The premaxilla (Pmx, Figs. 3D, 5A,D, and 6D) extends into the rostrum but does not reach into the tip as a small remnant of the premaxillary cartilage remains (Pmx crt, Figs. 5B and 6A,B,D). Caudally, the dorsal aspect of the premaxilla is perforated by a foramen, presumably for a branch of the infraorbital nerve and its associated vessels. Caudally the premaxilla ends in a narrow process, rostral to the external bony nares (Ebn, Figs. 5A and 6A). The premaxilla is not fused to the maxilla but these two bones are adjacent to each other.
The maxilla (Max) is expanded rostrally and caudally (Figs. 3C,D, 5A–D, and 6B,D). Rostrally, the maxilla reaches nearly as far as the premaxilla and forms most of the rostrum overlapping the premaxilla in lateral view. The maxilla also forms most of the palate, leaving a long alveolar groove (Alg, Figs. 3C and 5B) laterally to house the developing teeth. Tooth buds are not visible and were probably not mineralized at this stage.
The vomer is ossified at this stage (Vom, Figs. 3D and 5A,C); it is a long, narrow bone in the median plane, wedged between the nasal cartilage dorsally, and the maxilla, palatine, and pterygoid bones ventrally. The vomer does not extend as far rostrally as the maxilla, but it reaches beyond the basisphenoid (Bsp) caudally (Figs. 3D, 5A, and 6A). The nasal bone (Nas) is a small circular ossification just medial to the anterior part of the frontal and caudal to the external bony nares (Figs. 5B and 6B).
The lacrimal (Lac) is a small square bone in the rostral rim of the orbit (Figs. 3C, 5D, and 6D). The lacrimal process is located at the corner of the lacrimal bone and projects into the orbit. The lacrimal is fused with the jugal (Jug), a needle-shaped bone that extends along the entire ventral side of the orbit (Figs. 3C, 5B,D, and 6B,D).
The palatine (Pal) contributes to the caudal section of the palate (Figs. 3D, 5A,C, and 6A). At midline, the palatine is caudal to the maxilla and rostral to the pterygoid (Ptg, Figs. 3D, 5A,C, and 6A). In the infraorbital region, the palatine forms the lateral wall of the nasal cavity. The pterygoid forms the caudal portion of the palate and forms a hook similar to that in the adult (Mead and Fordyce, 2009).
The frontal bone (Fro) is considerably larger than in the previous stage and extends medially (Figs. 3C,D, 5A,B,D, and 6A,B,D). Rostrally, the frontal bone is overlapped by the maxilla (Figs. 3C, 5D, and 6D). The frontal bone also forms a distinct supraorbital ridge (Mead and Fordyce, 2009), and this ridge ends as the sharp posterior orbital process (Pop, Figs. 3C, 5B, and 6B,D), which is already developing in these Stenella fetuses. Caudally, the frontal is adjacent to the parietal (Par), but it does not overlap the frontal bone (Figs. 3C,D, 5A,B,D, and 6A–D). The parietal is an oval bone and makes up most of the lateral side of the braincase. Ventral to the parietal is the squamosal (Squ, Figs. 3C, 5B,C, and 6B,D). The squamosal consists of a pars medialis (Prm, Fig. 5D) that will develop into the lateral wall of the braincase, and a pars lateralis (Prl, Fig. 5D) that will form the zygomatic process of the squamosal bone. The interparietal bone (Inp) stretches toward the medial plane, making up part of the dorsal roof of the braincase (Figs. 3C,D, 5A,B,D, and 6A–D). There are large fontanelles between the interparietal and frontal bones (Figs. 5D and 6D) and also between the parietal and supraoccipital bone (Figs. 5D and 6C).
The dentary (Den) is ossified almost to the rostral tip but does not reach caudally to the squamosal bone (Figs. 3C,D, 5A–D, and 6A,B,D). The dentary, in its rostral two-thirds, covers the diminishing Meckel's cartilage medially, laterally, and ventrally; it only covers the lateral side of Meckel's cartilage for its caudal third (Figs. 3D, 5A, and 6A).
