The American Genus Penetrigonias Tanner & Martin, 1976 (Mammalia: Rhinocerotidae) As a Stem Group Elasmothere and Ancestor of Menoceras Troxell, 1921

The American Genus Penetrigonias Tanner & Martin, 1976 (Mammalia: Rhinocerotidae) As a Stem Group Elasmothere and Ancestor of Menoceras Troxell, 1921

Zitteliana A 52 (2012) 79 The American genus Penetrigonias Tanner & Martin, 1976 (Mammalia: Rhinocerotidae) as a stem group elasmothere and ancestor of Menoceras Troxell, 1921 Kurt Heissig Zitteliana A 52, 79 – 95 Bayerische Staatssammlung für Paläontologe und Geologie, Richard Wagner Str. 10, München, 23.05.2012 D-80333 Munich, Germany Manuscript received E-mail: [email protected] 03.02.2012; revision accepted 17.04.2012 ISSN 1612 - 412X Abstract A detailed character study of the early Oligocene skull of Penetrigonias dakotensis (Peterson, 1920) kept in the Bavarian State Collec- tion of Paleontology and Geology at Munich, Germany, and comparisons of this specimen with geologically older species of the genus has resulted in the recognition of features characterizing this lineage of small-sized rhinoceroses of North America. Morphological trends recognizable in the upper cheek teeth of P. dakotensis are present also in the early Elasmotheriini, suggesting that the genus Penetrigonias may represent a stem group representative of this tribe. At the same time, the more advanced characters of the latest early Oligocene species P. dakotensis suggest that the genus Menoceras with its Eurasian and American species may represent the successor of this species. A cladistic analysis of the characters of early Rhinocerotidae from America results in a scheme explaining how and when this family may have split into its different tribes. Key words: Elasmotheriini, North America, Menoceras, Penetrigonias, Paleogene, Phylogeny, Rhinocerotidae Kurzfassung Eine genaue Untersuchung des unteroligozänen Schädels von Penetrigonias dakotensis (Peterson, 1920) in der Bayerischen Staats- sammlung für Paläontologie und Geologie und Vergleiche mit älteren Arten der Gattung erlauben es, den typischen Merkmalsbestand die- ser phyletischen Linie kleinwüchsiger Nashörner Nordamerikas zu erfassen. Spezifische morphologische Trends der oberen Backenzähne stimmen mit denen früher Elasmotheriini überein, so dass die Gattung Penetrigonias als Stammgruppen-Vertreter dieses Tribus gelten kann. Zugleich lässt die jüngste, unteroligozäne Art, P. dakotensis, in ihren fortschrittlichen Merkmalen erkennen, dass die untermiozäne Gattung Menoceras mit ihren amerikanischen und eurasischen Vertretern wohl als Nachkomme dieser Art anzusehen ist. Eine cladistische Analyse der Merkmale früher Rhinocerotidae Amerikas ergibt ein Schema, das zeigt wie und wann sich die Familie in ihre verschiedenen Tribus aufgespalten haben könnte. Schlüsselwörter: Elasmotheriini, Nordamerika, Menoceras, Penetrigonias, Paläogen, Phylogenie, Rhinocerotidae 1. Introduction son, 1920). The specimen today is deposited in the Bavarian State Collection of Paleontology and Geol- In 1998, the Bavarian State Collection in Munich ogy in Munich, Germany, under accession number (BSPG) acquired an unprepared, vertically com- BSPG 1998 I 34. pressed skull of a small rhinoceros from Dakota. The combination Penetrigonias dakotensis (Peter- Unfortunately, the precise age and provenance of son, 1920) was first used by Prothero (2005: 29), who the fossil have not been reported by the merchants. also assigned to this species (as a junior synonym) The only indication provided reads “90 km south of the Eocene type species P. hudsoni Tanner & Mar- Roundup, Dakota”, which, however, does not make tin, 1976. The characters of the Munich specimen, sense. The adhering matrix, a greenish, heavily cal- however, are so peculiar that a more detailed study cified sandstone, apparently is the typical sediment on the phylogenetic significance is mandated. The of the Protoceras channels of the Whitneyan. After fundaments of this analysis are, on the one hand, the preparation, the dentition allowed the identification survey of the known specimens of the genus by Pro- of the specimen as Penetrigonias dakotensis (Peter- thero (2005: 29–35) and, on the other hand, the casts Zitteliana A 52 (2012) 80 of several specimens obtained from the South Da- ence, Denver, CO. kota School of Mines in Rapid City. For comparison • FMNH: Field Museum of Natural History, with Old World Elasmotheriini the specimens housed Chicago, IL. in the Bavarian State Collection from Anatolia and • IVAU: Instituut voor Aardwetenschappen, Pakistan, as well as a sample of casts from the same Rijks Universiteit Utrecht, collection were used, completed by published illus- • LACM: Natural History Museum of Los Angeles trations of material from the works of various authors. County, CA. • LSUMG: Louisiana State University Museum of Geology, Baton Rouge, LA. 2. Material and Methods • MHNT: Muséum d’Histoire Naturelle, Toulouse • PIUU : Paleontological Institute , University of 2.1 Specimens included in this study Uppsala • ROM: Royal Ontario Museum, Toronto • BSPG : Bavarian State Collection of Paleontol- • SMNH: Saskatchewan Museum of Natural ogy and Geology History, Regina, Sask. • Penetrigonias dakotensis: 1998 I 34 • TMM: Texas Memorial Museum, University of • Menoceras zitteli: AS 340 holotype, casts of Texas, Austin, TX. specimens from Paulhiac (1968 XVI 144), Velleron • UCMP: University of California Museum of (1968 XVI 141) and Flörsheim (Orig. M 6748) Paleontology, Berkeley, CA. • Menoceras arikarense: 1964 X 107, 108 • UNSM: University of Nebraska State Museum, • Caementodon oettingenae: 1956 II 364, 1968 XVI Lincoln, NE. 57 • Bugtirhinus praecursor: casts 1968 XVI 58 • Hispanotherium grimmi: holotype 1968 VI 1, 2 2.2 Methods and others The terminology used for the morphological de- Specimens in other collections used for direct com- scription of the dentition follows Osborn (1898: 107), parison: modified by Heissig (1969: 11–12), and that for the • AMNH: American Museum of Natural History, skeletal elements primarily Heissig (1972: pl. 13). New York The different grades of molarisation of the upper • Penetrigonias dakotensis: 1110 holotype premolars are named according to Heissig (1969: • F:AM: Frick Collection in the AMNH 15–16, fig. 4). Upper teeth are asigned capital let- • Penetrigonias.hudsoni: 105019 ters, while lower teeth are indicated by lower case • MNB: Museum für Naturkunde der Humboldt- letters: I, i = incisors, C, c = canine, P, p = premolars, Universität Berlin M, m = molars, D, d = deciduous molars, combined • Penetrigonias dakotensis: Ma 42545 with numbers. E.g. dc = lower deciduous canine, D4 • MNHN: Muséum National d’Histoire Naturelle, = fourth milk molar. Paris The cladistic analysis was performed without the • Pleuroceros pleuroceros: Type skull use of mathematical processes, but rather by weight- • NHM : The Natural History Museum, London ing the importance of characters. The characters • Bugtirhinus praecursor: M 15361 are sorted into three categories, i.e. (a) “unique”, (b) • NMB: Naturhistorisches Museum Basel “important but homoplastic in different rhino clades” • Menoceras zitteli: Figured specimen of Viret and (c) others of unknown importance. This method (1958: fig. 77) may be somewhat subjective, but selecting and sepa- • SDSM: South Dakota School of Mines and Tech- rating characters is generally subjective, regardless nology, Rapid City of whether they subsequently are treated mathemati- • Penetrigonias hudsoni: 5331 cally or not. Furthermore, no argument can be pro- • Amphicaenopus platycephalus duced to justify the equal weighting of characters. • SMF: Forschungsinstitut und Naturmuseum Sen- Characters occurring in all members of a large clade ckenberg, Frankfurt/M (e.g. a tribe) must be treated as more important • Menoceras zitteli: M 3748, M 6773 than characters that differ in closely related species. • UCBL: Centre de Palóntologie stratigraphique et Both are of different importance as general trends in Paléoécologie, rhino evolution, as, for instance, the molarisation of • Université Claude Bernard Lyon, Villeurbanne premolars or the reduction of incisors, which do not • Menoceras zitteli: cast of the cheek teeth of the allow reversals. Genera or species without known Paulhiac skull 9600, autapomorphies are accepted as valid to keep the taxonomy stable, if they cannot be synonymized. Al- Additional specimens cited from the literature are though this approach does not concur with the cla- housed in the following collections: distic orthodoxy, it is useful especially if only a few • DMNH: Colorado Museum of Nature and Sci- characters are known for the species considered. Zitteliana A 52 (2012) 81 2.3 Special characters fractures on both sides, but there seems to be no contact of the maxilla with the frontal. The frontal re- Many of the taxa included in this study show a gion is very broad above the orbits, rapidly tapering considerable variability in characters that are rather in front and to the rear. The temporal lines occur in stable in most rhinos. In the upper molars the posi- the form of thick and broad ridges, which unite abo- tion of the secondary folds is inconsistent, and thus ve the tips of the zygomatic arches to form a long, the dominating fold sometimes arises from the me- but not elevated sagittal line. A small postorbital pro- taloph as a “crochet”, while in other specimens it is cess and a moderately developed supraorbital swel- borne from the ectoloph as a “crista”. In the upper ling are present. The occiput is not preserved. premolars there occur irregular crests in much higher The robust jugal contacts the lachrymal; the fora- frequency than in other rhino groups. If a crest ari- men lacrimale is not preserved. The posterior end of sing from the lingual side of the mesostyle reaches

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