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Ontogeny and relationships of Trinucleoidea (Trilobita)

Published online by Cambridge University Press:  20 May 2016

Brian D. E. Chatterton
Affiliation:
1Department of Geology, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
Gregory D. Edgecombe
Affiliation:
1Department of Geology, University of Alberta, Edmonton, Alberta T6G 2E3, Canada
Stephen E. Speyer
Affiliation:
21676 West Village Way, Tempe, Arizona 85282
Allen S. Hunt
Affiliation:
3Department of Geology, University of Vermont, Burlington, 05405-0122
Richard A. Fortey
Affiliation:
4Department of Palaeontology, The Natural History Museum, London SW7 5BD, Great Britain

Abstract

Protaspides and later growth stages are described and discussed for the Ordovician trinucleid trilobite Cryptolithus tesselatus Green and the raphiophorid Lonchodomas chaziensis Shaw. A small triangular rostral plate is described for a single protaspid instar in both Cryptolithus and Lonchodomas. The presence of this small sclerite in the ontogeny of these taxa supports the origin of the Trinucleoidea from the Ptychopariida through reduction in and then fusion or loss of the rostral plate. Earlier and later growth stages have fused ventral sclerites, with no signs of connective sutures. All members of the Trinucleoidea with known ontogenies share similar small, ovoid asaphoid protaspides, with distinct axes and varying numbers of sharp, conical to subtubular, submarginal spines on the dorsal exoskeleton and marginal spines on the hypostome. Two protaspid instars are identified in both Cryptolithus and Lonchodomas, sharing many characters that indicate homologous levels of development. A single origin for the median suture of the Asaphida is supported, with an anterior rostellum as its precursor.

Type
Research Article
Copyright
Copyright © The Paleontological Society 

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