The most basal lepidosauriforms and lepidosaurs to scale

Lepidosauriform fossils are extremely rare in the Mesozoic and Paleozoic.
In the Earliest Permian we find Tridentinosaurus (Fig. 1; Dal Piaz 1931,1932; Leonardi 1959), a taxon ancestral to the pseudo-rib-gliders of the Late Permian (Coelurosauravus) through the Early Cretaceous (Xianlong) and close to the origin of all other lepidosauriforms, including living snakes, lizards and the tuatara (genus: Sphenodon).

Figure 1. Basal lepidosauriformes to scale from Tridentinosaurus (Earliest Permian) to Huehuecuetzpalli (Early Cretaceous). Subtle differences lump and split these taxa into their various clades.

Figure 1. Basal lepidosauriformes to scale from Tridentinosaurus (Earliest Permian) to Huehuecuetzpalli (Early Cretaceous). Subtle differences lump and split these taxa into their various clades.

 

Sometime during the Early Permian
the Lepidosauria split between the Sphenodontia + Drepanosauria and the Tritosauria + Protosquamata in the large reptile tree (LRT, 1381 taxa).

Short-legged
Jesairosaurus, in the Early Triassic, nests basal to the clade of slow-moving, arboreal drepanosaurs. On another branch, Megachirella (Middle Triassic) and Gephyrosaurus (Early Jurassic) are basal members of the Sphenodontia.

Long-legged
and probably arboreal Saurosternon and Palaegama, (both Late Permian) are the earliest known Lepidosauria, but they are basal to the Tritosauria + Protosquamata clades.

Figure 5. Subset of the LRT focusing on the Tritosauria. Note the separation of one specimen attributed to Macrocnemus.

Figure 5. Subset of the LRT focusing on the Tritosauria. Note the separation of one specimen attributed to Macrocnemus.

Late-surviving, long-legged basal Tritosauria
include tiny Tijubina and Huehuecuetzpalli (both Early Cretaceous). This clade gave rise to giant Tanystropheus, exotic Longisquama and volant Pteranodon.

Tiny and long-legged Late Permian
Lacertulus is the basal taxon in the previously unrecognized clade Protosquamata, the parent clade to the extant Squamata. This taxon documents the antiquity of this clade.

Going back to the Early Permian
we have a long-torso, short-legged specimen, MNC TA-1045, that nests in the LRT just outside the extant Squamata (Iguana). MNC TA-1045 was found alongside the genus Ascendonanus (MNC-TA0924), a basal archosauromorph diapsid with a shorter torso you can see here. The MNC TA-1045 specimen pushes the genesis of the lepidosaurs back to the Early Permian, nearly coeval with the basalmost lepidosauriform shown in figure 1, Tridentinosaurus.

The Lepidosauromorph-Archosauromorph dichotomy
was already present in the Viséan (Early Carboniferous, 330 mya), so the new Lepidosauromorpha had 30 million years to diverge into captorhinomorphs, diadectomorphs, millerettids and lepidosauriforms by the time Tridentinosaurus first appears in the Earliest Permian (300 mya).

Late surviving,
but basalmost lepidosauromorphs include Sophineta , Paliguana and Coletta (all Early Triassic). These taxa have an upper temporal fenestra not seen in outgroup taxa.

Proximal outgroups for the Lepidosauriforms
include the late-surviving owenettids: Barasaurus (Late Permian) and kin, Owenetta (Late Permian) and kin, and the late-surviving macroleterids (Middle Permian) and nycteroleterids (Middle Permian) before them.

At least that’s what the data says so far.
With every new taxon the tree grows stronger and more precise, so the odds of changing the tree topology with additional taxa continue to drop. Looking forward to seeing more Paleozoic arboreal lepidosauromorph discoveries as they arrive.

References
Dal Piaz Gb. 1932 (1931). Scoperta degli avanzi di un rettile (lacertide) nei tufi compresi entro i porfidi quarziferi permiani del Trentino. Atti Soc. Ital. Progr. Scienze, XX Riunione, v. 2, pp. 280-281. [The discovery of the remains of a reptile (lacertide) in tuffs including within the Permian quartz porphyry of Trentino.]
Leonardi P 1959. Tridentinosaurus antiquus Gb. Dal Piaz, rettile protorosauro permiano del Trentino orientale. Memorie di Scienze Geologiche 21: 3–15.

www.reptileevolution.com/reptile-tree.htm

 

 

 

 

Megachirella: Not at the origin of squamates. Lacertulus is older.

We looked at Lacertulus and the origin of the Squamata in the Late Permian
here in October 2011.

We looked at the splitting of the Tritosauria from the Protosquamata
here in December 2014.

Siimòes et al. 2018
proposed to nest Megachirella watchtleri (Fig. 1) at the origin of squamates in the Middle Triassic, 75 million years earlier than the previously known oldest squamate fossils. They reported, “For the first time, to our knowledge, morphological and molecular data are in agreement regarding early squamate evolution, with geckoes—and not iguanians—as the earliest crown clade squamates. Divergence time estimates using relaxed combined morphological and molecular clocks show that lepidosaurs and most other diapsids originated before the Permian/Triassic extinction event, indicating that the Triassic was a period of radiation, not origin, for several diapsid lineages.”

Figure 1. New µCT scans of Megachirella from Simoes et al. 2018.

Figure 1. New µCT scans of Megachirella from Simoes et al. 2018.

Unfortunately
|they did not include relevant taxa. According to the large reptile tree (LRT, 1224 taxa, www.reptileevolution.com/reptile-tree.htm) Megachirella nests at the base of the Rhynchocephalia (= Sphenodontia) along with Pleurosaurus (excluded from the Simoes team study) when many more relevant taxa are included.

Figure 2. Megachirella nests in the middle of this cladogram, that also nests turtles between rib gliders and choristoderes.

Figure 2. Megachirella nests in the middle of this cladogram, that also nests turtles between rib gliders and choristoderes.

 

Lacertulus is older (Late Permian) and more directly related to squamates.

FIgure 2. Megachirella (Renesto and Posenato 2003) is a sister to the BSRUG diapsid.

FIgure 3. Megachirella (Renesto and Posenato 2003) is a sister to the BSRUG diapsid and reconstructed here.

Nesting turtles with rib gliders
(Coelurosauravus) only hints at major flaws in the Simoes et al. cladogram topology. Nesting Sophineta and Palaegama close to and basal to Megachirella confirms findings made years earlier by the LRT. Marmoretta is also close, but nests within the Rhynchocephalia in the LRT.

Figure 2. Pleurosaurus and Palaeopleurosaurus skulls compared to those of sister taxa.

Figure 2. Pleurosaurus and Palaeopleurosaurus skulls compared to those of sister taxa.

Tijubina (which Simoes redescribed in 2012) is also missing from the Simoes et al. 2018 study.

Figure 1. Palaegama is basal to Coelurosauravus ('rib' gliders), Megachirella (rhynchocephalians), Lacertulus (protosquamates) and Tijubina (tritosaurs)

Figure 5. Palaegama is basal to Coelurosauravus (‘rib’ gliders), Megachirella (rhynchocephalians), Lacertulus (protosquamates) and Tijubina (tritosaurs)

 

 

References
Simòes T, and 8 co-authors 2018. The origin of squamates revealed by a Middle Triassic lizard from the Italian Alps. Nature 557: 706â709 (2018)

Publicity
https://www.livescience.com/62693-mother-of-lizards-fossil.html