Shallow-water brittle-star (Echinodermata: Ophiuroidea) assemblages from the Aptian (Early Cretaceous) of the North Atlantic: first insights into bathymetric distribution patterns

In spite of their excellent preservation potential and abundance, brittle-star microfossils are still an underexploited source of alpha-taxonomical data. Knowledge on the Lower Cretaceous fossil record of the ophiuroids is particularly patchy, hampering the use of the ophiuroids as a model organism to explore macroevolutionary, taphonomic and other further-reaching aspects. Here, we describe three ophiuroid assemblages mostly based on dissociated lateral arm plates from the early Aptian of Cuchía (Cantabria, northern Spain), and the latest Aptian of Wizard Way (Texas, USA). A total of eleven species were identified. Ten species are new to science, three of which (Ophioleuce sanmigueli sp. nov., Ophiozonella eloy sp. nov. and Ophiodoris holterhoffi sp. nov.) are formally described as new. The two Spanish assemblages are dominated by an ophionereidid and an ophiolepidid, and the Texan one by an ophionereidid and, to a much lesser extent, an ophiacanthid assemblage. Our analysis reveals that the eastern (Cuchía) and western (Texas) North Atlantic faunal spectra were not fundamentally different from each other during the Aptian. We furthermore present the first clear bathymetric gradient in the ophiuroid fossil record, comparing the Texan assemblage with a recently discovered coeval fauna from middle bathyal palaeodepths of Blake Nose, western North Atlantic, and show that Aptian shallowwater (<200 m) and deep-sea ophiuroid communities were clearly distinct. Finally, we argue that the Aptian shallowwater assemblages, although dominated by families which typically occur in present-day mid- to low-latitude shallow seas, have no modern equivalents in terms of family-level composition.