Comparative Anatomy of the Petrosal Bone of Dichobunoids, Early Members of Artiodactylamorpha (Mammalia)

Abstract

Among Artiodactylamorpha, dichobunoids are some of the oldest fossil species that have been associated with Artiodactyla, the crown clade that includes hippopotamids, camelids, suoids, ruminants, and cetaceans. These important fossil species are known from early Eocene rocks of North America, Europe, and Asia, but their phylogenetic position has yet to be well resolved. Before generating such a phylogeny, it is first critical to document all of the anatomy of known dichobunoid fossils. Here we use CT scans to describe previously undescribed anatomy of the petrosal bone, a complex part of the mammalian skull that contains many variable and phylogenetically informative features. Results show that these extinct species share a number of features that are not documented in modern species including a lateral process of the epitympanic wing constituting the medial border of the piriform fenestra, and a tegmen tympani foramen that may have given passage to the ramus superior of the stapedial artery. Future comprehensive phylogenetic studies may show that many of these characters are plesiomophic for Artiodactylamopha. Some species (Diacodexis, Homacodon and ?Helohyus) exhibit a dorsolateral exposure of the mastoid region of the petrosal on the temporal part of the cranium. This uncommon feature has, to our knowledge, not been reported in another euungulate group.

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Acknowledgement

We thank R. O’Leary (AMNH), Meng Jin (AMNH), C. Argot (MNHN), and B. Marandat (UM2) for access to the collections, J. Thostenson, R. Rudolph, and M. Hill for acquisition of raw CT data at the AMNH, and A-L Charruault and R. Lebrun for acquisition of raw CT scan data at the UM2. We are grateful to J Wible, J Geisler, and one anonymous reviewer for their enriching comments on earlier versions of the manuscript. We also thank the American Museum of Natural History for use of the high resolution CT-scanner (NSF MR1–R2 0959384 to N. Landman, D. Ebel, and D. Frost), and the Montpellier Rio Imaging platform (Montpellier, France) for use of their Skyscan machine. This is ISE-M publication 201X-XX. This research was supported by the ANR funding project Palasiafrica, headed by L. Marivaux.

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Correspondence to M. J. Orliac.

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Fig. S1
figure11

Paired stereo images of the petrosal bone of Diacodexis ilicis (AMNH 16141) in A) ventrolateral, B) dorsomedial, C) medioventral, and D) anterior views. Scale = 2 mm. (JPEG 177 kb)

Fig. S2
figure12

Paired stereo images of the petrosal bone of Acotherulum saturninum (MNHM Qu 16366) in A) ventrolateral, B) dorsomedial, C) medioventral, and D) anterior views. Scale = 2 mm. (JPEG 198 kb)

Fig. S3
figure13

Paired stereo images of the petrosal bone of Dichobune leporina (MNHN Qu 16586) in A) ventrolateral, B) dorsomedial, C) medioventral, and D) anterior views. Scale = 2 mm. (JPEG 178 kb)

Fig. S4
figure14

Paired stereo images of the petrosal bone of Homacodon vagans (USNM 482369) in A) ventrolateral, B) dorsomedial, C) medioventral, and D) anterior views. Scale = 5 mm. (JPEG 223 kb)

Fig. S5
figure15

Paired stereo images of the petrosal bone of ?Helohyus plicodon (USNM 13079) in A) ventrolateral, B) dorsomedial, C) medioventral, and D) anterior views. Scale = 5 mm. (JPEG 226 kb)

Fig. S6
figure16

Paired stereo images of the petrosal bone of Gobiohyus orientalis (USNM 26277) in A) ventrolateral, B) dorsomedial, C) medioventral, and D) anterior views. Scale = 2 mm. (JPEG 195 kb)

Fig. S7
figure17

Paired stereo images of the petrosal bone of A) Diacodexis ilicis (AMNH 16141); B) Acotherulum saturninum (MNHM Qu 16366); C) Homacodon vagans (USNM 482369); D) Dichobune leporina (MNHN Qu 16586); E) ?Helohyus plicodon (USNM 13079); and F) Gobiohyus orientalis (USNM 26277) in ventrolateral view. Scale = 2 mm for A, B, D, F and scale = 5 mm for C and E. (JPEG 325 kb)

Fig. S8
figure18

Paired stereo images of the petrosal bone of A) Diacodexis ilicis (AMNH 16141); B) Acotherulum saturninum (MNHM Qu 16366); C) Homacodon vagans (USNM 482369); D) Dichobune leporina (MNHN Qu 16586); E) ?Helohyus plicodon (USNM 13079); and F) Gobiohyus orientalis (USNM 26277) in dordomedial view. Scale = 2 mm for A, B, D, F and scale = 5 mm for C and E. (JPEG 343 kb)

Fig. S9
figure19

Paired stereo images of the petrosal bone of A) Diacodexis ilicis (AMNH 16141); B) Acotherulum saturninum (MNHM Qu 16366); C) Homacodon vagans (USNM 482369); D) Dichobune leporina (MNHN Qu 16586); E) ?Helohyus plicodon (USNM 13079); and F) Gobiohyus orientalis (USNM 26277) in anterior view. Scale = 2 mm for A, B, D, F and scale = 5 mm for C and E. (JPEG 262 kb)

Fig. S10
figure20

Paired stereo images of the petrosal bone of A) Diacodexis ilicis (AMNH 16141); B) Acotherulum saturninum (MNHM Qu 16366); C) Homacodon vagans (USNM 482369); D) Dichobune leporina (MNHN Qu 16586); E) ?Helohyus plicodon (USNM 13079); and F) Gobiohyus orientalis (USNM 26277) in medioventral view. Scale = 2 mm for A, B, D, F and scale = 5 mm for C and E. (JPEG 273 kb)

Fig. S11
figure21

Homacodon vagans (USNM 482369) left petrosal in situ, ventral view. Scale = 1 cm. (JPEG 114 kb)

Fig. S12
figure22

Gobiohyus orientalis (USNM 26277) right petrosal in situ (reversed to be shown from left side), ventral view. Scale = 1 cm. (JPEG 106 kb)

Fig. S13
figure23

?Helohyus plicodon (USNM 13079) right petrosal in situ (reversed to be shown from left side), ventral view. Scale = 1 cm. (JPEG 120 kb)

Fig. S14
figure24

3D reconstruction of endocranial features of Homacodon vagans (USNM 482369): endocranial cast in pink, tegmen tympani canal in red. The white dotted line corresponds to the petrosal outlines. Scale = 1 cm. (JPEG 27 kb)

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Appendices

Appendix 2

Table 2 Museum accession numbers and scanning parameters of the specimens included in this study

Appendix 1

Table 3 Alphabetical list of anatomical terms used and illustration of these terms in the different figures

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Orliac, M.J., O’Leary, M.A. Comparative Anatomy of the Petrosal Bone of Dichobunoids, Early Members of Artiodactylamorpha (Mammalia). J Mammal Evol 21, 299–320 (2014). https://doi.org/10.1007/s10914-014-9254-9

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Keywords

  • Middle ear
  • Eocene
  • Euungulata
  • micro CTscan