Chinlestegophis and the origin of caecilia

Yesterday Pardo et al. 2017
described two conspecific and incomplete amphibians in the lineage of caecilians, Chinlestegophis jerkinsi (DMNH 56658, DMNH 39033, Figs. 1, 3). These long-sought specimens were discovered in the late 1990s preserved in Late Triassic burrows.

This is really big news!
Congratulations to the Pardo team!!

From the abstract:
“Here, we report on a small amphibian from the Upper Triassic of Colorado, United States, with a mélange of caecilian synapomorphies and general lissamphibian plesiomorphies. We evaluated its relationships by designing an inclusive phylogenetic analysis that broadly incorporates definitive members of the modern lissamphibian orders and a diversity of extinct temnospondyl amphibians, including stereospondyls. Stem caecilian morphology reveals a previously unrecognized stepwise acquisition of typical caecilian cranial apomorphies during the Triassic. A major implication is that many Paleozoic total group lissamphibians (i.e., higher temnospondyls, including the stereospondyl subclade) fall within crown Lissamphibia, which must have originated before 315 million years ago.”

The diagnosis:
“Small stereospondyl with a combination of brachyopoid and caecilian characteristics.”  Stereospondyls were generally large, flat-skulled aquatic taxa that had simplified and rather weak vertebrae in which the intercentrum was topped by a neural arch and the pleurocentrum was reduced to absent. According to Wikipedia, “All lepospondyls have simple, spool-shaped vertebrae that did not ossify from cartilage, but rather grew as bony cylinders around the notochord.” 

This is the opposite of
Reptilomorphs, in which the pleurocentra are large and the intercentra are smaller. Reptilomorphs generally were smaller and better adapted to terrestrial environments.

In the LRT traditional stereospondyls
(Fig. 5, pink) are mid-sized basalmost tetrapods, aquatic with a weak backbone because they are not far from fish with fins. Temnospondyls have stronger limbs and stronger backbones (Fig. 5, yellow), but typically remain large and aquatic.

Reptilomorphs 
(Fig. 5, orange) tend to be smaller with stronger limbs and vertebrae and reduce their dependence on water. Both lepospondyls (including living amphibians) and reptiles arise from this clade in the LRT.

Few microsaurs
were included in the Pardo et al study (Fig. 4) and the topology of their tree is very different from the present topology. Caecilians nest with lepospondyl microsaurs in the large reptile tree (LRT, 2014).

In addition
several skull bones are identified differently here (Fig. 1) than in the Pardo et al. study (Fig. 3). Pardo et al. identify an otic notch (that hole in the temporal region). Here that appears to be the space left open after the supratemporal has popped out during taphonomy. The supraorbital bones are all re-identified and both the lacrimal and quadratojugal are now listed in the present identification of bones. Based on conversations with Pardo and others, bone identification on several taxa may be the cause of the differing tree topologies.

Figure 1. GIF movie showing the two skulls of Chinlestegophis from Pardo et al. 2017 with DGS colors applied to both along with a revised set of bone labels

Figure 1. GIF movie showing the two skulls of Chinlestegophis from Pardo et al. 2017 with DGS colors applied to both along with a revised set of bone label based on phylogenetic bracketing among the previously excluded microsaurs close to caecilians.

Outgroup taxa should help identify the bones.
Pardo et al. recover Rileymillerus and Batrachosuchus as outgroup taxa within a large clade that includes Eryops and Sclerocephalosaurus at one base and Trimerorhachis and Greererpeton at the very base. By contrast, the LRT recovers Microbrachis and ultimately Utegenia as outgroup taxa. Microsaurs, Microbrachis and Utegenia were not mentioned in the Pardo et al. report.

First step: Learn about Rileymillerus
As usual, I knew nothing about this taxon earlier this week. Now, according to the LRT Rileymillerus nests with Oestocephalus and Ophiderpeton, two other long-bodied microsaurs with round cross-section skulls, not included in the Pardo et all study.  The apparent loss or lack of bones in the temporal region may be homologous with the lateral temporal fenestra in Ophiderpeton. That’s a rare trait among basal tetrapods.

Figure 3. Rileymillerus from Bolt and Chatterjee 2000 with colors applied.

Figure 2. Rileymillerus from Bolt and Chatterjee 2000 with colors applied. Note the lack of bone on both sides of the temples in this specimen, as in Ophiderpeton. The color (DGS) identify of the bones here is not in complete accord with Bolt and Chatterjee. As you can see, the skull has many cracks, which makes finding the sutures that much more difficult.

Unfortunately
Pardo et al. excluded most of the taxa that the LRT found were most closely related to the clade Chinlestephos + (caecilians + lysorophians) That includes Microbrachis and the rest of the microsaurs. They had good reason for doing so (see below).

