The Earliest Arthropod Trace Fossils and Their Significance

The trace fossil, or ichnofossil record provides a different perspective on animal evolution compared to that of body fossil, as it is typically preserved in different facies and lacks the bias towards mineralised body parts. As trace fossils record behaviours, they often cannot be assigned to particular producers, except in rare cases where they are found in association with them. However, they play an important role in the study of evolution and biostratigraphy and are particularly useful in stratigraphic studies of otherwise unfossiliferous rocks. Rusophycus is a rare exception as it is a trace fossil that is typically assigned to a trilobite tracemaker. However, this does not exclude other arthropods from being considered as potential producers of Rusophycus. Indeed, the trace appears before the first trilobite body fossils and is found after their disappearance. The earlier occurrences of Rusophycus are also the earliest undoubted signs of arthropods and are from the Terreneuvian. Pre-trilobitic forms seem to be different from their younger counterpart in morphological aspect, but this has not been formally quantified. This study investigates these early Rusophycus to obtain a clearer picture of what the pretrilobitic Rusophycus record is like and to search for temporal trends in their morphology. Pretrilobitic occurrences are also compared with younger ones to quantify their differences and try to interpret their significance. Pre-trilobitic Rusophycus appear to have lower length:width ratios than the younger ones. Moreover, they are also, on average wider than the younger Rusophycus. This is significant as it could result from a change in producer and probably in appendages used for digging. It could as well reflect a reduction in the number of appendages used. However, these low ratios could also be the result of taphonomic variation and the preservation of only the deeper parts of the traces. A reflection on the potential producers of the pre-trilobitic Rusophycus is also provided based on what is known of the morphology of early arthropods. While no definitive conclusion can be made, the probability that at least some of the earlier Rusophycus were not made by trilobites appears to be clear