Buchtel College of Arts and Sciences (BCAS)
Date of Last Revision
3370:497 Honors Project in Geology
Number of Credits
Bachelor of Science
Date of Expected Graduation
The crinoid skeleton is characterized by a complicated, highly porous microstructure known as stereom. Details of stereomic microstructural patterns are directly related to the distribution and composition of connective tissues, which are rarely preserved in fossils. However, certain portions of the crinoid skeleton have never been studied with respect to stereomic microstructure. In particular, spines are common features on the crowns of many Paleozoic crinoids but had not previously been studied in detail with respect to stereomic microstructure. This study focused on pirasocrinid cladid crinoids, a common group with numerous identifiable crown spines, including spines on the arms and anal sac. The specific goals of this project were (1) to describe the microstructure of anal sac spines and interpret aspects of soft-tissue anatomy; and (2) to describe biologically relevant stereomic patterns associated with regeneration of broken spines.
Pirasocrinid anal sac spines consist of three major zones: the primary spine shaft; a sculptured region with short, meandering protuberances; and an articular ridge, where the spine articulated to the rest of the anal sac. The spine shaft is characterized by dense stereom indicating little interpenetration by connective tissues. The protuberances of the sculptured zone are equally dense, but the intervening valleys are characterized by labyrinthine stereom, suggesting articulation by short ligamentary fibers. The complex sculpturing of this region may reflect an increase in anchorage points for connective tissues that did not penetrate into the interior of skeletal plates. The articular ridge is characterized by dense crenulae, suggesting little penetration by connective tissues, and valleys indicating penetration by short ligamentary fibers, similar to the sculptured zone. This indicates that spines were articulated at their bases to the rest of the tegmen by a combination of physical interlocking of dense crenulae and articulation by ligaments in valleys.
Planes of regeneration are similar in both anal sac and brachial spines. Major patterns include (1) significantly larger pores between rods of stereom than the dense rectilinear stereom pattern present on unbroken or fully regenerated spines; (2) large, triangular outgrowths of stereom that eventually coalesce to form a layer extending outward from the regeneration plane; and (3) concentric “sheaths” of triangular stereomic outgrowths at similar stages of development throughout the entire diameter of the spine. These suggest that spine regeneration involved a complicated set of processes operating across the plane of regrowth rather than from the interior to the exterior.
James Thomka, Ph.D.
Honors Faculty Advisor
Smith, Hannah and Thomka, James, "Scanning Electron Microscope Study of Microstructure and Regeneration of Upper Pennsylvanian Cladid Crinoid Spines" (2019). Williams Honors College, Honors Research Projects. 998.