It’s the summer, 320 million years ago. Deep underwater, a tiny tentacle-bearing creature retreats into its spiral shell. Suddenly, a tremor! Organisms scurry for their lives. Soon after, a huge submarine landslide shakes the sea floor all around. Who will survive? Why, fossils of course!

Image: 1. Nautilus, a modern distant relative of goniatites

Fossil goniatites have an fascinating story to tell. Technically defined as members of the order Goniatitida, goniatites are extinct cephalopods ranging from the middle of the Devonian to the end of the Permian.

Related to squid and cuttlefish, these animals were very diverse in terms of size, shape and sculpturing. They had an external shell, divided internally into several chambers. The animal resided in the last formed chamber, the body chamber.

The distribution of goniatites in space and time is better known than their biology. In fact, the pearly Nautilus is the only known living descendant of externally shelled cephalopods. But, however tempting it may be to draw palaeoecological inferences from Nautilus, it must be noted that it is not phylogenetically closely related to goniatites.

Goniatites are a prime tool in biostratigraphy, since they evolved rapidly and achieved widespread distribution. The fossil remains of these marine-dwelling organisms are mostly found concentrated in dark shale horizons in the Carboniferous Shannon Basin, Western Ireland.

Here, the organic-rich shales contain a mixed fauna, including goniatites and bivalves. Each band has a distinctive faunal assemblage, usually with a dominant goniatite species, that allows correlation within the basin.

For instance, the index fossils employed in the biostratigraphy of the Ross Sandstone, which crops out mainly in the Loop Head Peninsula, Co. Clare, Ireland, are: Phillipsoceras paucicrenulatum, Phillipsoceras circumplicatile, Vallites henkei and Homoceratoides prereticulatus.

My current research in Trinity College Dublin aims to develop a detailed and coherent biostratigraphic framework for the Ross Sandstone Formation, a succession of deep-water turbidites which has proven to be an instructive analogue for oil and gas-bearing sequences elsewhere.

Anthea Lacchia

Ph.D. student, Department of Geology, Trinity College Dublin

@AntheaLacchia

anthealacchia.wordpress.com

Nautilus image source: http://en.wikipedia.org/wiki/Image:Nautilus_profile.jpg