Convergent evolution in planktic graptolites: independent origin of the dicranograptid morphology in the Hirnantian (latest Ordovician)

Abstract

A new genus and species of planktic graptolite, Anjigraptus wangi gen. et sp. nov., is described from the Upper Ordovician (Hirnantian) sponge-dominated Anji Biota of Zhejiang Province, China. The new genus has a biserial proximal portion and two uniserial stipes distally, the same tubarium form as Dicranograptus. The new genus has probable pattern H development and lacks introverted thecae; it can thus be readily distinguished from the biostratigraphically important Late Ordovician graptolite Dicranograptus, the Late Ordovician Diceratograptus, and the Ordovician–Silurian Neodicellograptus. As Anjigraptus wangi gen. et sp. nov. appears to lack a nema in its uniserial portions and can be distinguished from all other biserial graptolites in the same assemblage, we conclude that the new taxon is not a teratomorphic form of a previously known species, but represents convergent evolution of the dicranograptid morphology. The holotypes of two species of Neodicellograptus (N. siluricus and N. spinosus) are re-illustrated. Lucy A. Muir [lucy@asoldasthehills.org ], Department of Natural Sciences, Amgueddfa Cymru – National Museum Wales, Cathays Park, Cardiff CF10 3NP, UK; Yuandong Zhang†* [ydzhang@nigpas.ac.cn ], CAS Key Laboratory of Economic Geology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, 39 East Beijing Road, Nanjing 210008, PR China; Joseph P. Botting‡ [acutipuerilis@yahoo.co.uk ], Department of Natural Sciences, Amgueddfa Cymru – National Museum Wales, Cathays Park, Cardiff CF10 3LP, UK; Xuan Ma† [mxnjues1990@126.com ], CAS Key Laboratory of Economic Geology and Stratigraphy, Nanjing Institute of Geology and Palaeontology, 39 East Beijing Road, Nanjing 210008, PR China. †also affiliated with University of Chinese Academy of Sciences, Beijing 100049, PR China. ‡ also affiliated with Nanjing Institute of Geology and Palaeontology, 39 East Beijing Road, Nanjing 210008, PR China.