F OSSIL I MPRI NT • vol. 75 • 2019 • no. 3–4 • p p. 484–503 (for merly ACTA M USEI N ATI O N ALIS PR A G AE, Series B – Historia Naturalis)

SKELETAL A NATO MY OF T HE BASICRA NIU M A N D AU DITORY REGI O N I N THE METACHEIRO MYID PALAEANODONT METAC HEIR O MYS ( MA M MALIA, PH OLID OTA M ORPHA) BASED ON HIGH-RES OLUTI ON CT SCANS

TI M O T H Y J. G A U DI N 1,* , J O H N R. WI B L E 2 , KE N NET H D. R OSE 3 , R OBERT J. E MRY 4 , MIC HELLE SPAUL DI N G 5

1 D epart ment of Biology, Geology & Environ mental Science, University of Tennessee at Chattanooga, 615 McCallie Ave, Dept. 2653, Chattanooga, T N, 37403-2598, US A; e- mail: Ti mothy- Gaudin @utc.edu. 2 S e cti o n of M a m m als, Carnegie Museu m of Natural History, 5800 Bau m Boulevard, Pitts burgh, PA, 15206, US A; e- m ail: wi bl ej @ c ar n e gi e m n h. or g. 3 C enter for Functional Anato my and Evolution, The Johns Ho pkins University School of Medicine, Balti more, M D, 21205, US A; e- m ail: k dr os e @j h mi. e d u. 4 D e p art m e nt of P al e o bi ol o g y, M R C 1 2 1, N ati o n al M us e u m of N at ur al Hist or y, S mit hs o ni a n I nstit uti o n, P. O. B o x 3 7 0 1 2, W as hi n gt o n, D. C., 2 0 0 1 3, U S A; e- m ail: e mr yr @si. e d u. 5 P ur d u e N ort h w est U ni v ersit y, S c h w ar z H all, 1 4 0 1 S. U.S. 4 2 1, W est vill e, I N, 4 6 3 9 1, U S A; e- m ail: ms p a ul di @ p n w.e d u. * c or res pon din g author

G a u di n, T. J., Wi bl e, J. R., R os e, K. D., E mr y, R. J., S p a ul di n g, M. ( 2 0 1 9): S k el et al a n at o m y of t h e b asi cr a ni u m a n d a u dit or y r e gi o n i n t h e m et a c h eir o m yi d p al a e a n o d o nt Metacheiro mys ( Ma m malia, Pholidota morpha) based on high-resolution C T scans – Fossil I mprint, 75(3-4): 484–503, Praha. ISS N 2533–4050 (print), ISS N 2533–4069 (on-line).

A bstract: Cr a ni al s k el et al m at eri al of t h e E o c e n e p al a e a n o d o nt Metacheiro mys marshi was exa mined using high-resolution C T s c a ns. T h e pr es e nt st u d y r e pr es e nts t h e frst ti m e t h at C T s c a ns h a v e b e e n c o n d u ct e d o n s k ulls of t his e xti n ct f oss ori al m a m m al. The bony osteology of the auditory region is described in detail, including the ectoty mpanic and entoty mpanic, the petrosal i n b ot h ty m p a ni c a n d e n d o cr a ni al vi e ws, a n d th e mi d dl e e ar ossi cl es. T h e res ults of this in v esti g ati o n c o n fr m a n u m b er of derived rese mblances bet ween palaeanodonts and xenarthrans, including a large entoty mpanic ele ment in the medial wall of the auditory bulla, the presence of an anteroventral process of the teg men ty mpani, and a postte mporal canal. Ho wever, the present study also provides novel derived auditory features linking palaeanodonts and pangolins, consistent with current understanding of palaeanodont phylogenetic relationships, including the absence of an ectoty mpanic stylifor m process, a posterolaterally oriented aperture to the cochlear fossula, and a convex mallear head / concave incudal head. Several autapo morphic features c h ar a ct eri zi n g t h e a u dit or y ost e ol o g y of Metacheiro mys are also noted. The presence of a large, spherical mallear head, and of a c a p a ci o us t y m p a ni c c a vit y e xt e n d e d i nt o si n us es i n s urr o u n di n g b o n es, li k el y r e pr es e nt a d a pt ati o ns f or f oss ori alit y, c o nsist e nt with palaeobiological inferences dra wn fro m the postcranial anato my of Metacheiro mys . Key wor ds: Metacheiro mys , Palaeanodonta, P h oli d ot a m or p h a, a u dit or y r e gi o n, ost e ol o g y, s k ull, mi d dl e e ar ossi cl es

R e c eiv e d: A pril 4, 2 0 1 9 | A c c e pt e d: S e pt e m b er 1 7, 2 0 1 9 | Iss u e d: D e c e m b er 3 0, 2 0 1 9

Introduction most recently by a co mprehensive analysis of placental phylogeny based on molecular and morphological data Palaeanodonta is a relatively s mall, extinct group of ( O’ L e ar y et al. 2 0 1 3). fossorial, likely myr mecophagous ma m mals with strongly Partially due to their reduced dentitions, palaeanodonts reduced dentitions ( Rose 2006). They are restricted are unco m mon fossils, although many of the taxa are kno wn te mporally to the Paleogene, and are kno wn geographically across the Laurasian continents, though the best records fro m skulls and partial skeletons, perhaps because of their derive fro m western North A merica ( Rose 2006, 2008). burro wing habitus ( Rose 2008). The best kno wn taxon, Pr e vi o us w or k v ari o usl y alli e d t his e xti n ct cl a d e wit h eit h er the middle Eocene metacheiro myid palaeanodont genus the extant placental order Pholidota (the pangolins), the Metacheiro mys , is represented by t wo different species ( M. extant clade Xenarthra (ar madillos, anteaters, and sloths), m ars hi a n d M. dasypus ; Rose 2008) and multiple, well- or b ot h i n t h e n o w d ef u n ct “ E d e nt at a ”, b ut a c o ns e ns us h as preserved skeletons and skulls. The cranial anato my was e merged that they are more closely related to pangolins described in detail in Si mpson’s (1931) co mprehensive ( Rose et al. 2005, Gaudin et al. 2009). This is supported monograph. The ear region and basicranial anato my in

D OI 10.2478/if-2019-0029 4 8 4 Te xt-fi g. 1. a, c – Metacheiro mys marshi , A M N H 1 3 1 7 7 7, s k ull i n v e nt r al, ri g ht l at e r al vi e w; b – Metacheiro mys marshi , U S N M- P 452349, skull in ventral vie w. Abbreviations: pa – porus acousticus, ptf – postte mporal fora men. particular were also described by Patterson et al. (1992), i n R os e a n d E mr y 1 9 9 3: f g. 7. 1), alt h o u g h s o m e of th es e using Si mpson’s speci mens and a ne wer speci men fro m ne wer speci mens have been scored in phylogenetic studies the U.S. National Museu m that retained preserved ear that dra w on details fro m the cranial anato my of this taxon ossicles. Additional, and in so me cases more co mplete (e.g. Gaudin 1995, Gaudin and Wible 1999, Gaudin et al. and better preserved cranial material of Metacheiro mys is 2 0 0 9, O’ L e ar y et al. 2 0 1 3). T h e pr es e nt st u d y will f o c us o n h o us e d i n s e v er al m aj or U. S. m us e u m c oll e cti o ns, b ut m ost o n e s u c h s p e ci m e n, a n e arl y c o m pl et e s k ull a n d s k el et o n of of this material re mains undescribed (e.g. skull illustrated M. m ars hi fro m the A merican Museu m of Natural History.

