Taphonomy and environmental distribution of <i>Pseudophillipsia</i> in the middle Permian Kamiyasse Formation of the Southern Kitakami Terrane, Japan

Abstract Evidence suggests that trilobites experienced moderate diversification during the middle Permian, of which Pseudophillipsia Gemmellaro, 1892 is the most successful, with an unusually high number of species. However, it remains unclear whether their abundance reflects a stratigraphic trend or is specific to their habitat. To address this, we conducted a taxonomic study of Pseudophillipsia from the middle Permian (Capitanian) Kamiyasse Formation of the Southern Kitakami Terrane, Japan, and examined the burial processes to understand their habitat. Careful taxonomic analysis identified two species, Pseudophillipsia ( Pseudophillipsia ) spatulifera Kobayashi and Hamada, 1980 and Pseudophillipsia ( Carniphillipsia ) cf. Pseudophillipsia ( Carniphillipsia ) raggyorcakaensis Qian, 1981. The trilobites occur in both sandstone and mudstone, preserved as complete outstretched or enrolled specimens as well as disarticulated specimens, the majority of which are pygidia. Sedimentary facies indicate that the sandstone layer was formed in a shallow marine environment close to the lower shoreface, whereas the mudstone layer represents a slightly deeper environment, occasionally altered by storm flows. Based on biostratinomic features, the outstretched specimens with convex-up orientation must be autochthonous, whereas the enrolled specimens are interpreted as para-autochthonous, likely transported by storm flows. The greater the bioturbation, the greater the likelihood of the trilobite skeleton being disarticulated, particularly in mudstone layers. These findings suggest that Pseudophillipsia ( Pseudophillipsia ) spatulifera inhabited both sandy and muddy substrata, whereas Pseudophillipsia ( Carniphillipsia ) cf. Pseudophillipsia ( Carniphillipsia ) raggyorcakaensis was restricted to sandy environments. Given the limited geographic extent of the Kamiyasse Formation, we hypothesize that the appearance of Pseudophillipsia reflects a change in the sedimentary environment.