The evolution of Limnoscelis from Milleretta and Orobates

Figure 1. Limnoscelis based on Berman et al. 2010.

Wikipedia reports that Limnoscelis (Williston 1911) was a large (1.5m) diadectomorph (a type of reptile-like amphibian) from the Early Permian. They report, distinct from other diadectomorphs, Limnoscelis appears to have been a carnivore, but one without claws. Palaeos likewise nests Limnoscelis as an anamniote.

On the other hand…
The large reptile tree nests Limnoscelis and other diadectomorphs deep within the Reptilia.  Here we’ll take a look at Limnoscelis with a few of its closest ancestors, Orobates and Milleretta. Tseajaia and Tetraceratops are a sister clade to Limnoscelis.

Figure 2. Milleretta (RC14 and RC70 specimens), Orobates and Limnoscelis. 1. long anterior teeth. 2. Orbit loses dorsal exposure.

Limnoscelis paludis (Williston 1911) Late Pennsylvanian, 1.5m in length. Distinct from Orobates, the skull of Limnoscelis had a deeper premaxilla with more robust premaxillary fangs and a higher naris. The rostrum was longer. The orbit was relatively smaller. As in Milleretta a depression appeared between the ectopterygoid and pterygoid and the palate was otherwise similar. The neural spines were expanded. The elongated posterior process of the ilium is larger. The anterior caudals had smaller transverse processes. More posterior vertebrae had ribs.

Figure 3. Milleretta, Orobates and Limnoscelis. Lower images are to scale. Note the development of the posterior ilium process and broader cheek bones in Orobates and Limnoscelis. The expanded ribs of Milleretta are not retained in these taxa.

The literature hasn’t made the connection from Milleretta to Orobates and Limnoscelis, hence the need for a large reptile tree. When you put them together, though, the similarities start to shine through. The evolution of Orobates is one of creating a giant Milleretta. The evolution of Limnoscelis is one of creating a giant Orobates, without the girth of the diadectids.

Funny that in doing so, Limnoscelis started fooling paleontologists into thinking it was an amphibian of sorts, but one that didn’t look like any amphibians anyone has ever seen.

So that’s how you get one carnivore from out of the diadectomorpha. Limnoscelis is a milleretid.

References
Berman DS Reisz RR and Scott D 2010. Redescription of the skull of Limmoscelis paludis Williston (Diadectomorpha: Limnoscelidae) from the Pennsylvanian of Canon del Cobre, northern New Mexico: In: Carboniferous-Permian Transition in Canon del Cobre, Northern New Mexico, edited by Lucas, S. G., Schneider, J. W., and Spielmann, New Mexico Museum of Natural History & Science, Bulletin 49, p. 185-210.
Romer AS 1946. The primitive reptile Limnoscelis restudied American Journal of Science, Vol. 244:149-188
Williston SW 1911. A new family of reptiles from the Permian of New Mexico: American Journal of Science, Series 4, 31:378-398.

wiki/Limnoscelis