STRUCTURE AND CONSTRUCTION OF MACHAERIDIAN SCLERITES

The Lower Ordovician Kanash Shale from the Great Basin has yielded a large number of well preserved machaeridian (Plumulites sp.) sclerites exhibiting traces of microstructure. Together with primarily sclerites of Turrilepas from the Silurian of Gotland and previously known information the emerging picture of sclerite structure and construction is more inclusive and may also help to shed light on the group’s affinity.

Machaeridian sclerites possess at least two calcite layers where the thin outer layer is produced by lamellar deposition along the growing margin in contrast to the thicker inner layer consisting of calcite elements radiating from the umbo, a feature clearly visible in the Kanash material. This inner layer seemingly grew by addition and incorporation of new elements as the sclerite widened. The distinct granulation universally seen on the inner surfaces of the sclerites is a feature of the inner layer as well, and in well preserved sclerites the granulation appears tightly aligned with the radiating elements. Marginal spines when present do not directly appear to be formed from either layer, but may rather be a third component enveloping the margin and producing the doublure found on the inner surface, especially evident in turrilepadid sclerites.

A possible homolog to the pattern found in machaeridian sclerites could be that of the problematic taxon Multiplacophora whose sclerites seem to posses two calcite layers. The inner layer displays structures similar to the radiating elements of machaeridian sclerites along its margins where they protrude beyond the edge of the overlying outer layer. Although larger they would seem to indicate a similar type of construction.