CRANIAL MORPHOLOGY AND PHYLOGENETIC RELATIONSHIPS OF TRIGONOSTYLOPS WORTMANI, AN EOCENE SOUTH AMERICAN NATIVE UNGULATE

R.D.E. MACPHEE, SANTIAGO HERNANDEZ DEL PINO, ALEJANDRO KRAMARZ, ANALIA 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 HERNANDEZ DEL PINO IANIGLA, CCT-CONICET Mendoza, Argentina

ALEJANDRO KRAMARZ Seccién Paleontologia de Vertebrados, Museo Argentino de Ciencias Naturales Bernardino Rivadavia, CONICET, Buenos Aires, Argentina

ANALIA M. FORASIEPI IANIGLA, CCT-CONICET Mendoza, Argentina

MARIANO BOND Departamento Cientifico de Paleontologia 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

PAB SER ACE cs csch ceuriyins geape sxartqnweace deccamtea §appet su “inane os th ceebans "eeape scarigtngrars de ccagica: S-apsts ug "tnanaes on euetytns aeaphcg 5 TRO UGTIONY, #1 Acs rene Ging nc ae ee Rig ats ak glee hated SONNE ang tn hee REG te alee An SE, 5 Marterialscind Merh@elsa, srsriso 28h ae tea Sete So alae i WAR ly RSs ap ae Bila OR os AA 7 COMPArAative Set acme, marge; ata ceeke Meee aes Tiel Boh ee a eal eae PE. 7 Trigonostylops and Astrapotheriidae: Physical Status of Selected Specimens............. Z Trigonostylopsiwor tiiant 2oMN HNP? 28 700% aparece Sean atten weaee sty es a aches eaaetaee 7 Other eign ostviops: Specinens). Scccaue ances tutius nous a wy aig Souda halle ake agueng. Soleo Solan ee 8 SUPA OLCEING! SOC STIS pg enn ed Save tthe a a leche ater a eke welsh Mies Brees Co eh gg ye 15 PGS Tris GAL ek Bede C8 ox AGA IONE Bh ee tana Biel io. ee A Ae INE DNR Une aise 16 INethIOd-O fESCRIPLIO thet... 4 dae atl « «eM ane tha Sade ahee wt das a agenda ate « «ene tig Saas 16 WSE-Otr INA Clans ati aac ata Merl tar aware me tka Sym, Je okie ale Meare tye ck een ae Ame: 16 GerebtabandsVascular Reconsteu cH ane se. s. wear w tek : trpesch Bets te Gene oleate area Ratton fa arom 16 TOTTI CL ACU TG soia oe var het <oe-wate recor ew weg scrapie scene <A Taber Calaaee ers eee a Pa eon 24 Gomparative: Cranial Morphology-of. TrigOnOstylops. ieee se Fos oe ME mentee Day ov es eel eke eS 24 terre nes Vasculature 4 cteet RN NEL ti abana shad « Rie a haccet ya ahd SMa Pri ahaa ana ne 24 PRUCORICS® 2d yee whee uct hace ie ly A's MRC OE NA a Us le ns Se PEEN BS I EN ly de Vee PERA ane 26 FritermaltSarotic- Artery 6 va 1.168 Rew atdooge Woh eka tea dace AY tpg as Rady th ee 26