4 8 5 Although much anato mical detail fro m Metacheiro mys material is less co mplete than that of Metacheiro mys . has been recorded through traditional methods of anato mical Because of the hypothesized sister-group relationship st u d y a n d d es cri pti o n, li k e a n y f ossil t a x o n, s o m e d at a r e m ai n bet ween palaeanodonts and pangolins, we have also used i n a c c essi bl e t o s u c h m et h o ds, i n p art b e c a us e of i n c o m pl et e the latter in our co mparisons belo w, both extant taxa and preservation and post morte m distortion, but also because the latest Eocene pangolin Patrio manis a mericana , which of inco mplete preparation and the dif fculty of preparing has been described in detail by Gaudin et al. (2016), and delicate skull structures and interior skull spaces. Modern is the only fossil pangolin represented by a well-described C T-s c a n ni n g t e c h n ol o g y all o ws f or t h e e xtr a cti o n of d et ail e d and illustrated auditory and basicranial region. In addition, i nf or m ati o n fr o m f ossils t h at is n ot t y pi c all y a v ail a bl e fr o m because of the previously hypothesized link bet ween m or e tr a diti o n al m et h o ds of e x a mi n ati o n, i n cl u di n g t h e n as al palaeanodonts and xenarthrans (Si mpson 1931), and in cavity and its delicate nasal turbinate bones (e.g. Fernicola particular the rese mblances noted bet ween these for ms in et al. 2012, Van Valkenburgh et al. 2014), braincase and their auditory osteology (see Patterson et al. 1992, Gaudin cr a ni al si n us es ( e. g. Bill et et al. 2 0 1 7, B os c ai ni et al. 2 0 1 8), 1995), we have also e mployed xenarthrans for co mparative and the inner ear (e.g. Billet et al. 2015). These data can purposes, in particular the extant ar madillos Dasypus and provide ne w insights not only for co mparative anato my and Euphractus , whose auditory and basicranial anato my is phylogeny, but also the functional morphology of extinct particularly well kno wn ( Wible and Gaudin 2004, Wible organis ms. Previously, no C T scans of palaeanodont skulls 2010), and is si milar to that of palaeanodonts in a nu mber have been published. The goal of the present study is to of r es p e cts. collect and analyze high-resolution C T-scans of several s k ulls of Metacheiro mys , in order to better assess the bony I nstit utio nal a b breviatio ns a n at o m y of its b asi cr a ni u m a n d a u dit or y r e gi o n, a n d t o b ett er A MNH A merican Museu m of Natural History, Ne w understand the phylogenetic, evolutionary, and functional Yor k, N Y, U S A i m pli c ati o ns of t his a n at o m y. C M Carnegie Museu m of Natural History , Pitts b ur g h, P A, U S A US N M-P P al e o m a m m al c oll e cti o n, U. S. N ati o n al M us e u m Materials and methods of N at ur al Hist or y, S mit hs o ni a n I nstit uti o n, T wo speci mens of Metacheiro mys were micro CT- Was hi n gt o n, D. C., U S A scanned and digitally reconstructed: Metacheiro mys UT C M U ni versit y of Tennessee at Chattanooga Natural m ars hi , A M N H 131777, and M. m ars hi , US N M-P 452349 Hist or y M us e u m, C h att a n o o g a, T N, U S A ( Text- fg. 1). These skulls were manually co mpared to YP M-P U Princeton University Collection, Yale Peabody other well-preserved Metacheiro mys s k ulls, i n cl u di n g M. Museu m, Ne w Haven, CT, US A m ars hi , US N M-P 26132, M. m ars hi , A M N H 12560, and Metacheiro mys s p., Y P M- P U 1 8 1 0 7. All f v e s k ulls d eri v e fro m the Bridger For mation of Wyo ming ( Bridgerian Descri ptio n N A L M A, middle Eocene, 46–50 mya). T h e frst s k ull, A M N H 1 3 1 7 7 7, w as C T-s c a n n e d wit h a Overvie w 2010 G E phoenix nano-focus v|to me|x s240 at the A merican Metacheiro mys h as a f ull y ossi f e d a u dit or y b ull a Museu m of Natural History’s Microscopy and I maging including a tubular external acoustic meatus that nearly Facility ( MIF). We used a bea m voltage of 225k V and reaches the lateral margin of the skull ( Text- fgs 1, 2). The produced 1559 16-bit TIFF i mages, each representing 0.04 pri mary ele ment of the bulla and tubular meatus is the milli m et ers of s p e ci m e n t hi c k n ess. ectoty mpanic. Si mpson (1931) and Patterson et al. (1992) The second speci men, US N M-P 452349, was CT- agreed that another ele ment, the entoty mpanic, contributed scanned at the University of Chicago Microscopy and t o t h e b ull a, b ut t h e y dis a gr e e d a b o ut t h e si z e of t h at el e m e nt. I maging Facility, also using the 2010 G E phoenix nano- Si mpson (1931) restricted the entoty mpanic to the medial focus v|to me|x s240k V scanner. Voltage was at 190k V and and antero medial walls of the bulla, whereas Patterson et 2021 16-bit TI F F i mages were generated, each representing al. (1992: 58) extended the entoty mpanic into the posterior 0. 0 2 6 milli m et ers of t hi c k n ess. w all, alt h o u g h n oti n g t h er e w er e o nl y “ v a g u e i n di c ati o ns of All slices for both speci mens were converted to 8-bit a di visi o n b et w e e n e nt ot y m p a ni c a n d m ast oi d i n t his ar e a ”. using Adobe Photoshop, cropped and further processed The difference in opinion highlights the proble m that parts ( bri g ht n ess a n d c o ntr ast) wit h I M A G EJ, a n d t h e n i m p ort e d of s e v er al b asi cr a ni al el e m e nts ar e f us e d i n Metacheiro mys , into AVIZ O 9.0 for reconstruction, seg mentation, and as is oft e n t h e c as e a m o n g a d ult e xt a nt m a m m als. a n al ysis. All s e g m e nt ati o n w as d o n e b y h a n d, sli c e b y sli c e, Although C T data have provided a ne w perspective on exa mining the material in all three directional planes, with t his pr o bl e m, t h e y h a v e n ot mir a c ul o usl y r es ol v e d t h e li mits n o i nt er p ol ati o n utili z e d. of the basicranial bones in the areas of fusion noted by A wide variety of taxa are e mployed here for co mparative previous authors. For positing osseous boundaries based on p ur p os es. T h er e is i nf or m ati o n o n t h e a u dit or y ost e ol o g y of textural differences and morphology in the areas of fusion, a so me what older metacheiro myid palaeanodont, the late w e r el y m or e o n t h e st u d y of A M N H 1 3 1 7 7 7 u n d er t h e Paleocene through early Eocene genus Palaeanodon ( R os e microscope, and not the C T scans. Regarding the auditory 2008), which was described and illustrated by Matthe w b ull a, s ut ur es o n t h e ri g ht si d e of A M N H 1 3 1 7 7 7 c o n fr m t h e (1918) and Patterson et al. (1992), although the available c o m p ositi o n of t h e b ull a i n t h e a nt eri or a n d n e arl y t h e e ntir e

4 8 6 Te xt-fi g. 2. Metacheiro mys marshii , A M N H 131777, dra wing of basicraniu m in ventral vie w with isosurface fro m C T scans of left petrosal inserted (co mpare with Si mpson 1931: fig. 7). Much of the mastoid exposure on the speci men’s left side is da maged. Nu mbers 1 to 4 indicate depressions that based on the right side include a thin layer of entoty mpanic; 1 to 3 are bet ween petrosal a n d basiocci pital a n d 4 is petrosal o nly. T he w hite arro w i n t he lo wer left passes t hro ug h a ca nal bet wee n t he petrosal a n d exocci pital for the auricular branch of the vagus nerve. Abbreviations: ab X – grooves and fora mina for auricular branch of vagus nerve, as – alisphenoid, astp – alisphenoid ty mpanic process, bo – basioccipital, bs – basisphenoid, ea m – roof of external acoustic meatus, ec – ectoty mpanic, en – entoty mpanic, eo – exoccipital, es – epity mpanic sinus of squa mosal, f m – fora men magnu m, fo – fora men ovale, gf – glenoid fossa, hf – hypoglossal fora men, ips – fora men for inferior petrosal sinus, ljf – lateral jugular fora men, me – mastoid exposure of petrosal, mjf – medial jugular fora men, mt – muscular tubercle, mtc – musculotubal canal, oc – occipital condyle, pa – porus acousticus (hidden), pas – parasphenoid, pgp – postglenoid process, pr – pro montoriu m of petrosal, ps – presphenoid, s mf – stylo mastoid fora men, sof – superior orbital fissure, sq – squa mosal, tca – ty mpanic canaliculus, th – ty mpanohyal, t m – tubular external acoustic meatus. medial wall; the proble m is the posterior wall where there sinus. Second, there are an excessive nu mber of fora mina ar e ar e as of n o s ut ur es or o nl y p arti al s ut ur es t h at c a n n ot b e a n d c a n als i n t h e b asi cr a ni u m, m or e t h a n w e h a v e t y pi c all y fully traced. The issues of fusion in the bulla also confront encountered in other ma m mals. Identifying all of these has the pri mary ele ments of the ty mpanic roof, the petrosal b e e n c h all e n gi n g. and squa mosal, mainly in the posterolateral corner of the One observation in A M N H 131777 and US N M-P basicraniu m. T wo other issues co mplicate our understanding 452349 outside the ear region that we want to record of t h e b asi cr a ni u m i n Metacheiro mys . First, t h e p etr os al a n d concerns a lo w midline crest on the anterior aspect of the squa mosal have been highly pneu matized, resulting in the ventral surface of the basisphenoid ( Text- fg. 2). This crest for mation of large accessory air cha mbers in both bones, a w as als o ill ustr at e d o n t h e t y p e of Metacheiro mys dasypus , squa mosal epity mpanic sinus ( Text-fgs 2, 3) and a mastoid A M N H 11718, by Si mpson (1931: fg. 5), and is present in

4 8 7 Te xt-fi g. 3. Metacheiro mys marshi , US N M-P 452349, coronal sections fro m C T scans. a – section 590 of 2020 through the anterior most ty mpanic cavity sho wing air spaces in the entoty mpanic and squa mosal; b – section 898 of 2020 at level of the fenestra vestibuli sho wing the mastoid sinus. Abbreviations: bo – basioccipital, bs – basisphenoid, cp – crista parotica, ec – ectoty mpanic, en – entoty mpanic, es – epity mpanic sinus of squa mosal, fv – fenestra vestibuli, hyf – hypophyseal fossa, m – malleus, ms – mastoid sinus, pr – pro montoriu m, sq – squa mosal, tc – ty mpanic cavity.

4 8 8 M. m ars hi , U S N M- P 2 6 1 3 2. N ot hi n g i n t h e C T s c a ns of t h e Al o n g its a nt er ior border, the ectoty mpanic in A M N H t wo speci mens studied here suggests that this is a separate 1 3 1 7 7 7 is d eli mit e d fr o m m e di al t o l at er al b y s ut ur es fr o m ele ment. Nevertheless, one of the authors of this report the entoty mpanic, alisphenoid, and squa mosal, although has recently published on the occurrence of a little kno wn in the anterolateral corner the ectoty mpanic is fused to the midline bone called the parasphenoid that occurs in so me postglenoid process of the squa mosal. The antero medial e xt a nt m a m m als a n d is t h o u g ht t o b e pr es e nt i n s o m e e xti n ct m ar gi n of t h e e ct ot y m p a ni c is r o u n d e d, wit h o ut a st ylif or m ma m mals ( Wible et al. 2018). We suggest that this midline pr o c ess, a r es e m bl a n c e t o e xt a nt p a n g oli ns ( e. g. P h at a gi n us cr est i n Metacheiro mys li k el y is a p ar as p h e n oi d f us e d t o t h e t etr a d a ct yl a , C M 7 3 8 4; S m utsi a t e m mi n c kii ; Gaudin and basisphenoid. Wi bl e 1 9 9 9: f g. 4) b ut u nli k e ar m a dill os (s e e Wi bl e 2 0 1 0: f gs 8, 9) a n d ot h er x e n art hr a ns ( P att ers o n et al. 1 9 9 2). Al o n g Ectoty mpanic its m e di al b or d er, t h e e ct ot y m p a ni c i n A M N H 1 3 1 7 7 7 is The right auditory bulla is well preserved in both deli mited by a roughly horizontal suture fro m the ventral A M N H 131777 and US N M-P 452349, but the left has been m ar gi n of t h e e nt ot y m p a ni c ( Te xt- f g. 4), e x c e pt at t h e largely re moved in both speci mens to reveal the interior of postero medial corner where no suture is evident, although t h e t y m p a ni c c a vit y ( Te xt- f g. 1). R e g ar di n g t h e e xt eri or, t e xt ur al diff er e n c es s u g g est t h e c o nti n u ati o n of t h e s ut ur e i n Si mpson (1931) noted the “ fask-shaped” auditory bulla, t h e s a m e h ori z o nt al pl a n e. I n t h e l at er al p art of t h e p ost eri or r o u n d e d a n d i n f at e d m e di all y, a n d e xt e n di n g l at er all y w all, p arti al s ut ur es a p p e ar t o d eli mit t h e r e ar of t h e m e at al i nt o a lo n g, tu b ul ar e xt er n al a c o usti c m e at us. T h e in f at e d tube fro m ele ment(s) that either support it or abut it; the portion of the bulla is longer than it is wide, thus differing i d e ntit y of t h e el e m e nt(s) is c o nsi d er e d b el o w. I n t h e m e di al fro m the more spherical shape co m mon a mong many p art of t h e p ost eri or w all, s ut ur es ar e n ot e vi d e nt, a n d w e ar e ar madillos ( Patterson et al. 1989) and those pangolins with uncertain if the ectoty mpanic has a direct contact with the a n i n f at e d e ct ot y m p a ni c ( e. g. S m utsi a t e m mi n c kii ; Gaudin p etr os al or if a sli v er of e nt ot y m p a ni c i nt er v e n es. a n d Wi bl e 1 9 9 9: f g. 4). T h e e ct ot y m p a ni c is t h e pri m ar y The lateral opening to the tubular external auditory ele ment of the bulla, as noted by Si mpson (1931) and meatus see ms rather s mall relative to the size of the skull, Patterson et al. (1992), and accounts for nearly all of the but it is actually larger in proportion than in either the t y m p a ni c f o or th at is visi bl e in v e ntr al vi e w ( Te xt- f g. 2). e xt a nt p a n g oli n P h at a gi n us tri c us pis or t h e e xt a nt ar m a dill o

