For a long time, paleontologists have struggled to explain the stark differences between the wing shapes of hummingbirds and swifts. The two bird families are descended from a common ancestor, but they have distinct flight characteristics—hummingbirds hover while swifts glide. Now, a recently discovered fossil that might help scientists land on an answer. Here’s Gemma Tarlach, writing for D-brief:
The fossil — that of Eocypselus rowei — is notable particularly for the preservation of its feathers, making reconstruction of the wings far more accurate than from skeletal remains alone.
E. rowei, found in the fossil-rich Green River Formation in southwestern Wyoming, lived 50 million years ago and was less than five inches long from head to tail.
The fossil’s wing shape is unlike either extant family but the size is similar to both, suggesting that’s what evolved first. For more detail, you can also read the original paper.