A 104-Ma record of deep-sea Atelostomata (Holasteroida, Spatangoida, irregular echinoids) – a story of persistence, food availability and a big bang

Fossil deep-sea macrobenthos is scarce due to the rarity of onshore deep-sea sediments. Therefore, hypothesized migrations of shallow shelf taxa into the deep-sea after phases of mass extinction or oceanic anoxic events (onshore-offshore pattern) is not constrained by the fossil record. To resolve this conundrum, we investigated 1,475 deep-sea sediment samples (Atlantic, Pacific, Southern oceans; 200 - 4,700 m paleo-water depth). Ca. 41,500 spine fragments from Holasteroida and Spatangoida (Atelostomata) document a continuous occurrence of the groups since 104 Ma (late early Cretaceous). Literature records suggest even an older age (115 Ma). A gradual increase of spine tip morphotypes occurs since the Albian. An abrupt reduction in spine size and morphological inventory following the Cretaceous-Paleogene (K-Pg) Boundary Event is expression of a “Lilliput Effect”, related to nourishment depletion in the deep-sea. The post-event recovery lasted at least 5 Ma. Post-K-Pg Boundary Event assemblages seem to evolve progressively without any morphological breaks towards modern deep-sea assemblages. This observation is 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 pre-Campanian values, 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 persists in the Cenozoic.