Avian Incipient “Monomorphichnus” in Modern Dune and Ephemeral Pond Environments

In the realm of Ichnology, a noticeable disproportion exists between the buoyant Paleoichnology and the much less developed Neoichnology. Neoichnological experimentation is likewise poorly developed, despite its enormous potential in paleoethologic and taphonomic applications, yet it was an emergent discipline in the dawn of Ichnology thanks to the work of Buckland, Darwin and Nathorst.

The ichnogenus Monomorphichnus consists of asymmetric trackways composed only of raker impressions, organized into sets of elongated scratch marks, either isolated or repeating laterally. They exhibit a distinctive morphology as a result of arthropod appendages grazing the sediment. Monomorphichnus was typically produced in Paleozoic marine (shallow sublittoral) environments. Nevertheless Monomorphichnus is, in many instances, just a partial taphonomic expression of Dimorphichnus, due to preservational bias of the pusher impressions in undertrack basal layers.

Here we report sets of raker impressions left by the wings of landed Passeriformes birds in two different modern, terrestrial environments (coastal dune and ephemeral pond) in Lepe (Huelva, SW Spain). The sets are isolated and consists of three to four (up to ten, in uncommon cases) elongate raking marks, 3-8 cm long, slightly lanceolate in shape and describing a straight to gently curved path (marks are longer and curved in dune sand, but shorter and straight in pond mud). Morphologically, these structures are attributable to incipient “Monomorphichnus”. They are found associated with bird footprints, as well as with V-shaped markings obliquely penetrating the substrate, deeper toward the V apex, which are interpreted as peck marks.

This avian “Monomorphichnus” reflect a peculiar way of searching food; namely a bird grazes the sediment surface with remiges, removing the upper layer to force small preys to emerge, e.g. small beetles and larvae of other insect groups (traces of escaping preys are also visible). It illustrates the principle that different organisms can produce similar traces.