Blind-snake evolution in dorsal, lateral and palatal views

Today’s blogpost was sparked by
Fachini et al. 2020, who described a new, rather large (~1m) Cretaceous blind-snake lacking a skull, Boipeba.

Unfortunately,
this is a taxon I cannot add to the large reptile tree (LRT, 1765+ taxa; subset Fig. 1) because all included blind-snakes are scored on skull-only traits.

The Fachini et al. cladogram of blind-snakes
differs markedly from the LRT (Fig. 1) in that Fachini et al. nests the most derived taxa in the LRT at basal nodes and vice versa. They also nest extant burrowing snakes basal to extant terrestrial snakes instead of splitting the two at the genesis of snakes.

Fachini et al. report,
“Blindsnakes (Scolecophidia) are minute cryptic snakes that diverged at the base of the evolutionary radiation of modern snakes.”

By contrast the LRT recovers all terrestrial snakes (extant AND extinct) diverging from burrowing snakes at the origin of snakes (Fig. 1). Fachini et al. report both adding and deleting Tetrapodophis from their analysis resulting in no topological changes.

In doing so, Fachini et al. cited an abstract we looked at earlier (Caldwell et al. 2016), proving workers sometimes do cite abstracts.

On this disagreement, we agree:
“there is disagreement between morphological and molecular phylogenetic analyses with regard to their phylogenetic position and monophyly.” 

As often reported here genomic testing too often leads to false positives as compared to phenomic (trait-based) testing in deep time paleo studies. Seemingly the paleo community has not yet realized this, or taken evasive action on this, despite often writing about it (e.g. Fachini et al.).

On this statement we disagree:
Fachini et al. report, “The origin of blindsnakes is unclear.”

In the LRT blind-snakes (burrowing snakes) clearly arise from Tetrapodophis (snake outgroup), Najash and Loxocemus (Figs. 2–4). We looked a phylogenetic problems in snake origins earlier here, here and here in 2013.

Figure 4. Subset of the LRT focusing on snakes. Compare to figure 3.

The most derived blind-snake taxa
in the LRT are instead basal taxa in the cladogram of Fachini et al.

Figure 2. Evolution of blind-snake skulls in dorsal view. The white and black backgrounds are not significant, but represent the original background for the photos and drawings.

How was it possible that Fachini et al.
inverted the order of blind-snakes given Najash as a common outgroup taxon? I don’t know. It does not make sense. Given the clear similarity of python-like Loxocemus to Najash (Figs. 2–4) how is it possible that nearly blind Anomochilus through Typhlops (Figs. 2–4) nested closer to Najash? They were following a long tradition.

Figure 3. Evolution of blind-snake skulls in lateral view.

When you examine these three illustrations
of blind-snake skull evolution in dorsal, lateral and palatal views (Figs. 2–4, primitive at the top of each), remember, Fachini inverts the order (i.e. primitive at the bottom). In this way Fachini et al. separate python-like Najash and Pachyrhachis from extant pythons, like Boa, with burrowing snakes separating them. They may need to rethink that hypothesis of interrelationships.

Figure 4. Evolution of blind-snake skulls in palatal view.

Trends in blind-snake evolution recovered by the LRT:

1. naris shortens and migrates from anterodorsal to anteroventral
2. sharp snout becomes blunt, then round
3. vomernasal fenestra shrink and disappear
4. premaxilla rotates to the palate
5. maxilla shrinks and fuses to premaxilla
6. dentary shortens
7. all marginal teeth reduce and disappear
8. frontal enlarges vs parietal
9. parietal retreats away from orbit
10. circumorbital bones disappear (then reappear only in Liotyphlops)
11. parietal loses sagittal crest
12. palatine becomes mobile with transverse axial rotation
13. ectopterygoid shrinks
14. pterygoid loses teeth and becomes gracile loose strut
15. basisphenoid and basioccipital become bulbous
16. squamosal and supratemporal shrink and disappear
17. quadrate stretches and leans posteriorly
18. coronoid enlarges

Backstory
Blind snakes were first added to the LRT years ago. Many were not colorized as they are now (Figs. 2–4). Some new insights were gained by reviewing the taxa after coloring the drawings and recoloring the photos in a consistent manner.

Most snake workers label the squamosal the supratemporal. This topic was examined earlier here in 2018. The dorsal view image documents the reduction and disappearance of the supratemporal (bright green tiny bony near occiput) prior to the reduction and disappearance of the squamosal (magenta larger bone near quadrate)

The quadrate of Typhlops and Liotyphlops (Fig. 3) informed the previously misunderstood fusion of the quadrate with the stem-like articular in Leptotyphlops (Fig 3).

