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

 

 

 

 

A deeper extension for the Lepidosauria

By definition
the Lepidosauria includes Rhynchocelphalia (Sphenodon), Squamata (Iguana), their last common ancestor and all descendants. By this definition pterosaurs and kin are lepidosaurs because they nest between rhychocephalians and iguanids in a traditionally unrecognized clade the Tritosauria (Fig. 1).

Figure 1. Subset of the LRT focusing on the Lepidosauria. Now the drepanosaur clade lumps with the rhynchocephalians in the crown group. Extant lepidosaurs are in gray.

Figure 1. Subset of the LRT focusing on the Lepidosauria. Now the drepanosaur clade lumps with the rhynchocephalians in the crown group. Extant lepidosaurs are in gray.

While reviewing
the large reptile tree (LRT, 1087 taxa, subset Fig. 1) following the addition of Avicranium, the base of the Rhynchocephalia  shifted back to include Jesairosaurus, and the drepanosaursSaurosternon and Palaegama, which formerly nested as outgroup Lepidosauriformes now nest basal to the tritosaurs, pro-squamates and squamates within the Lepidosauria, based on the traditional definition.

With this change
the non-lepidosaur Lepidosauriformes are reduced to just the glider clade, Coletta, Paliguana, and Sophineta, taxa with a diapsid skull architecture. These remain stem lepidosaurs. The membership of the clade Lepidosauriformes do not change.

Remember,
despite their diapsid temporal morphology, these are not members of the clade Diapsida, which is restricted to Archosauromorph ‘diapsids’ only. Petrolacosaurus is a basal member of the monophyletic Diapsida. The clade name ‘Lepidosauriformes’ includes all lepidosauromorphs with upper and lateral temporal fenestrae. If you know any traditional paleontologists who still think lepidosaurs are related to archosaurs, please show them the LRT.

Once a definition for a clade is made
then the next step is to see which taxa fall under than definition… and then to see if that definition is a junior synonym for a previously published definition based on clade membership. Remember, traditional traits may not give you monophyly, but phylogenetic analysis always will.

And
yes, I do review all the scores in the LRT and announce updates when they are made.

 

Lacerta: where is the upper temporal fenestra?

Lacerta viridis (Fig. 1) is a common extant lizard that has more skull bones than is typical for most tetrapods. It also loses the upper temporal fenestra found in other lizards, by posterior expansion of the postfrontal.

Figure 1. Lacerta viridis skull from Digimorph.org and used with permission. Here the enlargement of the postfrontal basically erases the former upper temporal fenestra. Several novel ossifications appear around the orbit and cheek.

Figure 1. Lacerta viridis skull from Digimorph.org and used with permission. Here the enlargement of the postfrontal basically erases the former upper temporal fenestra. Several novel ossifications appear around the orbit and cheek.

This Digimorph.org image
was colorized in an attempt at understanding the skull bones present here. The extant Lacerta nests with the larger extinct Eolacerta in the large reptile tree (918 taxa).

40 species are known of this genus.
Fossils are known from the Miocene (Čerňanský 2010). The tail can be shed to evade predators. This lizard is an omnivore. The curled quadrate frames an external tympanic membrane (eardrum). With the premaxillae fused, Lacerta has nine premaxillary teeth, with one in the center.

Not sure why this lizard developed extra skull bones.
It is found in bushy vegetation at woodland and field edges, and is not described as a burrower or a head basher.

Other diapsid-grade reptiles that nearly or completely lose the upper temporal fenestra include:

  1. Mesosaurus
  2. Chalcides
  3. Acanthodactylus
  4. Phyrnosoma
  5. Minmi

References
Čerňanský A 2010. Earliest world record of green lizards (Lacertilia, Lacertidae) from the Lower Miocene of Central Europe. Biologia 65(4): 737-741.
Linnaeus C 1758.
Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.

Lacerta viridis images online
wiki/Lacerta

Earliest(?) stem squamate – SVP abstracts 2016

Klugman and Pritchard 2016
believe they have found the earliest lepidosaur stem squamate (see below). The large reptile tree finds earlier stem squamates (Fig. 1, click here to enlarge).
Earlier 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.

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.”
Notes
  1. In the LRT Megachirella (Middle Triassic) is an earlier basal rhynchocephalian. Bavariasaurus (Late Jurassic) and
  2. Lacertulus (Late Permian; Fig. 2) are basal stem squamates. Ascendonanus (Rößler et al. 2012), an Early Permian lepidosaur in the iguanid 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.
  3. 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 1. Lacertulus, a basal squamate from the Late Permian

Figure 2. Lacertulus, a basal squamate from the Late Permian

References
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

The water-walker, the rib-glider and the frill-neck are all cousins!

Update March 5, 2016 with the addition of the cladogram from the large reptile tree.

Some reptile oddballs
>do< nest together. In this case, the extant “Jesus” lizard, Basiliscus, the rib-glider, Draco, and the frill-neck, Chlamydosaurus now nest together in the large reptile tree.

