30 April 2010

Fossil Vertebrate of the Month: Platanistoid Dolphin

Modern dolphins are by many measures the most successful group of cetaceans: they are diverse, intelligent, and in many cases have proven more resistant to anthropogenic change than their larger relatives. Some dolphins have even colonized freshwater environments. These 'river dolphins' are often referred to as platanistoids, a name based on the modern genus Platanista that inhabits the Ganges and Indus Rivers (other genera inhabit the Amazon, La Plata, and - until recently - Yangtze Rivers), but there has been much debate about whether or not all river dolphins are actually related, as was originally thought. If the world's living and extinct river dolphins really are the product of separate colonizations of freshwater habitats, then they represent a striking example of convergent evolution: platanistoids share many morphological characteristics, perhaps the most striking being a long, pointed rostrum (or snout; this feature makes them similar in form to many other fish-eating vertebrates, such as ichthyosaurs and swordfish). The specimen at left, an as-yet unnamed platanistoid from the mid-Miocene of Oregon, exhibits this characteristic rostrum. However, it was uncovered from the Astoria Formation, a marine unit from the Oregon Coast, making it a saltwater freshwater dolphin. This implies that at least one lineage of river dolphins evolved its unusual morphology before migrating inland. To see this specimen, drop by the U of O's Museum of Natural & Cultural History's Paleolab exhibit, where it will be on display until this summer; if you're in Seattle, some very nice skulls of the similar (but unrelated) Eurhinodelphis are on display at the Burke Museum's Cruisin' the Fossil Freeway exhibit until the end of May.