Te xt-fi g. 4. Metacheiro mys marshii , A M N H 131777, basicranial isosurface fro m C T scans in oblique posteroventral vie w: left petrosal in blue and s mall piece of left entoty mpanic in red. The white arro w on the left petrosal passes through a canal traversed by the ty mpanic nerve. Abbreviations: ab X – fora men for auricular branch of vagus nerve, aptt – anteroventral process of teg men ty mpani, as – alisphenoid, bo – basioccipital, bs – basisphenoid, ctp – caudal ty mpanic process, ea m – squa mosal roof of external acoustic meatus, ec – ectoty mpanic, en – entoty mpanic, eo – exoccipital, hf – hypoglossal fora men, ips – fora men for inferior petrosal sinus, ljf – lateral jugular fora men, me – mastoid exposure of petrosal, mjf – medial jugular fora men, mt – muscular tubercle, oc – occipital condyle, pcf – posterior carotid fora men, pp – paroccipital process of petrosal, pr – pro montoriu m of petrosal, ptp – postty mpanic process of squa mosal, s mf – stylo mastoid fora men, sq – squa mosal, stf – stapedial artery fora men, tca – ty mpanic canaliculus, th – ty mpanohyal.

4 8 9 Table 1. Select skull measure ments ( m m) of Metacheiro mys marshi ( A M N H 13177) and co mparative taxa. Abberviations: ln – a ntero posterior le ngt h, d p – vertical de pt h.

Metacheiro mys marshi P h at a gi n us tri c us pis Euphractus sexcinctus Measure ments A M N H 131777 UT C M 1695 UT C M 1491 S k ull Gr e at est s k ull l e n gt h 6 4. 6 * 7 2. 1 1 1 9. 2 Por us ac ustic us m a xi m u m l n 3. 3 3. 8 4. 4 m a xi m u m d p 2. 0 1. 7 4. 2 r ati o l n/ d p 1. 7 2. 2 1. 1 * missi n g pr e m a xill a

Euphractus sexcinctus ( Tab. 1). It is ovate in shape, ( Te xt- f g. 3 a), a g ai n as in Euphractus sexcinctus , wit h t h e el o n g at e d a nt er o p ost eri orl y ( Te xt- f g. 1), as i n t h e f or m er e nt ot y m p a ni c li ni n g f o ur cir c ul ar d e pr essi o ns i n t h e m e di al t a x o n a n d i n c o ntr ast t o t h e r o u n d o p e ni n g f o u n d i n t h e l att er. aspect of the pro montoriu m of the petrosal (see petrosal A d diti o n all y, it pr oj e cts f art h er l at er all y at its a nt eri or m ar gi n b el o w). t h a n its p ost eri or o n e, gi vi n g it a sli g htl y o bli q u e ori e nt ati o n At the antero medial aspect of the bulla in ventral vie w i n v e ntr al vi e w ( Te xt- f g. 2). Fr o m t h e C T s c a ns of b ot h i n A M N H 1 3 1 7 7 7 is a tri a n g ul ar s h a p e d e x p os ur e of t h e A M N H 131777 and US N M-P 452349, the bone in the foor e nt ot y m p a ni c, w hi c h a b uts t h e b asis p h e n oi d a nt er o m e di all y of t h e t u b ul ar m e at us is as m u c h as 4. 5 ti m es t hi c k er t h a n t h at a n d t h e alis p h e n oi d a nt er ol at er all y ( Te xt- f g. 2). Alt h o u g h i n t h e t y m p a ni c f o or. Als o, t h e i nt er n al att a c h m e nts of t h e the bone appears thick fro m the exterior, the C T scans reveal ty mpanu m, the crista and sulcus ty mpanica, are positioned t h at this p art of th e e nt ot y m p a ni c is h oll o w ( Te xt- f g. 3 a), where the thick tube meets the la minar foor and result in a a c c o m m o d ati n g a n a c c ess or y air s p a c e c o n f u e nt wit h t h e m e m br a n e wit h a n o bli q u e ori e nt ati o n, 5 0° t o t h e h ori z o nt al. ty mpanic cavity posteriorly. The anterolateral margin of the entoty mpanic here slopes dorsolaterally and ends at Entoty mpanic an aperture bet ween the entoty mpanic, ectoty mpanic, and Patterson et al. (1992), Gaudin (1995), and others (e.g. alisphenoid. This opening is for the musculotubal canal Gaudin et al. 1996, Gaudin 2004, Rose et al. 2005) have (= Eustachian or pharyngoty mpanic tube), which connected noted the strong rese mblance bet ween the entoty mpanic of with the nasopharynx by extending anteroventro medially Metacheiro mys and many living xenarthrans, in particular a cr oss t h e e nt ot y m p a ni c a n d alis p h e n oi d, t h e l att er s h o wi n g the ar madillos (Patterson et al. 1989, Wible and Gaudin s o m e c o n c a vit y f or t h e t u b e ( Te xt- f g. 2). It s h o ul d b e n ot e d 2004), and the strongly contrasting morphology of the here that Gaudin (1995) mistakenly coded the musculotubal ele ment when present in living pangolins, which tends to canal as lying bet ween ectoty mpanic, entoty mpanic and b e s m all a n d n o d ul ar ( G a u di n a n d Wi bl e 1 9 9 9). As i n s o m e pt er y g oi d i n Metacheiro mys . ar m a dill os ( e. g. Euphractus sexcinctus ; Wible and Gaudin As noted above, it is the co mposition of the posterior 2 0 0 4), t h e e nt ot y m p a ni c is t h e m aj or el e m e nt i n t h e m e di al b ull ar w all t h at w as c o nt e nti o us wit h pri or a ut h ors: Si m ps o n wall of the ty mpanic cavity of Metacheiro mys ( Te xt- f gs 3, (1931) excluded the entoty mpanic fro m the posterior wall 4), in which the C T scans of A M N H 131777 and US N M-P and Patterson et al. (1992) extended the entoty mpanic into 4 5 2 3 4 9 r e v e al it is a l a mi n ar str u ct ur e w e d g e d b et w e e n t h e t h e p ost eri or w all as f ar as t h e t y m p a n o h y al, w hi c h is sit u at e d ectoty mpanic ventrally and the basioccipital and petrosal r o u g hl y i n t h e p ost eri or w all’s mi d p oi nt ( Te xt- f gs 2, 4). We dorsally. The entoty mpanic is pri marily concave medially ar e n ot c o n vi n c e d t h at eit h er vi e w is c orr e ct, wit h t h e c a v e at and convex laterally. Euphractus sexcinctus sho ws the that due to the lack of co mplete sutures, our interpretation r e v ers e. S ut ur es d eli mit t h e e nt ot y m p a ni c i n t h e m e di al w all belo w is taken as tentative until additional infor mation is i n A M N H 1 3 1 7 7 7 e x c e pt p ost eri orl y, w h er e t h e b o n e li es f ort h c o mi n g. b et w e e n t h e e ct ot y m p a ni c a n d p etr os al. I n t h e a bs e n c e of a F or d es cri pti v e p ur p os es, w e di vi d e t h e p ost eri or w all i n s ut ur e h er e, w e disti n g uis h t h e e nt o- a n d e ct ot y m p a ni c b y t h eir A M N H 1 3 1 7 7 7 int o m e di al a n d lat er al se g m e nts wit h th e different surface textures, rough and s mooth, respectively. A ty mpanohyal the divider. In the medial seg ment, there is co mparable surface textural difference does not distinguish littl e t o p o gr a p h y t o disti n g uis h el e m e nts; a f ai nt, h ori z o nt al the entoty mpanic and petrosal, but there is an oblique line sea m, in the sa me plane as the suture bet ween the ecto- of i n f e cti o n b et w e e n w h at w e i d e ntif y as a d ors all y c o n v e x and entoty mpanic in the medial wall on the right side, petrosal and ventrally concave entoty mpanic. This line may deli mit t wo ele ments, presu mably ectoty mpanic and p ass es t hr o u g h a s m all a p ert ur e i n t h e b ull a i d e nti f e d b el o w p etr os al, b ut w e c a n n ot r ul e o ut t h e p ossi bilit y t h at a sli v er of as the posterior carotid fora men, which we reconstruct t h e e nt ot y m p a ni c i nt er v e n es b et w e e n t h e t w o. I n t h e l at er al b et w e e n t h e t w o b o n es ( Te xt- f g. 4). If o ur i nt er pr et ati o n s e g m e nt, as s e e n o n t h e ri g ht si d e, a s m all, di gitif or m pi e c e of the posterior part of the medial bulla wall is correct, the of bone of rougher texture than its neighbors abuts the back entoty mpanic tapers in height posteriorly and the petrosal of t h e t u b ul ar m e at us wit h s o m e str et c h es of p ot e nti al s ut ur es d o es t h e r e v ers e, t a p eri n g i n h ei g ht a nt eri orl y. T h e C T s c a ns bet ween the t wo. This digitifor m piece in turn underlies a of t h e t w o Metacheiro mys als o s h o w t h at t h e d ors al m ar gi n broader, s moother bone dorsally that is certainly petrosal, of the entoty mpanic in the medial wall is infolded laterally a g ai n wit h f ai nt i n di c ati o ns of p ot e nti al s ut ur es b et w e e n t h e

4 9 0 Te xt-fi g. 5. Metacheiro mys marshi , A M N H 131777, left petrosal isosurface fro m C T scans in ty mpanic vie w; the postty mpanic process of the squa mosal and so me possible entoty mpanic are also included. a – isosurface; b – line dra wing with labels. Nu mbers 1 to 4 indicate depressions on medial flange. Abbreviations: a ms – antero medial septu m, aptt – anteroventral process of teg men ty m pa ni, cct – ca nal for c hor da ty m pa ni nerve, ci – crista i nterfe nestralis, cof – coc hlear foss ula, c p – crista parotica, ct p – ca u dal ty mpanic process, en? – possible entoty mpanic, epc – epity mpanic crest, e w – epity mpanic wing, fc – facial canal, fv – fenestra

