The Miocene deep-water echinoids Toxopatagus italicus and Neoholaster albensis gen. and sp. nov., implications for holasteroid phylogeny and palaeoecology

Holasteroids form a very diverse clade among irregular echinoids. Despite the fact that they are known from a wide variety of Lower Cretaceous to Present environments, their (palaeo)biology is poorly understood in general. We present the results of cladistic, taxonomic, palaeoecological and biostratigraphic studies of two holasteroids from Miocene deep-water deposits of northern Apennines (Italy): the monotypic genus Toxopatagus and a new genus and species described as Neoholaster albensis gen. and sp. nov. Previously unknown taxonomic characters inferred from newly discovered material allowed the refining of some phylogenetic relationships within Holasteroida. We moved the late Burdigalian–Langhian Toxopatagus from Hemipneustidae to Pseudholasteridae, a family hitherto known from Cretaceous to Oligocene records. Toxopatagus proved to be morphologically and phyletically related to the Cretaceous–Oligocene Australasian genus Giraliaster. These genera share relevant morphological characters, including a primitive protosternous plastron and a subanal fasciole. The Langhian–upper Tortonian taxon Neoholaster albensis gen. and sp. nov. was found to be morphologically close to the Cretaceous genus Holaster. However, cladistic analysis indicated no robust phyletic relationship between these taxa, nor with other known genera of Holasteridae, thus revealing a ghost lineage with phylogenetically related taxa yet to be discovered. This is possibly due to the patchy fossil record, likely enhanced by the biotic crises that occurred during the intricate evolution of the end Cretaceous–early Cenozoic Western Tethys. Biotic crises may have reduced the abundance of Neoholaster ancestor populations, lowering the chances of fossilization, or may have restricted populations to as-yet unknown refuge areas. The identified lineage Giraliaster–Toxopatagus showed an additional instance of holasteroid migration to the deep sea. Even Neoholaster inhabited bathyal bottoms. In deep environments, the studied echinoids probably took advantage of the high concentration of organic carbon from previously formed sapropels and localized sources such as wood-fall deposits and/or from marine snow produced by phytoplankton blooms. Based on their morphological features, an epibenthic and a partially burrowing mode of life were deduced for Neoholaster.