EARLY BURST IN FORM BUT NOT FUNCTION IN EARLY PALEOZOIC BIVALVES: DISTINCT TRENDS IN TAXONOMY, MORPHOLOGY AND ECOLOGY

Despite originating in the Cambrian Explosion, Class Bivalvia underwent a macroevolutionary lag during its initial ecological diversification. Here, we test whether the Early Paleozoic bivalves exhibited an early burst in morphological disparity relative to taxonomy and ecology. Our analyses show that from the Cambrian to the Early Ordovician, bivalves expanded rapidly in morphospace (constructed from outlines of the shell’s commissural profile for 176 Cambrian and Ordovician genera) but added relatively few ecological functions. Morphospace occupation in the Late Ordovician was similar to that of the Early intervals, despite a 50% increase in taxonomic diversity with much turnover and continued functional diversification, reflecting strong morphological stability following the establishment of the six major bivalve clades.

Only 16 out of the 79 Late Ordovician genera fall outside of the convex hull formed by the 53 genera in the Early Ordovician, indicating that most of the morphospace had been explored early in the diversification, albeit by different genera. These 16 genera all belong to functional groups present in the Early Ordovician, suggesting a discordance between morphological and functional evolution. Half of these morphologically novel genera come from one functional group with moderate taxonomic richness, implying uneven morphological evolution among functional groups. Bursts of morphology were also concentrated within certain families and clades: ~50% of the outlying genera belong to Family Ambonychiidae (Clade Pteriomorphia), with the rest distributed as ~one genus per family (also mostly Pteriomorphia). Neither the richest families nor the richest functional groups accumulated the greatest degree of morphological disparity. Further, the two genera founding new functional groups, e.g., chemosymbiosis, lie in intermediate positions between those of older genera in the morphospace. Thus, morphological innovation is neither necessary nor sufficient for ecological diversification. This result emphasizes the potential for biological diversification to follow distinct trajectories as measured by different currencies, and in particular that rapid evolution along one axis does not necessitate similar trends in another.