and Acanthostega. The cleithrum (upper blade of the shoulder) is fused to the scapulocoracoid (lower plate of the shoulder, in front of the shoulder socket), unlike in most tetrapods. Conversely, the shoulder girdle is independent of the skull, unlike in most fish.

Early four-limbed vertebrates are often referred to as tetrapods, according to a trait-based definition of the term as commonly used by most paleontologists. Devonian