The fish trace fossil Undichna from the Cretaceous of Spain

Abstract

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.

First record of Lower Triassic Undichna spp. fish swimming traces from Emei, Sichuan Province, China

“…Comparison: These traces match the morphology of the specimen Undichna unisulca described by Gibert et al [16]. Those specimens were from Spain and were Cretaceous in age but without sharply incised grooves or corresponding hypichnial ridges.…”

“…Their hypichnial ridges were narrower and lacked the shallow central furrow (refer to text and Figure 4c [16,18]). The wavelength and amplitude of the traces we found were more constant and had more regular symmetry than those specimens described by Gibert et al [16], Morrissey et al [18] and Soler-GijÓn et al [17]. However, the former were smaller in dimension and shorter in length than the latter.…”

“…Discussion: The fossil Undichna unisulca encountered in Gibert et al [16] was unlikely to be the same undertrail of the species of Undichna isp in our samples which is generally more complex. Undichna unisulca from Member 1 in the Jialingjiang Formation of Longmendong Section was assumed as there was no other species like Undichna britannica (Higgs, 1988) that had group of regular sinusoidal waves within this member [3].…”

“…However, the former were smaller in dimension and shorter in length than the latter. The ratio amplitude/wavelength（A/）was approximately twice that compared to the specimens described by Gibert et al [16] and Morrissey et al [18].…”

“…Undichna mostly appears in the terrestrial sedimentary beds [16], particularly in lake deposits [4]. However, in more recent years, more have been found within marine deposits [5,7,8,17].…”