The realization that the unique appearance of circumorbital bones in Liotyphlops (Fig. 3) was a reappearance and a reversal solved an identity problem first posed by Rieppel, Kley and Maisano 2009.

Skull photos above are from Digimorph.org
and used with permission.

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Bonus unrelated paleo news item for your enjoyment:
Some recent ‘stunning fossil finds” are listed and shown here:
https://www.boredpanda.com

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References
Fachini TS et al. (5 co-authors) 2020. Cretaceous blind snake from Brazil fills major gap in snake evolution. iScience ISCI 101834. https://doi.org/10.1016/j.isci.2020.101834
Rieppel 0, Kley NJ and Maisano JA 2009. Morphology of the skull of the white-nosed blindsnake, Liotyphlops albirostris (Scolecophidia: Anomalepididae. Journal of Morphology, 270, 536-557.

digimorph/Leptotyphlops
wiki/Leptotyphlops
digimorph/Liotyphlops
wiki/Liotyphlops
digimorph/Typhlops
wiki/Typhlops

Atractaspis: a burrowing snake not related to other burrowing snakes

Snake origins have traditionally perplexed paleontologists,
but not the large reptile tree (LRT, 1745+ taxa; Fig. 2). In the LRT snakes arise from tiny Tetrapodophis and Barlochersaurus (both with tiny vestigial limbs) following a series of aquatic taxa and a basal split from geckos. At their LRT genesis, snakes split into terrestrial and fossorial (burrowing) taxa.

Then along comes the burrowing viper, Atractaspis
(Fig. 1) studied in µCT scans by Strong, Palci and Caldwell 2020. “The genus Atractaspis, known commonly as the burrowing asp, is a fossorial lineage within the Colubroidea (= cobras, vipers and rattlesnakes), the most deeply nested clade of extant snakes.”

Figure 1. Atractaspis skull µCT scan from Strong, Palci and Caldwell 2020. Note the immature fangs ready to take the place of the main fangs and otherwise toothless jaws.

While sharing many traits with traditional burrowing snakes,
Atractaspis nests with the rattlesnake, Croatlus in the LRT. So, no big surprises here. The LRT confirms traditional snake topology, that is, once they lose their limbs and become snakes. Prior to that, taxon exclusion has made snake origins the enigma it remains in the traditional academic literature.

From the Strong, Palci and Caldwell abstract:
“Comparative osteological analyses of extant organisms provide key insight into major evolutionary transitions and phylogenetic hypotheses. This is especially true for snakes, given their unique morphology relative to other squamates and the persistent controversy regarding their evolutionary origins.”

re: Persistent controversy? Click here.

“However, the osteology of several major snake groups remains undescribed, thus hindering efforts to accurately reconstruct the phylogeny of snakes. One such group is the Atractaspididae, a family of fossorial colubroids. We herein present the first detailed description of the atractaspidid skull, based on fully segmented micro‐computed tomography (micro‐CT) scans of Atractaspis irregulars.”

See figure 1.

“The skull of Atractaspis presents a highly unique morphology influenced by both fossoriality and paedomorphosis. This paedomorphosis is especially evident in the jaws, palate, and suspensorium, the major elements associated with macrostomy (large‐gaped feeding in snakes).”

Figure 2. Subset of the LRT focusing on geckos and their sister snake ancestors. Atractaspis (not listed here) nests with Croatlus among the terrestrial snakes.

Continuing from Strong, Palci and Caldwell:
“Comparison to scolecophidians—a group of blind, fossorial, miniaturized snakes—in turn sheds light on current hypotheses of snake phylogeny. Features of both the naso‐frontal joint and the morphofunctional system related to macrostomy refute the traditional notion that scolecophidians are fundamentally different from alethinophidians (all other extant snakes). Instead, these features support the controversial hypothesis of scolecophidians as “regressed alethinophidians,” in contrast to their traditional placement as the earliest‐diverging snake lineage.”

In the LRT, fossorial (burrowing) snakes follow tradition and diverge from all other snakes at the genesis of snakes. Therefore, Atractaspis is convergent with fossorial snakes, but keeps that wide-gape elongate quadrate (red in figure 1) from its viper ancestors.

Figure 3, From 2015, prior to the insertion of Tetrapodophis: The origin of snakes alongside the origin of mosasaurs and the origin of Lanthanotus, all in phylogenetic order. Not to scale. Note the branching off of burrowing snakes at the origin of snakes near Dinilysia and Pachyrhachis.

Continuing from Strong, Palci and Caldwell:
“We propose that Atractaspis and scolecophidians fall along a morphological continuum, characterized by differing degrees of paedomorphosis. Altogether, a combination of heterochrony and miniaturization provides a mechanism for the derivation of the scolecophidian skull from an ancestral fossorial alethinophidian morphotype, exemplified by the nonminiaturized and less extreme paedomorph Atractaspis.”

