Snakes in a Burrow: Fossil Rattles Origin of Snakes

Last week, paleontologists published a study in the journal Science Advances revealing a possible habitat origin for modern snakes. This study was based on an exciting morphological discovery in a fossil snake that could help scientists understand why snakes developed limbless bodies and distinct sensory systems.

The habitat of ancestral snakes has been debated between several studies, pointing to either aquatic or terrestrial origins. Studying the habitat of fossil snakes is limited to inference based on qualitative morphological analyses of the fossils themselves, or by interpretation of the depositional environment of the fossil, which can be problematic if the fossils were potentially displaced post-death.

This new study, led by Hongyu Yi from the University of Edinburgh and Mark A. Norell from the American Museum of Natural History, produced a quantitative morphological analysis examining the inner ear across all lineages of snakes, including the Cretaceous snake Dinilysia patagonica, found in South American deposits. Yi and Norell concluded from their study that D. patagonica was a burrower, and that ancestral snakes likely occupied a fossorial (burrowing) ecological niche.

What does the inner ear have to do with burrowing? And how did Yi and Norell determine that D. patagonica was burrowing?

The inner ear is responsible for hearing and balance in all snakes. And sound waves are slowed down when they are moving through a soft, dense material like dirt. So obviously, animals like us humans don’t hear very well when buried because our ears haven’t evolved to detect sound waves or vibrations through dirt.

Loxocemus bicolor, the Mexican burrowing python. Image courtesy Wikipedia.
Loxocemus bicolor, the Mexican burrowing python. Image courtesy Wikipedia.

Modern burrowing snakes, such as the Mexican burrowing python (Loxocemus bicolor) or the sunbeam snake (Xenopeltis), have a distinct morphology of the inner ear that allows heightened senses underground. The inner ear is composed of a spherical vestibule, foramen ovale, and semicircular canals. In burrowing snakes, the spherical vestibule is massive compared to terrestrial and aquatic snakes, and contains a large sacular otolith (“ear stone”), that helps the brain interpret substrate vibrations. The larger the otoliths in the vestibule, the more sensitive the snake is to ground vibrations with low frequencies. These otoliths also help orient the snake and make them sensitive to angular rotations while impeding high speeds.

Yi and Norell examined x-ray computed tomography (CT) scans of 34 species of modern and fossils snakes, as well as 10 species of lizards and worm lizards (amphisbaenians), a group of limbless squamates not closely related to snakes. Using these scans, they built 3-D virtual models of the endocast of the bony inner ear labyrinth. Their results show that D. patagonica shares a inner ear morphology with modern burrowing snakes, with a large spherical vestibule that occupies most of the space delineated by the shape of the semicircular canals. They also interpreted a cast of a large sacular otolith, suggesting that these snakes were actively burrowing and sensing lower frequency vibrations associated with burrowing in a substrate.

The braincase and inner ear of D. patagonica (MACN-RN 1014). (A) Braincase of D. patagonica, showing the right otic region in lateral view. (B) X-ray CT model of MACN-RN 1014, with the inner ear highlighted in blue. (C) Bony inner ear of D. patagonica. FO, foramen ovale; LR, lagenar recess; SC, semicircular canal; V, vestibule. Scale bars, 5 mm.
The braincase and inner ear of D. patagonica (MACN-RN 1014). (A) Braincase of D. patagonica, showing the right otic region in lateral view. (B) X-ray CT model of MACN-RN 1014, with the inner ear highlighted in blue. (C) Bony inner ear of D. patagonica. FO, foramen ovale; LR, lagenar recess; SC, semicircular canal; V, vestibule. Scale bars, 5 mm.

In several recent studies hypothesizing the evolutionary relationships of snakes and other squamates, Dinilysia patagonica is recovered either as the sister taxon to all crown-group snakes or in a basal position within crown-group snakes. This is a critical position in helping determine the habitat origin of modern snakes. Using predictive models for snake habitat based on vestibular shape, Yi and Norell estimated a high probability (93.4%) that D. patagonica was a burrower, and also recovered a high probability (70.1%) that the hypothetical ancestor of snakes was also burrower and a low probability (0.02%) that the hypothetical ancestor was aquatic. Even removing D. patagonica from their analysis, they recovered the same results with high probability of a burrowing common ancestor.

Modern snakes originated as burrowers, based on their inner ear morphology. (A) Snake skulls in right lateral view, showing that the inner ear (orange) locates inside the braincase and opens to the stapes (blue) in the middle ear. Ear and skull models are not to scale. (B) Inner ear of Laticauda colubrina, an aquatic species. (C) Ptyas mucosa, terrestrial generalist. (D) Xenopeltis unicolor, a burrowing species. (E) Hypothetical ancestor of crown snakes, predicted as burrowing with 70.1% probability. (F) D. patagonica, predicted as burrowing with 93.4% probability. (G) Phylogeny of all snakes and lizards in this study, adapted from Gauthier et al., Pyron et al., and Yi and Norell.
Modern snakes originated as burrowers, based on their inner ear morphology. (A) Snake skulls in right lateral view, showing that the inner ear (orange) locates inside the braincase and opens to the stapes (blue) in the middle ear. Ear and skull models are not to scale. (B) Inner ear of Laticauda colubrina, an aquatic species. (C) Ptyas mucosa, terrestrial generalist. (D) Xenopeltis unicolor, a burrowing species. (E) Hypothetical ancestor of crown snakes, predicted as burrowing with 70.1% probability. (F) D. patagonica, predicted as burrowing with 93.4% probability. (G) Phylogeny of all snakes and lizards in this study, adapted from Gauthier et al., Pyron et al., and Yi and Norell.

Dinilysia patagonica is the largest known burrowing snake, with a snout-to-tail length greater than 1.8 meters (5.9 feet). The largest modern burrowing snakes approach 1.6 meters (5.25 feet). D. patagonica likely foraged for buried eggs or other reptiles, similar to modern burrowing snakes, or even likely hunted small vertebrates aboveground. More importantly, a burrowing habitat has large implications on the loss of limbs, more likely to aid in movement through the substrate than for swimming. This study also suggests that ancestral snakes were able to use substrate vibrations to detect movement of prey as well as to avoid predators.

Read the original article here:

Yi H, Norell MA (2015) The burrowing origin of modern snakes. Science Advances 1, e1500743. DOI: 10.1126/sciadv.1500743 

References:

Gauthier JA, Kearney M, Maisano JA, Rieppel O, Behlke ADB (2012) Assembling the squamate tree of life: Perspectives from the phenotype and the fossil record. Bull. Peabody Mus. Nat. Hist. 53, 3–308.

Pyron RA, Burbrink FT , Wiens JJ (2013) A phylogeny and revised classification of Squamata, including 4161 species of lizards and snakes. BMC Evol. Biol. 13, 93.

Yi H, Norell MA, New materials of Estesia mongoliensis (Squamata: Anguimorpha) and the evolution of venom grooves in lizards. Am. Mus. Novit. 3767, 1–31.

 

 

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Published by Sarah Z. Gibson

Dr. Sarah Z. Gibson is a paleontologist and science communicator based in Minnesota. Her research focuses on the evolutionary history of ray-finned fishes from the Early Mesozoic. https://orcid.org/0000-0002-6784-3980

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