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Serie Correlación Geológica - 34 (1): 53 - 70 Tucumán, 2018 - ISSN 1514-4186 - ISSN on-line 1666-9479

Structural variation of the masseter muscle in Typothe- ria (Mammalia, ) Luis Marcelo SOSA1 and Daniel Alfredo GARCÍA LÓPEZ2

Resumen: VARIACIÓN ESTRUCTURAL DEL MÚSCULO MASETERO EN (MAMMALIA, NOTOUNGULATA). La gran diversidad morfológica del orden Notoungulata se expresa en rasgos notorios, como el tamaño corporal, incluyéndose aquí desde animales del porte de un pequeño roedor (e.g., Punapithecus) hasta formas de más de una tonelada (e.g., ). Se conocen ade- más, diferentes casos de convergencia con otros placentarios, reconociéndose formas rodentiformes y otras similares a lagomorfos, perisodáctilos y artiodáctilos. Esta notable diversidad se expresa también en rasgos específicos, como la región auditiva y el aparato masticatorio. En este trabajo se presenta un análisis sobre la evolución estructural de la zona de origen del músculo masetero enfocada en el subor- den Typotheria. Se seleccionó una muestra de 12 representantes de ese grupo más dos representan- tes de notoungulados tempranamente divergentes (Henricosborniidae y ). La muestra abarca un lapso de más de 43 Ma (Eoceno–Pleistoceno). Se establecieron tres condiciones principales relacionadas con el origen del masetero que muestran cierto ordenamiento temporal y filogenético. La Condición 0 (e.g., Colbertia, Oldfieldthomasia) se caracteriza por el origen puntual y restringido del masetero superficial y por una zona limitada para el masetero profundo, encontrándose exclusivamen- te en formas eocenas y siendo comparable al plan generalizado presente en ciertos marsupiales (e.g., Didelphis). La Condición 1 (e.g., Notopithecus, ) presenta un masetero profundo de desarrollo limitado y un masetero superficial asociado a un proceso descendente conspicuo, mostrando simili- tudes con (e.g., Oryctolagus). Finalmente, la Condición 2 (e.g., Griphotherion, Typotheriodon, , Paedotherium) es la más compleja, mostrando un espectro de variaciones relacionadas con diferentes grados de hipertrofia del masetero profundo y reflejando similitudes a nivel superficial con ciertos roedores (e.g., Sciuromorpha). Esta perspectiva más focalizada muestra la complejidad alcanzada por los Notoungulata y es necesaria para obtener un cuadro más completo de la evolución del grupo, el cual brinda la oportunidad de estudiar la historia natural de un linaje de raíz mesozoica, sometido a aislamiento geográfico y sobreviviente casi hasta tiempos recientes.

Abstract: The great morphologic diversity of the order Notoungulata is evident in remarkable traits, such as the body size, and this group includes from small, size (e.g., Punapithecus) to forms exceeding 1000 kg in weight (e.g., Trigodon). Moreover, several cases of convergence with other placentals are known within the group, with rodent-like and lagomorphs-like forms in Typotheria and animals similar to some Perissodactyla or Artiodactyla within Toxodontia. This remarkable diversity is also expressed in specific traits developed on certain parts of the skeleton, such as the auditory region or the masticatory apparatus. In this contribution we present an analysis on the structural evolution of the area of origin of the masseter muscle focused on the suborder Typotheria. A sample of 12 representatives of that group was selected, plus two representatives of early diverging notoungulates (Henricosborniidae and Notostylopidae). This sample covers a span of approximately 43 Ma (

______1) Facultad de Ciencias Naturales e Instituto Miguel Lillo, Universidad Nacional de Tucumán. Miguel Lillo 205 (CP 4000), San Miguel de Tucumán, . e-mail: [email protected] 2) Instituto Superior de Correlación Geológica (INSUGEO–CONICET). Facultad de Ciencias Naturales e Ins- tituto Miguel Lillo, Universidad Nacional de Tucumán. Miguel Lillo 205 (CP 4000), San Miguel de Tucumán, Argentina. e-mail: [email protected] SOSA AND GARCÍA LÓPEZ 54

–Pleistocene). On each case, the relative development and arrangement of the different parts of the masseter was determined evaluating the area of the muscular scars remaining on the zygomatic arch and its anterior root. Three main conditions related to the origin of the masseter muscle were deduced, showing some tempo- ral and phylogenetic arrangement. Condition 0 (e.g., Colbertia, Oldfieldthomasia) is characterized by a small origin of the superficial master and a small area for the deep master, is only present in Eocene representatives, and is comparable to the generalized plan present in some (e.g., Didelphis). Condition 1 (e.g., Notopithecus, Protypotherium) presents a moderately-developed deep master and an origin of the superficial masseter related to a conspicuous descendant process, being similar to some Lagomorpha (e.g., Oryctolagus). Finally, the condi- tion 2 (e.g., Griphotherion, Typotheriodon, Hegetotherium, Paedotherium) is the most complex, showing a spectrum of variations related to different stages of hypertrophy of the deep masseter which reflex similarities to certain (e.g., Sciuromorpha). This morphologic perspective shows the complexity achieved by the Notoun- gulata and brings a more complete frame on the evolution of this group, which represents one of the most characteristic elements of the South American fossil biota.

Key words: Structure. Masseter muscle. Typotheria. Notoungulata. . .

Palabras clave: Estructura. Músculo masetero. Typotheria. Notoungulata. Cenozoico. América del Sur.

Introduction truction of fossil communities. Regarding the order Notoungulata, some authors have alre- The order Notoungulata is one of the ady made important contributions in subjects most representative elements of the endemic such as paleodiet, body size, functional mor- mammalian fauna of the Cenozoic from South phology, and ontogeny (Elissamburu, 2004, America. The position of the clade within pla- 2012; Shockey et al., 2007, Croft and Weinstein, cental remained elusive for more than 2008; Billet et al., 2009; Townsend and Croft, a century; however, recent contributions have 2010; Cassini et al., 2010, 2011; Scarano et al., established their close relationship with Peris- 2011; Cassini and Vizcaíno, 2012; McCoy and sodactyla, forming the clade Panperissodactyla Norris, 2012; Armella et al., 2016). Nevertehe- within Laurasitheria (Buckley, 2015; Welker less, these studies are mostly focused on Neo- et al., 2015). Even so, relationships among the gene taxa, being the paleobiology of Paleogene different families of Notoungulata remain in representatives still poorly known. part unresolved and their position among other Within the Notoungulata, the subor- groups of South American (e.g., As- der Typotheria is particularly interesting, since trapotheria, , , Pyrothe- it represents a clear example of evolutionary ria) are still a topic of debate (Billet, 2010, 2011; convergence with extant orders such as Lago- Billet and Muizon, 2013; Kramarz et al., 2017). morpha and Rodentia (Simpson, 1967; Cerde- Notoungulates are conspicuously present in ño and Bond, 1998; Croft and Anaya, 2006). most of the fossil assemblages of the Cenozoic This convergence is largely expressed in the and their record shows an evolutionary history arrangement of the muscles of the masticatory marked by changes in morphology, taxonomic apparatus, which leads to the development of diversity, and exploration of ecologic niches structures such as zygomatic plates and mandi- (Simpson, 1967; García-Lopez, 2009; García- bular expansions. López and Powell, 2011; Giannini and García Masseter muscles fulfill a fundamental -López, 2014). role in mastication within mammals, forming Paleobiological studies are of great the masticatory apparatus along with the importance since they bring a more compre- temporal and pterygoid muscles (Turnbull, hensive framework, necessary for the recons- 1970; Crompton and Parker, 1978; Herring, 55 THE MASSETER MUSCLE IN TYPOTHERIA

2007; Druzinsky et al., 2011). This group of toungulata (Henricosborniidae and Notos- muscles has been classified in different ways tylopidae) where included in order to obtain by many authors. In this sense, one of the an evolutionary and morphofunctional pers- most complete and detailed surveys was per- pective on the basis of forms of poorly-di- formed by Turnbull (1970), who classified fferentiated ecomorphology. Within Typo- the masticatory apparatus in four groups theria, the sample includes taxa belonging according to the arrangements and relative to “”, , development of each element. These groups “”, , and correspond to carnivores, rodents, ungulates, . Though some of these cla- and a generalized group (including forms des are paraphyletic clusters, the use of their like the metatherian Didelphis). In this classi- names is maintained here for practical pur- fication is evident that the masticatory domi- poses. Table 1 shows the list of included nance of the different muscles reflexes the genera, analyzed material, and contributions feeding habits of the ; in this way, the consulted. predominance of the temporal muscle is cha- The arrangement of the different racteristic of carnivores, whereas strong mas- parts of the masseter muscle was established seter and pterygoid muscles are more typical analyzing the surface of the skull, in order in herbivores (Herring, 2007). to identity the structures usually associated In the present contribution we analyze to the origin of each muscular element. At the variation patterns in the zone of origin of this point, the nomenclature and morpholo- the different parts of the masseter muscle in gical frame used was that proposed by Tur- a sample of notoungulates. Although most of nbull (1970). Likewise, specialized literatu- the analyzed forms represent families within re focused on extinct and extant mammals Typotheria, some early-diverging taxa were was consulted (e.g., Cox and Jeffery, 2011; included (i.e., Henricosborniidae, Notostylopi- Crompton and Parker, 1978; Gambaryan dae). From this, we bring a morphologic clas- and Kielan - Jaworowska, 1995; Russel, sification and discuss the related evolutionary 1998; Getty, 2005; Herring, 2007; Druzins- and phylogenetic background. ky et al., 2011; Evans and de Lahunta, 2013). Following Turnbull (1970), the masseter muscle is divided in three parts: superficial Materials and methods masseter (pars superficialis), deep masseter Functional characterizations mentio- (pars profunda), and zigomaticomandibularis. ned in the text were performed based on Starting from this scheme, structures com- the direct study of the mentioned taxa. In monly associated to the origin of each part some cases, this analysis was helped with the were individualized, and the development consultation of specialized literature, parti- (based on the relative area) of each surface cularly those contributions where cranioden- was qualitatively determined. Finally, three tal traits were described for representatives discrete categories were determined and na- of Notoungulata (e.g., Scott, 1912; Simpson, med as Condition 0, 1, and 2. 1948, 1967; Billet et al., 2008, 2009; Billet, 2011; García-López, 2011; García-López Abbreviations and Powell, 2011). The selected sample com- DMO, area for the deep masseter ori- prehends 11 genera and is focused mainly on gin; SMO, area for the superficial masseter representatives of Typotheria. Additionally, origin; ZMO, area for the zygomaticoman- two taxa of early diverging families of No- dibularis origin. SOSA AND GARCÍA LÓPEZ 56

Results dition are characterized by the presence of a vestigial tubercle associated to the The three morphological conditions origin of the superficial masseter (SMO) (Condition 0, Condition 1, and Condition 2) and a relatively small, narrow, and simple determined in this study are differentiated by area for the origin of the deep masseter the arrangement and relative development (DMO), without divisions for this part of of the different parts of the masseter mus- the muscle. The zygomatic arch is usually cle. Likewise, there are variations regarding gracile, with the anterior root small and the relative position of some structures, par- laterally compressed. Four genera were ticularly those associated to the origin of the included here: Simpsonotus (Henricosbor- superficial masseter and its position - regar niidae), Colbertia (“Oldfieldthomasiidae”), ding the anterior root of the zygoma. Boreastylops, and (Notostylopi- dae). Condition 0 Simpsonotus (figure 1A), recorded Genera included within this con- in the Mealla Formation (late Paleoce-

