Bipedal Cretaceous lizard tracks

These are the oldest lizard tracks in the world…
(if you don’t consider Rotodactylus (Early Triassic) strictly a ‘lizard’ (= squamate). One rotodactylid trackmaker, Cosesaurus, is a tiny lepidosaur).
Figure 1. Bipedal lizard tracks from South Korea in situ.

Figure 1. Bipedal lizard tracks from South Korea in situ. They are tiny.

From the abstract
“Four heteropod lizard trackways discovered in the Hasandong Formation (Aptian-early Albian), South Korea assigned to Sauripes hadongensis, n. ichnogen., n. ichnosp., which represents the oldest lizard tracks in the world. Most tracks are pes tracks that are very small. The pes tracks show “typical” lizard morphology as having curved digit imprints that progressively increase in length from digits I to IV, a smaller digit V that is separated from the other digits by a large interdigital angle. The manus track shows a different morphology from the pes. The predominant pes tracks, the long stride length of pes, narrow trackway width, digitigrade manus and pes prints, and anteriorly oriented long axis of the fourth pedal digit indicate that these trackways were made by lizards running bipedally, suggesting that bipedality was possible early in lizard evolution.”
Actually, the lizard was not running.
Typically in running tracks the prints are very far apart and these tracks are sometimes left toe to right heel.
Figure 2. Original and new tracings of the bipedal lizard tracks from South Korea. PILs are added,

Figure 2. Original and new tracings of the bipedal lizard tracks from South Korea. PILs are added. Manual digit 4 and 5 appear to have shifted.

 The authors did not venture who made the tracks.
They reported, “based on the palaeobiogeographic distribution of facultative extant families, the lizard that produced S. hadongensis tracks could well have been a member of an extinct family or stem members of Iguania, which was present in the Early Cretaceous.”
Actually the closest match among tested taxa
is with Eichstaettisaurus (Fig. 1), a basal member in the lineage of snakes. And this clade is close to the origin of geckos. ReptileEvolutiion.com and the large reptile tree would have been good resources for the authors to use. Lots of lizard pedes were illustrated and scored there.
Figure 3. Originally pictured as a generic lizard (below), here Eichstattsaurus scaled to the track size walks upright.

Figure 3. Originally imagined  as a generic lizard (below), here Eichstattsaurus matched and scaled to the track size walks upright.

 Based on a phylogenetic analysis of the tracks
the closest match in the LRT is with Eichstaettisaurus, so a slightly larger relative made them. Distinct from the skeletal taxon, the trackmaker had a longer p2.1 than 2.1 and pedal digit 1 was quite short. Otherwise a good match in all other regards.
So why walk bipedally?
It was walking, not running, so escape from predation can be ruled out. Elevating the upper torso and head, like a cobra, can be intimidating to rivals, or just offer a better view over local plant life. This sort of flexibility could have helped them get into the trees and then to move to higher branches.
References
Lee H-J, Lee Y-N, Fiorillo AR &  LÃ J-C 2018. Lizards ran bipedally 110 million years ago. Scientific Reports 8: 2617. doi:10.1038/s41598-018-20809-z

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.

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.

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

Snake origins according to DNA studies

Figure 1. Cladogram of squamates from Streicher and Wiens 2017 highlighting the origin of snakes based on DNA. Unfortunately, only the closely related taxa are correctly nested here. See figure 2 for gradual accumulations of traits in all related taxa.

Figure 1. Cladogram of squamates from Streicher and Wiens 2017 highlighting the origin of snakes based on DNA. Unfortunately, only the closely related taxa are correctly nested here. See figure 2 for gradual accumulations of traits in all related taxa.

 

Why would Streicher and Wiens 2017
(Fig. 1) want to do this? They can’t use fossils. They’ll never find a gradual accumulation of traits, starting from ‘snakes with legs’. And… DNA does not work over large phylogenetic distances. They put their faith in DNA. They believed they would get an answer. Their prayers were answered, but the answer does not make sense. Their cladogram cannot be verified with morphological studies (Fig. 2). Morph studies can and do use fossils and do produce a gradual accumulation of traits. Morphology is, and will always be, the gold standard of phylogenetics.

