Fossil Friday: Is the Four-Legged Snake Tetrapodophis a Missing Link or Not?
The origin of snakes is a hotly debated topic in evolutionary biology. There are controversies about their phylogenetic position, their origin from aquatic vs burrowing (fossorial) ancestors, and of course transitional fossils (Caldwell & Lee 1997, Rieppel et al. 2003, Brandley et al. 2008, Apesteguía & Zaher 2006, Lee et al. 2007, Caldwell 2007, 2008, Zaher et al. 2009, 2023, Longrich et al. 2012, Zaher & Scanferla 2012, Palci et al. 2013a, 2013b, Palci 2014, Caldwell et al. 2015, Hsiang et al. 2015, Yi & Norell 2015, Da Silva et al. 2018, Xing et al. 2018, Garbergoglio 2019a, 2019b, 2019c). While some scientists are convinced that “snakes and mosasauroids/coniasaurs share a limbed common ancestor that was likely aquatic, not fossorial” (Caldwell 2008), other scientists strictly claim that “lizards could not have transitioned to snakes by any other evolutionary path than through fossoriality” (Da Silva et al. 2018). Either way, there should have existed transitional forms, which exhibit at least some typical features of basal snake anatomy combined with four fully developed fore and hind limbs to document the transition of limbless snakes from still quadrupedal walking ancestors.
Assumed Mesozoic stem snakes like the Madtsoiidae and Simoliophiidae as well as the very primitive Cretaceous genera Coniophis, Dinilysia, and Najash did only possess small external hind limbs (of which vestigial remnants are also retained in some living snakes like boids), but show no evidence of forelimbs (Tchernov et al. 2000, Zaher & Rieppel 2002, Rieppel et al. 2003, Apesteguía & Zaher 2006, Garberoglio et al. 2019c).
In their study of the evolution of the snake body plan Garberoglio et al. (2019c) commented:
This basal position of the limbed terrestrial and marine snakes indicates that snakes retain sizeable external hindlimbs and sacral contacts for a substantial time after their origin—from approximately 170 Ma to the youngest confirmed legged snakes, the simoliophiids, at approximately 100 Ma. This indicates that (i) the reduction and loss of the pectoral girdle and forelimbs probably occurred much earlier, given the definitive absence of these structures in simoliophiids and the lack of evidence for their presence in Najash, Dinilysia, and madtsoiids, and was probably a major event in the early radiation of snakes, and one that occurred well before crown (modern) snake origins; (ii) the “forelimb-absent and hindlimb-reduced” morphology was a stable and successful body plan, rather than a transient phase between limbed and limbless conditions; and (iii) the origin of crown snakes was characterized by a major reduction in the hindlimb and pelvis (including loss of sacral contacts).
This All Sounds Well and Good
That is, until you find that other modern studies (e.g., Tchernov et al. 2000, Rieppel et al. 2002, Zaher & Rieppel 2002, Longrich et al. 2012, Zaher & Scanferia 2012, Palci et al. 2013b in part, Hsiang et al. 2015, Garbergoglio et al. 2019a contra Garbergoglio et al. 2019c) do not even agree that the fossil Madtsoniidae and Simoliophidae or even Dinilysia are stem group snakes at all, but recover them nested among living snakes, which would imply that the hind limb reduction occurred more than once in crown group snake. Probably, mainstream evolutionary biologists would respond that such a reduction is simple and therefore not unlikely, as is proven by the fact that several other groups of amphibians and reptilians have produced legless forms. However, this is exactly the point: convergence is all over the place in the animal kingdom and is nothing but incongruent similarity that contradicts a unique nested hierarchy of similarities predicted by Darwinism. Furthermore, there is experimental evo-devo evidence from transgenic mice that the reduction of limbs is not a simple task at all but requires multiple codependent mutations (compare Kvon et al. 2016). Together with the complex changes in skull kinetics of snakes, this raises a significant waiting time problem, because even the geologically available window of time of several million years is orders of magnitude too short to accommodate the genetic changes for this re-engineering of the body plan, based on the mathematical toolkit of mainstream population genetics.