Figure 1. Taxa included in the Condition 0 and schematic interpretation of the area for the origin of the different parts of the masseter muscle. A- Simpsonotus praecursor, holotype (MLP 73-VII-3-11); B- Notostylops murinus (MACN-A 10494); C- Boreastylops lumbrerensis, holotype (MLP 78-V-6-5); D- Colbertia lumbrerense (PVL 4294). Not in scale. / Figura 1. Taxones incluidos en la Condición 0 e interpretación esque- mática del área de origen de las diferentes partes del músculo masetero. A- Simpsonotus praecursor, holotipo (MLP 73-VII-3-11); B- Notostylops murinus (MACN-A 10494); C- Boreastylops lumbrerensis, holotipo (MLP 78-V-6-5); D- Colbertia lumbrerense (PVL 4294). Las imágenes no están en escala. 57 THE MASSETER MUSCLE IN TYPOTHERIA ne of Jujuy Province, Argentina), pre- Condition 1 sents a small SMO, associated to a very The two genera included within this small tubercle located ventrally and la- group belong to the family Interatheriidae, terally on the anterior part of the zy- and represent a pre- form (Noto- goma. This area is located at the level pithecus; middle-late Eocene) and one Inte- of the M2 in ventral view. On turn, the ratheriinae, a more specialized subfamily pars profunda of the masseter muscle is with euhypsodont members (Protypotherium; originated more ventrally on the zy- middle ). The Interatheriidae were gomatic arch, in an anteroposteriorly small to medium sized typotheres which elongated zone, located immediately show morphological convergences with posterior to the SMO. woodchucks and . Additionally, one Boreastylops and Notostylops (fi- of the most remarkable synapomorphies gure 1B, C) are forms with a consi- of the family is the condition of the jugal derably greater body size regarding bone on the zygomatic arch, which is ex- other representatives of the Condition cluded from the ventral and anterior orbital 0; nevertheless, they follow the same margin by a large zygomatic process of the basic pattern. The pars superficialis of maxilla (Cifelli, 1993; Hitz et al., 2006). The the masseter muscle rises from a sma- maxilla also exhibits a conspicuous descen- ll tubercle on the anterolateral surface dant process developed on the ventral part of the anterior root of the zygomatic of the anterior root of the zygoma, which arch, at the level of the M2. The deep marks the SMO (although not included in masseter rises posteriorly, in a narrow this study, some interatheriids show a very area that ends posteriorly on the ante- developed process, such as ). In rior edge of the zone for the origin of turn, the DMO is not particularly large (al- the zygomaticomandibularis (ZMO), though in some cases it can be moderately which is developed ventrally and inter- large), differing from the Condition 2 des- nally on the body of the arch. cribed below. In Colbertia (figure 1D) the super- In Notopithecus (figure 2A) the anterior ficial maseter has it origin in a small and root of the zygoma is moderately enlarged. narrow elongated area located between The tubercle or descendant process is small the level of the M2 and the medial part and located at the level of the mesial edge of the M3 and bounded anteriorly by a of the M1 in ventral view. The superficial small tubercle on the zygomatic root. masseter rises in the descendant process This condition is very similar to that and runs on the external edge of the zy- of Simpsonotus; nevertheless, in Colbertia goma, over a thin crest. Medially and pos- the tubercle is slightly larger. This area teriorly, the DMO, which is flat and shows is immediately adjacent to the mediola- well-marked edges. teral edge of the DMO, which is a na- Protypotherium (figure 2B) presents a rrow area that occupies two thirds of more specialized arrangement than Notopi- the zygomatic process of the maxilla. thecus. The descendant process is larger in The ZMO is visible from the level of this case. The superficial masseter rises from the middle length of the DMO to the this process, which bears a short posterior zone adjacent to the mandibular joint. crest. The zygomatic process of the maxilla The ZMO is developed on the internal is larger and longer than in Notopithecus, rele- aspect of the zygomatic arch. gating the jugal to the middle-posterior part SOSA AND GARCÍA LÓPEZ 58

Figure 2. Taxa included in the Condition 1 and schematic interpretation of the area for the origin of the di- fferent parts of the masseter muscle. A- Notopithecus adapinus (MACN-A 10790); B- Protypotherium australe (PVL 2389). Not in scale. / Figura 2. Taxones incluidos en la Condición 1 e interpretación esquemática del área de origen de las diferentes partes del músculo masetero. A- Notopithecus adapinus (MACN-A 10790); B- Protypotherium australe (PVL 2389). Las imágenes no están en escala. of the arch. Two well-defined fossae (ante- masseter (that will be described below). rior and posterior) are visible on the ventral Griphotherion (figure 3A) shows the part of the zygomatic process. The anterior most basic arrangement within this con- fossa is the DMO and is tear-shaped, with dition. The zygomatic arch is robust and a sharp posterior edge. The posterior fossa roughly parallel to the anteroposterior axis is the ZMO. This is adjacent medially to the of the skull. The posterior root is narrow, DMO and is located on the medial aspect with a smooth medial edge and a well - de- of the arch reaching the area adjacent to the veloped postzygomatic dorsal crest. The glenoid fossa. anterior root is expanded forming an inci- pient plate, mainly evidenced by a flattened Condition 2 ventral surface on the zygomatic process In this condition the anterior root of of the maxilla. This root is also expanded the zygomatic arch is highly developed or anteroposterioly, extending from the mesial hypertrophied and laterally expanded. Mo- edge of the M1 to the distal edge of the reover, this condition is characterized by the M3. The SMO is small and is located on the absence of a tubercle or descendant process anteroexternal aspect of the zygomatic pro- associated to the SMO. Five genera were stu- cess, at the level of the M2. Although the died within this condition: an Eocene form position of the SMO is similar to that of the of indeterminate family, although possibly Condition 0, the main difference lies in the linked to the ancestral branch leading to He- absence of an associated tubercle. Then, the getotheria (Griphotherion; see García-López SMO is only manifested as a narrow, elon- and Powell, 2011), one “Archaeohyracidae” gated, and rugous surface. The DMO is a (), one Mesotheriidae (Typothe- continuous surface, without the divisions riodon), and two Hegetotheriidae (Hegeto- observed in other genera within this condi- therium and Paedotherium). In turn, there are tion (see below). differences among these genera regarding In Archaeohyrax (figure 3B), the ba- the position, origin, and subdivisions of the sic pattern is similar to that in Griphotherion; 59 THE MASSETER MUSCLE IN TYPOTHERIA

Figure 3. Taxa included in the Condition 2 and schematic interpretation of the area for the origin of the different parts of the masseter muscle. A- Griphotherion peiranoi, holotype (PVL 5903); B- Archaeohyrax patagonicus, holotype (MACN-A 52-617); C- Typotheriodon extensum (MACN-PV 2925); D- Hegetotherium mirabile (PVL 91); E- Paedotherium typicum (PVL 3386). / Figura 3. Taxones incluidos en la Condición 2 e interpretación esquemática del área de origen de las diferen- tes partes del músculo masetero. A- Griphotherion peiranoi, holotipo (PVL 5903); B- Archaeohyrax patagonicus, holotipo (MACN-A 52-617); C- Typotheriodon extensum (MACN-PV 2925); D- Hegetotherium mirabile (PVL 91); E- Pae- dotherium typicum (PVL 3386). Las imágenes no están en escala. nevertheless, the zygomatic plate is more and its posterior edge is marked by the con- laterally expanded, being similar to the de- tact between the jugal and the maxilla. The velopment observed in Mesotheriidae and fossa for the DMO is located medially and Hegetotheriidae. The SMO is located on the posteriorly to the SMO. As for Griphotherion, anterolateral aspect of the zygomatic arch there are no divisions in the DMO. SOSA AND GARCÍA LÓPEZ 60

