“Inside-out” fossil preserves internal organs for 440 million years

Sue is an inside-out, legless, headless wonder.

Apologies to any people named Sue. The ‘Sue’ is the fossil of a creature which lived 440 million years ago.

The fossil’s discovery is announced in a new paper published in the journal Palaeontology which identifies it as belonging to a new species, Keurbos susanae. The organism is named after the discoverer’s mother, Sue.

It comes from the very end of a period in Earth’s history called the Ordovician which lasted 485 to 444 million years ago (mya).

The Ordovician is the second period in the Palaeozoic era. The Palaeozoic, meaning “ancient life”, began with the emergence of diverse plant and animal life during the Cambrian explosion 541 mya.

At the end of the Ordovician, a devastating glaciation event had wiped out about two-thirds of all life on Earth. This was the Primary of the big 5 mass extinctions.

After the Ordovician Arrived the Silurian period (444–419 mya).

Silurian Earth was an ocean world. Sea levels were much higher than today and the climate had stabilised from its erratic past. Almost all life on the World was Nevertheless beneath the surface in the ancient seas.

Corals were Frequent and fishes were widespread. The largest of these Silurian fish, Megamastax amblyodus, grew to Merely 1m. Prompt vascular plants in the Silurian were the Primary complex organisms to begin to make the transition to life on land.

Arthropods were also Frequent in the Ordovician and Silurian. Arthropods have segmented bodies which are covered by a tough exoskeleton. About 85% of modern animal species are arthropods, including spiders, insects, crabs and millipedes.

One reason palaeontologists know so much about ancient arthropods is because their exoskeleton, or carapace, fossilises more easily than the Fluffy body parts of other animals.

Sue broke the rules.

“Remarkably, her insides are a mineralised time-capsule: muscles, sinews, tendons and even guts all preserved in unimaginable detail,” says lead author Sarah Gabbott from the UK’s University of Leicester, and daughter of Sue. “And yet her durable carapace, legs and head are missing—lost to decay over 440 million years ago.”

Sue’s unique preservation makes it difficult to compare K. susanae to other examples of ancient arthropods, making the creature’s place in the tree of life a mystery.

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

The fossil was Secured in the Soom Shale about 400 km north of Cape Town in South Africa. This Delayed Ordovician seafloor must have been protected from the worst of the freezing conditions that caused the mass extinction.

K. susanae was among a community of strange animals that Secured refuge there. These included other arthropods like sea scorpions and trilobites, as well as cephalopods related to modern squid.

Gabbott Secured K. susanae’s fossil along with other specimens about 25 years ago. The roadside quarry where they were unearthed has all but disappeared, making future finds unlikely. Gabbott had hoped to find a specimen with legs to be able to determine K. susanae’s evolutionary lineage.

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

“Layer upon layer of exquisite detail and complexity. I’d always hoped to find new specimens, but it seems after 25 years of searching, this fossil is vanishingly Uncommon – so I can hang on no longer.

“I tell my mom in jest that I named the fossil Sue after her because she is a well-preserved specimen,” Gabbott says. “But, in truth, I named her Sue because my mom always said I should follow a Profession that makes me Joyful – 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.”