In addition to the underbite, it possessed eyes that were tinier than those of today's porpoises. The extinct mammal therefore probably had poor eyesight, making its jaw probing all the more important to its survival.
The researchers think the animal was, at least in some ways, similar to today's freshwater river dolphins.
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"Today we don't find anything resembling river dolphins in the same kinds of habitats that Semirostrum likely occupied," Racicot said, adding that the porpoise lineage may have become more specialized over time.
She and her colleagues hint that more interesting animal discoveries are on the near horizon, thanks to 3-D imaging that allows for nondestructive analysis of museum specimens, some of which have been stored away for decades.
"Many exciting new species awaiting description are lying in museum collections, but the sort of detailed descriptions that are required to do full justice to them often take a lot of time," Racicot said.
Illustration: Bobby Boessenecker