Exton ialarotid GArety \t ae! Cem cliame,. wend 2 tt. ce. Gece nae. A Meech aie. wee: 28 Occipital -aivd VertebraleArteriess a6 See er Ree AD le ce cis ak cetyl ars 33 Abtetia: Diploctica VMiaOmask io. mata 2st murtyes aero e tani hea aah a ays Noma hana ack marta ea | 35 MSITIS Is oh ah i ree nitrd ce tsa OB, oc ie orttne RD ee ohn. ge ante Steet Saal Bh elie, tens Seale See whe: wt 35 ETassat Vat MO tlie’ .y HIS see te Recta. nies onc SIR aes BOA Any a rs et AST nats oN 37 Caudodorsal Attay teres 2.52. ote caret anda ste Se Raan ha dae este MAT aM cae 38 ROStEOVERt Cal, erate seme ED re Reese theo RN Rote Ota) aR ARN Pe ci riR RR ts a 46 Cerebrospinal Venus: Sy Stems eye rres Weyer since Lites eR eee toe acer en oe RR RR satel 46 PCERPLetiINg Pe VMIAtAATOM ay Nhe ae ache aes. goede sro eset: anoles ase seh enor oe Poets -arslygucactac gra ese Di Ontosenvidk@ ramalbRneumalt zation? avg te: Marita ss ee tanvier a MenieeNa s ceiaty ays ec wei Bh: Ba Rrontal Sinus Pevelopnient tev. oe, Se hagas sand Sele ee AeA, COA Oe ead 61 Palatal Diverticulum of Maxillary Palatine Process............... 0... e cess ee eee 61 Epitympanic Sinus, Extratympanic Sinus, and Other Pneumatic Spaces ............... 65 ROStral C FansUnisard WV ieSOCEAMUN Ts te. ccwelen atic Re tee a atNe Ma Mate olin Gude aie ehh 69 Dentitionande ppers law Ne eka. Secce acs ye Wire ay. enc Pree ue flo kote eRe as ¥ sete as 69 Major Dental Features of Trigonostylops Compared to Other SANUs............... 69 Absence of Upper Incisors and Identification of Canines .................0.0000 0} 7 Palatal :Eacialvand-Nasalt Repions 2.1 mcenetatty un uet am Loh, PAB Oth We why Mera ntincety ates 75 Palatal Architecture et a. nek tha chagy 8s eet Sere BS hats Boda ahangy Wa a Eat 75 PalatalpAl ate, Process? cece maa. neta eet en A ea Ray eet RNR Da, Sb tet en SEN 75 Jedceswatee a) | ete yne uae, Ml vn Pauses ile aa SiN noe Perey es. WO, Wed pen epeat eat ey 7 Rarer 2 77

Frontals? 2 cndol, acatet snide ne oa ee tae nent, ahaa entails eons ts Mite ane eae pena Soh «sc 80

Ria OEDILALBOralyiiT Aue sr escie tert rae See Bee Pr eit se ect edhe a Mate eens ane eae b bd a 80

UNE IAS ERLS gee ei RERR CP DN se RPE HR OR RSL S E N OEE.SE OR OS 81 CHOANAC so snca. «etna <S aa she 9 Seger D ace pata Pad oA Raah Gah ad ote oot iga an Gat Be 81 Orbitalkamd InitatemperalSReorons:. te. niles mera ce en cesta Reg diese skids ne: 82 Orbitanid' Orbital MOSAIC arr seca tees es ac ony eteen TR eae teen sentra ese Tet PER sN a re 83

2021 MacPHEE ET AL.: TRIGONOSTYLOPS MORPHOLOGY AND RELATIONSHIPS

Granieorbital Sulcus ahd Ethmoidal, Poramen <1, -ha.0.-cvyn oes vex Role es es 83 Lacrimal-Foramen and! Jugales, . ..vmereac 5 tad ster «tela aga exe «aaa bade 84

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Sulcus and Foramen for Maxillary Nerve and Vessels................. 00000 0ee 85 Foramensfor Malar Artery fas. cco. Saad beniAle agers thay Migfiaane dais aaa eb esos 85

Spheno palatine hOranien My ate: FF cect weer sae Tas Wee een var ne eet a's epee ee 86 Greaterand Lesser Palatines Nerves ands Vesselsiite oi aga od eeagtln We eased hang « 86

