THE CAMBRIAN MARINE ARTHROPOD RECORD: EVOLUTIONARY RADIATIONS, MIGRATIONS, EXTINCTIONS, AND EXCEPTIONAL PRESERVATION

Arthropods have held a place of eminence as the most diverse and numerous animals on Earth through nearly the entire Phanerozoic Eon. The roots of that phenomenal success story date to at least the Cambrian Period. Body fossils resembling arthropods are present in some Ediacaran biotas, and certain complex trace fossils close to the Cambrian (Terreneuvian Series) base (541 Ma) may have arthropod tracemakers. The oldest unequivocal arthropod body fossils are in Cambrian Stage 2 (c. 525 Ma). The first trilobites appear in Cambrian Stage 3 (c. 521 Ma), at an early phase in the time during which marine arthropods experienced broad diversification. Most major marine arthropod clades other than decapods make their first stratigraphic appearances in Stage 3 or Stage 4 (521-509 Ma), although to a large extent the apparent distributions of nonbiomineralizing or lightly mineralizing taxa are influenced by preservation in Lagerstätten deposits. Polymerid trilobites and agnostoids, having readily preservable, calcite-reinforced exoskeletons, became the dominant macrofossils of the Cambrian in mid-Stage 4 to Stage 5 (c. 512-509 Ma).

The fossil record of Cambrian arthropods shows a close correlation with variations in carbon and strontium isotopes, and with eustatic sea level history. Major arthropod radiations coincide in time with strong positive δ¹³C excursions and usually also with positive deflections in ⁸⁷Sr/⁸⁶Sr ratios. Mass extinctions coincide with strong positive or negative excursions. Konservat-lagerstätten, most of which are dominated by fossils of nonbiomineralizing arthropods, are overwhelmingly associated with transgressive and highstand systems tracts.

Cambrian arthropods provide evidence of glacial-interglacial cyclicity that affected oceanic circulation and influenced the fossil record. In tropical regions, disjunct biofacies of shelf-dwelling trilobites in warm water juxtaposed against cool-water-adapted trilobites in deeper offshore areas implies the presence of a thermocline linked to glaciation in polar Gondwana. Repeated, near-simultaneous migrations of agnostoid species, plus some cool-water-adapted polymerids, onto continental shelves during Cambrian Age 5 to Age 10 coincide with eustatic rises inferred to be associated with interglacial phases.