Note here how the exoccipitals participate in the formation of the occipital condyle. However, the same principles apply. The occiput usually has considerable help here from other elements. Actually, it appears that the articulation in more basal vertebrates was far more complex than in Recent tetrapods. The very complexity of this arrangement may have created the need for an otico-occipital fissure to prevent excessive strain on the various components.
Modern sharks are notably different.
We have spent some extra time with the sharks in order to focus on the structure of the occiput as a whole, without the distraction of individual bones. The Osteichthyans actually show a rather similar pattern of development, with mammals eventually arriving at an occipital structure that is somewhat similar to the neoselachian condition. I have been unable to come up with enough information on the basic actinopterygian or sarcopterygian plans to comment much except to make the following tentative points. The supraoccipital does not seem to have been part of the original complement of occipital ossifications. As in sharks, the dorsal aorta is lost from the occiput first, presumably because it is functionally replaced by the carotid arteries.