The era in which dinosaurs and feathers mixed wasn't a single geographical time period but two, according to a study further analyzing the troves of fossil finds in northeast China, the Guardian reported.

When the discovery of feather-wearing dinosaurs (the precursors to birds), among other finds, was first made less than twenty years ago in the same location, researchers grouped them into one ecological time period, or biota, because of how perfectly preserved the samples were -- some had their feathers intact, salamanders kept parts of their gills, mammals their fur, etc. They didn't think it was likely that two periods could create such ideal conditions (ideal as in a volcanic eruption). Also, many of the discoveries were made by farmers and fossil deals, which further disorganized the research, according to the Guardian.

Originally, scientists attributed the region's fossils to the Jehol Biota, a time period beginning and ending between 133 and 120 million years ago. Now, they credit an older layer to a biota that's only recently been recognized, the Daohugou Biota, named for the Chinese village that contains some of its earliest fossil specimens. It began around 160 million years ago and overlaps slightly with the Jehol.

One major difference between the two periods is the presence of birds. The Jehol Biota marks the animal's earliest appearance, while none have ever been found in the Daohugou. Yet, when birds first became their own entities, "you can barely tell them apart," said Dr. David Hone, the study's author.

Because he and fellow researchers now have a more accurate timeline of their fossils, it will help chart their evolution. Adding to the process is the fact that the two periods run into each other and thus shared, at times, a common environment.