Figure 3. Chinlestegophis diagram. Drawings produced by Pardo et al. At left bones colored as they labeled them. At right same bone colors rearranged to fit the new interpretation. See figure 1.

Figure 3. Chinlestegophis diagram. Drawings produced by Pardo et al. At left bones colored as they labeled them. At right same bone colors rearranged to fit the new interpretation. See figure 1. The lateral temporal fenestra is interpreted here as the spot on the skull that once held the supratemporal. No related taxa have a lateral temporal fenestra in either cladogram.

The Pardo et al. skull bone labels
differ from the present interpretation (Fig. 3). Even with such massive dissonance, Pardo et al. and the LRT both nest Chinlestegophis with caecilians and not far from Rileymillerus.

How can such a thing happen??
I can’t answer that at present. It’s frankly surprising.

Figure 4. Pardo et al. cladogram nesting caecilians as ultra-derived temnospondyls.

Figure 4. Pardo et al. cladogram nesting caecilians as ultra-derived temnospondyls. Taxa also present in the LRT are highlighted to show the general mixup of taxa that the LRT separates.

The drifting of the postorbital
In most tetrapods the postorbital is one of the circumorbital bones. In caecilians and their relatives the postfrontal takes over that spot and the postorbital drifts posteriorly, still lateral to the parietal. This observation may be one of the issues attending circumorbital and temporal bone identification arguments in this clade.

Figure 5. Basal tetrapod subset of the LRT. This cladogram includes microsaurs. When given the opportunity to nest with microsaurs, caecilians do so.

Figure 5. Basal tetrapod subset of the LRT. This cladogram includes microsaurs. When given the opportunity to nest with microsaurs, caecilians do so.

In their Supplemental Info
Pardo et al. added the traits for Chinlestegophis to the dataset of Maddin et al. 2012 (who earlier described Jurassic Eocaecilia) and found Chinlestegophis nested with Rileymillerus, close to the stem frog Micromelerpeton and strong-legged Acheloma all far from the caecilians and all derived from a sister to giant Eryops. This study did include microsaurs. Lots of them! Other mismatches include nesting the large reptile Limnoscelis between Seymouria and tiny Utaherpeton and Microbrachis, taxa that share few traits with each other in the LRT. Numerous other morphological mismatches also occur In Maddin et al. Evidently no one is using scaled reconstructions in their analyses as a final check on these mismatches. In the LRT caecilians nest with similar long-bodied, tiny-limbed taxa, which some claim is due to convergence. On a similar note, the LRT lumped and separated snakes from amphisbaenids while other trees failed to do this. So perhaps convergence is not the reason here when dealing with burrowing amphibians.

Figure 6. Maddin et al. cladogram featuring only two temnospondyls from the LRT. Here Chinlestegophis does not nest with caecilians.

Figure 6. Maddin et al. cladogram featuring only two temnospondyls from the LRT. Here Chinlestegophis does not nest with caecilians and Rileymllerus nests far from Oestocephalus.

A note from Jason Pardo
restates that the Maddin et al. study “found no close relationship between Eocaecilia and lepospondyls nor did we find a close relationship between Chinlestegophis and those taxa.”

Figure 6. Living caecilian photo.

Figure 7. Living caecilian photo. Lengths range from 6 inches to 5 feet.

All three cladograms
share few major branches in common. As everyone knows by now, the major branches are the more difficult ones to determine. And, if we can’t agree on the identify of the skull bones, of specimens, the tree topologies will have a hard time finding consensus.

Wikipedia reports,
“Currently, the three prevailing theories of lissamphibian (extant amphibians) origin are:

  1. Monophyletic within the temnospondyli
  2. Monophyletic within lepospondyli
  3. Diphyletic (two separate ancestries) with apodans (=caecilians) within the lepospondyls and salamanders and frogs within the temnospondyli.”

Figure 8. Skull of Microbrachis in several views. Here is where the postorbital leaves the orbit margin and drifts posteriorly. Compare to Chinlestegophis above.

Figure 9. Skull of Microbrachis in several views. Here is where the postorbital leaves the orbit margin and drifts posteriorly. Compare to Chinlestegophis above.

So… even the experts have not come to a consensus
on basal tetrapod topologies. The LRT agrees that the lissamphibia are monophyletic within the lepospondyli, matching option #2 above. There are many aspects of caecilians that need to be interpreted in light of their phylogeny. And we’re not coming to a consensus on that. Earlier we looked at the fusion of the cheek bones in caecilians here with the extant taxon Dermophis.

