|The evolution of ligament systems in the Bivalvia|
Waller, T.R. (1990). The evolution of ligament systems in the Bivalvia, in: Morton, B. (Ed.) The Bivalvia: Proceedings of a Memorial Symposium in honour of Sir Charles Maurice Yonge (1899-1986) at the 9th International Malacological Congress, 1986, Edinburgh, Scotland, UK. pp. 49-71
In: Morton, B. (Ed.) (1990). The Bivalvia: Proceedings of a Memorial Symposium in honour of Sir Charles Maurice Yonge (1899-1986) at the 9th International Malacological Congress, 1986, Edinburgh, Scotland, UK. Hong Kong University Press: Hong Kong. ISBN 962-209-273-X. 355 pp., more
In order to test models of the primary ligament and ligament evolution, ligament systems of (arrays of ligaments and their shelly supports) were surveyed throughout the Bivalvia with particular attention to layer distinctions, ligament-layer / shell-layer association, and the ontogeny of ligament layers and supports. The results of this survey suggest that: (1), the ligament system was opisthodetic rather than amphidetic; (2), lamellar ligament originated not as an uncalcified shell layer but as a repair material secreted in response to periostracal splitting along the dorsum and (3), fibrous ligament may be a modification of a primary shell layer but may be absent in the earliest Bivalvia of the Cambrian. The simplest ligament systems involve ligament attachment to unmodified shell margins, a condition present throughout the Protobranchia except for the Solemyidae and the extinct Ctenodontidae. From this protobranch stem group, two major types of ligament system evolved. One type, in which the ligament layers are supported by nymphae, evolved twice, once in the Ctenodontidae + Solemyidae and again in a clade that contains the subclasses Anomalodesmata, Palaeoheterodonta and Heterodonta. The second type evolved once and is characteristic of the subclass Pteriomorphia including the Isofilibranchia. Here the ligament system lacks true nymphae and is characterized instead by discontinuous ontogeny of fibrous ligament, paedomorphic truncation of the primary opisthodetic ligament, and reconstitution of the ligament system by repetition or expansion of the early postlarval ligament or by repetition of lamellar ligaments. A phylogeny of bivalve subclasses based on ligament-system evolution and corroborated by other apomorphies calls for a reassessment of the subclass placement of many early Palaeozoic taxa and for a re-examination of the early evolution of hinge dentition.