Fenestration in the Mesosaurus Skull – Piñeiro et al. 2012

Not an Anapsid After All
Traditionally Mesosaurus (Gervais P 1865) has been considered a basal reptile, an anapsid (Laurin and Reisz 1995, Modesto 1999) lacking any temporal fenestration. Most skulls are crushed and slightly scattered, requiring reassembly. It’s been confusing. Now a new paper by Piñeiro et al. (2012) provides strong evidence of a lower termporal fenestra in undisturbed material.

Graciela Piñeiro,

Figure 1. Graciela Piñeiro, lead author on the new mesosaurus paper.

The Piñeiro et al. (2012) Abstract
The Early Permian mesosaurids are the oldest known aquatic amniotes with an exclusively Gondwanan distribution. Although several hundred of complete skeletons have been discovered and intensively studied, the anatomy and taxonomic composition of the group, as well as its phylogenetic relationships remain controversial. Several well-preserved
mesosaurid specimens found in Uruguay justify a new anatomical reconstruction of the skull of Mesosaurus tenuidens, differing from earlier ones especially in the presence of a lower temporal fenestra. The significance of this structure for the evolution of temporal fenestration in amniotes is evaluated according to the two most recent phylogenetic hypotheses, in which mesosaurids are basalmost sauropsids or basalmost parareptiles. A synapsid-like fenestration may be the primitive condition for Amniota, and it may be also a basal condition for parareptiles, because recent phylogenies suggest a basal position for mesosaurids and lanthanosuchoids within that group, and both possess a lower temporal fenestra. Our results also give a moderately strengthened support for diapsid affinities of turtles.

Mesosaurus skull with lateral temporal fenestra.

Figure 2. Mesosaurus skull with lateral temporal fenestra. From Piñeiro et al. 2012.

The New Mesosaurus Skull
The lateral temporal fenestra of this Mesosaurus (Fig. 2) occurs between embayments of the jugal and squamosal. Some skulls may have closed off this fenestra. Others may produce a slightly different fenestra shape. This trait may be universal or not. It’s hard to tell, but at least some specimens of Mesosaurus had this trait.

Earlier I reconstructed the skull of Mesosaurus with a lateral termporal fenestra based on reassembling  in situ tracings by Modesto (2006). With or without this trait, mesosaurs nested between Claudiosaurus and kin and enaliosaurs (sauropterygians, ichthyosaurs and thalattosaurs). That was not the first time someone proposed a lateral temporal fenestra (reference escapes me at the moment [von Huene 1941), but finding a lateral temporal fenestra broke with current paradigm. Further study of the more primitive and shorter-toothed Stereosternum will hopefully vindicate the appearance of an upper temporal fenestra as well.

Missing Sister Taxa 
Pineiro et al. (2012) attempted to nest Mesosaurus in a pseudoclade of select taxa virtually all of which possessed a lateral temporal fenestra, whether bound ventrally by a jugal/ quadratojugal/ quadrate bar or not. This tree assumed the lateral temporal fenestra appeared only once without convergence. Turtles (a taxon without temporal fenestration) were also included. Sauropterygians and several basal, mesosaur-like, aquatic forms with temporal fenestration, including Claudiosaurus, Hovasaurus, Thadeosaurus and Wumengosaurus were not included. Also missing were Adelosaurus and Acerosodontosaurus closer to the araeoscelids. Importantly, the large reptile tree found these six taxa to be the closest sisters to mesosaurs (represented by Stereosternum). Excluding these taxa is an unfortunate oversight that could have taken the study to the next level because these six taxa displayed various forms of temporal fenestration and mesosaurs nested in the midst of them.

Disagreements on the Phylogenetic Placement
Piñeiro et al. (2012) placed (not nested) Mesosauridae between Synapsida (Eothyris at the base) and Sauropsida (Romeria, etc. at the base), several nodes away from Petrolacosaurus and Ichthyosauria. Piñeiro et al. (2012) also placed Mesosauridae between Milleretidae and the aforementioned Romeria, etc.) following traditional topologies by Laurin and Reisz (1995) for basal amniotes, Reisz et  al. (2007) for “parareptiles,” and Laurin (2004) for other taxa, from Modesto (1999) for mesosaurids. Missing from the above studies were more undulating aquatic reptiles (listed above) like mesosaurs.

What is Happening in This Clade?
The examples of sister taxa in the large reptile tree indicate that the initial appearance of the diapsid configuration in taxa like Eudibamus and Petrolacosaurus, was modified in derived taxa like Araeoscelis (loss of lateral temporal fenestra), Mesosaurus (loss of upper temporal fenestra) and Wumengosaurus and sauropterygians (loss of lower temporal bar).

What the Large Reptile Tree Indicates
The large reptile tree indicates that the diapsid configuration appeared at least twice by convergence and variations thereof also appear by convergence. The lower temporal bar appeared in certain lines and disappeared in others.

Why were turtles included in this study?
Turtles are anapsids derived from diadectomorphs like Stephanospondylus and have nothing to do with diapsids like Petrolacosaurus or Mesosaurus, according to the recovered tree. Rather than elevating mesosaurs to the ranks of derived taxa, the authors proposed lowering the synapsid-like fenestra trait to the basal ranks (see abstract above), suggesting (to them) that turtles lost their diapsid configuration during their evolution. That hypothesis is not supported by the present tree based on a magnitude more taxa that encompasses the entire Reptilia.

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.

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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