[Stiglr]

Try this:

Surface mode. Select all the surfaces in the first concentric ring around the top of the skull (that is, the very top center vertice and the first ring of vertices around it). Be sure you have only those surfaces selected, by spinning around in 3D mode and checking your left and front views.

Surface/Delete Surface Only. This will delete all the surfaces, but leave the vertices.

Now, select the vertices that describe the same (now empty) area.
Vertex/Create Convex Hull.

You may have to go into the head and get rid of a few surfaces that form a "floor" under all the vertices, but if you have no polygon limit pressure, don't bother.

Select your newly created surfaces, and the rest of the head, or whatever you want to all be one object and Merge them. Then do a Optimize Vertices and Optimize Surfaces to be sure you get rid of any double vertices (these were what likely caused this mess in the first place).

Then, Surface/Triangulate the entire object.