Temporary expansion to shelf depths rather than an onshore-offshore trend: the shallow-water rise and demise of the modern deep-sea brittle star family Ophiacanthidae (Echinodermata: Ophiuroidea)

Abstract

Hypotheses on the age and possible antiquity of the modern deep-sea fauna put forward to date almost all agree on the assumption that the deep-sea fauna is largely the result of colonisation from shallow-water environments. Here, the fossil record of the Ophiacanthidae, a modern deep-sea brittle star family with extensive fossil occurrences at shelf depths, is systematically traced against a calibrated phylogeny. Several lines of evidence suggest that the Ophiacanthidae originated and greatly diversified in the deep sea, with most extant clades having diverged by the end of the Triassic at the latest. During the Jurassic, the family temporarily invaded shelf environments, attaining relative abundances and diversities comparable to those found in coeval and modern deep-sea settings, and gradually declined in abundance subsequently, to become largely restricted to the deep-sea again. The pattern of temporary expansion to shelf environments suggested here underpins the potential of deep-sea environments to contribute significantly to shallow-water biodiversity; an aspect that has mostly been neglected so far. It is speculated that the large-scale ophiacanthid invasion of shelf environments around the Triassic-Jurassic boundary was initiated by a change from thermohaline to halothermal circulation, attenuating the thermal stratification of the water column and thus providing opportunities for enhanced vertical migration of marine taxa

Brittle stars from the British Oxford Clay: unexpected ophiuroid diversity on Jurassic sublittoral mud bottoms

“…Remarks.-The large, ear-shaped spine articulations with the well-developed sigmoidal fold, in combination with a vertical striation on the outer LAP surface and two to three small, apical oral papillae, unambiguously place the present specimen among the members of the family Ophiacanthidae as formerly understood (e.g., Thuy, 2013). Within this group, the densely granulated disk, the rounded triangular radial shields, the relatively small tentacle openings, the large, contiguous proximal dorsal arm plates, the ventroproximalward-protruding ventral portion of the LAPs, and the regularly spaced spine articulations positioned in notches of the elevated distal LAP portion strongly favor assignment to the extinct genus Dermocoma.…”

“…Modern representatives of the OphiomusiumOphiosphalma-Ophiolipus complex tend to be much more morphologically conservative. The Jurassic forms generally contrast with their modern relatives in showing a much larger range of morphologies, with the present one even superficially reminiscent of ophiacanthid morphologies with the large, prominent spine articulations, the constricted LAPs, and the long spines (e.g., Thuy, 2013).…”

“…They can be very abundant and represent a time-averaged, taphonomically unbiased record of the ophiuroid assemblages at a given time and are therefore the most likely candidates to reflect the true ophiuroid paleobiodiversity as exhaustively as possible (Thuy, 2013(Thuy, , 2015. Fossils of articulated ophiuroid skeletons, by contrast, are rare finds bound to exceptional depositional conditions and therefore provide highly patchy glimpses into the fossil record of the group.…”

First glimpse into Lower Jurassic deep-sea biodiversity: in situ diversification and resilience against extinction

“…To assess the origin, resilience and bathymetric range shifts of the deep-sea fauna, we classified the families/superfamilies of the Glasenbach assemblage as well as coeval shelf (less than 200 m palaeo-depth) faunas [20,21,[23][24][25][26][27][28][29][30][31][32][33][34][35][36] as extinct, or extant and, according to their present-day bathymetric distribution, typically deep, typically shallow, or without depth preference (eurybathic) (figure 3; electronic supplementary material, table 1). Our bathymetric classification was based on the averaged mean depth distribution of the extant species of the families/superfamilies (calculated by averaging the upper and lower distribution boundaries for each species and by computing the arithmetic mean of the obtained values for each family/superfamily): typically deep-averaged mean depth greater than 500 m; typically shallow-averaged mean depth shallower than 200 m; eurybathic-averaged mean depth between 200 and 500 m. The upper boundary of the deep sea is commonly set between 200 and 500 m, in line with the fading of seasonal variations in physical parameters (e.g.…”

Redescription of Hemieuryale pustulata von Martens, 1867 (Echinodermata, Ophiuroidea) based on Brazilian specimens, with notes on systematics and habitat association

“…Presently only one fossil species is known from the family, which belongs to the genus Hemieuryale (H. parva Kutscher & Jagt, 2000 †-from Lower Maastrichtian, Late Cretaceous). Thuy (2013) recently transferred the fossil species Sigsbeia lunaris (Hess, 1962) † (from Late Pliensbachian, Early Jurassic) to the extinct genus Inexpectacantha Thuy 2011 † (Ophiacanthidae).…”