_Dorsal Aspect_ (Figs. 1, 2)
In reconstruction, the skull measures approximately 8.0 mm.
dorsoventrally at the posterior end. The height diminishes anteriorly
to about 1.5 mm. at the premaxillary. The length is about 15.5 mm. in
the median line, or 24.0 mm. to the tip of the tabular, and the width
about 16.0 mm. posteriorly. The snout is blunt, continuing about 1-2
mm. anterior to the external nares. Each of the tabulars has a slender
posterior process 5.0 mm. long, which probably met the supracleithrum;
the intertabular space is about 8.5 mm. wide. The orbits are
approximately 5.5 mm. in diameter and extend from the maxillary to
within about 3.0 mm. of the midline dorsally. The pineal opening is 1.8
mm. anterior to the occipital margin of the skull.
Reduction of bones at the back of the skull seems to have eliminated
any dermal elements posterior to the squamosal, while enlargement of
the orbit has removed most of the postorbital series, leaving the
squamosal as the only cheekbone. There is apparently no jugal or
postfrontal.
The squamosal of _Acanthostega_ (Jarvik, 1952) is articulated under the
tabular and reaches forward and down, much as if it were an opercular
in reversed position. Internally, it must lie against the otic capsule
below the tabular, partially concealing the stapes. The bone that we
suppose to be the squamosal of _H. garnettense_ is of similar shape, of
about the same size and has internally an articular surface at one
corner, bounded by a pair of ridges in the shape of a V. This articular
surface probably fitted on a lateral process extending from the roof of
the neurocranium, over the front of the otic capsule.
The premaxillary extends posterolaterally to a distance 5.5 mm. from
the midline and attains a width at its broadest point of about 1.5 mm.
The posterior edge is slightly concave and in part forms the anterior
border of the naris.
[Illustration: FIG. 1. _Hesperoherpeton garnettense_ Peabody. Skull,
dorsal view. Postorbital processes of the neurocranium are shown in
dotted outline. KU 10295, x 4.]
The nasal is triangular and, with the lacrimal, forms the medial border
of the naris. The length of the medial side of the nasal bone is
approximately 5.0 mm., the transverse width is 3.8 mm., and the extent
of the posterolateral border is 5.5 mm.
The maxillary meets the premaxillary lateral to the naris, borders the
naris posteroventrally, and continues posteriorly beneath the orbit, of
which it forms the external border. The maxillary is about 8.5 mm.
long, and immediately anterior to the orbit has a maximum width of 1.3
mm.
The lacrimal fills the remaining rim of the narial opening between the
nasal and maxillary, and extends to the anterior edge of the orbit. The
length, from naris to orbit, is 4.2 mm.; the width ranges from 1.0 mm.
anteriorly to 2.5 mm. posteriorly.
[Illustration: FIG. 2. _Hesperoherpeton garnettense_ Peabody. Skull,
lateral view, showing relatively large orbit and absence of smaller
circumorbital bones. KU 10295, x 4.]
The external naris is approximately 1.0 mm. in diameter. It is slightly
anterodorsal to the internal naris and 4.0 mm. lateral to the midline.
The dorsal margin of the orbit appears to be formed by the frontal. The
anterior part of this margin, however, may be formed by a prefrontal,
which is not clearly set off by a suture. The frontal extends 3.8 mm.
in the midline, and anteriorly and laterally borders the nasal and
lacrimal, respectively. A faint pattern of pitting radiates on the
surface from the center of ossification of the frontal. There is also a
pit indicating the presence of a supraorbital sensory pore.
The parietal bones enclose the pineal opening, approximately 2.5 mm.
posterior to the suture with the frontal. The foramen is about 0.5 mm.
in diameter. Laterally the parietal meets the medial angle of the
postorbital and the medial border of the supratemporal. No bone of this
animal shows the deep pitting and heavy ornamentation characteristic of
many primitive Amphibia.
The postorbital meets the anterolateral corner of the parietal for a
distance of 0.5 mm., the anterior edge bordering the frontal bone and
the orbit for a combined distance of about 3.0 mm. The lateral margin
is slightly convex, and is probably interrupted behind by the anterior
point of the tabular. Medially, the concave margin of the postorbital
meets the supratemporal for about 3.5 mm.
The supratemporal is thus wedge-shaped and located between the parietal
and the postorbital. The posterior edge of the supratemporal protrudes
as a convex border slightly behind the end of the parietal, and
measures 3.0 mm. around the curve to the parietal suture.
[Illustration: FIG. 3. _Hesperoherpeton garnettense_ Peabody. A, left
squamosal, internal surface. B, left squamosal, external surface. C,
right tabular internal surface. D, right tabular, external surface. KU
10295, all x 4.]
The squamosal (Fig. 3 A, B) is a large, somewhat rectangular bone
extending from the back of the orbit to the posterior extremity of the
cheek. It outlines almost entirely the posterior border of the orbit,
the ventrolateral portion of the cheek region, and the lateral border
of the top of the skull behind the orbit. Dorsally, the squamosal meets
the anterior half of the tabular and the lateral border of the
supratemporal. Near the anteroventral edge of the squamosal there is a
small pit, probably related to a postorbital sensory pore in the skin.
