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Phylogenetic minaturization at the advent of the gracile Artiodactyla

Friday, July 5, 2024 8:03

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Recent housekeeping in the LRT
moved a big taxon, Homalodotherium (Fig 1), from the base of the gracile Artiodactyla (= sans hippos and oreodonts), to the Toxodon + Thomshuxleyia clade of the Marsupialia. New skull data (Fig 1) made this move possible and necessary.

Figure 1. The skull of Homalodotherium in three views. Now it’s a marsupial linking Interatherium to Thomashuxleyia and Toxodon. ” data-image-caption=”

Figure 1. The skull of Homalodotherium in three views. Now it’s a marsupial linking Interatherium to Thomashuxleyia and Toxodon.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/homalodotherium.skull588.jpg?w=169″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/homalodotherium.skull588.jpg?w=576″ tabindex=”0″ role=”button” class=”size-full wp-image-87248″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/homalodotherium.skull588.jpg” alt=”Figure 1. The skull of Homalodotherium in three views. Now it’s a marsupial linking Interatherium to Thomashuxleyia and Toxodon. ” width=”584″ height=”1038″ />

Figure 1. The skull of Homalodotherium in three views. Now it’s a marsupial linking Interatherium to Thomashuxleyia and Toxodon.

That left a small taxon,
Aumelasia (Figs 2, 3) at the base of the gracile Artiodactyla (= sans hippos and oredonts) in the LRT. Again, better skull data (Fig 3) became available. This second Messel Pit specimen was traced and reconstructed using DGS methods.

Figure 2. Aumelasia, a small Messel Pit taxon, now nests with Cainotherium at the base of the Artiodactyla in the LRT. ” data-image-caption=”

Figure 2. Aumelasia, a small Messel Pit taxon, now nests with Cainotherium at the base of the Artiodactyla in the LRT.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2022/03/aumelasia_gabineaudi_insitu588.jpg?w=300″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2022/03/aumelasia_gabineaudi_insitu588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-65308″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2022/03/aumelasia_gabineaudi_insitu588.jpg” alt=”Figure 2. Aumelasia, a small Messel Pit taxon, now nests with Cainotherium at the base of the Artiodactyla in the LRT.” width=”584″ height=”381″ />

Figure 2. Aumelasia, a small Messel Pit taxon, now nests with Cainotherium at the base of the Artiodactyla in the LRT.

Aumelasia gabineaudi
(Sudre 1979 (1980) Middle Eocene, 51–43mya, 42cm long (juvenile), skull 9.2cm long) was originally described as a basal artiodactyl (Dichobunidae) based on its dentition. Here it nests as a basal artiodactyl close to Cainotherium. The tail was quite long with 24 vertebrae (as in Merycoidodon Fig 4). The mandible was massive. The incisors were spatulate. The lower canine was small. Five fingers and four toes were present with longer metatarsals than metacarpals. The hind limb was longer than the fore limb. Three complete skeletons from the Messel Pit are known, plus other scraps from other localities. Aumelasia ate leaves, seeds and fruit.

Figure 2. Skull of Aumelasia traced and reconstructed using DGS colors. ” data-image-caption=”

Figure 2. Skull of Aumelasia traced and reconstructed using DGS colors.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/aumelasia_skull2.jpg?w=273″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/aumelasia_skull2.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-87250″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/aumelasia_skull2.jpg” alt=”Figure 2. Skull of Aumelasia traced and reconstructed using DGS colors. ” width=”584″ height=”643″ />

Figure 3. Skull of Aumelasia traced and reconstructed using DGS colors. Compare to Homalodotherium in figure 1 and you’ll see how these two unrelated taxa once nested together when data was less precise. This is a larger specimen than the one shown in figures 2 and 4.

The outgroup for the gracile Artiodactyla in the LRT
includes the oreodont, Merycoidodon (Figs 4, 5) and the new basal hippos, Rostriamynodon and Merycopotomus.

Figure 4. Skeletons of Merycoidodon and Aumelasia to scale and enlarged for comparison. ” data-image-caption=”

Figure 4. Skeletons of Merycoidodon and Aumelasia to scale and enlarged for comparison.

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/merycoidodon_aumelasia_recon588.jpg?w=223″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/merycoidodon_aumelasia_recon588.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-87255″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/merycoidodon_aumelasia_recon588.jpg” alt=”Figure 4. Skeletons of Merycoidodon and Aumelasia to scale and enlarged for comparison.” width=”584″ height=”787″ />

Figure 4. Skeletons of Merycoidodon and smaller specimen of Aumelasia to scale and enlarged for comparison.

When Merycoidodon and Aumelasia are shown together to scale
(Figs 4, 5) one gets the impression that little Aumelasia was about the size of a newborn Merycoidodon. This is the essence of phylogenetic miniaturization and its affect on the evolution of distinctly different descendant taxa, like the gracile artiodactyls.

Figure 5. Skulls of the oreodont, Merycoidodon, to scale with Aumelasia (2 specimens) demonstrating phylogenetic miniaturization with a bit of neotony at the base of the gracile Artiodactyla = pigs, deer, camel, etc. ” data-image-caption=”

” data-medium-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/aumelasia_merycoidodon.skull588-2.jpg?w=272″ data-large-file=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/aumelasia_merycoidodon.skull588-2.jpg?w=584″ tabindex=”0″ role=”button” class=”size-full wp-image-87266″ src=”https://pterosaurheresies.wordpress.com/wp-content/uploads/2024/07/aumelasia_merycoidodon.skull588-2.jpg” alt=”Figure 5. Skulls of the oreodont, Merycoidodon, to scale with Aumelasia (2 specimens) demonstrating phylogenetic miniaturization with a bit of neotony at the base of the gracile Artiodactyla = pigs, deer, camel, etc.” width=”584″ height=”644″ />

Long time readers
will remember that mysticetes (= baleen whales) are also even-toed ungulates/artiodactyls derived from even-toed hippos and even-toed desmostylians. Odontocetes are different. They have five fingers because they are not artiodactyls, but are instead derived from tenrecs, some of which live today and likewise echolocate with torsioned skulls.

References
Sudre J 1980. Aumelasia gabineaudi n.g. n.sp. nouveau Dichobunidae (Artiodactyla, Mammalia) du gisement d’Aumelas (Hérault) d’âge Lutétien terminal. Palaeovertebrata, Mémoire Jubilaire R. Lavocat 9:197–211.

wiki/Aumelasia– in German

Source: https://pterosaurheresies.wordpress.com/2024/07/05/phylogenetic-minaturization-at-the-advent-of-the-gracile-artiodactyla/

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