Locomotion in the earliest tetrapods: testing models of terrestriality

3.7亿到3.6亿年前的泥盆纪时期，脊椎动物从水到陆地的转变是进化史上最深刻的转变之一，最终导致了包括人类在内的所有陆地脊椎动物的出现。这些变化包括对骨骼的基本修改——从在水中支持游泳肌肉组织到在陆地上行走和跑步的坚固的承重支柱和悬臂，以及生理和感觉系统的变化。后者的变化也对骨骼产生影响，进而影响运动模式。在过去的几年里，我们对代表这一转变的动物的化石记录有了很大的改进。自从只有两个分类群（真鳍鱼和四足鱼石螈）的时代以来，它已经有了很大的改进：我们现在有了elpistostgids Panderichthys和Tiktaalik，以及四足棘螈。这些新发现的骨骼已经足够为人所知，可以对性格获得的关系和顺序进行假设。然而，这只是故事的一部分。现在很明显，两种泥盆纪的四足动物（棘螈和鱼螈）的骨骼已经得到了充分的了解，它们不仅与现代四足动物有根本的不同，而且彼此之间也有很大的不同。对于从水里出来的一些问题，它们代表了不同的答案。鱼石螈和棘螈是重要的进化标志，可以与其他关键的“过渡形式”相媲美，如始祖鸟（第一只鸟）、南方古猿（如“露西”）和陆行鲸（一种早期的“行走鲸”）。棘螈虽然有四肢和趾，但基本上仍然是水生动物。另一方面，人们普遍认为鱼石螈是一种强壮的陆行四足动物，但事实证明这是非常错误的：事实上，鱼石螈在骨骼上非常异常，其形态特征表明它既属于水生动物，也属于陆生动物。它的运动方式似乎更像海豹，而不是蝾螈，根据PI和同事的研究，它的骨骼似乎可以像哺乳动物一样，通过区域化的脊柱和分化的肌肉组织进行背腹弯曲。我们想要测试这两种不同的早期四足动物形态模型在数量上和三维上的准确性，这是我们之前的尝试无法做到的。为了做到这一点，我们将对标本进行最新开发的高科技方法，包括微型CT， CT和激光扫描，3D重建和操作软件，使我们能够第一次做到这一点。接下来，我们将检查骨骼的运动范围，以评估它们的能力，并量化生物力学性能的关键指标，如脊柱的区域刚度以及内部骨骼结构。为此，我们将把骨骼的特定部位——中央、神经弓、肋骨、四肢和腰带——与现代四足动物进行比较，试图利用现代定量技术，了解泥盆纪四足动物是如何移动的，它们最适合什么条件，从而最适应什么样的环境和生活方式。这将有助于阐明从水到陆地过渡的最早条件，并最终有助于更好地理解现代四足动物的骨骼是如何进化的。这不仅将增强我们对从水到陆地的过渡的理解，而且还将为四足动物皇冠群中真正的陆地成员的最终出现奠定基础和建立起始条件，从而为我们自己的远古祖先奠定基础。

The water - to - land transition made by vertebrates during the Devonian period, between 370 and 360 million years ago, was one of the most profound in evolutionary history, and ultimately allowed the appearance of all land vertebrates including humans. The changes involved fundamental modifications to skeletons - from supports for swimming musculature in water to robust weight-bearing struts and cantilevers for walking and running on land, as well as changes to physiological and sensory systems. These latter changes also had influences on the skeleton that in turn affected the locomotor patterns. Over the last few years we have a gained a much improved fossil record of the animals that represent this transition. It has vastly improved since the days when only two taxa were available (the fish Eusthenopteron and the tetrapod Ichthyostega): we now have the elpistostegids Panderichthys and Tiktaalik, and the tetrapod Acanthostega. These newly discovered skeletons are well enough known to permit hypotheses about relationships and sequences of character acquisition. However, this is only part of the story. It is now evident that the two Devonian tetrapods whose skeletons are adequately known (Acanthostega and Ichthyostega) differ radically not just from modern tetrapods, but from each other. They represent divergent answers to some of the problems of emerging from the water. Ichthyostega and Acanthostega are evolutionary icons of importance comparable to that of other critical 'transitional forms' such as Archaeopteryx (the first bird), Australopithecus (e.g., 'Lucy'), and Ambulocetus (an early 'walking whale'). Acanthostega though possessing limbs with digits was still basically aquatic. On the other hand, the popular image of Ichthyostega as a sturdy land-going quadruped has been shown to be quite erroneous: in fact it was highly aberrant skeletally, having morphological traits that suggest both aquatic and terrestrial specialisations. Its mode of locomotion appears to have been more like that of a seal than a salamander, and its skeleton, according to work by the PI and colleagues, seems to have allowed dorsoventral flexion using a regionalised vertebral column and differentiated musculature as in mammals. We want to test both these diverse models of early tetrapod morphology for their accuracy quantitatively and in three dimensions, which our earlier attempts were unable to do. This we will do by subjecting the specimens to recently developed high-tech methods including microCT, CT and laser scanning, 3D reconstruction and manipulation software that allow us to do this for the first time. Next, we will examine the range of movements of the skeletons to assess what they were capable of, and quantify key indices of biomechanical performance such as regional stiffness of the vertebral column as well as internal bone architecture. For this, we will compare particular parts of the skeleton - centra, neural arches, ribs, limbs and girdles - with modern tetrapods to try to understand, using modern quantitative techniques, how the Devonian tetrapods moved, to what conditions they were most suited, and thus to what environments and life styles they were best adapted. This will help to illuminate the earliest conditions of the transition from water to land and ultimately lead to greater understanding of how modern tetrapod skeletons evolved. This will not only enhance our understanding of the transition from water to land, but lay the foundations and establish the starting conditions for the eventual appearance of the truly terrestrial members of the tetrapod crown group, and thus our own remote ancestors.