4 9 1 t w o. O n t h e l eft si d e w h er e m ost of t h e b ull a w as r e m o v e d, mediolaterally and thickest dorsoventrally at the posterior this sliver of bone is exposed lateral to the ty mpanohyal. e n d, a n d t a p ers i n b ot h di m e nsi o ns t o w ar ds t h e a nt eri or p ol e Given the textural differences and the hints of sutures, this of the pro montoriu m. Unlike Palaeanodon or ar madillos, sli v er of b o n e is li k el y t h e e nt ot y m p a ni c. Alt h o u g h it s e e ms m ar ki n g t h e v e ntr al s urf a c e of t h e m e di al f a n g e a n d m e di al likely that the entoty mpanic in the lateral seg ment of the surface of the pro montoriu m are four large, roughly circular posterior wall connected to that in the medial wall, we are depressions, the anterior three of which extend medially c urr e ntl y u n a bl e t o c o n fr m t h at. L at er al t o t h e st yl o m ast oi d onto the basioccipital, the posterior most being entirely on fora men in the lateral seg ment of the posterior wall is a the petrosal ( Text- fgs 2, 5). Based on the C T scans of the s mall, anteriorly directed fora men on both sides of A M N H right sides of A M N H 131777 and US N M-P 452349 where 131777. On the left side, this fora men can be traced into a the bulla is in place, these four depressions are lined by a short canal that we interpret for the chorda ty mpani nerve t hi n l a y er of e nt ot y m p a ni c, w hi c h i n t ur n r o ofs a c c ess or y air ( Te xt- f g. 5). If t h e oss e o us el e m e nts ar e i d e nti f e d c orr e ctl y s p a c es of t h e t y m p a ni c c a vit y. At t h e a nt eri or a n d p ost eri or h er e, t h e c o urs e of t his c a n al is f ull y or p arti all y i n t h e s e a m ends of this ro w of four depressions are narro w septa. For b et w e e n t h e e nt ot y m p a ni c a n d p etr os al. descriptive purposes, based on their positions, we identify these as the antero medial and postero medial septa. Petros al Extending for ward fro m the anterior pole of the Usi n g A M N H 1 3 1 7 7 7, b ot h th e a ct u al a n d virt u al ( C T pr o m o nt ori u m a n d l at er al t o t h e a nt er o m e di al s e pt u m is t h e scans) fossil, we seg mented the left petrosal in Avizo for epity mpanic wing, which is not horizontal but do wnturned descriptive and illustrative purposes; this is the side where a nt eri orl y ( Te xt- f g. 5). T his ar e a is q uit e br o k e n i n A M N H most of the auditory bulla had been re moved prior to our 1 3 1 7 7 7 a n d, t h er ef or e, w e ar e n ot e ntir el y c ert ai n of t h e study ( Text- fgs 2, 4). The areas of greatest uncertainty (and anterior li mit of the epity mpanic wing. I m mediately lateral t h er ef or e le ast c o n f d e n c e) f or d eli miti n g t h e p etr os al ar e to the antero medial septu m is an oval depression, longer antero medially where nu merous cracks are present, laterally than wide, that includes the carotid fora men (see belo w). w h er e m ost of t h e s ut ur es wit h t h e s q u a m os al ar e f us e d, a n d Lateral to that is the anterior end of a distinct groove that posteroventrally where an entoty mpanic may or may not r u ns p ost er ol at er all y t o a f or a m e n, t h e hi at us F all o pii, f or t h e have been present. greater petrosal nerve ( Evans 1993, Wible 2010), a branch T h e t h eri a n p etr os al is g e n er all y s u b di vi d e d i nt o t h e p ars of t h e f a ci al n er v e; t his n er v e l eft t h e t y m p a ni c c a vit y vi a t h e cochlearis (for the cochlear duct and saccule) and the pars m us c ul ot u b al c a n al. Palaeanodon s p. h a d a m or e e xt e nsi v e canalicularis (for the utricle and se micircular canals). As epity mpanic wing than Metacheiro mys (see Patters o n et al. usual, the ventral (ty mpanic) surface of the pars cochlearis 1 9 9 2: f g. 1 8 A, B), w h er e as t h at i n P atri o m a nis was much i n A M N H 1 3 1 7 7 7 is d o mi n at e d b y th e pr o m o nt ori u m, th e l ess e xt e nsi v e (s e e G a u di n et al. 2 0 1 6: f g. 7). A d diti o n all y, cochlear housing ( Text- fg. 5). Fro m the C T scans, the in both Metacheiro mys a n d Palaeanodon , the epity mpanic cochlear duct coils through 450° (follo wing the method wing is continuous laterally with the teg men ty mpani (see in Ekdale 2013). Although often in the sa me horizontal b el o w); t his is n ot t h e c as e i n P atri o m a nis or li vi n g p a n g oli ns plane in ma m mals, the pro montoriu m in A M N H 131777 is ( e. g. P h at a gi n us tri c us pis , C M 5 7 8 3 3, M a nis j a v a ni c a , C M positioned such that its rear is ventral to the plane of the 40597). basioccipital whereas its anterior pole is well dorsal to that One of the more note worthy discoveries of the present pl a n e. As n ot e d b y P att ers o n et al. ( 1 9 9 2), t h e pr o m o nt ori u m study is the presence in Metacheiro mys of a s mall groove is r o u n d e d a n d gl o b os e, m u c h m or e s o t h a n t h at of t h e ol d er on the pro montoriu m for the internal carotid artery and m et a c h eir o m yi d Palaeanodon , b ut l ess t h a n t h at of t h e l at est a c c o m p a n yi n g n er v e ( Te xt- f g. 5). P att ers o n et al. ( 1 9 9 2) Eocene pangolin P atri o m a nis ( G a u di n et al. 2 0 1 6). H o w e v er, cl ai m e d t h at s u c h a gr o o v e w as missi n g, d es pit e t h e pr es e n c e t h e f or m er a ut h ors als o st at e d t h at “t h er e is n o i n di c ati o n of of a strong transpro montorial groove in the generally more shelving around the pro montoriu m, as occurs in ar madillos” plesio morphic and older Palaeanodon . They likely missed ( Patterson et al. 1992: 58). The C T scans reveal that this is the presence of the groove because it is very shallo w for n ot e x a ctl y t h e c as e, as t h er e is b ot h a m e di al f a n g e a n d a n m ost of its l e n gt h, o nl y b e c o mi n g b ett er d e f n e d i n its e pit y m p a ni c wi n g. posterior most reaches, medial to the cochlear fossula. This The medial fange extends the anteroposterior length of deeper area was likely either covered by matrix or the the pars cochlearis ( Text- fg. 4) and is more developed than entoty mpanic in previous studies. At the postero medial that found in the ar madillo Dasypus nove mcinctus ( Wi bl e corner of the pro montoriu m, the transpro montorial groove 2010), though perhaps not quite as extensive as that of the runs onto the ventral surface of the postero medial septu m, gi a nt ar m a dill o Priodontes maxi mus ( P att ers o n et al. 1 9 8 9) partially and then fully enclosed in a canal that opens at a or t h at of Palaeanodon ( P att ers o n et al. 1 9 9 2). P atri o m a nis s mall posterior carotid fora men in the bulla wall, which a p p e ars t o l a c k a m e di al f a n g e e ntir el y ( G a u di n et al. we interpret as bet ween the entoty mpanic and petrosal. 2 0 1 6). T h e m e di al f a n g e i n A M N H 1 3 1 7 7 7 is at its wi d est A nt eri orl y, t h e tr a ns pr o m o nt ori al gr o o v e is m or e diff us e, b ut

vestibuli, gica – groove for transpro montorial internal carotid artery, gsa – groove for stapedial artery, h F – hiatus Fallopii, me – mastoid exposure, mf – medial flange, pcf – posterior carotid fora men, p ms – postero medial septu m, pp – paroccipital process, pr – pro montoriu m, ptp – postty mpanic process of squa mosal, sf – stapedius fossa, sff – secondary facial fora men, s mf – stylo mastoid fora men, stf – stapedial artery fora men, tca – ty mpanic canaliculus, th – ty mpanohyal, to cf – to carotid fora men, ttf – tensor ty mpani fossa.

