Arthropods comprise the creatures with jointed legs, from fleas to crabs to the extinct trilobites to the extant but primitive horseshe crab. Since first appearing, probably during the Precambrian, their variability has been nothing short of astonishing, and their impact on the living earth enormous. Arthropods (Greek for jointed feet) comprise the largest phylum of animals and include the insects, arachnids, crustaceans, and many others as given in the table below. Approximately 80% of extant animal species are arthropods, with over a million modern species described and an extensive fossil record dating back to the base on the Cambrian. Because of their beauty and their extensive availablity in the fossil record, the trilobite is the unequivocal favorite among fossil collectors. Though trilobites dominated the Paleozoic marine environments, came back strong after several mass extinctions, they faded out by the end of the Permian, their niches on various marine environments taken over by their crustacean cousins. The spiders thrived on land as did the insects. Insects underwent an amazing adaptive radiation. Excluding microbial organisms, modern times are dominated by the insects, with beetles alone making up some 25% of known organisms.

Subphylum	Class	Common Examples
Trilobitamorpha (note1)	Trilobita	Trilobites and Relatives
	Aglaspidida or Aglaspida	Aglaspids
Chelicerata	Arachnida	Spiders, scorpions, harvestmen, ticks, and mites
	Merostomata	Horseshoe crabs and eurypterids.
	Pycnogonida	Sea spiders
Myriapoda	Archipolypoda (note 2)	early myriapod
	Chilopoda	Centipedes
	Diplopoda	Millipedes
	Pauropoda
	Symphyla	Garden centipedes
Hexapoda	Diplura
	Collembola	Springtails
	Protura
	Insecta
Crustacea	Branchiopoda	Brine shrimp
	Remipedia
	Cephalocarida	Horseshoe shrimps (no fossil record))
	Maxillopoda	Barnacles
	Ostracoda	Seed shrimp
	Malacostraca	Crabs, mole crabs, lobsters, isopods (woodlice and sowbugs), true shrimps, and Phyllocarids (?)
1 - Trilobitomorpha is a subphylum of the phylum Arthropoda that includes the trilobites. Originally a variety of peculiar forms, mostly from the lower Cambrian, were included as the Class Trilobitoidea. However, the many species do not appear to be closely related to the trilobites or, in many cases, to each other either, and are now generally placed in separate subphyla when classified at all. 2 - The Archipolypoda comprise the early Myriapods that are also often classified under the Diplopoda (millipedes). Archipolypods differed from millipedes, however, in having less coalesced segments, larger head reletive to the body, and large compound eyes. The fossil record indicates appearance of Archipolypoda in the Silurian and extinction at the end of the Carboniferous. Well-preserved specimens are known from the Mazon Creek Formation.