The urohyal : development and homology within osteichthyans

The formation of the unpaired structure ventral to the basibranchial region, the so-called urohyal, differs within osteichthyans. A cartilaginous preformed, unpaired "urohyal" is present in sarcopterygians. A three-tendon ossification is present in Polypterus. An %rohyal" or urohyal is absent in both Amia and Lepisos- teus. The urohyal formed as an unpaired ossification of the tendon of the sternohyoi- deus muscle is a feature of teleosts. A new structure, the parurohyal, arises as a double ossification of the tendon of the sternohyoideus muscle in siluroids; during ontogeny an anterodorsal crest or cup-like structure derives from the anterior basibranchial region and the tendon bone; therefore, the parurohyal is compound in origin. Judging from their formation and their distribution within osteichthyans the cartilaginous preformed "urohyal" and the teleostean urohyal are nonhomologous, whereas the urohyal and parurohyal are homologous. The urohyal is connected by ligaments with the ventral hypohyals in most teleosts, whereas it articulates with the ventral hypohyals in teleosts such as Anguilla and Chanos. The parurohyal is a synapomor- phy of siluroids. The parurohyal in siluroids is articulated with both ventral and dorsal hypohyals, and with the basibranchial region in catfishes such as diplomystids and ictalurids, whereas it articulates only with the ventral hypohyals in other catfishes such as trichomycterines. The passage of the hypobranchial artery through the hypobranchial foramen of the parurohyal is a unique feature of siluroids, like the absence of the basihyal bone. An ossified dorsal hypohyal appears late in ontogeny in Amia, as do tooth plates related to the medial side of the hyoid arch and branchiostegal rays in Amia, and tooth plates on the hyoid arch and branchiostegal rays in Elops (unique features within extant teleosts). Two ossified hypohyals, a synapomorphy of teleosts, are present early in ontogeny. There is intraspecific variation in the onset of ossification of the bones of the head, but the sequence of ossification between bones in a defined structural system is conserved (e.g., branchiostegal rays ossify first, then bones of the hyoid arch). The gill basket of gnathostome fishes is sup- ported by paired cartilaginous or bony elements which meet one or many unpaired median elements, the basibranchials, in the ventral midline. Ventral to the basibranchial(s), one unpaired median bone appears in some osteichthyans. Traditionally the unpaired me- dian bone ventral to the basibranchial(s) has been called the urohyal in actinopterygians and sarcopterygians. The "urohyal" in sarcoptery- gians is an endoskeletal bone, whereas the uro- hyal of actinopterygians is either a tendon bone (Polypterus) or a dermal bone (teleosts) (Jarvik, '54). Jarvik ('54: footnote on p. 26) hypothesized the existence of a subbranchial series of paired elements (e.g., the paired tendon bones of the urohyal in Polypterus, the radii posteriores in urodeles, and the paraglossum in birds), but he stopped short of connecting these with the shoul- der girdle. Patterson ('77) argued that the uro- hyal of Pholidophorus and other teleosts derives from a fusion of the interclavicle (dermal bone) with an endoskeletal dorsal plate that would require a sinking inwards of the interclavicle (delamination theory). The urohyal is missing in most primitive actinopterygians but is formed in all teleosts, with the exception of the osteoglosso- morph Gymnarchidae (Taverne, '79). Gardiner ('84) postulates a cartilaginous urohyal in primi- tive actinopterygians, which is therefore not fos- silized. The few descriptions of the teleostean urohyal do not mention its development; they only describe it as an unpaired element existing from early ontogeny (Kindred, '29; de Beer, '37;