taxa, ACCTRAN character state optimization. The search for the analysis represents the most inclusive phylogenetic the most parsimonious cladograms was made by tree- study of the family Balaenidae that has been attempted bisection-reconnection (TBR) with ten replicates and one hitherto. The balaenid taxa included in the cladistic analy- tree held at each step during stepwise addition followed sis have been described above under comparative data by bootstrap analysis with 100 replicates. A randomiza- and much more extensively by McLeod et al. (1993) and tion test was performed by PAUP to assess the distance Bisconti (2003a). of the results from 10,000 cladograms sampled equiproba- In previous studies, Barnes and McLeod (1984), bly from the set of all possible trees generated from the McLeod et al. (1993) and Bisconti (2000) found that original matrix. Character evolution and morphological the Grey whales of the family Eschrichtiidae were sister support at nodes were assessed by the evaluation func- to a clade including Neobalaenidae and Balaenidae. This tions of Hennig86 (Farris 1988; Lipscomb 1988). clade had been previously named Balaenoidea by Gray (1825). Because of these studies, balaenids were suggested to be among the most derived mysticetes. A recent Results morphology-based analysis of the phylogenetic relationships of the major radiations of living and fossil mys- General patterns. The TBR algorithm found six equally ticetes (Kimura and Ozawa 2002), together with new parsimonious trees, which were 143 steps long. Their results from molecular studies (e.g. A´ rnason and Ledje strict-consensus is shown in Text-figure 8A together with 1992; A´ rnason and Gullberg 1994; Gatesy 1998), sugges- the bootstrap tree that had the same length (Text- ted that the Eschrichtiidae and Neobalaenidae are more Fig. 8B). Tree statistics are provided in the caption of BISCONTI: DIMINUTIVE PLIOCENE WHALE 805

TABLE 4. List of non-balaenid cetaceans examined and references of morphological descriptions.

Taxon Repository No. References

Protocetus atavus SMNS 11084 (holotype) Fraas (1904); Kellogg (1936); Luo and Gingerich (1999) Zygorhiza kochii USNM 4748, 16638, 449538 Kellogg (1936); Uhen (1998) Aetiocetus polydentatus Barnes et al. (1994) Parietobalaena palmeri USNM 10677, 10909, 16570, Kellogg (1925, 1968b) 24883 Eschrichtius robustus USNM 364969, 364580, True (1904); Wolman (1985) 364973, 364970, 364977, 504305, 571931 NMB 42002 ZML St 13130, St 20350, 630 True (1904); Gambell (1985); Bisconti (2001) Balaenoptera physalus MSNT 251, 252, 253, 258, 255, 257 (newborn) ZMA 14927 (1–2), 14935 (1–2), 14947, 14950 (1-2), 23353 Megaptera novaeangliae MSNT 263 Van Beneden and Gervais (1868–79); True (1904); Winn and Reichley (1985); Clapham and Mead (1999) USNM 269982, 486175 (1–2), 13656 ⁄ 16252, 21492 ZMA 14952 (1–2), 14953 (1–2), 14964, 14965, 14966, 14967 Caperea marginata IRSN 1536 Beddard (1901); Baker (1985).

Text-figure 8. The randomization test (Text-fig. 9) sugges- clades branched from an unsolved polytomy (Text- ted that the results of the analyses were significantly dif- fig. 8A). In the bootstrap tree the polytomy was solved ferent from chance and confirmed the existence of a because Morenocetus parvus was attached to the clade phylogenetic structure in the data. formed by Balaena and Balaenella (Text-fig. 8B). The The analyses found a monophyletic suborder Mysticeti monophyly of the family Balaenidae received a boot- (bootstrap support of 100 per cent) in which the toothed strap support of 100 per cent. aetiocetid Aetiocetus polydentatus represented the most The monophyly of Eubalaena (including E. glacialis external taxon. TBR and bootstrap analyses converged and E. belgica) was confirmed in both the trees receiving toward the monophyly of all the baleen-bearing mysticetes a bootstrap support of 84 per cent. Balaenula (including included (bootstrap support of 99 per cent). The analyses B. astensis, B. balaenopsis and the Japanese B. sp.) was differed in minor details of the ingroup relationships. monophyletic and received a bootstrap support value of In both the trees, the ‘cetothere’ Parietobalaena palmeri 94 per cent with its three species branching from an was monophyletic with a clade including Eschrichtiidae unsolved polytomy. The monophyly of Eubalaena + and Balaenopteridae, and Neobalaenidae was sister to Balaenula was supported by a bootstrap value of 70 per Balaenidae supporting the superfamily rank taxon named cent. Balaenoidea by Gray (1825; Text-fig. 8A). This taxon had Both the analyses converged toward a monophyletic been previously reaffirmed by McLeod et al. (1993) and Balaena (bootstrap ¼ 67%) that included B. montalionis, Bisconti (2000) on morphological grounds and by Gatesy B. mysticetus and B. ricei. In the TBR strict-consensus tree (1998) based on cytochrome b DNA sequence analysis. The and in the bootstrap tree Balaena montalionis was sister Balaenoidea received a bootstrap support of 99 per cent. to a clade including B. mysticetus + B. ricei (supported by a bootstrap value of 55%). Ingroup relationships. The six equally parsimonious In all the cladograms, Balaenella brachyrhynus was sister cladograms found by TBR and presented as strict- to Balaena. This sister group relationship received a boot- consensus tree in Text-figure 8A represented the most strap support of 89 per cent. In the bootstrap tree, More- parsimonious solutions found in this work. Their strict- nocetus parvus was sister to a clade including Balaenella consensus will be considered as the optimal tree in the and Balaena; the corresponding bootstrap support value next sections. In that cladogram, balaenids are subdivi- was 65 per cent. ded into three clades: one clade is formed by Morenoce- tus parvus; a second includes Balaena and Balaenella; Character support at nodes. The character states were and a third comprises Balaenula and Eubalaena. These mapped onto the strict-consensus TBR tree (Text- 806 PALAEONTOLOGY, VOLUME 48