lati aIGC ran iis as ke "s, vex ee PA hg SEE sal es were eh WA a ae ENON Be Sei hi we, oe 87 Basicranial Foramina-ancyRelateds Features ca .samecin. Who sae haa paid tara yeh Beetles 87 Continuous Basicapsular Fenestra and Transiting Structures.................... 87 RetreartichlanForamen-and:Glasertaiy Fissure. 6.524 <4 c0dvee nw scagen 4a 9s ach etaca anys 90 CGaudal-Carotid Incistire/FOramems . ¢ sz-tees sper a4 cacacs + games ages cs tea ak eeena quack 90 Promontobial Vascular PEATEs... tah seh seas GET ng ach ROPE ME ht wale eA ec 91 RostraltGarotidelneisure POLAMEN gadmnat, 2 am okt Sr dntn tes nae ie et A ana tes h 05 Candal-Aperture of Pteryxoid: Canal. «mf iuatamecng woh ates soe eee; 94 Styloimastoid. ImeisurePOramien escent Ee he Ace Site a Me Lee Page ai iat eaten hs ee 95 ‘Lympanie Aperture of the:Prootic (arial ocx: es ok, . kc aee ee SNe seks plats: 95 COATTAUER wane nani. geteinhy typi sha she ale ealire ca letabad omy alg Matar tue nie: ahoe aectyiang. dsletey Suaeuet 97 Trackways for Ventral and DorsalsPétrosaliSinuses .v's..0034 6 ee ee we alee 97

ELV PO OLOSSAIAPOr Areca 25, Je Us ls he AT a BR 5rd dis Ae BNE I ali, Re Re oF Posttemporal: Vascular omiples™, myers... 35 $4-& ote ws ete dan agen e ah, « «MRP Aan coadon es 99 iyvnnpanic’hloorand Related eatires..! ee. aeea rr ct fea Me sa ata Ret eens s ae area. 29 Peprosal- ti. lym atone LOGIE. Fem, whch. ele ec tn aen ey 1a ice, ee ec even areiue eo 100

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Other Beaturesor yap aril Ga Ow tage tees wres is wee Ta ie RR et ae Bee Feely rola ral 101 ‘Fympanic oor anda elatedt Features .)al nots A5. emer Nahe aed EMR 4 2 LAG aba 101 Petrosalin® Tyimpanic: ROE ta Fn. cotec Pent aich, Me Ot yb Tlate n ee ia sly a RIA 101 Paracondylar Process, Hyoid Recess, and Tympanohyal....................4. 101 Lainbdeidal Process Gi Petrosal -ecenaee nen Shee eee cane peoN tA Rees) cle eae: eh 102 pagittalsCrest and MeMpPOral Tests. kta tre oat scha cat cna iy ae RRs we ceen Are te esl ate 102

Wine nar te tals resets aycetrann tons tte age tr cearthese ape Pi se trtenaas sh ota macs cioace “tampons martes 4 103 Neurologicaland: Associatedy Structures. ee fakin ee ee ten peak ohnas ea tates eens aie PA) 103 CATT MT ANCLO CASE Ste 2 tcttn uen th Reet, Recess ns MURS OES ROCA Ach tlny ote tre, RT ASC AR 103

Cranial NEtVeS>aye239 5.5.2. 1eta ne aetge han ad a oss RAR dae oot age Fam Bata 106 PRUCHEO EY APP ATAU Sys cette tem re Reese the OR NN ee GaN aR te REO Pt a erm Ren 106 SSCOU Ss GOVE TIAL, ce tes eee. tar uae EUG Ra eet AR AER RON saeatecly ne Te RU REO n) 106

SLADE 5 one ee essay. Medd ac lotto rant. Axtabere wcnce edd arigy fua- evade gto Ph aK aeGibangnaat o Sua 112 Discussion: Indicial'and* MorphologicaltInterpretation nics. Gane a nesseie spr eee wie pets 113 Vascular Indicia tev Sect. JG. ah scene ba We See as ee SRA Ey, RD, ARN co BO as 113 PAL Pe TiAl SSUPUICHUrES Pear eit mathe Pe uta th te EAA PIE 6 a VME A ARAM ch Bathe ode 113 VEMOUS: SEPUCUUILOS cc eee Ren cia et raters SARs ce ELMS he, tial ep Ay deg OE Ee ca raed eee eGS 119 PICU TAA CZ ALLO MeN TC Tari fe Me Oe Ree Nan ace ee eu SOS We RNS Res ee Ni eS 129 Phylogenetic: Amal ySesek- Lomo Seek eee ee tei cae teehee Nek cota echt eR creas 131 PEACH TEICOHOSEVI OP Seren ci Pe resoutsmiats Meee omen soctebnrete hintaan Rete taee 5. ook aniubeien aLmamimoant i, 131

frigonostylopsary thie Gomext oF SAN Ur Evolution ges «peeing Ae ERs hare ae 134

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CATON OTMEINGE ef oF, ese habntsan ee ee ls Rae ata ae Sly aes ARR fy SESE ea ce a ah 136 RELETENCES 13.4.5 7. Sa Sate « aia Ragoige Ml « + AMBTAa Sado aoten omar dada outa » «alias he Sa 136 Appendix almnGrLOssarar-aride NEES bea ctc. te -ce eae Ran Meh el ces a SEdh lg cin Sin A alee Rede REMI 148 Appendix 2. Entotympanics, Vasculature, and Other Features of the Caudal Cranium of Extant

