Insect trace fossils in aeolian deposits and calcretes from the Canary Islands: Their ichnotaxonomy, producers, and palaeoenvironmental significance

摘要

Insect trace fossils from Canary Islands have been known and discussed since the beginnings of the last century. The most common and widespread morphologies have been tentatively referred to the ichnogenera Celliforma, Palmiraichnus or Rebuffoichnus and attributed to bees, coleopterans, and locusts until now. Herein they are included in a new single ichnospecies: Rebuffoichnus guanche isp. nov., which can be distinguished from other ichnospecies by the presence of an antechamber. R. guanche are considered herein as coleopteran (Curculionidae or Scarabaeidae) pupation chambers. This is supported by the mostly horizontal orientation in soils, ellipsoid shape, dispersion of sizes, and particularly the internal surface of the wall chamber that is completely smoothed, even in both extremes. In addition, it is supported by the finding of one adult weevil in a similar trace fossil and palaeoenvironment of Australia. All these characters argue against their assignation neither, to egg pods of a locust species nor to bee cells, the latter also contradicted by the lack of spiral closure. The wall of R. guanche shows the same petrological features of those of the palaeosol in which specimens are found. In those specimens found in aeolian deposits, the wall is composed of sand grains similar to those found in the dunes or sand sheets. The diversity of microfabrics found in calcretes is higher and depends on the calcrete hostrock and the degree of development of the calcretes. In poorly developed calcretes the trace fossils reflect very well the calcrete hostrock, whereas in more developed ones they usually show peloidal/ooidal or laminar microfabrics. These similarities indicate that, in contrast to some previous reports, insects utilised as building material that of the surrounding soil, as expectable for a coleopteran pupation chamber. Lower numbers of R. guanche are recorded in calcretes from the western Canary Islands, where no aeolian deposits are present, in contrast to the high densities in the aeolian deposits of the eastern Islands. Coleopterans would first colonise thin soils with calcretes developed on basaltic or sedimentary rocks in young islands and later colonise dunes as they appear when the islands aged. The widespread presence of calcretes, aeolian deposits, and trace fossils in the easternmost and older Canary Islands is driven by both the geological evolution of the Islands and climatic conditions. Both the calcretes and pupation chambers formed during the relatively wetter periods within otherwise prevailing arid to semiarid conditions.