Article: The nematularium of Pseudoclimacograptus scharenbergi (Lapworth) and its secretion

The nematularium of Pseudoclimacograptus scharenbergi (Lapworth) is a three-vaned structure derived from its hollow nema. The apex of the structure is sealed. The vanes are solid, lack a thickened rim, and comprise thin, irregular lamellae that parallel the semicircular vane outline. Each lamella consists of a body of anastomosing fibrils overlain by a dense outer pellicle. Although the thecae have a bandaged cortex, the vanes lack cortical layers. The nematularium is strikingly irregular in shape and lamella geometry compared to P. scharenbergi thecae. The structure of the nematularium is inconsistent with its secretion by an enveloping epithelium but is explained well by the pterobranch model. Both the nema and the nematularium were probably secreted externally to soft tissue (nematocaulus) confined within the lumen of the nema. A similar mode of secretion could have produced most of the other structures derived from the nema of planktic graptolites. This 'naked' nematularium could not have added to the colony's buoyancy but would have added preferentially to the viscous drag forces that slowed its rate of sinking. Thus, the nematularium and a variety of other structures evolved by planktic graptolites may have helped these graptolites to maintain their preferred depth in the oceans.