Perissodachylans itt. eco. 4 sights Magia deta aes ees eaah in geo Saadeh « Mietatnt ae te aes oS 157 PROPIONIC G1 ALAC T NSE ae SAU8 ote ete Wee PO a cog he rire ote SORE oy ies ue pM ae hha cogs ta h wieaey 173 Appendine 4 Character) amon sviatiise te. wees Oh teers ee ob Ue snag ae tat na ies Ono ae nL ga 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 order, Trigonostylopoidea, coordinate with Astrapotheria, Notoun- gulata, Litopterna, and Pyrotheria. Simpson's classification was not favored by most later authors, and in recent decades trigonostylopids have been almost universally assigned to Astrapotheria. How- ever, his evaluation of the allegedly unique characters of Trigonostylops and its allies has never been systematically treated, which is the objective of this paper. Using computed tomography, the skull of Trigonostylops is compared, structure by structure, to a variety of representative SANUs as well as extant perissodactylans (which together comprise the clade Panperissodactyla) and the “condylar- thran” Meniscotherium. In addition to placing Simpson's character evaluations in a comparative context, we also provide detailed assessments of many vascular and pneumatization-related features of panperissodactylans never previously explored. Overall, we found that this new assessment strengthened the placement of Trigonostylops within a monophyletic group that includes Astrapo- therium and Astraponotus, to the exclusion of other SANU clades. Although Trigonostylops cannot be considered as morphologically distinct or unusual as Simpson thought, our comparative and phylogenetic analyses have helped to generate a number of hypotheses about character evolution

and function in SANUs that may now be fruitfully tested using other taxon combinations.

INTRODUCTION

Trigonostylops wortmani AMNH VP-28700 (figs. 1, 2), the only substantially complete skull of a representative of Trigonostylopidae, is the source of most of what we know about cranial morphology in this clade of early South Ameri- can placentals. George G. Simpson, on whose 1930-1931 Scarritt Patagonian Expedition this specimen was collected at a terminal Middle Eocene locality south of Lago Colhuéhuapi,' thought it was “one of the most extraordinary mammalian skulls ever discovered, being unusual in almost every detail and having some

' Simpson did not mention a specific locality or strati- graphic context in his 1933a paper. According to his field notes (Simpson Ms, p. 6), the skull (field number 60 = AMNH VP-28700) was recovered at Colhuéhuapi in section A3, lower part of the lower tuff (= marker bed Y), equivalent to level 2 in Cifelli’s (1985) figure 3, section I. The specimen was col- lected by Justino Hernandez, October 31, 1930.

striking characters otherwise quite unknown in the Class Mammalia’ (Simpson, 1933a: 1).

To modern eyes Simpson's exuberance may seem misplaced, for he was essentially refer- ring to features that would now be judged as relatively uninformative autapomorphies, however striking they might be on other grounds. However, given his “evolutionist” approach to systematics (e.g., Simpson, 1975), such characters were worth emphasizing because they represented “specializations not absolutely excluding the possibility of [discov- ering a] very remote relationship” (Simpson, 1933a: 18), in this case between Trigonostylops and its presumed closest relatives among South American native ungulates (hereafter, SANUs). In light of the geological age of AMNH VP-28700 and its alleged distinctiveness, Simpson reasoned that trigonostylopids must have diverged from the other main lineages of