“The phylogeny of scolecophidians is uncertain, due in large part to disagreement between morphological and molecular data.”

Suggestion based on evidence: Ignore molecular data.

Continuing from Strong, Palci and Caldwell:
“Interestingly, a recent morphological phylogeny (Garberoglio et al., 2019) focussing on extinct snakes recovered scolecophidians as nested within Alethinophidia in stark contrast to the more orthodox placement of this group as basally divergent among Serpentes. These results strongly highlight the importance of continued morphological and phylogenetic analyses of this group, including a renewed examination of potential alethinophidian affinities of scolecophidians.”

A valid set of outgroup taxa (Fig. 2) should settle this issue. Just add taxa to find out for yourself.

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References
Strong CRC, Palci A and Caldwell MW 2020. Insights into skull evolution in fossorial snakes, as revealed by the cranial morphology of Atractaspis irregularis (Serpentes: Colubroidea). Journal of Anatomy. https://doi.org/10.1111/joa.13295

Tiny Late Cretaceous Najash: basal to burrowing snakes

Garberoglio et al. 2019
bring us long awaited skull data and several new partial skeletons of a Late Cretaceous snake with legs, Najash rionegrina (Figs. 1, 2). It must be said, the only evidence of legs supplied by the current authors was a caption labeled tibia on a tiny straight bone near the edge of the matrix. Nevertheless, legs and hips were described earlier in other headless specimens of Najash (Apesteguía and Zaher 2006; Fig. 1).

Figure 2. The pelvis of the protosnake with legs, Najash, compared to Heloderma (burrower) and Adriosaurus (swimmer). Heloderma appears to share more traits with Najash.

Most of the new specimens
were found in layered sandstone related to migrating aeolian dunes, along with abundant rhizoliths (root systems encased in desiccated mineral matter) and burrows.

Figure 2. Najash compared to Tetrapodophis (the last snake with legs) and Loxocemus, an extant burrowing snake without legs.

From the authors’ abstract:
“the evolutionary versatility of the vertebrate body plan, including body elongation, limb loss, and skull kinesis. However, understanding the earliest steps toward the acquisition of these remarkable adaptations is hampered by the very limited fossil record of early snakes.”

That’s not true.
In the large reptile tree (LRT, 1602+ taxa, subset Fig. 3) snakes have a well documented ancestry back to Cambrian lancelets. The cladogram presented by the nine co-authors was steeped in tradition and lacking in appropriate outgroup taxa. Contra Garberoglio et al. 2019, Varanus and its monitor lizard kin are not part of snake ancestry in the LRT.

Figure 3. Subset of the LRT focusing on geckos and their sister snake ancestors.

Garberoglio et al. continue:
“These new Najash specimens reveal a mosaic of primitive lizard-like features such as a large triradiate jugal and absence of the crista circumfenestralis, derived snake features such as the absence of the postorbital, as well as intermediate conditions such as a vertically oriented quadrate. The new cranial data also robustly resolve the phylogenetic position of this crucial snake taxon, along with other limbed snakes.”

1. The authors’ cladogram did not nest Najash with burrowing snakes, as in the LRT, but at a much more primitive node.
2. Perhaps this is so because Tetrapodophis and Barlochersaurus were not mentioned in the text.
3. The quadrate was sharply bent posteriorly at a right angle, a trait only found in burrowing snakes.
4. I found no primitive lizard-like features here, other than legs and hips, traits found in Tetraphodophis and Barlochersaurus, the last common ancestors of all living snakes.
5. Najash is a crown-group snake in the LRT until additional untested taxa move it out.

Najash rionegrina (Apesteguía and Zaher 2006; Garberoglio FF et al. 2019; Late Cretaceous) is a tiny burrowing snake that retained a pelvis and hind limbs, transitional between Tetrapodophis and Loxocemus. The premaxilla was tiny, as in terrestrial snakes. The mandible rose anteriorly, as in burrowing snakes. The jugal and vomers were retained.

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References
Apesteguía S and Zaher H 2006.A Cretaceous terrestrial snake with robust hindlimbs and a sacrum. Nature. 440 (7087): 1037–1040.
Garberoglio FF et al. (eight co-authors) 2019. New skulls and skeletons of the Cretaceous legged snake Najash, and the evolution of the modern snake plan. Science Advances 2019(5):eaax5833, 8pp.

wiki/Najash

More DNA results for worm-snake interrelationships

Aurélien et al. 2018 bring us
molecular evidence for the paraphyly of Scolecophidia (burrowing miniaturized worm-snakes, Fig. 2), which they claim “represent the earliest branching clades within the snake tree.” Like others making the same claim before, this is a preposterous statement given the highly derived skulls of various burrowing snakes and the major lack of fossil stem snakes in their cladogram. The Aurélien et al. cladogram includes Varanus, Lacertidae and Anolis sp. as progressively more distant outgroup taxa, omitting (as in all DNA studies) even more fossil taxa.

