INVERTEBRATE ZOOLOGY IN THREE VOLUMES - VOLUME 3 - H.I. Shcherbak - 1997

DEUTEROSTOMIA

PHYLUM HEMICHORDATA

CLASS PTEROBRANCHIA

FOSSIL PTEROBRANCHS

Representatives of modern pterobranchs are exceedingly rare in the paleontological record: Rhabdopleura is known from the Cretaceous, and Cephalodiscus from the Ordovician. Most extinct species belong to fossil orders that until recently were grouped into a distinct phylum — Graptolitha (Graptolites, Fig. 166, c–e). Recent studies have shown that graptolites are an artificial group comprising various pterobranch taxa.

The vast majority of graptolites are known from the middle Cambrian to the early Carboniferous. They are preserved solely as skeletons composed of a Chitin-like substance or tanned Proteins (Scleroproteins). Graptolite colonies, or rhabdosomes, originated from a specialized chamber called the sicula, which was more or less conical in shape. The colony attached to the substrate by its pointed end, while a hollow thread extended from the expanded end, along with developing daughter thecae. The walls of the theca consisted of two layers: the inner layer was formed of transverse plates connected by a zigzag suture (as in modern Rhabdopleura), and the outer layer consisted of plates oriented along the longitudinal axis of the body. This Structure provided both strength and flexibility to the colony.

The Morphology of rhabdosomes was highly diverse. Benthic colonies could be unbranched or branched; star-shaped, spiral, reticulate, and other colony forms are known. Some rhabdosomes attached to floating objects, such as Algae, forming a so-called pseudoplankton. True planktonic colonies also existed, for instance, Lagonographus. At the center of a planktonic colony was a specialized structure — a pneumatophore (floating bladder) with a thin chitinoid wall, which was presumably gas-filled. In other planktonic species, a float served in place of the pneumatophore: a flattened structure with double reticulate walls stretched between the Branches of the rhabdosome. It is believed that sessile graptolites inhabited shallow coastal waters where clay sediments accumulated. It is in these deposits that the majority of graptolites have been discovered.