34 p. : 24 cm

"The braincase of Macrepistius arenatus from the Glen Rose Formation (Lower Cretaceous, Albian) of Texas resembles that of other caturids in the following ways: (1) It has a completely ossified interorbital septum and otic region; (2) It possesses both autopterotic ("opisthotic") and prootic ossifications; and (3) It has an enlarged intercalar that partly surrounds the vagal foramen. It differs from other caturids (and apparently from other actinopterygians) in having paired, partly fused, supraoccipitals that are clearly separated from the epiotics. The parasphenoid is toothless and lacks basipterygoid processes. The cranial wall is complete in the area of the trigeminofacialis chamber. The jugular canal is represented by an opening medial to the ascending process of the parasphenoid. A recess below the jugular canal that has separate openings for the palatine and hyomandibular nerves probably contained the facial ganglion. The trigeminofacialis chamber is here defined as the extramural space between the lateral commissure and the primordial lateral wall of the braincase. This chamber has never been secondarily divided into a pars ganglionaris and a pars jugularis in the actinopterygians (as originally claimed by Allis, 1919) although its boundaries and associated foramina have been modified in various ways. An attempt to work out the relationships of Macrepistius on the basis of shared derived characters has resulted in two hypotheses--a 'sister' relationship with either Osteorhachis or Heterolepidotus. Because of insufficient evidence it is not possible to favor either hypothesis. The analysis of caturid and other amiiform character states indicates, however, that the unique specializations of Macrepistius evolved from a caturid with pointed teeth, relatively narrow palate and infraorbitals and no supraoccipitals. Comparison of the neurocranial ossification pattern in Macrepistius with that in a variety of other actinopterygians has emphasized the basic uniformity of the pattern throughout the holosteans, halecostomes, and teleosts, as noted by Rayner (1948). This situation presumably reflects the disposition of the embryonic ossification sites in the palaeonisciform neurocranium, which, in the adult, is usually composed of two major ossifications. The persistence of numerous separate neurocranial bones in the higher actinopterygian groups probably represents a neotenic adaptation of prolonged differential growth of the braincase beyond the chondrocranial stage. These conclusions are based, in part, on neurocranial ossification patterns in an immature specimen of the palaeonisciform Cosmoptychius and in adult specimens of Birgeria and Polypterus. Factors that determine the location and number of neurocranial ossification centers in relation to the chondrocranial model, the organization of the ossification centers in relation to the formation of a particular bone, and finally, the conditions that promote or inhibit perichondral and endochondral ossification in a particular part of the neurocranium (or in the rest of the endoskeleton) are poorly understood. In the absence of pertinent experimental data we can do little more than speculate on these problems. There is, however, circumstantial evidence that the extent of perichondral and endochondral ossification represents a compromise between the ancestral genotype and factors such as stress, buoyancy, equilibrium, and protection"--P. 30-31

Title from caption

"April 21, 1971."

"The uncrushed and nearly complete holostean (caturid) braincase ... described below was collected in 1934 by Gayle Scott in the Lower Cretaceous (Albian) Glen Rose formation, 1 mile southeast of Boyd, Wise County, Texas."

Includes bibliographical references (p. 31-34)

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