Study of the embryology and ossification of modern bones predicts that fossil intermembranous bones (which ossify from connective tissue) will exhibit greater size variability than endochondral bones (which are formed from embryological cartilaginous precursors), because intermembranous bones are less tightly constrained by joints and articular surfaces. To evaluate this hypothesis, we measured multiple dimensions of 989 intermembranous bones (patellae and other sesamoids) of the saber-toothed cat Smilodon fatalis, the Ice Age lion Panthera atrox, the bison Bison antiquus, the horse Equus occidentalis, the camel Camelops hesternus, the ground sloths Paramylodon (=Glossotherium) harlani and Nothrotheriops shastensis from Rancho La Brea and from the late Pleistocene San Josecito Cave in Nuevo Leon, Mexico. These were compared to measurements of 811 endochondral bones (primarily astragali) of comparable size. Through statistical analyses (coefficients of variation, ANOVA, modified Levene's test, and t-tests) we found slight evidence of higher variability in many of the intermembranous bones of these taxa (21 out of 27 CVs were higher for intermembranous bones than endochondral bones), although this trend is not found in all taxa. Using a modified Levene's test, only Smilodon and some of the dimensions of horse and bison patellae are significantly more variable than the corresponding dimensions of the astragali. Although the results are mixed, at least some data show that intermembranous bones are not as tightly constrained by growth and by adjacent tissues as are endochondral bones. This evidence of relative variability is important in assessing how much variability is typical of a single species, and thus has taxonomic implications.