6 BULLETIN AMERICAN MUSEUM OF NATURAL HISTORY

SANUs no later than the earliest part of the Cenozoic. While they might have shared a general “condylarthran” ancestry with other SANU groups, he considered that such rela- tionships woiuld have otherwise been remote. In this Simpson differed from Ameghino (1901), Scott (1928, 1937a), and other authori- ties of the time who favored the view (on the basis of then quite limited dental evidence) that Trigonostylops was best regarded as an early astrapothere. In his view the taxon was sui generis, “as unlike litopterns as astrapoth- eres’ (Simpson, 1933a: 27). He changed his mind somewhat in a subsequent paper (Simp- son, 1934; see also Simpson, 1957), allowing that trigonostylopids might conceivably be astrapotheres in a general classificatory sense, but should be placed in a separate suborder. Later, Simpson (1967) withdrew even this con- cession, making Trigonostylops and several other taxa previously incertae sedis the sole representatives of order Trigonostylopoidea. His justification for this maneuver was basi- cally the same as before, i.e., their marked dif- ferences from other early SANUs in certain aspects of the dentition and ear region (see Rose, 2006). In his view the existence of such distinctions meant that only a general “patris- tic” or grade position for Trigonostylops could be established: “their resemblances [to astrapo- theres] could equally well be explained by remote common ancestry, such as within the Condylarthra, perhaps with a limited degree of convergence” (Simpson, 1967: 210).

In phylogenetic terms, little was actually clar- ified by Simpson's (1933a, 1957, 1967) analyses, and his later publications referencing trigono- stylopid relationships did not differ from the first in either content or conclusions. In these papers he did not attempt to provide a broad comparative framework for his assessments, limiting his nondental comparisons to a few of the better-known (and mostly late-diverging) taxa representing other orders. As a separate order Trigonostylopoidea was never widely endorsed by other workers, and more recent sys-

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tematic and morphological analyses have gener- ally recommended reincorporation of the family into Astrapotheria (e.g., Carbajal et al., 1977; Soria and Bond, 1984; Soria, 1988; Cifelli, 1985; Kramarz and Bond, 2009; Kramarz et al., 2017; Billet, 2010, 2011; Billet et al., 2015). At the same time, none of these later studies has pro- vided a thorough reinvestigation of the skull and dentition of T. wortmani, nor of the osteological features that Simpson regarded as “quite unknown” elsewhere among mammals.

In this contribution we not only reevaluate many of Simpson's morphological inferences in a wider context, but also describe cranial fea- tures of AMNH VP-28700 that would have been impossible for him to explore without modern imaging technology. We also provide evaluations of less complete cranial specimens of Trigonostylops in Argentinian and other col- lections that help to clarify morphological aspects that cannot be adequately studied on the AMNH skull. These new categories of infor- mation provide a basis for reassessing the phy- logenetic position of Trigonostylops (fig. 34), using a modified version of the character set developed by Billet et al. (2015). See p. 131, Phylogenetic Analyses, and appendix 3.

That such a study can be usefully undertaken at this time is a credit not only to a wide range of relevant systematic and comparative studies pub- lished in the last several decades, but also to the increased availability of micro-CT scanning for examining vascular, otic, and cerebral endocast morphology at unprecedented levels of visualiza- tion. Given new molecular evidence that at least some SANU groups are more closely related to Perissodactyla than they are to any other mam- malian order with extant representation (Owen, 1854; Welker et al., 2015; Buckley, 2015; West- bury et al., 2017), we include extensive compara- tive remarks on horses, tapirs, and rhinos to aid in the interpretation of trigonostylopid morphol- ogy. This allows us to conveniently convey new morphology, much of which does not (yet) lend itself to character construction and analysis because of the lack of comparable studies.

