Skeletalanatomyoftheb a Si Cra Ni Umandau Di Toryre Gi Onin
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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.