Grappling with Graptolites: Paleozoic "Sawtooth" Index Fossils —Are you Jelly?

      Graptolites (or more precisely Graptolithina) are extinct colonial marine animals that are an excellent index fossil for parts of the Paleozoic period; individual species denote well-defined time frames within, especially, the Ordovician, Silurian and Devonian periods. The fossil remains present mostly as shiny, graphite-looking sawtoothed pieces. A big paleontology mystery up until the 1980’s was what these pieces represented. Early descriptors, including Linnaeus, were mystified by the “writing on the rock,” considering even hieroglyphics as a possible origin. He included them in “inorganic mineralizations and incrustations which resembled actual fossils.” In 1768’s, Systematic Naturae he included one species as a possible plant fossil. He also included a figure of a "fossil or graptolite of a strange kind.”

           These serrated, delicate, detailed fossils, represent about 200 million years from the Cambrian through the Pennsylvanian portion of the Carboniferous. 

           These useful Index fossils denote rocks to within a million year period within the Paleozoic.  The Ordovician was a particularly prolific time for graptolites, with rapid changes in morphology. In addition, early graptolites were sessile but rapidly evolved to be floating animals. Their fossil remains are found in shale and clay stone. Another prolific Ordovician index fossil, conodonts or conodont elements, are found mostly in limestone and other CaCO3-rich environments.{ More on conodonts during our next post.}

      Paleontologists use them to correlate the age of rocks, particularly mudstones or shales, based on changes in the serrated blades. Oil and gas geologists found graptolites particularly helpful in correlating borehole samples throughout a geologic basin. But, we didn’t have fossilized portions of the entire animal, just the shiny sawtooth bits.

            The rather shocking discovery of soft parts of the animal revealed graptolites resemble present-day jellyfish. These floating colonial animals had a proto-neural tube. But, only the floating strands of detailed serrated portions in the fossil record led to many incorrect hypotheses of their origin. Now, we believe graptolites looked like a an odd-shaped jellyfish as seen in these artists’ renderings:

              This rendering is even more bizarre:

      

      From long, shiny bit of graphite-looking straight shapes

  to the more elaborate swirling shapes, graptolites gave geologists a solid index fossil for use in oil and gas correlation between oil wells, long before we knew what the whole animals looked like.

The type 

locality for the Ordovician is located here in the U.K. As Ordovician. Note also, Silures, type locality for the Silurian, is located just to the south. {The Silurian was a time of great silence on the earth; almost no volcanic activity has been recorded during this Ordovician-adjacent time.}

Have you seen either graptolites or conodonts in rocks in the field? Odd that these two most unusual creatures would be the basis for so much geologic correlation, without knowing what the whole animals looked like. The Ordovician was a quite wild ride.

Happy New Year to all visitors to Partial Ellipsis of the Sun;we are glad you are here!

To 2022, with Great Expectations for Groundhog Day, 2/2/22! 

Steph

Bonus: Looking ahead: Here’s an image of  conodont elements in an SEM image as well as an artist rendering of the elements in the inside the mouth of that Ordovician beast, the stuff of nightmares. Conodonts, microfossils varied and extraordinaire, will be featured in our next post.