Oldest Urchin

Researchers have uncovered a sea urchin fossil that pushes back a fork in its family tree by 10 million years, according to a new study.

A team from USC Dornsife found the fossil — Eotiaris guadalupensis — in collections of the Smithsonian Institution from the Glass Mountains of west Texas, where it had been buried in a rock formation that dates back as much as 268.8 million years.

“This fossil pushes the evolution of this type of sea urchin from the Wuchiapingian age all the way back to the Roadian age,” said David Bottjer, professor of earth sciences, biological sciences and environmental studies, and senior author of a paper announcing the study that appeared in Nature Scientific Reports.

The paper was a collaboration between labs overseen by Bottjer and Eric Davidson at the California Institute of Technology. Jeffrey Thompson, a Ph.D. student at USC Dornsife, found the fossils of Eotiaris guadalupensis in the Smithsonian collections and was the lead author on the study.

Two types of urchins

Eotiaris guadalupensis is a cidaroid, one of the two main types of sea urchins found in today’s oceans. The other group, the euechinoids, evolved wildly varying body types and accounts for almost all sea urchins alive today. Cidaroids, by contrast, look pretty much the same as they did millions of years ago. Both evolved from an ancestral group of echinoids known as the Archaeocidaridae, which are now extinct.

David Bottjer, professor of earth sciences, biological sciences and environmental studies at USC Dornsife.

The divergence of the two groups marks a major — and relatively abrupt — shift in the genetic organization of sea urchins.

“It’s not just the color of a moth’s wing changing,” said Bottjer, referring to the classic example of the peppered moth in England that, in the post-Industrial Revolution’s sooty skies, began to appear in a darker color as a result of natural selection. “We’re looking at tightly intertwined networks of genes that change together to cause major morphological differences.”

Pinning down the time at which the two groups diverged allows evolutionary biologists to better understand the processes that occur during major evolutionary changes.

Bottjer and Thompson further expanded on these findings at the 2015 Geological Society of America meeting in Baltimore, where they discussed paleogenomics. The burgeoning field aims to date when genes first evolve by tracking morphological innovations from the fossil record that are produced by known genes in modern organisms.

The current study was funded by the National Science Foundation (grant IOS1240626).