Deuterostome decay - taphonomic testing of fossil anatomy and phylogenetic placement

我们最早的鱼类祖先是如何、何时以及为什么进化的，这些问题对于理解我们在进化中的地位至关重要，但答案仍然难以捉摸，因为这些事件的化石记录很难阅读。我们将通过研究我们的原始动物亲戚的尸体在腐烂时是如何变化的来解决这个问题，从而为有争议的化石提供新的线索，这些化石包含了我们自己深层祖先的重要线索。我们和所有其他有脊椎的动物都被归类为脊索动物，与一些不太熟悉的动物坐在一起（海葵，橡子虫，翼鳃类）在生命之树的一个主要分支上，被称为后口动物。这个项目将解决一个基本的进化问题：我们最早的脊索动物祖先是什么样的，他们与最近的后口动物亲戚有什么不同？回答这个问题的主要困难是理解早期的后口动物：经过5亿多年的进化，所有后口动物的生活群体都以不同的方式改变了它们的身体。介于现存群体之间的进化分支都灭绝了，所以解剖学上的中间动物--它们可以引导我们沿着进化树追溯到那些早期的祖先--只能以化石的形式存在。不幸的是，这些动物缺乏坚硬的骨骼。它们的身体完全由软组织组成，通常在死亡时腐烂，没有留下任何化石记录。然而，尽管困难重重，人们还是发现了一些化石，这些化石似乎保存了古老的、早已灭绝的后口动物的遗骸。古生物学家面临的困难工作是正确地解释它们，这是科学界激烈争论的主题。我们的研究项目将采取一种新的方法来分析这些有争议但至关重要的化石后口动物。我们将研究不同后口动物的特征性软组织特征--如肌肉、尾巴、鳃裂和滤食结构--如何腐烂和变硬，并确定我们是否能可靠地在化石中识别这些特征。化石基本上是腐烂被冻结在时间里的快照，通过腐烂的现代后口动物，我们将建立一种血淋淋的照片-在不同腐烂阶段看起来有多重要-如果我们要识别化石的特征，我们需要它。所以我们会在实验室的控制条件下腐烂各种后口动物。当它们分解时，我们将仔细记录它们解剖结构的关键特征对腐烂的相对抵抗力，以及它们在外观和位置上的变化。我们还将研究早期后口动物的软组织特征是如何被保存下来的，使用扫描电子显微镜对罕见的、保存异常完好的化石遗骸进行详细研究。再次，我们将集中在重要的后口类字符，记录他们的视觉外观，位置和组成，以确定模式和每个重要功能的保存变化。通过分析和比较所有这些数据，我们将能够确定化石记录是否能够保存可识别的活后口动物群及其祖先的早期代表。或者他们的遗体腐烂得太多，太快，以至于允许他们被正确识别的字符没有被删除？从来没有人以这种方式研究后口动物化石（除了我们最近的研究，专注于早期脊椎动物）。我们提出的新项目建立在我们在这一领域的专业知识基础上，将使我们能够开发新的方法并产生新的数据，以解决我们提案核心的重要进化问题-我们自己的深层祖先如何，何时以及为什么。化石有可能为这些大问题提供一些答案，但要等到我们学会如何正确阅读腐烂遗骸的化石记录。

Questions of how, when and why our earliest fish-like ancestors evolved are fundamental to understanding our place in evolution, but answers remain elusive because the fossil record of these events is hard to read. We will address this problem by investigating how the bodies of our primitive animal relatives change as they decay, shedding new light on controversial fossils that contain important clues to our own deep ancestry.We, and all other animals with backbones, are classified as chordates, and sit together with some rather less familiar animals (sea squirts, acorn-worms, pterobranchs) on a major branch of the Tree of Life known as deuterostomes. This project will address a fundamental evolutionary question: what were our earliest chordate ancestors like, and how did they differ from their nearest deuterostome relatives? The principal difficulty in answering this question is understanding the early deuterostomes: after more than 500 million years of evolution, all the living groups of deuterostomes have modified their bodies in ways that make them quite different to one another. The evolutionary branches that sit between the living groups are all extinct, so the anatomically intermediate animals - which can lead us back down the evolutionary tree to those early ancestors - can exist only as fossils. Unfortunately these animals lacked hard skeletons. Their bodies were made entirely of soft tissues and generally rotted away when they died, leaving nothing for the fossil record. Against the odds, however, fossils have been found which seem to preserve the remains of ancient, long-extinct deuterostomes. The difficult job faced by palaeontologists is to interpret them correctly, and this is the subject of heated scientific debate. Our research project will take a new approach to analysing these controversial yet crucial fossil deuterostomes. We will study the way in which the characteristic soft-tissue features of different deuterostomes - such as muscles, tails, gill-slits and filter-feeding structures - rot and become fossilized, and determine whether we can reliably identify these characters in fossils. Fossils are essentially snapshots where decay has been frozen in time, and by decaying modern deuterostomes we will build up the kind of gory photofit - how important characters look at various stages of decay - that we need if we are to identify the characteristics of the fossils. So we will rot a variety of deuterostomes under controlled laboratory conditions. As they decompose we will carefully record the relative resistance to decay of the key features of their anatomy and how they change in appearance and position. We will also study how the soft-tissue characters of early deuterostomes actually became preserved, using a Scanning Electron Microscope to conduct detailed studies of the rare, exceptionally well-preserved fossil remains. Again, we will concentrate on important deuterostome characters, documenting their visual appearance, position and composition to determine the patterns and variability of preservation of each important feature. By analysing and comparing all these data we will be able to determine whether the fossil record can preserve recognizable early representatives of living deuterostome groups and their ancestors. Or did their remains rot too much, too fast, so that the characters that would allow them to be correctly identified were not fossilized? Nobody has ever investigated deuterostome fossils in this way (except our recent research, focussed on early vertebrates). The new project we propose builds on our expertise in this area and will allow us to develop novel methods and produce new data to address the important evolutionary questions at the heart of our proposal - the how, when and why of our own deep ancestry. Fossils have the potential to provide some answers to these big questions, but not until we learn how to correctly read the fossil record of rotted remains.