Why Lizards Are Not Diapsids

The Paradigm
Reptiles were originally divided and sorted according to the number of openings in the temple and cheek regions of the skull. Some reptiles had one in the temple. Others had one in the cheek. Some had none. Reptiles with openings in the temple and cheek were considered diapsids, which described the “two arches” between and below the two openings.

Recent Studies Supporting the Paradigm
Traditional computer-assisted phylogenies, beginning with Gauthier et al. (1988) and Laurin (1991), placed the araeoscelidans (Petrolacosaurus and Araeoscelis) and the younginids (Youngina, Thadeosaurus and Orovenator) at the base of both the lepidosaurs (squamates + sphenodontians) and the archosauromorphs (prolacertiformes + rhynchosauria + archosauriforms). Paleothyris was identified as a precursor to the araeoscelidans. In this scenario diapsid openings appeared apart from the synapsid opening. The lower temporal arch, beneath the lateral temporal fenestra, was retained in Sphenodon and disappeared in squamates.

But is this true?

The Heretical View Supported by a Larger Dataset
The present large study, employing many more taxa, nested one Youngina at the base of the Prolacertiformes, which includes the Archosauriformes. Other Youngina nested within the Archosauriformes. Lepidosaurs nested elsewhere, far from Petrolacosaurus and Youngina. According to the present large study, lepidosaur precursors, including Owenetta, Paliguana, Gephyrosaurus and Meyasaurus, did not have a lower temporal arch, at least, not a complete one. Some lepidosaurs (such as Sphenodon, rhynchosaurs, pterosaurs and certain basal scleroglossans, such as Tianyusaurus) developed a lower temporal arch on their own, often independently of one another).

Reptile family tree

Figure 1. Click to enlarge. Here lepidosaurs nest far from Petrolacosaurus and the rest of the Diapsida.

Thus lepidosaurs can, at best, be considered quasi-diapsids. True diapsids, those related to and descending from Petrolacosaurus include the araeoscelids, enaliosaurs (Claudiosaurus, Mesosaurus and a host of marine reptiles) and younginiforms (Thadeosaurus through the Archosauriformes).

True Diapsid Precursors Were Derived from Basal Synapsids
The true diapsids were derived from a lineage of synapsids apart from the lineage of therapsids and eupelycosaurs beginning with Heleosaurus, which was originally described as a diapsid, ironically enough. In this scenario the lateral temporal fenestra was already present when the upper temporal opening appeared in Spinoaequalis, Eudibamus and Petrolacosaurus. Changes to the diapsid appearance occurred almost immediately with Milleropsis, Mesosaurus and Araeoscelis, but a more conservative branch that retained the diapsid configuration ultimately led to Youngina and the archosaur diapsids living today, the birds and crocs.

The large study provides genus-based precursors and successors from the first tetrapods to living taxa and all sister taxa resemble one another in a gradual spectrum of morphologies, echoing the evolutionary process. Other prior studies did not go into this depth, nor did other studies include the present gamut of taxa.

Definitions in Need of Revision
Laurin (1991) defined the Diapsida as the most recent common ancestor of araeoscelidians, lepidosaurs and archosaurs and all its descendants. According to the present results, the definition is now redundant with the Amniota and Reptilia.

Benton (1985) defined Neodiapsida as Youngina and all species more closely related to it than to Petrolacosaurus. According to Benton (1985) this definition likewise is in need of revision because it too contains lepidosaurs.

Gauthier, Kluge & Rowe (1988) defined Sauria as the most recent common ancestor of Lepidosauria and Archosauria and all of its descendants. Now that definition is redundant with Reptilia.

Gauthier (1994) defined Sauropsida as “Reptiles plus all other amniotes more closely related to them than they are to mammals,” based on traditional cladograms that indicated a basal split between the Synapsida and Sauropsida. Here the basal split was between archosauromorphs (which included synapsids) and lepidosauromorphs, so this definition defines a paraphyletic assemblage.

Many more definitions are no longer valid based on the new nestings and branchings recovered in the new tree. We’ll discuss these in future blogs.

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
Benton MJ 1985. Classification and phylogeny of diapsid reptiles. Zoological Journal of the Linnean Society 84: 97-164.
Callaway JM 1997.
 Ichthyosauria: Introduction, in JM Callaway & EL Nicholls (eds.), Ancient Marine Reptiles. Academic Press, pp. 3–16.
Gauthier J, Kluge AG and Rowe T 1988. The early evolution of the Amniota. In Michael J. Benton (ed.) The phylogeny and classification of the tetrapods, Volume 1: amphibians, reptiles, birds: 103-155. Oxford: Clarendon Press.
Gauthier J, Estes R and DeQueiroz K 1988. A phylogenetic analysis of Lepidosauria; pp. 15-98 in R. Estes and G. Pregill (eds.), Phylogenetic Relationships of the Lizard Families. Stanford University Press, Stanford, California.
Laurin M 1991. The osteology of a Lower Permian eosuchian from Texas and a review of diapsid phylogeny. Zoological Journal of the Linnean Society 101:59-95.
Laurin M and Reisz R 1995. A reevaluation of early amniote phylogeny. Zoological Journal of the Linnean Society, 113: 165–223.
Modesto SP and Anderson JS 2004.
The Phylogenetic Definition of Reptilia. Systematic Biology 53(5):815-821.
Reisz RR, Modesto SP and Scot DMT 2011. A new Early Permian reptile and its significance in early diapsid evolution. Proceedings of the Royal Society, London B doi:10.1098/rspb.2011.0439

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