Figure 1. Draco volans. Note the anterior maxillary fangs, and the antorbital fenestra between the lacrimal and prefrontal, traits shared with Chlamydosaurus (Fig 2).

Figure 1. Draco volans. Note the anterior maxillary fangs, and the antorbital fenestra between the lacrimal and prefrontal, traits shared with Chlamydosaurus (Fig 2).

Draco (Fig. 1) and Chlamydosaurus (Fig. 2) are particularly interesting
as both share anterior maxillary fangs and an antorbital fenestra between the prefrontal and jugal (rather than between the lacrimal and maxilla as in other taxa with an antorbital fenestra). A long list of shared character traits unites these two still quite different lizards.

Figure 2. Chlamydosaurus also has anterior maxillary fangs and an antorbital fenestra between the prefrontal and lacrimal, as in Draco (Fig.1).

Figure 2. Chlamydosaurus also has anterior maxillary fangs and an antorbital fenestra between the prefrontal and lacrimal, as in Draco (Fig.1).

Basiliscus
is (at this point in the proceedings) a sister to the last common ancestor. But with that parietal crest, it has definitely evolved apart from the above two taxa for a long time.

Figure 3. Basiliscus, the "Jesus" lizard, does not share as many traits as Draco and Chlamydosaurus do, but is related, given the short list of Iguanids currently employed.

Figure 3. Basiliscus, the “Jesus” lizard, does not share as many traits as Draco and Chlamydosaurus do, but is related, given the short list of Iguanids currently employed.

We’ve seen this before, 
where and when some odd little reptiles shared more traits with each other than with any other tested reptiles. Members of the Fenestrasauria (Cosesaurus, Kyrgyzsaurus, Sharovipteryx, Longisquama, and pterosaurs) also include bipeds with dorsal frills. One of them also glided with outstretched ribs and legs, although distinct from Draco. In Sharovipteryx, the hind legs were much longer than the ribs.

Figure addendum 1. Cladogram of the Iguania, the sister taxa of the Scleroglossa, both members of the clade Squamata, a subset of the clade Protosquamata, the sister taxon to the Tritosauria.

Figure addendum 1. Cladogram of the Iguania, the sister taxa of the Scleroglossa, both members of the clade Squamata, a subset of the clade Protosquamata, the sister taxon to the Tritosauria. Many more scleroglossans are shown in the large reptile tree at ReptileEvolution.com.

We don’t have 
close prehistoric relatives for Draco or Chlamydosaurus yet. So at this point the evolution of rib-gliding or frill-spreading is not yet a gradual demonstration. But the other shared traits are, to my knowledge, unique synapomorphies.

I will update the cladogram this weekend.

 

Purbicella, a basal scleroglossan from the Purbeck Limestone

Figure 1. Purbicella in situ (palatal view) and traced using DGS, then reconstructed using those tracings. Gray areas are unknown. If you think this looks like a generalized, plesiomorphic scleroglossan, you're right! Here colorizing the bones helps identify sutures and paired elements. That's a right pterygoid covering much of the paired frontals. The teeth are blunt.

Figure 1. Purbicella in situ (palatal view) and traced using DGS, then reconstructed using those tracings. Gray areas are unknown. If you think this looks like a generalized, plesiomorphic scleroglossan, you’re right! Here colorizing the bones helps identify sutures and paired elements. That’s a right pterygoid covering much of the paired frontals. The teeth are blunt.

A few years ago
a rather complete lizard skull (BGS GSb581) was described (Evans et al. 2012) from the Purbeck Limestone (Late Jurassic to Early Cretaceous) of England. It was originally excavated more than a century ago and assigned to the genus, Paramacellodus. Evans et al. renamed it Purbicella. Their cladistic analysis nested Purbicella with Lacertoidea: (Lacertidae (including Acanthodactylus), Teiidae (including Tupinambus), Gymnophthalmidae (including Gymnophthalmus), and the burrowing Amphisbaenia (including Amphisbaena)), not Paramacellodus, which nested with skinks. Evans et al. based their nesting on a partial data matrix of Conrad (2008).

The large reptile tree nested Purbicella between Acanthodactylus and Liushusaurus. The large reptile tree recovered the above listed ‘lacertoid’ taxa as members of a paraphyletic clade, some preceding Purbicella in various clades and others succeeding it.

While Purbicella is Late Jurassic/Early Cretaceous, it must have had its origins much earlier, in the Late Carboniferous, because a descendant taxon, Ascendonanus, is Early Permian.

References
Conrad JL 2008. Phylogeny and systematics of Squamata (Reptilia) based on  morphology. Bulletin of the American Museum of Natural History 310:1–182.
Evans SE, Jones MEH and Matsumto R 2012. A new lizard skull from the Purbeck Limestone Group (Lower Cretaceous) of England. Bull. Soc. géol. France, 2012, t. 183(6):517-524.

