ESCALATION AMONG FOSSIL MARINE ARTHROPODS: DID TRILOBITES LOSE THE ARMS RACE?

The escalation hypothesis explains patterns of morphologic change in several groups of fossil marine arthropods (decapods, malacostracans) through the Mesozoic and Cenozoic, but analogous "escalated morphologies" are not evident among trilobites, the predominant Paleozoic marine arthropods. This suggests at least two explanations: 1) predation pressure was not a driving force in trilobite morphologic evolution, or 2) the evolutionary response of trilobites to predation was swamped out by other constraints.

Trilobites, lacking appendages suited to the task, apparently did not play the role of shell-crushing predator. They were, however, cast as prey by cephalopod, fish, and other arthropod predators (e.g., Anomalocaris), so the first alternative seems unlikely. Enrollment and spines evolved presumably as anti-predatory responses. There is, however, no evident rhyme or reason to documented trends in trilobite enrollment and spinosity, despite the presumed increase in predation pressure throughout the Paleozoic. The number of thoracic segments, an aspect of trilobite morphology related to ecdysis, better predicts trilobite survivorship than do the presumed "escalated" responses. Trilobite ecdysis differed in pattern and process from ecdysis in other arthropods, and those attributes unique to trilobites may account for differences in escalation response. The competing selection pressures of predation and ecdysis, with their different optimal morphological responses, may have essentially randomized the evolutionary response of trilobites, precluding a morphological "end point" or optimal body plan in trilobite evolution.