The influence of divergent reproductive strategies in shaping modularity and morphological evolution in mammalian jaws.

Marsupial neonates are born at an earlier developmental stage than placental mammals, but the rapid development of their forelimbs and cranial skeleton allows them to climb to the pouch, begin suckling and complete their development ex utero. The mechanical environment in which marsupial neonates develop is vastly different from that of placental neonates, which exhibit a more protracted development of oral muscles and bones. This difference in reproductive strategy has been theorized to constrain morphological evolution in the oral region of marsupials. Here, we use 3D morphometrics to characterize one of these oral bones, the lower jaw (dentary), and assess modularity (pattern of covariation among traits), morphological disparity and rates of morphological evolution in two clades of carnivorous mammals: the marsupial Dasyuromorphia and placental fissiped Carnivora. We find that dasyuromorph dentaries have fewer modules than carnivorans and exhibit tight covariation between the angular and coronoid processes, the primary attachment sites for jaw-closing muscles. This pattern of modularity may result from the uniform action of muscles on the developing mandible during suckling. Carnivorans are free from this constraint and exhibit a pattern of modularity that more strongly reflects genetic and developmental signals of trait covariation. Alongside differences in modularity, carnivorans exhibit greater disparity and faster rates of morphological evolution compared with dasyuromorphs. Taken together, this suggests dasyuromorphs have retained a signal of trait covariation that reflects the outsized influence of muscular force during early development, a feature that may have impacted the ability of marsupial carnivores to explore specialized regions of morphospace.

The Influence of Climatic Variability on Morphological Integration, Evolutionary Rates, and Disparity in the Carnivora.

Biodiversity is unevenly distributed in space and time. One possible explanation for this is the influence of climate on the ecology, evolution, and morphology of taxa. Here we investigated the link between climatic variability and phenotypic integration, rates of morphological evolution, and disparity (morphological diversity) in three carnivoran clades (Canidae, Felidae, and Mustelidae). We gathered landmark data from the lower jaw and extracted current temperature and precipitation data from range maps. We found a significant negative relationship between climatic variability and integration for canids and felids. Among canids, variability in temperature was the key climatic variable, while in felids it was a combination of variability in temperature and precipitation. In both cases, relatively variable climates were associated with low phenotypic integration. We also found evidence for a negative association between climatic variability and both disparity and rates of morphological evolution in canids and mustelids. Selection can drive the evolution of jaw shape along lines of least resistance defined by patterns of integration, and this study suggests that climate may be a predictor of phenotypic integration. As a result, taxa in more variable regions (e.g., temperate, montane) may be more evolvable and more able to respond to fluctuating environmental conditions over a period of generations.