References
Bolt JR and Chatterjee S 2000. A New Temnospondyl Amphibian from the Late Triassic of Texas. Journal of Paleontology 74(4):670-683.
Maddin HC, Jenkins FA, Jr, Anderson JS 2012. The braincase of Eocaecilia micro podia (Lissamphibia, Gymnophiona) and the origin of Caecilians. PLoS One 7:e50743.
Pardo JD, Small BJ and Huttenlocker AK. 2017, Stem caecilian from the Triassic of Colorado sheds light on the origins of Lissamphibia. PNAS: 7 pp. www.pnas.org/cgi/doi/10.1073/pnas.1706752114

 

The weird skull and affinities of Brachydectes

Before you read any further, check out Jason Pardo’s letter below. He’s the expert. I’m only a freshman when it comes to this very unusual taxon and its kin. 

This post was updated February 8, 2017 with new identifications of several skull bones. This did not change the nesting of Brachydectes with Eocacilia. 

Further updated March 18, 2017 with new skull bone identities for Brachydectes

Brachydectes newberryi (Cope 1868, AMNH 6941; latest Carboniferous; 300 mya; Fig. 1-4) was long considered a lysorophian amphibian with a tiny skull, an extremely long snake-like torso, vestigial limbs and a very short tail. You find them in eastern Kansas.

Figure 1. Brachydectes overall and skull in four views.

Figure 1. Brachydectes overall and skull in four views.

A recent PlosOne article
by Pardo and Anderson (2016) studied the skull of Brachydectes (Fig. 3) using micro CT scanning. They report, “Contra the proposals of some workers, we find no evidence of expected lissamphibian synapomorphies in the skull morphology in Brachydectes newberryi, and instead recognize a number of derived amniote characteristics within the braincase and suspensorium. Morphology previously considered indicative of taxonomic diversity within Lysorophia may reflect ontogenetic rather than taxonomic variation.” Later they wrote, “an expansive phylogenetic analysis is outside the scope of this study and will appear elsewhere.” 

Earlier
in the large reptile tree (LRT), Brachydectes nested between Adelospondylus and Eocaecilia, which also has a long snake-like torso, but composed of far fewer and individually much longer vertebrae and a distinct skull architecture. A large, but not exhaustive, selection of basal amniotes was tested and none attracted Brachydectes as much as the two lissamphibians listed above, given the prior data of a line drawing of the skull (Fig. 2) by Marjanovic and Laurin 2013 derived from Wellstead C F 1991.

Figure 1. Brachydectes skull data from a line drawing produced by Marjanović and Laurin 2013. Most leposponysls have a very narrow parasphenoid process and large interptyergoid vacuities, but eocacaecilians expanded this bone and reduced the vacuities like Brachydectes did. 

Figure 2. Brachydectes skull data from a line drawing produced by Marjanović and Laurin 2013. Most leposponysls have a very narrow parasphenoid process and large interptyergoid vacuities, but eocacaecilians expanded this bone and reduced the vacuities like Brachydectes did.

Figure 1. Brachydectes newberryi has some difficult to identify bones just aft of the orbit due to fusion and reduction. Brachydectes (Laysorophus) elongatus (Fig. 2) provides Rosetta Stone clues as to what is happening in this clade.

Figure 1. Brachydectes newberryi has some difficult to identify bones just aft of the orbit due to fusion and reduction. Brachydectes (Laysorophus) elongatus (Fig. 2) provides Rosetta Stone clues as to what is happening in this clade.

The new data 
(Figs 2,3 ) are not too far off from the Wellstead C F 1991 data. Notably the tabular no longer extends ventrally alongside the squamosal as it does in the larger specimen. Does this represent a break? or fusion? Or phylogenetic difference? Below (Fig. 3) is the new data on KUVP 49541, plus a reinterpretation of skull sutures based on the micro CT scans. The nesting of the new Brachydactes does not shift in the LRT. It is still a lissamphibian close to microsaurs and caecilians. That’s a broad range, indicative of a long list of yet to be found taxa.

Pardo and Anderson’s reconstruction
(Fig. 3) does not include the coronoid or lateral exposure of the splenial.  Pardo and Anderson note the single supraoccipital compares well with that of various basal reptiles, and indeed it does.  The occipital arch of other lissamphibians consists of only paired exoccipitals,.. until you include microsaurs.

More on supraoccipital homologies
According to Pardo and Anderson, “the presence of a well-developed median supraoccipital is restricted to the amniote crown and recumbirostran ‘microsaurs’. Although the supraoccipital of Brachydectes and ‘microsaurs’ has traditionally been considered convergent with the amniote supraoccipital, new data from μCT have demonstrated that the ‘microsaur’ supraoccipital shares a number of morphological details with early amniotes, and early eureptiles in particular, and is likely homologous with the amniote element. This homology does not extend far down the amniote stem, as seymouriamorphs lack a supraoccipital and ‘anthracosaurs’ generally exhibit paired elements within the synoptic tectum.” 

