200 Years. 200 Stories. Story 102: “Meet Your Sister ”

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photo of a fossil of Tiktaalik
The holotype (the type specimen that defines the species) of Tiktaalik roseae. Photo by Nathan Pierson/ANSP.

Meet Your Sister

The fossil record will always be incomplete, so it is very difficult—if not impossible—for scientists to identify the “direct ancestor” for a species. On the other hand, it is possible to identify species that are closely related. Evolutionary biologists use the term “sister group” to identify the closest-known relative for a particular species or group of species. For instance, the Deinonychosauria (a group of bird-like dinosaurs that includes Deinonychus, Troodon, and Velociraptor) is considered the sister group of birds. Closer to home, chimpanzees and bonobos form the living sister group for modern humans. (The situation becomes more complicated if you include our fossil relatives, such as Neanderthals and “Heidelberg Man,” because scientists are still debating who is related to whom.)

Tiktaalik roseae, which was co-discovered by Academy paleontologist Dr. Ted Daeschler, is the best-known member of the sister group for all tetrapods (limbed animals that include amphibians, reptiles, birds, mammals, and their common ancestors). Unlike a frog, a dinosaur, or a rat, Tiktaalik has fins rather than legs. On the other hand, the internal bones of its fins are more like those of tetrapods than those in the fins of other fishes. Unlike other fishes, the skull of Tiktaalik is not directly connected to its shoulders. Instead, it has a neck. There are many other features that Tiktaalik shares with tetrapods, but one of the more remarkable features involves an internal bone of the skull called the hyomandibula. In fish, this bone is part of the connection between the skull and jaws, but Ted, fellow Academy scientist Dr. Jason Downs, and their colleagues discovered that the hyomandibula of Tiktaalik had begun to get smaller. It is even smaller in fossil tetrapods from the Late Devonian and eventually it is co-opted for part of the inner ear, the stapes.

Visit the first floor of the museum today to see a display complete with a cast of Tiktaalik, field notes from its discovery, and information on the Devonian period!

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