Identification of fossil worm tubes from Phanerozoic hydrothermal vents and cold seeps

One of the main limitations to understanding the evolutionary history of hydrothermal vent and cold seep communities is the identification of tube fossils from ancient deposits. Tube-dwelling annelids are some of the most conspicuous inhabitants of modern vent and seep ecosystems, and ancient vent and seep tubular fossils are usually considered to have been made by annelids. However, the taxonomic affinities of many tube fossils from vents and seeps are contentious, or have remained largely undetermined due to difficulties in identification. In this study, we make a detailed chemical (Fourier-transform infrared spectroscopy and pyrolysis gas-chromatography mass-spectrometry) and morphological assessment of modern annelid tubes from six families, and fossil tubes (seven tube types from the Cenozoic, 12 Mesozoic and four Palaeozoic) from hydrothermal vent and cold seep environments. Characters identified from these investigations were used to explore for the first time the systematics of ancient vent and seep tubes within a cladistic framework. Results reveal details of the compositions and ultrastructures of modern tubes, and also suggest that two types of tubes from ancient vent localities were made by the annelid family Siboglinidae, which often dominates modern vents and seeps. Our results also highlight that several vent and seep tube fossils formerly thought to have been made by annelids cannot be assigned an annelid affiliation with any certainty. The findings overall improve the level of quality control with regard to interpretations of fossil tubes, and, most importantly, suggest that siboglinids likely occupied Mesozoic vents and seeps, greatly increasing the minimum age of the clade relative to earlier molecular estimates.

制约人类解析热液喷口（hydrothermal vent）与冷泉（cold seep）群落演化历史的核心挑战之一，便是从古代沉积地层中准确识别管化石。管栖环节动物（annelid）是现代喷口与冷泉生态系统中最为显眼的栖息类群之一，而古代喷口与冷泉产出的管状化石通常被认为是环节动物所遗留。然而，诸多产自喷口与冷泉环境的管化石的分类亲缘关系仍存在争议，或因鉴定难度极高而长期未能明确。本研究针对6个科的现生环节动物栖管结构，以及采自热液喷口与冷泉环境的化石管（包含新生代（Cenozoic）7种管型、中生代（Mesozoic）12种管型与古生代（Palaeozoic）4种管型），采用傅里叶变换红外光谱（Fourier-transform infrared spectroscopy）与热解气相色谱-质谱联用法（pyrolysis gas-chromatography mass-spectrometry）开展了详尽的化学表征与形态学分析。本研究将上述实验鉴定得到的特征，首次在支序分类框架（cladistic framework）下探索古代喷口与冷泉管化石的分类系统学问题。研究结果不仅阐明了现生栖管的组成与超微结构细节，同时表明，两处古代喷口环境产出的两类管化石，正是由在现代热液喷口与冷泉生态系统中占据优势类群地位的帚毛虫科（Siboglinidae）环节动物所形成。本研究同时指出，此前被认为是环节动物遗留的若干喷口与冷泉管化石，目前无法确定其与环节动物的亲缘关系。总体而言，本研究成果提升了管化石解释的质控水平；尤为重要的是，研究表明帚毛虫类可能早在中生代便已栖息于喷口与冷泉环境中，这将该演化支的最小生存年代相较于此前的分子生物学估算结果大幅提前。