Osmoregulation in Alligatoroidea: shifting the paradigm untethers biogeographic questions

Abstract

ABSTRACT A pervasive maxim in herpetology is that alligators (Alligatoridae) are poor salinity regulators relative to their taxonomic kin, crocodiles (Crocodylidae), evidenced by the purported lack of osmoregulatory abilities of modern Alligator and caimans, which has led to the assumption that freshwater obligation is synapomorphic across the clade, to include extinct members of Alligatoridae, or even Alligatoroidea. Presented here is a brief review of osmoregulation and the fossil record of alligatoroids and a suggestion that salt-tolerance may have been more widespread in the group than is widely assumed, based upon the observations that 1) modern alligatorids are increasingly reported from brackish and marine environs and possess osmoregulatory abilities attained through different regulatory pathways than their crocodile counterparts, and 2) fossil members of the group are routinely (or exclusively) found in or near marine depositional settings, suggesting plausibility that earlier members of the clade were more ocean-going than their extant descendants. Starting with fewer assumptions about the osmoregulatory abilities of extinct taxa may help solve continuing problems in eusuchian biogeography, namely, the dispersals of alligatoroids into Europe and Asia, and multiple dispersal events to and from pre-Interchange South America. Secondary loss of salinity tolerance in Alligator may be adaptive in continental habitats.