4 9 2 appears to pass lateral to the anteromedial septum towards the fenestra cochleae, and this aperture in the posterior the oval depression on the epitympanic wing. In the CT wall of the promontorium. The aperture is positioned scans of both specimens, the bone in this oval depression is near the posterolateral corner of the promontorium and is damaged, obscuring any evidence for the further course of directed posterolaterally, a resemblance to Palaeanodon sp. the internal carotid on the petrosal. However, in the CT scans (Patterson et al. 1992: fg. 18), Patriomanis (Gaudin et al. of USNM-P 452349, anterodorsal to this part of the petrosal, 2016: fg. 7), and living pangolins (e.g. Phataginus tricuspis, a small carotid foramen penetrates the basisphenoid and CM 57833, Manis javanica, CM 40597). In the dorsal opens in the anterolateral aspect of the hypophyseal fossa. roof of the aperture and extending posteriorly beyond the Just lateral to the well-marked portion of the groove plane of the aperture is a triangular concavity, the cochlear for the internal carotid is a second small groove, also well fossula. The fossula is delimited posteriorly by a tall, thick marked posteriorly but becoming shallower anteriorly. This medial wall and a short, narrow lateral wall. The medial groove is initially parallel to the transpromontorial groove wall is part of the caudal tympanic process described with but diverges toward the fenestra vestibuli, representing a the pars canalicularis below, which resembles the condition groove for the stapedial artery (Text-fg. 5). It reaches the for this process in Smutsia gigantea (CM 5764) and Manis middle of the rim of the fenestra vestibuli and onto a small javanica (CM 40597), but not Dasypus (Wible 2010) where triangular prominence breaking the plane of the otherwise the process is broadly separated from the cochlear fossula. oval fenestra; this prominence is absent in USNM-P 452349. Separating the aperture of the cochlear fossula from the Posteromedially, the stapedial groove extends onto the fenestra vestibuli in AMNH 131777 is the near vertical crista posteromedial septum, posterodorsal to the internal carotid. interfenestralis. The space posterior to the cochlear fossula The stapedial groove leads to a canal that opens dorsal to and crista interfenestralis represents the postpromontorial the posterior carotid foramen on the bulla wall at a stapedial tympanic sinus. foramen (Text-fg. 4), which we interpret as entirely within The parts of the petrosal lateral and posterior to the the petrosal. Given the presence of separate foramina for promontorium in ventral view represent the pars canalicularis, the internal carotid and stapedial arteries in the bulla wall, which has been greatly complicated by pneumatization. The the bifurcation resulting in these vessels was outside of the course of the facial nerve on the pars canalicularis is a useful tympanic cavity, an unusual condition in mammals (Wible point of reference. Commencing at the anterolateral aspect 1987). In Palaeanodon, the groove for the stapedial artery of the fenestra vestibuli is a tubular facial canal (Text-fg. 5), arises from the internal carotid on the promontorium and directed posterolaterally, that transmitted the main trunk of is deeper, shorter, and positioned more posteriorly and the facial nerve from the cavum supracochleare housing the dorsally (Patterson et al. 1992: fg. 18), relative to that of geniculate ganglion within the pars cochlearis. A facial canal Metacheiromys. None of the living xenarthrans or pangolins is also known to occur in some extant pangolins (Manis possess a stapedial artery, at least as adults (Guth 1961, javanica, M. crassicaudata, and variably present in M. Bugge 1979, Wible 1984, 1987), though a reduced artery pentadactyla and Phataginus tetradactyla; Gaudin and Wible may be present in juveniles or embyros (Schneider 1955, 1999), but is unknown in other pangolins, in armadillos or Wible 1984, Patterson et al. 1992) or in some fossil taxa other xenarthrans, or in Palaeanodon (Patterson et al. 1992). (Gaudin et al. 2015). Regarding the internal carotid, only After a 2 mm course the facial canal opens via the secondary vermilinguan anteaters, among the living Xenarthra, possess facial foramen into a groove walled laterally by a prominent a transpromontorial internal carotid artery (Guth 1961, crest, the crista parotica (Text-fg. 5), most of which is hidden Patterson et al. 1992, Gaudin 1995, 2004). In the sloths and in ventral view on the left side of AMNH 131777 by the cingulates, the artery runs along the ventromedial edge of squamosal roof of the tubal meatus (Text-fg. 2). The facial the petrosal (Guth 1961, Patterson et al. 1989, 1992, Gaudin groove bends posteroventrally and opens on the exterior at 1995, 2004), as it does in pangolins (Kampen 1905, Gaudin the stylomastoid foramen dorsolateral to the tympanohyal and Wible 1999, Gaudin et al. 2009, 2016). (Text-fgs 2, 5). We interpret the stylomastoid foramen as As usual, two large openings are present in the lateral and entirely in the petrosal, as in Palaeanodon (Patterson et posterior aspects of the promontorium, the fenestra vestibuli al. 1992), Patriomanis (Gaudin et al. 2016), and Dasypus and the aperture of the cochlear fossula, respectively (Text- (Wible 2010), but in contrast to Euphractus (Wible and fg. 5). The larger fenestra vestibuli has a stapedial ratio Gaudin 2004) and extant pangolins (Jollie 1968), in which (length to width; Segall 1970) of 1.99 (left side of AMNH the ectotympanic forms a portion of the opening. 131777) and 1.83 (left side of USNM-P 452349). In The part of the pars canalicularis extending anteriorly comparison, the stapedial ratio of living armadillos is quite from the crista parotica in therians is the tegmen tympani similar (1.9–2.0; Wible and Gaudin 2004, Wible 2010), (DeBeer 1929). In AMNH 131777, the posterior part of whereas that of sloths and pangolins is somewhat lower (1.5– the tegmen tympani is represented by a low crest, the 1.6; Gaudin 2011, Gaudin et al. 2016). The fenestra vestibuli epitympanic crest (Text-fg. 5), originating from the facial is directed anteroventrolaterally; the rim accommodating canal, and a shelf anteromedial to it that is continuous with the footplate of the stapes is recessed, producing a shallow the epitympanic wing described above. Most of this surface vestibular fossula except at its posteriormost aspect. The long is fat except for a small concavity anteromedial and parallel axis of the fenestra vestibuli is oblique, from posterodorsal to to the epitympanic crest. It is likely that the tensor tympani anteroventral. The other opening, the aperture of the cochlear muscle arose from this entire surface, but the signifcance fossula, is more irregular in shape, like a narrow dome with of the small concavity on it is enigmatic. A similar, but a straight dorsal side. The cochlear fossula itself is the transversely much broader shelf appears in roughly the same space between the secondary typmanic membrane, covering position in Palaeanodon (Patterson et al. 1992). This shelf is

divided by a crest that runs anteriorly and ventrally. The area Gaudin and Wible 1999). It may also be present in the medial to this crest is identifed by Patterson et al. (1992) middle Eocene pangolin Eomanis (Rose et al. 2005, Gaudin as the fossa for the tensor tympani muscle. In Dasypus et al. 2009), but is clearly absent in the late Eocene pangolin (Wible 2010), the tensor tympani attaches to the petrosal Patriomanis (Gaudin et al. 2016). The sinus is variably anterior to the anteroventral process of the tegmen tympani present in Xenarthra, including euphractine armadillos, (aptt = processus crista facialis of Patterson et al. 1992, anteaters, and most sloths (Patterson et al. 1989, 1992, Gaudin 1995), rather than medially or posteromedially, as in Gaudin 1995, 2004), where it tends to extend anteriorly over palaeanodonts, whereas in Patriomanis, which lacks an aptt, the glenoid, as in palaeanodonts. the tensor tympani attaches to the small epitympanic wing Posterior to and smaller than the epitympanic recess is an of the petrosal and the lateral surface of the promontorium oval depression bounded by the facial canal anteriorly, crista (Gaudin et al. 2016). parotica laterally, pars cochlearis medially, and a low crest Anterior to the epitympanic crest, the tegmen tympani extending from the crista interfenestralis posteriorly (Text- bears an elongate, more ventrally positioned platform fg. 5). Dorsal to these structures, the remaining part of the (roughly 4 × 1.5 mm) that is obliquely oriented from gyrus of the lateral semicircular canal is visible bounding posterolateral to anteromedial as well as downturned the depression. Where the gyrus disappears above the crista anteriorly (Text-fg. 5). The ventral surface of this platform interfenestralis, it would have been joined by the gyrus for the has an oval depression with a much smaller round depression posterior semicircular canal. This depression circumscribed posterior to it. We identify this platform as the anteroventral by the lateral semicircular canal is subdivided into a larger, process of the tegmen tympani (aptt), which is generally deeper posterior part and a smaller, shallower anterior part. considered to be a synapomorphy of Xenarthra (Gaudin and The former is the fossa for the stapedius muscle, but we McDonald 2008). The aptt of Metacheiromys resembles that cannot account for the function of the smaller anterior part. of Priodontes (Patterson et al. 1989) in size and shape, with The stapedius fossa is deeply impressed and opens almost the giant armadillo having, proportionally, one of the largest directly ventrally. Patterson et al. (1992) did not describe aptt known among extant and extinct xenarthrans. An aptt is the stapedius fossa in Palaeanodon, and from the stereopairs not known to occur among any living or extinct pangolins in their paper (fg. 8A-B) we are uncertain of its position. (Gaudin and Wible 1999, Gaudin et al. 2016). However, In Patriomanis, the stapedius fossa is very shallow and there is a distinction between the aptt of Metacheiromys faces anterolaterally as well as ventrally. It is even more and armadillos; in the latter, it provides a contact surface anteriorly oriented in extant pangolins (Gaudin et al. 2016). for the ectotympanic (Wible 2010), whereas in the CT scans In xenarthrans, the fossa is also generally shallower with of both Metacheiromys, the ectotympanic does not abut a more lateral orientation (see Gaudin 1995, Wible and there. Palaeanodon has a ridge with no platform in the same Gaudin 2004). position that the aptt occurs in Metacheiromys (Patterson et Posterior to the stapedius fossa and the pars cochlearis al. 1992). is a large space produced by pneumatization of the pars Adjacent to the crista parotica, facial canal, and canalicularis, the mastoid sinus. This space is not visible in epitympanic crest is a large, oval opening between the our illustrations but is bounded posteriorly by the bullate petrosal and squamosal (Text-fg. 2). This opening, the mastoid exposure of the pars canalicularis, stretching foramen pneumaticum of Patterson et al. (1992), leads into between the posteromedial septum (and posterior carotid a much larger space produced by pneumatization of the and stapedial foramina) and the roof of the tubal external squamosal bone, including anteriorly over the glenoid fossa, acoustic meatus. The medial extent of the mastoid exposure to form an accessory middle ear chamber, an epitympanic is marked by a distinct suture with the exoccipital (Text-fg. sinus (Klaauw 1931). In addition to being a conduit to the 2) that originates at the posterolateral margin of the medial epitympanic sinus, the foramen pneumaticum serves as jugular foramen (see description of foramen below). The the epitympanic recess, the location of the mallear-incudal lateral extent of the mastoid exposure is marked by a suture articulation (Klaauw 1931). The pars canalicularis forms the with the squamosal that follows the line of the nuchal crest anterior, medial, and posterior walls of the epitympanic recess but fuses before it reaches the basicranial surface. From and the medial wall of the dorsally contiguous epitympanic the CT scans of both specimens, the mastoid sinus and the sinus. In the posterior aspect of the epitympanic recess is a squamosal epitympanic sinus communicate broadly deep small depression on the anterior surface of the crista parotica to the petrosquamous suture on the side of the braincase. opposite the position of the secondary facial foramen that According to Patterson et al. (1992), a smaller mastoid sinus represents the fossa incudis, which accommodated the crus is present in Palaeanodon, and it does not communicate breve of the incus (the fossa is hidden by the crista parotica with the epitympanic sinus. in Text-fg. 5). The dorsal margin of the fossa incudis is Two knobby prominences project from the ventral aspect marked by a horizontal prominence, the anteriormost part of the mastoid exposure, a smaller one near the midpoint and of the gyrus for the lateral semicircular canal. Palaeanodon a larger one at the lateral extreme (Text-fgs 4, 5). The smaller has a squamosal epitympanic sinus resembling that of prominence, the tympanohyal (hyoid articulation of Simpson Metacheiromys but differs in that the foramen pneumaticum 1931), is oval with two subequal ventral surfaces, a rounded is entirely in the squamosal and the epitympanic recess anterolateral one and a fatter posteromedial one that lies in (labeled tegmen tympani? in Patterson et al. 1992: fg. 18C) an oblique plane (Text-fg. 5); the latter surface has the facet is medial to it. An epitympanic sinus is well developed in that would have articulated with the stylohyal in life. The all extant pangolins, though it tends to extend posteriorly larger prominence is digitiform and curved posteroventrally rather than anteriorly into the squamosal (Gaudin 1995, with a small basal heel recessed from the ventral basicranial

