Morphological and phylogenetic changes of the hyobranchial apparatus from fishes to basal tetrapods.
Final Report Abstract
The basic arrangement of hyobranchial skeletal elements has been conserved to a large degree across the fish-to-tetrapod transition. The plesiomorphic condition of the tetrapod hyobranchium is remarkably fish-like and may be reconstructed as follows: one slender, rod-like basibranchial with expanded anterior and posterior ends, a pair of small, stout hypohyals, one pair of elongate, flattened ceratohyals, four pairs of rod-like hypobranchials, four pairs of curved, deeply grooved ceratobranchials with denticulate branchial platelets, and short epibranchials. The most prominent changes in the hyobranchium during the fish-to-tetrapod transition are as follows: reduction of the basibranchial ossifications to one basibranchial bone, whereas basihyal and urohyal remained cartilaginous; morphological simplification of basibranchial and hypobranchials; restriction of toothbearing pharyngeal platelets to the ceratobranchials; weaker degree of ossification (especially of the epiphyses) of skeletal elements and no ossified articulation facets; and reduction of distal hyobranchial elements. The fish-like hyobranchial apparatus was retained in further tetrapod evolution not only in early growth stages (larvae), but was present also in adults of different lineages (stem-tetrapods, temnospondyls, stem-amniotes). Thus, three or four pairs of cartilaginous or ossified ceratobranchials in basal tetrapods do not necessarily represent a larval or paedomorphic character, respectively, as was often suggested in analogy to extant salamanders. Rather, it represents the plesiomorphic state of the adult hyobranchium in tetrapods. According to these results, hypothesis 2 ("hyobranchium-supported land feeding") can be refuted and hypothesis 1 ("jaw prehension land feeding") is supported. In basal tetrapods, many functions of the hyobranchium of their fish-like ancestors were preserved (see above): the support of internal gills and of branchial teeth, as well as dorsoventral rotation of the apparatus (manily by rectus cervicis and geniohyoideus muscles) for mouth opening and buccal pumping for breathing and feeding. This was accomplished by those skeletal elements that were already present in fishes, although their number was reduced, and also several hyobranchial muscle like rectus cervicis (=sternohyoideus), geniohyoideus and intermandibularis were retained according to the Extant Phylogenetic Bracket and muscle attachment sites. In spite of the specializations present in the anatomy of Gerrothorax, the reconstruction of parts of its cranial and hyobranchial muscles has underlined that aquatic feeding in basal tetrapods was basically similar to that in extant urodeles and osteichthyans. However, due to the reduction of the more distal skeletal hyobranchial elements and the opercular apparatus, hyobranchial movements were mainly dorsoventral and lateral movements as in fishes did not take place. A hyobranchial apparatus with several posterior branchial arches reminiscent to those of fishes was retained also in primarily terrestrial basal tetrapods, whereas neomorphic structures to support tongue feeding developed later in evolution. Unfortunately, it has not been possible to identify these neomorphic structures in basal tetrapods due to a lack of preservation and their probably feebly ossified or cartilaginous nature.
