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Title: A New Order of Fishlike Amphibia From the Pennsylvanian of Kansas
Author: Eaton, Theodore H. (Theodore Hildreth), 1907-1981, Stewart, Peggy Lou
Language: English
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Copyright Status: Not copyrighted in the United States. If you live elsewhere check the laws of your country before downloading this ebook. See comments about copyright issues at end of book.

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UNIVERSITY OF KANSAS PUBLICATIONS

MUSEUM OF NATURAL HISTORY

Volume 12, No. 4, pp. 217-240, 12 figs.
May 2, 1960


A New Order of Fishlike Amphibia
From the Pennsylvanian of Kansas

BY

THEODORE H. EATON, JR., AND PEGGY LOU STEWART


UNIVERSITY OF KANSAS
LAWRENCE
1960



UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY

Editors: E. Raymond Hall, Chairman, Henry S. Fitch,
Robert W. Wilson


Volume 12, No. 4, pp. 217-240, 12 figs.
Published May 2, 1960


UNIVERSITY OF KANSAS
Lawrence, Kansas


PRINTED IN
THE STATE PRINTING PLANT
TOPEKA, KANSAS
1960

28-2495



A New Order of Fishlike Amphibia
From the Pennsylvanian of Kansas

BY

THEODORE H. EATON, JR., AND PEGGY LOU STEWART

INTRODUCTION


A slab of shale obtained in 1955 by Mr. Russell R. Camp from a
Pennsylvanian lagoon-deposit in Anderson County, Kansas, has yielded in
the laboratory a skeleton of the small amphibian _Hesperoherpeton
garnettense_ Peabody (1958). This skeleton provides new and surprising
information not available from the holotype, No. 9976 K. U., which
consisted only of a scapulocoracoid, neural arch, and rib fragment. The
new specimen, No. 10295 K. U., is of the same size and stage of
development as the holotype and it is thought that both individuals are
adults.

The quarry, University of Kansas Museum of Natural History Locality KAN
1/D, is approximately six miles northwest of Garnett, Anderson County,
Kansas, in Sec. 5, T. 19S, R. 19E, 200 yards southwest of the place
where _Petrolacosaurus kansensis_ Lane was obtained (see Peabody,
1952). The Rock Lake shale, deposited under alternately marine and
freshwater lagoon conditions, is a thin member of the Stanton limestone
formation, Lansing group, Missourian series, and thus is in the lower
part of the Upper Pennsylvanian.

Peabody (1958) placed _Hesperoherpeton_ in the order Anthracosauria,
suborder Embolomeri, family Cricotidae. Study of the second and more
complete specimen reveals that _Hesperoherpeton_ is unlike the known
Embolomeri in many important features. The limbs and braincase are more
primitive than those so far described in any amphibian. The vertebrae
are comparable to those of Ichthyostegalia (Jarvik, 1952), as well as
to those of Embolomeri. The forelimb is transitional between the
pectoral fin of Rhipidistia and the limb of early Amphibia. The pattern
of the bones of the forelimb closely resembles, but is simpler than,
that of the hypothetical transitional type suggested by Eaton (1951).
The foot seemingly had only four short digits. The hind limb is not
known.

The new skeleton of _Hesperoherpeton_ lies in an oblong block of limy
shale measuring approximately 100 × 60 mm. After preparation of the
entire lower surface, the exposed bones and matrix were embedded in
Bioplastic, in a layer thin enough for visibility but giving firm
support. Then the specimen was inverted and the matrix removed from the
opposite side; this has not been covered with Bioplastic. The bones lie
in great disorder, except that some parts of the roof of the skull are
associated, and the middle section of the vertebral column is
approximately in place. The bones of the left forelimb are close
together but not in a natural position. The tail, pelvis, hind limbs
and right forelimb are missing. Nearly all the bones present are
broken, distorted by crushing, incomplete and scattered out of place,
probably by the action of currents. The complete skeleton, in life,
probably measured between 150 and 200 mm. in length.

The specimen was studied at the Museum of Natural History, University
of Kansas, with the help of a grant from the National Science
Foundation, number NSF-G8624. The specimen was discovered in the slab
by Miss Sharon K. Moriarty, and was further cleaned by the authors. Mr.
Merton C. Bowman assisted with the illustrations. We are indebted to
Dr. Robert W. Wilson for critical comments.


SKULL

_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, × 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, × 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 × 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, × 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,
× 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, × 4. External surfaces are pitted;
broken surfaces are coarsely stippled.]


VERTEBRAE (Fig. 7)

The vertebrae that are visible from a lateral view are crushed and
difficult to interpret. It is possible, nevertheless, to see that the
trunk vertebrae resemble those of Ichthyostegalia (Jarvik, 1952, Fig.
13 A, B), except that the pleurocentra are much larger. A few parts of
additional vertebrae can be seen, but they are so scattered that it is
impossible to be sure of their original location. Therefore comparisons
between different regions cannot yet be made.

The U-shaped intercentrum encloses the notochord and occupies an
anteroventral position in the vertebra. Anteriorly, each intercentrum
articulates with the pleurocentra of the next preceding vertebra by
slightly concave surfaces. Dorsolaterally there is an articular surface
for the capitulum of the rib.

The two pleurocentra of each vertebra are separate ventrally as well as
dorsally, but form thin, broad plates of about the same height as the
notochord. The lateral surface appears to be depressed, allowing,
perhaps, for movement of the rib. Above each pleurocentrum, on the
lateral surface of the neural arch, there is a short diapophysis for
articulation with the tuberculum of the rib.

