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 [[email protected]], Department of Natural Sciences, Amgueddfa Cymru – National Museum Wales, Cathays Park, Cardiff CF10 3NP, UK; Yuandong Zhang†* [[email protected]], 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‡ [[email protected]], Department of Natural Sciences, Amgueddfa Cymru – National Museum Wales, Cathays Park, Cardiff CF10 3LP, UK; Xuan Ma† [[email protected]], 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.

Keywords:

• Graptolithina
• convergence
• evolution

Acknowledgements

The sponsors had no input into the study design; the collection, analysis and interpretation of data; or the writing of the manuscript. Yuanfeng Hu, Yuan Wang and Xuejing Wu are thanked for their assistance on fieldwork. We are grateful to Blanca Toro and an anonymous reviewer for their helpful comments.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

LAM’s visit to Nanjing was funded by a Chinese Academy of Sciences (CAS) President’s International Fellowship Initiative [PIFI; grant no. 2018VCB0014]; JPB’s research was also supported by a CAS PIFI [grant no. 2016VEB006]. Further support was provided by grants from CAS and the Ministry of Science and Technology of China [no. XDB10010100, no. 2013FY111000] to ZYD.