We looked at
the previous DaSilva et al. 2018 study that took on the origin of snakes using DNA  and a few fossils earlier here (and see figure 1.)

The LRT ancestors of snakes
were recovered here (subset Fig. 1) five years ago.

Figure 1. Subset of the LRT focusing on squamates and snakes. Note how many key taxa in the origin of snakes have been omitted by the previous DaSilva et al. study.

Distinct from DNA studies 
the large reptile tree (LRT, 1308 taxa; subset in Fig. 1) employs fossil taxa and skeletal traits. In doing so burrowing snakes are recovered as a derived monophyletic clade displaying a gradual accumulation of derived traits in derived taxa (Fig. 2). And that makes sense. We expect a gradual accumulation of derived traits in derived taxa, and that’s exactly what you get in the snake section of the LRT. We learned earlier to distrust DNA studies when dealing with distantly related tetrapods, and this is yet another case of the same problem.

Figure 2. Heloderma, Lanthanotus, Anilius, Cylindrophis, Uropeltis, and Leptotyphlops to scale. Boxed scales are enlarged. The first two are not related to the worm snakes.

Figure 3. Click to animate. Leptotyphlops jaws move medially, not up and down. For this reason alone, Leptotyphlops is the most derived snake, not the most primitive one. A long list of other traits support that nesting.

References
Aurélian M et al. (5 co-authors) 2018. Molecular evidence for the paraphyly of Scolecophidia and its evolutionary implications. Journal of Evolutionary Biology DOI: 10.1111/jeb.13373 online here.
DaSilva FO et al. (7 co-authors) 2018. The ecological origins of snakes as revealed by skull evolution. Nature.com/Nature Communications (2018)9:376  1–11. DOI: 10.1038/s41467-017-02788-3 pdf

The identity of the ‘supratemporal’ in snakes

Adding the rattlesnake
Cortalus (Fig. 1), to the large reptile tree (LRT, 1226 taxa) nested it with Boa, the boa constrictor. A typical review of snake skull bones ensued, since it had been awhile since I looked at snakes.

Interestingly
some traditions (but not all, see Fig. 2) anchor the quadrate on a bone identified as the supratemporal (Fig. 3). Others label that bone a squamosal.

In the LRT
several snake-ancestor taxa, including Pontosaurus (Figs. 3, 4), show the squamosal (magenta) remains the quadrate anchor, as in all other tetrapods. The more medial supratemporals become vestiges and disappear in higher snakes.

Figure 1. Crotalus the timber rattlesnake in two views. The light blue dot is the last remains of the jugal.

So, is that anchor bone
the supratemporal? or is it the squamosal? Some authors label it one way (Figs. 2, 8). Others label it the other. The problem is, despite what other tetrapods do with that bone, it lies close to the parietal in snakes, like the supratemporal. Squamosals usually frame the upper temporal fenestra. Validated ancestral taxa with both bones provide the solution.

Figure 2. Is the anchor bone for the quadrate a supratemporal or squamosal? Some authors go one way. Others go the other (see figure 8). The LRT indicates the squamosal is correct. So do ontogenetic studies. See text for discussion.

Pontosaurus has both
bones (Fig. 4) and nests basal to snakes. Pontosaurus lesinensis  (Gorjanovic-Kramberger 1892; Caldwell 2006) was a larger sister to Adriosaurus with a longer, deeper tail.

Figure 3. Pontosaurus skull in situ with relevant bones labeled. Note the squamosal leans into the parietal.

A reconstruction of Pontosaurus
(Fig. 4) clarifies the position and size of the squamosal, jugal, postorbital and supratemporal, matching those of other related taxa. Note the squamosals are pinched medially, the first step toward making those bones line against the cranium (parietal and other bones), as in snakes.

Figure 4. The skull of Pontosaurus reconstructed. Here the squamosal bends medially, reducing the size of the upper temporal fenestra, which is absent/confluent with the lateral temporal fenestra in living snakes. This taxon is basal to Tetrapodophis, Dinilysia and Pachyrhachis.

So where did the supratemporal identification come from?
I have not found the original source yet. Perhaps it came from the hypothesis that snakes are derived from mosasaurs (Fig. 6), but it is not easy to see how this happened.

