While the endocranium is an integral part of the skull in mammals, birds and reptiles, its connection to the roofing parts of the skull is more loose in the lower vertebrates.
In Agnathans and Chondrichthyes, the skull lacks the skull roof dermal elements, their whole cranium being composed of the endocranium, properly called a chondrocranium.
In most Osteichthyes, the skull is only loosely joined, and the endocranial elements do not form a unit with the skull roof.
While the endocranium is an integral part of the skull in mammals, birds and reptiles, its connection to the roofing parts of the skull is more loose in the lower vertebrates. In Agnathans and Chondrichthyes, the skull lacks the skull roof dermal elements, their whole cranium being composed of the endocranium, properly called a chondrocranium. In most Osteichthyes, the skull is only loosely joined, and the endocranial elements do not form a unit with the skull roof.
Endocranium