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ECDYSOZOA: ARTHROPODS 363

Box 14.1 Classifi cation 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 compro-

mise. 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, length-
wise, 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 append-
ages, the first being the chelicerae or pincer-like appendages, giving the group its name), and the

opisthosoma with an extended tail or telson
• ?Cambrian to Recent
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,

dragonfl ies, 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 of morphological disparity not really exceeded
(see Box 14.8); some have even been assigned during the next 500 million years of evolu-
to new phyla, emphasizing the expansive tion. Moreover, our knowledge of the Cam-
nature of the explosion, truly evolution’s “big brian arthropod record, particularly that of
bang”. Stephen Jay Gould, in his bestseller soft-bodied organisms, is probably not nearly
Wonderful Life argued that morphological as complete as that of the modern fauna and
disparity during the Cambrian was greater we should expect further surprises as more
than at any time since. Nevertheless, cladistic, Cambrian Lagerstätten are investigated (Box
and phenetic analyses of both morphological 14.2).
and taxonomic criteria suggested otherwise
(Briggs et al. 1993). Rather, the morphologi-
cal disparity among the Cambrian arthropods SUBPHYLUM TRILOBITOMORPHA
is not markedly different from that seen across The trilobitomorphs are highly derived arthro-
living taxa, they just look stranger to us. But pods lacking specialized mouthparts, and
it is nonetheless remarkable that very early in with tagmata comprising the cephalon, thorax
their history arthropods attained high levels and pygidium, together with trilobitomorph