CRANIAL MORPHOLOGY AND PHYLOGENETIC RELATIONSHIPS OF TRIGONOSTYLOPS WORTMANI, AN EOCENE SOUTH AMERICAN NATIVE UNGULATE R.D.E. MACPHEE, SANTIAGO HERNÁNDEZ DEL PINO, ALEJANDRO KRAMARZ, ANALÍA M. FORASIEPI, MARIANO BOND, AND R. BENJAMIN SULSER BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY CRANIAL MORPHOLOGY AND PHYLOGENETIC RELATIONSHIPS OF TRIGONOSTYLOPS WORTMANI, AN EOCENE SOUTH AMERICAN NATIVE UNGULATE R.D.E. MacPHEE Department of Mammalogy/Vertebrate Zoology and Richard Gilder Graduate School, American Museum of Natural History SANTIAGO HERNÁNDEZ DEL PINO IANIGLA, CCT-CONICET Mendoza, Argentina ALEJANDRO KRAMARZ Sección Paleontología de Vertebrados, Museo Argentino de Ciencias Naturales Bernardino Rivadavia, CONICET, Buenos Aires, Argentina ANALÍA M. FORASIEPI IANIGLA, CCT-CONICET Mendoza, Argentina MARIANO BOND Departamento Científico de Paleontología Vertebrados, Museo de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina R. BENJAMIN SULSER Department of Mammalogy/Vertebrate Zoology and Richard Gilder Graduate School, American Museum of Natural History BULLETIN OF THE AMERICAN MUSEUM OF NATURAL HISTORY Number 449, 183 pp., 42 figures, 6 tables Issued April 19, 2021 Copyright © American Museum of Natural History 2021 ISSN 0003-0090 CONTENTS Abstract.............................................................................5 Introduction.........................................................................5 Materials and Methods................................................................7 Comparative Set...................................................................7 Trigonostylops and Astrapotheriidae: Physical Status of Selected Specimens .............7 Trigonostylops wortmani AMNH VP-28700 .....................................7 Other Trigonostylops Specimens ................................................8 Astrapotheriid Specimens ....................................................15 Agenda..........................................................................16 Method of Description..........................................................16 Use of Indicia ...............................................................16 Cerebral and Vascular Reconstruction..........................................16 Nomenclature...............................................................24 Comparative Cranial Morphology of Trigonostylops......................................24 Interpreting Vasculature ...........................................................24 Arteries ......................................................................26 Internal Carotid Artery ......................................................26 External Carotid Artery ......................................................28 Occipital and Vertebral Arteries ..............................................33 Arteria Diploetica Magna.....................................................35 Veins . 35 Emissary and Diploic Veins...................................................37 Caudodorsal Array ..........................................................38 Rostroventral Array..........................................................46 Cerebrospinal Venous System .................................................46 Interpreting Pneumatization .......................................................51 Ontogeny of Cranial Pneumatization .............................................51 Frontal Sinus Development . .61 Palatal Diverticulum of Maxillary Palatine Process...............................61 Epitympanic Sinus, Extratympanic Sinus, and Other Pneumatic Spaces ...............65 Rostral Cranium and Mesocranium .................................................69 Dentition and Upper Jaw........................................................69 Major Dental Features of Trigonostylops Compared to Other SANUs ...............69 Absence of Upper Incisors and Identification of Canines .........................73 Palatal, Facial, and Nasal Regions ................................................75 Palatal Architecture . .75 Palatal Alate Process ......................................................75 Premaxillae ..............................................................77 Frontals .................................................................80 Infraorbital Foramina . .80 Nasals ...................................................................81 Choanae .................................................................81 Orbital and Infratemporal Regions ...............................................82 Orbit and Orbital Mosaic ..................................................83 2 2021 MacPHEE ET AL.: TRIGONOSTYLOPS MORPHOLOGY AND RELATIONSHIPS 3 Cranioorbital Sulcus and Ethmoidal Foramen ................................83 Lacrimal Foramen and Jugal ...............................................84 Sphenoorbital Fissure .....................................................85 Sulcus and Foramen for Maxillary Nerve and Vessels ..........................85 Foramen for Malar Artery .................................................85 Sphenopalatine Foramen...................................................86 Greater and Lesser Palatine Nerves and Vessels . .86 Caudal Cranium..................................................................87 Basicranial Foramina and Related Features ........................................87 Continuous Basicapsular Fenestra and Transiting Structures....................87 Retroarticular Foramen and Glaserian Fissure ................................90 Caudal Carotid Incisure/Foramen...........................................90 Promontorial Vascular Features.............................................91 Rostral Carotid Incisure/Foramen...........................................93 Caudal Aperture of Pterygoid Canal.........................................94 Stylomastoid Incisure/Foramen .............................................95 Tympanic Aperture of the Prootic Canal .....................................95 Canal X . .97 Trackways for Ventral and Dorsal Petrosal Sinuses ............................97 Hypoglossal Foramen . .97 Posttemporal Vascular Complex ............................................99 Tympanic Floor and Related Features.............................................99 Petrosal in Tympanic Floor .............................................. 100 Ectotympanic .......................................................... 100 Other Features of Tympanic Floor ........................................ 101 Tympanic Roof and Related Features ........................................... 101 Petrosal in Tympanic Roof ............................................... 101 Paracondylar Process, Hyoid Recess, and Tympanohyal ...................... 101 Lambdoidal Process of Petrosal ........................................... 102 Sagittal Crest and Temporal Crests ........................................ 102 Interparietal............................................................ 103 Neurological and Associated Structures ......................................... 103 Brain Endocasts . 103 Cranial Nerves . 106 Auditory Apparatus .......................................................... 106 Osseous Labyrinth . 106 Stapes ................................................................. 112 Discussion: Indicial and Morphological Interpretation ................................. 113 Vascular Indicia ................................................................ 113 Arterial Structures . 113 Venous Structures............................................................ 119 Pneumatization Indicia.......................................................... 129 Phylogenetic Analyses . 131 Placing Trigonostylops . 131 Trigonostylops in the Context of SANU Evolution................................... 134 4 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY NO. 449 Acknowledgments................................................................. 136 References........................................................................ 136 Appendix 1. Glossary and Notes .................................................... 148 Appendix 2. Entotympanics, Vasculature, and Other Features of the Caudal Cranium of Extant Perissodactylans .................................................................. 157 Appendix 3. Character List ......................................................... 173 Appendix 4. Character/Taxon Matrix ................................................ 182 Reconstruction of Trigonostylops wortmani by Jorge Blanco. ABSTRACT In 1933 George G. Simpson described a remarkably complete skull of Trigonostylops, an Eocene South American native ungulate (SANU) whose relationships were, in his mind, quite uncertain. Although some authorities, such as Florentino Ameghino and William B. Scott, thought that a case could be made for regarding Trigonostylops as an astrapothere, Simpson took a different position, emphasizing what would now be regarded as autapomorphies. He pointed out a number of features of the skull of Trigonostylops that he thought were not represented in other major clades of SANUs, and regarded these as evidence of its phyletic uniqueness. Arguing that the lineage that Trigonosty- lops represented must have departed at an early point from lineages that gave rise to other SANU orders, Simpson reserved the possibility that Astrapotheriidae might still qualify (in modern terms) as its sister group. Even so, he argued that the next logical step was to place Trigonostylops and its few known allies in a separate