Cambrian origin but no early burst in functional disparity for Class Bivalvia

Abstract

Both the Cambrian explosion, more than half a billion years ago, and its Ordovician aftermath some 35 Myr later, are often framed as episodes of widespread ecological opportunity, but not all clades originating during this interval showed prolific rises in morphological or functional disparity. In a direct analysis of functional disparity, instead of the more commonly used proxy of morphological disparity, we find that ecological functions of Class Bivalvia arose concordantly with and even lagged behind taxonomic diversification, rather than the early-burst pattern expected for clades originating in supposedly open ecological landscapes. Unlike several other clades originating in the Cambrian explosion, the bivalves' belated acquisition of key anatomical novelties imposed a macroevolutionary lag, and even when those novelties evolved in the Early Ordovician, functional disparity never surpassed taxonomic diversity. Beyond this early period of animal evolution, the founding and subsequent diversification of new major clades and their functions might be expected to follow the pattern of the early bivalves—one where interactions between highly dynamic environmental and biotic landscapes and evolutionary contingencies need not promote prolific functional innovation.

Ecological structure of diversity-dependent diversification in Phanerozoic marine bivalves

“…Throughout their evolutionary history, bivalve diversification has entailed both functional novelty and continued subdivision of those functions [ 73 , 74 ]. However, epifauna have attained less functional variety than have the infauna [ 23 ], which may help to explain both the lower standing diversity of the group today and the stronger diversity-dependence in its origination rate throughout the Phanerozoic.…”

“…High-dimensional functional groups of genera (i.e. combinations of motility, tiering, attachment, and feeding) have not been comprehensively assigned to all genera at sufficient temporal resolution [ 23 , 73 , 74 ]; thus, we cannot yet fully analyse the evolutionary sequence of niche subdivision and the effects of niche packing on diversity dynamics [ 80 ].…”

Unique functional diversity during early Cenozoic mammal radiation of North America

“…To evaluate the potential relationship between global climate and local mammalian FD, δ 18 O values were taken from the most recent compilation of mean global temperature through the Cenozoic [25]. The δ 18 O values represent the proportion of the heavier to lighter oxygen isotope ( 18 O/ 16 O) from deep-sea benthic foraminifera [25,43]. Higher ratios suggest colder global temperatures and more glaciation, and vice versa [25,43].…”

“…Thus, taxonomic diversity does not always provide adequate information about community dynamics or function [15]. An alternative approach to infer processes driving community structure uses functional diversity (FD) [16], a 'taxon-free' method that ordinates species in multidimensional trait space. Evaluating changes in multidimensional trait space can be beneficial because the changes in community structure are more directly associated with ecosystem functioning [11][12][13][16][17][18].…”

“…An alternative approach to infer processes driving community structure uses functional diversity (FD) [16], a 'taxon-free' method that ordinates species in multidimensional trait space. Evaluating changes in multidimensional trait space can be beneficial because the changes in community structure are more directly associated with ecosystem functioning [11][12][13][16][17][18]. Functional traits directly correspond to the role of a species in the community.…”