FUSED OR HYPERTROPHIED BRACHIALS: THE UNUSUAL ARMS OF THE PETALOCRINIDAE (SILURIAN-DEVONIAN, CRINOIDEA)

Ambulacra on crinoid free arms are typically underlain by multiple brachial plates arranged in a distally projecting repeated series. Where arms branch, a single brachial plate (axillary) typically underlies the bifurcation. The Petalocrinidae has unique arms comprised of only two brachials. The first is a typical brachial, which is short and non-axillary. However, the remainder of the arm is a single, subtriangular or cylindrical second primibrachial plate. The ambulacra branch multiple times on this second primibrachial. Is this unique arm construction the result of fusion of arm brachials or hypertrophy of a single second primibrachial plate? This question was tested using polarized light microscopy. Thin sections were prepared on second primibrachials of Petalocrinus inferior, Petalocrinus stenopetalus, and Spirocrinus circularis. Thin sections were oriented from proximal to distal roughly along the plane of the ambulacra in Petalocrinus specimens and through the elongate dimension of an arm of Spirocrinus. With the nichols crossed, most of each second primibracial has unit extinction, indicating a single plate. However, the distal edge of brachials have undulatory extinction. This suggests that this region of the brachial plate has incompletely fused distal brachials and supports the interpretation that the unusual arms of petalocrinids were formed by fusion of brachial plates.