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.
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.