Enigmatic Early Cretaceous ootaxa from Western Europe with signals of extrinsic eggshell degradation

Abstract

Crocodiloid eggshell is considered to be one of the most conservative among amniotes. This contrasts with the high body diversity observed within the crocodylomorph lineage, which extebds from the Triassic to the present. This incongruence raises a fundamental question in palaeology: is the crocodylomorph eggshell structure that conservative, or are there variations in this structure that have been misidentified in the fossil record or remain undiscovered to taphonomic biases?In this paper we re-examine eggshells from the Barremian of northern Spain that were previously assigned to chelonians. We erect a new oogenus and oospecies, Mycomorphoolithus kohringi, characterized by thin eggshells with mushroom-shaped or inverted cone shell units with blocky extinction with smooth or slightly undulating outer surface, covered by a highly variable number of pores of irregular size and shape. These variations in the pore opening pattern are here interpreted as evidence of degradation of the eggshell during embryo development, a process that has only been described in modern alligatorids. After discarding its chelonian and dinosaurian affinities, we identify them as related to Krokolithidae, but with enough differences to justify exclusion from this oofamily. In addition, eggshells from the Berriasian of England previously reported as dinosaurian-spherulitic eggshells, are here assigned to undetermined oospecies of Mycomorphoolithus. Thus, the record of Mycomorphoolithus extends throughout most of the Lower Cretaceous. This long-surviving oogenus may represent eggshells of the non-eusuchian crocodylomorphs that are abundant in the microfossil sites where Mycomorphoolithus eggshells are found.