The palaeobiogeographical significance of the Silurian and Devonian trilobites of Japan

Trilobites are widely represented in Siluro-Devonian strata of the South Kitakami, Hida Gaien and Kurosegawa Paleozoic terranes of Japan. This thesis presents a detailed systematic palaeontology of four groups of these trilobites: the Proetida (and Aulacopleurida), the Phacopidae, and the Encrinurinae. Proetids are represented by twelve species within eight genera, with three species under open nomenclature, and a single species of aulacopleurid; phacopids by six species – two in open nomenclature; and encrinurids by ten species within three genera, including three described under open nomenclature, and one new species.

A palaeobiogeographical analysis of the Siluro-Devonian trilobites of Japan is presented, including six groups with detailed recent systematic analysis – the Proetida, Aulacopleurida, Phacopidae and Encrinurinae analysed herein – and the Illaenidae and Scutelluidae. It also presents preliminary palaeogeographical data on Japanese Siluro-Devonian trilobite groups that are less well-studied, the Cheiruridae, Calymenidae, Lichidae and Odontopleuridae.

In terms of palaeogeographical signature, Silurian illaenids and scutelluids show generic and species links with the Australian part of the Gondwana palaeocontinent, but not – apparently – between the three Japanese terranes. By contrast, Silurian encrinurids appear to exhibit a pan-tropical signature. Devonian phacopids, and possibly proetids, suggest generic-level links with the North China palaeocontinent, Devonian cheirurids and calymenids suggest generic-level links South China and Australia, and Devonian lichids strong generic links with Australia.

The disparity in biogeographical connections of different groups may in part reflect the fragmentary biostratigraphical record of Japanese trilobites, but also appears to reflect their lithofacies ranges and palaeoecology. This cautions against their use for palaeobiogeographical assessment without a more complete understanding of their palaeoenvironmental context.

The Silurian and Devonian trilobites of Japan also have biostratigraphic significance, particularly in the Devonian, where trilobite biostratigraphy, along with detrital zircon geochronology, effectively constrains the age of the Naidaijin and Nakazato formations of the Kurosegawa and South Kitakami terranes respectively.