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Osteology of the Middle Eocene Ceratomorph Hyrachyus Modestus (Mammalia, Perissodactyla)

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Osteology of the Middle Eocene Ceratomorph Hyrachyus Modestus (Mammalia, Perissodactyla) OSTEOLOGY OF THE MIDDLE EOCENE CERATOMORPH HYRACHYUS MODESTUS (MAMMALIA, PERISSODACTYLA) BIN BAI Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, CAS, Beijing; and State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, CAS, Nanjing JIN MENG Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, CAS, Beijing; and Division of Paleontology, American Museum of Natural History, New York YUAN-QING WANG Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, CAS, Beijing; and College of Earth Science, UCAS, Beijing HAI-BING WANG Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate Paleontology and Paleoanthropology, CAS, Beijing; and UCAS, Beijing LUKE HOLBROOK Department of Biological Sciences, Rowan University, Glassboro, NJ; Division of Paleontology, American Museum of Natural History; and Department of Vertebrate Zoology, Academy of Natural Sciences of Drexel University, Philadelphia BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY Number 413, 68 pp., 34 figures, 22 tables Issued June 9, 2017 Copyright © American Museum of Natural History 2017 ISSN 0003-0090 CONTENTS Abstract.............................................................................3 Introduction.........................................................................3 Materials and methods................................................................5 Abbreviations........................................................................5 Description and comparison...........................................................5 Skull . .5 Lower jaw .......................................................................10 Axial skeleton ....................................................................13 Cervical vertebrae..............................................................13 Thoracic vertebrae . .18 Lumbar vertebrae . 22 Sacrum .......................................................................22 Ribs ..........................................................................23 Sternum ......................................................................24 Bones of the thoracic limb .........................................................25 Scapula .......................................................................25 Humerus......................................................................26 Radius........................................................................27 Ulna..........................................................................30 Carpals .......................................................................31 Metacarpals ...................................................................35 Phalanges .....................................................................38 Bones of the pelvic limb ...........................................................40 Innominate bones..............................................................40 Femur ........................................................................43 Patella ........................................................................45 Tibia .........................................................................45 Fibula ........................................................................48 Tarsals ........................................................................54 Metatarsals ....................................................................57 Discussion .........................................................................57 Origin of Hyrachyus . 58 From Hyrachyus to other Rhinocerotoidea ........................................60 Functional analysis . 60 Neck .........................................................................60 Dorsal vertebrae . .60 Scapular and forelimb function ..................................................61 Innominate and hindlimb function...............................................61 Perspectives ........................................................................62 Acknowledgments...................................................................62 References..........................................................................63 Appendix 1: List of the postcranial anatomical terms ....................................65 2 ABSTRACT The middle Eocene ceratomorph Hyrachyus has been considered a pivotal genus in cerato- morph evolution, either as a transitional form from tapiroids to rhinocerotoids, giving rise to all later rhinocerotoids, or else as the sister taxon to other rhinocerotoids. Thus,Hyrachyus has been commonly chosen as an outgroup in phylogenetic analyses of rhinocerotoids. However, little has been published on the osteology of Hyrachyus, even though well-preserved craniodental and post- cranial specimens of this taxon have been in collections for decades. Here, we describe and illus- trate the cranial and postcranial osteology of Hyrachyus modestus, based mainly on the exceptionally preserved specimens housed at the American Museum of Natural History, specifi- cally AMNH FM 12664. Our bone-by-bone description provides detailed information on the osteological morphology of Hyrachyus, which should be useful for phylogenetic analyses of both rhinocerotoids and perissodactyls in general, because it provides one of the more complete and best-preserved examples of the skeleton of an earlier Eocene perissodactyl. The cranial morphology ofHyrachyus modestus shows a shallow narial notch, a lacrimal con- tacting the nasal, and a sphenorbital fissure closely situated to the anterior opening of the alisphe- noid canal. In the basicranial region, there is a mastoid exposure of the petrosal between the occipital and the squamosal, and the posttympanic process and paracondylar process are partly fused. The postcranial morphology of Hyrachyus modestus includes the following features: The cervical region of the vertebral column is relatively short compared to the rest of the vertebral column. The lumbar vertebrae have concave-convex embracing prezygapophyses and postzyg- apophyses. The scapula has a distinct acromion process. The humerus has a greater tubercle that does not elevate above the head, and the deltoid tuberosity and deltopectoral crest are weak. The scaphoid and lunar facets of the radius are confluent. The olecranon of the ulna extends postero- proximally. The manus is functionally tetradactyl, with a complete fifth manual digit. The innomi- nate bone has a long, narrow coxal tuberosity. The greater trochanter of the femur is elevated proximally above the head. The femur has a long, narrow, and symmetric trochlea. The patella has a moderately anteroposteriorly deep base. The intercondyloid eminences of the tibia are equal in height, and the extensor sulcus of the tibia is relatively deep. The fibula has a relatively slender shaft with expanded ends. The pes has three functional digits. The calcaneus does not contact the navicular, nor does the Mt III contact the cuboid. Comparisons between the skeleton of Hyrachyus modestus and those of the early tapiroid Hep- todon, the hyracodontid Triplopus, the paraceratheriid Juxia, and the rhinocerotid Uintaceras were also investigated. These results indicate thatHyrachyus probably did not derive from Heptodon, but from a more basal group of ceratomorphs. Furthermore, distinct differences between the skeletons of Hyrachyus and Triplopus (the earliest representative of Hyracodontidae) suggest that hyracodontids were not descended from Hyrachyus. However, Hyrachyus-like ancestors probably gave rise to other non-hyracodontid rhinocerotoids. Like that of other Eocene perissodactyls, the postcranial morphology of Hyrachyus modestus exhibits adaptations that suggest that cursorial locomotion was already present early in perissodactyl evolution. INTRODUCTION Hyrachyidae, recognized four genera and 12 species; however, Radinsky (1967b) recognized The genusHyrachyus is a common cerato- only one genus and two species as valid in morph in the late early Eocene and middle North America. The two species, Hyrachyus Eocene in North America and Asia, and plays a modestus and Hyrachyus eximius, mainly differ pivotal role in understanding the origin of rhi- in size, and Radinsky (1967b) suggested that nocerotoids (Radinsky, 1967b; Prothero et al., they gave rise to the hyracodontids Triplopus 1986). Wood (1934), in his revision of the and Fostercooperia, respectively. As suggested 3 4 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 413 by Schoch (1984), Wood’s (1934) revision prob- modestus (AMNH FM 12664) as “a fine skull ably oversplit the Hyrachyidae, and yet Radin- and skeleton, which deserves monographic sky (1967b) probably oversynonymized the treatment.” AMNH FM 12664 is a well-pre- taxa. Emry (1990) suggested the validity of H . served skeleton, missing the humeri and pes affinis based mainly on its smaller size. Other and with some vertebrae and ribs broken. A authors have recognized as many as eight spe- comprehensive bone-by-bone description of cies of Hyrachyus from China (Radinsky, 1965b; this specimen is valuable for future phyloge- Chow and Qi, 1982; Huang and Qi, 1982; Qi, netic analyses of perissodactyls, especially when 1987; Huang and Wang, 2002).
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