The latitudinal diversity gradient (LDG), wherein the number of species and higher taxa peaks in the tropics and decreases toward the poles, is the best-documented large-scale diversity pattern on Earth, but hypotheses explaining the standard LDG must also account for rare ''contrarian'' taxa that show diversity maxima outside of the tropics. For marine bivalves, one of the few groups that provide spatially explicit temporal data on a global scale, we show that a major contrarian group, the Anomalodesmata, unexpectedly exhibits the same large-scale dynamics as related clades having normal LDGs in two key respects. First, maxima in standing genus diversity and genus origination rates coincide spatially. Second, the strength of a clade's present-day LDG is significantly related to the proportion of its living genera that originated in the tropics during the late Cenozoic, with the contrarian gradient strength at both species and genus level predicted quantitatively by the values for the other clades. Geologic age distributions indicate that the anomalous LDG results from origination that is damped in the tropics rather than heightened in the temperate zones. The pervasive role of spatial origination patterns in shaping LDGs, regardless of the position of their diversity maxima, corroborates hypotheses based on clades showing standard gradients and underscores the insights that contrarian groups can provide into general principles of diversity dynamics.bivalves ͉ biodiversity ͉ Anomalodesmata ͉ tropics ͉ extinction E xceptions to the rule are a valuable challenge to theory, as they often represent a severing of covariation among potentially confounding variables. One of the most pervasive and best-documented large-scale biotic patterns on Earth is the latitudinal diversity gradient (LDG), wherein the number of species and higher taxa peaks in the tropics and decreases toward the poles (1-6). Known for a wide range of marine and terrestrial taxa (7) and from several intervals in the geologic past (8), the LDG has a attracted a multitude of explanations derived from a wide variety of scientific disciplines (9-12). Virtually all of this work has focused on groups showing standard LDGs, with many potential controlling variables inevitably correlated with latitude and therefore with one another. However, the relatively small subset of taxa showing weak or even reversed gradients (7, 10, 13-16) relative to the standard LDG must also be accounted for in any theory of the formation of the global LDG. Here, we take advantage of the well sampled biogeography and rich fossil record of shallow-water marine bivalves to test evolutionary models for the LDG by using a major contrarian order, the Anomalodesmata. By comparing its evolutionary dynamics to five related orders showing variations of the standard LDGs, from the strong gradient of the veneroids [the dominant component of the sister group to Anomalodesmata; supporting information (SI) Fig. 7] to the weak gradient of the carditoids, we show that spatial patterns of origina...