New ‘Mesenosaurus’ closer to other taxa in the LRT

Maho, Gee and Reisz 2019
introduce us to a new Early Permian skull-only specimens (OMNH 73208, OMNH 73209 and OMNH 73500; Figs. 1, 2) they attribute to a new species of an old genus, Mesenosaurus efremovi

Figure 1. ?Mesenosaurus efremovi, left and right sides from Maho et al. 2019. Colors added using DGS techniques.

Figure 1. ?Mesenosaurus efremovi, left and right sides from Maho et al. 2019. Colors added using DGS techniques. Note the antorbital fossa without a fenestra.

The original art in Maho et al. 2019
(Fig. 1) was used to create this reconstruction (Fig. 2) using DGS techniques, using the best bones from the left and right to make this reconstruction. There are no surprises here.

Figure 2. ?Mesenosaurus efremovi reconstructed using DGS techniques.

Figure 2. ?Mesenosaurus efremovi reconstructed using DGS techniques. See figure 1.

After testing OMNH 73209
(Figs. 1, 2) in the large reptile tree (LRT, 1592 taxa, subset Fig. 4) this specimen nests a node away from three other Mesenosaurus specimens. So, distinct from Maho et al. 2019 (Fig 3), this specimen is not congeneric with other Mesenosaurus in the LRT (Fig. 4).

Figure 3. From Maho et al. 2019, their cladogram of Mesenosaurus relationships.

Figure 3. From Maho et al. 2019, their cladogram of Mesenosaurus relationships.

Further complicating matters,
Maho et al. contend that Mesenosaurus is a varanopid (Fig. 3). This is an outmoded traditional hypothesis invalidated in 2014 by the LRT. Mesenosaurus and kin are Protodiapsids, nesting between the basalmost synapsid, Vaughnictis and archosauromorphs with a diapsid skull architecture, labeled Diapsida in the LRT.

Figure 4. Subset of the LRT focusing on the clade Protodiapsida nesting between basalmost synapsids and archosauromorph diapsids.

Figure 4. Subset of the LRT focusing on the clade Protodiapsida nesting between basalmost synapsids and archosauromorph diapsids.

As we learned earlier in 2011
lepidosauriformes also developed a diapsid skull architecture by convergence, here labeled Lepidosauriformes. That news has not reached the three authors, Maho, Gee and Reisz, nor have they tested a sufficient number of pertinent taxa to recover that basal dichotomy, known for the last eight years.

Figure 1. Mesenosaurus skulls compared to sisters Heleosaurus and Mycterosaurus. Note the greater angularity of the skull shapes along with the wider posterior skulls in derived taxa (toward the bottom). The SGU specimen needs better data on the squamosal, which is illustrated as missing its ventral/lateral portion here.

Figure 5. Mesenosaurus skulls compared to sisters Heleosaurus and Mycterosaurus. Note the greater angularity of the skull shapes along with the wider posterior skulls in derived taxa (toward the bottom). The SGU specimen needs better data on the squamosal, which is illustrated as missing its ventral/lateral portion here.

According to the LRT
the various Mesenosaurus specimens (Fig. 5) demonstrate a wider variety of skull shapes than do many congeneric taxa.

Figure 2. A subset of the large reptile tree showing the relationships of protosynapsids, synapsids, protodiapsids and diapsids. Traditionally nested with synapsids as varanopids, the protodiapsids have rarely, if ever, been tested with diapsids.

Figure 6. A subset of the large reptile tree showing the relationships of protosynapsids, synapsids, protodiapsids and diapsids. Traditionally nested with synapsids as varanopids, the protodiapsids have rarely, if ever, been tested with diapsids.

Varanopids have a smaller clade membership
than the authors suppose when more taxa are tested. All they have to do is add taxa to see their varanopid clade become a dichotomy (Fig. 6) leading to mammals on one line and dinosaurs and giant sea reptiles on the other. All this was overlooked by the PhDs.


References
Maho S, Gee BM, Reisz RR 2019. A new varanopid synapsid from the early Permian of Oklahoma and the evolutionary stasis in this clade. R. Soc. open sci. 6: 191297. http://dx.doi.org/10.1098/rsos.191297

https://pterosaurheresies.wordpress.com/2014/03/03/basal-diapsida-and-proto-diapsida/

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