444-million-year-old fossil named 'Sue' has a story to tell

A tiny fossil with an unusually well-preserved interior is rewriting what we know about ancient life. Discovered 25 years ago and affectionately named “Sue,” the fossil is unlike most others from its time.

Instead of showing off a tough outer shell like other arthropods, this creature gives a rare peek inside – offering a glimpse at muscles, tendons, and guts from the Ordovician period, 444 million years ago.

The fossil, now officially named Keurbos susanae, was found in South Africa’s Soom Shale, a rock layer north of Cape Town that was formed after a massive global extinction event.

During that time, a freezing climate wiped out about 85% of Earth’s species. Somehow, a few marine creatures survived in this basin, and Sue was among them.

This unusual fossil was recently studied by scientists from the University of Leicester.

Keurbos susanae fossil is all guts

“Sue is an inside-out, legless, headless wonder,” said Professor Sarah Gabbott from the University of Leicester’s School of Geography, Geology and the Environment.

“Remarkably, her insides are a mineralized time-capsule: muscles, sinews, tendons and even guts all preserved in unimaginable detail. And yet her durable carapace, legs and head are missing – no douubt lost to decay over 440 million years ago.”

Sue stands out because her internal features are fossilized, rather than her external shell. This is rare.

Most fossilized arthropods – members of a group that includes spiders, lobsters, and millipedes – preserve hard body parts like exoskeletons. In Sue’s case, the soft tissues were sealed away by unique conditions on the ancient sea floor.

The lack of oxygen, combined with toxic hydrogen sulfide in the sediments, likely helped create an environment where her soft tissues were not decomposed, but could remain intact.

Scientists believe a strange chemical process was involved in preserving her this way.

Still a mystery in the family tree

While researchers are confident Sue was a primitive marine arthropod, exactly where she fits into the evolutionary story is still unclear.

“We are now sure she was a primitive marine arthropod but her precise evolutionary relationships remain frustratingly elusive,” Professor Gabbott explained.

Her unique preservation makes it hard to match her with other known species from the time.

Most comparable fossils only show hard outer shells or limbs, offering little internal detail for comparison. With no modern equivalent and no other specimens like her, Sue remains a puzzle.

No other fossils with the same level of detail have turned up in over two decades of searching. And the original quarry where Sue was found – a small roadside site – has all but vanished.

Personal tribute, 25 years in the making

“This has been an ultramarathon of a research effort,” said Professor Gabbott. “In a large part, because this fossil is just so beautifully preserved, there’s so much anatomy there that needs interpreting.”

She spent years studying every layer of the fossil, hoping to one day find another with more complete features like a head or legs. But as time passed, she realized this might be a once-in-a-lifetime find.

“I’d always hoped to find new specimens but it seems after 25 years of searching this fossil is vanishingly rare – so I can hang on no longer,” she said.

And so she named the fossil Keurbos susanae, or Sue, after her mother.

“Especially as, recently, my mom 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,'” she shared.

“I tell my mom in jest that I named the fossil Sue after her because she is a well-preserved specimen. But, in truth, I named her Sue because my mom always said I should follow a career that makes me happy – whatever that may be. For me, that is digging rocks, finding fossils and then trying to figure out how they lived, what they tell us about ancient life and evolution on Earth.”

Sue may not have her head or legs, but her legacy – both scientific and personal – is unforgettable.

The full study was published in the journal Palaeontology.

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