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.
Highlights
The deep sea is by far the most widespread environment on Earth, yet our knowledge of the composition, biodiversity and origin of deep-sea biota remains remarkably poor (e.g. Gage & Tyler 1991)
The results strongly suggest that most of the early evolution of the Ophiacanthidae took place in the deep sea, and that the shelf fossil record documents a temporary expansion of the bathymetric distribution to shallower depths rather than an onshore origination followed by an offshore migration or retreat
Based on a careful comparison with lateral arm plates (LAPs) of Recent ophiacanthid species, and on the observation by Thuy & Stöhr (2011) that LAP morphology generally reflects phylogenetic relationships in ophiuroids, it is possible to identify a number of ophiacanthid lineages (Fig. 1), based on similarities in LAP morphology
Summary
The deep sea is by far the most widespread environment on Earth, yet our knowledge of the composition, biodiversity and origin of deep-sea biota remains remarkably poor (e.g. Gage & Tyler 1991). Debates have largely centred on two competing hypotheses: deep-sea biota are either relatively recent, resulting from multiple turnovers linked to large disruptions of the deep-sea environment (Menzies et al 1973; Jacobs & Lindberg 1998; European Journal of Taxonomy 48: 1-242 (2013). All attempts at explanation share the same potential pitfall: they almost invariably imply that the deep-sea fauna originated from shallow-water ancestors (Menzies et al 1973; Jacobs & Lindberg 1998; Smith & Stockley 2005; Strugnell et al 2008). It has become widely accepted practice to ignore the possibility of deep (offshore) to shallow (onshore) patterns of groups and to eclipse the role of the deep sea in most macroevolutionary processes
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