Locomotion in the earliest tetrapods: testing models of terrestriality

脊椎动物在3.7亿至3.6亿年前的泥盆纪时期所做的水到陆地的转变是进化史上最深刻的转变之一，并最终允许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根本性修改-从水中游泳肌肉组织的支撑到陆地上行走和跑步的坚固承重支柱和杠杆，以及生理和感觉系统的变化。这些变化也对骨骼产生了影响，进而影响了运动模式。在过去的几年里，我们已经获得了代表这一转变的动物化石记录。从只有两个分类群（真鳍鱼和四足动物鱼石螈）的日子以来，它已经有了很大的改进：我们现在有了长柄鱼石螈科的Panderichthys和Tiktaalik，以及四足动物的长柄鱼石螈。这些新发现的骨骼是众所周知的，足以允许有关关系和字符获取序列的假设。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，它们的骨骼已经被充分了解（石属和鱼石螈属），不仅与现代的四足动物有着根本的不同，而且彼此之间也有着根本的不同。它们代表了对从水中出现的一些问题的不同答案。鱼石螈和鱼石螈是重要的进化标志，与其他关键的“过渡形式”，如始祖鸟（第一只鸟），南方古猿（例如，陆行鲸（Ambulocetus，早期的“行走的鲸鱼”）。虽然有四肢和趾，但扁石基本上仍然是水生的。另一方面，人们普遍认为鱼石螈是一种强壮的陆地四足动物，但事实证明这是完全错误的：事实上，它的生殖器官非常异常，具有水生和陆生的形态特征。它的运动模式似乎更像海豹而不是蝾螈，根据PI及其同事的工作，它的骨骼似乎允许使用区域化的脊柱和分化的肌肉组织进行背腹屈曲，就像哺乳动物一样。我们想测试这两种早期四足动物形态的不同模型在定量和三维上的准确性，这是我们早期的尝试无法做到的。我们将通过对标本进行最近开发的高科技方法来实现这一目标，包括microCT，CT和激光扫描，3D重建和操作软件，使我们能够首次做到这一点。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，如脊柱的局部刚度以及内部骨结构。为此，我们将把这些骨骼的特定部分--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，它们最适合什么样的条件，从而最适合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，并最终导致更好地了解现代四足动物骨骼是如何进化的。这不仅将增强我们对从水到陆地过渡的理解，而且为四足动物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，从而为我们自己的遥远祖先奠定基础。

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.