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A Turtle-y Cool Fossil Find

Carlsbad Sea Turtle Shell Fills 40-Million-Year Gap in Fossil Record

When the dinosaurs went extinct 66 million years ago, three quarters of the planet’s wildlife went down with them. In the ocean, one in three species didn’t survive—at least that’s what the fossil record tells us. 

For years now, scientists have puzzled over one group of marine animals in particular: the sea turtles. Prior to that mass-extinction event, there were enormous (think car-sized) sea turtles milling about the world’s oceans. But at the moment in time when the dinosaurs disappeared, called the Cretaceous-Paleogene Boundary, evidence of sea turtles in the Pacific Ocean drops off, only to be picked up again a whopping 40 million years later. 

What gives? Were sea turtles in the Pacific Ocean the one in the three that didn't make it? Did it really take 40 million years for surviving sea turtles in other oceans to repopulate the Pacific? Were turtles here all along, swimming below the radar?  

Turns out the answer was sitting in a small box, on the third shelf of a large cabinet, deep in The Nat’s paleontology collection.

Another piece in the puzzle, another drop in the bucket 

About five inches wide and shaped like a boomerang, a fossilized section of a sea turtle’s top shell was unearthed during construction of the Bressi Ranch housing development near Carlsbad, California in 2004 

Originally noticed by co-author and Berkeley Paleontologist Patricia Holroyd while she was visiting our collection, the fossil has finally been added to the scientific record—and its age couldn’t be more perfect.  

Based on the rocks and fossils found around the bone, our paleontology team concluded the turtle shell fossil was from the middle Eocene, which is in the very center of that 40-million-year "turtle gap." This suggests Pacific sea turtles not only survived the mass-extinction event of the late Cretaceous, but they were probably here all along.

"This fossil is just the first evidence of sea turtles’ existence in this time and place, but it suggests there may be much more to find," says Ashley Poust, a post-doctoral researcher with the San Diego Natural History Museum and lead author on the new paper describing the fossil in Acta Palaeontologica Polonica. "If big, easily recognizable animals like sea turtles can be hiding in the fossil record of the North Pacific, there may be other unexpected fossils yet to be discovered." 

Natural history collections are a trove of untold scientific stories. By mining The Nat’s fossil collection, Ashley has described 2 new species in the past three years, with about five more on the way. Discoveries like this are not uncommon at museums all around the world.  

Though these fossilized bones are far from a complete skeleton, they represent another drop in the bucket of knowledge, says Ashley. The fossil also has some physical differences compared to modern turtles, which suggests there could be more undescribed, ancient sea turtle species out there. 

"Now researchers can go into their future work knowing that sea turtle fossils might be present in certain rock formations," Ashley adds. In paleontology, every bit of new information, no matter how small, helps us fill in the chapters of the long story of life on Earth. 

This discovery wouldn’t have happened if not for the unique relationship between San Diego’s development entities and The Nat’s PaleoServices Department. Our paleontology experts are on-site during major construction projects around Southern California every day of the year, ensuring San Diego’s fossilized past is preserved and explorable for future generations.  

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Posted by Cypress Hansen, Science Communications Manager on May 5, 2023

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