We have to stop wasting time
on methods that do not work over large phylogenetic distances. Rant. Rant. Rave. Rave.

Figure 2. Subset of the large reptile tree focusing on lepidosaurs and snakes are among the squamates.

Figure 2. Subset of the large reptile tree focusing on lepidosaurs and snakes are among the squamates.

Here’s how you know, at first glance,
how the Streicher and Wiens cladogram produces odd, mismatching sisters.

  1. Derived taxa usually do not appear at the base of major clades: Dibamus, Typhlops
  2. Mismatches usually do not nest close to one another: Bipes & Lacerta, Python & Typhlops, Dibamus & Sphenodon

Streicher and Wiens will never find out
that snake ancestors had legs using DNA. Those just never shows up in molecules. Their paper’s title: “Phylogenomic analyses of more than 400 nuclear loci resolve the origin of snakes among lizards families” do not resolve the origin of snakes.

Snakes arise
from near the very beginning of a rapidly diversifying Scleroglossa. The snake clade split from the gekko clade shortly after the origin of the Squamata. Derived burrowing snakes with jaws that pull prey items in appear in derived taxa, not as basal plesiomorphic forms. When basal taxa are bland and plesiomorphic, that’s a good sign that you’re doing something right.

References
StreicherJW and Wiens JJ 2017. Phylogenomic analyses of more than 400 nuclear loci resolve the origin of snakes among lizards families. Biology Letters 13: 20170393.
http://dx.doi.org/10.1098/rsbl.2017.0393

 

 

Snake origins tied to nocturnal lizards

Emerling 2017 report:
The earliest snakes lost numerous light-associated genes. Evolutionary analyses suggest dim-light adaptation in snakes preceded leg loss.

From the abstract
The evolutionary origins of snakes involved the regression of a number of anatomical traits, including limbs, taste buds and the visual system, and by analyzing serpent genomes, I was able to test three hypotheses associated with the regression of these features. The final hypothesis addressed is that the earliest snakes were adapted to a dim light niche. I found evidence of deleted and pseudogenized genes with light-associated functions in snakes, demonstrating a pattern of gene loss similar to other dim light-adapted clades. Molecular dating estimates suggest that dim light adaptation preceded the loss of limbs, providing some bearing on interpretations of the ecological origins of snakes.

Google ‘nocturnal lizards’ and what do you get?
Geckos. That confirms the results of the the large reptile tree that documents that, while snakes are not geckos, the two clades shared a last common ancestor before geckos were geckos and snakes ancestors still had legs.

References
Emerling CA 2017. Genomic regression of claw keratin, taste receptor and light-associated genes provides insights into biology and evolutionary origins of snakes.
Molecular Phylogenetics and Evolution 115: 40–49.
doi: https://doi.org/10.1016/j.ympev.2017.07.014
http://www.sciencedirect.com/science/article/pii/S1055790317300131
(free pdf)

Tetrapodophis is not a snake! – SVP abstracts 2016

Not a snake?
Well, what if Tetrapodophis (Martill et al. 2015) was the closest proximal sister to Dinilysia and the rest of the snakes? That way it didn’t have to be a snake, but ‘the next best thing’. That’s what the large reptile tree (LRT) recovered a year ago, and that cladogram tests a large gamut of other candidates. The outgroup taxa for Tetrapodophis in the LRT include the likely aquatic pro-snakes Pontosaurus, Adriosaurus and Aphanizocnemus, all of which also had tiny legs.

