Fossil Sea Urchin Preserves Evidence of an Attack from a Mosasaur
Sixty-six million years ago, in a shallow sea in what is now Denmark, a sea urchin lay partially submerged on the seabed, when a keen-eyed mosasaur spotted it and went in for the kill. The marine reptile grabbed the sea urchin and bit it, but for some reason, the attack was aborted, the invertebrate was dropped and the little sea urchin survived the encounter with the apex predator. How do we know all this? A remarkable fossil has been discovered by amateur geologist Peter Bennicke at Stevns Klint, a famous UNESCO World Heritage Site, one of the few places in the world where rock layers mark the Cretaceous-Palaeogene boundary providing evidence to support the extra-terrestrial impact event that contributed to the demise of the Dinosauria.
Fossil Provides Evidence of a Mosasaur Attack
Picture Credit: Everything Dinosaur
Preserving Evidence of Predator/Prey Interactions
The stretch of chalk cliffs at Stevns Klint on the Danish island of Zealand (Sjaelland), was granted World Heritage Site status by UNESCO in 2014. The chalk deposits record the K/T boundary and the cliffs provide a record of the faunal turnover from the very end of the Cretaceous (Maastrichtian faunal stage), through to the earliest stage of the Palaeogene Period (Danian faunal stage). Sea urchin fossils are relatively common at this location, but the specimen found by Mr Bennicke is very special as it records evidence of predator/prey interaction.
The curator at the nearby Geomuseum Faxe, Jesper Milàn stated:
“It’s really an exciting find this, not only is there an exciting story to tell about it, but it also provides important information about how the animals in the Cretaceous sea lived and who ate who. It is such a find that helps put meat and blood on the otherwise dry fossils, when you can suddenly see such a small everyday drama caught in the stone.”
The Echinocorys Specimen Showing Evidence of a Mosasaur Attack
Picture Credit: Jesper Milàn
What Type of Mosasaur Attacked the Sea Urchin?
The round tooth marks are located near the top of the Echinocorys specimen, suggesting that the attack came from above and it is likely that the sea urchin was partially exposed out of the sediment on the sea floor when the attack occurred. An examination of the morphology of the tooth marks and their spacing indicates that the attacker had slender teeth, that were circular in cross-section and that these teeth were spaced relatively far apart in the jaw. Two types of hypercarnivorous mosasaurids are known from Denmark – Mosasaurus hoffmanni and Plioplatecarpus spp. It could be speculated that one of these types of mosasaur was responsible for the attack.
A Mosasaurid Specimen is Used to Demonstrate the Sea Urchin Attack
Picture Credit: TV OST
Lucky Escape for the Sea Urchin
Although a mosasaurid grabbed the sea urchin, it apparently abandoned the attack. Hypercarnivores such as M. hoffmanni and Plioplatecarpus probably preyed on a variety of vertebrates and invertebrates, but their teeth are not really suited to crushing the shell of an Echinocorys. Recently, Jesper Milàn in collaboration with other scientists, reported the discovery of a single broken tooth of a mosasaur called Carinodens minalmamar. The tooth crown was found in the uppermost Maastrichtian chalk strata at Stevns Klint, indicating that this Mosasaur probably lived around 50,000 years before the deposition of the iridium rich K/Pg boundary material. The shed tooth is reported to have come from the 11th or 13th position in the jaw. The tooth represents the northernmost occurrence of the genus Carinodens found to date. Carinodens minalmamar, was a very different type of predator compared to Mosasaurus hoffmanni and Plioplatecarpus, it was a specialist shell-eater (durophagus). The short, thick and rounded teeth of this type of mosasaur would have made quick work of the test of an Echinocorys.
Examples of the Teeth of Carinodens spp.
Picture Credit: Holwerda and Jagt
The sea urchin may count itself fortunate to have been attacked by a mosasaur more used to catching fish, sea birds and other marine reptiles. If a mosasaur such as Carinodens had grabbed the Echinocorys, then it is likely that the sea urchin would not have survived.
An exhibit telling the story of the sea urchin and who tried to eat it will open at the Geomuseum Faxe in February 2019.
To read an article about Stevns Klint being granted UNESCO World Heritage Site status: Famous KT Boundary gets UNESCO World Heritage Site Status