Figure 1. Mesosaurus up to 1 meter in length, was long considered an anapsid, but the temporal fenestrae were secondarily infilled from a basal diapsid configuration, as in Claudiosaurus.
Mesosaurus is a Problem for Paleontologists
Here’s the paradigm: Despite its many derived traits, Mesosaurus has long been nested with various other basal reptiles. This long-snouted, needle-toothed, aquatic Permian reptile apparently had no temporal fenestrae, according to Modesto (2006, 2010) and others. Therefore Gauthier (1988) and Modesto (1999) nested Mesosauridae with other such taxa, members of the Captorhinidae and Millerttidae, two basal herbivores without an aquatic niche. Laurin and Reisz (1995) nested Mesosauridae between synapsids and turtles. Laurin and Reisz (2004) nested Mesosaurus with Acleistorhinus (Figure 2), another herbivore sister to Milleretta. None of these proposed taxa even vaguely resemble mesosaurs. None of these studies included Claudiosaurus, ichthyosaurs and thalattosaurs.
Figure 2. Mesosaurus (left) and Acleistorhinus (right) were nested as sister taxa by Laurin and Reisz (2004) despite their many differences. Claudiosaurus and other enaliosaurs were not included in that study.
Not an Anapsid, but a Diapsid
A reconstruction of Mesosaurus (Figure 1) appears to retain at least a sliver of a lateral temporal fenestra. A reconstruction of the more primitive and typically ignored Stereosternum, another mesosaur, appears to retain a complete diapsid configuration. A reconstruction of yet another mesosaur, Wumengosaurus, retains a diapsid configuration, but with the loss of the lower temporal bar by reduction of the quadratojugal.
The Cleithrum
Mesosaurus had a tiny cleithrum, a sliver of bone dorsal to the clavicle on the leading edge of the scapula. It shares this trait with Petrolacosaurus and Claudiosaurus.
The Limbs
The structure of the ankles made walking on land impossible, thus relatives should be looked for among aquatic taxa, not terrestrial herbivores, like Acleistorhinus
It Takes a Larger Study
A larger study of reptiles, and the largest one so far, nests Stereosternum within the basal aquatic diapsids, between Claudiosaurus, and Wumengosaurus, which was basal to ichthyosaurs and thalattosaurs. Other studies did not offer mesosaurs the opportunity to nest elsewhere. When you expand the inclusion list, as was done here, the opportunity for a correct nesting increases.
Figure 2. The nesting of the mesosaur, Stereosternum, in the Reptile Family tree at the base of the aquatic clade, the Enaliosauria.
Below are skeletal images of the sisters of Mesosaurus in phylogenetic order. The similarities are obvious and follow a gradual evolutionary sequence. The reduction and closure of the temporal fenestrae are also found in sister taxa including Araeoscelis and Pachypleurosaurus (not shown here).
Figure 3. The sisters of Mesosaurus from the basal diapsid, Petrolacosaurus to the basal ichthyosaur, Utatsusaurus.
Check out the various taxon names in www.reptileevolution.com for more details. As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.
Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.
References:
Gervais P 1865. Du Mesosaurus tenuidens, reptile fossile de l’Afrique australe. Comptes Rendus de l’Académie de Sciences 60:950–955.
Laurin M and Reisz RR 1995. A reevaluation of early amniote phylogeny. Zoological Journal of the Linnean Society 113:165-223.
Modesto SP 1999.Observations on the structure of the Early Permian reptile.
Modesto SP 2006. The cranial skeleton of the Early Permian aquatic reptile Mesosaurus tenuidens: implications for relationships and palaeobiology. Zoological Journal of the Linnean Society 146 (3): 345–368. doi:10.1111/j.1096-3642.2006.00205.x.
Modesto SP 2010. The postcranial skeleton of the aquatic parareptile Mesosaurus tenuidensfrom the Gondwanan Permian. Journal of Vertebrate Paleontology 30 (5): 1378–1395. doi:10.1080/02724634.2010.501443.
The Pterosaur Heresies 