surface. Sutures between the petrosal and squamosal could then went through a short, vertical canal in the petrosal not be traced in relation to this larger prominence in AMNH before entering the opening next to the posterior carotid and 131777. However, the CT scans of USNM-P 452349 reveal stapedial foramina; the tympanic nerve is the only structure that the curved, digitiform process is on the squamosal with this course in extant mammals (see Evans 1993: (labeled as possible squamosal by Simpson 1931: fg. 7), fg. 19–20). whereas the basal heel is on the petrosal; consequently, Lateral to the posterior carotid, stapedial, and tympanic we identify the former as the post-tympanic process of apertures, Simpson (1931: fg. 7) labeled another foramen the squamosal and the latter as the paroccipital process of posterodorsal to the tympanohyal; this is also present in AMNH the petrosal (Text-fgs 4, 5). The paroccipital process lies 131777 and is likely vascular. The remaining two foramina of anterolateral to the distal end of tympanohyal, as is the case Simpson’s four (1931: fg. 7) are also present on the right side in the armadillo Euphractus (Wible and Gaudin 2004) and of AMNH 131777 but are supplemented by six more. These was probably also the case in Palaeanodon (based on fg. are related to the convoluted course of the auricular branch 18 in Patterson et al. 1992, though the paroccipital process of the vagus nerve and accompanying vessel (see Davis and is not labeled). In Dasypus (Wible 2010) and Patriomanis Story 1943: fg. 2). In AMNH 131777, this nerve left the vagus (Gaudin et al. 2016), the paroccipital process is relatively far at the medial of the two jugular foramina and ran laterally in posterior or posterolateral to the tympanohyal. As discussed a groove on the petrosal in the rim of that foramen. The nerve with the entotympanic above, we interpret most of the bone then went through a canal between the petrosal and exoccipital visible on the ventral surface of the mastoid exposure on the in the bony strut between the two jugular foramina (Text-fg. left petrosal of AMNH 131777 as petrosal, with the possible 2). Lateral to the lateral jugular foramen are two foramina; presence of some entotympanic posterolaterally. This part we reconstruct the nerve in the larger of these two, while of the petrosal represents a greatly infated caudal tympanic the smaller likely held an accompanying vessel. The lateral process (Text-fgs 4, 5; MacPhee 1981). opening for this neurovascular bundle is immediately dorsal Posterodorsal to the medial half of the mastoid exposure to the stylomastoid foramen where a small, dumb-bell shaped is the jugular foramen (posterior lacerate foramen of Simpson aperture opens into two grooves passing medial to the post- 1931 and Patterson et al. 1992) between the petrosal and tympanic process of the squamosal. The nerve likely occupied exoccipital. Although sutures do not delimit the components the larger medial groove and the accompanying vessel the of the occipital bone, we identify this as exoccipital given its smaller lateral groove. One more small foramen opens dorsal positional relationship to the adjacent hypoglossal foramen, to the dumb-bell shaped one and was likely vascular. which is generally enclosed in that bone. The jugular The last basicranial foramen we consider lies anteromedial foramen on the right side of AMNH 131777 is divided into to the medial jugular foramen. It is largely hidden in ventral subequal-sized medial and lateral foramina by septa arising view, is directed anteriorly, and is bounded medially by the from both the petrosal and exoccipital (Text-fg. 2); the prominent muscular process of the basioccipital and laterally same likely occurred on the left side but there has been some by the petrosal (Text-fgs 2, 4). This opening corresponds damage. Based on the position of other nervous conduits to that described by Patterson et al. (1992: 58) as for the discussed below, we reconstruct the medial foramen as internal carotid artery based on YPM-PU 18107. Given the nervous, transmitting the glossopharyngeal, vagus, and transpromontorial internal carotid course reported above, accessory nerves, and the lateral as venous, transmitting the this foramen transmitted something other than the internal internal jugular vein. Among extant mammals, some (e.g. carotid. It is situated where many mammals have a separate Homo sapiens; Standring 2008) show a similar positional opening for the inferior petrosal sinus (e.g. Monodelphis; relationship of structures at the jugular foramen, while others Wible 2003; Euphractus; Wible and Gaudin 2004). We (e.g. Pteropus; Giannini et al. 2006) show the reverse, with were able to trace a canal leading forward from this foramen the vein anteromedial to the nerves. The jugular foramen in in USNM-P 452349 initially between the basioccipital USNM-P 452349 differs in that the opening is only partially and petrosal and then between the basioccipital and divided by a prominence on the petrosal. Simpson (1931: entotympanic. However, we were unable to trace it further fg. 7) reconstructed only a single jugular foramen for rostrally due to specimen incompleteness. Nevertheless, the Metacheiromys, and a single foramen is present in USNM-P most likely occupant of this foramen and canal is the inferior 26132, whereas in YPM-PU 18107, a single foramen is petrosal sinus. There is an aperture in a similar position in present on the right, but on the left side of the specimen the Patriomanis that has also been identifed as a foramen for foramen is at least partially divided. Only a single foramen the inferior pertrosal sinus (Gaudin et al. 2016) is present in Palaeanodon (Patterson et al. 1992), but it is The endocranial surface of the petrosal is often proportionately much larger than that of Metacheiromys. overlooked because of accessibility issues and, consequently, On the rear of the petrosal ventral and ventrolateral to the is not as well studied as the tympanic surface. With access to jugular foramen, Simpson (1931: fg. 7) noted the presence the endocranium in the CT scans, we include a description of four small foramina that he thought were probably of the endocranial petrosal surface of AMNH 131777 (see vascular. The most medial of these is in the position of the Text-fg. 6). The division between the pars cochlearis and two foramina that we identifed above in AMNH 131777 as pars canalicularis is not as striking in the endocranial view, the posterior carotid and stapedial foramina. AMNH 131777 with the former positioned anteroventromedially and the has a third opening dorsal to these other two that we suggest latter posterodorsolaterally. is for the tympanic nerve, a branch of the glossopharyngeal The primary feature on the pars cochlearis is the internal nerve (Text-fgs 2, 4). Whatever passed through this third acoustic meatus (Text-fg. 6), which is subcircular and opening came from the medial jugular foramen above and deeper than wide, with the transverse crest deeply recessed,

Te xt-fi g. 6. Metacheiro mys marshi , A M N H 131777, left petrosal isosurface fro m C T scans in endocranial vie w. a – shaded dra wing; b – li ne dra wi ng wit h la bels. A b breviatio ns: a p p – a pex partis petrosae, cc – coc hlear ca nalic ul us, cr p – crista petrosa, cs – cere bral surface, ia m – internal acoustic meatus, lji – lateral jugular incisure, me – mastoid exposure, mji – medial jugular incisure, saf – s u barc uate fossa, si ps – s ulc us for i nferior petrosal si n us, soev – s ulc us for occi pital e missary vei n, sss – s ulc us for sig moi d si n us, tc – tra nsverse crest, to ptc – to postte m poral ca nal, va – vesti b ular a q ue d uct. as it is i n e xt a nt p a n g oli ns ( e. g. S m utsi a gi g a nt e a , C M 5 7 6 4), Lateral to the fora men acusticu m superius is the prefacial b ut n ot Dasypus ( Wi bl e 2 0 1 0). D ors al t o t h e tr a ns v ers e cr est c o m miss ur e, w hi c h is t hi c k wit h a pr o mi n e nt crist a p etr os a. is the s maller fora men acusticu m superius, which includes The crista petrosa does not contact any bone dorsally as it t h e f a ci al f or a m e n a nt er ol at er all y a n d t h e s u p eri or v esti b ul ar d o es i n P atri o m a nis a n d e xt a nt p a n g oli ns w h er e it pr o vi d es area postero medially. Ventral to the transverse crest is the t h e b as e f or t h e ossi f e d t e nt ori u m ( G a u di n et al. 2 0 1 6). larger fora men acusticu m inferius, which includes the The crista petrosa leads anteriorly to a projection fro m spiral cribrifor m tract medially and the inferior vestibular the pars cochlearis, the apex partis petrosae, as occurs in area posterolaterally; a separate fora men singulare was not S m utsi a gi g a nt e a ( C M 5 7 6 4) b ut n ot Dasypus ( Wi bl e 2 0 1 0). dis c er ni bl e b ut t his m a y h a v e b e e n a n iss u e of s c a n r es ol uti o n. The broad surface lateral to the crista petrosa contacts the

4 9 6 Te xt-fi g. 7. Metacheiro mys marshi , US N M- P 452349, basicraniu m isosurface fro m C T scans in oblique ventral vie w, sho wing middle e a r ossi cl es as p res e r v e d. O n t h e s p e ci m e n’s l eft si d e, t h e ossi cl es a re ess e nti all y i n lif e p ositi o n, wit h t h e st a p es l a r g el y hi d d e n i n t h e fe nestra vesti b uli; t he left malle us is bro ke n a n d re prese nte d largely by t he mallear hea d. O n t he s peci me n’s rig ht si de, most of t he floor of auditory bulla has been re moved to expose the malleus, which has shifted posteriorly fro m the life position. Abbreviations: aptt – anteroventral process of teg men ty mpani, bo – basioccipital, bs – basisphenoid, ea m – squa mosal roof of external acoustic m e at us, e o – e x o c ci pit al, gf – gl e n oi d f oss a, i – i n c us, m – m all e us, m h – m all e a r h e a d, o c – o c ci pit al c o n d yl e, p r – p r o m o nt o ri u m of petrosal, s h – sta pe dial hea d, t m – part of t u b ular exter nal aco ustic meat us. cerebru m and is for med by the roof of the epity mpanic wing this larger space is the pro minence posterior to the internal anteriorly and the roof of the teg men ty mpani posteriorly. acoustic meatus and the posterior margin is the sulcus for P ost er o m e di al t o t h e i nt er n al a c o usti c m e at us is t h e c o c hl e ar the sig moid sinus. The subarcuate fossa in S m utsi a gi g a nt e a c a n ali c ul us, b y w hi c h t h e p eril y m p h ati c d u ct g ai ns e ntr a n c e ( C M 5764) and Dasypus ( Wi bl e 2 0 1 0) is n ot cir c u ms cri b e d t o t h e i n n er e ar. by the anterior se micircular canal and extends onto the The pri mary features on the pars canalicularis are the neighboring exoccipital and squa mosal bones. subarcuate fossa anteriorly and the sulcus for the sig moid Running ventro medially posterior to the subarcuate fossa si n us p ost eri orl y, w hi c h t o g et h er o c c u p y m ost of t h e visi bl e in A M N H 131777 is the broad sulcus for the sig moid sinus. s urf a c e ( Te xt- f g. 6). We ar e n ot e ntir el y c ert ai n of t h e li mits of Entering the sulcus for the sig moid sinus fro m posteriorly the subarcuate fossa, the depression housing the petrosal lobe is a s maller, horizontal sulcus trans mitting the occipital of the para focculus of the cerebellu m. If the fossa is de fned e miss ar y v ei n ( Te xt- f g. 6). T his o p e ns o n t h e o c ci p ut at as the depression circu mscribed by the anterior se micircular the mastoid fora men bet ween the pars canalicularis and canal ( Gannon et al. 1988), then it is a s mall, subcircular supraoccipital. At the level of the internal acoustic meatus, depression subequal in size to the internal acoustic meatus the sulcus for the sig moid sinus bends ventrally and i n A M N H 1 3 1 7 7 7. H o w e v er, t his s m all d e pr essi o n is p art of continues to wards the posterior jugular fora men in a sulcus a larger renifor m-shaped space, the boundaries of which are jointly for med by the pars canalicularis and exoccipital. n ot w ell d e f n e d e x c e pt al o n g its a nt er ol at er al m ar gi n, w hi c h The sig moid sinus divides into subequal distributaries: the is for med by the sharp crista petrosa. The anterior margin of internal jugular vein runs ventrally out the lateral jugular

