Lepidosauria: by definition and trait-by-trait

The best way to define a clade, like the Lepidosauria,
is “the last common ancestor and all of its descendants.”

Lizard expert, Dr. Susan Evans, Department of Anatomy and Developmental Biology, University College London, London, United Kingdom provided the following description online at AccessScience.com.

Lepidosauria
A subclass of living and extinct diapsid reptiles. Lepidosauria and their immediate ancestors constitute the Lepidosauromorpha, one of the two major clades of the Diapsida.”

Unfortunately,
In the large reptile tree (LRT, 1406 taxa) reptiles with a diapsid skull architecture are not monophyletic. One type has its genesis in the Late Carboniferous with taxa like Petrolacosaurus in the Archosauromorpha. The other type has its genesis in the Early Triassic with taxa like Paliguana in the Lepidosauromorpha.

Dr. Evans continues
“By definition,
Lepidosauria includes the last common ancestor of the living squamates (lizards, snakes, and the limb-reduced burrowing amphisbaenians) and the New Zealand tuatara (Sphenodon) and all of its descendants. 
This is a narrower definition than that used in older literature, where Lepidosauria was used as a catchall group for all non-archosaurian diapsids.”

“Lepidosaurs differ from archosaurs in that the lower temporal arcade (the inferior border of the lower temporal fenestra) is typically incomplete, there is never an antorbital fenestra in front of the orbit or a mandibular fenestra in the lower jaw, and the teeth are generally fused in position (acrodont or pleurodont) rather than implanted in sockets (thecodont).”

The LRT invalidates this description because fenestrasaurs, including pterosaurs, have an antorbital fenestra without a fossa and the lower temporal arcade is often complete. Sphenodon and rhynchosaurs also have a complete lower temporal arcade.

According to Dr. Evans
“Other lepidosaurian characters include:
(my comments follow in parentheses)

  1. a specialized skin-shedding mechanism (not preserved in fossil taxa)
  2. paired male hemipenes (not yet preserved in fossil taxa)
  3. fracture planes in the caudal vertebrae that allow the tail to be shed if grabbed by a predator (= caudal autotomy. So many fossil tetrapods lack part of their tail, but this never happens in fenestrasaurs and pterosaurs, which interlock tail vertebrae with accessory processes. Many lizards, like chameleons and lepidosaurs do not develop this trait).
  4. and specialized knee, foot, and ankle joints that improve locomotion (all knee, foot and ankle joints improve locomotion, but note that many lepidosaurs lose their knees, feet and ankles)
  5. As in mammals, the ends of lepidosaurian long bones develop separate centers of ossification (epiphyses) that fuse to the shaft at the end of skeletal growth. (Not present in juvenile Huehuecuetzpalli or juvenile/embryo pterosaurs).

In the LRT, the last common ancestor
of all Lepidosauria (Sphenodon + Iguana) is close to the Earliest Permian Tridentinosaurus (Fig. 1) and/or Early Triassic Sophineta. After that, almost every morphology evolved, from giant sea lizards, to tiny and giant air lizards, to dime-sized geckos, to limbless taxa, to long-necked taxa, to bipeds, to plant-eaters and predators, etc. etc.

Figure 1. Tridentinosaurus at 26.5 cm long is an Earliest Permian ancestor to Late Permian Coelurosauravus and Late Triassic Icarosaurus.

Figure 1. Tridentinosaurus at 26.5 cm long is an Earliest Permian ancestor to Late Permian Coelurosauravus and Late Triassic Icarosaurus.

I hate to say it,
but Dr. Evans needs to add taxa, double-check and update her description/definition and here list of lepidosaur taxa. Just a suggestion…

References
AccessScience > Articles > Paleontology Fossil reptiles > Lepidosauria

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