(Before It's News)
we looked at the wider and narrower definitions of the term ‘stem’.
Figure 1. CLICK TO ENLARGE. Stem taxa are closest ancestors to living taxa. Here basal diapsids and marine enaliosaurs are stem archosaurs. Triceratops is a stem bird. Captorhinids are stem turtles. Pterosaurs are stem squamates. The colors here indicate the wider definition of ‘stem’.
From the Klugman and Pritchard abstract (abridged)
“Crown group lepidosaurs are highly diverse: they comprise more than 7,000 globally distributed extant species of lizards and snakes (Squamata), plus the single rhynchocephalian genus Sphenodon. The earliest known lepidosaurs are rhynchocephalians from the Late Triassic of Europe, (1) and this group quickly diversified and achieved a global distribution by the end of the Triassic. In contrast, early squamates have a sparse fossil record; their first representatives are found in the Early-Middle Jurassic of Laurasia (2). Although Rhynchocephalia and Squamata diverged in the Middle Triassic, a 40-50 million years ghost lineage exists for Squamata. Jurassic squamates are already considerably derived, and have already diversified into their extant groups, which testifies to a substantial gap in the known fossil record. Here we report on a new lepidosaur from a Norian microvertebrate site in Petrified Forest National Park, Arizona. This fossiliferous locality is from the Upper Blue Mesa Member of the Chinle Formation, and is dated to 221 mya. The depositional environment is a shallow anoxic lake, where skeletal elements preserved are disarticulated and often fragmentary. The site has yielded a diverse small vertebrate fauna, including the new lepidosaurs and several undescribed rhynchocephalians. Skeletal elements are represented by numerous small, delicate pleurodont maxilla and dentaries. We integrated the material of the new lepidosaurs into phylogenetic analyses of Permo-Triassic Diapsida and Mesozoic Lepidosauromorpha, using maximum parsimony, maximum likelihood, and Bayesian analysis. All analyses support the new taxon as the sister taxon to all other Squamata, (3) substantially reducing the ghost lineage of Squamata. This discovery indicates that the absence of squamate fossils in their early evolutionary history could be caused in part by collection bias towards larger, more robust specimens. This taxon provides a look into the early evolutionary history of squamates. It also adds direct evidence of yet another major lineage of extant terrestrial vertebrates to originate in the Triassic.”
- In the LRT Megachirella (Middle Triassic) is an earlier basal rhynchocephalian. Bavariasaurus (Late Jurassic) and
- Lacertulus (Late Permian; Fig. 2) are basal stem squamates. TA104 (Rößler et al. 2012), an unnamed Early Permian lepidosaur close to Saniwa in the varanid clade is the earliest lepidosaur I have encountered yet, although this is based on low-rez data. Based on these nestings, the original radiation of lepidosaurs must have occurred in the Permian and then enjoyed very long period of stasis. Lacertulus is the oldest known lepidosaur and older than any Late Triassic Petrified Forest taxa. It does not have pleurodont (fused to the jaw) teeth.
- The LRT is an analysis that includes a long list of pro or proto-squamates and tritosaurs that are sisters to the Squamata. Palaegama (Late Permian), Tridentinosaurus (Early Permian) and Saurosternon (Latest Permian) are sisters to the Lepidosauria and they are basal to the highly derived Late Permian taxon, Coelurosauravus. So the original radiation of lepidosaurs and their lepidosauriform sisters must have been in the Early Permian. If one deletes Sphenodon, then another stem squamate would be Macroleter (Middle Permian). Earlier than this and you get into stem turtles.
Figure 2. Lacertulus, a basal squamate from the Late Permian
Klugman B and Pritchard AC 2016. Earliest stem-squamate (Lepidosauria) from the Late Triassic of Arizona. Abstract from the 2016 meeting of the Society of Vertebrate Paleontology.
Rößler R, Zierold T, Feng Z, Kretzschmar R, Merbitz M, Annacker V and Schneider JW 2012. A snapshot of an early Permian ecosystem preserved by explosive volcanism: New results from the Chemnitz Petrified Forest, Germany. PALAIOS, 2012, v. 27, p. 814–834