The fish trace fossil Undichna from the Cretaceous of Spain

The Early Cretaceous Spanish localities of El Montsec and Las Hoyas have yielded the fish trails Undichna britannica and Undichna unisulca ichnosp. nov. respectively. The former consists of two intertwined waves, and was probably produced by the elopiform Ichthyemidion vidali. Undichna unisulca is characterized by having only a single sinusoidal wave, and was most probably produced by the pycnodontiforms Eomesodon and/or Macromesodon. Reliable criteria that distinguish this ichnospecies from the putative single-waved undertrails of more complex trails include the presence of hyporeliefs below undisturbed lamination and the preservation of epireliefs with lateral levees. The Spanish occurrences represent the first mention of this ichnogenus in post-Palaeozoic strata, and demonstrate that Undichna is not restricted to the Carboniferous–Permian, as previously thought. The lacustrine depositional setting of the two Cretaceous localities is consistent with the known palaeoenvironmental distribution of this ichnotaxon. The disparity between the stratigraphical record of Undichna and the broad temporal and palaeoenvironmental distribution of fishes capable of producing similar sinusoidal structures presumably reflects a taphonomic filter. Preservation of Undichna is favoured by: absence or scarcity of infaunal burrowers; presence of a very fine-grained, plastic, semiconsolidated substrate; low-energy bottom conditions; and relatively rapid burial with no associated erosion. Freshwater settings present these taphonomic constraints more frequently than other environments; thus preservation of Undichna is favoured in lakes, alluvial swamps and inner regions of estuaries.