Australian Museum Journal The functional morphology of Neotrigonia margaritacea (Bivalvia: Trigoniacea), with a discussion of phylogenetic affinities

Shortform:
Morton, 1987, Rec. Aust. Mus. 39(6): 339–354
Author(s):
Morton, Brian
Year published:
1987
Title:
The functional morphology of Neotrigonia margaritacea (Bivalvia: Trigoniacea), with a discussion of phylogenetic affinities
Serial title:
Records of the Australian Museum
Volume:
39
Issue:
6
Start page:
339
End page:
354
DOI:
10.3853/j.0067-1975.39.1987.173
Language:
English
Date published:
31 December 1987
Cover date:
31 December 1987
ISSN:
0067-1975
CODEN:
RAUMAJ
Publisher:
The Australian Museum
Place published:
Sydney, Australia
Subjects:
MOLLUSCA: BIVALVIA; EVOLUTION
Digitized:
23 January 2009
Available online:
27 February 2009
Reference number:
173
EndNote package:
EndNote file
Title page:
Title page (123kb PDF)
Complete work:
Complete work (2358kb PDF)

Abstract

Neotrigonia margaritacea is one of six living Australian species of the Trigoniacea, an otherwise extinct group of Mesozoic 'cockles' which dominated shallow inshore seas worldwide. The Trigoniacea have usually been grouped with the freshwater Unionacea, each within its own order, in the sub-class Palaeoheterodonta. This taxonomic decision is largely based on hinge and shell structure, but other morphological evidence does not support such a contention, Neotrigonia being characterised by a lack of mantle fusion and possession of 'filibranch' ctenidia, the Unionacea by well developed siphons and 'eulamellibranch' ctenidia. Such morphological discrepancies sustain lively debate in the literature, but seem to have been resolved when the ciliary pathways on the ctenidia of both groups were described to be uniquely similar

This study re-examines living N. margaritacea and investigates the histology of the ctenidia in particular, and concludes that in terms of structure and ciliary pathways, Neotrigonia is unique, and that its affiliations lie not with eulamellibranch bivalves but with the filibranch bivalves of the Pteriomorphia

The myophorid origin of the Trigoniacea is undisputed, as is the widely held view that the group is terminal, i.e. it has not given rise to other bivalve lineages. It is noted, moreover, that some palaeontologists regard the Palaeoheterodonta as an artificial assemblage and the conclusion of this study supports the view that the Trigoniacea and Unionacea are not closely related. I believe the Trigoniacea to represent another line of pteriomorph evolution: perhaps sharing some remote palaeotaxodont ancestor with the Unionacea, but in no greater sense than that the same ancestors are believed to be those of probably all living bivalve groups.