PECTORAL GIRDLE
Hecht and Ruibal (Copeia, 1928:242) make a strong point of the nature of the pectoral girdle in Notobatrachus, as described recently by Stipanicic and Reig (1955, 1956) from the Jurassic of Patagonia, and quite rightly recommend that the significance of the arciferal and firmisternal types of girdle be restudied. That of Notobatrachus is said to be firmisternal; in view of the arciferal condition in the supposedly primitive Leiopelma, Ascaphus, Bombinator, etc., this comes as a surprise. Is the firmisternal girdle, as seen in Rana, Bufo, and others, actually the ancestral type, and has the arciferal been derived from something like this?
In the figures given by Stipanicic and Reig the ossified parts of the girdle are figured in detail (Fig. 8) and Reig's discussion of it is thorough. The decision to call it firmisternal was taken with some hesitancy, for no median elements are indicated, and the position and shape of those seen is closely similar to the ossified parts in Ascaphus and Leiopelma; there is no bony sternum or omosternum. It is safe to suppose that some cartilage lay in the midline between the clavicles and coracoids, but there is no evidence as to its extent, rigidity, or degree of overlapping if any. Apparently, then, there is not sufficient reason to infer that this Jurassic frog had a pectoral girdle comparable with the modern firmisternal type.
Piveteau (1955:261) remarks that the only living Anuran that can be compared usefully with Protobatrachus (Triassic) with regard to its pectoral girdle is Ascaphus. Again, the extent of cartilage in Protobatrachus (Fig. 8) can only be inferred, and there are no median elements. The agreement with Ascaphus includes the presence, in both, of a separate coracoid ossification situated posterior to the ossified "scapulocoracoid" (actually scapula). This ossification is evidently that shown in Notobatrachus as "coracoid." Direct comparison of the three genera with one another suggests that if we use the term arciferal for any, we should use it for all.
In the remote predecessor of Anura, Amphibamus of the Pennsylvanian, the pectoral girdle was less substantial than in many of its contemporaries, but it contained the primitive median interclavicle in addition to the clavicle, cleithrum, and scapulocoracoid. (The figure of Watson, 1940, and that by Gregory, 1950, are of individuals of different ages, the latter being older.) It is clear that the paired elements of such a girdle were held rigid by their attachment to the interclavicle, via the clavicles. Subsequent elimination of the interclavicle in the Anuran line of descent, and decrease of ossification, left a girdle like that of Protobatrachus, Notobatrachus, Ascaphus and Leiopelma. But in several advanced families a more rigid median "sternum," of one or two bony pieces plus cartilage, is developed secondarily, possibly (as Cope, 1889: 247, suggested) in correlation with axillary amplexus.
Among Urodela no dermal bones occur in the pectoral girdle. There is usually a scapulocoracoid ossified as a single piece, from which a thin cartilaginous suprascapula extends dorsally and a broad cartilaginous coracoid plate extends medially, overlapping the one from the opposite side; a precoracoid lobe of this reaches forward on either side, and a median, posterior "sternum" of cartilage may make contact with the edges of the two coracoids. In Siren and Amphiuma two centers of ossification are found for each scapulocoracoid, and in Triton and Salamandra three. Probably the more dorsal and lateral of these represents the primitive scapula and the other one (or two) the primitive coracoid.
Comparing the girdle of a salamander with that of a frog, the closest similarity can be seen between Ascaphus and a salamander in which the scapula and coracoid ossify separately. Both have the median "sternum" in contact with the coracoid plates. The major difference, of course, is the lack of clavicle and cleithrum in the salamander.