This new information on the sympatry of small and large azhdarchid species led us to look anew at other geological horizons that yield azhdarchid fossils.

In this way what is called a “geological horizon,” or “zone,” is marked off, and its exact position in the series of formations is fixed.

In a more restricted sense it is used to connote certain thin layers of bony fragments, which occur upon well-defined geological horizons.