Classes of the Phylum Arthropoda (currently in Atlas)


What is an arthropod?
The most abundant group of invertebrates to ever inhabit the planet, arthropods were highly prolific in the Ordovician and remain the most diverse and numerous invertebrates today.  Arthropods can inhabit marine, aerial, and terrestrial environments.  They typically have jointed appendages and are characterized by ecdysis, or growth by molting.

Arthropods comprise 75 to 80 percent of the more than one million described modern animal species. Key modern arthropods groups include the insects, crustaceans (ex., crabs and lobsters), and arachnids (ex., spiders and scorpions).

Physical attributes
Arthropods are incredibly diverse in their forms and habitats. However, all are characterized by the presence of a segmented body, typically with jointed legs. Their body is covered with a chitinous exoskeleton.  In some groups (such as trilobites and ostracodes) this exoskeleton is reinforced by calcite, resulting in high preservation potential. The appendages of arthropods have highly diverse functions and are generally modified to the environment in which they live. Arthropods also undergo the process of ecdysis, or molting, in which the animal sheds its exoskeleton in order to accommodate new growth.

Why are arthropods so successful?
Arthropods have been a highly successful phylum throughout their evolutionary history due to a variety of advantageous attributes. They have an external, multi-layered skeleton, which acts as a form of protection against predators, UV radiation, and other harmful agents. This exoskeleton is also an anchor for muscles, allowing for greater strength and mobility. Additionally, arthropods have short life spans which promotes many generations in a short period of time.  This facilitates rapid adaptation to environmental changes and open niches.

Because the chitinous exoskeleton of arthropods has generally low preservation potential, the fossil record of most clades is best documented within deposits of exceptional preservation (Lagerstätten).  Some famous arthropod-dominated Lagerstätten include the Burgess Shale and Chenjiang Biota from the Cambrian, the Mazon Creek deposit from the Pennsylvanian, the Solnhofen deposit from the Jurassic and the Green River and Florissant deposits from the Eocene.

Which arthropods can be found near Cincinnati?
Trilobites are particularly numerous in the Cincinnati strata.  Fragments of trilobite carapaces occur in all layers, but complete trilobites occur mainly in the fine-grained shale layers.  Ostracods, crustaceans with calcite valves generally only a few mm in length, also occur throughout the Cincinnati region.  Megalograptus, a raptorial eurypterid, is also known from the Richmondian strata.