The ventral arms of the squamosal and the supraoccipital region of the exoccipital are the first occipital bones to appear. Ossification follows in the regions of the semicircular canals and occipital condyles. The dorsal end of the ventral arm of the squamosal and the posterior arm of the squamosal ossify as a unit at the same time the quadratojugal appears. Shortly thereafter the anterior arm of the squamosal ossifies, the distal part of the columella appears, and the anterior and lateral parts of the auditory region ossify.
The angular and dentary of the lower jaw appear concurrently with the dentigerous bones. Initially, the angular is short and broad; the articular surface is absent, and the anterior end is slightly overlapped by the dentary. The mentomecklians do not ossify until approximately the same time that the quadratojugal appears in the upper jaw.
The genus Smilisca is characterized by the following combination of cranial osteological characters: (1) A large amount of bone is involved in the skull and a minimal amount of cartilage and/or secondarily ossified cartilage; co-ossification is absent. (2) The skulls are uniformly broad with angular lateral margins, and truncate anteriorly. (3) An internasal septum and quadratojugals are present. (4) A well-developed squamosal minimally extends one-fourth the distance from the dorsal end of the quadrate to the maxillary, and maximally is separated from the maxillary by a suture. (5) The ethmoid is large; the distance between the anterior end of the ethmoid and the anterior edge of the premaxillary varies between 15 and 20 per cent of the total length of the skull.
On the basis of cranial osteology two species-groups can be recognized within the genus Smilisca. The sordida group, comprising S. sordida and puma, is characterized by a broad skull in which the lateral margins of the maxillaries are relatively straight anterior to the orbit. The moderate-sized nasals are rounded anteriorly, and bear relatively short, sometimes blunt, maxillary processes. The long axes of the nasals are not parallel to the maxillaries. The ethmoid is proportionately small in the sordida group. The bony part of the ethmoid terminates near the anterior edge of the orbits and does not extend anteriorly between the nasals; the entire anterior margin of the ethmoid is separated from the nasals by cartilage. The squamosals are generally small. They are narrow in dorsal view, and minimally extend one-fourth the distance from the dorsal end of the quadrate to the maxillary, and maximally, two-thirds the distance. The tegmen tympani are relatively small (Fig. 8).
Fig. 8. Dorsal views of the skulls of the species of Smilisca: (A) S. baudini (KU 68184); (B) S. puma (KU 68636); (C) S. phaeota (KU 41090); (D) S. sila (KU 80625); (E) S. cyanosticta (KU 55938), and (F) S. sordida (KU 36765). ×1.5.
In contrast to the tendency for reduction of cranial parts in the sordida group, the baudini group, constituted by S. cyanosticta, phaeota, and baudini, is characterized by more ossification of the cranial elements. The skull is broad; the lateral margins are less angular and are gently curved, rather than straight as in the sordida group. The nasals tend to be larger with the long axes parallel to the maxillary. Anteriorly the nasals are pointed, and posteriorly they bear long, delicate palatine processes extending to the maxillary. The ethmoid is fully ossified, extends anteriorly between the nasals, and laterally is separated by a suture from the nasals if the latter are fully ossified. The squamosals are large, and wide in dorsal view. They minimally extend one-fourth the distance from the dorsal end of the quadrate to the maxillary, and maximally are sutured to the maxillary. The tegmen tympani are massive.