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.
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 Ardeosaurus and other stem snakes close to stem geckos.
There’s no guesswork, no loose ends at the LRT!
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:
- skull (too) long – as in Pachyrhachis — how long is too long?
- mandible straight – dental side or ventral side? Pachyrhachis is similar.
- no subdental ridge – inner side of maxilla not visible in fossil and may be a synapomorphy of snakes proper.
- intramandibular joint is not preserved – even so, it looks a lot like Pachyrhachis.
- 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?
- 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.
- zygosphenes are not observed – those may show up in snakes proper
- the neural spines are tall – but not on all vertebrae. I see tall neural spines on the sacral region of Pachyrhachis?
- rib heads are not tubercular – this may come with snakes proper.
- lymphapophyses [lumbar transverse processes] are expanded sacral processes – available data does not show this detail
- Scales are not present – this is in contrast with the original observation
New observations by Caldwell et al include
- 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.
- features of the suspensorium – no details given by the authors.
- orbit size and margin – Pontosaurus 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.
- an elongate retroarticular process – In Pontosaurus the RP is elongate. In Pachyrhachis it is absent. In Tetrapodophis it is vestigial and transitional.
- position of the splenial apex at the terminus of the tooth row – that’s a hard one to judge in candidate taxa
- enlarged first metapodials – they are more robust in Pontosaurus and absent in Pachyrhachis, which makes Tetrapodophis, again, transitional
- reduced carpal and tarsal ossification – to be expected if you’re losing your limbs
- 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.
- 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.
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