Condition 2 shows higher singu- des -size forms from the Miocene and larities in representatives of the Neogene, , the condition shows the higher such as Typotheriodon (figure 3C), in which degree of complexity, whit subdivisions the zygomatic plate is much wider (laterally in the different parts of the masseter and expanded) and its surface reaches the distal particularly in the pars profunda. The SMO part of the M3. In these forms the SMO is is a small irregular area located on a facial located in a small surface below the lacrimal process of the maxilla (or horizontal spine; bone and laterally on the most basal part of Billet, 2011) located adjacent to the anterior the root of the zygoma (in ventral view). and dorsal end of the anterior zygomatic Then, the position of the SMO is more root and lateral to the infraorbital foramen. dorsal to that observed in Griphotherion and There is another wrinkled small area, loca- Archaeohyrax. The DMO is very developed, ted on the anterolateral surface of the arch. occupying most of the hypertrophied zygo- This secondary attachment surface (SAS) matic plate. This part apparently shows two could be interpreted as an origin for an ac- subareas, an internal or medial one, which is cessory branch of the superficial masseter; larger, and an external or lateral, being sma- nevertheless, such trait must be considered ller and parallel to the latter. Posteriorly, the with caution (see Discusion). ZMO is narrow and reaches the jugal-squa- The DMO shows a remarkable mosal contact. The described morphology development, indicating a high hypertro- is strongly homogeneous among the repre- phy for this muscle. Moreover, the DMO sentatives of Mesotheriidae, and the same is divided in three subareas distributed on arrangement can be found in a large zygomatic plate. The first subarea forms such as . is the larger subdivision and is located on Within Hegetotheriidae (figure 3D, the anterior surface of the zygomatic pla- E), the Condition 2 shows an even higher te, reaching dorsally the SMO. The second development of the zygomatic plate, and subarea is immediately posterior to the for- hence of the masseter muscle. The SMO is mer and faces ventrally. It is smaller than the dorsal in the zygoma and is associated to a first subarea and both are separated by an protuberance of the maxilla (very develo- oblique crest which anterolateral end rea- ped in some cases). ches the SAS mentioned above. Finally, the Hegetotherium, from the middle third subarea is located adjacent to the pos- Miocene, shows the mentioned arrange- terolateral end of the zygomatic plate, being ment. The SMO is relatively small and is developed partially on the jugal bone. It is located on the posterodorsal area of the narrow and anteroposteriorly elongated, be- maxilla, adjacent to the nasal and anterior ing the smallest of the three subareas. to the lacrimal bone. The DMO occu- pies most of the zygomatic plate; this is a continuous area and hence the muscle Discussion was very robust. Posteriorly, the ZMO Turnbull (1970) included the order is narrow and reaches the vicinity of the Notoungulata in the Specialized Group II mandibular joint. Considering this arran- of his classification (“-grinding” gement, the pattern in Hegetotherium can or “mill” type), which was integrated main- be regarded as an “advanced variant” of ly by extant ungulates (Perissodactyla). that observed in Archaeohyrax. However, this author stated on his contri- In Paedotherium, a genus that inclu- bution that the knowledge on the muscular 61 THE MASSETER MUSCLE IN TYPOTHERIA structure and functional characterization the Condition 0, matches with one of the of Notoungulata was inadequate and that groups proposed by that author (precisely, extensive surveys were not being develo- the Generalized Group). ped at the time. At this point, and in ac- cordance with the doubts expressed by Comparisons and comments Turnbull, the observations performed on As mentioned above, Condition 0 is the present contributions indicates that the restricted to Paleogene forms in which the inclusion of the entire order Notoungula- masseter muscle shows a simpler arrange- ta within the specialized group of modern ment (regarding other conditions here de- ungulates implies inconsistencies related to fined) and is not particularly hypertrophied. different arrangements in the masticatory In turn, the temporal muscle is proportiona- muscles and functional variations. Turn- lly large in these forms. bull characterized his Specialized Group Following the pattern and arrange- II as that in which “mill action, or grin- ment of the different parts of the masseter, ding motion, predominates” (Turnbull, Condition 0 can be considered as part of 1970: 160), and mentioned three mastica- Turnbull’s Generalized Group. This author tory movements associated: “fore-and-aft included here extant representatives of Me- direction”, “medial and lateral direction”, tatheria and Eutheria such as Didelphis (large and “vertical (orthal) direction”. Morpho- American opossum) and Echinosorex (moon- logical evidence (related to occlusal relief rat). In these animals the SMO is located on and arrangement of the temporomandi- a usually small osseous protuberance deve- bular joint) indicates that this combination loped ventrally on the anterior zygomatic of movements can be present in some no- root, just below the orbit. This protuberan- toungulates; however, other representati- ce can be part of both the maxilla and the ves of the order show marked singularities. jugal (as in Didelphis) or just the maxilla, and Moreover, Turnbull’s Specialized Group it is homologous to the vestigial descendant II presents a characteristic reduction of process or tubercle that represents the SMO the temporal muscle combined to a great in the representatives of the Condition 0. development of the masseter. This could As for the deep masseter, its origin is be compared to the condition present in located on the ventral edge of the zygoma- representatives of Pachyrukhinae or Meso- tic arch (occupying a large portion of the theriidae (here included on the Condition 2 body) in the representatives of the Genera- and traditionally considered as “rodent-li- lized Group. Posteriorly the DMO reaches ke” forms) but hardly to Paleogene forms the ZMO. This arrangement is similar in the (such as Griphotherion or Colbertia) or even genera that were included in the Condition certain Neogene genera (e.g. Protypotherium) 0, and both the DMO and the ZMO are an- which show a great development of the teroposteriorly elongated. temporal muscle, associated to strong sa- Within the representatives of the gittal crests and, occasionally, moderate to Condition 0, the arrangement of the mas- low development of the masseter. In this ticatory apparatus is particularly interesting sense, we consider that the order Notoun- for the family Notostylopidae. This clade gulata cannot be satisfactory included as (here represented by Notostylops and Boreas- a whole in just one of the morphologic tylops) includes forms that show a diastema groups proposed by Turnbull. Moreover, (very large in some genera) separating the just one on the conditions here established, anterior from premolars and molars SOSA AND GARCÍA LÓPEZ 62