The tabular (Fig. 3 C, D) is pointed anteriorly, where it probably fits
against the lateroposterior edge of the postorbital. The dorsal part of
the bone flares out and down, forming a small otic notch at a point
halfway back. Posteriorly, the flange attains a dorsoventral width of
2.0 mm. at the edge of the notch. The slender posterior process of the
tabular which continues beyond the flange is approximately 0.5 mm. in
diameter and 5.0 mm. long.
_Ventral Aspect_ (Fig. 4)
The palatal view of the skull shows the paired premaxillary, maxillary,
palatine, pterygoid, and quadrate bones. The openings for the internal
nares, the ventral orbital fenestrae, and the subtemporal fossae are
readily recognized. The quadrate processes extend posteriorly leaving a
large gap medially at the posterior end of the skull.
[Illustration: FIG. 4. _Hesperoherpeton garnettense_ Peabody. Palate
reconstructed; ventral aspect at left, showing teeth, dorsal aspect at
right. KU 10295, x 4.]
The left quadrate appears to be in place on the posterior prong of the
pterygoid. The dorsal side of the quadrate is grooved between two
anterolaterally directed ridges. The groove, which probably held the
end of the stapes, extends about half the width of the quadrate itself.
The width of the quadrate is 4.0 mm., the length is 4.5 mm. medially
and about 2.0 mm. laterally. In ventral view the quadrate appears to
project laterally, but is incomplete and its shape uncertain. The
distance from the posterior end of the quadrate to the visible
posterior edge of the orbital fenestra, which opens ventrally, is 10.0
mm.
This region between the quadrate and the orbit is occupied by a
pterygoid with three projections. Anteriorly, the pterygoid outlines
most of the posterior edge of the orbit (a distance of about 6.5 mm.).
A lateral process separates the orbit from the subtemporal fossa. A
posteriorly directed edge defines the fossa, which extends about 6.5
mm. anteroposteriorly. The lateral process of the pterygoid terminates
10.0 mm. from the midline. Both the lateral and posterior pterygoid
processes are approximately 2.0 mm. wide. The greatest width of the
subtemporal fossa is about 2.0 mm. The medial border of the orbital
fenestra is missing, but apparently consisted of the pterygoid for at
least the posterior half.
Along the posterior edge of the orbital fenestra, there is a narrow,
dorsally projecting flange of the pterygoid. The lateral opening of the
orbit is approximately 7.5 mm. wide.
The remaining border of the orbital fenestra on the anterior and medial
sides is formed by a bone occupying the position of palatine and vomer;
for convenience we designate this as palatine. When reconstructed in
its probable position in relation to the pterygoid, the left palatine
lacks a section, on its medial and posterior edges, measuring about 2.5
mm. by 9.0 mm. The lateral margin of the palatine is convex; about 5.5
mm. anterior to the orbit this margin curves into a strong anteriorly
pointing projection, medial to which is seen the internal narial
opening. The remaining anterior edge is slightly convex, smoothly
rounded, and meets the midline about 9.0 mm. anterior to the pterygoid.
The void area medial to the palatine and anterior to the pterygoid does
not fit any bone which we can recognize as the parasphenoid. It is thus
suspected that this area is covered in part by the missing edge of the
palatine and partly by an anteromedial extension of the pterygoid. Of
course a parasphenoid may also have been present.
The position, length, and shape of the premaxillary shown in palatal
view (Fig. 4) are primarily based upon the dorsal appearance since
ventrally most of it cannot be seen. At the point where it forms the
anterior border of the internal naris, the premaxillary is slightly
wider than the maxillary and seems to become narrower as it approaches
the midline.
The ethmosphenoid, which we cannot identify, may have been exposed in a
gap between the premaxillary and the palatine. The gap measures
approximately 8.0 mm. wide and ranges up to 1.0 mm. anteroposteriorly.
The maxillary begins at a suture with the premaxillary lateral to the
naris and continues posteriorly, bordering the orbit with a width of
about 1.2 mm. It then tapers to a point approximately 2.0 mm. anterior
to the lateral projection of the pterygoid. The width of the maxillary
at this point is 0.8 mm. and the posterior end is broken; probably when
complete it approached the pterygoid, and either met the latter or had
a ligamentous connection with it. As nearly as can be determined, the
total length of the maxillary is approximately 12.0 mm.
The teeth on the maxillary are small and seem to be in two longitudinal
rows. The palatine bears two large, grooved teeth anteriorly; the first
is approximately 1.0 mm. posteromedial to the naris and the second is
about 3.0 mm. posterior and slightly lateral to the naris. The flat
ventral surfaces of the palatine and pterygoid bear numerous small
teeth distributed as shown in Fig. 4.