Anyway, because of the fact that no known fossil stem snakes had retained four functional limbs, the discovery of a putative four-legged snake was a real sensation. Eight years ago my good colleague Prof. David Martill, with whom I co-authored several articles and monograph of the fossils of the Lower Cretaceous Crato Formation in Brazil (Martill et al. 2007), described from this locality the long sought missing link of snake evolution in the prestigious journal Science (Martill et al. 2015; also see this YouTube video): the beautifully and completely preserved fossil shows a very elongate snake-like animal with small fore and hind limbs, both with five well-developed digits. They named the 110-million-year-old fossil Tetrapodophis, which means four-legged snake. Their phylogenetic analysis placed it at the very base of the snake lineage. The scientists also identified burrowing adaptations that arguably would support an origin of snakes from fossorial rather than aquatic ancestors. Of course, the discovery was much celebrated in news reports as “a snake version of Archaeopteryx” (e.g., University of Portsmouth 2015, Christakou 2015a, Evans 2015, Yong 2015)
Yong (2015) commented in National Geographic:
Their analysis produced a family tree in which Tetrapodophis came after the earliest known snakes like Eophis, Parviraptor, and Diablophis, but is still very much a snake. But how could that be? Eophis and the others only have two legs, so how could four-legged Tetrapodophis have come after them? The answer is that evolution doesn’t proceed along simple, straight lines.
This is typical Darwinist doublespeak for the simple fact that the fossil does not fit within a Darwinian scenario without ad hoc hypotheses to explain away conflicting data. But in this case there is another little problem: all known fossils of the Jurassic earliest stem snakes (Portugalophis, Eophis, Parviraptor, and Diablophis) do not even have the body region preserved, which would show if they possessed limbs or not (see Caldwell et al. 2015), so that the above statement is also incorrect. Never believe Darwinist pop science journalists, who are notoriously sloppy with facts. But apart from this glitch, the real blows against Tetrapodophis were yet to come.
A First Blow
The first blow came with a very heated discussion about the legal status of the fossil (Christakou 2015b), even though there was no proof that any laws were broken with the export and sale of the fossil. I personally consider this discussion as very much ridiculous, given the fact that countless fossils from the quarries in Brazil were grinded for cement production or used for pavements, without any local scientist caring for this natural heritage. Now, museums around the world, which invested a lot of money and preparation effort to preserve these fossils for science, are now asked to repatriate the collections to Brazilian museums with a very poor track record of proper conservation and curation, all in the name of protecting alleged national treasures of poor countries from evil Western exploitation. Unfortunately, the pervasive woke agenda has taken hold of paleontology as well, which impairs scientific progress and has made fossil collecting to a very frustrating endeavour.
A Second Blow
The second blow came with another silly dispute about the deposition of the specimen. The specimen was deposited in a public museum in Germany as a loan by a private collector, who temporarily limited access to the specimen because it was damaged during the examination with synchrotron micro-CT scanning. Even though access was later restated and the specimen was also meticulously documented with drawings and photographs in the original publication, some scientists made absurd statements like “if the fossil can’t be studied, it doesn’t exist … I don’t want to mention the name Tetrapodophis ever again” (Gauthier quoted in Gramling 2016). Other researchers said the “original paper should not have been published, because the fossil was not officially deposited in a museum or other repository”. This reflects a common politically correct sentiment among scientists against private collections, even though many private fossil collections are better curated and more easily accessible to scientists than public collections in certain dubious countries with a reputation of sloppiness and corruption. Nevertheless, these bigoted scientists rather would leave sensational finds of high scientific value undescribed forever, than having scientific publications based on material in private collections. I find this bizarre, scientifically reprehensible, and politically highly problematic (anti private property in favor of statist bureaucracy). After all, using the same kind of reasoning we could purge all paleontological knowledge from science that is based on museum specimens that were destroyed during World War II, or lost by the post when they were sent on loan to foreign scientists, or simply cannot be relocated in the museum archives anymore, which happens way more often than many people may think. Many scientists seem to have lost their common sense nowadays.
