Experimental decay of onychophorans - lobopodian anatomy and arthropod origins

这个项目的目标很简单。通过在受控条件下腐烂天鹅绒蠕虫(指甲虫)，我们将产生开始正确解释龙虾化石记录所需的数据。在生命树中准确定位龙足类动物有可能解决一个重大的进化问题：节肢动物的起源。节肢动物可以说是地球上最成功的动物：比其他任何物种都更多样、更丰富，它们对每个人来说都是重要和熟悉的。然而，节肢动物现存最近亲属的身份，以及节肢动物起源和早期进化的细节仍不清楚。与节肢动物不同的是，甲龙既神秘又神秘。他们胖乎乎的腿和身体环看起来就像一群超重的米其林人。最近一篇关于动物关系的通俗记述指出，“没有哪一个族群引发了比这更多的动物学争论”(塔吉2000，《生命的多样性》)--指甲虫在生命树上的确切位置仍然存在争议。令人惊讶的是，回答趾甲关系的问题掌握着解开节肢动物进化出现的钥匙，而这正是化石叶足类发挥主要作用的地方。这些已灭绝的软体生物(几乎所有寒武纪的生物)与甲壳类有许多重要的解剖学特征，但最近的进化分析表明，化石叶足类包括节肢动物、甲壳类和泛节肢动物(甲壳类和节肢动物都属于的更大的类群)的祖先。因此，在生命树中找到合适的化石位置，有可能揭示节肢动物和甲壳类动物获得重要特征的顺序。如果叶节肢动物的分支确实填补了现存的甲壳动物和节肢动物之间的空白，我们也许能够解决主要节肢动物分支之间的关系。这一潜力只有在正确放置大叶藻的情况下才能实现，这需要关于它们如何腐烂的新信息。目前关于大叶藻放置位置的大部分分歧是因为我们不了解腐烂过程在石化之前是如何影响它们的身体的。对其他生物的研究表明，腐烂会迅速改变重要解剖特征的外观。随着软组织腐烂和塌陷，身体各部分的形状和并置--这是进行解剖学比较的关键标准--发生了显著的变化。其他功能完全腐烂了。我们需要新的数据，以便在解释它们的解剖学时，能够考虑到这些变化，这些变化将在某种程度上影响到所有的贝叶类化石。我们将采用一种新的方法来研究动物是如何腐烂的，这是我们实验室最近开发的。我们将在受控的实验室条件下腐烂甲鱼，并在分解过程中定期仔细记录它们的重要解剖特征(其中许多特征与化石叶脚类相同)。由此，我们将确定特征的衰变速度和顺序；它们何时以及如何并置、形状和外观发生变化。这将使我们能够建立识别龙虾化石中腐烂转化特征的标准，并重新评估这些有争议的动物的解剖学和进化关系(包括保存异常完好的新材料)。这也将允许我们进一步测试从我们正在进行的衰退实验中发展出来的一个假说：软体动物在进化上具有重要解剖特征的衰退不是随机的--最有助于识别进化关系的特征最有可能迅速衰退。如果这种模式普遍存在，那么在重建化石进化关系方面，这是一种重要的、但以前未被认识到的偏见。

The aims of this project are simple. By rotting velvet worms (onychophorans) under controlled conditions we will generate the data required to start correctly interpreting the fossil record of lobopodians. Accurate placement of lobopodians in the Tree of Life has the potential to resolve a major evolutionary problem: the origin of the arthropods. Arthropods are arguably the most successful animals on Earth: more diverse and abundant than any other group, they are important and familiar to everyone. Yet the identity of the arthropods' nearest living relatives, and the details of arthropod origins and early evolution remain unclear. In contrast to arthropods, onychophorans are both obscure and enigmatic. With their fat legs and body annulations they resemble a conga-line of overweight Michelin-men. A recent popular account of animal relationships noted that 'no group has prompted more zoological debate' (Tudge 2000, The Variety of Life) - exactly where onychophorans sit in the Tree of Life remains controversial. Surprisingly, answering the question of onychophoran relationships holds the key to unlocking the evolutionary emergence of the arthropods, and this is where fossil lobopodians have a major role to play. These extinct, soft-bodied organisms (almost all of Cambrian age) share a number of important anatomical features with onychophorans, but recent evolutionary analyses suggest that fossil lobopodians include the ancestors of arthropods, of onychophorans, and of panarthropods (the larger group to which both onychophorans & arthropods belong). Consequently, finding the correct places for fossil lobopodians in the Tree of Life has the potential to reveal the sequence in which important characteristics of arthropods and onychophorans were acquired. If lobopodian branches do fill the gap between living onychophorans and arthropods, we may be able to resolve relationships between the major arthropod branches. This potential can only be realised with correct placement of lobopodians, and this requires new information about how they decayed. Much of the current disagreement over the placement of lobopodians arises because we don't understand how the process of decay affected their bodies prior to fossilization. Studies of other organisms show that decay rapidly alters the appearance of important anatomical features. As soft tissues rot and collapse the shape and juxtaposition of body parts - crucial criteria for anatomical comparison - change significantly. Other features rot away completely. We need new data so that these changes, which will have affected all fossil lobopodians to some degree, can be taken into account when interpreting their anatomy. We will employ a new approach to the experimental study of how animals decay, recently developed in our lab. We will rot onychophorans under controlled lab conditions and carefully record their important anatomical features (many of which they share with fossil lobopodians) at timed intervals as they decompose. From this we will determine the rate and sequence of decay of features; when and how their juxtaposition, shape and appearance change. This will allow us to establish criteria for the recognition of decay-transformed features in fossil lobopodians and reassess the anatomy and evolutionary relationships of these controversial animals (including exceptionally well-preserved new material). It will also allow us to further test a hypothesis developed from our ongoing decay experiments: that the decay of evolutionarily important anatomical features of soft bodied animals is not random - features that are most useful for recognizing evolutionary relationships are the most likely to decay rapidly. If this pattern is widespread it is an important yet previously unrecognised bias in reconstructing the evolutionary relationships of fossils.