Arthropods Arthropods are the most diverse phylum today and probably also in the geologic past. They are a highly specialized group which are characterized by their bilaterally symmetrical body, paired appendages and a chitinous (calcite in some groups) exoskeleton. As the exoskeleton once produced remains inert, arthropods must periodically shed their exoskeleton during molting and re-precipitate a larger one in order to accommodate their larger size.

TRILOBITOMORPHS Like other arthropods, trilobitomorphs are characterized by numerous jointed and paired appendages. The calcitic and/or chitinous exoskeleton of trilobites consists of three lobes: a central axial lobe, and two lateral pleural lobes the exoskeleton of trilobites can be divided lengthwise into three regions: a fused head segment called a cephalon; a fused tail segment called a pygidium which sometimes bears spines, and a mid region or thorax consisting of numerous segments.

Paleoecology and Life Habits Trilobites are very common in marine limestones and shales of the early Paleozoic, especially from the Cambrian Period. Most trilobites were epifaunal crawlers. Although they occupy a wide variety of exclusively marine habitats, specific life habits are difficult to discern by morphology alone. Nonetheless, several aspects of trilobite morphology can indeed provide some clue as to the life habit or activity. Examples include the elongated cephalic shield of the example which may have aided in ploughing through sediments. Although most trilobites are considered to have been benthic, the small size and non-descript morphology of agnostid trilobites suggests that these (along with some others) may have been nektonic or nekto-benthic.\ Enrolling of trilobites may certainly have been a defensive mechanism.

Taxonomy According to some texts, trilobites are considered to have phylum status and are divided into eight Orders. A less radical classification treats trilobites as a Superclass or Class with two orders: the Polymerida and the Agnostida. The Polymerida are by far the most diverse of the two in regards to species diversity and also morphologic and ecologic types. The Polymerids can be identified by their larger size, a well defined cephalic region with eyes and facial sutures, and a large number of thoraxic segments. An easier way to identify Polymerids is by default; if its not a agnostid (easy to identify) then it's a Polymerid. All of the images you have seen thus far are Polymerids, here is another example from this diverse group. Agnostid trilobites are easily recognizable by their small size, few thoraxic segments (usually around two), and a cephalon without eyes which is superficially similar in morphology to the pygidium. Furthermore, agnostids lack facial sutures.

Superclass CRUSTACEA Crustaceans are a diverse group of Arthropods that include familiar forms such as barnacles, crabs, and shrimp. They occur in a wide variety of marine and fresh-water habitats from the deep sea to ephemeral lakes and streams. Apart from the ostracodes and barnacles which have a calcite exoskeleton, most crustaceans have a limited fossil record. Crustaceans are characterized by having two pairs of antennae and gills and additional biramus appendages.

Superclass CHELICERATA The Cheilicerata include all arthropods with pincer-like chelicerae which aid in food gathering and/or locomotion and uniramus appendages. Common chelicerate organisms that are still with us include scorpions, horseshoe crabs, and spiders.

Class MEROSTOMATA Merostomes are a numerically minor group both today and in the geologic past. Although only three living genera are known today, all belonging to the horseshoe crabs, the merostomes were only moderately diverse during the Early Paleozoic due to the abundance of eurypterid forms. Two orders you should know are the Eurypterida and Xiphosurida

Order EURIPTERIDA The eurypterids include small to large-sized merostomes with a scorpion-like exoskeleton Although they occur in a variety of normal marine to freshwater environments, they, like the ostracodes, are known from physically stressed environments such as hypersaline or brackish habitats. The eurypterids were especially common during the Silurian period and were known to occur in great numbers from the nearby Manlius Syracuse Formations.

Order XIPHOSURIDS The xiphosurids are characterized by a relatively large prosoma and partially fused opisthosoma. Although the common horseshoe crab Limulus is a typical Recent example, representatives are known as far back as the Cambrian. T he xiphosurids have apparently always been confined to near-shore marine environments. Like the eurypterids, xiphosurids have two sets of eyes: a compound pair and smaller ocelli.