Species of the bivalved mollusc genus Caryocorbula are often difficult to identify because they are morphologically conservative and because of apparently continuous morphologic transitions between species. In addition, the size and shape of fossil members of the bivalve mollusc Caryocorbula are often associated closely with the environmental conditions present when these animals lived. Quantitative analyses of Caryocorbula from the Caribbean that lived during the Neogene (the last 23 million years) show a strong correlation of valve shape with size so that a clam species of a given size typically has a particular valve shape. This association of size and shape is called allometry and explains the conservative and continuous nature of morphology among species in the genus.

The persistence of this allometric trend in Caryocorbula species despite speciation and extinction events, and imperfect geographic and temporal sampling, indicates that Caryocorbula shape was constrained in some way. The variety of shell shapes that evolve in Carycorbula may be limited because only a narrow range of shapes is functional for these bivalves. Constraints may be important to Caryocorbula because these bivalves have left and right valves that differ in size and shape. The inequivalved shell may predispose Caryocorbula for constrained shape because it may either create space limitations for the animal within the shell or cause problems for maintaining a tight fit between left and right valves as the bivalve grows.

As previous work indicates, however, environmental conditions, particularly nutrient availability, may control the spatial and temporal distribution of large Caryocorbula species. If shape were genetically linked to size in these bivalves, then selection for larger size may result in a restricted range of valve shapes each time large species evolve. In other words, specific environmental conditions are necessary preconditions for the evolution of large Caryocorbula, but the shape constraints placed on these bivalves limit their resulting morphologic variety.

Although constraints appear to play an important role in Caryocorbula evolution, selection could still play an important role in the evolution of Caryocorbula morphology. The more streamlined and elongate shape of large species may be an adaptation for more efficient burrowing. Alternatively, the more compact and triangular shape of small species may help stabilize shells so that these small bivalves can better maintain their position in soft sediments of the sea floor.