RADTKE, G., KEUPP, H. 2016. Imbricate radial sculpture: a convergent feature within externally shelled cephalopods. Palaeontology, 59, 3, 409-421. DOI: 10.1111/pala.12233
Gregor Radtke, and Helmut Keupp- Gregor Radtke - Institute of Geological Sciences Palaeontology Freie Universität Berlin Department of Earth Sciences Berlin Germany (Email: gradtke@zedat.fu-berlin.de)
- Helmut Keupp - Institute of Geological Sciences Palaeontology Freie Universität Berlin Department of Earth Sciences Berlin Germany
- Issue published online: 19 April 2016
- Article first published online: 01 January 1970
- Manuscript Accepted: 12 January 2016
- Manuscript Received: 12 March 2015
Radial sculptural elements (e.g. ribs, lirae), formed by imbrication of two succeeding shell lamellae are found in members of both the Nautiloidea (Cymatoceras) and Ammonoidea (Phylloceratinae and Aspidoceratinae). Their formation involves periodic cessation of shell growth due to weak to moderate withdrawal of the shell secreting mantle. The radial lirae (0.5–1.5 mm in width) of Phylloceratinae and Aspidoceratinae (Aspidoceras and Pseudowaagenia) are created by the succession of sigmoid lamellae of the organic periostracum or of the outer prismatic layer, respectively. Each lira has a characteristic adorally‐projecting, scythe‐like appendage, arising from its crest. The prismatic lirae of Aspidoceras and Pseudowaagenia are analogous to the larger scaled pseudoribs of Cymatoceras. Garland‐like lamellae of the outer prismatic layer form the radial lirae of Mirosphinctes and Epaspidoceras (Aspidoceratinae), but these lack a conspicuous, projecting scythe‐like appendage. Additional prismatic cement is formed within adoral, oval hollow spaces of scythe‐appendage‐bearing lirae, either through diagenetic crystal growth, remote biomineralization or as a component of the dorsal shell. In Aspidoceratinae these prismatic infillings are replaced by a continuous herringbone layer, accompanied by a reduction of the lirae.
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