The blogging professor, Darren Naish (1998-2009), wrote online here: Mosasauroids (mosasaurids and all of their close relatives) are platynotan lizards that, in the most recent analyses, share 40 shared derived characters with snakes (Ophidia) – therefore they share a single ancestor and form the clade Pythonomorpha Cope, 1869.”

Figure 5. Tylosaurus and other mosasaurs. Note the small supratemporal and large squamosal. Workers, like ED Cope and Darren Naish, found many similarities between mosasaurs and snakes, all by convergence according to other workers and the LRT.

According to Wikipedia
“Pythonomorpha was originally proposed by paleontologist ED Cope (1869) as a reptilian order comprising mosasaurs, which he believed to be close relatives of Ophidia (snakes). [However] Many recent authors demonstrate a closer relationship between mosasaurs and varanid lizards, like Varanus.” The latter relationship is supported by the LRT, which derives snakes from a long list of long-bodied taxa not related to mosasaurs and not tested by earlier workers, including tiny, four-legged Tetradophis (Fig. 6).

Figure 6. Snake skull evolution from Adriasaurus to the rattlesnake, Crotalus. Given the lack of a coronoid in Dinilysia and its presence in sister taxa makes me wonder if it has been displaced. The yellow bone, labeled the pterygoid, is a good candidate, but making judgements like that from a drawing is risky.

Pachyrhachis (Fig. 7) is a basal snake
with an expanded cranium, either reducing the twin posterior parietal processes or perhaps filling the space between them (because the cranium is relatively longer). Such a cranium appears to be inherited from tiny Tetrapodophis, which is preserved flat. In both taxa the squamosals lie upon the expanded cranium. The postorbital is fused to the postfrontal and no longer contacts the squamosal. The upper temporal fenestra is thus absent and/or confluent with the lateral temporal fenestra at the pre-snake/snake transition.

Figure 7. Pachyrhachis, a basal snake in which the postorbital no longer contacts the squamosal, which lies close to the parietal, losing the upper temporal fenestra.

Ontogeny
Werneburg and Sánchez-Villagra 2015 tested homology hypotheses by examining snake and lizard embryos then reported, “The ‘supratemporal’ of snakes could be homolog to squamosal of other squamates, which starts ossification early to become relatively large in snakes.” The authors wrongly reported, “We included for the first time Varanus, a critical taxon in phylogenetic context.” Varanus is not related to snakes in the LRT. The authors did not include Pontosaurus, Tetrapodophis and several other snake ancestors listed in the LRT. Without these taxa, the correct phylogenetic framework was not present in their study.

Figure 8. Snake skull form Andjelkovic et al. 2017 mislabeling the squamosal as a supratemporal.

 

Longtime readers may remember
I took a stab at this issue several years ago, but that was before the present tree topology of snake relations was established. Once I realized that I followed an invalid tradition back then, I trashed that post to avoid further confusion. I discover errors all the time, most often in my own work. I then repair those errors and misconceptions, which is the basis of good science, as everyone knows. Sadly, not every paleontologist follows this dictum.

On that note…
You might remember another traditional bone I-D mistake was also found in turtles,
in which the supratemporal was misidentified as the squamosal. So, yes, widespread misidentification can happen, and a wide gamut phylogenetic analysis illuminates such problems. So, bottom line, once again taxon exclusion is the single source of this long-standing problem. 

References
Caldwell MW 2006. A new species of Pontosaurus (Squamata, Pythonomorpha) from the Upper Cretaceous of Lebanon and a phylogenetic analysis of Pythonomorpha. Memorie della Società Italiana di Scienze Naturali e del Museo Civico di Storia Naturale di Milano 34: 1–42.
Cope ED 1869. On the reptilian orders Pythonomorpha and Streptosauria. Proceedings of the Boston Society of Natural History 12:250–266.
Gorjanovic-Kramberge D 1892. O fosilnih cetaceih hrvatske i kranjske. Rad. Jugoslavenske Akademije Znanosti i Umjetnosti 111:1-21.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.
Werneburg I and Sánchez-Vilagra MR 2015. Skeletal heterochrony is associated with the anatomical specializations of snakes among squamate reptiles. Evolution 69(1):254-63. doi: 10.1111/evo.12559. Epub 2014 Dec 17.

wiki/Crotalus
wiki/Boa_constrictor
wiki/Pontosaurus
wiki/Pythonomorpha

Snake evolution: new paper suffers from taxon exclusion

DaSilva et al. 2018
bring us a new perspective on snake evolution that employs molecules, physical traits, embryos, fossils, CT scans… a huge amount of data and labor… perhaps all for nought because they excluded so many pre-snake taxa (Fig. 2). And their results do not produce a gradual accumulation of derived traits (Fig. 1), even when they omit the mosasaur skulls listed at their base of snakes. Here I added that missing mosasaur skull.

