Snakes are incredibly diverse, with more than 3,400 living species found on land and in the water. But little is known about how the first serpents looked and behaved. Now scientists at Yale University have reconstructed what the ancestor of snakes may have been like — a nocturnal land-dwelling predator that had tiny legs with ankles and toes.
The scientists analyzed fossils, genes and anatomy from 73 living and extinct species of reptiles. These included representatives from all the major living groups of snakes as well as lizard relatives of snakes. They sought to “get as broad a picture of the entirety of snake diversity as possible,” says researcher Allison Hsiang, a phylogeneticist at Yale.
By comparing the ages of these species and identifying similarities and differences, the scientists developed a family tree for snakes, shedding light on what their ancestor were like and how they evolved. The number and variety of fossils they examined make it the most comprehensive family tree for snakes to date. “We were able to extrapolate backwards in time as to what the most likely behavior of the ancestral snake was,” Hsiang says. The scientists detailed their findings online May 20 in the journal BMC Evolutionary Biology.
Their work suggests that the first snake hunted stealthily at night, sinking needle-like hooked teeth into soft-bodied prey no bigger than its head and swallowing them whole. It had not yet developed the ability to kill prey much larger than itself using constriction. “We could look at preserved fossil snake teeth and infer what type of prey they hunted, which further informed our inference of the feeding behaviors of the ancestral snake,” Hsiang says. Based on the placement of fossil serpents with tiny hind limbs on the snake family tree, Hsiang and her colleagues reinforced arguments in the field that the ancestor of snakes retained tiny legs as well.
The findings also suggest that snakes first evolved on land rather than in the water, providing adding to an ongoing debate. The researchers assert that snakes originated in warm moist forests on the ancient supercontinent of Laurasia about 128.5 million years ago, a period that coincides with the rapid appearance of many species of mammals and birds on Earth.
“I think this is the best attempt yet to infer snake evolution by combining what we know about the genetics and anatomy,” says evolutionary biologist Michael Lee at the University of Adelaide in Australia, who did not take part in this research.
Snakes apparently survived the cosmic impact that ended the Age of Dinosaurs quite well. Henophidia, the largest superfamily of snakes, diversified greatly right after the mass extinction, likely taking advantage of the relatively empty landscape that the formerly dominant dinosaurs left behind.
A key part of snakes’ success has been their wide range of habitats, which is at least partly the result, notes Hsiang, of their capacity to invade aquatic and marine environments. “The way snakes move on land actually closely approximates the way that long, slender fish like eels swim, which may have made it easier for them to adapt to moving in water,” Hsiang notes.
This study is not necessarily the final word on snake evolutionary history, Hsiang says. She cites some recently described snake-like fossils that may push back the date for the point at which the snake lineage first diverged from other lizards says. Other work suggests that snakes are closely related to extinct marine mosasaurs, which could affect the inference as to whether snakes evolved on land or in the seas.
“I think their reconstruction is plausible, but so are other interpretations,” Lee says, noting the wide variety of species near the base of the snake evolutionary tree. “Extrapolating the form of the ancestor from this assortment of morphologies can be challenging.”