INVESTIGATING ONTOGENY IN THE EOCENE CRINOID CONOCRINUS THORENTI

Crinoids are considered the most basal of living echinoderms and understanding their development is key to unraveling echinoderm development as a whole. The stalked bourgueticrinids are commonly accepted as paedomorphic comatulids from both molecular and morphological data, and recent work conducted on stalked isocrinids and stalkless comatulids has shown that many traits retained in juvenile isocrinids and comatulids are characteristic of adult bourgueticrinids. The group therefore has unique potential for conveying information on juvenile and basal traits of the Articulata, the only crinoid clade to survive the P/T extinction, and their divergent origin in the Paleozoic. However, despite the studies conducted on stalked isocrinids and stalkless comatulids, little has been done on modern bourgueticrinids due to their deep sea habitat. Fossil bourgueticrinids are more accessible, but few studies have attempted to constrain where their variation might be ontogenetic vs a new species, and of the studies conducted, most focused on the change of a single feature between species rather than within.

Conocrinus are modern stalked crinoids with a fossil record stretching back to the Eocene. They have been found in multiple locations, and are a well-defined taxon, being characterized by high basal plates and low radial plates, discreet sutures, fused basal plates, and a conical to sub-cylindrical shape. However, large collections of specimens show a wide range of variation in theca shape. A recent acquisition of over 280 C. thorenti theca from the Eocene in northeastern Spain demonstrate this disparity and provide a substantial number for a morphological study focused on ontogeny. Here we examine the relationships of basal plate height vs basal plate width, radial plate heights vs basal plate height, diameter of central cavity, and overall theca height vs theca width for potential changes during growth. Changes in height vs width followed a linear pattern while other features such as the shape of the theca showed no strong link to theca size. The identification of key features and better understanding of bourgueticrinid ontogeny may lead to insights into early Mesozoic crinoids development and potentially clarify the murky waters surrounding species identification.