Protocetus atavus 7(0 fi 1), 8(0 fi 1), 12(0 fi 1), 22(0 fi 1) 44(0 fi 1) Zygorhiza kochii Aetiocetus polydentatus and 47(0 fi 1). These characters are related to the devel- Parietobalaena palmeri opment of a wide and flat rostrum, lack of the mandibu- Eschrichtius robustus Megaptera novaeangliae lar symphysis, development of an anterior groove for Balaenoptera physalus Mysticeti the mental ligament, presence of a complex structure (the Caperea marginata Morenocetus parvus infundibulum; see Fraser and Purves 1960) surrounding baleen-bearing Balaenella brachyrhynus the foramen ‘pseudo-ovale’, uniquely derived shape of the Mysticeti Balaena montalionis tympanic membrane (which is shaped as a glove finger; Balaena ricei Balaenoidea Balaena mysticetus Fraser and Purves 1960), and development of monop- Balaenula astensis hiodonty (which is also shared with odontocetes). Char- Balaenidae Balaenula balaenopsis Balaenula sp. acters 9, 10, 21, 25, 54 and 60 are ambiguously A Eubalaena glacialis reconstructed and cannot be confidently used to support Eubalaena belgica the monophyly of Mysticeti. Protocetus atavus Zygorhiza kochii The baleen-bearing mysticetes are diagnosed by the fol- Aetiocetus polydentatus lowing characters: 11(0 fi 1), 13(0 fi 1), 14(0 fi 1), Parietobalaena palmeri 100 52 15(0 fi 1), 16(0 fi 1), 18(0 fi 1), 55(0 fi 1) and 70 Eschrichtius robustus 100 Megaptera novaeangliae 73(0 fi 1). From this analysis, baleen-bearing mysticetes 100 Balaenoptera physalus are uniquely characterized by the lack of teeth, the pres- Mysticeti Caperea marginata Morenocetus parvus ence of baleen plates together with their vascular comple- 65 Balaenella brachyrhynus baleen-bearing 99 89 ment on the ventral surface of the maxilla, the lack of a Balaena montalionis Mysticeti 67 55 Balaena ricei close articulation of dentary and squamosal, the presence 100 Balaena mysticetus Balaenoidea of a lateral squamosal crest functioning as an attachment Balaenula astensis 94 Balaenula balaenopsis site for neck muscles, the lack of the hypoglossal foramen 70 Balaenidae Balaenula sp. (shared with odontocetes; however, the foramen is pre- B 84 Eubalaena glacialis Eubalaena belgica sent in Balaenula astensis; see Bisconti 2000, 2003a), the straight to slightly concave profile of the glenoid fossa of TEXT-FIG. 8. Phylogenetic analysis of the Balaenidae showing the relationships of Balaenella brachyrhynus. A, strict-consensus the squamosal in lateral view (which is transformed into of six equally parsimonious trees found by tree-bisection- a highly concave profile in Balaenopteridae), and a longer reconnection (TBR). B, bootstrap tree based on TBR results; supraorbital process of the frontal. bootstrap values are located above the branches. Tree statistics The clade including Parietobalaena palmeri, Eschrichtius are the same for both trees: tree length, 143; consistency index robustus, Balaenoptera physalus and Megaptera novaeangli- (CI), 0.7552; homoplasy index (HI), 0.2448; CI excluding ae (‘cetotheres’, eschrichtiids and balaenopterids) is sup- uninformative characters, 0.7518; HI excluding uninformative ported by the following characters: 27(0 ⁄ 1 fi 1), characters, 0.2482; retention index (RI), 0.8818; rescaled 51(0 ⁄ 1 fi 1), 53(0 ⁄ 1 fi 1) and 60(0 ⁄ 2 fi 2). Balaenopte- consistency index (RC), 0.6659. rids and eschrichtiids share four characters: 23(0 fi 1), 24(0 fi 1), 25(0 ⁄ 1 fi 1) and 26(0 fi 1). Characters Fig. 8A) by the DOS Equis function of Hennig86, which 21(1), 22(1), 26(0 fi 1) and 44(1) are shared by Caperea also provided the reconstructions of ancestral character marginata and the clade including ‘cetotheres’, eschrichti- states at nodes (Lipscomb 1988). In this way, it was poss- ids and balaenopterids. These characters include the lack ible to study patterns of character evolution in different of coalescence of the endocranial opening of the facial clades. canal into the internal acoustic meatus at adulthood, the Characters 1–3 support the monophyly of the order presence of a sagittal crest on the supraoccipital, the pres- Cetacea (see Appendix for a character list); they have ence of a squamosal cleft and the presence of a flat ros- been extensively treated by other authors (Van Beneden trum. The coalescence of the internal acoustic meatus and 1886; Fraser and Purves 1960; Kellogg 1965, 1968a; Ford- the endocranial opening of the facial canal has been des- yce and Barnes 1994; Luo and Gingerich 1999; Bisconti cribed in the late ontogeny of balaenopterid whales (Bis- 2001, 2003a) and are not discussed again here. Of course, conti 2001, 2003b); the meatus and facial canal are widely the monophyly of the Cetacea is supported by more than separated during adulthood in some fossil ‘cetotheres’ four morphological characters (e.g. Messenger and McGu- such as Parietobalaena palmeri, Diorocetus hiatus and Mes- ire 1998; Luo and Gingerich 1999; O’Leary and Geisler ocetus longirostris, and in the neobalaenid Caperea margi- 1999) but a discussion on cetacean monophyly is outside nata. Among balaenids, a morphological formation that the scope of the present paper. looks like a sagittal crest has been described in a Pliocene The monophyly of the suborder Mysticeti is supported Eubalaena sp. from Tuscany (Bisconti 2002). The squ- by the following characters: 5(0 fi 1), 6(0 fi 1), amosal cleft is a suture observed within the squamosal of BISCONTI: DIMINUTIVE PLIOCENE WHALE 807