_Braincase and Occipital Region_ (Fig. 5)
The parts of the neurocranium are scattered, disconnected and
incomplete, but it is possible to make out a number of features of the
otico-occipital section with fair assurance. In posterior view the
notochordal canal and foramen magnum are confluent with each other, and
of great size relative to the skull as a whole. The notochordal canal
measures 2.8 mm. in diameter, and the foramen magnum about 4.0 mm. The
crescent-shaped supraoccipital rests on the upright ends of the
exoccipitals, but between the latter and the basioccipital no sutures
can be seen. Probably the whole posterior surface of the braincase
slanted posteroventrally; consequently the rim of the notochordal canal
was about 3.0 mm. behind the margin of the parietals.
The U-shaped border of the notochordal canal is a thick, rounded bone,
comparable in appearance to the U-shaped intercentra of the vertebrae.
This bone apparently rested upon a thinner, troughlike piece (Fig. 5 B)
forming the floor of the braincase. The latter is broad, shallow,
concave, open midventrally and narrowing anteriorly to form a pair of
articular processes. Since no sutures can be seen in this structure, it
probably is the ventral, ossified portion of the basioccipital. Watson
(1926, Fig. 4 B) illustrates the floor of the braincase in
_Eusthenopteron_, with its more lateral, anterior portion labelled
prootic, but in our specimen the corresponding part could scarcely have
formed the anterior wall of the otic capsule, being entirely in the
plane of the floor. The two articular surfaces anteriorly near the
midline suggest that a movable joint existed between the
otico-occipital part of the braincase and the ethmosphenoid part, as in
Rhipidistia (Romer, 1937). We have found nothing in the specimen that
could be referred to the ethmosphenoid; it may have been unossified.
[Illustration: FIG. 5. _Hesperoherpeton garnettense_ Peabody, KU 10295,
x 4. A, occipital view of skull; B, basioccipital bone in dorsal
(internal) view.]
The otic capsules appear to have rested against lateral projections of
the basioccipital. The single otic capsule that can be seen (the
right) is massively built, apparently ossified in one piece, with a
shallow dorsomedial excavation, probably the vestige of a supratemporal
fossa. On the lateral face is a broad, shallow depression dorsally, and
a narrower, deeper one anteroventrally; these we suppose to have
received the broader and narrower heads of the stapes, respectively.
The posterior wall of the otic capsule we have designated opisthotic in
the figure. Anterior to the otic capsule the lateral wall of the
braincase cannot be seen, and may not have been ossified.
The roof of the braincase is visible in its ventral aspect, extending
from approximately the occipital margin to a broken edge in front of
the parietal foramen, and laterally to paired processes which overlie
the otic capsules directly behind the orbits (see dotted outlines in
Fig. 1). Each of these postorbital processes, seen from beneath,
appears to be the lateral extension of a shallow groove beginning near
the midline. Presumably this section of the roof is an ossification of
the synotic tectum. It should be noted that the roof of the braincase
proper is perfectly distinct from the overlying series of dermal bones,
and that the parietal foramen can be seen in both. The roof of the
braincase in our specimen seems to have been detached from the
underlying otic capsules and the occipital wall.
The bone that we take to be the stapes is blunt, flattened (perhaps by
crushing), 5.0 mm. in length, and has two unequal heads; its width
across both of these is 4.0 mm. The length is appropriate to fit
between the lateral face of the otic capsule and the dorsal edge of the
quadrate; the wider head rests on a posterodorsal concavity on the otic
capsule, and the smaller fits a lower, more anterior pit. Laterally the
stapes carries a short, broad process that probably made contact with a
dorsally placed tympanic membrane. Thus the bone was a hyomandibular in
the sense that it articulated with the quadrate, but it may also have
served as a stapes in sound-transmission. It contains no visible canal
or foramen.
_Mandible_ (Fig. 6)
The crushed inner surface of the posterior part of the left mandible
and most of the external surface of the right mandible are preserved in
close proximity. Although the whole length of the tooth-bearing margins
is missing, some parts of six elements of the right mandible can be
seen. The pattern of sutures and the general contour closely resemble
those of _Megalichthys_ (Watson, 1926, Figs. 37, 38) and other known
Rhipidistia.
The anteroposterior length of the mandible is about 23.8 mm., and the
depth is 3.8 mm. The dentary extends approximately 17.6 mm. back from
the symphysis, and its greatest width is probably 2.0 mm. Its lower
edge meets all the other lateral bones of the jaw. The splenial and
postsplenial form the curved anteroventral half of the jaw for a
distance of about 9.0 mm. The fragmented articular, on the posterior
end of the jaw, is 4.0 mm. long and 2.0 mm. deep, exhibiting a broken
upper edge; presumably the surface for articulation with the quadrate
was a shallow concavity, above the end of the articular.
[Illustration: FIG. 6. _Hesperoherpeton garnettense_ Peabody. Right
mandible, lateral view, KU 10295, x 4. External surfaces are pitted;
broken surfaces are coarsely stippled.]