It has been proposed that the extant bivalve family Plicatulidae was derived from Prospondylidae during the Triassic, but the repeated evolution of species with transitional morphologies makes the differentiation between the two families difficult. Based on new material from the Hettangian Germig Formation of southern Xizang (Tibet), a new subgenus Persia (Nyalamia) and a new species Eoplicatula nianduoensis are proposed. P. (Nyalamia) is closely allied to the prospondylid Persia s.s. in shell outline and external ornamentation, but it is distinguished by the presence of resilial teeth and stronger crura, which enhances the mechanical strength of the hinge region. Hence, P. (Nyalamia) is phylogenetically linked to Prospondylidae but evolved character traits that are more typical of Plicatulidae. We suggest that the evolution of additional hinge elements was an evolutionary response to increased predation pressure during the initial Mesozoic marine revolution, which fostered iteration in the evolution of strongly interlocking hinges in cementing bivalves. Whether Early Jurassic Harpax evolved from Eoplicatula or from P. (Nyalamia) is currently unresolved, but if the latter option is confirmed, the taxonomy of Plicatulidae would be further complicated. This study reveals the complex evolutionary relationship between the two families and highlights the role of intensified predation pressure during the Mesozoic in promoting increased structural complexity of the hinge systems in cementing bivalves.
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