Deep sea fossil spines reveal life after huge mass extinction

Ancient sea urchins reveal a tale of resilience in the face of catastrophe 104 million years ago.
Sade Agard
A sea urchin (Echinoidea) fossil.
A sea urchin (Echinoidea) fossil.


The deep sea, a realm shrouded in mystery and darkness, is often considered a relatively young ecosystem in the grand history of our planet. However, recent research published in PLOS ONE has unveiled a revelation that challenges this notion. 

Scientists from the University of Gottingen, Germany, have provided the first fossil evidence of a stable colonization of the deep sea floor by higher invertebrates, dating back at least 104 million years.

Ancient deep sea secrets

The key to this discovery lies in the fossilized spines of irregular echinoids, commonly known as sea urchins

These remarkable findings not only shed light on the ancient inhabitants of the deep sea but also offer insights into their evolution in response to ever-changing environmental conditions.

To uncover these secrets of the deep, the research team meticulously examined over 1,400 sediment samples extracted from boreholes in the Pacific, Atlantic, and Southern Oceans, representing former water depths. 

These depths ranged from 200 meters to a staggering 4,700 meters below the surface. Among the samples, they unearthed more than 40,000 fragments of spines, categorized as belonging to the group of irregular echinoids based on their unique structure and shape.

Deep sea fossil spines reveal life after huge mass extinction
A range of sea urchin spines from different periods of the Earth's history illustrating the diversity of shapes.

Still, the study's investigation didn't stop there. To paint a more vivid picture of the past, the scientists recorded detailed characteristics of these ancient spines, such as their shapes and lengths. 

Additionally, they scrutinized the thickness of around 170 spines from two distinct time periods. These measurements indicated the total mass of sea urchins in their deep-sea habitat, effectively quantifying their biomass from ancient times to the present.

Significantly, the fossil spines provide compelling evidence that irregular echinoids have inhabited the deep sea since the early Cretaceous period, a staggering 104 million years ago. 

Surviving the dinosaur mass extinction

But these ancient relics also tell a story of resilience in the face of catastrophe. 

That is, around 66 million years ago, a catastrophic meteorite impact triggered a worldwide mass extinction event, forever altering life on Earth, including the extinction of the dinosaurs. 

This event also cast a shadow on the deep sea, resulting in significant disturbances. The spines, once thick and diverse in shape, bore the marks of change, becoming thinner and less varied after the cataclysmic event. 

Scientists call this phenomenon the "Lilliput Effect," where smaller species gain a survival advantage after mass extinctions, possibly due to limited food resources on the deep-sea floor.

"We interpret the changes in the spines as an indication of the constant evolution and emergence of new species in the deep sea," explained lead author Dr. Frank Wiese from the Department of Geobiology at the University of Göttingen in a press release.

Additionally, he highlights a remarkable correlation: "About 70 million years ago, the biomass of sea urchins increased. We know that the water cooled down at the same time."

"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."

In conclusion, the team has unveiled the layers of time, providing a unique glimpse into the deep sea's ancient history. Most importantly, this study offers valuable insights into how it may respond to the challenges of our changing world.

The complete study was published in PLOS ONE and can be found here.

Study abstract:

Deep-sea macrobenthic body fossils are scarce due to the lack of deep-sea sedimentary archives in onshore settings. Therefore, hypothesized migrations of shallow shelf taxa into the deep-sea after phases of mass extinction (onshore-offshore pattern in the literature) due to anoxic events is not constrained by the fossil record. To resolve this conundrum, we investigated 1,475 deep-sea sediment samples from the Atlantic, Pacific and Southern oceans (water depth ranging from 200 to 4,700 m), providing 41,460 spine fragments of the crown group Atelostomata (Holasteroida, Spatangoida). We show that the scarce fossil record of deep-sea echinoids is in fact a methodological artefact because it is limited by the almost exclusive use of onshore fossil archives. Our data advocate for a continuous record of deep-sea Atelostomata back to at least 104 Ma (late early Cretaceous), and literature records suggest even an older age (115 Ma). A gradual increase of different spine tip morphologies from the Albian to the Maastrichtian is observed. A subsequent, abrupt reduction in spine size and the loss of morphological inventory in the lowermost Paleogene is interpreted to be an expression of the “Lilliput Effect”, related to nourishment depletion on the sea floor in the course of the Cretaceous-Paleogene (K-Pg) Boundary Event. The recovery from this event lasted at least 5 Ma, and post-K-Pg Boundary Event assemblages progress—without any further morphological breaks—towards the assemblages observed in modern deep-sea environments. Because atelostomate spine morphology is often species-specific, the variations in spine tip morphology trough time would indicate species changes taking place in the deep-sea. This observation is, therefore, interpreted to result from in-situ evolution in the deep-sea and not from onshore-offshore migrations. The calculation of the “atelostomate spine accumulation rate” (ASAR) reveals low values in pre-Campanian times, possibly related to high remineralization rates of organic matter in the water column in the course of the mid-Cretaceous Thermal Maximum and its aftermath. A Maastrichtian cooling pulse marks the irreversible onset of fluctuating but generally higher atelostomate biomass that continues throughout the Cenozoic.

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