Figure 1. Figure 1 from DaSilva et al. 2018 with mosasaur skull added here. Note the complete lack of a gradual accumulation of traits leading to snakes and the very derived snake skull placed at the base of all snakes. No wonder they omitted adding the mosasaur skull to the parade of pre-snakes. IF you were part of this study and failed to raise your hand at this RED FLAG, then do better next time.

The DaSilva et al. abstract:
“The ecological origin of snakes remains amongst the most controversial topics in evolution, with three competing hypotheses: fossorial; marine; or terrestrial. Here we use a geometric morphometric approach integrating ecological, phylogenetic, paleontological, and developmental data for building models of skull shape and size evolution and developmental rate changes in squamates. Our large-scale data reveal that whereas the most recent common ancestor of crown snakes had a small skull with a shape undeniably adapted for fossoriality, all snakes plus their sister group derive from a surface-terrestrial form with non-fossorial behavior, thus redirecting the debate toward an underexplored evolutionary scenario. Our comprehensive heterochrony analyses further indicate that snakes later evolved novel craniofacial specializations through global acceleration of skull development. These results highlight the importance of the interplay between natural selection and developmental processes in snake origin and diversification, leading first to invasion of a new habitat and then to subsequent ecological radiations.”  Fossorial = burrowing.

The DaSilva et al. Supplementary Data reports:
“To include a large dataset of squamate specimens, including extant, fossil, and embryonic taxa (see details below as well as Fig. 1 (main text) and Supplementary Fig. 1), we used a composite phylogenetic hypothesis based on the most recent molecular as well as combined molecular and morphological studies on squamate evolution.”

As readers know by now,
molecular data fails at large phylogenetic distances. It produces false positives. Even so, their large number of physical traits (691 morphological characters and 46 genes) should have given them a good cladogram… unless they omitted huge swaths of taxa.

Which is what they did (Fig. 2).

Even though they used fossil and embryological data,
their results do not produce a gradual accumulation of traits (Fig. 1). Nor do they employ appropriate outgroup taxa, either for squamates or for snakes (Fig. 2).

Without these key transitional taxa,
the authors have no idea what the basalmost squamates and snakes should look like. Here’s what the large reptile tree (LRT, 1152 taxa) recovered (Fig. 1):

Figure 1. Subset of the LRT focusing on squamates and snakes. Note how many key taxa in the origin of snakes have been omitted by the DaSilva et al. study.

If nothing else,
I hope readers gain a critical and skeptical eye toward published material. Sometimes it’s not what they say, but what they omit that spoils their results.

The LRT is a good base
to begin more focused studies in tetrapod evolution. It covers virtually all the possible candidates so workers can have high confidence that their more focused studies include relevant taxa and exclude irrelevant taxa.

For more information on snake origins
click here and/or here, along with links therein.

References
DaSilva FO et al. (7 co-authors) 2018. The ecological origins of snakes as revealed by skull evolution. Nature.com/Nature Communications (2018)9:376  1–11. DOI: 10.1038/s41467-017-02788-3 pdf

News at the genesis of snakes: Tetrapodophis ‘highly suggestive’ as aquatic

A year ago
Martill et al. 2015 described the stem snake, Tetrapodophis (Fig. 1) and considered it a burrowing squamate. A new paper by Lee et al. 2016 reports that Tetrapodophis had aquatic adaptations.

Figure 1. Tiny Tetrapodophis at full scale if your monitor produces 72 dpi images (standard on many monitors).

This new study confirms 
what you read here, here and here when we nested Tetrapodophis with the following aquatic pre-snaketaxa: Pontosaurus, Adriosaurus and Aphanziocnemus in the large reptile tree (subset Fig. 2).

From the Lee et al abstract

“The exquisite transitional fossil Tetrapodophis – interpreted as a stem-snake with four small legs from the Lower Cretaceous of Brazil – has been widely considered a burrowing animal, consistent with recent studies arguing that snakes had fossorial ancestors [not so here]. We reevaluate the ecomorphology of this important taxon using a multivariate morphometric analysis and a reexamination of the limb anatomy. Our analysis shows that the body proportions are unusual and similar to both burrowing and surface-active squamates. We also show that it exhibits striking and compelling features of limb anatomy, including enlarged first metapodials and reduced tarsal/carpal ossification – that conversely are highly suggestive of aquatic habits, and are found in marine squamates. The morphology and inferred ecology of Tetrapodophis therefore does not clearly favour fossorial over aquatic origins of snakes.”

Figure 2. Scleroglossan subset of the large reptile tree. Generalists taxa duplicated in the Yi and Norell tree are shown in bright green. Burrowers shared in the Yi and Norell tree are in dark green. Legless taxa are black. Vestiges are in gray. Unknown are striped.

