Assessment of morphological variation in the early Cambrian trilobite Protoryctocephalus

Studies focused on morphological variation are key for chrono- and biostratigraphy. Morphological variability can be induced by biological and taphonomical biases. Their distinction can become a difficult task due to the lack of population control. Here, we analyse three populations belonging to the genus Protoryctocephalus, two populations from China (P. arcticus and P. balangensis) and a third population of P. arcticus from Greenland. Protoryctocephalus balangensis was collected from the lowermost part of the Kaili Formation where fossils are tectonically deformed while P. arcticus from the 'Tsinghsutung' and Greenland specimens are non-deformed. Geometric morphometrics analysis shows that these three populations indeed belong to the same species. However, the large allometry measured in P. balangensis compared with the 'Tsinghsutung' population and the Greenland population, which is represented by meraspid and early holaspid individuals, suggested that the morphological variation is taphonomically controlled and non-biological. The results suggest that Protoryctocephalus arcticus from the 'Tsinghsutung' Formation is suitable for international correlation, and further studies focus on different aspects such as ontogeny, segment development or fluctuating asymmetry and modularity.