Eurypterids were the pre-eminent predators of the Silurian seas. Resembling giant water scorpions, they typically measured 5 to 17 inches, but the largest version (Pterygotus from the late Silurian) reached 6 to 8 feet in length. The eurypterids terrorized the early vertebrate denizens of shallow seas, estuaries, and lakes. In addition to pincers on some of the preoral appendages, the eurypterid could flex its thorax up and forward, like the terrestrial scorpion, to attack with its spiny telson, which was probably poisonous.
Eurypterids first appear in the early Ordovician, peak in number and diversity by the late Silurian, and then decline through the Carboniferous. They meet their ultimate demise along with the trilobites and 95% of all extant species during the great Permian extinction. Early eurypterid fossils occur with marine invertebrates during the Ordovician, but the Silurian, they appear in sediments from brackish lagoons and bays. By the Pennsylvanian period, they have moved to freshwater swamps as seen by associated plant fauna as well as freshwater mussels and snails.
To understand the rise and decline of the eurypterids and their change in territories, one must understand their prey. The most primitive of vertebrates, the jawless fish or Agnatha, appeared and flourished in the Silurian period. These primitive fish lacked lateral fins and were consequently slow moving, sitting ducks for a larger predator. The Agnatha were adapted to bottom dwelling habits, scraping the bottom and sucking up food (referred to as benthonic). The Agnatha developed strong external armor formed from bony plates, resulting in the alternate name of ostracoderms (shell skins). The armor was the only defense against the ravages of the eurypterids. These fish congregated in shallow waters and were ultimately followed by their predator. The acanthodians and placoderms were armored fish that developed mobile jaws and paired lateral fins. The fins allowed much faster swimming to elude predators and also allowed adaptation to larger bodies of water such as oceans and thus evade the more confined and less mobile eurypterids. As these new prototypes dominated in the Devonian period, the eurypterids began a steady decline in number of species and of individuals, never to recover.
Eurypterids belong to the phylum Arthropoda and the superclass Chelicerata. Their closest living relative is the so-called living fossil,the horseshoe crab (Limulus Polyphemus). In analogy to the eurypterids, the horseshoe crab undergoes multiple moulting stages, mates and deposits eggs in shallow bays like the Chesapeake, and can survive out of water in the intertidal flats.
Arthropods outnumber all animals by species, by individuals, and by prospects for evolutionary continuation. Some 80% of known animal species are arthropods, most of these insects. Arthropods first appear in the early Cambrian 550 to 600 million years ago. Rapid development of disparity resulted in wild experimentatioik of arthropod forms in the Cambrian. The Burgess Shale in British Columbia contains fossils of more than 20 distinct arthropod designs, yet only 4 basic designs survive the extinction at the end of the Cambrian. Of the four superclasses, the trilobites were extinguished at the end of the Permian. The remaining three are the insects (Hexapoda), the crustaceans, and the chelicerates. Spiders and scorpions are terrestrial chelicerates (Aracnida), while the eurpterids and horseshoe crabs belong to the Merostomata or aquatic chelicerates. Like all chelicerates, the eurypterids have 6 pair of appendages. The first pre-oral appendage has small pincers or chelae. The next four appendages are for walking, and the last appendage functions as a paddle. Different genera may be distinguished by the shape of the cephalic shield. The thorax is composed of several segments which terminate in a spiny tail or telson, which is conjectured to have been poisonous. Of course, we have no proof of poison and the hypothesis was suggested by analogy to the terrestrial scorpion which it most resembles. Figure I shows the reconstruction of a typical euryptid. Most fossils represent only the moulted exoskeleton and the number of moults may have varied by species, although 9 may be average. Most trilobite fossils are also moulted exoskeletons. The eurypterid has two large compound eyes on the lateral dorsal carapace along with two small simple eyes in a mor medial position.
Eurypterids were first discovered in great quantities during canal construction at Lockport, New York in the early nineteenth century. These fossils are quite rare otherwise, with few whole specimens. The most prolific collecting occurs in the Bertie limestone formation in western New York State near Buffalo (which includes the aforementioned canal site). The Bertie limestone has produced most of the known complete specimens of late Silurian age. The most common species from this formation is Eurpterus remipes lacustris. Pterygotus is the most infamous genus by virtue of its size,- 6 to 8 feet, which makes it the largest arthropod ever.
Reconstruction of a typical eurypterid, Baltoeurypterus tetragonophthalmus (Fischer)
showing the terminology used in this paper. (Adapted from Treatise on invertebrate
paleontology, R.C. Moore, ed., part P, Arthropoda 2:25, 1955.)
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Chelicerata by L. Stormer in Treatise on Invertebrate Paleontology. R.C. Moore (ed.). University of Kansas Press, 1955.
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Fossils of Ontario Part 3: The Eurypterids and Phyllocarids. M.J. Copeland and T.E. Bolton. Royal Ontario Museum Publications: Toronto, 1985.