Apparently primitive rhipidistian characters in _Hesperoherpeton_ are:
Braincase in two sections, posterior one containing an expanded
notochordal canal; lateral series of mandibular bones closely
resembling that of _Megalichthys_, as figured by Watson (1926); tabular
having long process probably articulating with pectoral girdle; lack of
movement between head and trunk correlated with absence of occipital
condyle; sensory pits present on frontal and squamosal.
Although we are unable to separate, by sutures, the vomers from the
palatines, the palatal surface of these bones and of the pterygoids is
studded by numerous small teeth, as in Rhipidistia (Jarvik, 1954) and
some of the early Amphibia (Romer, 1947). The stapes apparently reaches
the quadrate, and could therefore serve in hyostylic suspension of the
upper jaw.
The pectoral limb has an axial series of bones carrying hooklike
flanges on their posterior edges. The other bones of the limb show
little modification of form beyond the nearly flat, aquatic type seen
in Rhipidistia. No distinct elbow or wrist joints are developed.
Characters of _Hesperoherpeton_ common to most primitive Amphibia, in
contrast with Crossopterygii, are: Nares separated from edge of jaw;
stapes having external process that may have met a tympanic membrane,
thus giving the bone a sound-transmitting function. Apparently none of
the opercular series was present.
There are two large palatal teeth, slightly labyrinthine in character,
adjacent to each internal naris. The scapulocoracoid, as shown by
Peabody (1958), is Anthracosaurian in structure, as are the
long-stemmed clavicles. The limbs have digits rather than fin-lobes,
although the digital number apparently is four and the number of bones
in the manus is less than would be expected in a primitive amphibian.
The vertebrae are similar to those of Ichthyostegids, as described by
Jarvik (1952), except that the pleurocentra are much larger.
In addition to this remarkable combination of crossopterygian and
amphibian characters, _Hesperoherpeton_ is specialized in certain
features of the skull. The orbits are much enlarged, probably in
correlation with the diminutive size of the animal, and this has been
accompanied by loss of several bones. The frontal and squamosal nearly
meet each other, and both form part of the rim of the orbit. The bones
of the posterior part of the dermal roof are greatly reduced, and there
is none behind the squamosal except the projecting tabular; there is no
indication of quadratojugal, jugal, intertemporal or postparietal. The
foramen magnum is enormous. The external surfaces of the bones of the
skull are nearly smooth.
Is it possible that the "primitive" and "specialized" features of this
animal are actually larval? Are they not just the kind of characters
that would be expected in an immature, aquatic embolomere of
Pennsylvanian time? For several reasons we do not think this is the
case. Except for the anterior part of the braincase, there is no
indication that the skeleton was not well ossified. The postaxial
processes on the humerus, ulna and ulnare could scarcely have been
larval features only, since they are so clearly homologous with those
in adult Rhipidistia; a larval limb should indeed be simple, but its
simplicity is unlikely to involve paleotelic adult characters. The
scapulocoracoid of our specimen is of practically the same shape and
size as that in the only other known individual, the type; this would
be probable if both were adults, but somewhat less likely if they were
larvae of a much larger animal. The form of the stapes, tabular and
otic notch suggest a functional tympanic membrane, which could not have
occurred in a gill-breathing larva. On the other hand, an adult animal
of pigmy size might be expected to have large orbits, large otic
capsules and a large foramen magnum.
We conclude that _Hesperoherpeton_ lived and sought food in the weedy
shallows at the margin of a pond or lagoon, and that for much of the
time its head was partly out of water (Fig. 12). The animal could
either steady itself or crawl around by means of the paddlelike limbs,
but these probably could not be used in effective locomotion on land.
Like the Ichthyostegids, it probably swam by means of a fishlike tail.
[Illustration: FIG. 12. _Hesperoherpeton garnettense_ Peabody. Probable
appearance in life. x 0.5.]