New Middle Jurassic lizard – svp abstracts 2013

From the abstract
Conrad et al. (2013) wrote: “The last three decades has seen a dramatic increase in our knowledge regarding the earliest evolution of the major squamate clades, but most known fossils are Cretaceous or younger. The earliest known squamates are the incompletely known Parviraptor, Eichstaettisauridae, Ardeosaurus, and the Paramacellodidae with their osteodermal armor. We report on a new late Middle Jurassic lizard from the Shishugou Formation of China representing the oldest complete squamate skeleton. The animal possesses vomerine teeth, a rectangular frontal, and incipient cusps on its marginal teeth. The preserved hind limb is very elongate. The entire body was encased in osteoderms.

The resultant phylogenetic hypothesis finds a “gecko-morphotype” (unarmored, relatively large-eyed, morphs with limbs of intermediate length and simple, insectivore-style teeth) to be ancestral for squamates. Our new lizard is recovered as a basal episquamate, related to lateratans, anguimorphs, and iguanomorphs.

The Late Jurassic saw the rise of therian mammals and coelurosaurian dinosaurs. At the same time, squamates enter the fossil record in both the gecko-morphotype and armored forms (e.g., Paramacellodus and our new taxon). We suggest that the selective pressure from this changing fauna may have helped “push” squamates into new morphotypes. Many known Late Jurassic and Cretaceous episquamates possess long legs (e.g., Bavarisaurus, Saichangurvel) and/or extensive osteodermal armor (e.g. Paramacellodus). These pressures may have contributed to the marginalization of the previously diverse and widespread rhynchocephalians.”

Notes
Conrad et al. are following Hedges (2005), a DNA study, in nesting geckos at the base of the squamata, iguania nesting higher. Hedges found legless Dibamus as the most basal squamate. Of the remaining taxa (Bifurcata), the gekkonids form a basal lineage. The Unidentata, squamates that are neither dibamids nor gekkonids, are divided into the Scinciformata (scincids, xantusiids, and cordylids) and the Episquamata. These include Laterata (Teiformata, Lacertiformata, and Amphisbaenia, with the latter two joined in Lacertibaenia) and Toxicofera (iguanians, anguimorphs and snakes). So, distinct from the large reptile tree, Hedges (2005) links Iguana and snakes on the basis of DNA.

It is unfortunate that Conrad et al. used a DNA tree that differs so much from a morphological tree because fossils cannot be tested for DNA.

Lacertulus

Figure 1. Lacertulus, a basal tritosaur lepidosaur, also at the base of the Squamata.

It is also unfortunate that Conrad et al. do not recognize the third squamate clade, the Tritosauria, that reached their origin in the Permian (Lacertulus, Fig. 1) and reached their acme in the Triassic. Tritosaurs dwindled in diversity into the Cretaceous when only Huehuecuetzpalli and pterosaurs survived. These too became extinct by the end of the Cretaceous.

Figure 3. Click to enlarge. Reconstruction of TA1045, an unnamed Early Permian pre-lizard from Germany. Close to Dalinghosaurus, this genus had shorter legs and a longer torso. In grey above are the transverse belly scales.

Figure 2. Click to enlarge. Reconstruction of TA1045, an unnamed Early Permian pre-lizard from Germany. Close to Dalinghosaurus, this genus had shorter legs and a longer torso. In grey above are the transverse belly scales.

TA 1045 (Fig. 2) is an unnamed tritosaur lepidosaur from the Early Permian. Considering the antiquity of TA 1045 and Lacertulus (both Permian) and the diversity of Triassic tritosaurs and Middle Jurassic squamates, there are certainly many more basal squamates  out there to be found in Triassic strata. Odd that they have not been discovered as yet.

I’m looking forward to seeing the new armored lizard to nest it morphologically.

References
Conrad J, Wang Y, Xu X, Pyron A and Clark JG. 2013. Skeleton of a heavily armored and long legged middle Jurassic lizard (Squamata, Reptilia). Journal of Vertebrate Paleontology  abstracts.
Hedges VN 2005. The phylogeny of squamate reptiles (lizards, snakes, and amphisbaenians) inferred from nine nuclear protein-coding genes. CR Biology 328(10-11):1000-1008. Epub 2005 Oct 27.

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