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

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

From the Caldwell et al. 2016 abstract:
“A very tiny fossil specimen (195mm TL) of a long bodied (~160 presacrals), long
tailed (~112 caudals), limb-reduced, squamate was recently described as the first known
four-legged snake, Tetrapodophis amplectus. Snake affinities were proposed based on 24 features, of which only 13 were actually tested in that phylogenetic analysis. First hand examination has produced counter observations and interpretations to both morphology and proposed affinity. We find the skull to be long, the mandible straight, there is no subdental ridge, an intramandibular joint is not preserved, the teeth are not snake-like, and are taphonomically displaced not recumbent. The high precloacal vertebral counts are not exclusive to snakes (eg., dibamids ~135; amphisbaenians ~175), zygosphenes are not observed, the neural spines are tall, rib heads are not tubercular, “lymphapophyses” are expanded sacral processes, and scales are not present. New anatomical observations include a high cervical count, features of the suspensorium, orbit size and margin, an elongate retroarticular process, position of the splenial apex at the terminus of the tooth row, enlarged first metapodials, reduced carpal and tarsal ossification, intercentra in the neck and tail, and reduced limb articulation surfaces. The skull and skeleton are not snake like. The original phylogenetic analysis found snakes and Tetrapodophis as sister to the Mosasauria, but concluded burrowing habits and origins for snakes. The hypothesis of a burrowing habit for Tetrapodophis is falsified by both phylogeny and morphology. The limbs are very small, lightly constructed, show delayed mesopodial ossification similar to a variety of aquatic reptiles, along with enlarged first metapodials, elongate phalanges, and weak girdles. The limbs were ineffective paddles, and the tail was long (burrowing snakes have very short tails, as do amphisbaenids and dibamids), which we interpret as indicating that Tetrapodophis employed anguilliform locomotion in water or on land. It had large eyes and has small fish-like vertebrate bones in its gut, not large mammals. Evidence for anatomies consistent with constriction are not observed.”

First of all,
except for a weak finish (see below) this is a good, detailed abstract, the way all abstracts should be.

The weak finish
You can’t claim this is NOT a snake without also suggesting what it IS. It needs a phylogenetic analysis for that. Luckily for us, the LRT has nested Tetrapodophis at the base of all snakes, derived from a sister to Pontosaurus, an elongate aquatic squamate ultimately derived from Ardeosauruand other stem snakes close to stem geckos.

There’s no guesswork, no loose ends at the LRT!

Figure 2. 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.

Figure 2. 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.

In the LRT
Tetrapodophis is not a snake, but the closest outgroup taxon. Or is it the basalmost snake? Depends on your definition and what trait defines snake from non-snake. So Tetrapodophis may not have all the traits snakes have (think only of those legs for the moment), but it has many more snake-like traits than any other tested taxon. Here are some notes based on the Caldwell et al. abstract:

  1. skull (too) long – as in Pachyrhachis — how long is too long?
  2. mandible straight – dental side or ventral side? Pachyrhachis is similar.
  3. no subdental ridge – inner side of maxilla not visible in fossil and may be a synapomorphy of snakes proper.
  4. intramandibular joint is not preserved – even so, it looks a lot like Pachyrhachis.
  5. the teeth are not snake-like, not recumbent – recumbent teeth appear in Dinilysia and Pachyrhachis, but may also be present in Tetrapodoophis. Dear reader, you decide. Is this transitional?
  6. The high precloacal vertebral counts are not exclusive to snakes (eg., dibamids ~135; amphisbaenians ~175) – but note they’re not saying Tetrapodophis is either of these.
  7. zygosphenes are not observed – those may show up in snakes proper
  8. the neural spines are tall – but not on all vertebrae. I see tall neural spines on the sacral region of Pachyrhachis?
  9. rib heads are not tubercular – this may come with snakes proper.
  10. lymphapophyses [lumbar transverse processes] are expanded sacral processes – available data does not show this detail
  11. Scales are not present – this is in contrast with the original observation

