Prior to the beginning of the Silurian Period, there were extensive mud flats in the Rochester area. This mud solidified to form the Queenston shales which are exposed in the Genesee River Gorge and also in the Niagara River Gorge.
The oldest Silurian rock exposed in the Genesee Gorge is the Grimsby sandstone which has evidence of the burrowings of ancient sea worms.
As the Silurian inland seas became deeper, the environment became more favorable for animal life. At times, populations of small shellfish were very dense and their remains occasionally constitute a fossil "coquina," or shell rock, consisting of millions of fragments of shells. The "pearly shell" limestone layer of the Sodus shale is a splendid example.
The inland seas over the Rochester area fluctuated frequently in depth as the land masses of the region rose and sank over periods of many thousand years. At times, there were brackish water mud flats and shallow lagoons of water so polluted with hydrogen sulfide and iron and lacking in food and oxygen that few forms of animal life could exist. The black Williamson shale is an excellent example. Its predominant fossils are graptolites that can be used to correlate similar rock layers in Sweden, Scotland and Bohemia. The Furnaceville hematite is another example of the product of a polluted water environment. In this instance, the fossils represent a dwarf fauna.
Later in the Silurian period, the seas became deeper and the water much clearer and favorable for underwater life. Brachiopods, the "moss" animals (bryozoa) and corals were abundant along with a variety of gastropods, cephalopods and ostracods. Mollusks (Pelcypods), which are very common marine animals of today, were uncommon in the Silurian Period. It was in the late Silurian Period that ancient "sea scorpions," Eurypterids, left fossil evidence.
Near the end of the Silurian Period, forces deep within the earth's crust lifted the land in Western New York above sea level though deep inland seas existed to the east.
Much later, at the start of the Middle Devonian Period, a warm, shallow sea again covered the region. Extensive populations of crinoids ("sea lilies," and brachiopods carpeted the sea floor. Lung fishes, considered the first air breathers, joined the armored fishes. These curious lung fish are considered the stock from which higher vertebrates have evolved. Coral accumulations were building up to great size in the Devonian though not as great as the reefs formed in other parts of the world. Many small coral deposits can be found in the Le Roy-Batavia area.
The rock layers in the southern part of the Genesee Country particularly in the Letchworth Park area and the southern half of Canadaigua and Keuka Lakes are shales, siltstones and sandstones that were formed from muds and sands deposited by streams, often in deltaic deposits, that flowed westward from higher lands to the east. Fossils are not common, but include pelecypods, brachiopods, gastropods and cephalopods. Occasional plant fragments can be found.
If a scuba diver could go backward over 350 million years in time and explore the shallow inland seas that covered this region, the underwater world would have been a marvelous sight rivaling the scenes that one finds today in the warm seas of the world. Though one cannot go backward into time, one can see the magnificent, expertly displayed restorations on undersea life of the Silurian and Devonian Periods in the Genesee Region in the Rochester Museum of Arts and Sciences on East Avenue.
Fossils are either the remains of animals and plants, or the impressions made by them which are preserved in rock. Fossils may be classified according to their kind of fossilization. Fossils may be actual remains, such as insects in amber, frozen mammoths, shells and shark teeth, or the original organic structure may be replaced partially or completely by mineral matter. Petrified wood is an example. Other fossils are molds of the exterior or interior form, or mineral casts of the interior structures. Some fossils are only impressions of carbonized plants and leaves.
The majority of fossils to be found in Western New York are of the invertebrate type; i.e., "lower animals" with no backbone, although some primitive fish and plants have been found.
PORIFERA, or sponges (fig. 11a. Hydnoceras). These are multicellular animals which secreted a skeleton of horny fibers or very small needle-like spicules (fig. 11b Siliceous spicule network of a recent glass sponge). Sponges usually are poorly preserved and occur in varied shapes and sizes, rendering them difficult to recognize. The most common fossil sponge impressions found in Western New York are impressions of siliceous sponges.