225

200

175

150

125

100

75 number of cladograms 50

0 1 10 19 28 37 46 55 64 73 82 91 100 109 118 127 136 145 154 163 172 181 190 199 208 217 226 235 244 253 262 271 280 289 298 307 316 325 334 343 352 361 370 379 388 397 steps

TEXT-FIG. 9. Randomization test. The histogram depicts the tree length distributions of 10,000 cladograms equiprobably sampled from the set of all possible cladograms based on the character · taxon matrix presented in the Appendix. This result combined ten separate randomization tests each of which generated 1000 random cladograms. Mean tree length, 351.8; mean standard deviation, 24.53. The arrow indicates the tree length of the most parsimonious cladograms found by TBR whose strict-consensus is presented in Text-figure 8A. balaenopterids, neobalaenids and some late ‘cetotheres’; Characters 29(0 fi 1), 45(0 fi 1), 46(0 fi 1), 47(1 fi from the present analysis the presence of a squamosal 2), 48(0 fi 1), 49(0 fi 1), 50(0 fi 1), 51(0 ⁄ 1 fi 2), cleft should be interpreted as a convergence in different 52(0 fi 1), 53(0 fi 2) and 54(1 fi 2) support the mysticete clades. Character 44 is concerned with the ori- monophyly of the family Balaenidae. Balaenids are entation of the dorsolateral surface of the maxilla in mys- uniquely characterized by the possession of a wide manus, ticetes: balaenids are unique in having the anterior anterior torsion in the dentary, mylohyoidal sulcus along portion of the maxilla characterized by a transversely the ventromedial surface of the dentary, incomplete in- compressed and dorsoventrally depressed dorsolateral sur- fundibulum around the foramen ‘pseudo-ovale’, location face; in neobalaenids, the dorsolateral surface of the max- of the pterygoid near the posterior border of the skull, illa is horizontal and not dorsoventrally depressed as in presence of a well-developed ventral lamina of the ptery- balaenids; however, as in balaenids, the maxilla in neoba- goid fossa, palatines partially covering the pterygoids, laenids has a transversely compressed dorsolateral surface long baleen, dorsoventral orientation of the squamosal, (character 44(0 fi 1)). dorsoventrally compressed round window, long and shal- The monophyly of the superfamily Balaenoidea is low stylomastoid fossa, and long and triangular lateral supported by the following characters: 17(0 fi 1), projection of the anterior process of the periotic. 27(0 ⁄ 1 fi 2), 28(0 fi 1), 31(0 fi 1), 33(0 fi 1), Among Balaenidae, the monophyly of Balaenula is sup- 34(0 fi 1), 35(0 fi 1), 38(0 fi 2), 40(0 fi 2), 43(0 fi 1) ported by characters 70(0 fi 1), 71(0 fi 1) and and 56(0 fi 1). These include a suite of morphological 72(0 fi 1); these characters have been described in the transformations related to hearing and feeding: low and introduction of this paper and in Bisconti (2003a). The flattened tympanic bulla with a low sygmoid process, ele- monophyly of Eubalaena (including here only E. glacialis vated neurocranium and deep skull, well-developed ros- and E. belgica) is due to the sharing of characters 68 tral arch, short zygomatic process of the squamosal, low (presence of a dome on the anterior portion of the supra- and round angular process of the dentary, relative posi- occipital: 0 fi 1) and 69 (squared exoccipital in lateral tion of the posterolateral corner of the exoccipital and the view: 0 fi 1). Balaenula and Eubalaena form a monophy- postglenoid process, and superimposition of the supraoc- letic group diagnosed by the following synapomorphies: cipital onto the parietal (this pattern is different from that 62 (abrupt depression of the premaxilla in the anterior of Balaenopteridae, Eschrichtiidae and ‘cetotheres’ in half of the rostrum: 0 fi 1), 63 (irregular profile of the which the supraoccipital is developed in between the par- skull due to the development of a distinctive apex ietals). Characters 57 and 59 are reverted in the Eubalae- between supraoccipital and frontal: 0 fi 1) and 67 na + Balaenula clade. (spreading of the parietal onto the supraorbital process of 808 PALAEONTOLOGY, VOLUME 48 the frontal medially). Characters 57(1 fi 0) and support received by its inclusion in the first clade as sister 59(1 fi 0) are interpreted as unique reversions in the to Balaenella + Balaena. The phylogenetic analysis presen- Balaenula + Eubalaena clade. ted here conflicts with the previous results published by The monophyly of Balaena is unambiguously suppor- Bisconti (2000) in which a clade including the genera ted by two synapomorphies: 65 (raising of the nasals Morenocetus and Balaenula was discovered. As discussed together with the proximal rostrum: 0 fi 1) and 66 in Table 3, the synapomorphies proposed to support that (presence of a crest-like relief on the parietal squama: clade revealed plesiomorphic character states and are no 0 fi 1). Balaena mysticetus and B. ricei are more closely longer useful. related than each is with B. montalionis because of the The results of the new analysis strongly support a close rounder morphology of the anterior border of the supra- relationship of Eubalaena and Balaenula, two genera well occipital (60(1 fi 4)). Balaenella brachyrhynus and Balae- represented in Pliocene and Pleistocene sediments of Eur- na form a monophyletic group characterized by the ope, USA and Japan (see Bisconti 2000 and 2002 for following unambiguous synapomorphies: 58(0 ⁄ 1 fi 1), reviews of the fossil records of these genera). They show 60(0 fi 1), 61(0 ⁄ 1 fi 2) and 64(0 fi 1). These characters the highest morphological distance from Caperea margi- support the view that Balaenella and Balaena share the nata, the sister taxon to Balaenidae. In fact, in both posterior orientation of the lateral process of the maxilla genera, a complete restructuring of the skull occurred (which, in lateral view, seems to be located under the that was characterized by the interruption of the general supraorbital process of the frontal), distal absence of the curvature of the premaxilla, the transverse elongation of infraorbital plate, and glenoid fossa of the squamosal the supraorbital process of the frontal, the transverse located posterior to the posterior apex of the lambdoidal expansion of the anterior portion of the supraoccipital, crest. A marked transverse constriction of the anterior and a higher elevation of the vertex leading to the inter- portion of the supraoccipital is uniquely shared by Balae- ruption of the continuous curvature of the dorsal profile nella brachyrhynus and Balaena montalionis. Characters of the skull in lateral view. In Balaenula the retention of 57, 59 and 62 are shared by Balaenella, Balaena and Cape- the primitive round anterior border of the supraoccipital rea marginata. (shared with Morenocetus parvus and Caperea marginata) was paralleled by the development of a suite of derived character states affecting the temporal and the exoccipital DISCUSSION regions of the skull (see also Bisconti 2003a). The clade including the genera Balaena and Balaenella The discovery of Balaenella brachyrhynus in the Lower retained the posterior orientation of the supraorbital pro- Pliocene of the North Sea region helps our understanding cess of the frontal, the distal absence of the ascending of the evolution of body size in balaenid whales. In partic- temporal crest from the supraorbital process of the fron- ular, the phylogenetic analysis presented here suggests that tal, and a round exoccipital as in Caperea marginata and the origin of the gigantic size typical of the living Right Morenocetus parvus, together with a regular profile of the and Bowhead whales was attained independently in these skull in lateral view, continuously arched rostrum, and different clades. A small size was a common condition in lateral process of the maxilla located under the frontal Pliocene balaenids (such as in Balaenula and Balaenella) in lateral view. The absence of the distal portion of the and in the Lower Miocene Morenocetus parvus. Pliocene infraorbital plate represents an apomorphic feature absent species belonging to the extant genera were also commonly in the other balaenid clades. Unfortunately, it is imposs- small with respect to the gigantic sizes of the living balae- ible to score this character for Morenocetus parvus because nids (Bisconti 2000, 2002; Westgate and Whitmore 2002). the maxilla is absent. The extinction of all the small balaenids took place poss- Balaenella brachyrhynus and Balaena montalionis share ibly before the end of the Pliocene in all the oceans of the a transversely constricted anterior portion of the supraoc- world, erasing much of the Pliocene diversity of the family. cipital whereas a marked constriction is absent in Balaena The reasons for this large-scale (in geographical terms) mysticetus and B. ricei (Westgate and Whitmore 2002). extinction are not yet completely understood but theoreti- This character is not, however, sufficient to support the cal models are beginning to emerge (Bisconti 2003a) that monophyly of Balaenella brachyrhynus and Balaena deserve further investigation. montalionis to the exclusion of Balaena ricei and B. mys- One of the most important results of the present paper ticetus, and this reinforces the establishment of a different is the unambiguous definition of two main balaenid radi- genus for the Tilburg skull. ations: one includes Balaenella brachyrhynus plus the The generic differences characterizing Balaenella brac- genus Balaena, and the other includes the genera Eubalae- hyrhynus are observed in the nasal bones, the rostrum na and Balaenula. The phylogenetic position of Morenoce- and the lower inclination of the supraoccipital. While the tus parvus is still uncertain, given the low bootstrap last character is independent of the phylogenetic position BISCONTI: DIMINUTIVE PLIOCENE WHALE 809