New observations by Caldwell et al include

  1. high cervical count – how many is too high? I count 5 in Tetrapodophis. That’s 3 less than in outgroup taxa, like Pontosaurus. The number of cervicals in a snake has been a long-standing question, once tentatively answered here.
  2. features of the suspensorium – no details given by the authors.
  3. orbit size and marginPontosaurus has a tall jugal. Pachyrhachis as a deep postfrontal-postorbital. Tetrapodophis has a transitional vestigial jugal and a deeper postfrontal. The orbit size is relatively similar to that in Pachyrhachis and Dinilysia.
  4. an elongate retroarticular process – In Pontosaurus the RP is elongate. In Pachyrhachis it is absent. In Tetrapodophis it is vestigial and transitional.
  5. position of the splenial apex at the terminus of the tooth row – that’s a hard one to judge in candidate taxa
  6. enlarged first metapodials – they are more robust in Pontosaurus and absent in Pachyrhachis, which makes Tetrapodophis, again, transitional
  7. reduced carpal and tarsal ossification – to be expected if you’re losing your limbs
  8. intercentra in the neck and tail – Since no related taxa have intercentra, this could be due to neotony, retaining embryonic traits, due to phylogenetic miniaturization and thus is, again, a transitional trait that appears in tiny transitional taxa, then disappears with greater size in descendants.
  9. reduced limb articulation surfaces –  to be expected if you’re losing your limbs

The original phylogenetic analysis
(Martill et al. 2015) found snakes and Tetrapodophis as sister to the Mosasauria. That is not supported by the LRT.

The hypothesis of a burrowing habit
for Tetrapodophis is falsified by both phylogeny and morphology. The limbs are very small, lightly constructed, show delayed mesopodial ossification similar to a variety of aquatic reptiles. That is supported by the LRT.

In July of 2015 I reported
Like non-snakes, Tetrapodophis retained a postorbital, squamosal and lacrimal. A broken jugal was also found.” Along with the nesting at the base of all snakes in the LRT, this is one more time that taxonomic placement of enigma taxa in the LRT were later confirmed by professional paleontologists.

Transitional taxa, like Tetrapodophis,
help focus our definitions of clades. Everyone knows that no snakes have legs, but transitional basal forms do — as expected because this is micro evolution at work…small steps, always, and often in miniaturized taxa.

References
Caldwell MW, Reisz RR, Nydam RL, Palci A and Simoes TR 2016. Tetrapodophis amplectus (Crato Formation, Lower Cretaceous, Brazil) is not a snake. Abstract from the 2016 meeting of the Society of Vertebrate Paleontology.
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

Science Mag news link here.

NatGeo news link here.

wiki/Pachyrhachis
wiki/Tetrapodophis

Stem geckos? Or stem snakes? – SVP abstract 2016

Earlier Simoes et al. 2016 published their paper and today their SVP abstract of Eichstaettisaurus and Ardeosaurus, two Jurassic squamates. Here’s how the LRT (subset Fig. 1, complete tree here) recovered geckos, snakes and stem snakes like Eichstaettisaurus and Ardeosaurus, the dual subjects of Simoes et al. 2016 paper and abstract.

Figure 1. Subset of the LRT focusing on geckos snakes and stem snakes that nest close to geckos.

Figure 1. Subset of the LRT focusing on geckos snakes and stem snakes that nest close to geckos.

From the Simoes et al. 2016 abstract:
“Late Jurassic lizards from Solnhofen, Germany, are some of the oldest known articulated lizard specimens in the world (1), and are also the most complete Jurassic squamates. These specimens are thus very important to our understanding of early squamate evolution, with valuable information regarding morphology, taxonomy, and phylogeny. Eichstaettisaurus schroederi and Ardeosaurus digitatellus are two of the best  preserved species from that locality, the former being represented by the most complete Jurassic lizard specimen known anywhere in the world. Despite their relevance to broad questions in squamate evolution, their morphology has never been described in detail, and their systematic placement has been under debate for decades. Here, we provide the first detailed morphological description, species level phylogeny and functional morphological evaluation of E. schroederi and A. digitatellus. We identified previously undescribed features of E. schroederi linking this taxon to gekkotans, such as the Meckelian canal being closed and fused medially, ectopterygoid lying dorsal to transverse process of pterygoid, and autopodial digit symmetry. Using a revised and updated dataset containing 610 characters and 193 taxa (2), we corroborate their initial placement as geckoes—stem gekkotans, more specifically. This is of fundamental importance to the early evolution of squamates, as it demonstrates the existence of yet another major extant squamate clade (Gekkonomorpha) in the Jurassic (3). Additionally, both taxa illustrate a number of climbing adaptations (e.g. shape of unguals, penultimate phalanges, and body proportions), which indicates a scansorial lifestyle arose earlier in the evolution of geckos than previously known. Autopodial modifications associated with digital hyperextension and adhesive toepads (e.g. depressed and reduced intermediate phalanges, and arcuate penultimate phalanges), which provide geckoes with a highly sophisticated climbing apparatus, are not present. Therefore, our findings further suggest that morphological adaptations for scansoriality evolved in geckoes prior to the first known occurrence of adhesive toepads in the Cretaceous. Our results provide support from the fossil record to most molecular and combined evidence estimates of the origin of most major clades of squamates, including geckoes, which usually place divergence times for their stem back in the Jurassic or the Triassic.”

