Fossilized babies of ancient crocodile-like predators uproot understanding of how animals adapted to the land
Life on our planet began in the water. Eventually, one branch of the fish family tree developed legs and came up on land. These early four-legged animals, the tetrapods, were the forebears of today's mammals, birds, reptiles ...
Scientists have long thought that the earliest of these occasional land dwellers were like modern amphibians, in that they hatched from eggs, underwent a tadpole phase and then transformed into their adult bodies. But in a new study in the journal Science, researchers announce the discovery of fossilized baby early tetrapods, which skipped the tadpole metamorphosis scientists had expected to see.
The finding means that the first land-dwelling vertebrates were less like modern amphibians than had previously been thought—upending scientists' understanding of how animals conquered the land.
"When a lot of us were in high school, we were taught this simplified story of evolution: that some fish evolved into amphibians, and some of those amphibians evolved into reptiles, and some of those reptiles evolved into mammals. And our study shows that this basic underlying premise, that the first four-legged vertebrates grew up like amphibians, is wrong," says Jason Pardo, a research associate at the Field Museum and the study's co-lead author.
"This is the first time we've had these early, early hatchling animals. This discovery is really a testament to the power of Mazon Creek, the site where these fossils came from," says Arjan Mann, the Field Museum's assistant curator of early tetrapods and the study's other co-lead author.
Baby crocodile-like early tetrapods called embolomeres. New fossil evidence suggests that these embolomeres did not undergo a metamorphosis the way that modern amphibians do when growing up, which challenges a long-standing scientific belief that amphibians, reptiles, and mammals evolved from animals that had a tadpole stage. Credit: Berit Godring