March 8. Graptolites

Graptolites are another group of animals that got started during the Cambrian, and survived the end Cambrian extinction to expand dramatically during the Ordovician.

Ordovician graptolites.
Photo by Steve Henderson,
used with permission.

Graptolites are a class of the phylum Hemichordata, a fairly obscure phylum that includes modern acorn worms and pterobranchs, which are worm-like filter feeders. There are only a handful of these surviving members of the Hemichordata, but there are hundreds of graptolite species.

Their fossils are distinctive, little linear branches that look like they have sawteeth along at least one side. Some are short single segments, some branch into V or Y shapes or more complex fan-like arrangements, and some are organized into spirals. Most of them are only a few centimeters long, but that may be because larger colonies broke apart after death.

I just said colonies – and they were colonial, like corals. That means that the individual animal elements aren’t really independent, and can’t survive apart from the colony. The individuals, called zooids, were tiny, almost microscopic, connected by a thin nerve-like structure. The whole colony was probably planktonic, floating on the sea surface or in the upper waters – and that made them incredibly widespread, which in turn makes them excellent index fossils world wide for identifying precisely what part of the Ordovician you’re in. You might not know the time to an accuracy of a million years, but you can know with incredible accuracy that you’re in the Pendeograptus fruticosis zone – or whatever – and where that zone is in relation to the other parts of the Ordovician. We’d call that very precise relative dating of rock layers.

They’re called graptolites – which means “writing on rock” – because the broken, flattened segments are often fossilized in ways that look like hieroglyphs or other bits of writing in the rock.

Graptolites were incredibly abundant during the Ordovician, and were decimated by the end Ordovician extinction, but survived. They finally died out during the Early Carboniferous, called the Mississippian in the United States, around 315 million years ago. So they had a good run, about 180 million years or so. It isn’t clear exactly how the modern pterobranchs are related to graptolites – as descendents, or cousins or maybe even actually representing surviving graptolites.

I’d also like to express my thanks for the shout-out by Helena, Montana, earth science teacher Rod Bensen. You can check out his blog with excellent resources for science teachers. 

—Richard I. Gibson

Photo: Ordovician graptolites from Womble Shale, Arkansas. Photo by Steve Henderson, used with permission.

http://en.wikipedia.org/wiki/Graptolithinia
http://www.princeton.edu/~achaney/tmve/wiki100k/docs/Graptolite.html
http://discovermagazine.com/1993/jul/itsaliveanditsag249#.UxUKzYWKKAw
http://www.ucmp.berkeley.edu/chordata/hemichordata.html
http://museumvictoria.com.au/discoverycentre/infosheets/marine-fossils/graptolites/