Noteworthy:
In the LRT, microsaurs are sisters to the clade that includes Adeospondylus, Brachydectes and Eocaeceila. That’s a great deal of phylogenetic distance, but not as great as any other pairing in the LRT. Perhaps more taxa will fill the apparent gaps someday.

Figure 4. Four sizes of Brachydectes in situ. Here, unfortunately, the authors have penned in the sutures, negating any possibility of any reviewer to judge whether they were drawn correctly or not.

Figure 4. Four sizes of Brachydectes in situ. Here, unfortunately, the authors have penned in the sutures, negating any possibility of any reviewer to judge whether they were drawn correctly or not.

Pardo and Anderson also report
“neurocranial morphology does not support a close relationship between Brachydectes and lissamphibians.” Admittedly, Brachydectes is indeed quite different from its sisters…yet it is not closer to other tested taxa in the LRT. If you look at various microsaurs and other lissamphibians, you get a wide range of morphologies at every node.

By noting various key features in contention with the traditional relationship. Pardo and Anderson essentially ‘put the cart before the horse.’ They waited to do the phylogenetic analysis, when they should have done that analysis before publishing. Homoplasy is rampant in tetrapods. I think they fell prey to yet another example. Only analysis, at present, settles all issues.

Pardo and Anderson then report, 
“Morphology of the braincase of Brachydectes suggests a close relationship with the brachystelechid ‘microsaurs’ Carrolla craddocki  and Quasicaecilia texana, within the Recumbirostra.” These two are new to me and untested in the LRT. Wikipedia nests them with Batropetes, which has long legs, and a horned-lizard type body, only distantly related to Brachydectes in the LRT. The skull of Quasicaecilia is shown here, but no post-crania is shown. Recumbirostran microsaurs, are considered the earliest known example of adaptation to head-first burrowing in the tetrapod fossil record. I wish the sister candidates offered by Pardo and Anderson were long and snake-like, but they are not. Deletion of post-cranial traits from the LRT does not shift the placement of Brachydectes within the LRT.

Figure 3. Original interpretation of Brachydectes, KUVP 49541, by Pardo and Anderson. Colors added for clarity and to match micro CT scan.

Figure 5. Original interpretation of Brachydectes, KUVP 49541, by Pardo and Anderson. Colors added for clarity and to match micro CT scan.

References
Carroll RL 1967. An Adelogyrinid Lepospondyl Amphibian from the Upper Carboniferous: Canadian Journal of Zoology 45(1):1-16.
Carroll RL and Gaskill P 1978. The order Microsauria. American Philosophical Society, Philadelphia, 211 pp.
Cope ED 1868. Synopsis of the extinct Batrachia of North America. Proc Acad Nat Sci 20: 208–221. doi: 10.5962/bhl.title.60482
Jenkins FA and Walsh M 1993. An Early Jurassic caecilian with limbs. Nature 365: 246–250.
Jenkins FA, Walsh DM and Carroll RL 2007. Anatomy of Eocaecilia micropodia, a limbed caecilian of the Early Jurassic. Bulletin of the Museum of Comparative Zoology 158(6): 285-366.
Marjanović D and Laurin M 2013. The origin(s) of extant amphibians: a review with emphasis on the “lepospondyl hypothesis”. Geodiversitas 35 (1): 207-272. http://dx.doi.org/10.5252/g2013n1a8
Pardo JD and Anderson JS 2016. Cranial Morphology of the Carboniferous-Permian Tetrapod Brachydectes newberryi (Lepospondyli, Lysorophia): New Data from µCT. PLoS ONE 11(8): e0161823. doi:10.1371/journal.pone.0161823. online here.
Wellstead C F 1991. Taxonomic revision of the Lysorophia, Permo-Carboniferous lepospondyl amphibians. Bulletin of the American Museum of Natural History 209: 1–90.

wiki/Eocaecilia
wiki/Brachydectes
wiki/Adelospondylus

Reconstruction from jumbled scraps: the squamate, Kuroyuriella

Figure 1. The skull of Kuroyuriella reconstructed from bone scraps (above), most of which are layered on top of one another. Not all elements are identified, but enough are to nest this taxon with Ophisaurus.

Figure 1. The skull of Kuroyuriella (represented by two specimens of different size) reconstructed from bone scraps (above), most of which are layered on top of one another. Not all elements are identified, but enough are known to score and nest this taxon with Ophisaurus.

