Conodont, Graptolite, Hemichordata
conodont SYLLABICATION: co·no·dont NOUN: 1. A member of an extinct group of small primitive fishlike chordates, preserved primarily in the form of their conelike teeth. 2. A fossil tooth of this chordate. Conodonts are the most widespread Paleozoic microfossils and are important for biostratigraphic indexing. ETYMOLOGY: Greek knos, cone; see k- in Appendix I + –odont
graptolite SYLLABICATION: grap·to·lite NOUN: Any of numerous extinct colonial marine animals chiefly of the orders Dendroidea and Graptoloidea of the late Cambrian to the early Mississippian periods, whose fossil remains are often used to date the rocks of the Silurian and Ordovician ages. ETYMOLOGY: Greek graptos, written (from graphein, to write; see graphic) + –lite (from the resemblance of the fossils' impressions on shale to markings on a slate).
Hemichordata (hmkôr´´d´t) (KEY) , small phylum of marine invertebrates closely related to both the echinoderms (phylum Echinodermata) and chordates (phylum Chordata). Acorn worms, class Enteropneusta, are the most common hemichordates. The body is composed of an anterior, conical proboscis, a short collar, and a long, wormlike trunk. Gill clefts in the pharynx and a hollow nerve cord in the collar suggest relationship to chordates, while the ciliated larvae are similar to those of echinoderms. Hemichordates live in burrows or under objects in shallow water and feed on detritus. Balanoglossus gigas may reach a length of nearly 5 ft (1.5 m), but most species are considerably smaller. 1 Also in the phylum is the class Pterobranchia, whose members are tiny deep-sea creatures that form colonies by secreting a collagenous exoskeleton. They are considered more primitive than the enteropneusts. Pterobranchs have been identified with graptolites, hemichordates of the class Graptolithina (formerly believed to be extinct) since the discovery in 1992 of a living pterobranch identical to fossil graptolites.