Graptolithina

Article By:

Berry, William B. N. Museum of Paleontology, University of California, Berkeley, California.

Mitchell, Charles E. Department of Geology, State University of New York, Buffalo, New York.

Last reviewed:January 2020

DOI:https://doi.org/10.1036/1097-8542.298400

A group of marine organisms that were common in the early Paleozoic (the Late Cambrian to Early Devonian periods). Graptolites became extinct in the late Paleozoic Era. They were minute animals that built communal skeletons. Each graptolite colony contained from two to many hundreds of separate graptolite animals (referred to individually as zooids). All of the zooids within a single colony (a rhabdosome) were formed by asexual budding from the founder zooid, which probably grew from a fertilized egg. Thus, each colony started with a sexually produced animal and then enlarged by budding new zooids from one another. The result was a spreading rhabdosome composed of a few to many hundreds of minute tubes (thecae), each containing its own zooid (Fig. 1). Graptolite thecae exhibit a range of diameters (30 μm–2 mm), but are most commonly about 0.5–1.0 mm in diameter and several times this length. The thecae consist of the protein collagen, which is organized into two main building blocks. The most prominent are the fuselli, which have the form of narrow half-rings stacked one upon the other like a set of semicircular bricks. Collectively, the fuselli formed the main substance of the thecal tube. The second building block was deposited on the outside of the theca in the form of bandagelike strips or thin continuous sheets (Fig. 2). Also built of collagen fibers, this second layer (the cortex) is much thinner, but probably added greatly to the strength of the theca.

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