of the Tilburg skull seemingly related to the small size of to the Balaena + Balaenella clade than to the Eubalae- the whale, the morphology of the nasal bones and the na + Balaenula clade. The analysis also confirmed the rostrum strongly support a generic distinction. The max- position of Caperea marginata as sister to the family Bala- illa of B. brachyrhynus is depressed anterior to the frontal enidae. whereas in Balaena it is highly arched and projects dor- Balaenella brachyrhynus is sister to Balaena. Its mor- sally, anterior to the frontal (see Bisconti 2003a). The phology, together with its phylogenetic position, docu- nasal bones of Balaenella brachyrhynus show the highest ments an unsuspected diversity among the small degree of reduction observed among the whole suborder balaenids of the Pliocene and suggests that small size has Mysticeti; these bones are small, short and horizontal, been a common condition among the balaenids just a few and differ from the corresponding bones of the other million years ago. Balaenidae, which are usually rectangular and bear a notch on the anterior border (see images in True 1904; Acknowledgements. I am indebted to Frans Ellenbroek (direc- Cummings 1985; see also Bisconti 2000, 2003a). The tor of NMB, Tilburg, Holland) who allowed me to study the function of the small, delicate nasals of Balaenella brac- specimen, and Marie-Cecile Van De Wiel (NMB) who assisted hyrhynus, if any, is not yet understood. me during my work and provided help on several occasions. I The discovery of the Tilburg skull adds important thank Klaas Post (Natuurmuseum Rotterdam, Holland) very much for making it possible for me to study the Dutch col- information on the morphological evolution of Balaeni- lections of fossil mysticetes, providing literature, making useful dae and documents an unsuspected diversity among the comments on an early draft of the manuscript, and kindly small balaenids. Three genera are currently known that assisting me during my visits to Holland. Thanks are also due are uniquely characterized by small species: Morenocetus, to Olivier Lambert for his help during my study of the IRSN Balaenella and Balaenula. The systematic position of the collection and to David Bohaska who assisted me during the small genus Balaenotus is not yet clear because it is too study of the USNM collection. Albert Sanders (The Charleston poorly known. Future studies should provide convincing Museum, Charleston, South Carolina) provided a thorough evidence on the phylogenetic placement of the Lower and insightful review of the manuscript: his help is gratefully Miocene Morenocetus parvus in order to make it possible acknowledged. Mark Uhen (Cranbrook Institute of Science, to assess the divergence times of the living genera from Bloomfield Hills, Michigan) and Oliver Hampe (Museum fur their last common ancestor. This information is crucial Naturkunde, Berlin, Germany) carefully reviewed the manuscript, providing suggestions that significantly improved the for conservation biologists in order to test the estimated quality of the paper. This work has been supported by rates of evolution and the genetic health of the living MURST funds, being a contribution of the Pisa Unit (Unit balaenid populations (Rooney et al. 2001). coordinator Walter Landini, Dipartimento di Scienze della Terra, University of Pisa) to the research project ‘Palaeobioge- ography of Central Mediterranean from Miocene to Quater- CONCLUSIONS nary’ (national coordinator Danilo Torre, Dipartimento di Scienze della Terra, University of Florence). In this paper, a new genus and species of a balaenid whale has been described and compared with all the living and fossil members of the family Balaenidae. The spe- REFERENCES cimen concerned is from the Lower Pliocene near Antwerp (Belgium) and is named Balaenella brachyrhynus. ABEL, O. 1941. Vorlaüfige mitteilungen uber die Revision der This whale is characterized by being the smallest represen- fossilen mystacoceten aus dem tertia¨r belgiens. 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APPENDIX 2. Air sinus: 0, absent; 1, present around the tympanic bulla (Fraser and Purves 1960; Luo and Gingerich 1999). Morphological characters used in the phylogenetic analysis 3. Ascending temporal crest: 0, absent; 1, present. This crest is located on the supraorbital process of the frontal in mysticetes; it can be located on the posterodorsal edge of the pro- The following is an annotated list of the character states used cess (Oligocene toothed and baleen-bearing mysticetes), at in the cladistic analysis. A brief discussion of the characters is the middle of the process (‘cetotheres’, eschrichtiids, balae- provided only for characters described here for the first time nids, and neobalaenids) or along its anterior border (balaeno- or not used in previous cladistic analyses of the mysticetes. pterids) (Fordyce and Barnes 1994). Otherwise, the characters are referred to the appropriate litera- 4. Parietal and squamosal are bulged into the temporal fossa: 0, ture. no; 1, yes. The bulging of the parietal and squamosal into the 1. Suprameatal area of petrosal: 0, low; 1, high. Luo and Ginge- temporal fossa is observed in basilosaurid archaeocetes, ‘ce- rich (1999) gave accurate descriptions of the petrotympanic totheres’, and eschrichtiids. complexes of early cetaceans. They showed that in basilosau- 5. Plate-like infraorbital process of the maxilla: 0, absent; 1, pre- rids the suprameatal area of the petrosal is high and flat-to- sent (Messenger and McGuire 1998; Sanders and Barnes slightly concave. Sanders and Barnes (2002a, b) described this 2002b). same condition in Oligocene baleen-bearing mysticetes. A 6. Mandibular symphysis: 0, present; 1, absent (groove for men- high suprameatal area of the petrosal is present in several tal ligament present) (Messenger and McGuire 1998; Sanders Miocene ‘cetotheres’ (see Kellogg 1965, 1968a; Van Beneden and Barnes 2002b). 1886) and some balaenids (Bisconti 2003a). In balaenopterids 7. Tympanic membrane: 0, present; 1, modified into glove finger the suprameatal area of the petrosal is comparatively lower (Fraser and Purves 1960; Messenger and McGuire 1998). (Bisconti 2001). BISCONTI: DIMINUTIVE PLIOCENE WHALE 813