Figure 1. From Simoes et al 2016, their cladogram of the squamates separate varanids from mosasaurs, link snakes to skinks and shows how close pre-snakes are to basal geckos.

Figure 1. From Simoes et al 2016, their cladogram of the squamates separate varanids from mosasaurs, link snakes to skinks and shows how close pre-snakes are to basal geckos.

Notes

  1. In the LRT lepidosaurs extend back to the Early Permian (TA 1045 specimen, close to Saniwa) and Lacertulus (Late Permian).
  2. The Ardeosaurus/Echstaettisaurus clade lies outside the geckos, inside the Pro-serpentes in the LRT as a single clade, but 3 ways by Simoes et al.
  3. No wonder those two taxa form a separate clade outside the geckos in the Simoes et al. report. The do so as well in the LRT, at the base of all snake and pro snakes.
  4. The LRT is a single, fully resolved tree, not the consensus of 3174 MPTs.

References
Simoes TR, Caldwell MW, Nydam RL and Jimenez Huidobro P 2016. Osteology, phylogeny and functional morphology of two Jurassic lizard species indicate the early evolution of scansoriality in geckoes. Abstract from the 2016 meeting of the Society of Vertebrate Paleontology.
Simões TR, Caldwell MW, Nydam RL and Jiménez-Huidobro P 2016. Osteology, phylogeny, and functional morphology of two Jurassic lizard species and the early evolution of scansoriality in geckoes. Zoological Journal of the Linnean Society (advance online publication) DOI: 10.1111/zoj.12487 http://onlinelibrary.wiley.com/doi/10.1111/zoj.12487/fullwiki/Ardeosaurus

New paper on Ardeosaurus and Eichstaettisaurus as geckos

Ardeosaurus and Eichstaettisaurus (Fig. 1) have been traditional enigmas in paleo studies. Here is some progress from Simões et al. 2016, who nest these two with geckos, as they were initially placed. The large reptile tree nests these two at the base of snakes, as sisters to the gecko clade. So very close!
Eichstattisaurus and Ardeosaurus.

Figure 1. Eichstattisaurus and Ardeosaurus. Two Jurassic lizards in the lineage of snakes – but very close to geckos.

From the Simões abstract:
“Late Jurassic lizards from Solnhofen, Germany, include some of the oldest known articulated lizard specimens, sometimes including soft tissue preservation. These specimens are thus very important to our understanding of early squamate morphology and taxonomy, and also provide valuable information on squamate phylogeny. Eichstaettisaurus schroederi and Ardeosaurus digitatellus are two of the best-preserved species from that locality, the former being represented by one of the most complete lizard specimen known anywhere in the world from the Jurassic. Despite their relevance to broad questions in squamate evolution, their morphology has never been described in detail, and their systematic placement has been under debate for decades. Here, we provide the first detailed morphological description, species-level phylogeny, and functional morphological evaluation of E. schroederi and A. digitatellus. We corroborate their initial placement as geckoes (stem gekkotans, more specifically), and illustrate a number of climbing adaptations that indicate the early evolution of scansoriality in gekkonomorph lizards.”

