Early Jurassic to Late Cretaceous evolution of Zoophycos in the French Subalpine Basin (southeastern France)

The complex and enigmatic trace fossil Zoophycos is recorded worldwide, from Cambrian to Holocene. The possible evolution of the ichnofossil has already been proposed by some authors, but their works concerned the very large group of trace fossils related to Zoophycos (structures that could be assigned potentially to several ichnogenera) recorded in several depositional basins. This paper focuses on the Subalpine Basin of southeastern France, where excellent outcrops facilitate the study of changes in Zoophycos that occurred between the Early Jurassic and Late Cretaceous. Thousands of specimens have been studied in detail. It appears that several trace fossils, classically related to Zoophycos, are so different that they could actually represent separate ichnogenera. But a group of them can be considered as belonging to a single ichnogenus, as they share the same morphological characters: a thin layer of bioturbated sediment (lamina), constituted by a network of primary and secondary lamellae and bordered by a marginal tube. The outline may be simple or lobate. The general architecture is a flattened cone, spirally coiled around a virtual central axis, without a vertical tunnel, and constructed upward, or, rarely, downward, in the sediment. One or several subsequent whorls are observed. The producers could have been specialized deposit feeders (sipunculans?), following a K-selected strategy. All these trace fossils are found in fine- to medium-grained limestones and marls. Inside this group, subtle differences make it possible to distinguish four morphotypes; they differ in size and how lobate they are, and they could be assigned to true ichnospecies. They succeed in time with slow and gradual changes and two trends are documented. These Zoophycos underwent evolutionary and environmental changes during the Jurassic-Cretaceous interval, becoming more lobate and larger, as they migrate to deeper and deeper environments. The four morphotypes share the same basic morphology, result from the same general activity, and are restricted to the same depositional basin. Therefore a unique clade of Mesozoic Zoophycos producers could be proposed. These animals have constructed structurally elaborate burrow systems that record complex and varied behavior. These trace fossils can also be regarded as extended phenotypes, that is adaptive projections of the organisms' body and activity. The changes of the ichnofossils documented in the basin can thus reflect real changes in the organisms themselves and in their ethology: their phylogenetic history is mirrored to a certain extent in the succession of morphotypes. So, complex trace fossils, such as Zoophycos, are not only biosedimentary structures, but true mirrors of the evolutionary trends of their producers. This opens a new biological perspective for ichnological studies. (C) 2002 Elsevier Science B.V. All rights reserved.