2021 MacPHEE ET AL.: TRIGONOSTYLOPS MORPHOLOGY AND RELATIONSHIPS

MATERIALS AND METHODS

COMPARATIVE SET

The comparative set (table 1) lists specimens that were extensively used to investigate morpho- logical features or to ascertain character states, together with their technical names, institutional acronyms, and other data. For abbreviations used in figures 1-42, see table 2. Tables 3-6 provide data on specimens and other information relevant to analyses in the text. Taxon names for extant perissodactylans follow Grubb (2005). For fossil taxa, the names utilized are ones judged to be valid and current. We are aware that “condylar- thrans” do not constitute a natural group, but as this paper is not centrally concerned with the ulti- mate ancestry of SANUs, it is adequate for our purposes to employ this term, but in quotes. Pan- perissodactyla minimally includes extinct and extant members of Perissodactyla as usually defined plus the SANU orders Notoungulata and Litopterna, as established by proteomic and genomic evidence presented by Welker et al. (2015) and Westbury et al. (2017). Other tradi- tional SANU orders, sometimes grouped with the foregoing in superorder Meridiungulata, may also be members of Panperissodactyla (e.g., Pyrothe- ria); so might other taxa (e.g., Cooper et al., 2014; but see Rose et al., 2019), although delimiting this larger potential clade is not the focus of this paper. “No data” (ND) specimens of extant species (table 1) are ones, usually from zoos, that lack a native geographical origin; they were used for scanning and dissection in several instances because of their excellent preservation and availability for these purposes.

TRIGONOSTYLOPS AND ASTRAPOTHERIIDAE: PHYSICAL STATUS OF SELECTED SPECIMENS

Although it is not feasible to comment on the physical condition of each of the numerous fos- sils utilized in this study, we have done so below for most of the high-quality trigonostylopid and astrapotheriid specimens available to us:

TRIGONOSTYLOPS WORTMANI AMNH_ VP- 28700. Simpson (1933a: 1) described AMNH VP-28700 as being “nearly complete with preser- vation unusually favorable for study” (figs. 1A-E; 2A, B). This assessment is substantially correct, but it should be noted that, as was common prac- tice at the time, a fair amount of reconstruction was undertaken to improve the fossil’s appear- ance. On the specimens left side (fig. 1D), much of the face, floor of the orbit, and rostral part of the zygomatic arch are reconstructed in plas- ter, their shapes based on the somewhat more intact right side. Plaster likewise holds together the nasal cavity’s surviving right wall, which is minutely fractured and crushed inward (fig. 24). The meso- and neurocranium are internally much fragmented, although the caudal cranium is in surprisingly good condition. Some sutures and plaster/bone contacts were traced in India ink, perhaps by Simpson (e.g., fig. 25). During the original preparation of the specimen, matrix was removed from all external surfaces; foram- ina were subjected to especially deep cleaning, with inevitable (if mostly minute) loss of bone as a result. Matrix lodged in the endocranium was removed by tunneling through the foramen magnum, which resulted in additional damage to some structures (e.g., to internal walls of extra- tympanic sinuses, so that they now artificially open into the cranial cavity; figs. 11, 12).

The basicranium as it appears at present (fig. 26) differs slightly from the illustration in Simp- sons (1933a: fig. 5) paper. This indicates that AMNH VP-28700 has undergone additional preparation by unknown hands since Simpson’s time, especially on the specimen’s right side where additional work on the auditory region has obviously occurred. Internally, the portion of the skull that housed the olfactory bulbs and adjacent parts of the forebrain are extensively damaged and could not be usefully reconstructed (fig. 8). The ubiquitous presence of mineral sin- ter on internal surfaces complicated endocast rendering (e.g., figs. 11, 12).

The skull may have been discovered in two or more parts, or perhaps it broke after collection or

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NO. 449

FIG. 1. Trigonostylops wortmani AMNH VP-28700, skull: A, ventral; B, dorsal; C, right lateral; D, left lateral; and E, caudal views (on this and facing page). In A, rectangle encloses palatal alate process (see fig. 20). For details of orbital and basicranial regions, see figures 25 and 26.

during preparation. In any case, the pieces were expertly glued together, albeit with a slight loss of bone as the pieces were fitted along fractures. The most obvious break can be seen ventrally, where the fracture line passes from side to side just caudal to the opening of the choanae. Large fractures were treated with a filler (¢polyethylene glycol) at some stage of preparation.

OTHER TRIGONOSTYLOPS SPECIMENS. Three additional cranial fragments ascribed to T’ wort- mani were available for study: a partial face (MLP 52-X-5-98; fig. 21), a partial rostrum (MACN Pv 47 [cast of MNHN-CAS 187]), and another incomplete rostrum in poorer condition but pre- serving parts of the nasals and maxillae (MACN

A 11078). A nearly complete jaw, MPEF PV 5483 (fig. 3), is also briefly described and illustrated in connection with the summary of dental features (see p. 69, Dentition and Upper Jaw).