Last but Not Least
The third and truly fatal blow came with several studies by paleontologist Michael Caldwell and his colleagues, who strongly disputed the fossorial adaptation of Tetrapodophis in favor of an aquatic adaptation, and also disputed the snake-affinity in favor of a determination as dolichosaurid lizard (Lee et al. 2016, Caldwell et al. 2016, 2021, Paparella et al. 2018), which is an extinct group of marine reptiles. The press immediately reported that the big splash was a case of mistaken identity (Geggel 2016), and that the assumed missing link between lizards and snakes has been debunked (Hodžić_2021, Lyle 2021, Strauss 2022).
One might think that this misidentification is a minor issue because dolichosaurids are often considered as close relatives of snakes within a clade Pythonomorpha. However, this opens a whole new can of worms, as the status of Pythonomorpha is subject of considerable scientific controversy itself. The Wikipedia article on Pythonomorpha has a good summary of this mess, of which the long story short is as follows: Pythonomorpha was proposed by the famous 19th century paleontologist Edward Drinker Cope (1869) to include mosasaurs and snakes as assumed close relatives, while snakes were believed to have evolved from burrowing lizards. This was rejected by most 20th century experts, who instead suggested that the closest relatives of mosasaurs are monitor lizards (e.g., Russell 1967). Then, Pythonomorpha was resurrected by Michael Caldwell and Michael Lee in the late 1990s (Caldwell & Lee 1997, Lee 1997, 1998, Caldwell 1999, Lee & Caldwell 2000) to include the aquatic extinct reptile groups aigialosaurs, mosasaurs, dolichosaurs, coniasaurs, as well as snakes, which were postulated as derived from aquatic ancestors (also see Pierce & Caldwell 2004, Palci & Caldwell 2007, Caldwell 2008, Paparella et al. 2018). This was again questioned by anatomical evidence (Zaher & Rieppel 1999) and more recent phylogenetic studies, which again placed mosasaurs with monitor lizards and snakes with skinks (Conrad 2008) or with anguinids and iguanas (Simões et al. 2020), or placed mosasaurs more basal than monitor lizards and snakes (Gauthier et al. 2012). On the other hand, more recent analyses (Palci et al. 2013a, Palci 2014, Martill et al. 2015, Reeder et al. 2015) recovered snakes and mosasaurs as sister groups and thus supported a monophyly of Pythonomorpha. The latter would also be congruent with the growing for a monophyletic group called Toxicofera, which would include all squamates with poison glands such as iguanas, monitor lizards, gila monsters, and snakes, under exclusion of skinks, gekkos, and true lizards. However, as always in phylogenetics there is also strong conflicting evidence presented by opponents of the Toxicofera hypothesis (Hargreaves et al. 2015, Mongiardino Koch & Gauthier 2018, Joffroy 2022). If we would draw a least common denominator of all published trees of the past decades, the result would be an unresolved polytomy (as I described in a recent Fossil Friday article about arachnid phylogeny), which rather agrees with the expectations of creationists than those of evolutionary biologists.
A New Turn of Events
Considering this uncertainty it is hardly a surprise that this year a new study brought a new turn of events and again confirmed the status of Tetrapodophis as most basal snake (Zaher et al. 2023). The authors remarked that “Our scorings of Tetrapodophis were based on first-hand observations of the original specimen, and our interpretations of the anatomy and attendant scoring disagree significantly with those of Caldwell et al. (2021), as do our phylogenetic results.” It does not take too much of a prophetic gift to predict that we will not have to wait very long for a rebuttal by Caldwell and his team.
Whoever may ultimately win this dispute, there is no doubt that the true reason for all this uncertainty is the simple fact that the pattern of similarities between different animal groups does not fall into a neatly ordered set of nested hierarchies as often pretended by hardcore Darwinists. In reality, incongruent patterns are all over the place. A snake-like body plan is a good example: Among tetrapod vertebrates a snake-like body only originated in caecilians (Gymnophiona) among lissamphibians and in squamates among amniotes. However, within amniotes this body plan originated not less than 26 times independently (Brandley et al. 2008, Evans 2015). Tetrapodophis could be a missing link to at least two of them respectively, or just might be number 27, who knows. What is also interesting is that scientists found “statistically significant support for at least six examples of the re-evolution of lost digits in the forelimb and hind limb” ((Brandley et al. 2008), which shows the level of absurdity that is readily accepted only to preserve the evolutionary paradigm.
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