avialans

Avialans is a broad term used in paleontology and evolutionary biology to refer to the group of animals that includes modern birds and their extinct ancestors. This group encompasses all descendants of the most recent common ancestor of Archaeopteryx and modern birds. The defining characteristic of avialans, and a key evolutionary innovation, is the development of feathers, though not all early avialans possessed flight-capable wings.

The fossil record provides significant insight into the evolutionary journey of avialans. Many early avialans, such as Archaeopteryx and other feathered dinosaurs, exhibit a mosaic of reptilian and avian features. These include teeth, long bony tails, and claws on their wings, alongside the presence of feathers. Over time, these features were gradually modified or lost in later avialan lineages, leading to the specialized anatomy of modern birds.

The evolutionary origin of avialans is firmly rooted within the theropod dinosaurs, a diverse group of bipedal, mostly carnivorous dinosaurs. Specifically, avialans are considered to be part of the maniraptoran clade, which also includes dromaeosaurs (like Velociraptor) and troodontids. This relationship is supported by numerous anatomical similarities observed in their skeletal structures and the discovery of feathered non-avian theropods that bridge the gap between these groups. The study of avialans is crucial for understanding the transition from dinosaurs to birds and the diversification of life in the Mesozoic Era and beyond.