(or in some cases a strong reduction of the a higher development, given the relative anterior premolars and the canine). This trait surface of the DMO. is commonly found in mammals with some The basic pattern characteristic of kind of dental specialization (e.g., hypsodon- the Condition 1 can be compared to that ty) and implies a clear division between the present in the order Lagomorpha. Most function of incisors and postcanines during representatives of this group present a mastication (Hildebrand, 1974), as well as a lateral development of the anterior root dominance of the masseter and pterygoid of the zygoma and a conspicuous descen- muscles (Greaves, 2008). The case of Notos- dant process, referred as masseteric spine tylopidae is notable, since the of this by Wible (2007). This structure, located family retain brachyodont postcanine teeth, on the ventral side of the zygomatic pro- enlarged (although not hypsodont) incisors, cess of the maxilla, is homologous to the and well - developed (probably dominant) descendant process of the Interatheriidae temporal muscles. In this sense, notostylo- (figure 4). However, a clear difference be- pids show a singular combination of brach- tween these two clades is that the zygo- yodont teeth, a generalized arrangement of matic process of the maxilla is shorter the masticatory apparatus, development of in Lagomorpha, being well developed in diastemata, and reduction of dental pieces. Typotheria and particularly in Interathe- Considering the basal position of this family riidae. Moreover, one of the characters in the phylogenetic context of the Notoun- that define the family Interatheriidae is gulata, evidenced in different contributions the isolation of the jugal from the ventral (e.g., Cifelli, 1993; Billet, 2010, 2011), these edge of the orbit, which is formed enti- traits constitute remarkable characters. Addi- rely by the maxilla (Cifelli, 1993; Hitz et tionally, dental and other cranial features of the Notostylopidae had lead to the proposal of some interesting phylogenetic affinities, not previously considered (e.g., close rela- tionship with the enigmatic clade ; see Billet, 2010, 2011). Regarding Condition 1, this is ob- served in representatives of Interatherii- dae, a family of Typotheria. These forms present a conspicuous descendant pro- cess developed on the anterior root of the zygomatic arch which can be particularly large in some cases, as in the genus Intera- therium. In the analyzed taxa, the disposi- tion of the masseter muscle represents a pattern similar to that of the Condition 0; Figure 4. Lateral view of the skull in Interatheriidae nevertheless, in this case the descendant and Lagomorpha. The horizontal arrow points to the process were the SMO is present (hen- masseteric spine. A- Protypotherium australe; B- Romero- ce homologous to the small tubercle or lagus diazi (modified from Wible, 2007). Not in scale. / Figura 4. Vista lateral del cráneo en Interatheriidae y La- wrinkled area of the Condition 0) is cons- gomorpha. La flecha horizontal señala la espina masetérica. A- picuous. Moreover, although not hyper- Protypotherium australe; B- Romerolagus diazi (modifi- trophied, the deep masseter shows also cado de Wible, 2007). Las imágenes no están en escala. 63 THE MASSETER MUSCLE IN TYPOTHERIA al., 2006). In turn, lagomorphs generally riorly to the Eocene/ boundary. had a well-developed jugal bone and the In Griphotherion, an Eocene typothere, the maxilla is restricted to the anterior part SMO is restricted to a small ventral area of the zygoma. The DMO is also simi- while in mesotheriids and non-pachyrukhi- lar in both groups; in Lagomorpha this ne hegetotheriids it is located on the anterior area of origin runs posteriorly from the and dorsal zone of the zygoma. Moreover, ventrolateral aspect of the jugal, while in in Pachyrukhinae, the SMO is associated to Interatheriidae the DMO shows a similar a sharp process lateral to the infraorbital fo- arrangement but is located on the ventro- ramen. This process was referred as hori- lateral aspect of the maxilla. zontal spine by Billet (2011), who confirms Teeth functional adaptations also its homology regarding the descendant pro- differ between Lagomorpha and Intera- cess of other notoungulates, based on the theriidae. The presence of diastema and observation of basal pachyrukhines. a particular arrangement of the glenoid Notwithstanding the morphological fossa (oblique and not strongly differen- variations expressed above, the most com- tiated from the surrounding structures) plex trait in Condition 2 is related to the are found in Lagomorpha (Wible, 2007). great development of the DMO, which is In turn, interatheriids usually do not pre- laterally and dorsally expanded forming a sent well-developed diastema (except for structure that can be regarded as a zygo- Interatherium), dentition turns from bra- matic plate. In Paleogene taxa, such as Gri- chyodont in basal forms to hypsodont in photherion, the DMO is relatively simple but post-Eocene genera, and the axis of the in Neogene representatives of Mesotheri- temporomandibular joint is clearly trans- idae and Hegetotheriidae (and particularly verse. Thus, in this case is not possible to in Pachyrukhinae) the DMO shows divi- establish a clear correlation between the sions that imply a great functional comple- development of diastemata, the structural xity for the deep masseter. modifications of the masticatory appara- Development of zygomatic plates tus, and the masticatory movements per- can be observed in other mammals, but formed by these two groups. Condition 2 can be particularly compared Finally, some characters of the Condi- with vombatid marsupials and within Eu- tion 2 can be compared to protrogomorph theria, certain rodent groups. Vombatidae and sciuromorph rodents, a fact consistent () are herbivorous marsupials of with the image of “rodent-like forms” tradi- the semiarid savannahs of Australia (Wool- tionally applied to many of the taxa that we nough, 1998). Their diet includes mainly include here (particularly within Mesothe- grasses (e.g., Poa; Evans et al., 2006). The riidae and Hegetotheriidae). Nevertheless, presence of zygomatic plate, the develop- the Condition 2 is more heterogeneous and ment of diastemata, and euhypsodont teeth complex than the previous conditions, co- represent common traits between vomba- vering a broader spectrum of comparisons. tids and pachyrukhines, the latter being in- In a general sense, it includes representati- cluded among the most specialized forms ves that show a great development of the of the Condition 2. Crompton et al. (2008) anterior zygomatic root, exhibiting a wide described the control of mandibular mo- surface for the insertion of the deep part vements in Lasiorhinus latifrons (a species of the masseter. In turn, the SMO differs of ) and characterized its mastica- between taxa recorded previously or poste- tory apparatus. In their classification, the- SOSA AND GARCÍA LÓPEZ 64 se authors named the superficial masseter al., 2007; Cassini et al., 2010). The masti- as external superficial masseter, the deep catory apparatus in rodents is highly spe- masseter as internal superficial masseter, cialized, showing strong modifications of and the zygomaticomandibularis muscle the masseter muscle (and a wide variety of as deep masseter. Following those authors morphologies within the order) affecting the SMO of Vombatidae is located dorsa- the morphology of the zygomatic arch and lly on the anterior root of the zygomatic infraorbital foramen. Given the different arch. The DMO is wide and run posteriorly arrangements derived from these complex on the ventral edge of the zygoma. Finally, adaptations, the masticatory apparatus in the ZMO is located posteriorly on the me- rodents vary from generalized forms that dial aspect of the arch. This arrangement is does not show strong modifications of the very similar to that present in Mesotheriidae anterior zygoma and the infraorbital fo- and non - pachyrukhine hegetotheriids, and ramen (e.g., Protrogomorpha, Sciuromor- results in a comparable general morpholo- pha), to specialized conditions in which the gy that can be appreciated when skulls are infraorbital foramen is highly modified for compared in ventral view (figure 5). the passage of rami of the masseter (e.g., Regarding the comparison with ro- , Myomorpha). In this dents, several comments can be made sense, similarities between the morphology when Condition 2 is considered. As we in representatives of the Condition 2 and mentioned above, previous authors have rodents are somewhat superficial, being li- usually noted the similarities between mited to the development of a zygomatic Neogene typotheres and rodents or even plate (with the concomitant development regarded these extinct taxa as “rodent-li- of the corresponding part of the masse- ke” notoungulates (e.g., Elissamburu, ter), the presence of diastema, enlarged in- 2004; Croft and Anaya, 2006; Reguero et cisors, and hypsodonty. A deeper analysis

