A better sister for Astrapotherium: Meniscotherium

Revised Dec 5 2016 with new text and images. 

These are difficult taxa to nest:
The tusks of Astrapotherium (Fig. 1; Burmeister 1879; Hatcher 1902) are canines. The premaxilla is missing. The mandible of Astrapotherium really does stick out quite a bit further than the rostrum.

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.

Figure 1. Revised interpretation of Astrapotherium skull with premaxilla absent or transverse and large canines tusks replacing old canine tusks. 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.

Finally a sister!
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 Phenacodus.

Figure 2. Meniscotherium skull. In this is a smaller predecessor to Astrapotherium note the genesis of maxillary tusks here and then longer dentary when the teeth are matched to occlude correctly.

Figure 2. Meniscotherium skull. In this is a smaller predecessor to Astrapotherium note the genesis of maxillary tusks here and then longer dentary when the teeth are matched to occlude correctly. Note the lack of contact between the jugal and squamosal.

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 canine tusks in Meniscotherium.

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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3 thoughts on “A better sister for Astrapotherium: Meniscotherium

  1. There is some debate about the nostrils, some say they are on the edge of the trunk as usually, some other ones believe it could be just opening above, before the trunk’s edge.

      • it’s not without interest – I am finishing to do an Astrapotherium sculpture… (about trunk, Macrauchenia bears a very unusual skull, a bit like a dolphin’s one, if first Amerindiands did some pictures of them, it could help to know how they looked alilke..)

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