The Cranial Anatomy of the Miocene Notoungulate Hegetotherium mirabile (Notoungulata, Hegetotheriidae) with Preliminary Observations on Diet and Method of Feeding Dakota E. McCoy1 and Christopher A. Norris2 1 Corresponding author: Yale College, Yale University, P.O. Box 200735, New Haven CT 06520-0735 USA —e-mail: [email protected] 2 Division of Vertebrate Paleontology, Peabody Museum of Natural History, Yale University, P.O. Box 208118, New Haven CT 06520-8118 USA —e-mail: [email protected] ABSTRACT The skull and mandible of the Miocene hegetotheriine notoungulate Hegetotherium mirabile are described. Although conventional models of hegetothere biology propose that these animals filled a lagomorph-like ecological niche, study of the cranial and dental anatomy of Hege- totherium suggests that this extinct South American ungulate may have fed by breaking through wood in order to eat xylophagous grubs and other organisms. This mode of life is seen in two extant mammals, the monotypic Malagasy primate Daubentonia madagascariensis and diprotodont marsupials of the genus Dactylopsila. It has also been proposed as a feeding mecha- nism for two extinct groups of mammals: the “proteutherian” apatemyids from the Paleogene of Europe and North America, and the enigmatic marsupial Yalkaparidon from the early Miocene of Australia. Collectively, these living and extinct taxa have been proposed as mammalian wood- peckers. They are defined by a suite of morphological characteristics, including unusually devel- oped and continually growing incisors, molars adapted to have a constantly sharp edge, exaggerated klinorhynchy, deep zygomatic arches and a shortened snout. Hegetotherium mirabile shares almost all characteristics common to the mammalian woodpeckers, although it lacks sev- eral features that are likely to be adaptations for an arboreal, nocturnal lifestyle. This suggests that although the feeding techniques of this species were similar to those of the mammalian wood- peckers, H. mirabile occupied a diurnal, cursorial niche, feeding on grubs and insects within fallen trees and logs. KEYWORDS Convergence, Hegetotherium, Dactylopsila, Daubentonia, apatemyids, klinorhynchy, mam- malian woodpecker Introduction interatheres. As described by Patterson and Pas- cual (1968), the notoungulates underwent an The Notoungulata, an extinct group of endemic early—Late Paleocene and early Eocene—“adap- South American ungulates, were the most diverse tational dichotomy” between large ungulates and and successful archaic order of mammals, with at the smaller rodent- and lagomorph-like forms. least 13 families and over 140 genera represented Later, post-Oligocene, there were four dominant in the fossil record (Simpson 1967; Patterson and families of notoungulates: toxodontids, intera- Pascual 1968; Cifelli 1993; Croft 1999; Croft and theriids, mesotheriids and hegetotheriids (Croft Anaya 2006). They filled a wide variety of evolu- and Anaya 2006). tionary niches in South America during the early There are two recognized subfamilies of hege- and middle Cenozoic, ranging from the gigantic totheriid: the small Pachyrukhinae and the larger rhinolike toxodonts to the small, rodentlike Hegetotheriinae. Although they share certain Bulletin of the Peabody Museum of Natural History 53(2):355–374, October 2012. © 2012 Peabody Museum of Natural History, Yale University. All rights reserved. • http://peabody.yale.edu Downloaded From: https://bioone.org/journals/Bulletin-of-the-Peabody-Museum-of-Natural-History on 19 Dec 2019 Terms of Use: https://bioone.org/terms-of-use Access provided by Harvard University 356 Bulletin of the Peabody Museum of Natural History 53(2) • October 2012 Figure 1. Lateral view of the skulls of Miocene notoungulates. A, Hegetotherium mirabile. B, Pachyrukhos moy- ani. C, Protypotherium australe. Modified from Sinclair 1909. Scale bar equals 10 mm. similarities with other groups of small to medium- between the tibia and fibula is much less than in sized notoungulates (Figure 1), such as the pachyrukhines and the tibia runs parallel to the mesotheriid typotheres, hegetotheres possess a fibula for much of its length, resulting in a wider suite of distinctive morphological synapomor- distal articular surface (Croft et al. 2004). In addi- phies (Cifelli 1993; Croft 2000; Croft et al. 2004). tion to these synapomorphic characters, hege- These are hypselodont (rodentlike, rootless, and totheres are also characterized, although not ever-growing) first incisors, premolars and molars; distinguished, by a broad and raised zygomatic a flattened lingual edge of the molars (which cre- arch, to which the jugal is entirely restricted; a rel- ates a roughly rectangular occlusal surface); the atively unspecialized auditory region; and