THE MORPHOLOGY OF THE UNUSUAL FUSED COLUMNAL COMPLEX IN THE STEM OF BROCKOCYSTIS

The stems of glyptocystitoid rhombiferans are conservative throughout their history from the Middle Cambrian to the Late Devonian. In nearly all species the stem is divided into a proximal and distal portion with a gradational transition. Proximally, the stem is heteromorphic with inner and outer columnals that taper distally. The distal portion of the stem is homeomorphic with barrel-shaped columnals, and in most groups the stem is not affixed to the substrate by a holdfast. In Brockocystis, the transition between proximal to distal stem is abrupt and marked by a large, conical, fused element that is unknown in other glyptocystitoid taxa.

A large number of these Brockocystis fused columnal complexes were collected from the Middle Silurian Brassfield Formation of central Kentucky. These conical columnal complexes show a great deal of variation in external morphology, ranging from short and wide to tall and narrow. A principle component analysis shows that all of the shape variance of these elements is drawn from a single morphological population, rather than representing different species. In all cases the proximal end is wider with a sunken facet where the distal end of the proximal stem articulates. The distal end bears a small facet for articulation of the distal stem. The lumen is somewhat narrow and tapers distally.

Details of the internal morphology were studied in thin sections cut parallel to the axis of the element through the lumen. These thin sections show that the fused columnal complexes are formed from inner and outer columnals arranged similarly to the proximal stem. The inner columnals are thin and of uniform thickness throughout the element and are confined to the edge of the lumen. In most specimens, the distal few outer columnals bear greatly thickened flanges that wrap around the more proximally positioned inner and outer columnal pairs, suggesting that columnals are incorporated into the structure proximally. In one of the sections, flanges from all of the outer columnals were fused into the structure. It is unclear what the function of the fused element is, but it may relate to isolating the proximal and distal portion of the stem.