Figure 5. Ventral view of the skull in Mesotheriidae, Hegetotheriidae, and Vombatidae, showing the general mor- phology and the development of zygomatic plates. A- Typotheriodon extensum (MACN-PV 2925); B- Hegetotherium mirabile (modified from Scott, 1912); C- Lasiorhinus latifrons (modified from Crompton et al., 2008). Not in scale. / Figura 5. Vista ventral del cráneo en Mesotheriidae, Hegetotheriidae y Vombatidae mostrando su morfología general y el desarrollo de placas cigomáticas. A- Typotheriodon extensum (MACN-PV 2925); B- Hegetotherium mirabile (modificado de Scott, 1912); C- Lasiorhinus latifrons (modificado de Crompton et al., 2008). Las imágenes no están en escala. 65 THE MASSETER MUSCLE IN TYPOTHERIA reveals a different disposition of the mus- cles, particularly regarding the superficial masseter. The SMO is always more dorsal in the rostrum of typotheres developing a zygomatic plate, being different from the morphology of most rodents, where the SMO is low and more anterior on the fa- cial aspect of the maxilla. Additionally, the passage of fibers of the masseter through the infraorbital foramen is not present in representatives of the Condition 2 (or any other notoungulate). Some superficial convergence can be also traced between the heterogeneous na- ture and increasing complexity of the mus- cular packages in Condition 2 and the va- riable arrangements exhibited by different rodent groups. For example, in beavers (Aplodontia rufa, Protrogomorpha) the mus- cular arrangement is far simpler than that Figure 6. Comparisson of skull shape and general mas- present in animals like woodchucks (Mar- ticatory apparatus arrangement in lateral view between mota monax, Sciuromorpha) (Druzinsky, a basal notoungulate and a rodent. A- Griphotherion peira- 2010) and squirrels (Turnbull, 1970). This noi; B- Aplodontia rufa (modified from Druzinsky, 2009). Not in scale. / Figura 6. Comparasión entre la forma del increasing complexity is comparable to that cráneo y la disposición general del aparato masticatorio entre un observed from the Paleogene representa- notoungulado basal y un roedor; A- Griphotherion peiranoi; tives of Condition 2 to the more specia- B- Aplodontia rufa (modificado de Druzinsky, 2009). Las lized and post- forms of the imágenes no están en escala. group. Nevertheless, evolutionary trends seem to be different between rodents and cene, dental and muscular traits are similar taxa included in Condition 2. In basal for- to the extant Aplodontia and hence different ms, such as Griphotherion, the development from the basal forms of the Condition 2. (although incipient) of a zygomatic plate This is also observed in Rhombomylus, an was not parallel to the acquisition of den- Eocene genus proposed as sister group of tal specializations (e.g., hypsodonty, diaste- Rodentia (Meng et al., 2003). Both Ischyro- mata, etc.). In that case, development of mis and Rhombomylus present a primitive more complex muscles precedes morpho- pattern for the masseter muscle and an in- logical and functional modifications of the cipient zygomatic plate but develop dias- basic dental pattern of the order. In turn, temata and fully hypsodont teeth. Then, in the generalized condition in rodents, as in this case dental specialization precedes in Aplodontia rufa, the incipient zygomatic hypertrophy and division of the masseter plate (with a muscular arrangement similar muscle, a trend that becomes evident in to that of Griphotherion) in combined with other groups (e.g., Sciuromorpha). hypsodont teeth and development of dias- Sciuromorph rodents, like Marmota temata (figure 6). In fossil protrogomorph monax, present a well-developed zygomatic genera, such as Ischyromis from the Oligo- plate but do not exhibit particular conver- SOSA AND GARCÍA LÓPEZ 66 gences with taxa of the Condition 2, such Conclusions as mesotheriids and pachyrukhines. Some traits can be identified as common to both It is evident that the masticatory appa- morphologies, including hypsodont den- ratus underwent a wide range of changes tition, enlarged incisors, and presence of and specializations among notoungulates. diastemata but, although there is hypertro- This structural evolution shows some de- phy of the deep masseter in both cases, the gree of independence from the acquisition areas of origin are considerably different. of derived dental characters, although in In summary, the pattern of attach- some cases there is a trend toward an increa- ment of the masseter muscle and different sing complexity of the masticatory muscles degree of dental specialization observed in associated to hypsodonty and development Condition 2 is singular for these notoun- of other traits like diastemata. gulates and morphologic convergences Some of the arrangements exhibi- with extant lineages are only superficially ted by the analyzed taxa do not show evi- expressed. Therefore, the term “rodent-li- dent morphological convergence regarding ke” used for most typotheres indicating extant forms. In turn, certain Neogene ge- convergence with Rodentia cannot be con- nera show a clear convergence related to sidered extensively accurate. In this sense, complex structures. both Condition 0 and Condition 2 show Certain morphological patterns characters comparable to a relatively broad were circumscribed to determinate families spectrum of groups, including clades out- (e.g., Interatheriidae, Notostylopidae) whe- side Eutheria, such as Didelphidae and reas others are phylogenetically widespread. Vombatidae. The case of Condition 2 is notable, achie- Finally, it is also worth noting that ving a high degree of complexity and com- Paedotherium shows, besides the anterodorsal prising a group of lineages whose radiation SMO, another attachment area located on the spans from the middle Paleogene to the end lateral and external surface of the arch, be- of the Neogene. low and anteriorly to the orbit (see figure 3E). This presumably represents an additional Acknowledgements punctual area for the attachment of a mus- cular bundle, indicating a subdivision of the We thank the editor and two anony- superficial masseter (that would be divided in mous reviewers whose valuable comments superior and inferior parts). Gambaryan and greatly improved the manuscript. For access Kielan-Jaworowska (1995) described a simi- to specimens we thank Alejandro Kramarz lar division in the mesozoic multituberculates (Museo Argentino de Ciencias Naturales Nemegtbaatar gobiensis and Catopsbaatar catops- Bernardino Rivadavia, Buenos Aires), Mar- doides; in these taxa the superficial masseter is celo Reguero (Colección Paleontología de divided in two rami (anterior and posterior) Vertebrados, Museo de La Plata), and Pablo along the zygomatic arch. In turn, this kind Ortiz (Colección Paleontología de Verte- of subdivision in the superficial masseter is brados Lillo, Universidad Nacional de Tu- not usual in extant mammals. Although more cumán). This work was funded by the Fun- studies are needed in order to assess this pos- dación Miguel Lillo; Agencia de Promoción sible trait in pachyrukhines, it certainly illus- Científica y Tecnológica (FONCyT) [PICT trates the high complexity achieved by this 407; PICT 2016-3682]; Consejo Nacional particular group of eutherians. de Investigaciones Científicas y Técnicas 67 THE MASSETER MUSCLE IN TYPOTHERIA

