. Chordate morphology. Morphology (Animals); Chordata. sheath and constriction of the notochord in the basal plate, notochord reaching dorsal margin of dorsum sellae and pro- jecting from it (at least in younger stages), dorsum sellae forming between ends of parachordals, epihyal not ex- tending up lateral to head vein (no laterohyal involved), presence of orbital margin of cartilages, presence of nasal and labial cartilages, and separate posterior superficial ophthalmic foramen. Acanthodians The acanthodians are represented by spines and plates in Upper Silurian deposits, but it is not until
. Chordate morphology. Morphology (Animals); Chordata. sheath and constriction of the notochord in the basal plate, notochord reaching dorsal margin of dorsum sellae and pro- jecting from it (at least in younger stages), dorsum sellae forming between ends of parachordals, epihyal not ex- tending up lateral to head vein (no laterohyal involved), presence of orbital margin of cartilages, presence of nasal and labial cartilages, and separate posterior superficial ophthalmic foramen. Acanthodians The acanthodians are represented by spines and plates in Upper Silurian deposits, but it is not until Devonian strata that much of their structure is preserved. The head (Figure 5-21) was covered by small scales with larger plates forming a circumorbital ring. The several gill slits opened separately behind folds; the most anterior fold or operculum was supported by mandibular rays. The endocranium had a large anterior orbit, little or no ethmoid region, and a large postorbital oticoccipital section. The endocranium was largely cartilaginous with subperichondral calcification in some species, and in others perichondral ossification shading into subperichondral calcification, which appears to be the more primitive condition. These ossifica- tions included a pair of exoccipitals lying behind the vagus foramen. There was a basioccipital enclosing the dorsal aorta, and in front of this a basisphenoid. A parasphenoid sheathed the ventral surface and extended forward below the orbits. Its anterior end was associated with five small plates. The postorbital process was ossified and the proc- esses of either side were joined by a section of ossified roof. There was a lateral commissure below the orbital process. The palatoquadrate had quadrate, metapterygoid, and autopalatine centers of ossification. It articulated with the postorbital process and with a basal process of the para- sphenoid. The two palatoquadrates were not joined ante- riorly. The mandible consisted of an anterior and p
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