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Life in the Time of the Marcellus

Written by Jonathan Squibb | Apr 19, 2011 5:36 PM

While the Marcellus Shale is in the news today because of what it contains, it may be helpful to explain what it is.

The Marcellus rock layer is mostly shale that settled on the bottom of an inland sea around 390 million years ago in the Middle Devonian epoch, over a period of about 2 million years. Today’s Allegheny Plateau and the adjacent ‘ridge and valley’ portion of the Appalachians were at the time an area of very low elevation, termed the Appalachian Basin. While this area is most notable for the coal that formed from swampy forests about 90 million years later, the Marcellus was deposited at a time when the basin was filled with an unusually deep sea devoid of oxygen, somewhat like the Baltic Sea today.

The sea had deepened because North America was buckling under the force of Europe, which had just attached itself to Canada and Greenland. The upward bend that formed the new Acadian Mountains between the two continents produced a corresponding downward bend in the Appalachian basin. As the new mountains began to erode, they began to fill the Appalachian basin with sediment, starting in the area of the Catskills in New York and flowing to the southwest.

The Marcellus shale contains few large fossils because the depth and lack of oxygen in the sea kept it sparsely inhabited. However, the same dearth of oxygen also prevented the dead plankton and bacteria that fell to the seafloor from decaying; these organisms are still with us today in the form of natural gas trapped within the pores of the rock.

Those large fossils that are found in the Marcellus tend to be in small sub-layers of limestone that formed in brief periods of lower sea levels. Brachiopods, organisms that resemble scallops or mussels but are very different biologically than mollusks, were very common; the brachiopod genus Lingula, present in the Marcellus formation, survives to this day with only small alterations. Bivalve mollusks, snails, crinoids (‘Sea Lilies’) and trilobites such as the Pennsylvania state fossil Phacops rana lived on the seafloor. Above them lived shelled cephalopods: the nautiloids, whose modern-day relative is the nautilus; and the ammonites, which are extinct, but are more closely related to the modern octopus and squid.

Small fossils similar to those found in the Marcellus shale can be found at a public fossil-digging site along Bear Hole Trail at Swatara State Park near Grantville, PA. The exposed rock layer is brightly colored shale from the Mahantango Formation, which sits atop the Marcellus and is only a few million years younger.

Although Marcellus fossils consist mainly of creatures with shells, there were a wide variety of fish alive at the time; the Devonian period is often called “the Age of Fishes” because of their diversity. Most notable were the placoderms, armored fish with jaws. More heavily-armored placoderms shuffled along the seafloor, while more flexible species prowled the water above. Also present were acanthodians, nicknamed “spiny sharks” for their shape and the large spines that supported their fins. Though these now-extinct types of fish were dominant at the time, relatives of today’s fish had also appeared, including sharks, ray-finned fish and the lobe-finned fish that are the relatives of modern coelacanths, lungfish, and land vertebrates.

However, the most important changes to life on Earth in the Devonian period took place on land. At the beginning of the Devonian, 416 million years ago, land plants were tiny green stems without true leaves or roots, or were non-vascular plants like mosses and liverworts. By 390 million years ago when the Marcellus shale was formed, plants such as the Maine state fossil Pertica quadrifaria had grown to around 6 feet tall and had small leaflike branches to take in more sunlight. The first known forest, found in Gilboa, New York, dates to only 5 million years later and consists of the first known tree, Wattieza.

Wattieza grew to around 25-40 feet high and looked somewhat like a modern tree fern. Its trunk bulged outwards at the bottom to support its weight, as it lacked true roots. By the end of the Devonian 360 million years ago, forests were filled with the tree Archaeopteris, which had strong, deep roots and a sturdy trunk. Although plants still reproduced entirely by spores, which restricted them to wet lowlands, the growth of forests provided food for larger land animals and increased the oxygen in the atmosphere, producing long-lasting ecological and climate changes.

As for Devonian land animals, wingless insects and arachnids were common, feeding on the small plants and on each other. The first four-legged land vertebrates had previously been thought to have evolved near the end of the Devonian, the time of their earliest surviving fossil skeletons. However, in 2009 Polish researchers found the salamander-like tracks of a creature several feet long walking across a tidal mudflat, preserved in a 395 million year old rock.

One other unsolved mystery of the early and middle Devonian (and the preceding Silurian period) concerns a fossil called Prototaxites that defies categorization. At first sight it appears to be a petrified tree trunk up to 25 feet in height. However, it is not only older than the first trees, but under a microscope it reveals a fungus-like substructure of tangled fibers, rather than the orderly fibers of plants. The scientist who first identified the fossil in the 19th century mistook it for a rotten tree trunk; in following decades theories have variously identified the organism as an enormous alga, fungus or lichen towering over the tiny plants of the time. The most recent theory postulates that the fossilized columns are actually mats of small plants called liverworts that had rolled up like a carpet under the force of the wind. Under modern conditions this seems ridiculous, but primitive plants were not strongly rooted to the ground, and there were no trees to break the force of the wind.

So, whenever you turn on a gas stove, remember that what is cooking your dinner may have been alive at a time when the land itself was Earth’s great new frontier.


PHOTO: The brachiopod genus Lingula, present in the Marcellus formation, survives to this day with only small alterations. Scale line = 1 centimeter. (Credit: Laurie VanVleet, Ithaca City School District, from the collection of the Paleontological Research Institution [PRI], Ithaca, New York). View the original on Flickr.

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