Australia: The Land Where Time Began

A biography of the Australian continent 

Snake Origins

The question of the origin of snakes is still being debated. The contending proposals are that snakes are related closely to the amphisbaenians, in witch case they arose from terrestrial burrowers, or that they belong in the anguimorph clade, making them closely related to the mosasaurs and aigialosaurs (mosasauroids), indicating they were primitively marine.

Snakes belong to the Squamata and are known to have originated from lizards, as it is evident legs were present in the ancestral forms, the reason the legs were lost being the basis of the debate over their origin. It the limbs were lost as the result of a burrowing habit, they would have arisen from burrowing lizards, such as legless lizards, and if the legs were lost because they swam with a serpentine motion, using their legs less, then they have arisen from marine forms. The marine aigialosaurs and dolichosaurs had limbs that were reduced.

A re-description of Pachyrhachis, a mid-Cretaceous form from Israel, that is about 1.5 m in length and has 146 presacral vertebrae (Caldwell & Lee, 1997). The hind limb is much reduced and the forelimb is completely lacking, and the pelvis is small. They determined that Pachyrhachis was a snake that was basal to the living forms, also claiming that snakes were anguimorphs that were close to mosasauroids. This finding was repeated in a phylogenetic analysis of the Squamata that was more detailed (Caldwell, 1999). Among the characters shared by snakes and mosasaurs was reduced ossification of the pelvis and hind limbs, reduced metakinesis and mesokinesis, pterygoid teeth that were enlarged and half way along the lower jaw, a hinge that allows some lateral movement. Detailed cladistic analyses of snakes and squamates respectively reaffirmed this view (Lee & Scanlon, 2002; Lee et al., 2004).

The dispute began with close questioning of the link between snakes and mosasauroids. A new snake, Haasiophis, from the Cretaceous, that had limbs was found, but it was found that the limbed snakes fitted into Macrostomata, the main clade of living snakes, in the cladogram some distance from the base (Tchernov et al., 2000). Haasiophis, that looked somewhat similar to a boa, is about 1 m long and had broad ribs, lacks forelimbs and has much reduced hind limbs that consisted of a femur, tibia, fibula, astragalus, calcaneum and the remains of 4 rudimentary toes, and a small pelvis. The small, highly kinetic skull had double rows of recurved teeth, with a mobile pre-maxillary-maxillary contact, characteristic of a snake, the marginal teeth being fused into sockets, and 140 presacral vertebrae comprising the elongated body, with a shoulder girdle, but lacked forelimbs.

Many of the suggested homologies between mosasauroids and snakes, such as the braincase and the lower jaw, were also questioned by others (Rieppel & Zaher, 2000a, b), suggesting that snakes and amphisbaenian lizards are closely related, the traditional view, with shared features such as the skull bones, in which loss, reduction and consolidation, braincase enclosure, the adductor muscles being displaced dorsally, and in the limbs and girdles, loss and reduction and uniformity along the vertebral column. Pachyrhachis and Haasiophis were redescribed, reaffirming their original findings (Zaher & Rieppel, 2002; Rieppel et al., 2003) respectively.

The disputed issues are:

  • Are the snakes with limbs basal to all known snakes in the phylogeny or are they nested higher, among the Macrostomata?
  • Are snakes most closely related to mosasauroids or amphisbaenians?

How characters are defined and used in cladistic analysis accounts for much of the difference in results. If emphasis is placed on intramandibular joint characters, it emphasises a relationship between snakes and mosasauroids, snakes become closer to amphisbaenians if there is highlighting of the reduction and loss of skull elements. The biological model for understanding snake origins is then driven by cladistic results - did snakes lose their limbs and evolve long, thin bodies as an adaptation to burrowing, or was it an adaptation to making their serpentine swimming in the sea more efficient? There are also tentative molecular results from studies on living reptiles that have snakes as a sister group to (Anguimorpha + Iguania) (Harris, 2003), which differs from the results obtained from the study of fossils. See links 1, 2 and 3.

The snake, Haasiophis terrasanctus, with very small hind limb has been found in Israel and dated to the mid-Cretaceous

Sources & Further reading

  1. Benton, Michael J., 2005, Vertebrate Palaeontology, 3 rd ed., Blackwell Publishing.

Links

  1. Discovery challenges snake origins
  2. Legged snake fossil may uproot family tree
  3. New fossil snake with legs reported in Science
 
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Last updated 15/08/2011 

 

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading