Pennaceous feather

The pennaceous feather is a type of feather present in most modern birds and in some other species of maniraptoriform dinosaurs. A pennaceous feather has a stalk or quill. Its basal part, called a calamus, is embedded in the skin. The calamus is hollow and has pith formed from the dry remains of the feather pulp, and the calamus opens below by an inferior umbilicus and above by a superior umbilicus. The stalk above the calamus is a solid rachis having an umbilical groove on its underside. Pennaceous feathers have a rachis with vanes or vaxillum spreading to either side. These vanes are composed of a high number of flattened barbs, that are connected to one another with barbules.

The barbules are tiny strands that criss-cross on the flattened sides of the barbs. This forms a kind of miniature velcro-like mesh that holds all the barbs together, stabilizing the vanes.

Pennaceous feathers on the forelimb or wing are often attached strongly due to stresses related to flight or other activities. This strong attachment is accomplished by ligaments under the skin, which in some birds and feathered dinosaurs results in raised bumps or marks along the rear forelimb bone (ulna). These bumps, called quill knobs (ulnar papillae) are often used as an indirect indication of strongly-attached forelimb feathers in fossil species, and can also indirectly indicate the number of secondary regimes in a given specimen.

Flight feathers (remiges and rectrices) are specialized types of pennaceous feathers, adapted for high loadings and often strongly asymmetric for improved flight performance.