Class OSTRACODA

Although ostracodes are a minor zoological group, they are often found as fossils in large numbers. Although they occur in a variety of normal marine to freshwater environments, they typically characterize physically stressed environments such as hypersaline or brackish habitats. Ostracode species are often long-lived limiting the usefulness in biostratigraphy.

Ostracodes have a bivalved carapace enclosing the body which is usually calcified. Similar to pelecypod bivalves, the left and right valves of ostracodes are joined on the dorsal margin by a flexible ligament. The interior of the ostracode valve often supports muscle attachment scars which are of two types: (i) the centrally located adductor muscle scars, and (ii) the dorsally located operator muscle scars. The operator muscles control the various appendages of the ostracode animal. The exterior of an ostracode valve may be smooth or ornamented with various spines, knobs, or pits. Apart from the differences outlined above, ostracodes differ from bivalves in that they, the ostracodes, are usually smaller and do not have growth lines in their shell (recall arthropods secrete their exoskeleton at once and not by incremental growth).

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. The skeleton of merostomes is characterized by three main regions: (i) the prosoma which is a fused cephalon and thorax combined, (ii) the opisthosoma which incorporates the abdomen and (iii) the tail spine called the telson. A unifying character among cheliceratids, all merostomes have two prosomal (preoral) pincer-like appendages called chelicerae for gathering and crushing food. The prosoma also have up to five pairs of walking legs behind the chelicerae. 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 (see Figure 3 below). 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.

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Figure 3 - General Eurypterid Morphology

From McRoberts (1998)

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Eurypterids have two pairs of eyes on the prosoma: the smaller, centrally located ocelli, and (ii) the larger lateral compound eyes. The telson are often found as disarticulated remains. The eurypterids have one pair of large appendages which served as swimming paddles behind the uniramus walking legs. The prosomal appendages can clearly be seen on the following image. Examine closely this well preserved, and rather complete specimen.

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. The 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.

ETC...

Very rarely, other non calcitized arthropods are preserved as fossils. The hexapods (including the insects), which are the most diverse superclass of any phylum, very rarely are fossilized due to their delicate exoskeleton.