Arthropoda (jointed leg)

Included within the phylum Arthropoda are lobsters, trilobites, ostracodes, barnacles, crabs, spiders, and insects. All are characterized by externally jointed legs. Their bodies are usually elongate; segmented; divided into head, thorax and abdomen; and are bilaterally symmetrical. A mouth and esophagus are located in the head and lead to a stomach, digestive glands, intestines, and anus. The anus is located at the rear of the animal. The central nervous system is composed of a brain that forms a ring a round the esophagus and two chains of nerve ganglia that extend along the lower surface of the animal. A valved, pulsating tube acts as a heart and circulates blood through an open circulatory system. Respiration is carried on by external gills or tracheae (internal cavities in the body, as in insects), depending upon the animal. Eyes are usually present, and are simple and/or complex (there may be many individual lenses in each eye). Reproduction is sexual. In order to grow, the animal must molt (shed) its exoskeleton (its outer protective covering) and a secrete a new, larger one. Molting mean s that one arthropod was capable of producing many fossils during its life time, and that growth stages are known for many fossil arthropods. Arthropods appeared in the Early Cambrian (ca. 520 million years ago) and persist in great numbers at present in a wide range of sizes, forms, and environments.

  1. Trilobita (class) - The body of trilobites was covered and protected by a hard shell that was divided lengthwise into three parts or lobes. "Trilobite" ("three lobed ") refers to these three longitudinal lobes. The middle or axial lobe separates two pleural (or lateral) lobes. Between the pleural and each axial lobe is a trough-like depression. All three of these lobes are segmented along the length of the animal. This segmentation allowed most species to curl up for protection. The shell is further divided into a head shield (cephalon), thorax (the segmented body), and a tail plate (pygidium). A pair of eyes is present on the head in most trilobites. The stomach lay immediately under the middle lobe of the head. A pair of jointed legs extended from each lateral segment of the thorax. These legs served a second function in respiration as gill-bearers. Other appendages of the trilobite induded the antennae and mouth parts. These parts and the legs are seldom preserved because they were not mineralized. Trilobites appeared in the Early Cambrian (520 million years ago) and became extinct at the end of the Paleozoic (245 million years ago). The first completely preserved trilobites, with legs, antennae, digestive track, and even muscle fibers, were found in the 18905 near Rome, N.Y.

  2. Eurypterida (order) - The typical eurypterid was an elongate animal with six pairs of appendages that protruded from the cephalothorax (the fused head-thorax portion). They resembled their living relatives, the horseshoe crabs. Phylogentically and anatomically, eurypterids are closely related to scorpions and spiders.
    The fossil remains usually consist of parts of the hard outer covering of: 1) the cephalothorax, 2) the laterally segmented abdomen, and 3) the segmented appendages. A large lateral pair of eyes and a pair of tiny medial eyes are on the cephalothorax. The anterior pair of appendages were modified into pinchers in some species; in others, they were greatly reduced. The following three pairs of appendages were walking legs. The fifth pair was a balancing apparatus. The usually larger, often paddle-like sixth pair was likely used in swimming, anchoring, or burrowing. Most eurypterids were bottom dwellers in shallow seas, but some lived in brackish and fresh water. Giant trackways known from sandstones in New Brunswick and lllinois are thought to have been produced by eurypterids that crawled across swampy land. Some of these arthropods reached a length of three meters (ten feet), but most were from ten to thirty centimeters (4-12 inches) long. They lived from the Late Ordovician (445 million years ago) through the Permian (245 million years ago), but were rare after the Devonian (345 million years ago). Fossils of these animals are more abundant and well represented in Late Silurian rocks in New York than anywhere else in the world. For this reason, the eurypterid, Eurypterus remipes, was designated the New York State Fossil in 1984.

  3. Ostracoda (order) - Ostracodes are among the smallest arthropods. Their body plan consists of a very small, shrimp-like body with eyes, legs, and tail housed within two shells (valves). They are very abundant in marine and fresh-water environments. Paleontologists use their remains for relative time correlation. Ostracodes appeared in the Early Cambrian (ca. 515 million years ago) and are common in modern oceans and lakes.

Next >> Echinodermata (spiny skin)


The New York State Geological Survey (NYSGS) is a bureau of
the New York State Museum in the New York State Education Department.