State Key Laboratory of Palaeobiology and Stratigraphy, Nanjing Institute of Geology and Palaeontology and Center for Excellence in Life and Paleoenvironment, Chinese Academy of Sciences, No. 39 East Beijing Road, Nanjing 210008, People's Republic of China.
Department of Earth Sciences, Mountjoy Site, Durham University, South Road, Durham DH1 3LE, UK
Proc Biol Sci. 2018 Sep 26;285(1887):20181780. doi: 10.1098/rspb.2018.1780.
Hyoliths are a taxonomically problematic group of Palaeozoic lophotrochozoans that are among the first shelly fossils to appear in the Cambrian period. On the basis of their distinctive exoskeleton, hyoliths have historically been classified as a separate phylum with possible affinities to the molluscs, sipunculans or lophophorates-but their precise phylogenetic position remains uncertain. Here, we describe a new orthothecide hyolith from the Chengjiang Lagerstätte (Cambrian Series 2 Stage 3), Sun, Zhao et Zhu gen. et sp. nov., which exhibits a non-mineralized attachment structure that strikingly resembles the brachiopod pedicle-the first report of a peduncular organ in hyoliths. This organ establishes a sessile, suspension feeding ecology for these orthothecides and-together with other characteristics (e.g. bilaterally symmetrical bivalve shell enclosing a filtration chamber and the differentiation of cardinal areas)-identifies hyoliths as stem-group brachiopods. Our phylogenetic analysis indicates that both hyoliths and crown brachiopods derived from a tommotiid grade, and that the pedicle has a single origin within the brachiopod total group.
石鳖是一类在分类上存在问题的古生代担轮动物，它们是最早出现在寒武纪的有壳化石之一。基于其独特的外骨骼，石鳖类在历史上被归类为一个单独的门，可能与软体动物、星虫或腕足动物有关——但它们的确切系统发育位置仍不确定。在这里，我们描述了来自澄江生物群（寒武系第 2 统第 3 阶）的一种新的正形贝类石鳖，Sun, Zhao et Zhu gen. et sp. nov.，它具有非矿化的附着结构，与腕足动物的腕足惊人地相似——这是石鳖类中首次报道的茎足器官。这个器官为这些正形贝类建立了一个固着的、悬浮取食的生态位——与其他特征（例如，双侧对称的双壳贝壳包围一个过滤腔室和区分主区）一起，将石鳖类鉴定为腕足动物的干群。我们的系统发育分析表明，石鳖类和冠腕足动物都起源于一个托莫蒂类，并且腕足动物总群中的茎足具有单一的起源。
