Mesosaurus Mistake?

The recent interpretation of a lateral temporal fenestra in Mesosaurus (Piñeiro et al. 2012, Fig. 1) was accompanied by a new interpretation of the premaxilla/maxilla suture. Added to this problem, the published restorations of the Mesosaurus skull in lateral and dorsal views don’t match (Fig. 1). Note the shifting of the fenestrae among other sutural changes.  Unfortunately, no sister taxa have the premaxilla/nasal configuration that Piñeiro et al. (2012) propose.

Mesosaurus skull with lateral temporal fenestra.

Figure 1. Mesosaurus skull with lateral temporal fenestra. From Piñeiro et al. 2012. Note the many differences in bone shape not attributable to the two distinct views. More rigorous attention would have aligned the fenestra and sutures in both views of these apparently freehand sketches.

Mesosaurus skull reconstructed based on data from Modesto (2006).

Figure 2. Mesosaurus skull reconstructed based on data from Modesto (2006).

Modesto
had it right.

Modesto (2006) delineated and identified the premaxilla and nasal of Mesosaurus (Fig. 2) and these sutures also agree with those of sister taxa, as recovered by the large reptile tree. With or without the skull, mesosaurs don’t shift within the tree.

Rostrum of Mesosaurus (dorsal view).

Figure 3. New tracings of the Piñeiro et al. (2012) rostral skull of Mesosaurus. On the left the bones are colored in the new interpretation. At right blue indicates published interpretation of nasals, unlike those of any other sister.

The split or separation of the nasals seen in thalattosaurs and sauropterygians (by convergence) has its origin in mesosaurs. In these taxa the elongated premaxilla contacts (sometimes just barely) the frontals. In ichthyosaurs, like wumengosaurs and mesosaurs, the premaxilla does not contact the frontals. Even so, it does split the nasals at least anteriorly.
Don’t Forget the von Huene (1940) Interpretation.
von Huene (1940) spent a considerable amount of time examining several Mesosaurus specimens and he was the first to interpret a lateral temporal fenestra (Fig. 4). He also interpreted nasals extending anteriorly a short distance beyond the nares as in the Piñeiro (2012) interpretation.

Almost overlooked, von Huene’s interpretations of the rostrum are much narrower than those of Piñeiro et al. (2012) indicating some variation in the specimens they were looking at. Certainly these are distinct species. Perhaps distinct genera?

Mesosaurus according to von Huene 1940.

Figure 4. Mesosaurus according to von Huene 1940. Color added to nasals in blue and premaxilla in yellow. This interpretation of the premaxilla/nasal suture matches that of Piñeiro et al. (2012).

Which is Correct?
Neither Piñeiro et al. (2012) nor von Huene (1940) compared their specimens to sisters recovered by the large reptile tree in which the premaxilla bisects the nasals. The image below is recolorized from Piñeiro (2012) showing two interpretations: 1) a longer premaxilla and 2) a possible interdigitating nasal and premaxilla. Hard to say given the data (Fig. 2).

Two reinterpretations of the premaxilla/nasal suture in a recolorized restoration of Mesosaurus

Figure 5. Two reinterpretations of the premaxilla/nasal suture in a recolorized restoration of Mesosaurus by Piñeiro et al. (2012). See Fig. 1 for the original. Also more squamosal has been restored on one side. It is not only possible, but likely that there is variation in several aspects of Mesosaurus from quarry to quarry and from species to species.

What’s Interesting
Here you see variation in interpretation between three paleontologists (one being the Piñeiro team). Look around and you’ll see the maxilla extends beyond mid orbit in two interpretations, but not the Modesto (2004) one. The tooth count differs in all three. The rostral proportions differ. The lateral temporal fenestra differs. The premaxilla/nasal suture differs. The frontal/nasal suture differs. Granted, they are not all looking at the same specimens. And now you can see why it is so important in phylogenetic analysis to use specimens for taxa, even if incomplete.

The Good Thing Is…
Mesosaurus will never again be lumped with basal anapsids like pareiasaurs and captorhinids. Hopefully…

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 Stereosternum tumidum Cope. Palaeontol. Afr. 35, 7–19.
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.
Piñeiro G, Ferigolo J, Ramos A and Laurin M 2012.
 Cranial morphology
of the Early Permian mesosaurid *Mesosaurus tenuidens* and the evolution of
the lower temporal fenestration reassessed. Comptes Rendus Palevol.
von Huene F 1941. Osteologie und systematische Stellung von Mesosaurus. Palaeontogr. Abt. A. 92, 45–58.

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