The left forelimb is the only one present and appears to be nearly
complete, although the elements are scattered almost at random. The
only parts of the forelimb known to be missing are two subterminal and
two terminal phalanges, probably of the first and third digits, and the
proximal end of the second metacarpal. The smooth and relatively flat
surfaces suggest an aquatic rather than terrestrial limb; only the
proximal half of the humerus bears any conspicuous ridges or
depressions. As we restore the skeleton of the limb, several features
are remarkable: The humerus, ulna, and ulnare align themselves as the
major axis of the limb, each carrying on its posterior edge a process
or flange comparable to those in the axial series of a rhipidistian
fin. The remaining elements take positions comparable to the diagonally
placed preaxial radials in such a fin. The digits appear to have been
short, perhaps with no more than two phalanges. There is only one row
of carpals present (the proximal row of other tetrapods). A second and
third row would be expected in primitive Amphibia; if they existed in
_Hesperoherpeton_ they must either have been wholly cartilaginous or
washed away from the specimen. Neither of these alternatives seems at
all likely to us in view of the well-ossified condition of the elements
that are present, and the occurrence of both the proximal carpals and
the metacarpals. The space available for metacarpals probably could not
have contained more than the four that are recognized.
[Illustration: FIG. 11. _Hesperoherpeton garnettense_ Peabody. Left
forelimb, showing characters of both a crossopterygian fin and an
amphibian foot. KU 10295, x 4.]
The proximal end of the humerus is more rounded anteriorly than
posteriorly, and has a thin articular border that bore a cartilaginous
cap as the primary surface for articulation with the scapulocoracoid.
Although the unfinished surface of the head extends down the anterior
margin about a third the length of the humerus, the shaft has been
broken and so twisted that the distal part is not in the same plane as
the proximal. Immediately posterior to the cartilaginous cap is a
round, deep notch bordered posteriorly by the dorsal process of the
head.
The shaft is longer and narrower than would be anticipated in a
primitive amphibian limb (cf. Romer, 1947). The distal end bears two
surfaces for articulation with the radius and ulna. The full extent of
the former surface was not determined because the more anterior part of
the expanded end is represented only by an impression. The surface
nearest the ulna was partially rounded for articulation with that
element, the remaining posterior edge being broadly concave. The most
striking feature of the humerus is a slender hooklike process on the
posterior edge near the distal end, probably homologous with (1) the
posterior flange on the "humerus" in Rhipidistia, and (2) the
entepicondyle of the humerus in _Archeria_ (Romer, 1957) and other
tetrapods.
The radius is about the same width proximally as distally. The
curvature of the shaft is approximately alike on both sides. Distally
the surface is rounded for articulation with the radiale and perhaps
the intermedium.
The proximal end of the ulna is similar to that of the radius but is
slightly larger. Posteriorly, there is a short, broad expansion
resembling the entepicondyle of the humerus, and even more nearly like
the postaxial flanges in a crossopterygian fin.
The ends of the radiale are expanded and rounded, the entire bone being
approximately twice as long as wide. The three sides of the intermedium
are similarly convex. The surface of this bone is unfinished, showing
that it must have been embedded in cartilage. The ulnare is
conspicuously similar to the ulna in bearing a posterior hooklike
expansion, and is larger than the radiale.
The four metacarpals are slightly expanded proximally and distally.
Although measurements of length and width are tabulated below (Table
2), we are not certain of the sequence of these bones in the row.
The dimensions of the two proximal phalanges are alike. The shape of
these elements is similar to that of the metacarpals. The two terminal
phalanges are somewhat triangular in shape, the lateral edges being
concave and the proximal convex.