8. Foramen ‘pseudo-ovale’: 0, absent; 1, present and perforating depressed from the interorbital region of the frontal in bala- styliform process of squamosal (Barnes et al. 1994). enopterids, eschrichtiids, and related taxa. In ‘cetotheres’ 9. Sternum: 0, formed by manubrium and several seternebra; 1, and balaenids it gently descends from the interorbital region formed by manubrium only. In archaeocetes and odontocetes, (Zeigler et al. 1997). the sternum is formed by manubrium and some sternebra 21. Internal opening of facial canal coalesces into the internal (Kellogg 1936) whereas in mysticetes it includes the manubri- acoustic meatus during early ontogeny: 0, yes; 1, no. Biscon- um only (Tomilin 1967). ti (2001) demonstrated that the internal acoustic meatus 10. Number of ribs attached to the sternum: 0, several pairs; 1, and the internal opening of the facial canal are separate in one pair (True 1904; Tomilin 1967). the early ontogeny of Balaenoptera physalus. Subsequently, 11. Teeth in the adult: 0, present; 1, absent (Messenger and Bisconti (2003b) extended this observation to Balaenoptera McGuire 1998). edeni and B. borealis. These formations coalesce during the 12. Dental generations developed during embryology: 0, poly- late ontogeny of balaenopterids. In archaeocetes, balaenids ophiodonty; 1, monophiodonty (Messenger and McGuire and ‘cetotheres’, the internal opening of the facial canal does 1998). not coalesce and both are included in a common cavity at 13. Baleen plates: 0, absent; 1, present (Messenger and McGuire least during the latest ontogeny (Geisler and Luo 1996; Luo 1998). and Gingerich 1999; Bisconti 2003a). The coalescence is 14. Lateral squamosal crest: 0, absent; 1, present. The lateral absent in Caperea marginata. squamosal crest is observed on the dorsal edge of the zygo- 22. Sagittal crest on the anteriormost portion of the supraoccip- matic process of the squamosal and anterior to the caudal ital: 0, absent; 1, present. A sagittal crest is observed in apex of the lambdoidal crest in mysticetes; the term is used aetiocetids, some ‘cetotheres’ (including Parietobalaena by Kellogg (1965, 1968a). palmeri), neobalaenids, and in an Eubalaena sp. from the 15. Cranio-mandibular joint: 0, dentary and squamosal closely Pliocene of Tuscany (Bisconti 2002). articulate each other; 1, dentary and squamosal are not clo- 23. Ascending process of the maxilla: 0, absent; 1, definite and sely articulated. In baleen-bearing mysticetes, the glenoid long. State 1 is typical of Balaenopteridae, Eschrichtiidae, fossa of the squamosal is a wide concavity, which is not as and some advanced ‘cetotheres’ including Cetotherium rath- closely articulated with the dentary as in terrestrial mammals kei (Tomilin 1967; Pilleri 1986, his fig. 15, p. 25). In other and in archaeocete and odontocete cetaceans (Sanderson ‘cetotheres’ (including Parietobalaena palmeri), the ascending and Wassersug 1993); however, it seems that a close articu- process of the maxilla is tapered and broad (Sanders and lation of dentary and squamosal was present in the toothed Barnes 2002b). In balaenids and neobalaenids, the ascending mysticete Aetiocetus polydentatus (Barnes et al. 1994). process of the maxilla is short and not as definite as in the 16. Angular process of dentary: 0, high; 1, low; 2, very low. In above taxa (Tomilin 1967). archaeocetes, toothed mysticetes, and eschrichtiids the angu- 24. Interorbital region of frontal: 0, present; 1, absent. In bala- lar process of the dentary is well developed. In balaenids enopterids and some late ‘cetotheres’ (such as Cetotherium and ‘cetotheres’ it is lower (Bisconti 2000). In balaenopte- rathkei), the ascending processes of the maxillae superim- rids, the angular process is strongly reduced (Lambertsen pose onto the interorbital region of the frontal; therefore, et al. 1995). this region is obliterated. In the other mysticetes and archae- 17. Angular process of dentary in lateral view: 0, squared, ocetes this region is not obliterated because the ascending robust, pterygoid groove absent; 1, round, slightly-built, processes of the maxillae are short or absent (Bisconti pterygoid groove absent; 2, squared, slightly-built, pterygoid 2003b). groove present. A round angular process is present in Bala- 25. Posterior apex of the ascending process of maxilla: 0, anter- enidae (Bisconti 2000, 2003a). In balaenopterids, the angu- ior and far to postorbital corner of frontal; 1, close to post- lar process is rectangular and is separated from the orbital corner. In early ‘cetotheres’, such as Parietobalaena condylar area through a pterygoid groove Lambertsen et al. palmeri, Diorocetus hiatus and Pelocetus calvertensis, the 1995). ascending process of the maxilla is slightly superimposed 18. Supraorbital process of frontal: 0, short; 1, long. In archae- onto the anteromedial portion of the interorbital region of ocetes and some ‘cetotheres’ (such as the skull USNM the frontal and its posterior apex does not move far posteri- 187416 of Cetotherium megalophysum) the supraorbital pro- orly. In balaenopterids and later ‘cetotheres’ (such as Ceto- cess of the frontal is short. In the mysticetes included in this therium rathkei), the posterior apex of the ascending process work, it is longer (Bisconti 2003b). of the maxilla is very posterior on the dorsal wall of the 19. Supraorbital process of frontal: 0, broad; 1, slender; 2, very skull. broad. In archaeocetes, the supraorbital process of the fron- 26. Squamosal cleft: 0, absent; 1, present. The squamosal cleft is tal is broad; in mysticetes it is usually slender; in balaenopte- a widely distributed character observed in several mysticete rids it is very broad, allowing for the attachment of the taxa such as Neobalaenidae, Balaenopteridae and Eschrichtii- anterior portion of the muscle temporalis (Lambertsen et al. dae. 1995; Zeigler et al. 1997). 27. Shape of zygomatic process of squamosal in lateral view: 0, 20. Supraorbital process of frontal: 0, horizontal; 1, gently des- elongated, slender and subtle; 1, crescent-shaped; 2, slightly cending from the interorbital region of frontal; 2, abruptly pointed or round; 3, sharply triangular. In archaeocetes and depressed. The supraorbital process of the frontal is abruptly early ‘cetotheres’ (such as Parietobalaena palmeri) the zygo- 814 PALAEONTOLOGY, VOLUME 48

