Evaluating the plausibility of oceanic rafting by giant palaeognathous birds to Lanzarote during the early Pliocene

Abstract

The discovery of giant palaeognathous bird eggs and eggshells in Lower Pliocene deposits of Lanzarote (Canary Islands) raises a fundamental biogeographical question: how could large flightless birds have colonized an oceanic island that has never been connected to the adjacent African mainland? Although the fossil material provides unequivocal evidence for the former presence of these birds, no associated skeletal remains are currently known and the mechanism by which they reached the island remains unresolved. During the early Pliocene, northwestern Africa experienced warmer and wetter climatic conditions than today, with enhanced fluvial activity that could have favoured the mobilization of large volumes of plant debris and the formation of natural rafts entering the Atlantic Ocean. Here we combine geological and palaeontological evidence with simple hydrostatic calculations and probabilistic modelling to evaluate whether passive rafting across the Lanzarote Passage was physically and biologically plausible. Hydrostatic calculations indicate that natural vegetation rafts of realistic dimensions could support an adult bird of approximately 100 kg. Probabilistic simulations show that survival decreases with increasing transport duration, with adult individuals retaining higher survival probabilities than eggs under the modelled conditions. Bayesian analyses are used to explore parameter combinations compatible with successful transport while accounting for uncertainty. These results do not demonstrate that rafting occurred, but indicate that rare passive transport of adult giant palaeognathous birds is compatible with the geological, physiological and oceanographic constraints considered here.