EARLY ONTOGENY OF TRILOBITES: IMPLICATIONS FOR SELECTIVITY OF SURVIVORSHIP AT THE END-DEVONIAN CRISIS.

Our recent discoveries of both latest Devonian and earliest Carboniferous proetide larvae allow us to propose a new hypothesis to explain the survivorship of this group during the end-Devonian Hangenberg Event. The comparison of Lower Carboniferous larvae with latest Devonian ones clearly demonstrate that the size-range of planktonic larvae, very narrow in oldest larvae, widened. Some Lower Carboniferous planktonic larvae are 12.5% smaller than the smallest pre-Hangenberg ones, whereas others are 12.5% larger than the largest pre-Hangenberg ones. This extension of the planktonic size-range suggests that the Lower Carboniferous proetide trilobites may have had two planktonic larval stages which considerably increased the duration of the planktonic period, thus delaying the onset of metamorphosis. In the context of overall spread of hypoxic bottom waters poisoning benthic habitats in end-Devonian outer-shelf seas, the observed modification of the early ontogenetic life cycle might have constituted an advantageous strategy for survivorship and post-event recovery. Indeed the opportunity for planktonic larvae to migrate further and to reach other, potentially non-poisoned, habitats could have enabled these trilobites to survive the Hangenberg cisis.

Finally, it must be emphasized that proetide trilobite larvae from the Famennian and Lower Carboniferous share a particular morphology lacking in other trilobites such as phacopids. Indeed, proetide planktonic larvae are very bulbous and smooth whereas, after metamorphosis, benthic ones are slightly vaulted and possess well defined furrows that are clearly homologous with major adult features. In contrast, earliest larval stages of phacopid trilobites are disc-shaped and of low relief, already provided with adult-like anteriorly diverging proto-cephalic axial furrows; these forms might well have been already adapted to bottom life excluding a long preceeding planktonic period. We speculate that the absence or short duration of any planktonic larval stage was the ultimate cause of the extinction of the Phacopida at the end-Devonian Hangenberg Event.