Can undertracks show higher morphologic quality than surface tracks? Remarks on large amphibian tracks from the Early Permian of France

Abstract

In tetrapod ichnology, the morphologic quality of tracks is widely termed preservation, including both formational and post-formational processes; this study follows this interpretation. The term undertrack is generally referred to poorly-preserved tracks found on the underlying layers of the actual trampled surface, which usually shows better-preserved tracks. This study, using the most recent concepts and techniques in tetrapod ichnology, aims to give a significant contribution to the understanding of the undertrack formation. The studied material includes some large temnospondyl tracks from the Early Permian of the Lodeve Basin (France), a trackway type material of Opisthopus and a pes-manus couple type material of Laoporoides (nomen vanum), both assigned to Limnopus. These specimens show a peculiar feature: manual footprints that are more distinct, more complete, deeper and classifiable (better-preserved) on the underlying layers (up to two underlying layers) compared to the actual trampled surface, which shows shallower, indistinct and unclassifiable manual tracks (poorly-preserved). Pedal tracks on the actual trampled surface are deformed and thus unclassifiable (poorly-preserved), and nearly non-impressed in the underlying layers. This is probably the result of the trackmaker differential weight of manual and pedal impressions on water-saturated fine-grained laminated sediments. This is the first convincing fossil evidence of a better preservation of the undertracks in certain substrate conditions, confirming what is known from laboratory experiments. As a consequence, caution is suggested in the interpretation of undertracks and surface tracks, and the term undertrack should not be used as a synonym for poorly-preserved tetrapod tracks, although these terms often coincide.