The strange skull of Moeritherium, a pre-manatee with legs

Recent and current housekeeping the LRT
brings several taxa to renewed attention several years after their addition. Today, let’s take another look at the strange manatee ancestor in the large reptile tree (LRT) and elsewhere, Moeritherium (Figs. 1–3).

Figure 1. Moeritherium skeleton sans hands and feet.

Distinct from nearly all other vertebrates
except for a few fish (like Gymnothorax, the moray eel and Eurypharynx, the deep sea gulper eel), Moeritherium (Fig. 2) has an odd skull with an orbit nearly above the premaxilla, most of the teeth posterior to the orbit and a very longer cranium supporting a posteriorly set jaw joint.

Figure 2. Moeritherium skull animation. The first tooth in the maxilla is behind the oribit, distinct from almost all other tetrapods.

Ancestral to Moeritherium
are two manatee and elephant ancetors, Notostylops (Fig. 3) and Rostriamynodon (Fig. 4).

Figure 3. Procavia compared to Moeritherium. Such side-by-side comparisons are instructive when trying to figure out sutures from cracks. (I said trying!) You gotta admit, color helps. And no labels or abbreviations necessary.

Figure 4. Rostriamynodon skull in three views, Colors added.

Moeritherium lyonsi
(Andrews 1901; Eocene, 36 mya; 70 cm tall at shoulder) nests here as an ancestor to sea cows, like Dusisiren. Moeritherium is derived from a sister to Notostylops. Note the similar small tusks. The orbit was far forward, over the diastema. Huge jaw muscles filled the back half of the skull. The cervicals were long with robust fused ribs extending laterally. A long torso, tiny tail and short limbs are assembled from from bits and pieces of several specimens.

Figure 5. Another view of the strange skull of Moeritherium. Colors added here.

References
Andrews RC 1901 Tagebl. d. V. International Zool. Congr. Berlin 6: 41.
Andrews CW 1906. A Descriptive Catalogue of the Tertiary Vertebrata of the Fayûm, Egypt. Based on the Collection of the Egyptian Government in the Geological Museum, Cairo, and on the Collection in the British Museum (Natural History), London. 324 pp. Natural History Museum, London.
Delmer C, Mahboubi M, Tabuce R and Tassy P 2006. A New Species of Moeritherium (Proboscidea, Mammalia) from the Eocene of Algeria: New Perspectives on the Ancestral Morphotype of the Genus. Palaeontology. 49 (2): 421–434.
Matsumoto H 1923. A Contribution to the Knowledge of Mœritherium. Bulletin of the American Museum of Natural History 48(4): 97-140.

wiki/Moeritherium