Alleged vertebrate eggs from Upper Cretaceous redbeds, Gobi Desert, are fossil insect (Coleoptera) pupal chambers: Fictovichnus new ichnogenus

Abstract

Small egg-like structures from the Upper Cretaceous (Campanian) Djadokhta Formation of Mongolia were previously interpreted as casts of crocodile, lizard, and turtle eggs, or as inorganic nodules. Identical structures from coeval redbeds at Bayan Mandahu in northern China indicate the structures are not vertebrate eggs, nor of vertebrate origin. Comparisons with Recent and Quaternary beetle (Coleoptera) pupal chambers show close similarities in size, shape, and the presence of a round to irregular exit hole for the adult beetle. Most importantly, the Cretaceous structures are enveloped by a thin clay-rich zone, which is expected if constructed by beetle larvae but inexplicable in any egg or inorganic nodule model. Additional evidence contradicting a vertebrate egg origin for the structures includes (i) the structures are too small to have been laid by turtle or crocodile species occurring in the Bayan Mandahu redbeds; (ii) the structures are isolated, not in clutches or pairs; and (iii) unlike newly hatched soft-shelled lizard eggs, the Cretaceous structures are not collapsed and show a round to irregular exit hole rather than a slit. It is concluded that the egg-like structures are sand casts (steinkerns) of beetle pupal chambers, probably of Scarabaeidae, Tenebrionidae, or Curculionidae. The Cretaceous pupal chambers are assigned to a new ichnogenus, Fictovichnus, and new ichnospecies, Fictovichnus gobiensis and Fictovichnus parvus.