Pennsylvanian Period Ammonoid Populations Across the Midcontinent and Appalacian Basins

Finding a fossilized Ammondoid in the Appalachian Basin is a rare occurrence. I’ve managed to find only a handful over a large number of trips. I have recovered a single one from the Brush Creek limestone, and four from the Pine Creek limestone in four years of collecting. Mapes et al (1997) discussed this in detail in chapter 2-2 of Pennsylvanian Cephalopods of Ohio. They reported that “Of the more than 60,000 Pennsylvanian-age invertebrate fossils in the available Ohio collections, only 276 specimens are ammonoid cephalopods”. Yet in the midcontinent, workers have collected 500 to 1,000 ammonoids in a single day.

While nobody can know for sure why there were so many more nautiloids than ammonoids, the prevailing theory has to do with the environment at the time. The globe below shows a view of the Earth as it looked 300 million years ago. While this is a simple cartoon illustration, it was based on a mature determination by scientists who study the rocks and their magnetic position. When a rock forms from lava, a signature of the earth’s magnetic field at that time locks into the rock as it cools. While a rock may shift position after this, many of them do not, especially huge flood basalts. By measuring the embedded magnetic fields, scientists have worked backward to determine the position of the Earth’s tectonic plates.

The landmasses of Earth, 300 million years ago. The modern United States position is overlaid on the map with Pennsylvania colored yellow. Note that Pennsylvania is located south of the equator at this time.

Confirming the Plate Positions

Of course, there is no way to know for sure. Unless you were able to capture light strong enough to see the Earth at a position 300 million light-years away from us, true confirmation is impossible. But science uses measurements, and the combination of these measurements can tell a story. Scientists measure modern rocks and find that the grains within a point to magnetic North. The seafloor spreads under the Atlantic Ocean, and measurements made from the point of spreading and all the way to the coast confirm the theory. Local measurements also help. In places of marine limestone deposits, we interpret this as evidence of an ancient sea. We know that limestone forms in warm shallow seas, so the water was warm, shallow, and it had to be seawater. In places where mountain ranges rise, there was a plate collision that caused the bedrock to rise to great heights.

In western Pennsylvania, the environment was flat and had active deposition of sediments in the Late Carboniferous. The layers of limestone, sandstone, and shale tell the story of a changing world, where sea levels rose and fell over millions of years as water became locked up as ice in the South. We know about the ice because glaciers leave evidence, with deposits of much larger stones with striations on them. This is due to dragging the rock across the landscape inside a glacier.

CG-0317 - Order: Goniatitida? — CG-0317 – Order: Goniatitida? An unknown ammonoid, the latest and fifth ever collected by the author in four years of searching.

CG-0358 – Order: Goniatitida? Another unknown ammonoid. Shell ornament is visible in the top right of the figure. The pitted appearance of the shell is common at the locality in which these are found.

So What About the Ammonoids?

So in the midcontinent, the ocean existed as the Atlantic and Pacific exist today. There were likely continental shelves at the edge, but the water was deeper and the space was larger. It seems that the ammonoids preferred this environment, and only rarely would travel to the shallow warm sea inlets that formed when the ice melted and the seas rose. This would explain the lack of local ammonoid finds. The animals either could not hunt, were more hunted themselves, or both. Nautiloids seemed to flood these niches, they are common in rocks here.