Evolutional trends and palaeobiogeography of the Ordovician trilobite Ovalocephalus Koroleva 1959.

Ovalocephalus has a long stratigraphic range and wide geographical distribution in Ordovician peri-Gondwana. Based largely on the well-preserved specimens recently collected from China, all known forms are revised and listed. Phylogenetic analysis was conducted on the genus, involving 10 species. As suggested by the strict consensus tree, evolutional trends of the genus include mainly the isolation of the anterior glabellar portion anterior to S1, the forward shifting of eyes and the related lengthening (exsag.) of the posterior fixigena, the reduction of the number of pygidial axial segments and pleural abaxial rounded free tips, the shortening of the pygidial postaxial region, and the development of cranidial genal spines. Ovalocephalus may have originated in shallow-water sites of the South China Plate in the Early Floian, but migrated into the deep-water regions from the Darriwilian onwards. All the records of the genus from the Early Floian to Early Katian were confined to eastern peri-Gondwanan plates and terranes in low-latitude zones. It was only restricted to the South China, Tarim and North China plates until the Middle Darriwilian, but the Late Darriwilian eustatic sea-level rise and especially the Sandbian-Early Katian immense transgression may have brought about its dispersal to Alborz, Sibumasu and central Asian terranes. Following the closure of the Tornquist Sea, the genus was even able to spread to Baltica during the latest Katian, and the pre-Hirnantian warming (the Boda event) may have promoted a wider distribution of Ovalocephalus to western peri-Gondwana (the Taurides and Armorica terranes) in the then-high latitudes.