A better sister for Astrapotherium: Meniscotherium

This starts with a confession:
Earlier I, like everyone else before today, interpreted the tusks of Astrapotherium (Fig. 1; Burmeister 1879; Hatcher 1902) as canines. They’re not canines. They’re incisor tusks, like those of elephants. The original drawing (complete skeleton in Fig. 1) including some dotted lines indicating missing rostral material. That was conjectural and mistaken. The mandible of Astrapotherium really does stick out quite a bit further than the rostrum. It’s important to remember that not all drawings have to be interpreted the way they look. Mistakes can be made by anyone anywhere.

Figure 1. New interpretation of Astrapotherium skull with premaxilla and large incisor tusks replacing old canine tusks. The canines are absent.

Figure 1. New interpretation of Astrapotherium skull with premaxilla and large incisor tusks replacing old canine tusks. The canines are absent. That manual digit 1 that Hatcher 1902 did not like in his drawing (Fig. 1 toned beige) is actually a good fit and works in phylogenetic bracketing. 

And that’s not the only error
Like so many others before me, I also fell prey to errors that arise via taxon exclusion. I didn’t have one really good enough sister taxon for Astrapotherium. Now I do.

Meet Meniscotherium
(Figs. 2, 3; Cope 1874; Williamson and Lucas 1992; Middle Eocene 54-38 mya; 25-50 cm long), which Wikipedia describes as a dog-sized herbivore with hooves found as a pack of individuals.

Cooper et al. 2014 nested Meniscotherium with Phenacodus as a condylarth, a possible member of Afrotheria, perissodactyl. They did not test Astrapotherium.

Wible et al. 2007 nested Meniscotherium close to early cetioartiodactyls (an invalid clade) and close to early Carnivora. They, likewise, did not test Astrapotherium.

The LRT nests the clade of Astrapotherium + Meniscotherium between the clade of Edentates and the clade of Proboscideans + Sirenians. I will add those and several other taxa to the LRT when I have a little more time.

Figure 2. Meniscotherium skull. Note the genesis of premaxillary tusks here along with the diminishing canine (orange). This is a smaller predecessor to Astrapotherium.

Figure 2. Meniscotherium skull. Note the genesis of premaxillary tusks here along with the diminishing canine (orange). This is a smaller predecessor to Astrapotherium.

The retention of five fingers and five toes
is key to the phylogenetic nesting of these taxa. More derived taxa start losing digit 1. We can see the genesis of incisor tusks in this genus.

Figure 3. Meniscortherium skeleton. The fingers and toes are not known. This reconstruction differs from the original in that the pelvis is rotated more vertically.

Figure 3. Meniscortherium skeleton. The fingers and toes are not known. This reconstruction differs from the original in that the pelvis is rotated more vertically. Some specimens were 25 cm long. Others were 50 cm long estimated.

Meniscotherium is the smaller and more plesiomorphic
of the two and is found in earlier strata (Eocene, 50-38 mya) than Astrapotherium (late Oligocene, Middle Miocene, 28-15 mya).

Figure 4. Astrapotherium to scale with two specimens of Meniscotherium.

Figure 4. Astrapotherium to scale with two specimens of Meniscotherium.

References
Burmeister 1879. Description physique de al République Agentine, T. III 1879:517.
Cooper LN, Seiffert ER, Clementz M, Madar SI, Bajpai S, Hussain ST, Thewissen JGM 2014-10-08. 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.
Hatcher JB 1901. Report of the Princeton University Expeditions to Patagonia 1869-1899. Mammalia of the Santa Cruz Beds. IV. Astrapotheria. Scott WB ed. Vol. 6, Paleontology 3. Princeton, NJ Stuttgart 1909-1928.
Wible JR, Rougier GW, Novacek MJ, Asher RJ 2007. Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary. Nature 447: 1003–1006. doi: 10.1038/nature05854
Williamson TE, Lucas SG 1992. Meniscotherium (Mammalia, “Condylarthra”) from the Paleocene-Eocene of western North America. Bulletin of the New Mexico Museum of Natural History and Science 1: 1–54.

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