When provided disarticulated scraps,
start with the easy bones, then fill in the gaps in the puzzle. Sometimes, as in Kuroyuriella mikikoi (Evans and Matsumoto 2015, Early Cretaceous), there are enough parts to more or less recreate the skull most similar (among tested taxa in the large reptile tree) to that of Ophisaurus and basal to Myrmecodaptria and CryptolacertaEvans and Matsumoto nested Kuroyuriella  between Huehuecuetzpalli and the suprageneric clade Rhynchocephalia, both well outside the Squamata.

From the online paper:
“Together, SBEI 1510 and SBEI 1608, as type and referred specimen, characterize Kuroyuriella mikikoi as a small lizard having paired frontals with deep subolfactory processes; a median parietal without a parietal foramen, with sculpture of low relief, and with lateral shelves that restricted the adductor muscle origins to the ventral surface; upper temporal fenestrae that were at least partially closed by expanded postorbitofrontals; an unsculptured maxilla with a strongly concave narial margin; a large flared prefrontal; and a slender, relatively small pterygoid. In the shallow lower jaw, the teeth are closely packed, cylindrical, and pleurodont with lingual replacement; a subdental ridge is present; the dentary bears a tapering coronoid process that braces the coronoid, and has a posterior extension with a curved free margin; the surangular, angular, and splenial are all present and the surangular is shallow; the adductor fossa is open but not expanded; and the articular surface is asymmetrical.

In order to explore the affinities of Kuroyuriella mikikoi, it was coded into the matrix of Gauthier et al. (2012), as extended by Longrich et al. (2012) (184 characters coded out of 622, 70.4% missing data),

The consistent placement of Kuroyuriella on the squamate stem is problematic and probably artifactual, but whether the weighted analysis is giving a more accurate placement is uncertain. Of the derived character states possessed by Kuroyuriella, 76 [1] (postorbital partly occludes upper temporal fenestra), 364 [1] (dentary coronoid process extends beyond level of coronoid apex), 367 [2] (coronoid process of dentary overlaps most of anterolateral surface of coronoid), and 369 [2] (dentary terminates well posterior to coronoid apex) provide some support for placement of Kuroyuriella on the stem of scincids, and 129 [1] (prefrontal extends to mid-orbit), 104 [1] (absence of parietal foramen) and 385 [1] (posterior mylohyoid foramen posterior to coronoid apex) would be consistent with that placement. However, given the considerable difference between the results using equal weighting and Implied Weighting, Kuroyuriella remains incertae sedis pending recovery of more complete material.”

Figure 2. Ophisaurus, the extant glass snake or legless lizard is close to Kuroyuriella in the large reptile tree.

Figure 2. Ophisaurus, the extant glass snake or legless lizard is close to Kuroyuriella in the large reptile tree.

Here
Ophisaurus (Fig. 2) and Kuroyuriella both nest will within the Squamata, not ouside. of it in the large reptile tree. Reconstruction of the skull helps to ‘see’ this lizard as it was. I can’t imagine how difficult it would be to do try to establish traits  with a jumble of disarticulated bones.

As you’ll see, I think the parietal foramen was present. The parietal may have had longer posterior processes, now broken off.

References
Evans SE and Matsumoto R 2015. An assemblage of lizards from the Early Cretaceous of Japan. Palaeontologia Electronica 18.2.36A: 1-36
palaeo-electronica.org/content/2015/1271-japanese-fossil-lizards
http://palaeo-electronica.org/content/2015/1271-japanese-fossil-lizards

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.

Some thoughts on Sineoamphisbaena

One of the strangest (= most unlike its sister taxa) reptiles is Sineoamphisbaena, which nests in the large reptile tree at the base of the burrowing skinks that ultimately gave rise to amphisbaenids like Amphisbaena and Bipes.

Wikipedia reports: Sineoamphisbaena is an extinct genus of squamate of uncertain phylogenetic placement. Wu et al. (1993), Wu et al. (1996) and Gao (1997) proposed and argued that it was the oldest known amphisbaenian; this, however, was challenged by other authors, such as Kearney (2003) and Conrad (2008), who instead assignedSineoamphisbaena to the group of squamates variously known as Macrocephalosauridae, Polyglyphanodontidae or Polyglyphanodontia. A large-scale study of fossil and living squamates published in 2012 by Gauthier et al. did not find evidence for a particularly close relationship between amphisbaenians and Sineoamphisbaena; in their primary analysis Sineoamphisbaena was found to be the sister taxon of the clade containing snakes, amphisbaenians, the family Dibamidae and the American legless lizard. The primary analysis of Gauthier et al. did not support a close relationship between Sineoamphisbaena and polyglyphanodontians either; however, the authors noted that when all snake-like squamates and mosasaurs were removed from the analysis, and burrowing squamates were then added individually to it, Sineoamphisbaenagrouped with polyglyphanodontians. Gauthier et al. (2012) considered it possible that Sineoamphisbaena was a burrowing polyglyphanodontian.”

