The big boa in a sauropod nest: Sanajeh restored

Wilson et al. 2010 described a Late Cretaceous 3.5m snake,
Sanajeh indicus (GSI/GC/2901–2906), curled around a nest of titanosaur sauropod dinosaur eggs, including one hatchling (Fig 1). Pretty spectacular.

Figure 1. Fossil from Wilson et al. 2010 showing Sanajeh and dinosaur eggs in situ. Diagram from Wilson et al. 2010 distorted for a closest match to the fossil. Not sure why the authors chose to create the diagram they way they did. Usually in situ diagrams are exact. Note: the dorsal view of the skull in the diagram is more complete than the in situ skull (Fig 2).

From the Wilson et al. abstract:
“Recent phylogenetic analyses differ in their interpretations of the origin and interrelationships of snakes, resulting in polarized views of snake ecology, habit and acquisition of features associated with wide-gaped feeding (macrostomy).

Many, many workers have attempted to understand the origin of snakes.
In every case they cherry-picked taxa, instead of letting their own wide-gamut cladogram tell them where snakes nest.

By contrast
in 2011 the large reptile tree (LRT, subset Fig 7) nested snakes with Eichstaettisaurus.
In 2013 the LRT nested snakes with Adriosaurus and Ardeosaurus.
In 2015 and ever since (Fig 7), snake ancestors, like Norellius, nested with gecko like Tchingisaurus.

“Here, we report a new specimen of the Late Cretaceous nest predator Sanajeh indicus that helps to resolve the origin of macrostomy”.

Macrostomy = ‘big mouth’ refers the ability of some fish and snakes to engulf and swallow very large prey, even if that takes a while to do.

“The new specimen preserves an ossified upper temporal bar and a posteriorly expanded otooccipital region that lacks a free-ending supratemporal bone and retains a lizard-like palatomaxillary arch that allows limited movements during swallowing.

I don’t see an ‘ossified upper temporal bar’ in the fossil, but restoration of an extended squamosal (magenta Fig 2) produces an upper temporal bar. The occipital region is laterally expanded. The skull lacks a ‘free-ending supratemporal bone’ due to a misidentifcation. The traditional supratemporal is bright green in figure 2. The heretical squamosal is magenta (= hot pink) here. This traditional mistake is taught in university textbooks and was rectified here in 2018 based on homologus elements in previously overlooked snake precursor taxa (Fig 7).

Figure 2. Sanajeh skull from Wilson et al. 2010. Colors and restoration added here. Of course, the restoration is speculative, which is why these traits cannot be scored in the LRT. The extended restored squamosal (not a supratemporal) forms an upper temporal arch.

Wilson et al. continue:
“Phylogenetic analyses of a large-scale total evidence dataset resolve Sanajeh near the base of Pan-Serpentes, as the sister group of Najash, Dinilysia and crown-group Serpentes.

Too little is known of the Sanajeh skull (Fig 2) to attempt a nesting in the LRT. However a restoration based on existing elements indicates it likely nests between Boa and Crotalus (the rattlesnake). Restoration is speculation, not evidence worth scoring.

Figure 3. Subset of the LRT from 2018 showing stem snakes, snakes and their sister group, the geckos. Cherry-picking is minimized here by taxon inclusion. The squamosal is in magenta.

Wilson et al. continue:
“The Cretaceous Tetrapodophis and Coniophis represent the earliest-diverging members of Pan-Serpentes.”

Tiny Coniophis is known from a maxilla and dentary, not enough to enter the LRT. The burrowing snakes are all tiny and so is Coniophis. Tiny Tetrapodophis is a snake because it nests at the node (Figs 3, 7) that is the last common ancestor of all living snakes.

“The Cretaceous hindlimbed pachyophiids and Cenozoic Australian ‘madtsoiids’ are inside crown Alethinophidia, whereas mosasaurs are recovered invariably within anguimorphs”.

This was in the era of Pythonomorpha, when it was popular to nest snakes with mosasaurs. The LRT does not recover these two clades together.