 

Sirenoscincus mobydick: the only terrestrial tetrapod with ‘flippers’

Sakata and Hikida 2003
introduced us to a new and extant fossorial (burrowing) lizard (Sirenoscincus yamagishii. Fig.1). The authors described having “an elongated body and eyes covered by scales, lacking external ear openings and pigmentation through- out the body, resembles Cryptoscincus and Voeltzkowia. However it differs from these or any other scincid genera known to the present in having small but distinct forelimbs, each with four stout claws, and complete lack of hind limbs.”

Figure 1. Sirenoscincus-yamagishii, a new skink with forelimbs and no hind limbs. Note the four fingers.

Figure 1. Sirenoscincus-yamagishii, a new skink with forelimbs and no hind limbs. Note the four fingers.

Sirenoscincus is a very tiny lizard
with 53 presacral vertebrae and a tail longer than the snout vent length. The snout is pointed and the lower jaw is countersunk, like a shark’s mouth. The forelimbs are tiny with indistinct fingers and four stout claws. An outgroup taxon, Gymnophthalmus, also has tiny fingers and the medial one is a vestige.

Then a second Sirenoscincus species was discovered
S. mobydick (Miralles et al. 2012, Fig. 2; see online interview here). “The specicific epithet refers to Moby Dick, the famous albino sperm whale imagined by Herman Melville (1851), with whom the new species shares several uncommon characteristics, such as the lack of hind limbs, the presence of fipper-like forelimbs, highly reduced eyes, and the complete absence of pigmentation.”

Figure 3. Sirenoscincus mobydick.

Figure 2. Sirenoscincus mobydick.

S. mobydick has only five scleral ring bones, the lowest of any lizard. The authors reinterpreted several scale patterns on the holotype species. So, mistakes do happen, even at a professional level. Those mistakes get corrected and no one gets upset (hopefully unlike the blogosphere!).

Figure 2. Sireonscincus mobydick, named for its flippers, unique for any terrestrial tetrapod.

Figure 3. Sireonscincus mobydick, named for its flippers, unique for any terrestrial tetrapod. Colors added.

Fossorial skinks are often described by their scale patterns.
Unfortunately that doesn’t work with prehistoric skeletons, so I was only able to add only the bone traits of Sirenoscincus mobydick to the large reptile tree (subset shown in Fig. 7). The skeletal traits nested S. mobydick between two skinks Gymnophthalmus and Sineoamphisbaena, another taxon with forelimbs only (granted, the posterior half is not known). Like Sineoamphisbaena, Sirenoscincus prefrontals contact the postfrontals, unlike those of most lizards. In derived taxa the quadrate leans almost horizontally. That’s not the case with Sirenoscincus, which has a vertical but bent quadrate.

Figure 4. Sirenoscincus mobydick pectoral and pelvic girdles. Colors added.

Figure 4. Sirenoscincus mobydick pectoral and pelvic girdles. Colors (other then the original red) are added here.

Miralles et al. (2012) reported,  “Due to the absence of molecular data the phylogenetic position of the genus Sirenoscincus is still an enigma, even if we can reasonably claim it belongs to the Malagasy scincine clade.” In the last few days author, A. Miralles reported via email that molecular data have recently nested S. mobydick with skinks. 

Figure x. Chalcides guentheri and C. occellatus, two skinks were morphology quite similar to that of Sirenoscincus.

Figure 5. Chalcides guentheri and C. occellatus, two skinks with morphologies quite similar to that of Sirenoscincus. C. oscellatus has longer legs. Note the wrapping of the maxilla over the premaxilla which is continued in Sirenoscincus mobydick which has a smaller orbit. Also note the prefrontal and postfrontal are closer to contact in C. ocellatus.

An outgroup taxon is Chalcides (Fig. 5) where you’ll note the same long overlap of the maxilla over the premaxilla. A sister, Sineoamphisbaena also has an underslung mandible, but much more robust forelimbs (only the humerus is known). Could this be a redevelopment? Or has the cladogram missed something, needing more taxa perhaps, to fill this gap? No doubt new taxa will fill these various morphological gaps.

Figure 6. Sineoamphisbaena is a sister to Sirenoscincus in which the prefrontal contacts the postfrontal.

Figure 6. Sineoamphisbaena is a sister to Sirenoscincus in which the prefrontal contacts the postfrontal. The lower jaw is countersunk and the upper teeth don’t point down, they point in (medially).

New data has revised the relationship of skinks to reptiles in the large reptile tree (Fig. 7). Some to most of the confusion (here or earlier) likely results from the massive convergence in burrowing lizards. And some portion is also due to having good data (old line drawings) replaced by better data (rotating online images), often thanks to the good scientists over at Digimorph.org.

Figure 7. Here's where Sirenoscincus nests in the lizard family tree.

Figure 7. Here’s where Sirenoscincus nests in the lizard family tree.

References
Miralles A et al. 2012. Variations on a bauplan: description of a new Malagasy “mermaid skink” with flipper-like forelimbs only (Scincidae, Sirenoscincus Sakata & Hikida, 2003). Zoosystema 34(4):701-719.
Sakata S and Hikida T 2003. A fossorial lizard with forelimbs only: description of a new genus and species of Malagasy skink (Reptilia: Squamata: Scincidae). Current Herpetology 22:9-15.