The large reptile tree agrees with the original Wu et al. (1993) nesting, at the base of a clade of burrowing prehistoric lizards, some of which included amphisbaenids. Their analysis, unfortunately used suprageneric taxa and they recovered all legless taxa (including snakes) in one clade.

Figure 1. The lineage of Sineoamphisbaena with Chalcides as more primitive and Crythiosaurus + Spathorhynchus as more derived. The quadrate is orange.

Figure 1. The lineage of Sineoamphisbaena with Chalcides as more primitive and Crythiosaurus + Spathorhynchus as more derived. The quadrate is orange.

The temporal region of Sineoamphisbaena has been difficult to interpret because of its unique character and bone fusion patterns not quite like any other. Unlike most burrowing lizards, Sineoamphisbaena did not lose any temporal bones. It rearranged them, fusing some. Here (Fig. 2) is the original interpretation and some suggested reinterpretations.

Figure 2. The skull of Sineoamphisbaena as originally interpreted and as reinterpreted here with color coding matched to that of a more "normal" sister, Chalcides guentheri. Note the squamosal forms the posterior border of the upper temporal fenestra of both taxa.

Figure 2. The skull of Sineoamphisbaena as originally interpreted and as reinterpreted here with color coding matched to that of a more “normal” sister, Chalcides guentheri. Note the squamosal forms the posterior border of the upper temporal fenestra of both taxa. And the long jugal is really composed of the jugal + postorbital. It was not a stretch for the squamosal to contact the postfrontal. If it did fuse, then a crack in the specimen has put a question to that.

Distinct from the original interpretation,
the old postorbital is the new squamosal, continuing to border the posterior upper temporal fenestra. The old jugal is the new jugal + postorbital, matching Chalcides. The old squamosal is the new supratemporal, a bone considered missing originally. The old lacrimal is fused to the prefrontal from what I can tell by comparison to Crythiosaurus. The prefrontal and lacrimal fuses to the maxilla in Bipes.

Burrowing lizards,
evolved in a wide variety of ways and all, except Sineoamphisbaena, lose skull (temporal) bones. All appear to have evolved from a variety of the genus Chalcides because some retain a long low rostrum. Others, like Bipes, have a short blocky snout, but Bipes does not rotate its upper teeth medially as Sineoamphisbaena does. So that split likely preceded tooth rotation. It’s a little confusing with lots of convergence in a little clade due to their burrowing niche.

Figure 3. Chalcides, Crythiosaurus and Bipes with bones colored. Note, only the quadrate remains in Bipes. Other bones are lost or fused. Sineoamphisbaena lost the epipterygoid. Crythiosaurus nests basal to Bipes in the large reptile tree, but the extreme reduction of the quadrate is an autapomorphy.

Figure 3. Chalcides, Crythiosaurus and Bipes with bones colored. Note, only the quadrate remains in Bipes. Other bones are lost or fused. Sineoamphisbaena lost the epipterygoid. Crythiosaurus nests basal to Bipes in the large reptile tree, but the extreme reduction of the quadrate is an autapomorphy.

There may be another skink closer to Sineoamphisbaena, but I haven’t found it yet.

References
Gao K 1997. Sineoamphisbaena phylogenetic relationships discussed. Canadian Journal of Earth Sciences. 34: 886-889. online article
Kearney M 2003. The Phylogenetic Position of Sineoamphibaena hextabularis reexamined. Journal of Vertebrate Paleontology 23 (2), 394-403
Müller J, Hipsley CA, Head JJ, Kardjilov N, Hilger A, Wuttke M and Reisz RR 2011. Eocene lizard from Germany reveals amphisbaenian origins. Nature 473 (7347): 364–367. doi:10.1038/nature09919
Wu XC., Brinkman DB, Russell AP, Dong Z, Currie PJ, Hou L, and Cui G 1993. Oldest known amphisbaenian from the Upper Cretaceous of Chinese Inner Mongolia. Nature (London), 366: 57 – 59.
Wu X-C Brinkman DB and Russell AP 1996. Sineoamphisbaena hexatabularis, an amphisbaenian (Diapsida: Squamata) from the Upper Cretaceous redbeds at Bayan Mandahu (Inner Mongolia, People’s Republic of China), and comments on the phylogenetic relationships of the Amphisbaenia. Canadian Journal of Earth Sciences, 33: 541-577.

wiki/Sineoamphisbaena

Assembling the Squamate Tree of Life – part 1

Some of the heaviest hitters in paleontology joined forces to produce a 300-page paper (including tons of photos and the data matrix) of squamate phylogeny, including several fossil taxa. Gauthier et al. (2012) takes the reader through the history of squamate studies, discusses some of long standing problems and some of the new molecular studies. 141 extant and 51 extinct species were included. The outgroup consisted of three Rhynchocephalians. 610 characters were tested. 112 trees were recovered, chiefly at the base of the Iguania. The homoplasy index was 0.82, so a great deal of homoplay was present. This was a huge study and powerful due to its size.