A PDF has been requested.
We’ll see if they included any basal snakes in their analysis.
This just in. The PDF arrived
The Simoes et al. cladogram nests snakes within skinks, derived from amphisbaenids. I don’t see Tetrapodophis or other pre-snakes in their cladogram.
References
Broili F 1938. Ein neuer fund von ?Ardeosaurus H. von Meyer. S.-B. bayer. Akad. Wiss. München, math.-naturw. Abt. 97-114.
Conrad JL and Daza JD 2015. Naming and rediagnosing the Cretaceous gekkonomorph (Reptilia, Squamata) from Öösh (Övörkhangai, Mongolia). Journal of Vertebrate Paleontology 35:5, e980891
Conrad JL and Norell MA 2006. High-resolution x-ray computed tomography of an Early Cretaceous gekkonomorph (Squamata) from Öosh ( €Ov€orkhangai; Mongolia). Historical Biology 18:405–431.
Daza JD, Bauer AM and Snively E 2013. Gobekko cretacicus (Reptilia: Squamata) and its bearing on the interpretation of gekkotan affinities. Zoological Journal of the Linnean Society 167:430–448.ischen Akademie der Wissenschaften, München 1938: 97–114.
Evans SE, Raia P and Barbera C 2004. New lizards and rhynchocephalians from the Lower Cretaceous of southern Italy. Acta Palaeontologica Polonica 49:393-408.
Hoffstetter R 1953. Les Sauriens anté−crétacés. Bulletin de la Museum Nationale d’Histoire Naturelle 25: 345–352.
Kuhn O 1958. Ein neuer lacertilier aus dem fränkischen Lithographie−schiefer. Neues Jahrbuch für Geologie und Paläontologie, Monatshefte 1958: 437–440.
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.
Simões TR, Caldwell MW, Nydam RL and Jiménez-Huidobro P 2016. Osteology, phylogeny, and functional morphology of two Jurassic lizard species and the early evolution of scansoriality in geckoes. Zoological Journal of the Linnean Society (advance online publication) DOI: 10.1111/zoj.12487 http://onlinelibrary.wiley.com/doi/10.1111/zoj.12487/fullwiki/Ardeosaurus

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

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: PontosaurusAdriosaurus 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.

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

Dinilysia and the burrowing(?) origin of snakes

A recent paper
by Yi and Norell 2015 attempted to answer the controversy whether ancestral snakes were terrestrial burrowers or marine swimmers while looking at their ear vestibules. They looked at the stem snake Dinilysia (1.8m in length) along with other reptiles and noted that the spherical vestibule portion of the inner ear was large (Fig. 1), like those of other burrowing reptiles.

Then they put together a cladogram (Fig.1) that was missing pertinent taxa in the ancestry of snakes, as determined by the large reptile tree (Fig. 2).

Figure 1. Colored chart originally published in Yi and Norell 2015 with their cladogram of snake origins tied to burrowing, aquatic and generalst niches, I have added black bars for legless taxa and gray bars for vestigial leg taxa. The inner ear of Dinilysia is shown at the bottom.

Figure 1. Colored chart originally published in Yi and Norell 2015 with their cladogram of snake origins tied to burrowing, aquatic and generalst niches, I have added dark green bars for burrowers and light green for other taxa, black bars for legless taxa and gray bars for vestigial leg taxa. The inner ear of Dinilysia is shown at the bottom. It does resemble the inner ear of burrowers. Note the large number of legless taxa preceding leg taxa, revealing this as an untenable family tree. See figure 2 for a cladogram that makes more sense.

 

Legless taxa
are primitive in the Yi and Norell tree (Fig. 1). That marks their cladogram as untenable or at best, unlikely because legs would have to redevelop in Varanus and Heloderma in the Yi and Norell tree. By contrast, legless taxa are largely derived in the scleroglossan subset of the large reptile tree (Fig. 2, Bipes is a notable exception).

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.

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. Here legless taxa are largely derived/

The cladogram of Yi and Norell 
cannot tell us whether ancestral (= pre-Dinilysia) snakes were burrowers or not because Yi and Norell did not include any pre-Dinilysia ancestral snakes in their cladogram. That’s a major oversight.

