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

Urbanek, Adam, and Kenneth M. Towe. Ultrastructural Studies on Graptolites, 1: The Periderm and Its Derivatives in the Dendroidea and in Mastigograptus. Smithsonian Contributions to Paleobiology, number 20, 48 pages, 2 figures, 30 plates, 2 tables, 1974. —Organic skeletons of two dendroid graptolites and an aberrant sessile graptolite (Mastigograptus sp.), all Ordovician in age, were isolated chemically from the matrix and used subsequently for ultrastructural studies with the transmission election 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: fabric and tissue. 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. Fusellar, cortical, sheet, and crassal 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 Mastigograptus 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 Dictyonema 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 (autocortex). 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 rhabdocortex. The presence of a cortical component on the inner surface of the inner thecal walls in Acanthograptus 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 Mastigograptus 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 Mastigograptus 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 Acanthograptus sp. are made mainly of the crassal fabric. OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution's annual report, Smithsonian Year. SI PRESS NUMBER 5021. SERIES COVER DESICN: The trilobite Phacops rana Green. Library of Congress Cataloging in Publication Data Urbanek, Adam Ultrastructural studies on graptolites, 1: The Periderm and Its Derivatives in the Dendroidea and in Mastigograptus. (Smithsonian contributions to paleobiology, no. 20) 1. Graptolites. 2. Paleontology—Ordovician. I. Towe, Kenneth M. , joint author. II. Title. III. Series: Smithsonian Institution. Smithsonian contributions to paleobiology, no. 20. QE701.S56 no. 20 [QE840.5] 560'. 8s [563'. 71] 73-18097 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 • Price $1.35