The Sequence of Cranial Ossification in the California Toad, Bufo boreas (Amphibia, Anura, Bufonidae)

Abstract

The order of appearance of skull bones in Bufo boreas is described. Skull bones appear in the following sequence: exoccipitals, parasphenoid, frontoparietals, premaxillae, prootics, squamosals, angulares, dentaries, septompxillae, maxillae, nasals, pterygoids, prevomers, palatines, quadratojugals, mentomeckelians, columellae, and sphenethmoid. Overall skull devel- opmental patterns in B. boreas did not differ greatly from those in other generalized frogs; however, differences were noted in the specific developmental patterns of the frontoparietal, exoccipital, columella, and sphenethmoid. Differences in overall developmental patterns were noted when comparisons were made with Xenopus laevis, Ascaphus truei, Osteopilus septentri- onalis, Smilisca baudinii, and Triprion petasatus. Comparisons among all of the species studied to date indicate several instances of fusion of, or loss of ossification centers in the skull of Bufo boreas. A comparison of osteogenesis in field-collected and lab-reared B. boreas revealed that even though growth patterns and sequence of appearance is the same in both groups, head bones develop significantly earlier in toads collected in the field. Head bones also developed earlier in toads collected in the Owens Valley of eastern California than in the San Fernando Valley of southern California. * * *