Precambrian to Tremadoc biotas in the Caledonides

Summary The early Phanerozoic faunas of the Caledonides (late Precambrian to Tremadoc) are reviewed; they reflect the early history of the Iapetus Ocean. The long-established distinction between ‘Pacific’ and ‘Atlantic’ faunas is justified, but the latter are divisible into Avalonian and Baltic sub-faunas. Putative deep-water faunas have a more cosmopolitan distribution. Five time segments are considered: late Precambrian (Vendian), early, middle and late Cambrian, and Tremadoc. In the late Precambrian, consideration is given to the distribution of soft-bodied Ediacaran faunas. In the Cambrian and Tremadoc, interpretations are based primarily on trilobite distributions, although other fossils have biogeographical importance. Supposed transgressive-regressive cycles, which are connected with eustatic sea-level changes, influence faunal distributions. The perturbations giving rise to the biomeres recognized around the N American craton also appear to be recognizable elsewhere in the Caledonides; they may reflect eustatic changes of sea level.