DEAD CLADE STILL WALKING: FIRST OBSERVATIONS OF A LIVING CASSIDULOID ECHINOID, NORTH SULAWESI, INDONESIA

Cassiduloid echinoids reached their maximum species richness in the Eocene, constituting >40% of all echinoids. Since then, they have declined to only ~30 extant species, just ~3% of all living echinoids. Explanations for their decline include competition with clypeasteroids (sand dollars), their remarkably conserved morphology, Cenozoic cooling and/or stochastic events. Nonetheless, despite their decline they are still with us. While much is known about the biology of the typically larger and often-sympatric irregular urchins, little is known about the living cassiduloids. Among these, Oligopodia is an enigmatic genus with a poor Plio-Pleistocene fossil record, has uncertain relationships to other cassiduloids, and is currently represented by a single species―O. epigonus―which has the broadest distribution of any living cassiduloid. Finding living cassiduloid populations has been challenging, and our knowledge of their biology is restricted to a handful of publications. Bare tests of O. epigonus were unexpectedly found during a recreational dive off Bangka Island, North Sulawesi, Indonesia. The tests exhibited a range of post-mortem color alteration suggesting that they were derived from an in situ living population, which was found the following year buried in well-ventilated mixture of skeletal debris (coral, molluscs) and inorganic sediment. While observations were made on their morphology, taphonomy and predatory drill holes, the most striking discovery was behavioral―their burial rate was much slower than a similar-sized spatangoid (heart urchin) recovered in the same sediment. Spatangoids have a larger variety of spines that increases burrowing speed compared to a cassiduloid. Oligopodia epigonus is pale yellow, as the substrate, and has only been found infaunally. We hypothesize that this species is an environmental specialist restricted to a limited set of substrates, has patchy distribution and stay in the vicinity after they die. The highly conserved morphology of the group implies millions of years of intense stabilizing selection, further indicative of limited ecological tolerances. Stepping back, this effort highlights the long-recognized deep challenges of determining species’ ecological constraints, and thus the reasons for their evolutionary success and/or demise.