The tale of how whales walked into the seas is one of the most celebrated in evolutionary biology. Even our own fossil backstory, of arboreal apes that eventually began to walk upright, seems relatively unimpressive compared to how little mammals that scampered along the beach on four legs began a transition that would result in oceanic leviathans. But the land-to-sea switch isn’t the only one whales have undergone.
Not all whales live in the ocean. River dolphins – long-snouted whales with tiny eyes – inhabit turbid freshwater streams in South America and Asia. For a time, before evolutionary biology began using molecular techniques, it was thought that all of these similar cetaceans belonged to a singular group, but, as it turns out, they represent different lineages that all independently gave up life in the seas for one in freshwater. Dolphins have done this over and over again in waterways around the globe for millions of years, and a newly-named prehistoric species from Panama adds a little more definition to the outline of when South America’s river dolphins made the switch.
The prehistoric dolphin, described by Smithsonian Institution paleontologist Nick Pyenson and colleagues, didn’t actually live in a river. The geologic context in which it was found made it clear that this whale swam out in the open ocean off the Caribbean coast of Panama between 6.1 and 5.8 million years ago. But, at that time, this stretch of sea was not as it was now. The ocean was brimming with plankton and supported a much richer collection of creatures, possibly thanks to upwelling from the deep Pacific, and the dolphin snapped up fish in this highly-productive stretch of sea before the Panamanian Isthmus fully closed. Pyenson and coauthors drew from this fact to give the creature its name – Isthminia panamensis.
Relatively little of Isthminia was left in the rock. Just a skull, lower jaws, right shoulder blade, and two flipper bones. Much of the skeleton probably eroded away before it could be excavated in 2011. But enough remained to allow Pyenson and colleagues to outline how this dolphin lived and who it’s related to. While the size of the cetacean’s eyes and the wear on its teeth are most similar to open-ocean dolphins, the details of its anatomy indicate that it’s an ancient relative of today’s South American river dolphins in the genus Inia. In short, Isthminia was an early “river dolphin” that lived at sea.
Even though today’s river dolphins had marine ancestors, however, how Isthminia fits into this picture isn’t totally clear. Paleontologists have uncovered the first wave of river dolphin invasion in South America as occurring between 16 and 11 million years ago – a group called platanistids that later disappeared from this continent (but is represented by the Ganges River dolphin today). Isthminia could represent the beginnings of the second wave as dolphins became increasingly adapted to nearshore life towards the end of the Miocene, or, Pyenson and coauthors note, the dolphin could mark a reversal. In this case, an even earlier and as-yet-unknown group of dolphins threaded into South America’s rivers and, while most stayed, the ancestors of Isthminia went back to sea.
Regardless of which hypothesis turns out to be correct, there was no straight-line transition from the sea to the rivers. Just like the initial invasion of the water by whales was a tangle of different lineages that took to the water in disparate ways. Into the sea, to the rivers, and maybe even back again, fossil whales remind us that transcendent change is never as simple as it first seems.
To get a better look at the fossil, check out the Smithsonian X3D browser.