Reconstruction of the Herefordshire (Silurian) Lagerstatte biota

我们对生命历史的详细了解依赖于化石记录，尤其是那些非常罕见且在科学上具有重要意义的化石沉积物，它们不仅保存了动物的坚硬部分，还保存了它们的整个身体，柔软的部分和所有的一切。显示出这种特殊保存的地质沉积物被称为化石Lagerstätten，它们比正常的壳类化石记录更能告诉我们古代生物群和群落的信息；它们提供了了解过去生活的独特窗口。来自威尔士边境地区的赫里福德郡Lagerstätte就是这样一个沉积物，其中包含了生活在大约4.25亿年前志留纪时期的小型海洋无脊椎动物的壮观化石。这些动物被火山喷发的火山灰吞没后被保存了下来。动物本身很快就腐烂了，但它们的形状被忠实地记录了下来，最初是由灰烬本身记录的，然后是由在形成的空隙中生长的方解石晶体记录的。现在在火山灰层的硬结核中发现了这些晶体形状，这些化石不仅保存了整个动物的细节，而且几乎是独一无二的，它们是完全三维的，而不是被压扁的。赫里福德郡的化石不能通过机械制备或酸消化技术从岩石中完整地提取出来。也不能通过使用x射线计算机断层扫描（CT扫描）或磁共振成像（MRI）等最新技术获得它们的图像。相反，来自牛津大学、莱斯特大学、伦敦帝国理工学院和耶鲁大学的一组科学家用一种新颖的方法对它们进行了重建和研究。这涉及到计算机技术，矛盾的是，还涉及到化石本身的破坏。每次将标本磨去30微米，并对每个新暴露的表面拍一张照片。然后，数十到数百张这样的照片被用来制作一个高保真的“虚拟化石”，可以在电脑屏幕上旋转甚至解剖。化石本身在几毫米到大约5厘米的范围内，但通过快速成型技术，也可以用它们制作出放大的物理模型。赫里福德郡的动物可以追溯到我们对软体动物知之甚少的时期。寒武纪（约5.42 - 4.88亿年前）沉积的沉积物化石相对丰富Lagerstätten，泥盆纪（约4.16 - 3.59亿年前）沉积的化石相对丰富，但在其间的奥陶纪和志留纪沉积的沉积物中很少有化石。因此，赫里福德郡的化石在帮助填补生命史上的这一空白方面具有重要意义。该小组已经研究了赫里福德郡的各种动物，如蠕虫状的软体动物Acaenoplax和微小的，类似鲎的节肢动物Offacolus。来自Lagerstätte的这些化石和其他化石被证明是以前未知的进化谱系的代表，并帮助解决了关于灭绝动物和现存动物之间关系的争议。赫里福德郡的一些动物Lagerstätte仍有待调查。它们包括几种节肢动物，一种软体动物，两种棘皮动物，一种多毛类蠕虫，一种腕足动物（灯壳动物）和一种海绵，正是这些动物将成为一个新的研究项目的重点。单独来看，这些化石对于各自的主要动物类群来说，预计会像赫里福德郡已经研究过的化石一样令人兴奋，具有同样重要的科学意义。总之，他们的研究将首次使人们能够综合赫里福德郡动物群的组成、群落结构和生态，并将其与其他特别保存的动物群进行比较。这将使我们对4.25亿年前，志留纪时期的海底生命有一个无与伦比的了解。

Our detailed understanding of the history of life relies on the fossil record, and most especially on the very rare and scientifically important fossil deposits that preserve not simply the hard parts of animals but their entire bodies, soft parts and all. Geological deposits that show such exceptional preservation are known as fossil Lagerstätten, and these tell us much more about ancient biotas and communities than the normal shelly fossil record; they provide unique windows on past life. One such deposit is the Herefordshire Lagerstätte from the Welsh Borderland, which contains spectacular fossils of small marine invertebrates that lived about 425 million years ago, during the Silurian Period. These animals were preserved when they were engulfed in ash from a volcanic eruption. The animals themselves soon rotted away, but their shapes were faithfully recorded, initially by the ash itself, and then by crystals of calcite that grew within the resulting voids. These crystalline shapes are now found within hard nodules in the ash layer, and not only do the fossils preserve entire animals in fine detail, but almost uniquely they are fully three-dimensional rather than squashed flat. The Herefordshire fossils cannot be extracted whole from the rock by mechanical preparation or acid digestion techniques. Nor can images of them be obtained by use of more recent technologies such as X-ray computed tomography (CT scanning) or magnetic resonance imagery (MRI). Instead, they are reconstructed and studied, by a team of scientists from Oxford, Leicester, London (imperial College) and Yale universities, using a novel approach. This involves computer technology and, paradoxically, the destruction of the fossils themselves. Specimens are ground away 30 microns at a time, and a photograph is taken of each freshly exposed surface. Tens to hundreds of these photographs are then used to create a high-fidelity 'virtual fossil' in the round, which can be rotated or even dissected on a computer screen. The fossils themselves are in the range of a few millimetres to about five centimetres, but scaled-up physical models can also be made of them through rapid-prototyping technologies. The Herefordshire animals date from a period of time for which we have little knowledge of soft-bodied faunas. Sediments deposited during the Cambrian Period (about 542-488 million years ago) are relatively rich in fossil Lagerstätten, and those deposited during the Devonian (about 416-359 million years ago) reasonably so, but there are very few such horizons represented in sediments laid down in the intervening time, during the Ordovician and Silurian periods. Thus the Herefordshire fossils are of great importance in helping to fill in this gap in the history of life. Various of the Herefordshire animals, such as the worm-like mollusc Acaenoplax and the tiny, Limulus-like arthropod Offacolus, have already been researched by the team. These and other fossils from this Lagerstätte have proved to be representatives of previously unknown evolutionary lineages, and helped to resolve controversies about the relationships both of extinct animals and of those alive today. Some animals from the Herefordshire Lagerstätte remain to be investigated. They include several arthropods, a mollusc, two echinoderms, a polychaete worm, a brachiopod (lamp-shell) and a sponge, and it is these that will provide the focus of a new research programme. Individually, they are anticipated to be as exciting and as scientifically significant for each of their respective major animal groups as those Herefordshire fossils that have already been studied. Together, their study will allow, for the first time, syntheses of the composition, community structure and ecology of the Herefordshire fauna, and comparison of it with other exceptionally preserved faunas. This will give us an unrivalled view of life on the seabed 425 million years ago, during the Silurian period.