Picture the deep ocean floor: crushing pressure and zero light, and cold so absolute it feels like a place nothing could choose to live. Scientists long suspected that the bizarre creatures dwelling down there were relative newcomers, survivors of whatever catastrophe last scraped the abyss clean. Then a team started pulling tiny fragments out of ancient seafloor sediment, and the number they kept arriving at changed everything.
Boreholes across three oceans told a very old story
A research team led by the University of Göttingen provided the first fossil evidence for a stable colonization of the deep seafloor by higher invertebrates for at least 104 million years.
They did not find that evidence in a single lucky cliff face or quarry.
The researchers examined over 1,400 sediment samples from boreholes in the Pacific, Atlantic and Southern Ocean, representing former water depths of 200 to 4,700 metres.
Each borehole was like drilling a time machine straight into the planet. Layer by layer, the sediment held a record nobody had thought to read this way before.
Some cores came up pale and fine-grained, the compressed remains of ancient calm. Others were darker, denser, hinting at upheaval. Every centimetre of that material was a page in a story nobody had yet bothered to open.
The clue was almost too small to see
They found more than 40,000 fragments of spines, which they assigned to a group called irregular echinoids, based on their structure and shape.
These were not bones or shells in the dramatic fossil sense. They were tiny spines, pressed into ancient mud for tens of millions of years.
By measuring their shape, length and thickness, the team could read population health across deep time the way a doctor reads a chart.
The density of material packed into each sediment layer told them not just whether these animals were present, but how well they were doing in any given geological moment.
The picture that emerged was of an animal that simply never left.
Then the asteroid hit, and even the abyss felt it
The record running back 104 million years was remarkable on its own. But the spines held a second story inside them, written in the years right after the most violent event in recent Earth history.
There seemed to be a drastic change at the end of the Cretaceous period 66 million years ago, when a devastating asteroid impacted Earth and caused a mass extinction of dinosaurs and considerable disturbances in the deep sea , shown by spines that were thinner and less diverse in shape after the event than before.
The researchers interpret this as the “Lilliput Effect,” meaning that smaller species have a survival advantage after a mass extinction, leading to reduced body size.
Even miles underwater, far from the burning sky, the shockwave of that day reshaped life in the darkness below.
The creature at the center of it all: the humble sea urchin
The protagonist of this entire 104-million-year saga is the sea urchin, a creature most people know only as a spiky nuisance on a beach vacation or a delicacy on a sushi menu.
Fossil spines of irregular echinoids indicate their long-standing existence since the Cretaceous, as well as their evolution under fluctuating environmental conditions.
Lead author Dr. Frank Wiese noted that about 70 million years ago, sea urchin biomass increased at the same time the water cooled down. “This relationship between biomass in the deep sea and water temperature allows us to speculate how the deep sea will change due to human-induced global warming.”
That link between deep-sea biomass and water temperature now allows scientists to consider how the deep sea will change under human-induced warming, making these spiny survivors an early-warning system for what warming oceans may do next. The study was published in PLOS ONE.
What 104 million years of stubbornness actually means
Some theories hold that deep-sea ecosystems have collapsed and restarted after multiple mass extinctions, making today’s life in the abyss comparatively young.
But there is growing evidence that parts of this world are far older than anyone assumed.
The sea urchin data puts real fossil weight behind that idea. These animals did not recolonize the abyss after each disaster. They held on through all of them, shrinking when they had to and rebuilding when conditions allowed.
While the dinosaurs rose and then vanished, while continents drifted and seas rose and fell, a small spiny creature was down in the permanent darkness miles below, going about its business without interruption.
The team notes that the scarce fossil record of deep-sea echinoids is in fact a methodological artefact, limited by the almost exclusive use of onshore fossil archives.
Still, 104 million years of continuous presence is a story the spines tell with remarkable consistency, and one that may help scientists predict which deep-sea creatures stand the best chance of enduring the warming centuries ahead.