Fig. (15) Vertebrate brainstem vascular characters. The presence of paired versus unpaired basilar arteries and the dominance of transverse versus median penetrating arteries are indicated (+) for the different clades examined in this study. Whether the basilar arteries are paired along the entire hindbrain in the coelacanth is uncertain (+?). Our current hypothesis is that paired basilars are primitive for all vertebrates and that a single basilar arose 3 times independently in cartilaginous fishes, teleosts, and tetrapods. Loss of circumferential transverse arteries and restriction of penetrating branches to midline central arteries is a unique teleost innovation, demonstrating that the neurovascular anatomy of teleosts is clearly derived relative to all other vertebrates.