Until Panama do us part: new finds from the Pliocene of Ecuador provide insights into the origin and palaeobiogeographic history of the extant requiem sharks Carcharhinus acronotus and Nasolamia velox

The extant blacknose shark Carcharhinus acronotus is a small-sized, tropical to warm-temperate carcharhinid shark occurring along the western Atlantic coasts from North Carolina (USA) through the Gulf and Caribbean regions to Uruguay. Here, we report on two carcharhinid teeth from lower Pliocene (4.07–3.76 Ma) strata of the Upper Onzole Formation exposed in the vicinities of Camarones (northwestern Ecuador). These specimens are assigned to C. acronotus, of which they apparently represent the first occurrence in the Pacific Ocean. The blacknose shark is regarded as the sister group of the whitenose shark Nasolamia velox, an idiosyncratic carcharhinid that currently inhabits the eastern Pacific coasts from Baja California (Mexico) to Peru; furthermore, the divergence between C. acronotus and N. velox has been recently estimated at about 3.7 Ma, which matches well the final phases of formation of the Isthmus of Panama. In light of these data, our Ecuadorian specimens might document an early Pliocene phase in which the newly originated C. acronotus occurred West of the then-fading Panamanian Seaway, possibly as a consequence of occasional dispersal through the latter. Alternatively, they might represent the teeth of an as yet unnamed C. acronotus-like carcharhine, from which the the morphologically conservative C. acronotus and the highly autapomorphic N. velox later arose by vicariance as the Isthmus of Panama rose. A survey of the fossil record of these two taxa does not falsify either hypothesis. Further research on the fossil chondrichthyans from the Cenozoic marine successions of Ecuador will hopefully shed new light on this issue and, more generally, on the role played by the closure of the Panamanian Seaway as a macroevolutionary trigger in the late Cenozoic marine realm.