matic process of the squamosal is usually slender (see, e.g., 35. Extremely low conical process of tympanic bulla: 0, no; 1, Kellogg 1936, fig. 3, p. 23 and fig. 31a, p. 108). In balaeno- yes. State 1 observed in balaenids and neobalaenids only pterids, the zygomatic process of the squamosal is curved (Bisconti 2003a). ventrally and this gives the process a crescent shape (True 36. Tympanic bulla transversely enlarged: 0, no; 1, yes. State 1 1904, pls 3–4, 26–27, 31). This shape is also shared by the observed in balaenids and neobalaenids only (Bisconti aetiocetid Aetiocetus polydentatus (Barnes et al. 1990) and 2003a). the Oligocene baleen bearing mysticetes Micromysticetus rot- 37. Internal opening of facial canal: 0, wide, broad and shallow; hauseni and Eomysticetus whitmorei, together with an 1, small, cylindrical and deep (Geisler and Luo 1996). unnamed Oligocene mysticete from Japan (Sanders and Bar- 38. Length of the zygomatic process of squamosal: 0, long; 1, nes 2002a, b; Okazaki 1994). In balaenids and neobalaenids short; 2, very short. A short zygomatic process of the squ- the zygomatic process of the squamosal is stocky (see True amosal is observed in eschrichtiids and such ‘cetotheres’ as 1904, pls 43–44; Baker 1985). In eschrichtiids the zygomatic Cetotherium rathkei. In these taxa, however, a zygomatic process of the squamosal is triangular in lateral view (True process of the squamosal can be easily distinguished. In 1904, pl. 47). balaenids and neobalaenids the zygomatic process of the 28. Position of the posterolateral corner of the exoccipital rel- squamosal is even shorter and somewhat vestigial (McLeod ative to the posterior border of the postglenoid process in et al. 1993). ventral view: 0, close and posterior; 1, close and medial; 2, 39. Cervical vertebrae: 0, free; 1, fused. State 1 observed in far and posterior (Bisconti 2003b). In archaeocetes and Oli- balaenids and neobalaenids only (McLeod et al. 1993). gocene baleen-bearing mysticetes the posterolateral corner 40. Rostral arch: 0, absent; 1, rostrum slightly arched; 2, rostrum of the exoccipital is close and posterior to the posterior highly arched. State 1 is observed in eschrichtiids only border of the postglenoid process of the squamosal. In esc- (McLeod et al. 1993). The rostrum as a whole is highly hrichtiids and some ‘cetotheres’, such as Cetotherium rath- curved in Neobalaenidae and Balaenidae; a detail of the cur- kei and Mixocetus elysius (Kellogg 1934), this condition is vature (the curve of the premaxilla) is treated in character emphasized being the posterolateral corner of the exoccipi- 62. tal located far to the posterior of the posterior border of 41. Coronoid process of dentary: 0, present; 1, absent. The coro- the postglenoid process (state 3). In the other mysticetes noid process is absent in eschrichtiids, balaenids, and neoba- the posterolateral corner of the exoccipital is located closer laenids (McLeod et al. 1993). (state 1). 42. Mandibular condyle: 0, articular surface dorsal; 1, articular 29. Manus: 0, short; 1, long; 2, wide. A long manus is typical of surface posterior. State 1 is observed in eschrichtiids, balae- Balaenopteridae whereas a wide manus is observed in the nids, and neobalaenids (McLeod et al. 1993). living Balaenidae (Tomilin 1967). 43. Height of neurocranium: 0, low; 1, high. In balaenids the 30. Shape of the anterior process of the petrosal in dorsal view: neurocranium is high due to the enormous curvature of the 0, round; 1, squared; 2, triangular. Bisconti (2001, 2003a), rostrum. This character is absent in other mysticetes Van Beneden (1878, 1880, 1886), Kellogg (1936, 1965, (McLeod et al. 1993). 1968a), and Sanders and Barnes (2002a, b) gave adequate 44. Rostrum transversely compressed: 0, yes; 1, no, rostrum flat descriptions of the morphology of the anterior process of (McLeod et al. 1993). the petrosal in mysticetes and archaeocetes. 45. Mylohyoidal sulcus along the ventromedial surface of the 31. Parietal exposition at the cranial vertex: 0, absent; 1, present, dentary: 0, absent; 1, present (McLeod et al. 1993). parietal under the supraoccipital (Balaenoidea); 2, present, 46. Anterior torsion of the dentary: 0, absent; 1, present parietal divided posteriorly by the interposition of the supra- (McLeod et al. 1993). occipital (Balaenopteridae, Eschrichtiidae, ‘cetotheres’). In 47. Infundibulum: 0, absent; 1, complete; 2, incomplete. State 2 balaenids and neobalaenids the supraoccipital superimposes is observed in balaenids only (Fraser and Purves 1960). the parietal and the posteromedial portion of the interorbital 48. Posterior extension of the palatine: 0, anterior to posterior area of the frontal (Bisconti 2002). In balaenopterids, esc- border of the skull; 1, very close to the posterior border of hrichtiids and ‘cetotheres’ the supraoccipital is interposed in the skull. State 1 observed in balaenids only (Fraser and Pur- between the parietals. ves 1960). 32. Position of coronal suture: 0, anterior to the anterior border 49. Ventral lamina of the pterygoid: 0, absent; 1, present. State 1 of the supraoccipital; 1, posterior. As a consequence of the observed in balaenids only (Fraser and Purves 1960). superimposition of the supraoccipital onto the parietal and 50. Pterygoid partially covered by the palatine: 0, no; 1, yes the posteromedial region of the interorbital area of the fron- (Fraser and Purves 1960). tal, the coronal suture is posterior to the anterior border of 51. Stylomastoid fossa: 0, absent; 1, deep as a notch; 2, long and the supraoccipital in balaenids and neobalaenids (Bisconti shallow. The stylomastoid fossa is shaped as a deep notch in 2002, 2003b). several ‘cetotheres’ (e.g. Parietobalaena palmeri, Diorocetus 33. Main orientation of the squamosal: 0, horizontal; 1, dorso- hiatus) and balaenopterids (Bisconti 2003b). In balaenids it ventral (McLeod et al. 1993; Bisconti 2000, 2003a). occupies a long portion of the posterior process of the peri- 34. Dorsoventral compression of tympanic bulla: 0, absent; 1, otic and is shaped as a shallow concavity (Bisconti 2003a). present (generates a shallow tympanic cavity) (McLeod et al. 52. Oval window: 0, elliptical; 1, dorsoventrally compressed. In 1993; Bisconti 2003a). all the balaenids included into this analysis, the oval window BISCONTI: DIMINUTIVE PLIOCENE WHALE 815

