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Discovered 25 years ago, a 444-million-year-old marine arthropod fossil stumped paleontologists, as they couldn’t identify its exact species.
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Sarah Gabbot, who originally discovered the specimen, realized that the fossil had been preserved inside-out, meaning its muscles, tendons, and guts were exquisitely preserved while its limbs, carapace, and even head had dissolved away.
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Gabbot named the specimen Keurbos susanae—nickname ‘Sue’—in honor of her mother, whom she said always supported her love of paleontology.
Arthropods are the most successful animal group on the planet. These varying invertebrates make up roughly 85 percent of all animal life on the planet, and they have one of the most extensive and well-preserved fossil records of any animal group, with examples dating back some 518 million years ago to the Cambrian era—a.k.a. when complex life really boomed for the first time.
Fast forward some 73 million years to the end of the next geologic period (the Ordovician), and life meets its first bust. The first of five (or possibly six) mass extinctions in Earth history, the Late Ordovician mass extinction wiped out roughly 85 percent of all life on Earth, making it the second most deadly (after the Permian extinction—you don’t get the nickname “The Great Dying” for nothing). It was during this tumultuous biological period, that a certain arthropod met its end, eventually becoming entombed and fossilized in Soom Shale—a band of silts and clays located 250 miles north of Cape Town, South Africa. Although intense glaciation laid waste to the planet, this small pocket of the world continued to thrive even under icy threat.
Some 444 million years later, paleontologists unearthed this particular specimen, but it’s appearance didn’t match anything in the fossil record. That is, until Sarah Gabbott, a lead author on a study published in the journal Paleontology detailing this new species (named Keurbos susanae after the lead author’s mother, Sue), made the surprising discovery—the fossil was actually preserved inside-out.
“‘Sue’ is an inside-out, legless, headless wonder,” Gabbott said in a press statement.
“Remarkably her insides are a mineralised time-capsule: muscles, sinews, tendons and even guts all preserved in unimaginable detail. And yet her durable carapace, legs and head are missing—lost to decay over 440 million years ago.”
Although this fossil’s resting place included an anoxic environment (a necessary ingredient for fossilization to occur), it also contained hydrogen sulphide dissolved in the water. The researchers believe this chemistry likely dissolved away the carapace. Yet the mineral that perfectly preserved the marine arthropods insides—calcium phosphate—is the same mineral found in our bones and teeth. Gabbott told IFLScience that she’s still trying to work out the exact details of how this strange inside-out preservation took place.
Although ‘Sue’—not to be confused with another famous fossil of the T. Rex persuasion—provides an incredible glimpse at the organs and guts of an ancient arthropod, it’s difficult to know where to place the specimen on the tree of life—even 25 years after Gabbott first discovered it.
“This has been an ultramarathon of a research effort,” Gabbott said in a press statement. “In a large part because this fossil is just so beautifully preserved there’s so much anatomy there that needs interpreting. Layer upon on layer of exquisite detail and complexity.”
While the mystery remains, the naming of the species at least checks off one to-do on Gabbott’s list:
“Recently my mum said to me ‘Sarah if you are going to name this fossil after me, you’d better get on and do it before I am in the ground and fossilized myself.’”
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