'Viva la evolución!' Stingrays invaded South America
The New World freshwater rays, the Potamotrygonidae – literally the ‘river rays’ in ancient Greek – have been the focus of considerable evolutionary interest and even controversy over the last 20 years. This controversy was centered around a pretty obvious question – how did these stingrays get from the ocean to the middle of the Amazon river basin? Lots of hypotheses were pitched – all involving the idea of one or several ‘marine incursions’ – times during which continental South America was flooded by estuarine or even full saltwater millions of years ago. The question remained however – from what side of South America did these rays come from? The Atlantic? The Pacific? The Caribbean? And with that question came the hunt for the ancestors and living cousins of these river rays – because there would be a good chance that marine cousins of the freshwater rays were still living in the same area that millions of years previous had served as the origin of their marine invasion of the Amazon.
Alternate hypotheses of marine incursions into South America
In the mid to late 90s, my PhD advisor Nathan Lovejoy challenged the ingrained hypothesis at the time – that freshwater rays were Pacific in origin – finding that two species of strange rays (chupare whiprays – Himantura pacifica and H. schmardae), which straddled the Panamanian Isthmus, were the closest living relatives of Potamotrygon and friends. What this finding suggested is that freshwater rays had a Caribbean origin – and that this earlier Caribbean Sea formed a corridor between the Pacific and Atlantic oceans – hence why modern-day Himantura are split into two species, one on either side of the Isthmus of Panama.
Nathan Lovejoy's hypothesis of freshwater ray ancestry
But what was ancient South America like when these rays invaded? What did it look like?
…And most importantly when did these invasions happen?
Come back later and I’ll tell you what we know!
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