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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 supraorbital shelf). Hexanchiforms are the only extant elasmobranchs with a postorbital articulation, but do not occupy a basal position in modern morphological and molecular phylogenetic analyses. Amphistyly in hexanchiforms is therefore viewed as a derived state rather than a highly conserved feature.
No hyomandibular facet has been identified in “Cobelodus”, suggesting that its epihyal had only a ligamentous connection to the braincase. However, previous suggestions that symmoriiforms were aphetohyoidean (with a complete hyoidean gill slit and “unmodified” hyoid arch) are not supported by morphological evidence.
The systematic classification of symmoriiform sharks is in disarray. Symmoriiforms collectively are probably monophyletic, but within them only the family Falcatidae is characterized convincingly by synapomorphies. Remaining symmoriiforms have been traditionally classified as “stethacanthids” and “symmoriids”, based respectively on the presence or absence of a spine-brush complex, but that distinction seems artificial because no undisputable “brushless male symmoriids” or “brushed female stethacanthids” have been documented and because sex-linked dimorphism of the spine-brush complex has been demonstrated only in falcatids. The braincase in Cladoselache shares some unusual features with “Cobelodus”, suggesting that Cladoselache and symmoriiforms are closely related, but it has yet to be determined whether Cladoselache was morphologically platybasic or tropibasic.