Reflecting the Past in the Present: How Studying Living Crinoids Can Shed Light on Their Fossil Record

Abstract

Crinoids are one of the few clades on Earth that have both a well-documented fossil record and extant taxa with a similar morphology. Extant stalked crinoids are morphologically similar to fossil members from the Mesozoic onward, and provide an excellent study group for modern to fossil comparisons with the caveat that their deep bathymetric range left them relatively inaccessible until the last half century. With access to submersible technology, studies can now address questions from the fossil record, such as why stalked crinoids became restricted to depths greater than 100 meters, how largely sessile stalked crinoids orient themselves for feeding or protection, and what physical characteristics are useful to delineate stalked crinoid species. Long-term in situ observations, field experiments, and morphological data from a large population of Democrinus (suborder Bourgueticrinina) off Roatán, Honduras, were used to explore the ecology, functional morphology, and taxonomy of this stalked crinoid genus. Major areas of interest in this thesis are assessing predation as a causal factor in the bathymetric restriction of stalked crinoids observed in the fossil record, mobility in stalked crinoid clades, and a general assessment of the taxonomy of modern Democrinus. The post-Mesozoic bathymetric range restriction of stalked crinoids has long been attributed to an increase in predation intensity during the Mesozoic Marine Revolution. If predation was a determining cause, then we would expect to see that reflected in the intensity of predation on extant crinoids across bathymetric ranges. Stalked crinoids, limited to bathyal environments should encounter predators less frequently, and thus, face a lower predation intensity than feather stars found in shallow water environments. Documentation of predation intensity in the Roatán Democrinus population revealed that individuals encounter predators nearly 60 times less often than reported previously for shallow-water feather stars, confirming that the predation intensity is much lower at greater depths. Other stalked crinoids, i.e., isocrinids, are now known to be able to crawl and relocate, but this ability may be morphologically limited. Individuals from the population of Democrinus were dislodged, observed, and no individual was found to relocate from its original placement. However, all dislodged individuals regained a vertical upright posture, something previously thought impossible because of the absence of muscle in their stalk. Quantitative assessment of four biomechanical models showed that all were insufficient as the mechanism by which Democrinus might regain a vertical posture; it is likely instead that the tissues of the ligaments within the stalks have a contractile ability, even in the absence of muscle. Examination of submersible-collected specimens from the Roatán Democrinus population and comparison to over 350 existing specimens, photographs, and drawings of known Democrinus species revealed significant differences in the Roatán population, which is designated a new species accordingly. This study also more precisely define certain features of the cup and brachials, and establishes that the attachment structure, is likely associated with environment rather than phylogeny. This assessment of defining characteristics was used to amend the diagnosis for the genus Democrinus and the species D. rawsonii, and has implications for the reassessment of other extant and fossil species. Injury and regeneration rates cannot be determined from a static fossil, nor can fossil organisms’ responses to challenges, such as changing currents, be observed. Thus, studying extant crinoids to investigate aspects of their ecology, morphology and evolutionary history provides insights inaccessible from the fossil record.