Wonder why 
prior workers have not performed a phylogenetic analysis on this taxon that includes the above named aquatic squamates and Jucaraseps?

References

Lee MSY, Palci A, Jones MEH, Caldwell MW, Holme JD & Reisz RR 2016. Aquatic adaptations in the four limbs of the snake-like reptile Tetrapodophis from the Lower Cretaceous of Brazil. Cretaceous Research (advance online publication)

doi:10.1016/j.cretres.2016.06.004 online for sale

Martill DM, Tischlinger H and Longrich NR 2015. A four-legged snake from the Early Cretaceous of Gondwana. Science 349 (6246): 416-419. DOI: 10.1126/science.aaa9208

wiki/Tetrapodophis

Tetrapodophis, the four-legged snake – part 3

Just a little more to add
to what was already said about Tetrapodophis ( Martill, Tischlinger and Longrich 2015)  earlier here and here. Today we’ll show DGS tracings of the pectoral girdle (impression only, Fig. 1), manus, pelvis and pes.

Figure 1. Tetrapodophis pectoral region. The tips of the anterior ribs and the pectoral girdle were preserved as faint impressions traced here. Purple is clavicle. Yellow/green is interclavicle

What does that say about Tetrapodophis
that the forelimbs were ossified (Fig. 1), but the pectoral girdle was not ? The scapula, coracoid, clavicle and interclavicle are not mentioned in the text. Nor are they mentioned by their absence. Are they in the counter plate? I was able to trace colors over possible impressions of the pectoral elements and distal anterior ribs in the plate. But nothing is clear here.

Figure 2. Tetrapodophis manus extends beneath the ribs. Not sure what is happening under there, but the reconstruction (at right) is a best guess. The metacarpals are very short. The proximal phalanges are very long.

The manus of Tetrapodophis
is somewhat obscured beneath a few ribs (Fig. 2), but the reconstruction suggests a pattern of long proximal phalanges, short metacarpals and large claws (unguals).

Figure 3. The hind limbs of Tetrapodophis, here colorized to differentiate the digits (in which all phalanges are fused) from the metatarsals. At upper right is the reconstructed pelvis. Metatarsals in purple. Distal tarsals in red. 

The pelvis is preserved
along with the hind limbs (Fig. 3). Interesting that the pedal phalanges are all fused together within the five digits. Tetrapods don’t do this very often, if at all. But this is what happens prior to disappearance. The metatarsals were very short, not much longer than the distal tarsals.

References
Martill DM, Tischlinger H and Longrich NR 2015. A four-legged snake from the Early Cretaceous of Gondwana. Science 349 (6246): 416-419. DOI: 10.1126/science.aaa9208x

 

 

 

Tetrapodophis – new four legged very basal, very tiny snake – part 2

Earlier we looked at the skull of Tetrapodophis (Martill et al. 2015), a four-legged very tiny snake.

Figure 1. Tetrapodophis nests at the base of the clade of snakes in the large reptile tree. Note, the burrowing snakes are not basal in this tree. Rather these very specialized snakes are quite derived. There are more proto-snakes and basal snakes known, so this tree should be considered in that light.

A phylogenetic analysis nested Tetrapodophis at the base of all snakes (Fig. 1).

Figure 2. The skulls of pre-snakes, Tetrapodophis and snakes compared. The orbits move foreword. The jaw muscles enlarge. The upper temporal arch disappears.

National Geographic featured several Dave Martill quotes. Here are a few:

“And then, if my jaw hadn’t already dropped enough, it dropped right to the floor,” says Martill. The little creature had a pair of hind legs. “I thought: bloody hell! And I looked closer and the little label said: Unknown fossil. Understatement!”

“I looked even closer—and my jaw was already on the floor by now—and I saw that it had tiny little front legs!” he says.

“But no snake has ever been found with four legs. This is a once-in-a-lifetime discovery.”

“This little animal is the Archaeopteryx of the squamate world,” he says.

Martill thinks that Tetrapodophis killed its prey by constriction, like many modern snakes do. “Why else have a really long body?” he says.

Martill thinks that the snake may have used these “strange, spoon-shaped feet” to restrain struggling prey—or maybe mates.

There was a bit of controversy raised about this specimen. Read about it here at Forbes.com. It also includes an illustration of Tetrapodophis wrapped around a mouse-like mammal. Tiny prey bones were found in its gut.

Likely a Crato Formation fossil
Martill et al. thought the Tetrapodophis substrate was from the Crato Formation. Wikipeidea reports, “The Crato Formation is a geologic formation of Early Cretaceous age in northeastern Brazil‘s Araripe Basin. It is an important Lagerstätte (undisturbed fossil accumulation) for palaeontologists. The strata were laid down mostly during the early Albian age, about 108 million years ago, in a shallow inland sea. At that time, the South Atlantic was opening up in a long narrow shallow sea.”

