Monday, November 30, 2015

Dinilysia's inner ear suggests it was a burrowing species, implications for the evolution of snakes

Modern snake skull, with inner ear shown in orange. 
Photo Credit: Hongyu Yi
Modern snakes probably originated as habitat specialists, but it is controversial unclear whether they were ancestrally terrestrial burrowers or marine swimmers. In a new paper Yi and Norell (2015)  use x-ray virtual models of the inner ear to predict the habit of Dinilysia patagonica, a stem snake closely related to the origin of modern snakes. Previous work has shown that modern snakes perceive substrate vibrations via their inner ear. The study's  data show that D. patagonica and modern burrowing squamates share a unique spherical vestibule in the inner ear, as compared with swimmers and habitat generalists. The authors built predictive models for snake habitats based on their vestibular shape, which estimated D. patagonica and the hypothetical ancestor of crown snakes as burrowers with high probabilities. This study provides an extensive comparative data set to test fossoriality quantitatively in stem snakes, and it shows that burrowing was predominant in the lineages leading to modern crown snakes.

Comparisons between CT scans of the fossil and modern reptiles indicate that snakes lost their legs when their ancestors evolved to live and hunt in burrows, which many snakes still do today.

The findings suggest snakes did not lose their limbs in order to live in the sea, as has been previously suggested.

Scientists used CT scans to examine the bony inner ear of Dinilysia patagonica, a 2-metre long reptile closely linked to modern snakes. These bony canals and cavities, like those in the ears of modern burrowing snakes, controlled its hearing and balance.

They built 3D virtual models to compare the inner ears of the fossils with those of modern lizards and snakes. Researchers found a distinctive structure within the inner ear of animals that actively burrow, which may help them detect prey and predators. This shape was not present in modern snakes that live in water or above ground.

The findings help scientists fill gaps in the story of snake evolution, and confirm Dinilysia patagonica as the largest burrowing snake ever known. They also offer clues about a hypothetical ancestral species from which all modern snakes descended, which was likely a burrower.

Dr Hongyu Yi, of the University of Edinburgh's School of GeoSciences, who led the research, said: "How snakes lost their legs has long been a mystery to scientists, but it seems that this happened when their ancestors became adept at burrowing. The inner ears of fossils can reveal a remarkable amount of information, and are very useful when the exterior of fossils are too damaged or fragile to examine."

Mark Norell, of the American Museum of Natural History, who took part in the study, said: "This discovery would not have been possible a decade ago -- CT scanning has revolutionised how we can study ancient animals. We hope similar studies can shed light on the evolution of more species, including lizards, crocodiles and turtles."

Citation

Yi, H. &  M.A. Norell. The burrowing origin of modern snakes. Science Advances, 2015; 1 (10): e1500743 DOI: 10.1126/sciadv.1500743

No comments:

Post a Comment