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The Bivalvia — Proceedings of aMemorial Symposium in Honour of Sir Charles Maurice Yonge, Edinburgh, 1986. (Edited by Brian Morton). Hong Kong University Press, Hong Kong, 1990. THE EVOLUTIO N O F LIGAMEN T SYSTEM S IN TH E BIVALVI A Thomas R. Waller Department of Paleobiology, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, U.S.A. ABSTRACT In order to test models of the primary ligament and ligament evolution, ligament systems (arrays of ligaments and their shelly supports) were surveyed throughout the Bivalvia with particular attentio n t o layer distinctions, ligament-layer / shell-layer association , and the ontogeny of ligament layers and supports. The results of this survey suggest that: (1), the primary ligament system was opisthodetic rather than amphidetic; (2), lamellar ligament originated not as an uncalcified shel l 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 simples t ligamen t system s involv e ligamen t attachmen t t o unmodifie d shel l margins, a condition present throughout the Protobranchia except for the Solemyidae and the extinct Ctenodontidae . Fro m thi s protobranch ste m group , two major type s o f ligamen t systems evolved. One type, in which the ligament layers are supported by nymphae, evolved twice, onc e i n th e Ctenodontida e + Solemyida e an d agai n i n a clad e tha t contain s th e subclasses Anomalodesmata, Palaeoheterodonta and Heterodonta. The second type evolved once and is characteristic of the subclass Pteriomorphia including the Isofilibranchia. Here the ligamen t syste m lack s tru e nympha e an d i s characterized instea d b y discontinuou s ontogeny of fibrous ligament , paedomorphic truncation of the primary opisthodetic liga ment , an d reconstitution o f the ligament syste m b y repetition o r expansion o f 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. 50 THOMAS R . WALLE R INTRODUCTION One of the joys i n science i s to create theory tha t integrate s an d explains facts tha t otherwise would have no obvious relationship, and one of the frustrations is to discover facts that d o no t fi t theory . Frustratio n i s bor n fro m theory , bu t ne w theor y emerge s fro m frustration. This symposiu m wa s convene d t o commemorat e th e wor k o f C M . Yonge , wh o throughout his long career was a master theorist who sought to integrate many originally disjunct observation s o n morphology an d morphologica l function s i n the Bivalvia . Th e present contribution is concerned with the theoretical model of the primary (i.e. primitive) ligament i n the Bivalvi a an d its evolution, matter s tha t figure d prominentl y i n Yonge' s examinations of bivalve morphology from 195 3 (Owen et al., 1953; Yonge, 1953) through 1986 (Yonge and Allen, 1985 , and references therein). It was Yonge's thesis, summarized in his 1978 paper on the significance of the ligament in the classification of the Bivalvia, that the primary ligament was amphidetic, with a fibrous central portion (the inner ligament) beneath an outer non-fibrous par t that extended bot h anteriorly an d posteriorl y (th e anterio r an d posterio r oute r ligaments) , al l o f thi s bein g covered externally by the periostracum. He was also convinced that the three layers of the ligament (periostracum, outer ligament, and inner ligament) were homologues of three shell layers (periostracum, outer shell and inner shell). In a long series of papers, Yonge and his colleagues assumed that this anterior-posterior symmetry of the ligament was the prepattern from which all later ligament configurations evolved. It was assumed, for example, that in heterodonts with prosogyrous beaks, a vestige of the primary anterior outer ligament remains as a short spur twisted beneath the anterior part of the inner ligament (illustrated in Yonge, 1978, fig. 4a; see also Trueman, 1969 , fig. 52D). In the present...

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