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Hyopsodus: an Eocene pre-dog, not an archaic ungulate

Thursday, September 22, 2016 11:11
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(Before It's News)

Hyopsodus lepidus (H. paulus type specimen, Leady 1870; AMNH 143783; Eocene; Fig. 1) is traditionally considered an odd-toed ungulate, despite having a five-clawed manus and a four-clawed pes. Wikipedia also promotes this nesting, but unlike most ungulates, they report, “It is believed to have been swift and nimble, living in burrows, and perhaps able to use echolocation,” and it is shown climbing on a tree trunk. Another website lists Hyopsodus as a condyarth.

Here, in the LRT, 
Hyopsodus nests with Canis and more closely with Miacis (Fig. 2), two members of the Carnivora in the large reptile tree (LRT). Shifting Hyopsodus over to the Condylarthra adds nearly 30 steps. Those tiny feet beneath that long and wide body do not look to me like they could be used to excavate burrows or climb trees. Plus that short tail and long torso are not typical of climbing animals.

Figure 1. Hyopsodus as originally reconstructed (below) and as reconstructed here above in two views. This former condylarth now nests with dogs.

Figure 1. Hyopsodus as originally reconstructed (below) and as reconstructed here above in two views. This former condylarth now nests with dogs. Hyopsodus has more teeth than Canis and Miacis, and the canine is small and there is no carnassial, similar to the outgroup taxon, Protictis.

From the Orliac et al. 2012 abstract:
“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 [hearing]. The detailed analysis of the overall morphology of the postcranial skeleton of Hyopsodus indicates a nimble, fast moving animal that likely lived in burrows.” Sounds like a member of the Carnivora…

Figure 1. Miacis, an Eocene ancestor to extant dogs, such as Canis.

Figure 2. Miacis, an Eocene ancestor to extant dogs, such as Canis. Note the transverse premaxilla and tiny premaxillary teeth as in Hyopsodus.

Miacis has long been known as a dog ancestor.
And here it also nests with Canis (Fig. 3). So, compare Miacis to Hyopsodus (Fig. 2) and you’ll find very few differences.

Figure 3. Canis lupus, the wolf that gave rise to extant dogs through selective breeding.

Figure 3. Canis lupus, the wolf that gave rise to extant dogs through selective breeding. Note the five phalanges on the manus and four on the pes.

Several specimens and species are known
of Hyopsodus, most from just teeth and jaws.

References
Orliac MJ, Argot C and Gilissen E 2012. Digital Cranial Endocast of Hyopsodus (Mammalia, “Condylarthra”): A Case of Paleogene Terrestrial Echolocation? PlosOne v.7(2); 2012PMC3277592

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