Burgersdorp trace fossil surface

Containing one of the richest late Early to early Middle Triassic continental biotas globally, the Burgersdorp Formation of the main Karoo Basin in South Africa hosts a diverse vertebrate fossil assemblage. Comparatively, trace fossils in this unit are lesser known, and thus how the Burgersdorp biotas behaved and interacted with their palaeoenvironment remain enigmatic. Here we report on two Middle Triassic continental ichnofossil localities dominated by tetrapod scratch traces from the Anisian part (subzones B and C) of the Burgersdorp Formation in the main Karoo Basin (Eastern Cape, South Africa). Analyses of the trace fossils and associated sedimentary facies aim to identify the possible trace makers, their behaviours and the local depositional conditions in this southern part of Pangea. We establish a robust palaeoenvironmental context for the ichnofossils combining ichnological data with the sedimentological records of the trace fossil sites and the nearby type area of the Burgersdorp Formation. To facilitate comparisons with similar trace fossils in the future, we quantify the morphometric parameters of these Burgersdorp trace fossils in digital models. Our results show that bilobated, branching and cross-cutting furrow casts with abundant scratch traces: a) were generated within crevasse splay deposits adjacent to meandering river channels in a semi-arid setting, and b) can be interpreted as the claw-scratched ventral walls of a therapsid burrow system with a 3D network architecture. This well-developed vertebrate burrow network complex from the Middle Triassic of South Africa is among a handful of similarly complex vertebrate trace fossils in the global pre-Jurassic ichnological record. Collectively, these trace fossils indicate that the Triassic, and in particular the Middle Triassic, was an important period in the evolutionary history of the complex vertebrate burrowing.

南非主卡鲁盆地的博格斯多普组（Burgersdorp Formation）产出了全球最为丰富的早三叠世晚期至中三叠世早期陆相生物群之一，拥有种类繁多的脊椎化石组合。相较而言，该地层中的遗迹化石研究程度较低，因此博格斯多普生物群的行为模式及其与古环境的相互作用机制至今仍不明晰。本文报道了产自南非主卡鲁盆地（东开普省）博格斯多普组安尼阶（B、C亚带）的两处中三叠世陆相遗迹化石产地，其以四足动物抓痕遗迹为主要特征。通过对遗迹化石及其伴生沉积相的分析，旨在明确该泛大陆南部区域的潜在造迹生物、其行为模式以及当地的沉积环境条件。本研究结合遗迹学数据、遗迹化石产地及邻近博格斯多普组模式产地的沉积学记录，为该批遗迹化石建立了可靠的古环境背景。为便于未来与同类遗迹化石开展对比研究，我们通过数字模型量化了这批博格斯多普遗迹化石的形态参数。研究结果显示，这类带有大量抓痕的双叶状、分枝状且交叉切割的沟铸型：a）形成于半干旱环境下毗邻蜿蜒河道的决口扇沉积中；b）可被解释为具有三维网络结构的兽孔类（therapsid）洞穴系统的爪抓腹侧壁遗迹。这件产自南非中三叠世的发育完整的脊椎动物洞穴网络复合体，是全球侏罗纪前遗迹化石记录中为数不多的同类复杂脊椎动物遗迹化石之一。综合来看，这批遗迹化石表明，三叠纪，尤其是中三叠世，是复杂脊椎动物穴居行为演化历史中的重要阶段。