Certain fossils from the Ordovician and Silurian Periods of the Paleozoic Era resemble saw blades sketched on the rock in pencil.  As it turns out, these fossils appear all over the world.  Not only that, certain types of the fossils appear only during well-defined time periods, and are invaluable tools for dating and organizing rock strata from those periods. They are so important, that an entire lexicon of terms has been invented to describe their morphological characteristics.

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These peculiar fossils are the remains of graptolites, colonial animals that either floated as macroplankton in the ocean or lived benthically, attatched to the sea bottom.  Each colony, or "rhabdosome", was composed of one or more long bodies, or "stipe"s.   The stipes of planktonic species were arranged radially; the stipes of benthic graptolites had a dendritic habit.  The individual organisms, or "zooids", lived in little cups or "thecae" strung out along the stipes.  Each zooid was a clone (produced, presumably, by budding) of a nearby zooid, and presumably all of the zooids were descended from a single original organism, or "sicula"  (presumably produced sexually) which lived in a theca at the root of the rhabdosome.

    _     _     _     _     _     _     _     _     _     _     _     _     _
_.-' \_.-' \_.-' \_.-' \_.-' \_.-' \_.-' \_.-' \_.-' \_.-' \_.-' \_.-' \_.-' \
\_.--'\_.--'\_.--'\_.--'\_.--'\_.--'\_.--'\_.--'\_.--'\_.--'\_.--'\_.--'\_.--'
/ `--./ `--./ `--./ `--./ `--./ `--./ `--./ `--./ `--./ `--./ `--./ `--./ `--.
 `-._/ `-._/ `-._/ `-._/ `-._/ `-._/ `-._/ `-._/ `-._/ `-._/ `-._/ `-._/ `-._/

A planktonic rhabdosome's most important function was to keep its members floating by the plankton they fed on, and various species evolved different techniques to keep from settling to the sea bottom, and death.  Although some appear to have grown gas-filled bladders attatched at the center of a wheel of stipes, the most common adaptation was to reduce the number of stipes, so that by the mid-Silurian, single-stipe rhabdosomes that floated as a big conical coil had evolved.

421 million years ago, at the end of the Wenlock epoch of the Silurian, a period of rapid global cooling followed by rapid global warming caused a mass extinction that killed off 95% of all graptolite species.  A few dendroid forms lived on until the Mississippian Period.  Although graptolites had no hard parts to fossilize, if one fell into the right type of anoxic mud after death, its remains would turn into a thin carbonized layer in the shale formed from that mud.  Stipes would resemble saw blades with the thecae as the "teeth".  Just the right thing for a paleontologist to dig up and ponder several hundred million years later.

The graptolites' closest modern relatives appear to be the Pterobranchs, which closely resemble dendroid graptolites. Not only that, British pterobranch expert Peter N. Dilly recently showed that pterobranchs exhibit a type of exoskeleton found elsewhere only on graptolites, and claimed that pterobranchs were "living graptolites".

Another group, the Enteropneusta or acorn worms, has been associated with graptolites. Originally, the acorn worm's dorsal nerve chord made scientists think that it was a primitive ancestor of vertebrates, and so they and the graptolites were placed relatively close to the Urochordata (tunicates) and Chordata (vertebrates).  However, it was later shown that acorn worms are more closely related to echinoderms than chordates, and so they, the pterobranchs, and the graptolites were moved into their own phylumHemichordata.

In 2001, Claus Nielsen of the University of Copenhagen's Zoological Museum claimed that acorn worms really aren't related to the graptolites and pterobranchs at all. Nielsen separates the acorn worms into their own phylum Enteropneusta and elevates to another phylum, Pterobranchia.

Grap"to*lite (?), n. [NL, Graptolithus, from Gr. is engraved, written ( to write) + stone.] Paleon.

One of numerous species of slender and delicate fossils, of the genus Graptolites and allied genera, found in the Silurian rocks. They belong to an extinct group (Graptolithina) supposed to be hydroids.

 

© Webster 1913.

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