Collagen in the pterobranch coenecium and the problem of graptolite affinities

Amino-acid analysis of the extracellular skeleton (coenecium) of two pterobranchs (Hemichordata) Rhabdopleura normani and Cephalodiscus (C.) hodgsoni shows that both contain considerable quantities of collagenous material with relatively high hydroxyproline and low hydroxylysine levels. The appearance of the fibrous material in the skeleton of pterobranchs, although collagenous, differs from standard EM characteristics of collagen. The identification of the collagenous nature of the pterobranch skeleton with the presumed presence of collagen-like material in the periderm of fossil graptolites, taken in conjunction with other data, supports the hypothesis that both groups may be closely related phylogenetically.