Ocepeia: phylogenetic miniaturization at the base of the Mesonychidae
Though it looks like a tiny hippo,
Ocepeia (Figs 1, 2) now nests at the base of the Mesonyx clade in the large reptile tree (LRT, 2323 taxa, subset Fig 3).
Outgroup taxa include larger Coryphodon (Figs 2, 4) and Phenacodus (Figs 2, 4).
The Mesonyx clade is basal to oreodonts and hippos.
Figure 1. Ocepeia from a µCT scan. Colors added here.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2022/02/ocepeia_daouiensis_skull588.jpg?w=223″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2022/02/ocepeia_daouiensis_skull588.jpg?w=584″ tabindex=”0″ role=”button” class=”wp-image-64992 size-full” src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2022/02/ocepeia_daouiensis_skull588.jpg” alt=”Figure 1. Ocepeia from a µCT scan. Colors added here. ” width=”588″ height=”792″ />
Figure 1. Ocepeia from a µCT scan. Colors added here.
Ocepeia daouiensis
(Gheerbrant et al 2001, 2014; Paleocene, 60 mya; 9 cm skull length) is a phylogentically miniaturized basal mesonychid from Morocco. The pneumatized skull contains many air spaces. Slightly larger skulls have larger canines and so are considered male, but may just be older. This specimen has only a tiny canine on a broken maxilla. The molar teeth are relatively large and the jugal deepens below the orbit, hiding the posterior molars in lateral view.
Figure 2. Coryphodon and Ocepeia skulls to scale.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/coryphodon_skull588.jpg?w=154″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/coryphodon_skull588.jpg?w=524″ tabindex=”0″ role=”button” class=”size-full wp-image-88320″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/coryphodon_skull588.jpg” alt=”Figure 2. Coryphodon and Ocepeia skulls to scale. ” width=”584″ height=”1140″ />
Figure 2. Coryphodon and Ocepeia skulls to scale. Coryphodon is basal to elephants and sirenians in the LRT.
Note the dorsal eyes, as in hippos.
Ocepeia was found with aquatic taxa and was probably amphibious. Ocepeia is basal to oreodonts + hippos + slender artiodactyls. Hippos are basal to desmostylians and mysticetes.
So giraffes had robust, short-legged, aquatic ancestors, according to the LRT.
Welcome to Evolution 101.
Figure 3. Subset of the LRT focusing on Ocepeia and the mesonychids.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/ocepeia.cladogram588.jpg?w=144″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/ocepeia.cladogram588.jpg?w=492″ tabindex=”0″ role=”button” class=”size-full wp-image-88317″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/ocepeia.cladogram588.jpg” alt=”Figure 3. Subset of the LRT focusing on Ocepeia and the mesonychids.” width=”584″ height=”1216″ />
Figure 3. Subset of the LRT focusing on Ocepeia and the mesonychids.
Ocepeia post-crania is not known.
So closely related taxa provide the only clues (Fig 4). Descendant mesonychids include some of the larger mammals, like Andrewsarchus, and smaller taxa, like Ptolemaia.
Figure 4. Ocepeia compared to more or less related taxa. Phenacodus and Coryphodon provide phylogenetic clues to the post-crania of skull-only Ocepeia.
” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/large_scott_phenacodus.ocepeia588.jpg?w=148″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/large_scott_phenacodus.ocepeia588.jpg?w=506″ tabindex=”0″ role=”button” class=”size-full wp-image-88323″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/08/large_scott_phenacodus.ocepeia588.jpg” alt=”Figure 4. Ocepeia compared to more or less related taxa. Phenacodus and Coryphodon provide phylogenetic clues to the post-crania of skull-only Ocepeia. ” width=”584″ height=”1182″ />
Figure 4. Ocepeia compared to more or less related taxa. Phenacodus and Coryphodon provide phylogenetic clues to the post-crania of skull-only Ocepeia.
These present results
follow several months of housekeeping (= checking scores) in the LRT. Clades are falling into logical sequences, and the work continues.
References
Gheerbrant E, Amaghzaz M, Bouya B and Goussard F and Letenneur C 2014. Ocepeia (Middle Paleocene of Morocco): The Oldest Skull of an Afrotherian Mammal. PLoS ONE. 9 (2): e89739. doi:10.1371/journal.pone.0089739.
Source: https://pterosaurheresies.wordpress.com/2024/08/20/ocepeia-phylogenetic-miniaturization-at-the-base-of-the-mesonychidae/
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