Data from: Getting a head in hard soils: convergent skull evolution and divergent allometric patterns explain shape variation in a highly diverse genus of pocket gophers (Thomomys)

Background: High morphological diversity can occur in closely related
animals when selection favors morphologies that are subject to intrinsic
biological constraints. A good example is subterranean rodents of the
genus Thomomys, one of the most taxonomically and morphologically diverse
mammalian genera. Highly procumbent, tooth-digging rodent skull shapes are
often geometric consequences of increased body size. Indeed, larger-bodied
Thomomys species tend to inhabit harder soils. We used geometric
morphometric analyses to investigate the interplay between soil hardness
(the main extrinsic selection pressure on fossorial mammals) and allometry
(i.e. shape change due to size change; generally considered the main
intrinsic factor) on crania and humeri in this fast-evolving mammalian
clade. Results: Larger Thomomys species/subspecies tend to have more
procumbent cranial shapes with some exceptions, including a small-bodied
species inhabiting hard soils. Counter to earlier suggestions, cranial
shape within Thomomys does not follow a genus-wide allometric pattern as
even regional subpopulations differ in allometric slopes. In contrast,
humeral shape varies less with body size and with soil hardness. Soft-soil
taxa have larger humeral muscle attachment sites but retain an orthodont
(non-procumbent) cranial morphology. In intermediate soils, two pairs of
sister taxa diverge through differential modifications on either the
humerus or the cranium. In the hardest soils, both humeral and cranial
morphology are derived through large muscle attachment sites and a high
degree of procumbency. Conclusions: Our results show that conflict between
morphological function and intrinsic allometric patterning can quickly and
differentially alter the rodent skeleton, especially the skull. In
addition, we found a new case of convergent evolution of incisor
procumbency among large-, medium-, and small-sized species inhabiting hard
soils. This occurs through different combinations of allometric and
non-allometric changes, contributing to shape diversity within the genus.
The strong influence of allometry on cranial shape appears to confirm
suggestions that developmental change underlies mammalian cranial shape
divergences, but this requires confirmation from ontogenetic studies. Our
findings illustrate how a variety of intrinsic processes, resulting in
species-level convergence, could sustain a genus-level range across a
variety of extrinsic environments. This might represent a mechanism for
observations of genus-level niche conservation despite species extinctions
in mammals.