|Graptolite fossils from Presque Isle|
In a low outcrop not far from downtown Presque Isle, the pale rock feels gritty, but not coarse. This type of sediment is not the product of a deep ocean or a beach, but the sweet spot in between. It's born in an area where prevailing winds could blow off the warm top layers of an ocean, and reveal the cold nutrient rich waters below. Perhaps inconsequential to you or me, but life changing to a graptolite.
Certain shards of the rock slab host the shallow impressions of graptolite rhabdosomes, basically an apartment building for tiny animals called zooids. In Silurian time the complexes either rooted themselves in coastal sediment or floated in masses at the surface. The zooids may have reached out of the many holes in these colonial homes to grasp at and eat phytoplankton from the coastal waters, enriched by the upwelling of vital nutrients.
In a time when Caribbean-like islands stood sentry on the coast of North America habitats like this would have been commonplace in Maine. But, Maine was changing. Three hundred eighty million years ago the tectonic block that serves as our current coast, Avalon, docked with Maine. The conjunction crumpled the graptolites' ocean habitat. Driving fossil types of graptolites from fourteen in the Silurian to one after the collision. Eighty million years later the jaws of Pangea snapped shut. The huge landmass was inhospitable to the aquatic creature and by the time it had formed the graptolites were extinct.
The watery Silurian period was ideal for graptolites. The changing environment drove them out of Maine, and then to extinction. A sad moment, perhaps. But, the transition to dry land had some advantages, too. Plants left the coasts, invading Pangea, and in the process became trees. Amphibians bravely abandoned their shrinking watery habitats and evolved into reptiles. Graptolites didn't make it, but maybe that's okay. Perhaps their loss is our gain.
Dickson, Lisa, and Robert D. Tucker. Maine's Fossil Record: The Paleozoic. Augusta, ME: Maine Geological Survey, Dept. of Conservation, 2007. Print.
Koren', T. N., and R. B. Rickards. "Extinction of the Graptolites." Geological Society, London, Special Publications (1979): 457-66.
"Graptolites." Common Fossils of Oklahoma. Sam Noble Museum. Web. 20 Dec. 2014.