COELENTERATES, or corals. These animals usually had very tiny barrel shaped bodies (polyps) which secreted a calcium carbonate shell. During its lifetime the coral formed hard calcareous partitions inside its shell. With these partitions and an upward growth of the outer shell wall, the coral moved upward and sealed off its lower portion, adding to the strength and size of the shell. The corals were of two types, an individual animal such as the common "cup" or "horn" coral (figs. 11c & d), and a group of animals living together, such as the "honeycomb" coral (fig. 11e) where hundreds of individuals joined together into a "colonial" coral.
Two of the common corals found in the area are the "horn" coral (figs. 11c & d), Heliophyllum, and the "honeycomb" coral (fig. l1e).
BRYOZOA or moss animals. These are small colonial animals which secreted about themselves a calcareous protective and supporting structure. This structure is joined together in many ways to form chains, lace-like fans and rounded branches. They are often found encrusting stones and shells. Each tiny hole in the structure was the home of a microscopic individual. Most common of the bryozoa are the Fenestellid (fig. 11f), a fan-shaped, lace-like colony.
BRACHIOPODS. These bivalve shells are quite common fossils in Western New York. They occur in many shapes and sizes, from one quarter of an inch to over two inches. The two valves are different in size and shape, distinguishing them from pelecypods. The calcareous shell may be smooth surfaced or corrugated by radial or concentric lines. The hinge line, where the two shells are joined together, varies greatly in length and has much to do with the shape of the shell. An example of the wide hinge line is in the Mucrospirifer mucronatus (fig. 12a);and of a short hinge line, the Athyris spiriferoides (fig. 12b). The interior of one valve is seen in fig. 12c. Common brachiopods are Atrypa (fig. 12d), Spirifer (fig. 12e), and Rhipidomella (fig. 12f).
PELECYPODS, or clams. These mollusks are composed of a calcareous bivalved shell. The two valves of the shell are complementary and in most species nearly similar in size and outline. Each valve of the shell is a mirror image of the other. This helps differentiate between the pelecypods and the brachiopods. In order to obtain a mirror image of the brachiopod shell, a slice through the middle of each valve would have to be made, as both valves are of unequal shape and Leiopteria is a fine example of a local fossil clam (fig. 13a).
GASTROPODS, or snails. Usually found coiled in a left or right handed spiral shape, sometimes conical or saucer shaped. The outer shell layer is composed of calcite while the inner one, the pearly layer, is of aragonite. The gastropod shell normally is not well preserved as a fossil, due to the relatively soluble aragonite. Many Gastropod fossils are fillings (cores) of the shell interiors for this reason. Platyceras (fig. 13b) is one of the common gastropods found in Western New York.
CEPHALOPODS. These differ from the gastropods in that the interiors of the shell are divided into compartments by septae, the animal living in the open outer compartment. The pearly nautilus, octopus, squid, and cuttlefish are living representatives of this class. The extinct cephalopods had either straight, curved or coiled calcareous shells. Both straight and coiled fossil cephalopod shells are found in Western New York. An example of each type respectively are Michelinoceras (fig. 13c) and Tornoceras (fig. 13d).
TRILOBITES. These are extinct marine Arthropods with a variable number of segments, each of which carried two pairs of appendages; one pair for crawling or swimming, another pair acting as gills. They varied in size and shape, from a fraction of an inch to over two feet in length. The body was covered by a dorsal shield divided into three parts: the cephalon (head, fig. 13e), bearing large compound eyes; the thorax, composed of numerous segments which were articulated and allowed enrollment; and the pygidium, (tail, fig. 13f), rounded or pointed. Several types are found in this area, the most common being Phacops rana (fig. 14a). Two others,not quite as prevalent, are Greenops boothi (fig. 14b) and Dalmanites limulurus (fig. 14c).
CRINOIDS. Commonly known as "sea lilies." These marine invertebrates have the appearance of a plant. Covered with calcite plates (fig. 14d) and jointed arms, they lived in depths ranging from shallow reef waters to over 2100 fathoms. Their length ranges from a fraction of an inch to a fossil specimen over sixty feet in length. The stem was made up of circular plates or discs (fig. 14e) stacked in a column, which in turn supported a globular or cup shaped body with radiating arms. At times the crinoids were so abundant in the sea bottom that their skeletal remains make up a major part of the rock of some formations.
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