Juvenile phacopid trilobites from the Prague Basin (Czech Republic)

Abstract

Complete early ontogenetic stages of phacopid trilobites are very rare in the fossil record worldwide. Although phacopid trilobites were a common component of the Devonian communities inhabiting the Prague Basin, only two articulated juvenile individuals have been described so far. However, rich and diverse fauna, including common remains of meraspid and young holaspid specimens of the phacopid Pedinopariops superstes superstes, has been found in the late Emsian Daleje Shale (Daleje-Třebotov Formation) at the Ceřinka Quarry near Bubovice. The rather unusual association of skeletal fauna in the Daleje Shale collected from the Ceřinka Quarry probably reflects the specific position of the locality, which corresponds to the transition from shale facies to carbonate facies. This newly described association contains common molted young holaspids, one entire meraspid specimen, and one poorly preserved but entire late meraspid–early holaspid specimen. The young individuals of P. superstes superstes strongly resemble the ancestral Pedinopariops degener; with the exception of their smaller eyes, they are somewhat reminiscent of the coarsely tuberculated descendent Pedinopariops superstes superior. The principal differences between the early stages of P. superstes superstes and the adult specimens include the relatively coarse tuberculation of the dorsal exoskeleton, which is more strongly vaulted and shows a less advanced level of eye development in young specimens. However, co-occurring adult specimens appear to be typical P. superstes. Here, the newly collected material is supplemented with a description of two as yet unreported remains of juvenile Pedinopariops superstes from other localities, and one well preserved specimen of an entire meraspid of Reedops cf. bronni, as well as a detailed discussion of the two previously described specimens of Pedinopariops insequens and Prokops hoeninghausi. Possible causes of an obvious paucity of articulated meraspid and young holaspid specimens when compared with adult and subadult ontogenetic stages in phacopid trilobites are discussed.