What is a Graptolite?

Graptolites  are a group of colonial hemichordates, long considered extinct, although recent systematic work  has suggested the group is equivalent to modern pterobranchs (Mitchell et al., 2010). Although the most primitive members of the group are benthic and attached to the sea floor, one lineage of graptolites became planktonic around ~490 million years (MY), the base of the Ordovician period. We refer to any members of this lineage as graptoloids, also known as the planktic graptolites or the Eugraptoloidea (as defined by Maletz et al., 2008). Graptoloids underwent rapid diversification in the later Ordovician, suffered immense losses during the Hirnantian mass extinction and radiated again during the Silurian period. The last few lineages of graptoloids finally died out around ~418 MY, in the early Devonian.  During their relatively brief ~80 MY  history, the graptoloids experienced extremely rapid taxonomic turnover, with over two thousand species observed in the fossil record.

In terms of their construction and physiology, graptoloid colonies were essentially a ‘floating bee hive’. Zooids, probably very similar to the small suspension-feeding zooids of Rhabdopleura,  were apparently very motile. The preserved portion of graptolite colonies is a protein-rich material secreted and molded into desired components by each zooid. The most basic component was a repeated half-ring to form tubes; this is the same construction seen in some pterobranchs colonies. Thus, graptolite colonies are constructed objects, like bee hives, termite nests, packrat middens or bird nests.

Above, a hypothetical reconstruction of what a portion of a graptoloid colony with living zooids may have looked like, from Crowther and Rickards, 1977.

 

 

To the left, a relatively complete three-dimensional specimen of Saetograptus from Maletz, 1997.

© David Bapst, 2013.