The ichnogenus Tubotomaculum: an enigmatic pellet-filled structure from Upper Cretaceous to Miocene deep-marine deposits of southern Spain

The trace-fossil name Tubotomaculum has been extensively used to refer to spindle-shaped pellet-filled tubes present in Upper Cretaceous to Miocene deep-marine deposits of the western Mediterranean region. However, it has never been formally diagnosed, and accordingly it was regarded as a nomen nudum. In this paper, we formally introduce the ichnogenus Tubotomaculum, including the new ichnospecies Tubotomaculum mediterranensis. Bioglyphs, represented by scratch traces that may be present on the basal and lateral surfaces of the structure, suggesting production by crustaceans. The functional meaning of these structures challenges the simple model of a mining strategy. Instead, the storing of pellets to use them as a bacteria-enriched resource during times when organic detritus was scarce is suggested. The association with chemoautothrophic bacteria in modern analogs of Tubotomaculum provides a crucial piece of evidence to support the cache model. Integration of information from modern environments and the fossil record points to a connection between Tubotomaculum, mud volcanism, fluid venting, and hydrocarbon seeps. The presence of bioglyphs suggests firmgrounds that may have resulted from bottom current scouring of the sea sediment, leading to erosional exhumation of previously buried compacted sediment, which was therefore available for colonization by the infauna. However, an alternative scenario involves enriched fluids related to mud-volcanism resulting in reducing conditions that favored carbonate precipitation and nodule formation just a few centimeters below the sediment-water interface.

遗迹化石（trace fossil）名称Tubotomaculum曾被广泛用于指代分布于西地中海地区上白垩统至中新统深海沉积中的纺锤形颗粒充填管状物。然而该名称从未被正式诊断，因此被视为裸名（nomen nudum）。本文正式建立遗迹属（ichnogenus）Tubotomaculum，包含新遗迹种（ichnospecies）Tubotomaculum mediterranensis。该构造的基底与侧面表面可能存在由划痕构造构成的生物痕（bioglyphs），这暗示其造迹者为甲壳动物（crustaceans）。这类构造的功能意义挑战了单一的采矿策略模型，研究提出其更可能是为了储存颗粒，在有机碎屑匮乏时期将其作为富集细菌的资源加以利用。现代Tubotomaculum类似物与化能自养细菌（chemoautotrophic bacteria）的共生关系，为支持储存模型提供了关键证据。整合现代环境与化石记录的信息表明，Tubotomaculum与泥火山作用（mud volcanism）、流体逸出（fluid venting）以及烃类渗漏（hydrocarbon seeps）之间存在关联。生物痕的存在暗示存在硬底质（firmgrounds），其可能由海底流冲刷海底沉积物形成，导致先前埋藏的压实沉积物被侵蚀暴露，从而可供底内生物（infauna）定植。不过另一种假说则认为，与泥火山作用相关的富集流体可形成还原环境，有利于沉积物-水界面下方数厘米处的碳酸盐沉淀与结核形成。