Compound biogenic structures resulting from ontogenetic variation: An example from a modern dipteran

Resumen

Several families of Diptera are known to be active burrowers. Fly larvae of the genus Symplecta (family Limoniidae) were observed in a sandy substrate of an orange grove in Lepe, SW Spain. Traces produced by Symplecta display: (1), a proximal segment characterized by a winding furrow trail; (2), an intermediate part consisting of burrow segments arranged in zigzag; and (3), a distal part formed by a complex U-shaped burrow. The most complex morphology is present in the intermediate zigzag burrow system, which varies from irregularly sinuous and curved in its most proximal part to spiraled. In the fossil record, the trace would be ascribed to different ichnotaxa depending on the part preserved. In fact, this is a compound trace recording multiple behavioural patterns. The winding furrow shows affinities with Helminthopsis, the zigzag burrows with Treptichnus, and the U-shaped burrow with Arenicolites. Furthermore, the spiral portion shows some similarities with Spirophycus and Spirodesmos. Interestingly, formation of these compound traces is connected with different ontogenetic stages of the producer. The winding and zigzag segments are produced, respectively, by Symplecta larvae and pupae for feeding purposes, whereas the U-shaped burrow is excavated during the end of pupal stage for protection until adulthood. In the fossil record, this incipient compound trace potentially may occur in lake-margin, fluvio-estuarine and overbank deposits, including levee, crevasse splay and pond deposits, forming part of the Scoyenia or the Mermia ichnofacies. Structures similar to those produced by modern diptera are known from freshwater ichnofaunas since the Devonian