Morphology and Development of the Skull in the Frog Hyla septentrionalis

The skull of Hyla septentrionalis is solidly roofed and partly fused with the skin of the head. The roofing bones (nasals, frontoparietals, and ethmoid), and the maxillary flanges are involved in integumentary coossification. Co-ossified skin is characterized by bone deposition continuous between, and in, the dense connective tissue of the dermal layer of the skin, which effectively obliterates the lower dermal layer and unites the epidermis and superficial dermis with the underlying bone. The ethmoid is usually considered to be completely endochondral in origin; in order to explain its involvement with integumentary-cranial co-ossification, a dual origin from two ossification centers, one endochondral and the other intermembranous, is suggested. Development of the skull of Hyla septentrionalis resembles that of other hylids. Integumentarycranial co-ossification is not evident until all the cranial bones are well formed. Some variation in the extent of cranial development is correlated with sex and age.