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Cranial Anatomy of Kryptobaatar Dashzevegi (Mammalia, Multituberculata), and Its Bearing on the Evolution of Mammalian Characters

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Cranial Anatomy of Kryptobaatar Dashzevegi (Mammalia, Multituberculata), and Its Bearing on the Evolution of Mammalian Characters CRANIAL ANATOMY OF KRYPTOBAATAR DASHZEVEGI (MAMMALIA, MULTITUBERCULATA), AND ITS BEARING ON THE EVOLUTION OF MAMMALIAN CHARACTERS JOHN R. WIBLE Research Associate, Division of Vertebrate Zoology, American Museum of Natural History; Section of Mammals, Carnegie Museum of Natural History, 5800 Baum Boulevard, Pittsburgh, PA 15206 GUILLERMO W. ROUGIER Research Associate, Division of Paleontology, American Museum of Natural History; Department of Anatomical Sciences and Neurobiology, School of Medicine, University of Louisville, Louisville, KY 40292 BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY Number 247, 124 pages, 39 ®gures, 3 tables Issued February 28, 2000 Price: $10.90 a copy Copyright q American Museum of Natural History 2000 ISSN 0003-0090 CONTENTS Abstract ....................................................................... 4 Introduction .................................................................... 5 Materials and Methods .......................................................... 9 Descriptions ................................................................... 16 Premaxilla .................................................................. 16 Nasal ....................................................................... 19 Lacrimal .................................................................... 20 Frontal ..................................................................... 21 Maxilla ..................................................................... 23 Palatine .................................................................... 28 Pterygoid ................................................................... 30 Sphenoid Complex ........................................................... 31 Petrosal .................................................................... 36 Jugal ....................................................................... 43 Squamosal .................................................................. 43 Parietal ..................................................................... 46 Supraoccipital ............................................................... 46 Exoccipital .................................................................. 47 Basioccipital ................................................................ 48 Endocranium ................................................................ 48 Mandible ................................................................... 59 Vascular Reconstructions ....................................................... 62 Veins ....................................................................... 62 Arteries .................................................................... 67 Comparisons .................................................................. 72 Snout and Palate ............................................................ 72 Prenasal Process of Premaxilla .............................................. 72 Septomaxilla .............................................................. 74 Nasal Overhang and Anterior Nasal Notch ................................... 76 Nasal Foramina ........................................................... 76 Infraorbital Foramina ...................................................... 77 Palatine Foramina ......................................................... 77 Facial Process of Lacrimal .................................................. 78 Orbitotemporal Region ....................................................... 78 Orbital Mosaic ............................................................ 78 Orbital Foramina .......................................................... 80 Postorbital Process ......................................................... 82 Nasoparietal Contact ....................................................... 83 Jugal ..................................................................... 83 Basicranium and Lateral Braincase Wall ........................................ 83 Pterygopalatine Ridges and Troughs ......................................... 83 Ectopterygoid ............................................................. 84 Alisphenoid ............................................................... 85 Foramina for Branches of the Mandibular Nerve .............................. 86 Venous System ............................................................ 87 Arterial System ........................................................... 89 Pterygoid Canal ........................................................... 92 Petrosal .................................................................. 93 Hypoglossal Foramen ...................................................... 96 2 2000 WIBLE AND ROUGIER: CRANIAL ANATOMY OF KRYPTOBAATAR DASHZEVEGI 3 Endocranium ................................................................ 96 Cavum Epiptericum and Primary Braincase Wall .............................. 96 Hypophyseal Fossa, Tuberculum Sellae, and Jugum Sphenoidale ................ 98 Cribriform Plate ........................................................... 99 Conclusions ................................................................... 99 Acknowledgments ............................................................ 104 References ................................................................... 105 Glossary ..................................................................... 114 Index of Anatomical Terms .................................................... 121 We affectionately dedicate this publication to Mike Novacek, for his years of encouragement, support, and friendship. 4 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 247 ABSTRACT The cranial anatomy of the Mongolian Late Cretaceous multituberculate Kryptobaatar dash- zevegi is described based on exquisitely preserved specimens collected from Ukhaa Tolgod and Tugrugeen Shireh in the Gobi Desert by joint expeditions of the American Museum of Natural History and the Mongolian Academy of Sciences. Most sutural relationships are pre- served, enabling a bone-by-bone description of the skull and lower jaws exclusive of the nasal fossa and paranasal sinuses. A reconstruction of the principal components of the cranial ner- vous, arterial, and venous systems is facilitated by specimens with exposed endocranial sur- faces. Comparisons with previously described multituberculates, other Mesozoic mammaliaforms, and extant mammals allow an assessment of major topics in the evolutionary morphology of the multituberculate and mammaliaform skull, as well as identi®cation of Kryptobaatar as an appropriate model regarding most aspects of the multituberculate skull for future phylogenetic studies. Elements previously unknown or poorly known in multituberculates are described. Included are a complete jugal on the internal surface of the zygoma; the orbital mosaic and foramina, including the optic foramen, the metoptic foramen, the transverse canal, and the foramen for the pituito-orbital vein; and the endocranium with an extensively ossi®ed primary braincase wall formed by the pilae metoptica and antotica. The latter pila is very robust compared with its ossi®ed remnants in non-mammalian cynodonts and monotremes, suggesting that it is a derived multituberculate condition. The co-occurrence of the pilae metoptica and antotica in multituberculates is thus far unique among mammaliaforms, but agrees with the morphology expected to be primitive for Mammalia. This in turn implies an independent loss of an ossi®ed pila metoptica in monotremes, marsupials, and Vincelestes or the loss of the pila metoptica in the ancestry of multituberculates and therians combined with the independent reacquisition of a neomorphic pila metoptica in multituberculates and eutherians. The absence of several elements from the multituberculate skull, controversial in nature, is con®rmed, including the prenasal process of the premaxilla, the septomaxilla, the ectopterygoid, and the orbital process of the palatine. Also con®rmed is the presence of several controversial elements in the multituberculate skull, including an alisphenoid with a reduced contribution to the braincase and an anterior lamina expanded dorsal to the alisphenoid. Competing anatomical hypotheses for several elements are addressed, including the function of the lateral pterygo- palatine trough as muscle attachment and not for the auditory tube, the homology of the postorbital process on the parietal with that on the frontal, the identity of foramina in the anterior lamina as for mandibular nerve branches and not for the mandibular and maxillary nerves, and the function of the jugular fossa as primarily having housed a diverticulum of the cavum tympani and not large cranial nerve ganglia. The cranial arterial system in Kryptobaatar generally resembled that restored for other multituberculates and for other mammaliaforms, in particular the prototribosphenidan Vincelestes. Both Kryptobaatar and Vincelestes had a trans- promontorial internal carotid artery, a stapedial artery that ran through a bicrurate stapes, ramus inferior, ramus superior, and an arteria diploeÈtica magna. The cranial venous system in Kryp- tobaatar resembled that described for other Mongolian Late Cretaceous multituberculates and for monotremes with the major exits of the dural sinuses having been the prootic canal and the foramen magnum. A revised diagnosis of Kryptobaatar distinguishes it from other djadochtatherians (the grouping that includes 10 of the 11 genera of Mongolian Late Cretaceous multituberculates) by a
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