“Our results suggest that the wide-gape condition in mosasaurs and snakes might have evolved independently, as functionally distinct mechanisms of prey ingestion”.

The LRT also documents a separate and unrelated origin for mosasaurs and snakes.

“The intermediate morphology preserved in Sanajeh indicates that ingestion of large prey items (macrophagy) preceded wide-gaped, unilateral feeding (macrostomy), which appeared 35 Myr later, in the common ancestor of pachyophiids, Cenozoic Australian ‘madtsoiids’ and alethinophidians”.

The LRT documents a different origin for snakes than Wilson et al. envisioned with taxon exclusion in 2010 (Fig 4).

Figure 4. Cladogram from Wilson et al. 2010 nesting Sanajeh with several snake taxa. There are no outgroup taxa in this cladogram.

Wilson et al. cherry-picked outgroup taxa for their cladogram
(Fig. 4). “Consensus of the single most parsimonious trees derived from analyses employing Amphisbaenia and Varanoidea as outgroups.“

Those are not snake outgroups in the LRT. But then Wilson et al. published several years before snake ancestors were identified by the LRT. In any case, whenever someone cherry picks ingroup and outgroup taxa, the cladogram will inevitably suffer.

Figure 5. Xenopeltis. As in other tetrapods, the quadrate articulates with the squamosal, a bone missing in Sanajeh. Note the difference in teeth in Xenopeltis compared to Sanajeh.

Wilson et al. wrote,
“As in basal alethinophidian snakes such as Xenopeltis [Fig 5], the supratemporal has a wide articular surface for the quadrate on its lateral margin and a very short, free-ending posterior margin that does not extend posteriorly beyond the otic capsule. Importantly, the position of the quadrate articular facet, which is on the lateral surface of the supratemporal and located dorsal to the juxtastapedial recess, suggests that the jaw joint of Sanajeh was positioned lateral to the posterior margin of the braincase, as it is in basal snakes.”

As in other tetrapods, the supratemporal in Xenopeltis (Fig 5) does not articulate with the quadrate. The squamosal articulates with the quadrate. A restoration (Fig 2) might have helped Wilson et al. understand Sanajeh better. In the LRT a restored Sanajeh nests close to boa constrictors (appropriate for a baby sauropod killer).

Figure 6. Yurlunggur, an early boa from Australia. That transverse crack in the parietal made me misunderstand this area of the skull. Colors repaired here. The squamosal is hot pink/magenta. The supratemporal is bright green, distinct from most snake skull diagrams.

PS
While working on Sanajeh, another snake taxon, Yurlunggur (Fig 5, Scanlon 1992, Oligocene-Miocene, Australia, up to 6m long), became better understood, was rescored and now nests with Boa in the LRT.

Figure 7. Subset of the LRT from 2019 focusing on geckos and their sister snake ancestors. See the LRT for updates in the last three years.

Scanlon’s 1992 cladogram
(Fig 8) demonstrates the sort of problems snake workers experienced back in 1992 without a valid outgroup taxon or a series of valid outgroup taxa (Fig 7). Note the nesting of the extremely tiny and derived Leptotyphlops arising from a giant generalized terrestrial Dinilysia and then giving rise to decreasingly derived burrowing snakes ultimately giving rise to the same sort of terrestrial snakes represented by Pachyrhachis, Yurlunggar and Dinilysia. Essentially parts of Scanlon’s cladogram are upside-down. Try to guard against this.

Figure 8. Cladogram from Scanlon 1992. Colors added here. Always recheck your cladogram to make sure it is not deriving less derived taxa from more derived taxa, as in this cladogram. Compare to the snake subset of the LRT in figure 7.

References
Scanlon JD 1992. A new large madtsoiid snake from the Miocene of the Northern Territory. The Beagle, Records of the Northern Territory Museum of Arts and Sciences 9(1):49-60.
Wilson JA, Mohabey DM, Peters SE, Head JJ 2010. Benton MJ ed. Predation upon Hatchling Dinosaurs by a New Snake from the Late Cretaceous of India. PLoS Biology. 8 (3): e1000322. doi:10.1371/journal.pbio.1000322

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