MORPHOLOGY OF REGULAR ECHINOID SPINES USED TO INFER ENVIRONMENTAL PRESSURE AND ADAPTIVE CONVERGENT EVOLUTION

Echinoids, or sea urchins, are an extremely important component in both ancient and modern marine ecosystems. In the Modern, they occupy a wide range of morphological and ecological diversity, inhabiting waters from the equator to the poles, living in a shallow, tropical waters and depths of more than 5,000 metres. Echinoids evolved spines before the rigid test, the function of which likely played a major role in the evolutionary history of the group. Echinoids experienced rapid adaptive radiation in the Late Triassic and Jurassic, and radiated to fill a wide range of niches and morphologies. During this time, penta-radially symmetric “regular” echinoids evolved ever more elaborate spines shapes. These morphologies range from bulbous spines, some carrying a poison sack at the tip, to needle sharp spines which break and leave the tip embedded in the predator. Some forms carried very large, heavy spines which are hard to break and make the echinoid unpalatable. Though the function of spines has been nominally accepted as primarily for anti-predator defence, spines are also known aid in locomotion and camouflage in Modern groups. Therefore, the shape of the spine and internal structure in fossil taxa may also adapt in response to changing biological and abiological selective pressures in the Mesozoic to Modern oceans.

Here we develop a categorization of echinoid spine morphology, utilizing statistical methods to: 1) categorize spine into morphological groupings based on discretised morphological characters, 2) produce spine outlines using photographs for landmark geomorphometric analyses, and 3) apply a disparity analyses and generalised least squares model to detect correlations with spine morphology and habitat data, while accounting for the influence of phylogeny using phylogenetic PCA. We find spine shape and size to be highly variable within Mesozoic to Modern regular echinoid clades, with a high degree of convergence of general shape among disparate clades, reflecting the significant ecological and environmental selective pressure acting upon spine shape.