The inner ear morphology of the ‘condylarthran’ <i>Hyopsodus lepidus</i>

<div><p>We describe the bony labyrinth morphology of the Eocene ‘archaic ungulate’ <i>Hyopsodus</i><i>lepidus</i> (Bridgerian, North America) reconstructed from micro computed tomography scan data. Comparisons with the inner ear of the Eocene early diverging artiodactyl <i>Diacodexis</i> and perissodactyl <i>Xenicohippus</i> allow refining the picture of the ancestral inner ear morphology of Euungulates. These taxa are very close morphologically and mostly differ by slight differences in their semicircular canal angulations and profile. They all present a secondary crus and a low position of the plane of the lateral semicircular canal relative to the posterior semicircular canal. These two characters, considered as ancestral features for Theria, might be symplesiomorphies of Euungulata as well. <i>Hyopsodus</i> and <i>Xenicohippus</i> share characters also observed in other basal Equoidea, which would support the close relationship between these two taxa previously proposed in the literature. A functional study of the cochlea of <i>Hyopsodus lepidus</i> is also realised to discuss its putative ability of using terrestrial echolocation previously proposed in the literature. The morphology of the cochlea of <i>Hyopsodus lepidus</i> does not indicate a specialisation to sophisticated echolocation such as observed today in microchiropteran bats. However, its estimated audible range of frequencies (208 Hz to 76.8 KHz) would be compatible with terrestrial echolocation.</p></div

Endocast measurements for <i>Hyopsodus</i> (given in mm, and mm³ for endocast volume).

<p>R = right side of specimen; L = left side of specimen;</p><p>* = <i>H. paulus</i> (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030000#pone.0030000-Gazin2" target="_blank">[35]</a>, pl. 1);</p><p>** = <i>H. miticulus</i> (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030000#pone.0030000-Gazin2" target="_blank">[35]</a>, pl. 5).</p

Labelled endocast and basicranium of <i>Hyopsodus lepidus</i> (AMNH 143783).

<p>Reconstruction illustrated in left lateral (A), dorsal (B), right lateral (C), ventral with basicranium (D), ventral without basicranium (E), and posterior views. Abbreviations: cas, cavernous sinus; cc, condyloid canal; cdf, condyloid foramen; ci, colliculi inferior; cf, condylar foramen; cs, colliculi superior; fl, paraflocculus; flm, foramen lacerum medium; flp, foramen lacerum posterius; fo, foramen ovale; fp, fissure prima; hy, hypophysis; ips,inferior petrosal sinus; lal, lateral lobe of cerebellum; las, lateral sinus (or transverse sinus); los, longitudinal sinus; mf, mastoid foramen; mo, medulla oblongata; ms, mesencephalon; mv, mastoid vein; np, neopallium; ob, olfactory bulb; oc, occipital condyle; op, olfactory peduncle; os, occipital sinus; ot, olfactory tubercle; pb, petrosal bone; pgf, postglenoid foramen; pil, piriform lobe; rhp, rhinal fissure; sc, sinusal canal; tf, temporal foramen; ts, temporal sinus; vc, vermis cerebelli. Scale bar = 1 cm.</p

Data for brain size estimate of basal ungulates.

<p>EV, endocast volume; EBM, estimated body mass; EQ1, encephalization quotient using Radinsky's <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030000#pone.0030000-Radinsky2" target="_blank">[20]</a> equation; EQ2, encephalization quotient using Eisenberg's <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030000#pone.0030000-Eisenberg1" target="_blank">[39]</a> equation.</p

Simplified phylogenetic relationships among basal ungulates, modified after [<b>36, fig. 11B</b>].

<p>Simplified phylogenetic relationships among basal ungulates, modified after <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030000#pone.0030000-Ladevze1" target="_blank">[<b>36, fig. 11B</b>]</a>.</p

Dorsal view of some basal ungulates.

<p>Reconstructions made after the original cranial endocasts of: (A) <i>Phenacodus</i> (AMNH 4369), (B), <i>Meniscotherium</i> (AMNH 49082), (C) <i>Hyopsodus</i> (AMNH 143783), (D) <i>Pleuraspidotherium</i> (MNHN CR 252, 963), (E) <i>Arctocyon</i> (MNHN CR 700), (F) <i>Cebochoerus</i> (MNHN 34–1967), (G) <i>Hyracotherium</i> (AMNH 55267). All specimens normalized on the neopallium length. Proportions of the different parts indicated in percent of the total endocast length: black, rhinencephalon; red, neopallium; blue, mesencephalon; green, cerebellum. Not to scale.</p

Reconstruction of the lateral view of <i>Hyopsodus lepidus</i> compared with other basal ungulates.

<p>Reconstructions made after the original cranial endocasts of: (A) <i>Hyopsodus lepidus</i> (AMNH 143783), (B) <i>Pleuraspidotherium</i> (MNHN CR 252, 963; cast AMNH 39266); (C) <i>Phenacodus</i> (AMNH 4369); (D) <i>Meniscotherium</i> (AMNH 49082; USNM 19509); (E) <i>Arctocyon</i> (MNHN CR 700), modified after Russell and Sigogneau (1965). Not to scale. Abbreviations: same as <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0030000#pone-0030000-g002" target="_blank">Figure 2</a>.</p

Volcanic rocks likely to embed fossil vertebrates and associated morphological and/or taphonomic features, sorted by increasing temperature range.

<p>Preservation of the Karacaşar rhino (submitted to a ∼400–450°C temperature) is most likely tied to a pyroclastic density current, reminiscent of that of Pompeii-Herculaneum-Oplontis, of Vesuvius origin (79 AD).</p

Taphonomical processes involved in the preservation of the Karacaşar rhino.

<p>a. general view of the cranium and mandible. b. detail of the dorsal area of the cranium, with a broken rhino rib, trapped ‘upstream’ beside it. c. detail of baked dentine (brittle occlusal surface) and roots (hollow) of right P4-M2. d. labial detail of left p3-m1 showing deteriorated roots and intact enamel; note that p4 and m1 show mild enamel hypoplasia. Scale bar: 20 mm.</p

<i>Ceratotherium neumayri</i> (Osborn, 1900), Karacaşar, Late Vallesian of South Central Anatolia.

<p>Dimensions of the cranium HU-2011-1 (in mm).</p