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.