(CONICET) [PIP 489]; Secretaría de Cien- Cassini, G.H., Flores, D.A. and Vizcaino, S.F. 2011. cia, Arte e Innovación Tecnológica (Univer- Postnatal ontogenetic scaling of - sidad Nacional de Tucumán) [26/G522]. tine (Notoungulata, ) cranial morphology. Acta Zoologica. 93: 249-259. Cassini, G.H., Mendoza, M., Vizcaino, S.F. and Bar- go, M.S. 2010. Inferring habitat and feeding References behavior of early Miocene notoungulates from . Lethaia. 44: 153-165. Armella, M.A., García-López, D.A., Lorente, M., and Cerdeño, A. and Bond, M. 1998. Taxonomic revision Babot, M.J. 2016. Anatomical and systematic and phylogeny of Paedotherium and Tre- study of proximal tarsals of ungulates from macyllus (Pachyrhukynae, Hegetotheriidae, the (Northwestern Argen- Notoungulata) from the late Miocene to the tina). Ameghiniana, 53: 142-159. of Argentina. Journal of Vertebrate Billet, G. 2010. New observations on the skull of Palaeontology, 18: 799-811. (Pyrotheria, Mammalia) and Cifelli, R.L. 1993. The phylogeny of the native South Ame- new phylogenetic hypotheses on South Ame- rican ungulates. In: F.S. Szalay, M.J. Novacek rican ungulates. Journal of Mammalian Evolu- and M.C. McKenna (eds.), Phylo- tion, 17: 21-59. geny, Volume 2: Placentals, Springer Verlag Billet, G. 2011. Phylogeny of the Notoungulata (Mam- Editors, New York, pp. 195-216. malia) based on cranial and dental characters. Cox, P.G. and Jeffery, N. 2011. Reviewing the Mor- Journal of Systematic Palaeontology, 9: 481-497. phology of the Jaw-Closing musculature in Billet, G. and de Muizon, C. 2013. External and in- Squirrels, Rats, and Guinea Pigs with Con- ternal anatomy of a petrosal from the late trast-Enhanced MicroCT. The Anatomical Re- of Itaboraí, Brazil, referred to No- cord, 294: 915-928. toungulata (Placentalia). Journal of Vertebrate Croft, D.A. and Anaya, F. 2006. A new middle Mioce- , 33: 455-469. ne Hegetotheriidae (Notoungulta: Typothe- Billet, G., de Muizon, C. and Mamani Quispe, B. ria) and a phylogeny of the Hegetotheriidae. 2008. Late Oligocene mesotheriids (Mam- Journal of Vertebrate Palaeontology, 26: 387-399. malia, Notoungulata) from Salla and Lacayani Croft, D.A. and Weisntein, D. 2008. The first appli- (Bolivia): Implications for basal mesotheriid cation of the mesowear method to endemic pylogeny and distribution. Zoological Journal of South American ungulates (Notoungulata). the Linnean Society, 152: 153-200. Palaeogeography, Palaeoclimatology, Palaeoecology, Billet, G., Blondel, C. and de Muizon, C. 2009. 269: 103-114. Dental microwear analysis of notoungula- Crompton, A.W. and Parker, P. 1978. Evolution of tes (Mammalia) from Salla (Late Oligocene, the Mammalian Masticatory Apparatus. Ame- Bolivia) and discussion on their precocious rican Scientist, 66: 192-201. hypsodonty. Palaeogeography, Palaeoclimatology, Crompton, A.W., Lieberman, D.E., Owerkowicz, T., Palaeoecology, 274: 114-124. Baudinette R.V. and Skinner, J. 2008. Motor Bond, M. 1981. Un nuevo Oldfieldthomasiidae control of masticatory movements in the Southern (Mammalia, Notoungulata) del Eoceno Infe- hairy - nosed wombat (Lasiorihinus latifrons). rior (Fm. Lumbrera, Grupo Salta) del NW ar- In: C.J Vinyard, M.J Ravosa and C.E Wall, gentino. 2º Congreso Latinoamericano de Paleonto- Primate Craniofacial Function and Biology, logía (Porto Alegre, Brazil), Anais, 2: 521-536. Springer Editors, New York, pp. 83-111. Buckley, M. 2015. Ancient reveals evolutio- Druzinski, R.E. 2010. Functional anatomy of incisal nary history of the endemic South American biting in Aplodontia rufa and sciuromorph ‘ungulates.’ Proceedings of the Royal Society B, rodents - Part 1. Masticatory muscles, skull 282: 20142671. shape and digging. Cell, Tissues, Organs, 192: Cassini, G.H. and Vizcaíno, S.F. 2012. An approach 50-63. to the biomechanics of the masticatory appa- Druzinsky, R.E., Doherthy, A.H. and De Vree, F.L. ratus of early Miocene ( Age) 2011. Mammalian Masticatory Muscles: Ho- South American ungulates (, mology, Nomenclature and Diversification. Litopterna, and Notoungulata): moment arm Integrative and Comparative Biology, 51: 224-234. estimation based on 3D landmarks. Journal of Elissamburu, A. 2004. Análisis morfométrico y mor- Mammalian Evolution, 19: 9-25. fofuncional del esqueleto apendicular de Pae- SOSA AND GARCÍA LÓPEZ 68