Nevertheless,
Martill et al. consider Tetrapodophis closer to burrowing snakes, not aquatic ones. Distinct from its sea-going predecessors, Tetrapodphis had a longer torso than tail, like living snakes do. It also had a single row of belly scales, like snakes, preserved as soft tissue impressions.

Perhaps,
owing to its small size, Tetrapodophis had returned to the land, or shallows grading to swamps.

Video link 1. Dave Martill describing from large photos Tetrapodophis. Just a few minutes long.

Finally,
here’s Dave Martill in a video describing Tetrapodophis. Click to play.

References
Martill DM, Tischlinger H and Longrich NR 2015. A four-legged snake from the Early Cretaceous of Gondwana. Science 349 (6246): 416-419. DOI: 10.1126/science.aaa9208

Tetrapodophis – new four legged very basal, very tiny snake

A new paper by Martill, Tischlinger and Longrich (2015) brings us a really tiny, new Early Cretaceous snake, Tetrapodophis amplectus (Fig. 1, BMMS BK 2-2, ), with four limbs and all of its fingers and toes. The authors suggest this basal snake and thus all snakes evolved from burrowing rather than marine ancestors in accord with the  Longrich, Bullar and Gauthier (2012) assessment of another tiny snake, Coniophis, which is known from only a few skull parts. (Also see below.)

Unfortunately Tetradopodophis (so far based on skull traits only) nests in the large reptile tree between Adriosaurus + Pontosaurus and Dinilysia + Pachyrhachis + Boa, so an aquatic origin is recovered from the cladogram despite the extremely tiny size of Tetrapodophis (skull length about 1 cm, total length about 16 cm). Martill et al. used mosasaurs and several incomplete taxa (Eophis, Diablophis, Portugalophis and Parviraptor, none included in the large reptile tree) for outgroups and nested Tetrapodophis as a sister to Coniophis and basal to Najash, Dinilysia and all other snakes. The authors note, “As the only known four-legged snake, Tetrapodophis sheds light on the evolution of snakes from lizards. Tetrapodophis lacks aquatic adaptations (such as pachyostosis or a long, laterally compressed tail) and instead exhibits features of fossorial snakes and lizards: a short rostrum and elongation of the postorbital skull, a long trunk and short tail, short neural spines, and highly reduced limbs.”

I wonder if Tetrapodophis is a hatchling? Or does it represent yet another example of phylogenetic miniaturization at the origin of a major clade? It is similar in size to Jucaraseps, a more primitive lizard with snake affinities. Tetrapodophis may be a late surviving (Early Cretaceous) very basal snake with likely origins in the Middle Jurassic. DGS (digital graphic segregation) was helpful in pulling out details (Fig. 1) overlooked or ignored by the original authors.

Figure1. The skull of Tetrapodophis in situ and colorized (middle) as originally interpreted (below) and reconstructed using DGS (above). I have not seen the fossil, but examination of the photograph using DGS permits more details to be identified. This image will be tested for validity Monday. Only the major bones were identified here. The skull is about 1 cm in length.

The preparator did an excellent job on such a tiny (16 cm) specimen, unless it split naturally into part and counterpart. The specimen was in a private collection for decades before getting its museum number.

Like non-snakes, Tetrapodophis retained a postorbital, squamosal and lacrimal. A broken jugal was also found. Palatal fangs were present along with a deep coronoid process. There is a mass at the back of the throat that makes it difficult to identify the posterior palatal bones. The authors report, “BMMS BK 2-2 is distinguished from all other snakes by the following combination of characters: 160 precaudal and 112 caudal vertebrae, short neural spines, four limbs, metapodials short, penultimate phalanges hyper elongate and curved, phalangeal formula 2?-3-3-3-3? (manus) 2-3-3-3-3 (pes).”

Although DGS was able to pull lots of details out of this specimen, don’t expect the DGS detractors to applaud this example, although It would be nice to get a tip of the hat for this one. It’s a pretty striking example and only took an hour or two to do.

Figure 2. Tiny Tetrapodophis at full scale if your monitor produces 72 dpi images (standard on many monitors).

This is a major find and congratulations are due to the authors. More on this specimen in future blog posts.

References Longrich NR, Bullar B-A S and Gauthier JA 2012. A transitional snake from the Late Cretaceous period of North America. Nature 488, 205-208. Martill DM, Tischlinger H and Longrich NR 2015. A four-legged snake from the Early Cretaceous of Gondwana. Science 349 (6246): 416-419. DOI: 10.1126/science.aaa9208