The concepts of platytrabia/platybasia and tropibasia/tropitrabia in gnathostomes are reviewed. The terms platytrabia and tropitrabia refer to developmental states of the embryonic trabecular cartilages that can be determined only by ontogenetic studies. The terms platybasia and tropibasia originally had this meaning, but have subsequently taken on additional descriptive connotations involving morphological features in the prechordal part of the adult chondrocranium. However, platybasia and tropibasia are not synonymous with platytrabia and tropitrabia. In gnathostomes, platytrabia usually gives rise to a platybasic adult condition (but not invariably; e.g., Lepisosteus), and tropitrabia usually gives rise to the tropibasic condition (modern elasmobranchs may be an exception). Thus, ontogeny does not provide an absolute guide to the adult condition, nor does adult morphology provide an accurate means to assess the prior ontogenetic condition in gnathostomes. Platybasia and tropibasia are regarded here as useful morphological terms that can be applied to fossils or to extant forms for which ontogenetic data are not available (although it may still be possible to reach some ontogenetic conclusions, based on morphological observations).
A well-preserved but disarticulated fossil symmoriiform shark braincase from the Pennsylvanian of Arkansas is described under the informal generic designation “Cobelodus”, using digital reconstructions made from a high-resolution computerized-tomography (CT) scan. The braincase is morphologically tropibasic and clearly represents a departure from the common platybasic pattern found in elasmobranchs (e.g., Tamiobatis, Cladodoides, Orthacanthus).
The contribution made by the embryonic polar cartilage in “Cobelodus” was probably extensive (unlike in modern gnathostomes), as in the platybasic Paleozoic shark Cladodoides. Thus, tropibasia in “Cobelodus” seems to be superimposed on an already-specialized pattern of cranial morphology found in some early platybasic elasmobranchs. The basicranial arterial circuit in “Cobelodus” was highly modified, and its internal carotids could not have communicated with the cranial cavity via the bucco-hypophyseal chamber as in other elasmobranchs. Internal carotids either were absent or met the efferent pseudobranchials within the orbit before the combined vessel entered the cranial cavity via the orbital cartilage, but the arrangement was certainly not osteichthyan-like (where the combined internal carotid/efferent pseudobranchial arteries pass through the basisphenoid pillar).
“Cobelodus” and many other Paleozoic sharks possessed a postorbital palatoquadrate articulation (possibly strengthened by ligaments above the articulation in “Cobelodus”), on cartilage presumably formed in the embryonic lateral commissure. This arrangement differs from that in amphistylic hexanchiform sharks, where the lateral commissure is absent and there is no postorbital arcade; the postorbital articulation is located instead on the primary postorbital process (an outgrowth of the supr