The ectotympanic (Ect) is undergoing ossification (Figs. 3C,D, 5A–C, and 6B). It is horseshoe-shaped and its medial edge is expanding into the ventral middle ear wall. This medial edge of the ectotympanic has a rostral process. The caudal limb of the ectotympanic terminates in a semicircular and flat piece of bone. There is no sign of a sigmoid process or an involucrum.
The chondrocranium is disappearing at this stage; cartilage is retained mostly in the ventral midline. The cribriform plate is cartilaginous at this stage, but lacks the foramen for CN I present in the previous stage. The nasal cartilage (Nas crt) or mesorostral cartilage of Mead and Fordyce (2009) is large and triangular (Fig. 6A). It covers the vomer in the median plane of Fig. 6, however, the vomer is visible in Figs. 3D and 5A where the skull is cut more laterally exposing structures lateral to midline. The nasal cartilage continues caudally to the presphenoid (Pre), which is a large ossification center present in this area (Figs. 3D, 5A, and 6A). The ossification center for the presphenoid is continuous with that for the orbitosphenoid (Orb sph) in the ala orbitalis (Figs. 3C,D, 5A,B, and 6B). The ventral part of the orbitosphenoid is ossified and the optic foramen remains large (Opt for, Figs. 3D, 5A,B, and 6B). The dorsal part of the ala orbitalis remains cartilaginous and is surrounded by the semicircular rim of the orbit, as made by the frontal bone.
The ossification center of the basisphenoid (Bsp, Figs. 3D, 5A, and 6A) is separated by cartilage from the presphenoid rostrally and is distinct from the basioccipital bone (Boc) caudally (Fig. 6A). The ala temporalis is present, but faint and its connections to the ala orbitalis and the occipital arch are not visible. An oval ossification center for the ali temporalis is present (Alt, Fig. 6B).
Laterally, the exoccipital (Exo) is ossified (Figs. 3C,D, 5A,B, and 6B,D) and the rest of the occipital arch is cartilaginous and surrounds the foramen magnum. There are clear cartilaginous occipital condyles present at this stage. Dorsally, the supraoccipital bone (Soc) is ossified (Fig. 3C,D, 5A,B,D, and 6A–D). It is a diamond-shaped, bilateral ossification in LACM 94592. In LACM 94310, the supraoccipital bone is slightly larger and consists of three parts. Right (RSoc, Fig. 6C) and left (LSoc, Fig. 6C) ossifications are bilaterally paired dorsally and a single ventral ossification is present caudally (Soc, Fig. 6C). The supraoccipital bone stretches from the foramen magnum to where the interparietal and parietal bones meet. The supraoccipital bone does not overlap with any other bone (Fig. 6C). The supraoccipital is not fused to the interparietal bone at this time. The otic capsule is faint and displays no morphological details.
Only the most caudal portion of Meckel's cartilage remains continuous with the cartilaginous malleus (Mal) of the middle ear (Fig. 4C,D). The manubrium (Man) of the malleus is thick and points ventrocaudally. The accessory ossicle (Acc, Fig. 4C,D) is a densely ossified, circular structure that overlies, and is fused to, Meckel's cartilage (Mec, Fig. 4C,D). It is not fused to the ectotympanic as in adult odontocetes (Luo, 1998). The head of the malleus is distinct and separate from Meckel's cartilage and is located slightly ventral to Meckel's cartilage (Fig. 4C,D). The incus (Inc, Fig. 3C) is cartilaginous, with the crus longum (Crl, Fig. 4C,D) directed ventrally and articulating with the faintly visible cartilaginous stapes (Stp, Fig. 4C,D). The crus breve (Crb) points caudally (Fig. 4C,D). The crus longum is still more robust than the crus breve.
The hyoid is connected to the chondrocranium caudally. The bar-shaped stylohyoid (Sth) is ossifying and is between two cartilaginous sections of the hyoid (Figs. 3C,D, 4C,D, 5B, and 6B,D). The distal cartilage bar connects to the oval basihyoid cartilage (Bas crt), from which the thyrohyoid cartilage (Thh crt) extends caudally (Figs. 5B and 6B). The basihyoid and thyrohyoid are not ossified.