is markedly compressed along the dorsoventral axis (Bisconti lar curvature is observed in Balaena and Balaenella. Irregular 2003a). curvature is present in Eubalaena and Balaenula (McLeod 53. Lateral projection of the anterior process of petrosal: 0, et al. 1993). absent; 1, present and short; 2, present, triangular and long. 64. Distal portion of the infraorbital plate of the maxilla: 0, pre- The triangular projection of the anterior process of the pet- sent; 1, absent (Bisconti 2003a). rosal is well marked in living and fossil Balaenidae. It is not 65. Orientation of the nasals and the proximal rostrum: 0, preserved in Balaenula astensis and B. balaenopsis (Bisconti onward; 1, upward. State 1 is present in Balaena mysticetus, 2003a; see also Geisler and Luo 1996). B. montalionis and, presumably, in B. ricei (Bisconti 2003a). 54. Baleen plate length: 0, baleen short; 1, long (Eschrichtiidae, 66. Relief on the parietal squama: 0, absent; 1, present (Bisconti Neobalaenidae); 2, very long (Balaenidae) (McLeod et al. 2000, 2003a). 1993). 67. Spreading of the anterolateral portion of the parietal onto the 55. Glenoid fossa of squamosal: 0, short and concave; 1, ﬂat-to- emergence of the supraorbital process of the frontal: 0, slightly concave; 2, highly concave and long. State 1 is absent; 1, present. The spreading of the parietal onto the observed in balaenids, neobalaenids, eschrichtiids, and ‘ce- emergence of the supraorbital process of the frontal is totheres’. State 2 is typical of Balaenopteridae. observed in Balaenula and Eubalaena among the Balaenidae 56. Relative position of zygomatic process of squamosal and (Bisconti 2003a). postglenoid process: 0, postglenoid process lower than the 68. Dome on the supraoccipital: 0, absent; 1, present (Bisconti zygomatic process; 1, postglenoid process on the same hori- 2002). zontal plane as zygomatic process. In balaenopterines 69. Posterior outline of the exoccipital in lateral view: 0, round; and megapterines the postglenoid process is lower than the 1, squared. Squared exoccipitals are observed in Eubalaena zygomatic process of the squamosal (see True 1904, pls 3–4, skulls (Bisconti 2000, 2003a). 26–27, 31). 70. Position of the glenoid fossa of the squamosal relative to the 57. Distal presence of the ascending temporal crest on the orbit: 0, posterior; 1, under the orbit. State 1 is diagnostic of supraorbital process of the frontal: 0, yes; 1, no. The crest is Balaenula (Bisconti 2003a). absent in Caperea marginata, Morenocetus parvus and in 71. Height of the ventral surface of the exoccipital: 0, higher Balaenella and Balaena (see also Bisconti 2003a). than the orbit; 1, at the level of the orbit. State 1 is diagnos- 58. Lateral process of maxilla in lateral view: 0, anterior to the tic of Balaenula (Bisconti 2003a). supraorbital process of frontal; 1, under the supraorbital 72. Height of the ascending temporal crest: 0, low; 1, high. A process of frontal (Bisconti 2003a). high ascending temporal crest is observed in Balaenula (Bis- 59. Supraorbital process of the frontal: 0, transverse to the long conti 2003a). axis of the skull; 1, directed posteriorly (McLeod et al. 73. Hypoglossal foramen: 0, present; 1, absent. The presence of 1993). the hypoglossal foramen is known in Balaenula astensis (Bis- 60. Anterior process of the supraoccipital: 0, round and wide; 1, conti 2000, 2003a). squared and transversely constricted; 2, squared; 3, triangu- 74. Olecranon: 0, present; 1, absent. lar; 4, narrowly rounded. This character accounts for the 75. Acromion: 0, present; 1, absent. morphological diversity of the anterior portion of the supra- 76. Coracoid: 0, present; 1, absent. occipital in mysticetes. State 1 is observed in Balaenella and 77. Shape of the scapula: 0, high and short; 1, high and long. Balaena montalionis; state 2 is present in balaenopterids; 78. Antibrachium: 0, shorter than humerus; 1, longer than state 3 is found in ‘cetotheres’; state 4 is seen in Balaenula. humerus. See Westgate and Whitmore (2002) for a discus- 61. Position of the glenoid fossa relative to the posterior apex of sion of this character. the lambdoidal crest: 0, glenoid fossa located below the 79. Radius: 0, straight; 1, highly convex. The radius is anteriorly lambdoidal crest; 1, anterior; 2, posterior. The character is convex in living balaenids and in Balaena ricei (see Pilleri related to Bisconti’s (2003a) diagnosis of Balaenula. 1987; Benke 1993; Westgate and Whitmore 2002). 62. Curvature of premaxilla: 0, no curvature; 1, regular curva- 81. Pars cochlearis: 0, transversely short; 1, protruded cranially. ture; 2, irregular curvature. In Balaenula astensis and Eubal- A cranial protrusion of the pars cochlearis is observed in aena the anterior 25 per cent of the premaxilla is directed balaenopterids and in the living Eubalaena glacialis (Bisconti ventrally, interrupting the regular curvature of the rostrum 2001, 2003a). in that region (Bisconti 2000, 2003a). 82. Groove for the tensor tympanic muscle: 0, present; 1, absent 63. Curvature of the dorsal surface of the skull: 0, skull mainly (Luo and Gingerich 1999). straight; 1, regular curvature; 2, irregular curvature. A regu- 816 PALAEONTOLOGY, VOLUME 48

