Aquatic, terrestrial and aerial invertebrates, with a segmented body and jointed legs (hence the name) covered by chitin. Growth takes place through moults, so the chitin skeleton shows no growth lines.
subphylum TRILOBITOMORPHA (Cambrian to Permian)
In addition to the trilobites, this subphylum includes the class TRILOBITOIDEA, which are largely represented by fossils from the Middle Cambrian Burgess Shale of British Columbia. They appear to be a varied group, but to have similar appendages to trilobites.
class TRILOBITA (Cambrian to Permian)
Marine arthropods with a dorsal skeleton divided longitudinally into three lobes. The skeleton is also divided laterally into three parts: the head shield (cephalon), the thorax (which shows distinct segments) and the tail (pygidium). Each segment carried a pair of ventral appendages for locomotion and respiration. The appendages are not often preserved fossil, as their covering is not reinforced by calcite like the dorsal skeleton. Many trilobites were deposit feeders, living on the sea floor. Common trilobite traces include Cruziana (a linear trail with oblique scratch marks) and Rusophycus (an excavation of a resting place).
subphylum CHELICERATA (Cambrian to Present)
Terrestrial and aquatic arthropods, with the front pair of appendages developed as pincers. The body is divided into a cephalothorax (with six segments) and an abdomen (with up to twelve segments).
class MEROSTOMATA (Cambrian to Present)
Aquatic arthropods which include the king-crab (Limulus) and its relatives, and also an extinct group, the eurypterids, which include a species of Ptcrygotus which is over 1.8m long and is the largest known arthropod.
class ARACHNIDA (Silurian to Present)
Air-breathing chelicerates, which include the scorpions and the spiders.
subphylum CRUSTACEA (Cambrian to Present)
Arthropods with two pairs of antennae in front of the mouth. Body generally covered with a hard carapace. Respiration by gills. Some classes are rare or absent as fossils, and are omitted here. The cycloidea (Carboniferous to Triassic) have limpet-like shells and are perhaps related to the Crustacea.
class BRANCHIOPODA (Devonian to Present)
Crustacea in which the carapace may form a single dorsal plate or a bivalved shell usually about 5mm long. The branchiopods (literally gill-feet) use their limbs for breathing, and also for swimming and feeding; most swim or crawl; some burrow in mud. Many prefer fresh or brackish water to normal marine environments.
class OSTRACODA (Cambrian to Present)
Small, bivalved Crustacea with a calcified carapace hinged along dorsal margin, covering a body with few segments. Freshwater or marine; mostly benthic. Benthic ostracodes do not have a pelagic larval stage, and thus many species are unable to cross deep water.
class COPEPODA (Miocene to Present)
Elongate segmented carapace with prominent central articulation. Copepods are the most abundant of modern marine animals, but known early forms were all lacustrine.
class CIRRIPEDIA (Silurian to Present)
Adults permanently fixed, often with calcareous plates (the barnacles).
class MALACOSTRACA (Cambrian to Present)
A varied group of highly developed Crustacea; usually with a thorax of eight segments and an abdomen of six or seven segments, most of which bear appendages. Includes the decapods (lobsters, crabs and shrimps), the isopods and the amphipods. Mostly shallow marine, but some freshwater or terrestrial.
subphylum MYRIAPODA (Silurian to Present)
The centipedes and millipedes; terrestrial arthropods. The head has a single pair of antennae.
subphylum HEXAPODA (Devonian to Present)
The insects develop wings and have six walking legs. They are all air breathing.
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