Signs of Summer 3: Ancient Ecosystems Underfoot

H. Harder, Wikimedia

One of the Big Ideas in Biology that has grown clearer and more compelling to me over the years was very well expressed by the German biologist Ernst Haeckel back in the Nineteenth Century: “Nothing is constant but change. All existence is a perpetual flux of being and becoming.”

Organisms change, ecosystems change, climates change, genes change, proteins change: change is the common currency of an individual’s life experience and also the essence of all extended and interconnected life experiences. Last week, my daughter Marian and her friend Lee Drake were here for a few days on a short, summer break. Marian was on her way from Albuquerque to a month at a research station in Uganda, and Lee was taking a much needed respite from his extensive worldwide business traveling. They headed out one afternoon to go look for fossils and came back with a car load of shale from a road cut just north of Ford City.

We spread the rocks out on a table down in the basement and began to brush and clean the pieces. The fossils were stunning! I found my fossil books and we started putting names on some of the specimens. We also made inferences on the age and type of the shale layer that they were from based on the types of fossils that were present. Lee described the crunching feel of fossil-rich shale debris underfoot as they poked and explored the fresh road cut for specimens.

Pecopteris (D. Sillman)

There were three main types of fossils: a tree fern fossil called Pecopteris, a seed fern fossil called Neuropteris, and sphenopsid (“horsetail”) called Calamites. The abundance of these three plants helped us to determine that these rocks were part of the Mahoning shale layer. This shale was laid down about three hundred million years ago during the later (“Pennsylvanian”) portion of the Carboniferous Period. This site in which these shales formed was a swampy forest located on a delta plain near the coastline of one of the world’s incredibly extensive oceans (painting above by H. Harder). Now this ecosystem and its location does seem quite a change from the present day mixed hardwood forests that dominate our landlocked Western Pennsylvania, but there were other differences that were even more overwhelming!

Neuropteris (D. Sillman)

To begin with what we now call “Pennsylvania” was attached to a massive continental assemblage that included not only North America but also South America, Africa, Europe, Asia, Australia, Antarctica, and India. This was the giant continent called “Pangea,” and it contained all of the major land masses of the Earth! All of the rest of Earth was one, continuous ocean!

Further, Pangea was located far down in the southern hemisphere! “Pennsylvania” was on its northern coast and was located very near the equator! The climate here, then, was warm and tropical and had no “sun-seasons” of summer, fall, winter, or spring. Adding to the warmth of the climate was an atmosphere enriched in carbon dioxide (40% higher carbon dioxide levels than today) which trapped heat in an exaggerated greenhouse effect. Atmospheric oxygen levels were also much higher (35% of the atmosphere was molecular oxygen compared to 20% today). So each breath of air was different! Further, we were rotated about ninety degrees away from our present orientation (so our current “west” was our ancient “north”).

Calamites (D. Sillman)

The formation of Pangea came about via the collision of the many smaller continents that each moved along on their respective tectonic plates. When these continents collided they forced masses of continental materials in between them to fold upward thus making great mountain ranges. The range near “Pennsylvania” was one of the greatest lines of mountains ever formed on Earth. The present day Himalayas are thought to be similar to this ancient, amazing up-thrust of rock and crust. These mountains were/are the Appalachians which even as they were rising began to erode to form the rocks, gravels, sands, and silts than made up the soils out of which our ancient swamp forests grew. The steady flow of these eroded materials extended the ancient shoreline out into the shallow oceans and stretched and grew the swampy forests further and further away from the mountains.

The trees of these forests were also different. There were no flowering plants on Earth and there were no deciduous trees. The trees of this ancient swampy forest were giant versions of our present day ferns and horsetails. Fifty foot tall fern trees (like Pecopteris) and one hundred foot tall horsetails (like Calamites) formed an upper canopy layer with a ground cover of other, lower growing fern and fern-like species (like Neuropteris). The warm, wet climate encouraged plant growth and the high, atmospheric carbon dioxide levels accelerated it even more. Also, the dead plants did not readily decay so the fallen branches and trunks and fronds built up in the forming soils and their eventual sedimentary rocks. This carbon accumulation was the source for most of our present day coal, and oil, and natural gas deposits (they were the fossils of our “fossil fuels!”). These ancient swampy forests are often referred to as “coal forests” or “coal swamps.”

And, just to complete the picture of change, the Earth three hundred million years ago was spinning faster on its axis than it does today. So a “day” was only 23 hours long!

We took some of the pieces of shale outside and laid them next to one of my red maples. I have collected Pecopteris before from some of the shale exposed on the down-slope to the west of my house, so adding these new fossils to the site seemed appropriate. We stood under the red maple tree and listened to the Carolina wrens singing and chattering up in the branches and watched two gray squirrels chase each other across the yard. Three hundred million years ago there were no birds or mammals, there weren’t even any dinosaurs (they wouldn’t be around for another seventy million years!). The only land dwelling vertebrates were early amphibians (sometimes their fossilized jaw bones are found in this shale) although the first reptiles with their revolutionary, self-contained eggs were possibly just evolving. There were terrestrial insects, though, some of which that had attained immense sizes because of the high atmospheric levels of oxygen. Dragonflies with two and a half foot wing spans, eight foot long millipedes, and three foot long scorpions flew and scuttled about in the warm, wet coal swamps.

The ancient forest would have been noisy with its giant insects and there might have even have been a scent of the ocean in the thick, warm air. A thunderstorm was approaching and we went inside. The rain fell on the fossils in the shale and began their slow erosion into soil.

“There is nothing permanent except change” (Heraclitus).