Subphylum Trilobitomorpha

Trilobites probably played a crucial role in the evolution of living arthropods. Members of the now extinct subphylum Trilobitomorpha began roaming primeval seas in the Precambrian, reached their zenith in the late Cambrian with 4000-10,000 species, and then slowly went extinct around 230 to 275 mya. They were flattened, bilaterally symmetrical arthropods with bodies divided by longitudinal and transverse grooves unlike living arthropods (Fig. 1). Most adults were 2 to 7 cm in length, but giants of 50 cm are known. Their organ systems probably resembled those in modern arthropods. Smaller species were probably planktonic suspension feeders, whereas most species and all larger species were probably benthic deposit feeders or facultative predators. Unlike most other arthropods, trilobites

Arthropoda and Related Groups 61 TABLE I Estimates of Arthropod Diversity, with Comments on Certain Arthropod Taxa and Two Related Phyla"

Estimated number of

Taxon

species (families)

Biological features

lylum Arthropoda

1-5 million

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