Happily most of the Gauthier (2012) tree echoed the results of prior trees and the large reptile tree. At the base of both: Huehuecuetzpalli followed by Iguania and Scleroglossa with the latter divided into Gekkota, Scincomorpha and Anguimorpha. Major differences include: 1) Mosasaurs and their kin at the base of the Scleroglossa. 2) Eichstattisaurus at the base of the Gekkota, 3) Amphisbaenia as the sister to a 4) monophyletic Serpentes (snakes). The large reptile tree found 1) mosasaurs to nest with varanids, 2) Eichstattisaurus to nest with basal snakes close to mosasaurs and their kin, far from the Gekkota, 3) amphisbaenids as sisters to skinks, 4) and diphyletic clades of snakes arising from sisters to Lanthanotus and Adriosaurus.

The Gauthier et al. 2012 family tree of the squamates

Figure 1. Click to enlarge. The Gauthier et al. 2012 family tree of the squamates, color added for large clades.

We’ll look at these differences point by point in coming blogs and attempt to dissect the differences and why they occurred.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Gauthier, JA, Kearney M, Maisano JA, Rieppel O and Behkke ADB 2012. Assembling the Squamate Tree of Life: Perspectives from the Phenotype and the Fossil Record. Bulletin of the Peabody Museum of Natural History 53(1):3-308. online here.

Eichstattisaurus. Not a Gekkotan. A Snake Ancestor.

Eichstattisaurus and its sister, Ardeosaurus, were two small lizards found in Solnhofen limestones from the late Jurassic period, approximately 150 mya. Originally (Meyer 1860) and subsequently (Mateer 1982) these two were considered basal gekkotans, relatives of the living gecko, Gekko. Not much attention has been paid to either one. Both are typicall preserved complete and articulated, sometimes with some scalation and soft tissue preservation.

Eichstattisaurus and Ardeosaurus.

Figure 1. Eichstattisaurus and Ardeosaurus. Two Jurassic lizards in the lineage of snakes.

New Nesting
After entering the characters of both lizards, I was surprised to see that they nested not with Gekko, but with Adriosaurus, a hyper-elongated lizard with tiny limbs and a long neck, and Pachyrhachis, a basal snake with tiny hind limbs. This nesting, ancestral to snakes, has been largely overlooked by prior studies. I missed it too. The relationships is not obvious at first glance. I just finally got around to studying these two, fully expecting them to nest with Gekko.

Characters shared by members of this clade with Adriosaurus and Pachyrhachis include the orbit shape, the quadrate shape, supraoccipital fusion, converging temples, ectopterygoid shape, the absence of the retroarticular process, and the metatarsus configuration, among dozens of other traits that are shared with larger clades and by convergence with other reptilian clades.

The slender and elongated premaxillary ascending process was overlooked by Mateer (1982). If anyone has a palate view of either taxon, I’d like to see it.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References
Mateer NJ 1982. Osteology of the Jurassic Lizard Ardeosaurus brevipes (Meyer). Palaeontology 25(3):461-469. online pdf
Meyer H von 1860. Zur Fauna der Vorwelt. Reptilien aus dem lithographischen Schiefer des Jura in Deutschland mit Franchreich. Frankfurt-am-Main.

wiki/Ardeosaurus

The Origin of the Amphisbaenia

Updated August 12, 2014. Moving Cryptolacerta to a closer relationship to Heloderma, elevating Sineoamphisbaena to its place between skinks and amphisbaenids.

Amphisbaena literally means, “goes both ways.” The name is that of the mythological “Mother of Ants,” an ant-eating serpent with a head on both ends. Amphisbaena is also a genus within the Amphisbaenia. Like its mythological namesake, Amphisbaena can back up as easily as it moves forward, despite having no legs.

Amphisbaenia are the worm lizards, typically (with one exception, Figure 1) legless squamates that burrow and have a superficial resemblance to earth worms, including having their scales arranged in rings (annuli). Their right lung is reduced to make more room for the left lung. (In snakes the left lung is reduced.) The eyes are reduced and deeply recessed and the tail resembles the head. Amphisbaenians are so different from other squamates that they have been considered a third suborder, after lizards and snakes.

The primitive Amphisbaenian, Bipes.

