Sea Urchins: Newly Examined Fossils Represent Oldest Known Specimens Of Its Kind

After taking a closer look at a fossil sea urchin, Eotiaris guadalupensis, researchers from the University of Southern California (USC) reveal that the species diversified 10 million years earlier than previously thought.

"This fossil pushes the evolution of this type of sea urchin from the Wuchiapingian age all the way back to the Roadian age," David Bottjer, senior author of the recent study and a USC professor, explained in a news release.

For their study, USC researchers examined specimens from the Smithsonian Institution that were originally excavated from the Glass Mountains of west Texas. 

Echinoids, or sea urchins, are rare in the Paleozoic fossil record, which is why not much is known about the species' early diversification. Today, there are two main types of sea urchins that can be found in the ocean: cidaroids and euechinoids. Both species evolved from an ancestral group of echinoids known as Archaeocidaridae, which are now extinct. Cidaroids and euechinoids have drastically different morphological characteristics.

Cidaroids are characterized by widely separated spines, two symmetrical plates and they have no gills. They pretty much look the same as they did millions of years ago. On the other hand, euechinoids are more rigid sea urchins with interlocking plates. They evolved more widely with varying body types and account for almost all sea urchins alive today, according to the release. 

Researchers concluded that Eotiaris guadalupensis is a cidaroid and represents the earliest known specimen. This discovery has important implications regarding the timing of the divergence of echinoids and the genetic differences among the two sea urchin species. The rock formation the fossils were originally excavated from dates back at least 268.8 million years, signifying the specimens are ten million years older than the previously oldest known cidaroid, researchers explained. 

"It's not just the color of a moth's wing changing," Bottjer added. "We're looking at tightly intertwined networks of genes that change together to cause major morphological differences."

Narrowing down exactly when the two sea urchins evolved separately from one another helps biologists better understand the processes that occur during major evolutionary changes, the release noted. This also sheds light on when certain morphological genes emerged. Their findings will be presented at this year's annual Geological Society of America meeting in Baltimore. 

Their study was recently published in Scientific Reports.

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