4 9 7 Te xt-fi g. 8. Mi d dl e- e a r ossi cl es of Metacheiro mys marshi , U S N M- P 4 5 2 3 4 9. a – l eft m all e us ( p a rti al), i n c us, a n d st a p es i n v e nt r al vie w; b – right malleus in oblique anterior vie w (left) and oblique posterior vie w (right). Abbreviations: acr – anterior crus, c b – c r us b re v e, cl – c r us l o n g u m, f p – f o ot pl at e, i af – i nf e ri o r a rti c ul a r f a c et, i b – i n c u d al b o d y, l p – l at e r al p r o c ess, m h – m all e a r head, mn – manubriu m, mp – muscular process, n – neck, ol – osseous la mina (broken), pcr – posterior crus, sh – stapedial head, stf – sta pe dial fora me n, s uaf – s u perior artic ular facet. fora men and the condyloid vein runs posteriorly through E a r Ossi cl es t h e c o n d yl oi d c a n al i n t h e e x o c ci pit al (s e e E v a ns 1 9 9 3). I n t h e a nt eri or w all of t h e s ul c us f or t h e si g m oi d si n us at t h e l e v el of M all e us t h e i nt er n al a c o usti c m e at us is t h e v esti b ul ar a q u e d u ct f or t h e The right malleus of US N M-P 452349 is al most fully endoly mphatic duct, hidden by an irregular pro minence. preserved, with da mage to the osseous la mina and associated At the top of the sulcus for the sig moid sinus is a rostral process. It was found in the epity mpanic recess, fora men that laterally connects with the postte mporal r o u g hl y in sit u b ut m ost li k el y r ot at e d m e di all y ( Te xt- f gs canal ( Text-fg. 6). The well-developed postte mporal 7, 8). O nl y t h e h e a d of t h e l eft m all e us is pr es er v e d, still i n canal runs anteroposteriorly and is enclosed bet ween the c o nt a ct wit h t h e i n c us. D es cri pti o ns will b e b as e d pri m aril y pars canalicularis and squa mosal. Trans mitting the arteria on the right malleus, as the left preserves no additional diploëtica magna and acco mpanying vein ( Wible 1984, str u ct ur es. O n t h e l ar g e r o u n d e d h e a d of t h e m all e us t h er e ar e 1 9 8 7), t h e c a n al b e gi ns o n t h e o c ci p ut at t h e p ostt e m p or al fora men within the pars canalicularis ( Text- fg. 1) and t w o f a c ets f or arti c ul ati o n wit h t h e i n c us – a l ar g er s u p eri or continues for ward to the orbit. En route to the orbit, the f a c et t h at is l o n g er t h a n it is wi d e a n d sli g htl y c o n c a v e, a n d artery and vein provide ra mi te mporales that exit the skull a s m all er, r o u g hl y s q u ar e i nf eri or f a c et wit h sli g ht c o n v e xit y that perfuse the te mporalis muscle. A M N H 131777 has fve ( Text- fg. 8). The superior incudal facet is also larger than fora mina for ra mi te mporales on the right and four on the the inferior facet in ar madillos, but not pilosans, whereas left, t wo in the squa mosal bilaterally and the re mainder t h e c o n diti o n is v ari a bl e in li vi n g p a n g oli ns ( S e g all 1 9 7 3, in the parietal. A postte mporal canal is present in extant Patterson et al. 1992, Gaudin 1995). The angle bet ween xenarthrans but lacking in extant pangolins ( Wible 1984, the t wo incudal facets in Metacheiro mys is roughly 195° 1 9 8 7). A M N H 1 3 1 7 7 7 diff ers fr o m x e n art hr a ns in th at its ( measured posteriorly, taking the superior facet as the postte mporal fora men is within the petrosal rather than 0° line), which is to say, the t wo for m a convex articular bet ween the petrosal and squa mosal. s urf a c e, as i n e xt a nt p a n g oli ns, w h er e as i n x e n art hr a ns, as i n

4 9 8 other ma m mals, the incudal facets for m a concave surface v esti b uli ( Te xt- f g. 7). T his ossi cl e h as b e e n p arti all y ( S e g all 1 9 7 3, P att ers o n et al. 1 9 9 2, G a u di n a n d Wi bl e 1 9 9 9, da maged, with an inco mplete anterior crus and a da maged Wi bl e a n d S p a ul di n g 2 0 1 2). T h e v ast m aj orit y of t h e m all e ar footplate. It was previously described in so me detail by h e a d is n ot c o v er e d b y t h e i n c u d al f a c ets, b e c a us e t h e h e a d P att ers o n et al. ( 1 9 9 2), w h o n ot e d t h e l o w pr o fl e of t h e b o n e, its elf is b ul b o us a n d s p h eri c al, a n d q uit e l ar g e i n pr o p orti o n its wi dt h gr e at er t h a n its h ei g ht, as w ell as its w ell- d e v el o p e d to the re mainder of the bone. In fact, it is proportionately st a p e di al f or a m e n f a n k e d b y sli g htl y b o w e d, l at er all y s o m e w h at l ar g er t h a n t h e m all e ar h e a d i n li vi n g x e n art hr a ns convex crura, with an oval, dorsoventrally co mpressed head a n d p a n g oli ns ( S e g all 1 9 7 3, P att ers o n et al. 1 9 9 2), t h o u g h n ot a n d f o ot pl at e ( Te xt- f g. 8). It is r at h er si mil ar i n f or m t o t h e as l ar g e as t h at i n s e v er al t a x a of e p oi c ot h erii d p al a e a n o d o nts st a p es of m a n y li vi n g pl a c e nt al “i ns e cti v or es ” ( G a u di n et al. ( R os e a n d E mr y 1 9 8 3), i n w hi c h t h e m all e ar h e a d is e nl ar g e d 1 9 9 6), la c ki n g t h e d eri v e d m o di f c ati o ns t h at c h ar a ct eri z e as much as in living golden moles (chrysochlorids), which the stapes of extant pangolins and many living and fossil have relative to body size the largest mallear heads a mong x e n art hr a ns. T h e s c a n s h o ws t h at t h e f o ot pl at e is b ull at e ( as living ma m mals ( Mason 2013). i n gi b b o ns; s e e G a u di n et al. 1 9 9 6), w hi c h c o ntr asts wit h t h e The manubriu m of the malleus in Metacheiro mys is fat footplate occurring in extant xenarthrans and pangolins relatively long and strongly anteroposteriorly co mpressed, ( G a u di n et al. 1 9 9 6). t a p eri n g t o a p oi nt v e ntr all y, wit h a f at m ar gi n o n its l at er al a p e x ( Te xt- f g. 8). T h e s h a p e is q uit e si mil ar i n Dasypus a n d S m utsi a ( S e g all 1 9 7 3, P att ers o n et al. 1 9 9 2). T h e m us c ul ar Disc ussio n process on the manubriu m is not well developed, but the As n ot e d i n t h e I ntr o d u cti o n, t h e pr es e nt st u d y is t h e frst lateral process is much more pro minent, ending in a blunt p oi nt. A si mil arl y l ar g e l at er al pr o c ess is pr es e nt i n p a n g oli ns p u bli c ati o n t o e m pl o y C T-s c a n ni n g t e c h n ol o g y i n t h e st u d y of and euphractine ar madillos, though not in Dasypus ( S e g all cranial osteology in Palaeanodonta, and as such we focused 1 9 7 3, P att ers o n et al. 1 9 9 2). O nl y t h e b as e of t h e s m all, on the best kno wn taxon represented by the best preserved laterally concave osseous la mina is present, as the rest of cranial skeletal material, the early Eocene ( Bridgerian this thin structure, along with the rostral process, has not N AL M A) metacheiro myid genus Metacheiro mys . Alt h o u g h been preserved. the ear region and basicraniu m in this taxon have been described by previous authors, notably Si mpson (1931) and I n c us Patterson et al. (1992), and further detailed infor mation is c o d e d i nt o a n u m b er of p h yl o g e n eti c a n al ys es ( e. g. G a u di n T h e l eft i n c us of U S N M- P 4 5 2 3 4 9 is al m ost c o m pl et el y 1995, 2004, Gaudin and Branha m 1998, Gaudin and Wible pr es er v e d, still i n arti c ul ati o n wit h t h e m all e us, wit h t h e b o d y 1999, Gaudin et al. 2009, O’ Leary et al. 2013), the present of t h e el e m e nt h o us e d i n t h e e pit y m p a ni c r e c ess a n d t h e cr us st u d y h as r e v e al e d n e w d et ails u n a v ail a bl e t o pri or w or k ers, br e v e r e a c hi n g i nt o t h e f oss a i n c u dis ( Te xt- f gs 7, 8). T h er e is especially regarding the morphology of the endocranial d a m a g e t o t h e cr us l o n g u m; it is br o k e n b ef or e t h e ass u m e d surface of the petrosal and the morphology of the ear ossicles, start of the pedicle, the site of articulation with the head of a n d h as als o all o w e d us t o c orr e ct e arli er misi nt er pr et ati o ns the stapes. The extre me shortness of the crus breve appears of the auditory anato my of Metacheiro mys , no w evident n at ur al, gi v e n its pr es er v e d p ositi o n i n t h e f oss a i n c u dis. T h e b e c a us e of t h e d et ails r e v e al e d b y t his st u d y. cr us br e v e is m u c h s h ort er t h a n t h e cr us l o n g u m i n ci n g ul at es, A mong the more signi fcant corrections provided by the and is also quite short in Asian pangolins, whereas the t wo present study is the docu mentation of a transpro montorial processes are nearly equal in length in sloths and African s ul c us f or t h e i nt er n al c ar oti d art er y. P att ers o n et al. ( 1 9 9 2) p a n g oli ns ( S e g all 1 9 7 3, P att ers o n et al. 1 9 9 2). T h e b o d y of ass ert e d t h at Metacheiro mys l a c k e d s u c h a s ul c us, i n c o ntr ast t h e i n c us h as t w o arti c ul ar f a c ets f or t h e h e a d of t h e m all e us. t o t h e ol d er m et a c h eir o m yi d Palaeanodon , where the groove As might be expected given the mallear morphology, the is q uit e pr o mi n e nt. T h e C T s c a ns s h o w t h at Metacheiro mys t wo for m a concave articular surface as in extant pangolins, in fact has a shallo w transpro montorial sulcus, along with whereas in xenarthrans and other ma m mals the articular a s h all o w s ul c us f or t h e st a p e di al art er y ( Te xt- f g. 5). p orti o n of t h e i n c u d al h e a d is c o n v e x ( S e g all 1 9 7 3, P att ers o n The latter sulcus is also more pro minent in Palaeanodon et al. 1992, Gaudin and Wible 1999). The lateral articular ( P att ers o n et al. 1 9 9 2). T h e i nt er n al c ar oti d art er y e nt ers t h e f a c et is r e ct a n g ul ar, c o n c a v e, a n d r o u g hl y h alf t h e si z e of t h e ty mpanic cavity via a posterior carotid fora men bet ween medial articular facet. This latter facet is convex, for ming the entoty mpanic and petrosal in the postero medial bulla a s mooth “c” shape over the body’s anterior surface. The w all ( Te xt- f g. 4). P att ers o n et al. ( 1 9 9 2) cl ai m e d t h at very short crus breve extends posteriorly and dorsally fro m Metacheiro mys has a perbullar internal carotid passing t h e b o d y t o its bl u nt a p e x ( Te xt- f g. 8). T h e b as e of t h e cr us through a fora men in the medial wall of the entoty mpanic. longu m is preserved and sho ws the ele ment was projected The scans sho w that this opening actually acco m modated strongly anteriorly with a ventral co mponent. It is broken t h e i nf eri or p etr os al si n us ( Te xt- f gs 2, 4). before any indication of narro wing for the for mation of a The ne w infor mation brought to light by the present pedicle. The angle bet ween the t wo processes is roughly study also has i mportant i mplications for the syste matic 1 3 0°. relationships of palaeanodonts, long a controversial matter. As noted in the Introduction, historically palaeanodonts St a p es have been variously allied with the extant ma m malian The left stapes of US N M-P 452349 is roughly in situ, orders Xenarthra and Pholidota ( Rose and E mry 1993, Rose though it appears to have fallen slightly into the fenestra et al. 2005). More recently, a consensus has begun to e merge