antero- presence of cementum on the roots of the teeth posterial elongation of the basicranium (Simpson (also present on the crown of Hegetotherium); the 1967). presence of a deep sulcus on the labial surface of The distinctive nature of Hegetotherium was M1 and M2; and extended fusion of the tibia and highlighted in a recent morphometric analysis of fibula. In hegetotheriines the degree of fusion notoungulates, which revealed that the taxon Downloaded From: https://bioone.org/journals/Bulletin-of-the-Peabody-Museum-of-Natural-History on 19 Dec 2019 Terms of Use: https://bioone.org/terms-of-use Access provided by Harvard University Cranial Anatomy of Hegetotherium mirabile • McCoy and Norris 357 occupied an “exclusive” corner of the morpho- species of avian woodpeckers feed on wood- space separate from its relatives; its feeding meth- boring insects within (e.g., Maser and Trappe ods did not fit well into any classic grazing or 1984; Jackson et al. 2002). The unusual cranial browsing niche (Cassini et al. 2011). Hegetotheres features of Hegetotherium mirabile suggest that it have traditionally been compared to rodents and filled a similar niche, feeding on wood-boring rabbits (e.g., Patterson and Pascual 1968; Simp- insects in fallen trees and logs. son 1980; Reguero et al. 2007; Shockey and Anaya Institutional abbreviations are as follows: 2008). These animals are similar in size, and there AMNH, Paleontology Collection, American are some areas of the postcranium where the con- Museum of Natural History; AMNH M, Mam- vergence in morphology is remarkable; for exam- malogy Collection, American Museum of Natural ple, the bowing of the fibula in pachyrukhines History; YPM MAM, Mammalogy Collection, closely resembles the condition seen in modern- Peabody Museum of Natural History, Yale Uni- day leporoids (Sinclair 1909; Croft et al. 2004). versity; YPM VP, Vertebrate Paleontology Col- The cranial morphology, however, is quite dis- lection, Yale Peabody Museum; YPM VPPU, similar in hegetotheres, rodents and rabbits, and Vertebrate Paleontology (Princeton University) the traditional comparison between hegetotheres Collection, Yale Peabody Museum. and these modern groups is based on only super- ficial similarities. In particular, the morphology Materials and Methods of the hypertrophied I1 in Hegetotherium—in which the prognathous upper incisors converge The description of the hegeotheriine cranium is medially onto the midline to form a “pick-axe”- principally based on YPM VPPU 015542, a sub- like structure—is very different from the recurved, stantially complete skull and lower jaw of Hege- chisel-like incisors typical of rodents. totherium mirabile, with associated postcranial The distinctive I1 morphology of Hege- elements from the Miocene Santa Cruz formation totherium is shared with a small but taxonomi- of Argentina. Observations were supplemented cally diverse group of otherwise unrelated extant with reference to other specimens of Hege- and extinct mammals, including the primate totherium in the collections of the Peabody Daubentonia, striped possums of the genus Museum of Natural History, Yale University, in Dactylopsila, members of the proteutherian fam- New Haven, Connecticut, USA (YPM VPPU ily Apatemyidae, and Yalkaparidon, a marsupial 015505, YPM VPPU 015200, YPM VPPU of uncertain taxonomic affinities from the early 015298) and the American Museum of Natural Miocene Riversleigh deposits of Australia. These History in New York, New York, USA (AMNH animals, which have been characterized as “mam- 9223, AMNH 9159). Comparisons were made malian woodpeckers,” use their massive incisors with crania from other notoungulate taxa, includ- to break into wood in search of wood-boring ing the hegeotheriine hegetotheres Prohege- (xylophagous) grubs and other organisms (Cart- totherium caretti (YPM VPPU 021279, YPM mill 1976; Beck 2009). Incisor morphology and VPPU 021880, YPM VPPU 021882) and Sal- the presence of other key features identified by latherium altiplanense (published description in Cartmill (1976) and Beck (2009) to be typical of Reguero and Cerdeño 2005); the pachyrukhine mammalian woodpeckers support the hypothe- hegetothere Pachyrukhos moyani (YPM VPPU sis that Hegetotherium also adopted this mode of 015743, AMNH 9219, AMNH 9525, AMNH feeding, rather than conforming to the established 29605, AMNH 29570); the mesotheriid Tra- model of a rodentlike herbivore. The few mam- chytherus mendocencis (YPM VPPU 020687, malian woodpecker features that Hegetotherium YPM VPPU 021925, YPM VPPU 021927); the lacks are associated with nocturnal activity and an archaeohyracids Archaeohyrax suniensis (YPM arboreal mode of life, rather than with wood bor- VPPU 023701, supplemented by the published ing. Therefore, Hegetotherium