Hemichordates are soft-bodied animals that are close relatives to chordates. They possess a structure similar to a notochord, as well as several derived chordate features: a pharynx with multiple openings, a dorsal nerve cord, and a ventral blood vessel (Prothero, 1998, p. 333). Although they possess an elongated, bilaterally symmetrical body, which is very different than echinoderms radial symmetry, there is evidence to suggest that hemichordates are a sister group to the echinoderms. Some hemichordate larvae are very similar to echinoderm larvae. DNA studies also suggest a close relationship to the echinoderms (Waggoner, 1997, UCMP Berkeley). The most familiar hemichordates today are the acorn worms (class Enteropneusta). Acorn worms have a worm-like body with a proboscis used for burrowing and feeding. Graptolites (class Graptolithina) are the most important hemichordate fossil.

Index Fossils

Graptolites are fossil colonial organisms. The name graptolite is derived from the Greek. Grapto- means inscribed or written and -lite means stone (Borror, 1988, pp 43 & 54). This name comes from the fact that graptolite fossils reminded people of hieroglyphs inscribed in stone. Most graptolites are preserved as carbon films on Paleozoic black shales. They look like tiny hacksaw blades. Graptolites first appear in the Cambrian and become extinct in the Carboniferous. Long before anyone new the biology of the graptolites they became the most important index fossils for Ordovician and Silurian strata. Graptolites evolved rapidly and many were pelagic animals floating on oceans worldwide, feeding on plankton.

In 1948 Koszlowski determined the nature of graptolites by extracting three-dimensional specimens from cherts. These specimens were imbedded in paraffin and sectioned. A graptolite colony (rhabdosome) is divided into branches (stipes) that support many cup-like tubes (thecae). Thecae house individual organisms called zooids. At the base of the rhabdosome is a sicula that represents the zooid, which founded the colony. A long spine-shaped structure called the nema extends from the sicula and, in life, functioned as an attachment to some floating body (Prothero, 1998, p. 335).

Science Olympiad Fossil Event

The 2016 Science Olympiad Fossil List includes the class Graptolithina (Graptolites) in the Phylum Hemichordata.

Graptolites
Location Unknown

Bibliography

Borror, D.J. (1988). Dictionary of Word Roots and Combining Forms. California: Mayfield Publishing Company.

Prothero, D.R. (1998). Bringing Fossils to Life: An Introduction to Paleobiology. New York: McGraw-Hill.

Waggoner, B. (1997). Introduction to Hemichordata. California: UCMP Berkeley