The evolution of fossorial locomotion in the transition from tetrapod to snake-like in lizards

Dramatic evolutionary transitions in morphology are often assumed to be
adaptive in a new habitat. However, these assumptions are rarely tested
because such tests require intermediate forms, which are often extinct. In
vertebrates, the evolution of an elongate, limbless body is generally
hypothesized to facilitate locomotion in fossorial and/or cluttered
habitats. However, these hypotheses remain untested because few studies
examine the locomotion of species ranging in body form from tetrapod to
snake-like. Here, we address these functional hypotheses by testing
whether trade-offs exist between locomotion in surface, fossorial, and
cluttered habitats in Australian Lerista lizards, which include multiple
intermediate forms. We found that snake-like species penetrated sand
substrates faster than more lizard-like species, representing the first
direct support of the adaptation to fossoriality hypothesis. In contrast,
body form did not affect surface locomotion or locomotion through
cluttered leaf litter. Furthermore, all species with hindlimbs used them
during both fossorial and surface locomotion. We found no evidence of a
trade-off between fossorial and surface locomotion. This may be either
because Lerista employed kinematic strategies that took advantage of both
axial and limb-based propulsion. This, may have led to differential
occupation of their habitat, facilitating diversification of intermediate
forms.

形态学上剧烈的进化转变通常被认为是对新栖息地的适应。然而，这些假设极少得到验证，因为此类验证需要中间形态——而这些形态往往已灭绝。在脊椎动物中，细长无肢躯体的进化通常被假设为有助于在穴居（fossorial）和/或杂乱生境中的运动。然而，这些假设仍未得到验证，因为极少有研究考察身体形态从四足动物（tetrapod）到蛇形的物种的运动。在此，我们通过测试澳大利亚Lerista蜥蜴（包含多种中间形态）在地表、穴居及杂乱生境中的运动是否存在权衡，来验证这些功能假设。我们发现，蛇形物种穿透沙质基质的速度快于更似蜥蜴的物种，这是对穴居适应性假设的首次直接支持。相比之下，身体形态对地表运动或穿过杂乱落叶层的运动无显著影响。此外，所有具有后肢的物种在穴居和地表运动中均会使用后肢。我们未发现穴居与地表运动之间存在权衡的证据。这可能是因为Lerista蜥蜴采用了同时利用轴向推进和肢体驱动推进的运动学策略。这或许促成了它们对栖息地的差异化占据，从而推动了中间形态的多样化。