Mesosaur-oid Skulls

It’s a Unfortunate Tradition in Paleontology
Mesosaurus has been left pretty much alone in the fossil record, with few to no relatives close in morphology. That’s why it has been associated with such mismatches as pareiasaurs, captorhinids and other oddballs among the Anapsida (= basal Reptilia).

Here (Fig. 1) in a heretical presentation you haven’t seen in any prior publications, are a few mesosaur-oid cousins recovered from the large reptile tree. Here Mesosaurus nests among the basal enaliosauria (sea reptiles) and its quite derived, not primitive. Even so, a sister to Mesosaurus gave rise to several large clades of paddle-finned reptiles.

It’s best to let the evidence to the talking. Gosh they sure look like cousins, don’t they? Mesosaurus was not alone. Closing of the upper temporal fenestra was common in this clade, but not universal. Earlier we looked at evidence for an open lateral temporal fenestra.

Mesosaurus and its kin

Figure 1. Mesosaurus and its kin among the ichthyosaurs and thalattosaurs. Please note the similarities in the placement of the naris, the skull sutures and the general reduction (in many cases, not all) of the teeth. The closure of the upper temporal fenestra in Mesosaurus and thalattosaurs was by convergence with different sutural patterns. That’s evolution for ya!

Compare and contrast
Sometimes you have to get the entire family into the same picture to see the similarities and family ties. That’s the case here as I illustrate the relationships recovered by the large reptile tree simply by putting several similar skulls together following the patterns recovered by phylogenetic analysis. Other closely related forms (not included in the figure) reduced the rostrum, modified their teeth and the post-crania likewise evolved in different directions. These include Sinosaurosphargis and the placodonts and Vancleavea among the thalattosaurs. Mesosaurus itself was more specialized than Stereosternum and whatever unknown protomesosaurs derived from a sister to Claudiosaurus await their eventual discovery.

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.

3 thoughts on “Mesosaur-oid Skulls

  1. Which taxa do not belong? Please be specific. Where are the mismatches? To your point, mosasaurs, aquatic turtles and aquatic sphenodontians don’t nest with enaliosaurs. Here there is a gradual accumulation of characters. If there were not, THEN I would question the results. Mickey, you might be living off untested dreams of the past and paleo hero worship. You can help science by pointing out specific problems. I hope you will. This tree was able to separate one clade of snakes from another, so it handles convergence very well.

  2. I of course don’t know which aquatic taxa should go where, because other analyses have problems with suprageneric taxa (as you correctly point out) and coding accuracy (though at least not also affected by your photo manipulation technique), so I don’t trust them either. Limbless pygopodines and skinks didn’t nest with snakes+Anniella+amphisbaenians+dibamids in Gauthier et al.’s tree, but molecular data plus tests of eliminating taxa show convergence was still at work. Note your “two clades of snakes” are divided by lifestyle- burrowing vs. aquatic, which itself is an example of convergence. Vidal and David (2004) found that Uropeltis groups with Boa instead of Leptotyphlops by two nodes supported with 100% Bayesian posterior probability based on two nuclear genes. Similarly, Zhou et al. (2006) showed that Leptotyphlops groups with the non-burrowing Dinodon to the exclusion of Varanus with 100% probablity, but this time based on full mitochondrial genomes. So we have two independent lines of evidence which also agree with all other morphological analyses, which makes your competing hypothesis highly unlikely.

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