Deuterostome decay - taphonomic testing of fossil anatomy and phylogenetic placement

关于我们最早的鱼类祖先是如何、何时以及为什么进化的问题，对于理解我们在进化中的地位是至关重要的，但答案仍然难以捉摸，因为这些事件的化石记录很难读懂。我们将通过研究原始动物亲属的身体在腐烂时如何变化来解决这个问题，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。我们和其他所有有脊椎动物的动物被归类为脊索动物，与一些不太熟悉的动物(海鞘、橡子蠕虫、翼龙)一起坐在生命树的一个主要分支上，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子，他们与他们最近的后生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，后口动物的所有生物群体都以不同的方式修改了自己的身体，使它们彼此之间有很大的不同。位于生活群体之间的进化分支都已经灭绝，因此在解剖学上居于中间地位的动物--它可以引导我们沿着进化树回到早期的祖先--只能作为化石存在。不幸的是，这些动物缺乏坚硬的骨骼。他们的身体完全是由软组织组成的，通常在他们死后就腐烂了，没有留下任何化石记录。然而，尽管困难重重，化石还是被发现了，这些化石似乎保存着古老的、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解释它们，这是激烈的科学辩论的主题。我们的研究项目将采用一种新的方法来分析这些有争议但至关重要的后口化石。我们将研究不同后口动物特有的软组织特征--如肌肉、尾巴、鳃缝和滤食性结构--腐烂并成为化石的方式，并确定我们能否可靠地在化石中识别这些特征。化石本质上是腐烂在时间上被冻结的快照，通过腐烂的现代后肢动物，我们将建立一种血淋淋的照片拟合--人物如何重要地看待腐烂的各个阶段--如果我们要识别化石的特征，就需要这种照片。因此，我们将在受控的实验室条件下腐烂各种后口动物。随着它们的分解，我们将仔细记录它们的主要解剖特征的相对耐腐性，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实际上是如何被保存下来的，使用扫描电子显微镜对这些罕见的、保存异常完好的化石遗骸进行详细研究。同样，我们将专注于重要的后口生物角色，记录他们的视觉外观、位置和组成，以确定每个重要特征的保存模式和变异性。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现存后口生物群体及其祖先的可辨认的早期代表。或者，他们的遗骸腐烂得太多、太快，以至于让他们被正确识别的字符没有僵化？从来没有人用这种方式研究后口化石(除了我们最近的研究，重点是早期脊椎动物)。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和产生新的数据，以解决我们提案核心的重要进化问题--我们自己深厚的祖先如何、何时和为什么。化石有可能为这些大问题提供一些答案，但只有在我们学会如何正确阅读腐烂遗骸的化石记录之前。

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.