Euconodont hard tissue: preservation patterns of the basal body

Conodont elements, consisting of crown and basal tissue are the well-known fossilized hard parts of Conodonta (extinct marine chordates), but the taphonomic processes leading to decomposition or remineralization of the basal tissue are not well understood. Here we focus on the taphonomy of basal tissue, reviewing the published record and describing new material from Asia and Europe (248 occurrences globally). These include crown and basal tissue in conjunction, and isolated basal bodies showing different stages of preservation. Some isolated specimens resemble phosphate rings similar to those assigned to Phosphannulus universalis. High-resolution biostratigraphy indicates that the lamellar type of conodont basal tissue is found in all facies and depositional environments. Other basal tissue types, described in the literature as tubular, mesodentine, spherulitic or lamellar with canalules, are limited to the early Palaeozoic and found exclusively in siliciclastic deposits (with the exception of spherulitic tissue). Although the stratigraphic record of basal tissue spans the range of Euconodonta (Cambrian–Triassic), this study shows that most of the isolated plate and ring-like structures are derived from early Palaeozoic coniform conodonts. Basal tissue of platform-type elements has a much more fragile shape and is therefore rarely preserved as a recognizable isolated unit.

牙形石元素（Conodont elements）由齿冠与基部组织构成，是牙形动物（Conodonta，已灭绝的海洋脊索动物）广为人知的石化硬体组分，但学界对驱动基部组织分解或再矿化的埋藏学过程（taphonomic processes）尚未形成充分认知。本研究聚焦基部组织的埋藏学特征，系统梳理了已发表的相关记录，并报道了采自亚洲与欧洲的新材料（全球范围内共计248处产出位点）。这些标本既包含齿冠与基部组织协同保存的类群，也涵盖呈现不同保存阶段的孤立基部结构。部分孤立标本形态类似被归入通用磷环（Phosphannulus universalis）的磷酸盐环构造。高分辨率生物地层学研究显示，层纹状牙形石基部组织可在所有岩相及沉积环境中被发现。而文献中记载的其他基部组织类型，如管状、中牙本质（mesodentine）、球粒状或具小管的层纹状组织，则仅局限于早古生代地层，且几乎全部赋存于硅质碎屑岩沉积中（球粒状组织除外）。尽管基部组织的地层记录覆盖了真牙形石类（Euconodonta，寒武纪至三叠纪）的整个演化时限，但本研究表明，绝大多数孤立的板状及环构造均源自早古生代的锥状牙形石。台型牙形石元素（platform-type elements）的基部组织形态更为脆弱，因此极少以可识别的孤立单元形式被保存下来。