Digital Cranial Endocast of Hyopsodus (Mammalia, “Condylarthra”): A Case of Paleogene Terrestrial Echolocation?

We here describe the endocranial cast of the Eocene archaic ungulate Hyopsodus lepidus AMNH 143783 (Bridgerian, North America) reconstructed from X-ray computed microtomography data. This represents the first complete cranial endocast known for Hyopsodontinae. The Hyopsodus endocast is compared to other known “condylarthran” endocasts, i. e. those of Pleuraspidotherium (Pleuraspidotheriidae), Arctocyon (Arctocyonidae), Meniscotherium (Meniscotheriidae), Phenacodus (Phenacodontidae), as well as to basal perissodactyls (Hyracotherium) and artiodactyls (Cebochoerus, Homacodon). Hyopsodus presents one of the highest encephalization quotients of archaic ungulates and shows an “advanced version” of the basal ungulate brain pattern, with a mosaic of archaic characters such as large olfactory bulbs, weak ventral expansion of the neopallium, and absence of neopallium fissuration, as well as more specialized ones such as the relative reduction of the cerebellum compared to cerebrum or the enlargement of the inferior colliculus. As in other archaic ungulates, Hyopsodus midbrain exposure is important, but it exhibits a dorsally protruding largely developed inferior colliculus, a feature unique among “Condylarthra”. A potential correlation between the development of the inferior colliculus in Hyopsodus and the use of terrestrial echolocation as observed in extant tenrecs and shrews is discussed. The detailed analysis of the overall morphology of the postcranial skeleton of Hyopsodus indicates a nimble, fast moving animal that likely lived in burrows. This would be compatible with terrestrial echolocation used by the animal to investigate subterranean habitat and/or to minimize predation during nocturnal exploration of the environment

Analyse morphofonctionnelle du squelette postcrânien d'<i>Amphicyon major</i> (Mammalia, Carnivora, Amphicyonidae) du Miocène de Sansan (Gers, France) comparé à trois carnivores actuels : <i>Ursus arctos</i>, <i>Panthera leo</i> et <i>Canis lupus</i>

Le squelette postcrânien de l\u27amphicyonidé Amphicyon major de l\u27Helvétien moyen de Sansan (Gers, France) est décrit et comparé à des taxons actuels appartenant aux familles Ursidae (Ursus arctos), Felidae (Panthera leo) et Canidae (Canis lupus). Au membre antérieur, la scapula est très semblable à celle d\u27un ours, tout comme le coude suggérant des positions abductées de l\u27avant-bras et la capacité à supiner la main. L\u27épaule mobile et le triceps massif sont des traits partagés à la fois par les ursidés et les félidés. Une musculature puissante de la main, des métacarpiens courts et divergents, et une grande amplitude de flexion dorsale au poignet rappellent l\u27extrémité antérieure d\u27un ours et assurent une capacité de préhension. Au niveau du squelette axial, le complexe atlas/axis, la forme des vertèbres lombaires et une longue et lourde queue sont plus semblables à ce qui est observé chez les félidés qu\u27à n\u27importe quel autre carnivore actuel. Le bassin et le fémur, ainsi que le genou et l\u27articulation tibio-astragalienne sont aussi plus semblables à ce qui est observé chez un félidé que chez un ursidé, bien que l\u27ischion et le fémur d\u27Amphicyon permettent des positions plus abductées et plus dressées. La taille et la forme du tibia, du calcanéum et des métatarsiens sont plus semblables à celles d\u27un ours. Les métatarsiens courts, avec un Mt I divergent et une puissante musculature du pied, suggèrent également une capacité de préhension. Le squelette d\u27Amphicyon major, qui présente donc une série de similitudes morphologiques avec les ours et les félins actuels, mériterait le nom d\u27« ours-lion » plutôt que son nom actuel d\u27« ours-chien ».The postcranial skeleton of the amphicyonid Amphicyon major from the middle Helvetian of Sansan (Gers, France) is described and compared to extant taxa belonging to the families Ursidae (Ursus arctos), Felidae (Panthera leo), and Canidae (Canis lupus). From the perspective of the forelimb, the scapula is very similar to that of a bear, as well as the elbow joint suggesting habitually abducted postures of the arm and the ability to supinate the forefoot. The mobile shoulder joint and massive triceps are features shared by both extant ursids and felids. A powerful hand musculature, short and divergent metacarpals, and an important range of dorsifl exion of the wrist recall the forefoot of an ursid and indicate good grasping ability. On the axial skeleton, the complex atlas/axis, the shape of the lumbar vertebrae, and a long, heavy tail are more similar to the same anatomical regions in felids than to any other extant carnivore. The pelvis and femur, as well as the knee and tibio-astragalar joints, are also more similar to those of a felid than an ursid, although the ischium and femur of Amphicyon allowed more abducted and erected postures. The size and shape of the tibia, calcaneum, and metatarsals are more similar to those of a bear. The short metatarsals, with a divergent Mt I and a powerful hind foot musculature also suggest potential grasping ability. Therefore, the skeleton of Amphicyon major that displays a series of bear-like and cat-like morphological similarities would deserve the term of "bear-lion" instead of "bear-dog".</p

Holocene subfossil rodents from the Lavajaza Cave, Central Highlands of Madagascar

International audienc

Osteology and Functional Morphology of the Forelimb of the Marine Sloth Thalassocnus (Mammalia, Tardigrada)

International audienceThalassocnus is a genus of “ground sloths” known from Neogene deposits, for the great majority of specimens, of the Pisco Formation (Peru). Five species are recognized, their description being currently restricted, for the most part, to the skull, mandible, and dentition. The bones of the forelimb are here described, and compared among the species of Thalassocnus and to other pilosans. The main characteristics of the forelimb of Thalassocnus relative to other sloths are the shortness of the humerus and radius, and the specialized digits. Moreover, the late species of the genus are characterized by the development of the pronator ridge of the radius, stoutness of the ulna, widening of the proximal carpal row, and shortening of the metacarpals. Analogies with extant tetrapods are proposed in order to infer plausible aquatic functions of the forelimb of Thalassocnus. In addition to paddling, it is argued that the forelimb of Thalassocnus was involved in bottom-walking, a function similarly found in extant sirenians. However, the function of the forelimb of Thalassocnus differs drastically from that of the latter, since it was likely involved in an activity related to obtaining food such as uprooting seagrass rhizomes

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

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

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