Data from: Getting a head in hard soils: convergent skull evolution and divergentallometric 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.

研究背景：当自然选择青睐受内在生物学约束的形态时，近缘动物类群中可能产生极高的形态多样性。囊鼠属（Thomomys）便是绝佳范例：该属是哺乳动物中分类学与形态学多样性最高的类群之一。高度前倾（procumbent）、以齿挖掘的啮齿类头骨形态，往往是体型增大所引发的几何必然结果。事实上，体型更大的囊鼠属物种多栖息于更坚硬的土壤中。本研究采用几何形态测量学分析（geometric morphometric analyses），探究了土壤硬度（穴居哺乳动物（fossorial mammals）的主要外在选择压力）与异速生长（allometry，即体型变化引发的形态改变，通常被视为核心内在因素）对该快速演化哺乳动物支系的头骨与肱骨的相互作用。

研究结果：体型更大的囊鼠属物种/亚种多具备更为前倾的头骨形态，但存在部分例外——包括一种栖息于坚硬土壤的小型物种。与此前的学术推测相悖，囊鼠属的头骨形态并不遵循全属统一的异速生长模式，即便区域亚种群间的异速生长斜率也存在差异。与之相反，肱骨形态随体型与土壤硬度的变化幅度更小。栖息于软质土壤的类群拥有更大的肱骨肌肉附着位点，但保留了正齿（orthodont，非前倾）的头骨形态。在中等硬度土壤中，两对姊妹类群（sister taxa）通过对肱骨或头骨的差异化修饰产生演化分化。在最坚硬的土壤中，肱骨与头骨形态均通过发达的肌肉附着位点与高度前倾的特征得以塑造。

研究结论：本研究结果表明，形态功能与内在异速生长模式之间的冲突，能够快速且差异化地改变啮齿类的骨骼结构，尤其是头骨。此外，我们发现了栖息于坚硬土壤中的大、中、小型物种之间，门齿前倾性状趋同演化的全新案例。该趋同演化通过异速生长与非异速生长变化的不同组合实现，推动了该属内的形态多样性。异速生长对头颅形态的显著影响，似乎印证了“哺乳动物头骨形态分化源于发育变化”的学术推测，但该结论仍需个体发育研究加以验证。我们的研究结果阐明了多种内在过程如何通过引发物种层面的趋同演化，从而支撑类群在多样的外在环境中维持属级分布范围。这或许可以解释哺乳动物类群中“尽管物种发生灭绝，但属级生态位仍得以保留”的观测现象。