Cambaytherium: not a basal perissodactyl

Rose et al. 2014
considered pig-sized Cambaytherium thewissi (Eocene, 55 mya; 45-75 lbs; Fig. 1) to be a basal perissodactyl (odd-toed ungulates like Tapirus, Equus and Ceratotherium), but with four fingers. Rose et al. pieced together 200 bones from several individuals of which only some were published.

Figure 1. Cabaytherium does not nest as a basal perissodactyl, but between Hippopotamus and Anthracobune in the LRT. Note the long retroarticular process.

Figure 1. Cabaytherium does not nest as a basal perissodactyl, but between Hippopotamus and Anthracobune in the LRT. Note the long retroarticular process.

In their phylogenetic analysis,
Rose et al. report, “In these nine shortest trees, cambaytheres and anthracobunids were always united with perissodactyls to the exclusion of afrotheres, artiodactyls and phenacodontids, with cambaytheres always recovered at the base of the perissodactyl stem.”

Figure 3. Mesonychids including Cambaytherium nesting between Hippopotamus and Anthracobune. This is a subset of the LRT.

Figure 2. Mesonychids including Cambaytherium nesting between Hippopotamus and Anthracobune. This is a subset of the LRT.

Unfortunately
Rose et al. 2014 did not have access to Cooper  et al. 2014, which featured a more or less complete Anthracobune skull and mandible (Fig. 3). Among all mammals, no other taxa have such an elongate retroarticular process — except Cambaytherium. That and several other traits nested Cambaytherium in the large reptile tree (LRT, 1012 taxa) between Hippopotamus and Anthracobune, ultimately in the lineage of desmostylians and baleen whales.

The Rose et al. cladogram included both rhinos and horses, Unfortunately indricotheres were not included. Dang!

Figure 3. Anthracobune reconstructed with a larger skull to match the teeth on the mandible.

Figure 3. Anthracobune reconstructed with a larger skull (in color) to match the teeth on the mandible.

During this study 
I realized the skull I had used for Anthracobune did not match the teeth of the larger mandible. They were likely from different sized specimens. So I enlarged the skull and reidentified some teeth (Fig. 3). No change in tree topology took place following these changes.

Figure 3. An image falsely attributed to Cambaytherium.

Figure 4. An image falsely attributed to Cambaytherium that appears on the Internet.

Cambaytherium was found
on the marine coastline of island India, close to where odontocete whales were also evolving from terrestrial tenreci ancestors. like Pakicetus. Based on its relationships Cambaytherium was likely much more aquatic than is typical for perissodactyls, which makes it easer to reach the island continent. Ocepeia, another relative, was found in Morocco.

References
Cooper LN, Seiffert ER, Clementz M, Madar SI, Bajpai S, Hussain ST, Thewissen JGM 2014. Anthracobunids from the Middle Eocene of India and Pakistan Are Stem Perissodactyls. PLoS ONE. 9 (10): e109232. doi:10.1371/journal.pone.0109232. PMID 25295875.
Rose, KD et al. (8 other authors) 2014.
 Early Eocene fossils suggest that the mammalian order Perissodactyla originated in India. Nature Communications. 5 (5570). doi:10.1038/ncomms6570.

LA Times article online
wiki/Cambaytherium

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