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

Both the Cambrian Explosion, more than half a billion years ago, and its
Ordovician aftermath some thirty-five million years 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.