SPINY TRILOBITES - FORM AND FUNCTION THROUGH 300 MILLION YEARS

Marginal and dorsal spines, of varying length and form, evolved numerous times in trilobites, often in distantly related taxa. These must have served important functions as they were physiologically expensive to secrete, caused problems during molting, increased drag, impeded enrolment, and tangled with objects on the sea floor. Functions suggested for these spines, other than protection, include: sexual display/species recognition, ‘snowshoe' support on soft substrates, increased apparent size, camouflage, gas exchange through the exoskeleton, and form drag to provide support in the water column for pelagic species. Marginal spines usually spread horizontally or slightly ventrally to maximize ‘snowshoe' support and minimize drag by staying within the boundary layer, where current flows are reduced. Long dorsal and marginal spines often turn backward distally so that when the animal moved forward, or faced into a current, form drag was reduced in favor of less forceful friction drag. Examples of Ampyx specimens, all oriented in the same direction, in beds where other less spiny trilobites occur in random orientations, suggest that some taxa with very long spines in relation to their size had limited choice over orientation in a bottom current. One oft-repeated pattern is the presence of much longer marginal (macropleural) spines on some thoracic segments, frequently towards the middle and back of the thorax, usually when the animal is micropygous. These long spines were able to function as ‘snowshoe' support but also could have been raised by bowing the thorax to present a sharp deterrent to a predator approaching from behind. Long pairs of pygidial marginal spines probably functioned in the same manner. Spines were particularly common on trilobites during the latter halves of the Cambrian and Ordovician and the first half of the Devonian, coinciding with radiations of new groups of predators (respectively: arthropods and worms, cephalopods and starfish, fishes and ammonoids). Using spines for protection in an arms race with a predator was only successful in the short term, as comparatively smooth trilobites always outnumbered spiny trilobites. Highly specialized spiny trilobites were at high risk during episodes of mass extinction, explaining the numerous examples of iterative evolution of such spines in the Trilobita.