The occipital regions of the braincases of two gigantic ctenacanthiform sharks are described from the Finis Shale (Virgilian, Upper Pennsylvanian, ca. 300 Ma) of Texas. Their original braincase lengths are estimated to have been 42.8–68.2 and 33.9–64.8 cm, based on comparison with smaller, more complete ctenacanthiform specimens (e.g., Tamiobatis). In complete ctenacanthiform body fossils (e.g., Goodrichthys), braincase length represents approximately 10% of total body length. This suggests that the Finis Shale sharks attained lengths up to 7 m (as large as modern great white sharks and Cretaceous cardabiodontids) and body weights of 1500–2500 kg. As apex predators, such large sharks probably maintained a high hepatosomatic index and may even have utilized lamnid-like heat exchange retia in critical parts of the body (e.g., brain, eyes, stomach).