The Origin of Snakes Goes Way, Way Back

As we learned earlier, hypotheses on snake origins have suffered by not recognizing that living snakes are diphyletic. The tiny burrowing snakes (Scolecophidia, Aniliidae, Uropeltidae), like Cylindrophis, have a separate lineage through ancestors like Heloderma. The larger non-burrowing snakes (Macrostomata), like Pachyrhachis, have a lineage through Adriosaurus and Ardeosaurus (Fig. 1) according to the large reptile tree.

Wiki reports, “Based on comparative anatomy, there is consensus that snakes descended from lizards.”

Well, duh…

Figure 1. The origin of snakes going back to the most primitive reptile, Cephalerpeton. To learn more about these taxa, find them on reptileevolution.com

Figure 1. Click to enlarge. The origin of snakes – to scale – going back to the most primitive reptile, Cephalerpeton. To learn more about these taxa, find them on reptileevolution.com. Along the way several of these taxa were basal to new clades. The basal forms, many still unknown, are small and lizardy.

Wiki also reports, “The origin of snakes remains an unresolved issue. There are two main hypotheses competing for acceptance. 1) Burrowing lizard hypothesis; 2) Aquatic mosasaur hypothesis.”

As it turns out, based on the results of the large reptile tree, both are correct.

Wiki notes that Anilius is the most primitive known snake and the large reptile tree nests it closest to Lanthanotus, a sister to the ancestor of burrowing snakes. These are not directly related to boas, cobras and king snakes.

How far back can we trace the origin of snakes?
With the large reptile tree we can trace the origin of snakes back to Ichthyostega, but here (Fig. 1) we go back to the most primitive known reptile, Cephalerpeton. Because snakes are at one end of the lepidosaurormorph branch to get to snakes you pass through the basal taxa of all the other lepidosauromorphs, including diadectids, caseids, pareiasaurids, chelonians, gliding lepidosauriforms, lepidosaurs, tritosaurs (including pterosaurs and drepanosaurs) and squamates (including squamates). So, what we have are a series of mostly small lizardy forms (Fig. 1) that ultimately gave rise to a wide variety of derived forms. Several lizardy squamates lost their limbs. Snakes are just the most successful version of these.

What is Najash?
Described as a terrestrial Cretaceous protosnake with legs, Najash (Apesteguía and Zaher 2006) appears to share more pelvic traits with Heloderma than Adriosaurus, confirming our tree topology separating burrowers from the others, including swimmers.

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.

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, such as a curved ilium and ventrally narrowing pubis. The femur of Najash includes a very large process is not found in either candidate.

All it takes is another study with a similar gamut to confirm the diphyletic origin of living snakes.

References
Apesteguía S and Zaher H 2006. A Cretaceous terrestrial snake with robust hindlimbs and a sacrum. Nature 440: 1037-1040.

3 thoughts on “The Origin of Snakes Goes Way, Way Back

  1. Can you please list the characters that explicitly support these arrangements? I am especially interested in that Cryptolacerta is placed as a basal monstersaur rather than a lacertoid or amphisbaenian.

  2. Pingback: Is The Treatment In ‘The Well of the Souls’ Room? | Third News

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