dotherium (Mammalia, Notoungulata). Ameghi- Annals of Carnegie Museum, 84: 95-164. niana, 41: 363-380. McCoy D.E. and Norris C.A. 2012. The cranial ana- Elissamburu, A. 2012. Estimaciones de la masa cor- tomy of the Miocene notoungulate Hegeto- poral en géneros del Orden Notoungulata. therium mirabile (Notoungulata, Hegetothe- Estudios Geologicos, 68: 91-111. riidae) with preliminary observations on diet Evans, H.E. and de Lahunta, A. 2013. Miller’s Ana- and method of feeding. Bulletin of the Peabody tomy of the dog. Fourth Edition. Elsevier Editors, Museum of Natural History, 53: 355-374. Missouri, 850 pp. Meng, J., Hu, Y. and Li, C. 2003. The Osteology of Evans, M.C., Macgregor, C. and Jarman, P.J. 2006. Rhombomylus (Mammalia, Griles): Implica- Diet and feeding selectivity of common tions for phylogeny and evolution of . wombats. Wildlife Research, 33: 321-330. Bulletin of the American Museum of Natural His- Gambaryan, P.P. and Kielan-Jaworowska, Z. 1995. tory, 275: 1-247. Masticatory musculature of Asian taeniolabi- Pascual, R., Vucetich, M.R., and Fernández, J. 1978. doid multituberculates mammals. Acta Paleon- Los primeros mamíferos (Notoungulata, tologia Polonica, 40: 45-108. Henricosborniidae) de la Formación Mealla García-López, D.A. 2009. [Notoungulados del Pa- (Grupo Salta, Subgrupo Santa Bárbara). Sus leógeno del noroeste argentino: morfología y implicancias filogenéticos, taxonómicas y evolución. PhD thesis, Facultad de Ciencias cronológicas. Ameghiniana, 15: 367-390. Naturales e Instituto Miguel Lillo, Universi- Reguero, M.A., Candela, A.M. and Alonso, R.N. dad Nacional de Tucumán, 338 pp. Unpubli- 2007. Biochronology and Biostratigraphy of shed]. the Uquia Formation (Pliocene- Early Pleis- García-López, D.A. 2011. Basicranial osteology of tocene, NW Argentina) and its significance in Colbertia lumbrerense Bond, 1981 (Mammalia: the Great American Biotic Interchange. Jour- Notoungulata). Ameghiniana, 48: 3-12. nal of South American Earth Sciences, 23: 1-16. García-López, D.A. and Powell, J.E. 2011. Griphothe- Russel, A.P. 1998. The Mammalian Masticatory Appa- rion peiranoi, gen. et sp. nov., a new Eocene ratus: An introductory comparative exercise. In: S.J. Notoungulata (Mammalia, ) Karcher (ed.), Tested studies for laboratory from northwestern Argentina. Journal of Ver- teaching,19th Workshop/Conference of the tebrate Palaeontology, 31: 1117-1130. Association for Biology Laboratory Educa- Getty, R. 2005. Sisson y Grossman. Anatomía de los ani- tion(ABLE) Editors, Alberta, pp. 271-286. males domésticos. 5ta Edición. Masson Editors, Scarano, A., Carlini, A. and Illius, A. 2011. Intera- Barcelona, 2350 pp. theriidae (Typotheria;Notoungulata), body Giannini, N.P. and García-López, D.A. 2014. Eco- size and paleoecology characterization. Mam- morphology of Mammalian Fossil Lineage: malian Biology, 76: 109-114. Identifying Morphotypes in a Case Study of Scott, W.B. 1912. Mammalia of the Santa Cruz Beds. Endemic South American Ungulates. Journal Vol. 6. Part 2-3. Toxodontia. Reports of the of Mammalian Evolution, 21: 195-212. Princeton University Expedition to Patagonia, 6: Greaves, W.S. 2008. Mammals with a long diastema 111-300. typically also have dominant masseter and Shockey, B.J., Croft, D.A. and Anaya, F. 2007. Analy- pterygoid muscles. Zoological Journal of the Lin- sis of function in the absence of extant nean Society, 153: 625-629. functional homologues: a case study using Herring, S.W. 2007. Masticatory Muscles and the mesotheriid notoungulates (Mammalia). Pa- Skull. Archives of Oral Biology, 52: 296-299. laeobiology, 33: 227-247. Hildebrand, M. 1974. Analysis of vertebrate structure. Simpson, G.G. 1948. The beginning of the age of John Wiley & Sons, Inc Editors, 710 pp. Mammals in South America, Part 1, Intro- Hitz, R.B., Flynn, J.J. and Wyss, A.R. 2006. New basal duction, Systematics: Marsupalia, Edentata, Interatheriidae (Typotheria, Notoungulata, Condylarthra, Litopterna and Notioprogo- Mammalia) from the Paleogene of Central nia. Bulletin of the American Museum of Natural Chile. American Museum Novitates, 3528: 1-32. Hystory, 91: 1-232. Kramarz, A.G., Bond, M. and Rougier G.W. 2017. Simpson, G.G. 1967. The beginning of the age of Re - description of the auditory region of the Mammals in South America, Part 2, Syste- putative basal astrapothere (Mammalia) Eoas- matic: Notoungulata, concluded (Typotheria, trapostylops riolorense Soria and Powell, 1981. , , Notoungulata In- Systematic and Phylogenetic considerations. certae Sedis); Astrapotheria, Trygonostylo- 69 THE MASSETER MUSCLE IN TYPOTHERIA

poidea; Pyrotheria; Xenungulata; Mammalia Brace, S., Cappellini, E., Turvey, S.T., Regue- . Bulletin of the American Museum ro, M., Gelfo, J.N., Kramarz, A., Burger, J., of Natural Hystory, 137: 1-352. Thomas-Oates, J., Ashford, D.A., Ashton, Townsend, B. and Croft, D.A. 2010. Middle Miocene P.D., Rowsell, K., Porter, D.M., Kessler, B., Mesotheriine diversity at Cerdas, Bolivia and Fischer, R., Baessmann, C., Kaspar, S., Olsen, reconsideration of Plesiotypotherium minus. Pa- J.V., Kiley, P., Elliott, J.A., Kelstrup, C.D., Mu- leontologia Electronica, 13: 1-36. llin, V., Hofreiter, M., Willerslev, E., Hublin, Turnbull, W.D. 1970. Mammalian Masticatory Appa- J.J., Orlando, L., Barnes, I. and MacPhee, R.D. ratus. Fieldiana: Geology, 18: 153-305. 2015. Ancient proteins resolve the evolutio- Vucetich, M.G. 1980. Un nuevo Notostylopidae nary history of Darwin’s South American un- (Mammalia, Notoungulata) proveniente de la gulates. Nature, 522: 81-84. Formación Lumbrera (Grupo Salta) del no- Wible, J.R. 2007. On the cranial osteology of the roeste argentino. Ameghiniana, 17: 363-372. Lagomorpha. In: K.C. Beard and Z.X. Luo Wall, W.P. and Hayes, S.J. 1995. A biomechanical analy- (eds.), Mammalian Paleontology on a Global sis of mastication in the fossil rodent Ischyromys and Stage: Papers in Honor of Mary R. Dawson, its bearing on the origin of Sciuromorphs. In: V.L. Bulletin of Carnegie Museum of Natural History, Santucci and L. McClelland (eds.), National 39,Penssylvania, pp. 213-234. Park Service Paleontological Research, Uni- Woolnough, A.P. 1998. [The Feeding Ecology of ted States Department of the Interior, Na- the Northern hairy-nosed wombat, Lasior- tional Park Service, Natural Resources Publi- hinus krefftii (Marsupalia: Vombatiidae). PhD cation Office, Denver, Colorado, pp. 17-24. thesis, James Cook University, 333 pp. Unpu- Welker, F., Collins, M.J., Thomas, J.A., Wadsley, M., blished].

Recibido: 26 de Setiembre del 2017 Aceptado: 07 de Noviembre del 2017 SOSA AND GARCÍA LÓPEZ 70

Table 1. List of specimens and bibliographical sources. / Tabla 1. Lista de especímenes y fuentes bibliográficas.