Phylogeny and escalation in Mellopegma and other Cambrian molluscs

Michael J. Vendrasco, Artem V. Kouchinsky, Susannah M. Porter, and Christine Z. Fernandez

ABSTRACT

Mellopegma Runnegar and Jell is a widespread Cambrian stenothecid helcionellid mollusc that may represent the ancestral state of rostroconchs. New fossils provide details about the morphology and evolution of this genus.

Many specimens show healed wounds, indicating Mellopegma experienced frequent predation. Common scars near the sub-apical aperture indicate this area was a favoured target. Predation may have led to the formation of the strengthening zygion in this region of the possible descendent Eurekapegma MacKinnon. Mellopegma exhibits many anti-predator traits, and preliminary analyses herein show: 1) Mellopegma became better defensed through time via aperture narrowing; and 2) stenothecids show an increase in proportion of damaged shells from early to middle Cambrian deposits. This evidence is consistent with an early arms race between predators and molluscan prey.

Specimens from Siberia reveal that calcitic semi-nacre has a deeper history in the Mollusca than previously thought, consistent with the hypothesis that this shell microstructure occurs in both molluscs and brachiopods due to homology in the organic framework for shell formation. The shell of Mellopegma contained pores and the commonality of this trait among early molluscs suggests a porous shell may be primitive in Mollusca. The protoconch/teleoconch boundary is distinct in many specimens and indicates the fossils are of adult shells, and Mellopegma was lecithotrophic. One specimen of Mellopegma preserves the periostracum.

Mellopegma schizocheras sp. nov. is described from the middle Cambrian of Australia. Anabarella simesi MacKinnon is transferred to Mellopegma. Ribeiria junior Runnegar is removed from Rostroconchia and transferred to the new genus Acanthotheca.

Michael J. Vendrasco. Institute for Crustal Studies, University of California, Santa Barbara, California, 93106, USA. Current address: Department of Biological Science, California State University, Fullerton. P. O. Box 6850, Fullerton, California 92834-6850, USA.
Artem V. Kouchinsky. Department of Palaeozoology, Swedish Museum of Natural History, Box 50007, SE-104 05 Stockholm, Sweden.
Susannah M. Porter. Department of Earth Science, University of California, Santa Barbara, California, 93106, USA.
Christine Z. Fernandez. 14601 Madris Ave., Norwalk, California, 90650, USA.

KEY WORDS: new species; mollusc; Mellopegma; helcionellid; stenothecid; rostroconch

PE Article Number: 14.2.11A
Copyright: Palaeontological Association July 2010
Submission: 13 October 2009. Acceptance: 15 March 2011