EVOLUTIONARY TRENDS IN PALEOZOIC CRINOID CALYX COMPLEXITY

The crinoid calyx is a complex, multi-elemental structure composed of interlocking plates. The number of calyx plates is highly variable, however, ranging from 10 or fewer plates to more than 1000. Intriguingly, simplification of the calyx through reduction of the number of plates is a recurring pattern in many groups of crinoids. Some of the most striking examples of this trend occur in members of the diverse camerate family, Platycrinitidae, which includes well-known taxa like Platycrinites and Eucladocrinus. Although camerate crinoid calyces typically incorporate numerous fixed brachial and interradial plates, which may number in the hundreds, platycrinitids developed simplified calyces with as few as eight plates in total. Likewise, it has been recognized that the number of posterior plates in eucladids tends to decrease through time, particularly during the Mississippian. These parallel trends in calyx simplification across distantly related lineages represent an important component of crinoid morphologic evolution, yet little quantitative work has been done to characterize and understand this pattern.

We examined trends in crinoid complexity through time as measured by the total number of calyx plates. For each taxon studied, the number of plates was counted for each of the eight major components of the calyx (infrabasals, basals, radials, fixed brachials, fixed pinnules, interradials, and intrabrachials) and summed to determine the total number of calyx plates. Plate counts were compiled for Ordovician-Mississippian crinoids using specimens from the Smithsonian Institution National Museum of Natural History, the American Museum of Natural History, and published literature. A series of statistical analyses were conducted to identify trends in crinoid calyx complexity, focusing on changes in plate number (1) over the end-Ordovician mass extinction, and (2) through the Ordovician-Mississippian. Results provide a quantitative, multi-clade characterization of crinoid calyx complexity through time and can be used to determine whether calyx simplification is a ubiquitous and/or significant trend in crinoids. In addition, evaluation of plate number for different calyx components indicates heterogeneity in rates of morphologic evolution both among calyx components and across clades.