Cleidocephalic

Cleidocephalic is an adjective used in paleontology and comparative anatomy to describe a skull that lacks a clavicle, or collarbone. In tetrapods, the clavicle is typically a bone that connects the scapula (shoulder blade) to the sternum (breastbone). Its presence or absence can be an important characteristic for understanding evolutionary relationships and the functional anatomy of extinct and extant animals.

The term itself is derived from the Greek words "kleis" meaning key or collar and "kephalē" meaning head, thus referring to the head's connection, or lack thereof, to the clavicle. Many early tetrapods and some amphibians exhibit cleidocephaly, indicating a primitive or secondarily reduced condition of the pectoral girdle. For instance, some salamanders are cleidocephalic.

In contrast, many mammals, birds, and reptiles possess well-developed clavicles. In some species, the clavicle may be fused with other bones or even entirely absent, but the term cleidocephalic specifically focuses on the skull's association with this particular bone. The absence of a clavicle can have implications for shoulder mobility and limb function. Studying cleidocephaly helps paleontologists reconstruct the biomechanics and evolutionary history of extinct vertebrates.