Figure 1. The primitive amphisbaenian, Bipes.

Bipes, a Primitive Extant Amphisbaenian
Bipes (Figure 1) is a living amphisbaenian with strong front legs. The hand is stout, like that of a mole, with digits 2 and 3 the longest, digit 1 absent and digit 5 vestigial. The vestigial hind limbs do extend beyond the body wall. By contrast, in typical lizards digit 4 is the longest.

Extinct Burrowers
Several fossil taxa have been linked to amphisbaenians. Tamaulipasaurus lived during the Early Jurassic. Sineoamphisbaena and Crythiosaurus lived during the Late Cretaceous.  Spathorhynchus lived during the early Oligocene. Most of these are known from skulls and partial skulls. No hands yet known in any of these taxa.

Amphisbaenian Origins – part 1 – Sineoamphisbaena
Wu et al. (1993), Wu et al. (1996) and Gao (1997) proposed and argued that a round-skulled Late Cretaceous squamate, Sineoamphisbaenea (Figure 2), was the oldest known amphisbaenian. Unfortunately, it didn’t look very much like most amphisbaenians (Figure 2) which made accurate nesting something of a problem in the eyes of many.

Amphisbaenian Origins – part 2 – Not Sineoamphisbaena
Kearney (2003) argued that Sineoamphisbaena nested closer to Macrocephalosaurus and that Amphisbaena nested with Dibamus and snakes, not far from Gekko and the legless geckos, the Pygopodidiae.

 

Figure 2. Cryptolacerta and kin, including Heloderma and the Amphisbaenia.

Figure 2. Cryptolacerta and kin, including Heloderma and the Amphisbaenia.

Amphisbaenian Origins – part 3 – Cryptolacerta
Müller et al. (2011) argued that a new Eocene lizard, Cryptolacerta  (Figure 2), was the sister to the Amphisbaenia and both were sisters to Sineoamphisbaena and the Teiioidea, a lizard taxon that includes the skinks, Gymnophthalmus and Chalcides. 

Where Do Amphisbaenians Nest in the Large Study?
Here Cryptolacerta does indeed nest close to skinks and amphisbaenians, but it nests closer to Heloderma, another burrowing lizard. Müller et al. (2011) reported that Cryptolacerta had sealed up its upper temporal fenestrae with expansion of the very large postfrontal bone. I was unable to duplicate that reconstruction. Instead I found upper temporal fenestrae in the specimen. A GIF movie and comparative reconstructions can be found here. In any case, amphisbaenians do not add bone to their skulls, they lose bone.

Amphisbaenians nest close to skinks with Sineoamphisbaena nesting close to the base of the other amphisbaenians. In consideration of Kearney (2003), I deleted all amphisbaenians, then all skinks and amphisbaenians from the large study, but those tests failed to dislodge Sineoamphisabaenia form its node, which kept it far from Macrocephalosaurus.

Summary
While amphisbaenians are distinct from most other lizards, they are closer to skinks and legless skinks than to any other lizard taxa. More legless taxa will be added to the large tree as time goes by and I will report on each one in turn.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

References:
Cope ED 1894. On the genera and species of Euchirotidae. American Naturalist 28: 436-437.
Gao K 1997.
 
Sineoamphisbaena phylogenetic relationships discussed. Canadian Journal of Earth Sciences. 34: 886-889. online article
Kearney M 2003. The Phylogenetic Position of Sineoamphibaena hextabularis reexamined. Journal of Vertebrate Paleontology 23 (2), 394-403.
Müller J, Hipsley CA, Head JJ, Kardjilov N, Hilger A, Wuttke M and Reisz RR 2011. Eocene lizard from Germany reveals amphisbaenian origins. Nature 473 (7347): 364–367. doi:10.1038/nature09919
Wu XC., Brinkman DB, Russell AP, Dong Z, Currie PJ, Hou L, and Cui G 1993. Oldest known amphisbaenian from the Upper Cretaceous of Chinese Inner Mongolia. Nature (London), 366: 57 – 59.
Wu X-C Brinkman DB and Russell AP 1996. Sineoamphisbaena hexatabularis, an amphisbaenian (Diapsida: Squamata) from the Upper Cretaceous redbeds at Bayan Mandahu (Inner Mongolia, People’s Republic of China), and comments on the phylogenetic relationships of the Amphisbaenia. Canadian Journal of Earth Sciences, 33: 541-577.
Papenfuss TJ 1982. The Ecology and Systematics of the Amphisbaenian Genus Bipes. Occasional papers of the California Academy of Science 136: 1-42.

wiki/Cryptolacerta
wiki/Amphisbaenia
wiki/Bipes
wiki/Sineoamphisbaena