Study co-author Ryan McKellar of the Royal Saskatchewan Museum told the Christian Science Monitor about the peculiarities in the preserved dinosaur tail's feathers that made them so different from modern feathers.
"In this specimen, we're seeing barbs that are looking like they're branching off of each other."
As the dinosaur tail came with articulated vertebrae, that eliminated the chance that it belonged to an early bird. As National Geographic noted, modern birds and their ancestors from the Cretaceous era had pygostyles, or fused vertebrae in their tails that allowed unified movement in the tail feathers. McKellar describes the pygostyle as a feature that "you (may have seen) if you've ever prepared a turkey."
Given the lack of definition in the rachis and its peculiar design, the researchers concluded that the dinosaur tail feathers are "more similar to modern ornamental feathers" to those designed for flight. As mentioned above, flight feathers have better feature definition in the rachis, barbs, and barbules, optimizing them for the birds that carry them.
The researchers believe that if the dinosaur tail was completely covered with the type of feathers found preserved in the amber, that would have prevented the coelurosaur from flying. McKellar said that the feathers, if not used for flight, may have been used for temperature regulation, or "served a signaling function."
In all, the new study on the dinosaur tail and its feathers is consistent with an evolution model created in the 1990s by Yale University ornithologist Richard Prum.
According to the Christian Science Monitor, Prum theorized that there are five stages in feather evolution, starting with a mere filament in stage 1, and moving on to stage 2, which includes multiple filaments, also known as barbs. Once at stage 3, the model diverges, with stage 3a representing a rachis with barbs extending off it, and stage 3b featuring smaller barbules, but no rachis. This combines to form stage 3a+b later on, including feathers with a rachis, barbs, and barbules. And once at stage 4 and stage 5, the barbules become more complex and similar to those on today's birds of flight.
Prum, who was not involved with the new study, told the Christian Science Monitor that the preserved dinosaur tail's feathers are consistent with stage 3a+b feathers in his model.
"Maybe the fact that it does have barbules on its rachis may indicate that its rachis identity has not proceeded to that strong stage, but I don't see it as separate."