By contrast,
the large reptile (Fig. 2) tree indicates that many pre-snakes, like Adriosaurus, were not burrowers — BUT BUT BUT — teeny tiny Tetrapodophis was a likely burrower, derived from a series of much larger swimmers, like AdriosaurusTetrapodophis was the size and shape of living burrowing snakes AND Dinilysia was the proximal descendant taxon in the large reptile tree. No one has yet revealed what sort of inner ear vestibule Tetrapodophis had. But IF it followed the pattern shown by Yi and Norell and had a burrowing-type large vestibule, it would follow that its proximal descendant, Dinilysia, might have shared that trait, no matter what its niche was. So the large inner ear vestibule of Dinilysia indicates that it could have been a burrower (at least until other data, like its large size, trumps the Yi and Norell results) or it could have been the descendant of a burrower.

Unfortunately, the large vestibule of Dinilysia cannot tell us whether or not pre-Dinilysia taxa were burrowers or not. We should look at the vestibule in those taxa from the large reptile tree. IMHO tiny Tetrapodophis was a likely burrower and Dinilysia was not by virtue of its much greater size.

Remember when we wondered if pterosaurs were quadrupeds or bipeds? 
And then we discovered they were both!

Were pre-snakes aquatic or burrowing?
Again, they were likely both!

References
Yi H and Norell MA 2015. The burrowing origin of modern snakes. Research Article Science Advances 2015;1:e1500743  27 November 2015

 

 

SVP 9 – Origin of snakes: Da Silva

Da Silva 2015 takes on the origin of snakes using skull shape. Notes follow (*):

From the abstract:
“The origin of snakes is a contentious topic with three competing hypotheses: aquatic, terrestrial or fossorial*. The snake fossil record is poor with a few preserved complete skulls dated back to the Cretaceous**. Phylogenies using discrete morphological data and including fossils are contradictory regarding the ophidian ancestor***. Thus, alternative approaches that aid tracking down the lizard-snake ancestral transition are necessary****. Comparisons of quantitative data such as skull shape of extant and fossil taxa but also ontogenetic trajectories of skull development are relevant alternative approaches. In this study, we analyzed for the first time more than 600 extant and extinct taxa representative of all major Squamata families using two- and three-dimensional landmarked-based geometric morphometrics. We also mapped a consensus phylogeny onto the morphospace and estimated ancestral shapes with Parsimony. Lastly, we traced 61 skull
ontogenetic trajectories with principal component analysis. We first found that snakes and lizards occupy different parts of the morphospace, except for many convergent fossorial forms. Shape transitions are gradual and strongly linked with ecology. The first axis of variation largely accounts for changes in the braincase and quadrate. Interestingly, ancestral estimations recovered the most common ancestor of snakes as a small fossorial similar to Anomochilus*****, while Cretaceous snakes show intermediate skull shapes similar to boas and pythons. Ontogenetic trajectories of snakes and lizards are linear and  overall parallel phylogeny in snakes. Young embryos of Alethinophidia have similar shape to terrestrial adult lizards and trajectories are clearly peramorphic. Adults and embryos of Scolecophidia are located at the base of lizard ontogenetic trajectories, likely indicating neoteny. Altogether, our data indicate that skull shape and ecology are strongly connected, supporting the hypothesis that modern snakes lineages originated from a fossorial snake ancestor through an early transition from terrestrial lizards. Lastly, natural selection fine-tuned skull ecological function upon variation generated by heterochrony.”

*The large reptile tree solved this problem. The answer is secondarily terrestrial from aquatic ancestors. For more information, start here.
** Not so. the record is wonderfully complete. Da Silva just did not recognize it yet.
*** True. But then, none but the large reptile tree employ Jucaraseps and Tetraopodophis.
**** Not so. Just taxon inclusion and phylogenetic analysis will solve the problem.
***** No. Anomochilus is a highly derived burrowing form, not a basal snake in the large reptile tree.

References
Da Silva FO 2015. Skull shape supports a terrestrial – fossorial transition in the early evolution of snakes through heterochrony. Journal of Vertebrate Paleontology abstracts.