Early Arthropod Faunas

A huge variety of bizarre arthropod types formed much of the basis for the Cambrian explosion (see p. 249). Over 20 groups of arthropod have been described from the Mid

Box 14.1 Classification of arthropods

There are currently differences in the status given to the main arthropod groups. If the Arthropoda is in fact a superphylum, the following groupings are phyla. However, some authorities have assigned the following superclass status within the phylum Arthropoda. The classification here is a compromise. Basal to the phylum are a number of minor but evolutionarily important groups, such as the tardigrades (water bears), that are now known from the Cambrian.

Subphylum TRILOBITOMORPHA

• Trilobites and their relatives; animals with a cephalon, thorax and pygidium; the body, lengthwise, has an axial lobe and two lateral pleural lobes

• Cambrian to Permian

Subphylum CHELICERATA

• Large group with a body divided into two tagmata; the prosoma (which bears six pairs of appendages, the first being the chelicerae or pincer-like appendages, giving the group its name), and the opisthosoma with an extended tail or telson

Subphylum MYRIAPODA

• Includes the flexible centipedes together with the millipedes

• ?Ordovician, Silurian to Recent

Subphylum HEXAPODA

• Highly-diverse group, with a head, thorax and abdomen and six legs; includes the ants, beetles, dragonflies, flies and wasps

• Devonian to Recent

Subphylum CRUSTACEA

• Includes the bivalved phyllocarids; Early Paleozoic taxa were ancestral to the crabs, shrimps and lobsters

• Cambrian to Recent

Cambrian Burgess Shale and related deposits (see Box 14.8); some have even been assigned to new phyla, emphasizing the expansive nature of the explosion, truly evolution's "big bang". Stephen Jay Gould, in his bestseller Wonderful Life argued that morphological disparity during the Cambrian was greater than at any time since. Nevertheless, cladistic, and phenetic analyses of both morphological and taxonomic criteria suggested otherwise (Briggs et al. 1993). Rather, the morphological disparity among the Cambrian arthropods is not markedly different from that seen across living taxa, they just look stranger to us. But it is nonetheless remarkable that very early in their history arthropods attained high levels of morphological disparity not really exceeded during the next 500 million years of evolution. Moreover, our knowledge of the Cambrian arthropod record, particularly that of soft-bodied organisms, is probably not nearly as complete as that of the modern fauna and we should expect further surprises as more Cambrian Lagerstätten are investigated (Box 14.2).

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