MLP 52-X-5-98 is the only specimen of Trigo- nostylops discovered to date that substantially preserves the nasal aperture and rostral portion of the face. The rostrum has been distorted by lateral crushing, with the result that the left pala- tal process of the maxilla has been partly jammed under the right. There is also a significant frac- ture passing through the palate that principally affects the teeth, but has no other consequences for present purposes. There is some loss of bone on projecting margins, but the nasal aperture

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2021

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MacPHEE ET AL.: TRIGONOSTYLOPS MORPHOLOGY AND RELATIONSHIPS 13

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2021 MacPHEE ET AL.: TRIGONOSTYLOPS MORPHOLOGY AND RELATIONSHIPS 15

and the pillarlike projections in which the tusks were socketed are relatively well preserved.

ASTRAPOTHERIID SPECIMENS. Astrapoth- erium magnum AMNH VP-9278. Although the morphological gap between Middle Eocene Trigonostylops and Early-Middle Miocene Astrapotherium is wide indeed, it is important to be able to compare similarities and differ- ences visually. To enhance our descriptions we include illustrations of the complete, but par- tially reconstructed, skull of Astrapotherium magnum AMNH VP-9278 (figs. 4, 27A) that Simpson (1933a) used in making his compari- sons (see also Scott, 1928). Many of the areas of greatest morphological interest on this speci- men are either not intact or have been heavily reconstructed (e.g., most large processes, palatal area, auditory region). Fortunately, some of these regions are adequately preserved on specimens in the MACN collection (see below).

Astrapotherium magnum MACN A 8580. This is the only specimen of Astrapotherium for which cranial CT scans are currently available (figs. 13-15). It consists of a partial adult skull lacking the entire rostrum, the right zygomatic arch, and most of the occipital condyles (last named recon- structed in plaster). The auditory region is fairly well preserved on both sides.

Astrapotherium magnum MACN A 3208. This specimen is a markedly distorted skull of a sub- adult. The basicranium, nearly complete on dis- covery, was later dissected to enable better exploration of areas of interest. The left petrosal (fig. 27B) contains the stapes (fig. 33A-D), dis- lodged into the labyrinth (figs. 31, 33E) and reconstructed via CT segmental data.

Astrapotherium guillei MAPBAR 5322, recently described by Kramarz et al. (2019a), is an almost complete but laterally compressed skull preserving most of the bones and delicate structures of the basicranium, including the cranial portion of the hyoid apparatus and paracondylar processes. Unlike other Astrapotherium specimens used for comparisons, which derive from Early Miocene (Santacrucian) beds of southern Patagonia, A. guillei is Middle Miocene (Colloncuran) in age

and the youngest representative of the genus known to date.

Parastrapotherium sp. AMNH VP-29575 is not extensively referenced in our descriptions because the cranium is considerably damaged and much of it is reconstructed. Almost the entire facial region is plaster, including the nasal aperture and the rostral end of the palate, as are the zygomatic arches and much of the basicra- nium. Known errors in reconstruction include absence of apertures (adituses) in the recon- structed retromandibular processes. These aper- tures are consistently present in better-preserved skulls of both Astrapotherium and Parastrapo- therium; absence does, however, occur in Astra- ponotus (p. 51, Interpreting Pneumatization).

Astraponotus sp. MPEF PV 1279 and MPEF PV 1084, described by Kramarz et al. (2010), were found in late Middle Eocene (Mustersan) contexts in central Patagonia. MPEF PV 1279 preserves an almost complete rostrum, the left half of the palate with partial dentition, and the braincase and zygomatic arch, but lacks the entire caudal basicranium. MPEF PV 1084 is a partial skull with nearly complete palate, basicra- nium and occiput, along with the right side of the braincase and zygoma. Together, the two specimens provide significant information on much of the cranial anatomy of Astraponotus.

Scaglia kraglievichorum MMP M-207 is a juve- nile skull described by Simpson (1957, 1967), which he considered the basalmost astrapotheriid then known. The skull is somewhat distorted, but except for the petrosals and most