4 9 9 f a v ori n g t h e f or m er o v er t h e l att er ( e. g. G a u di n et al. 2 0 0 9, t h e s k ull. Metacheiro mys , like other palaeanodonts ( Rose O’ Leary et al. 2013), but the most i mportant data linking 2006), has long been ascribed a fossorial lifestyle based palaeanodonts to xenarthrans have co me fro m shared on its postcranial skeletal anato my. The auditory region r es e m bl a n c e i n t h eir a u dit or y a n at o m y ( P att ers o n et al. 1 9 9 2, w o ul d s e e m t o c o n fr m s u c h a n i nf er e n c e, e x hi biti n g si mil ar Gaudin 1995). Our observations con fr m a nu mber of unusual f oss ori all y-li n k e d a d a pt ati o ns, m ost n ot a bl y a n i n f at e d rese mblances in the auditory osteology of Metacheiro mys middle ear and an enlarged mallear head ( Text- fg. 8). and xenarthrans. These include a l ar g e e nt ot y m p a ni c el e m e nt A bulbous mallear head has been previously reported in of si milar construction (see Gaudin 1995), an epity mpanic epoicotheriid palaeanodonts ( Rose and E mry 1983), where sinus inside the squa mosal that extends anteriorly to ward it is gr ossl y e nl ar g e d as i n m ost g ol d e n m ol es ( M as o n 2 0 0 3, t h e gl e n oi d, a disti n ct, tri a n g ul ar a nt er o v e ntr al pr o c ess of t h e 2013) and so me talpids ( Mason 2006). The enlarge ment of t e g m e n t y m p a ni, a n d a lar g e m e di al f a n g e of t h e p etr os al. All the malleus in these extant subterranean for ms is considered b ut t h e l ast ar e q uit e li k el y d eri v e d r el ati v e t o t h e pri miti v e an adaptation for the detection of lo w-frequency seis mic condition for Placentalia ( O’ Leary et al. 2013). That said, vi br ati o ns ( M as o n 2 0 0 3). T h e gr e atl y i n f at e d mi d dl e e ar, the present study has also uncovered a nu mber of previously including large mastoid and epity mpanic sinuses and a unkno wn, distinctive rese mblances bet ween the auditory sizable ossi fed auditory bulla, occurs in other subterranean a n at o m y of Metacheiro mys a n d t h at of t h e li vi n g a n d e xti n ct taxa, e.g. kangaroo rats and talpid moles ( Webster and pangolins, consistent with the recent consensus regarding Plass man 1992, Mason 2006), where it too has been the sister-group relationship bet ween Palaeanodonta and vie wed as an adaptation to perceiving lo w frequency sound Pholidota. These ne w rese mblances include the absence of vi br ati o ns. a stylifor m process of the ectoty mpanic (co m monly present Although much ne w infor mation has been brought to in xenarthrans), an ovate, anteroposteriorly elongated light by the results of our C T scans, there re mains more p or us a c o usti c us, a p ost er ol at er all y ori e nt e d a p ert ur e t o t h e to learn about the auditory anato my of palaeanodonts in c o c hl e ar f oss ul a, t h e m e di al w all of w hi c h f or ms p art of t h e g e n er al, a n d e v e n Metacheiro mys its elf. F or e x a m pl e, gi v e n caudal ty mpanic process of the petrosal, a deeply recessed ti m e a n d s p a c e li mit ati o ns, t h e a ut h ors c h os e n ot t o att e m pt a internal acoustic meatus, an apex partis petrosae, and a r e c o nstr u cti o n of t h e b o n y l a b yri nt h of t h e i n n er e ar, t h o u g h mallear/incudal articulation that is convex on the mallear t h e s c a ns of t h e U S N M- P s p e ci m e n w o ul d p er mit us t o d o s o. head and concave on the incudal head. Once again, most Other studies have revealed the i mportant palaeobiological of these features are likely derived relative to the pri mitive insights that can be gleaned fro m such reconstructions (e.g. placental condition ( O’ Leary et al. 2013; the exceptions Bill et et al. 2 0 1 5, R uf et al. 2 0 1 6, B os c ai ni et al. 2 0 1 8 – t h e b ei n g t h e frst a n d p e n ulti m at e f e at ur es). T his r e pr es e nts t h e last study noted i mportant li mitations of such inferences as frst ti m e t h at a si g ni f c a nt n u m b er of li k el y d eri v e d a u dit or y w ell), b ut s u c h i nsi g hts m ust a w ait f ut ur e w or k. I n a d diti o n, rese mblances bet ween a palaeanodont and pangolins has the scans do not resolve all uncertainties concerning the b e e n i d e nti f e d, c o nsist e nt wit h t h eir p ut ati v e sist er- gr o u p ost e ol o g y of t h e e ar r e gi o n i n Metacheiro mys . F or e x a m pl e, relationship ( Gaudin et al. 2009, O’ Leary et al. 2013). we were unable to resolve the contrasting interpretations Whether these rese mblances are actual synapo morphies of of the entoty mpanic by Si mpson (1931) and Patterson et t h es e t w o cl a d es a w aits p h yl o g e n eti c a n al ysis. al. (1992), regarding the posterior extent of the ele ment The present study also highlights the uniquely derived and its contribution to the posterior bullar wall. There are n at ur e of t h e a u dit or y ost e ol o g y i n Metacheiro mys . Whereas a d diti o n al w ell- pr es er v e d s k ulls of Metacheiro mys t h at w er e so me unique features of this region have long been recognized inspected visually for the present study, but not scanned, ( e. g. Si m ps o n 1 9 3 1, P att ers o n et al. 1 9 9 2), m ost pr o mi n e ntl y, a n d ot h er w ell- pr es er v e d s k ulls i n ot h er m us e u m c oll e cti o ns t h e gr e atl y i n f at e d m ast oi d p orti o n of t h e p etr os al, m a n y t h at w er e n ot e x a mi n e d at all. It m a y b e t h at f ut ur e d et ail e d a d diti o n al p ot e nti al a ut a p o m or p hi es ar e i d e nti f e d h er e st u di es of t his m at eri al m a y h el p r es ol v e s u c h u n c ert ai nti es. f or th e frst ti m e. T h es e in cl u d e th e pr es e n c e of n u m er o us Moreover, there is less well-preserved but nonetheless sinuses, a mong the m not just the squa mosal epity mpanic interesting cranial material of earlier metacheiro myids, and mastoid sinuses, but also sinuses in the dorsal portion especially of Palaeanodon , and good cranial material as of the entoty mpanic; the presence of a large nu mber of well of the epoicotheriid palaeanodonts ( Rose 2006). C T f or a mi n a i n t h e b asi cr a ni u m ( e. g. ass o ci at e d wit h t h e c o urs e s c a ns a n d d et ail e d a n at o mi c al a n al ysis of t h es e t a x a h a v e t h e of the auricular branch of the vagus nerve); the presence of p ot e nti al t o a d d gr e atl y t o o ur k n o wl e d g e of t h e s yst e m ati cs, a p ar as p h e n oi d el e m e nt f us e d t o t h e b asi cr a ni u m; a n i n f at e d p al a e o bi ol o g y, a n d e v ol uti o n of t h e p al a e a n o d o nts as a w h ol e. bulla that is elongated anteroposteriorly; the presence of a tubular external auditory meatus; a greater petrosal nerve that exits through the musculotubal canal; stapedial and Ackno wledg ments internal carotid arteries that separate prior to entering the auditory bulla; and, a ventrally directed stapedius fossa. We would like to thank the editors for inviting us T h e m or p h ol o g y of t h e st a p es, wit h its pr o mi n e nt st a p e di al to participate in this festschrift. Gerhard Storch was an fora men, and the presence of a transpro montorial internal i mportant and acco mplished palaeontologist and a valued carotid artery, are likely plesio morphic aspects of the colleague. Speaking as lead author ( TJ G), he was not only a u dit or y a n at o m y ( O’ L e ar y et al. 2 0 1 3). tre mendously helpful in the develop ment of my research, I n a d diti o n t o t h e s yst e m ati c i m pli c ati o ns of t h e a u dit or y but also unfailingly kind. K D R also ackno wledges with anato my in Metacheiro mys , it is also the case that so me gr atit u d e t h e s u p p ort, e n c o ur a g e m e nt, a n d fri e n ds hi p of Dr. functional inferences can be dra wn fro m this portion of Storch over three decades.

5 0 0 We thank Z.- X. Luo and A. Isch of the University of Gaudin, T. J. (1995): The ear region of edentates and the Chicago, and M. Hill of the A merican Museu m of Natural phylogeny of the Tardigrada ( Ma m malia, Xenarthra). – History ( A M N H), for conducting the C T-scans. We thank Journal of Vertebrate Paleontology, 15(3): 672–705. R. O’ L e ar y a n d C. M e hli n g of t h e A M N H, a n d J. Str ot m a n htt ps:// d oi. or g/10.1080/02724634.1995.10011255 and M. Florence of the US N M, for facilitating loans of Gaudin, T. J. (2004): Phylogenetic relationships a mong t h e Metacheiro mys speci mens. Text- fgs 2, 3 and 4 were sloths ( Ma m malia, Xenarthra, Tardigrada): the cranio - skillfully executed by Paul Bo wden of the Carnegie Museu m dental evidence. – Zoological Journal of the Linnean of Natural History. This research was supported in part by S o ci et y, 1 4 0( 2): 2 5 5 – 3 0 5. 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