Ultrastructural Studies on Graptolites, 1: The Periderm and Its Derivatives in the Dendroidea and in <i>Mastigograptus</i>

Abstract

Organic skeletons of two dendroid graptolites and an aberrant sessile graptolite (<i>Mastigograptus</i> sp.), all Ordovician in age, were isolated chemically from the matrix and used subsequently for ultrastructural studies with the transmission electron microscope. Peridermal material of all forms investigated proved to be unusually well preserved as far as ultrastructural features are concerned, and it reveals a variety of fabrics and patterns classified into two structural categories: <i>fabric</i> and <i>tissue</i>. Following Kozlowski (1949) the term tissue is retained for larger structural components of the periderm, defined from a morphogenetic point of view. To distinguish the fusellar and cortical tissue, directly observed structures are classified as fabrics defined by the form of their unit elements and their spatial interrelations. <i>Fusellar, cortical, sheet</i>, and <i>crassal</i> fabrics were distinguished and characterized. Fusellar and cortical fabrics are both fibrillar but differ in fibril diameter. Fibrils are branched and wavy, or straight, producing a mesh, or showing a parallel arrangement and packed into layers. Sheet fabric is an electron dense, homogenous or densely reticulated material delimiting particular layers within the cortical tissue or producing an external pellicle on the fuselli. Crassal fabric is an electron dense and featureless material found in the sheath of stolons and as a secondary deposit inside the thecae in <i>Mastigograptus</i> sp. A given peridermal tissue is thus composed of more than one fabric, but with a predominance of either a fusellar or cortical one. In addition to an earlier observation (Towe and Urbanek, 1972) that cortical fibrils are collagen-like, arguments are presented in favor of the opinion that fusellar fibrils may also represent the collagen group of fibrous proteins. Frequently observed passages of a single fusellar fibril within the body of a fusellus, into a fibril of an outer lamella of the same fusellus, showing a typically cortical arrangement of the fibrils, seems to indicate the same chemical nature of the fibrous components in both fabrics. Differences in their physical organization are here ascribed to the changes in the composition of the matrix. The unexpected presence of a cortical coating on the inner thecal walls in <i>Dictyonema</i> sp. is explained through delayed growth of lateral thecae in the triad and very early deposition of cortical tissue over the outer surface of the young autotheca (<i>autocortex</i>). This wall is later overgrown by lateral thecae to become an inner thecal wall. A common cortical envelope secreted later over the outer surface of all thecae is called the <i>rhabdocortex</i>. The presence of a cortical component on the inner surface of the inner thecal walls in <i>Acanthograptus</i> sp. suggests, that at least in some dendroids also, the soft tissues inside the thecal cavity were capable of secretion of the cortical tissue. This changes the classical scheme of cortical tissue formation as proposed by Kozlowski (1949). The fusellar tissue in <i>Mastigograptus</i> sp. reveals unusual, erratic distribution of fuselli, which are present only on certain places in the thecal wall. Over large areas these are substituted by a peculiar layer of electron dense, homogenous material termed here the crassal fabric. This is interpreted as a presumed secondary resorption and subsequent substitution of the fusellum by the crassal layer. The basal disc of <i>Mastigograptus</i> sp. is composed of a material resembling that in cortical fabric but with fewer fibrils embedded in abundant matrix. The lower layer of the basal disc is provided with numerous filaments made of delicate fibrils. The sclerotized sheaths of stolons recognized in <i>Acanthograptus</i> sp. are made mainly of the crassal fabric.