Character · taxon matrix used in the phylogenetic analysis

Protocetus atavus ?000000000000000000000000000?000000000000000000000000-00--00000-0000000-000000000 Zygorhiza kochii 111100000000000000000000-0000000000000000000000000000-000-00100-000000010??????00 Aetiocetus polydentatus ?1111111??0100000000?11000000000000000000001001000000-000003100000000001000000000 Parietobalaena palmeri 111111111111111101111100-00000000000000001010010001010100003100000000000100000000 Eschrichtius robustus 111111111111111001111011113301000000010110010010001011110002000000000000100000000 Balaenoptera physalus 111111111111111221221111111022200000000001010010001010200002000000000000100011011 Megaptera novaeangliae 111111111111111221221111111022000000000001010010001010200002000000000000101111011 Caperea marginata 111111111111111111001100-12101111111121210110010000001111010111000000000100011011 Morenocetus parvus ?1101??11111111??101?000-021??111????21??01?112??????21111100???000000001???????? Eubalaena glacialis 111011111111111111111100-02111111111121210101121112122110000022000111000100000110 Eubalaena belgica ?1101?11??11111??111?000-021??111111?212?01???2111???211000002200011100010??????? Balaenula balaenopsis 111011111111111111110000-021?1111111121210101121112102110000022000100111100000000 Balaenula astensis 11101111??11111111110000-021?1111111?2?2101011211121?21100000220001001110??????00 Balaenula sp. (Japan) 111011111111111111110000-021?1111111121210101121112122110000022000100111100000000 Balaenella brachyrhynus ?1101?11??11111??1110000-021?1111111?2?2?010??211121221111112111000000001??????00 Balaena montalionis ?1101?11??11111??111?000-021??111????2?2?010??2111???21111112111110000001???????? Balaena ricei 11?0?1111?11111?????1000-021??111???1?12??1011211121?2???1?4211??10000??100000100 Balaena mysticetus 111011111111111111110000-021111111111212101011211121221111142111110000001?1100100