The margin of the neural spine is convex anteriorly and concave
posteriorly, the tip reaching a point vertically above the
postzygapophysis. The prezygapophysis of each vertebra articulates
with the preceding postzygapophysis by a smooth dorsal surface. One
nearly complete neural arch shows (Fig. 7 B) a pit above the neural
canal, clearly corresponding to the canal for a dorsal ligament shown
by Jarvik in _Ichthyostega_. Indeed this view of the neural arch and
intercentrum together brings out the striking resemblance between the
vertebrae of _Hesperoherpeton_ and those of the Ichthyostegids. The
rounded intercentrum in both is an incomplete ring enclosing the
notochordal canal.

[Illustration: FIG. 7. _Hesperoherpeton garnettense_ Peabody. A, End
view of incomplete vertebra, probably near anterior end of column. B,
Neural arch and intercentrum in end view, showing probable association.
C, Left lateral view of trunk vertebra. All figures: KU 10295, × 4.]

TABLE 1.--AVERAGE MEASUREMENTS OF THE TRUNK VERTEBRAE (in mm.).
NUMBERS IN PARENTHESES INDICATE THE NUMBER OF PIECES AVAILABLE FOR
MEASURING

----------------------------+------------+-------------+---------------
            PARTS           | Ant.-post. | Dors.-vent. | Transv. width
----------------------------+------------+-------------+---------------
Neural spine                |  1.5 (3)   |   3.0  (3)  |     --
----------------------------+------------+-------------+---------------
Neural spine and arch       |  2.0 (4)   |   4.5? (4)  |     --
----------------------------+------------+-------------+---------------
Neural canal                |  2.0 (4)   |   2.0  (1)  |    1.0 (1)
----------------------------+------------+-------------+---------------
Intercentrum                |  1.5 (5)   |   3.5  (4)  |    3.0 (1)
----------------------------+------------+-------------+---------------
Pleurocentrum               |  1.5 (3)   |   3.0  (2)  |     --
----------------------------+------------+-------------+---------------

The shape, in end view, of a partly preserved neural arch (Fig. 7 A)
seems to account for the incompleteness of the intercentrum just
mentioned; the ventral edge of the arch is emarginate in such a way as
to fit the dorsal surface of the notochord. The dorsal portion of this
neural arch is not present (either broken or not yet ossified), but
the opening of the neural canal is comparable in width to the foramen
magnum. Hence this vertebra may be one of the most anterior in the
column. In comparison with the trunk vertebrae seen farther posteriorly
it appears that there may be a progressive ossification of neural
arches toward their dorsal ends, and of intercentra around the
notochord, with probable fusion of the intercentra and neural arches in
the posterior part of the trunk. The notochord seems to have been
slightly constricted by the intercentra, but not interrupted.


RIBS

The proximal ends of the ribs expand dorsoventrally to a width
approximately four times that of their slender shafts. The tuberculum
and capitulum on each of the trunk ribs are separated only by a shallow
concavity. These two articular surfaces are so situated that the rib
must tilt downward from the horizontal plane. The shaft flares
terminally in some ribs, and the distal end is convex. Ribs in the
trunk region differ little if any in size. Five that can be measured
vary in length from 5.0 to 7.0 mm. One short, bent rib 3.5 mm. long
perhaps is sacral or caudal.


PECTORAL GIRDLE (Figs. 8, 9, 10)

The right scapulocoracoid is almost complete, and the left one is
present but partly broken into three pieces, somewhat pushed out of
position. With the advantage of this new material, we may comment on
the scapulocoracoid of _H. garnettense_ as described by Peabody (1958).
In size and contour, the slight differences between the type (KU 9976)
and the new skeleton (KU 10295) are considered to be no more than
individual variation. We have redrawn the type (Fig. 8) in order to
show the resemblances more clearly.

The small sections that were missing from the type are present in KU
10295. The jagged edge directly posterior to the area occupied by the
neural arch in the type extends 0.5 mm. farther back in our specimen.
The angle formed between the recurved dorsal ramus and the edge of the
ventral flange is seen in our specimen to be less than 90°. The glenoid
fossa, appearing as a concave articular surface for the cap of the
humerus, was in part covered by cartilage and shows as "unfinished"
bone (Peabody, 1958, p. 572); this area is more oval than triangular,
as Peabody thought. The obstruction of a clear view of this part of the
type is the result of the accidental position of a neural arch. The
raised portion immediately dorsal to the glenoid fossa exhibits an
unfinished surface, suggesting the presence of either cartilage or a
ligament.

[Illustration: FIG. 8. _Hesperoherpeton garnettense_ Peabody. Type
specimen redrawn. Right scapulocoracoid in external view (at left), and
internal view (at right). KU 9976, × 4.]


[Illustration: FIG. 9. _Hesperoherpeton garnettense_ Peabody. Right
scapulocoracoid in external view, showing part of interclavicle, and
position occupied by clavicle. The specimen is flattened and lies
entirely in one plane. KU 10295, × 4.]


[Illustration: FIG. 10. _Hesperoherpeton garnettense_ Peabody. Right
clavicle in external view. Anterior edge to right. KU 10295, × 4.]

The right clavicle is complete, and resembles a spoon having a slender
handle. The dorsal tip of the handle is L-shaped. The expanded ventral
part is convex externally, and rested upon the anteroventral surface of
the scapulocoracoid. The lateral edge next to the "stem" is distinctly
concave, abruptly becoming similar in contour to the opposite edge, and
giving the impression of an unsymmetrical spoon. The left clavicle is
present in scattered fragments, its dorsal hooklike end being intact.

The posterior end of the interclavicle lies in contact with the right
scapulocoracoid. There are short lateral processes at the point where
the interclavicle was overlapped by the clavicles, but we cannot be
sure of the extent of this bone anteriorly or posteriorly.

The presumed left cleithrum, a long rectangle, is approximately equal
in length to the rodlike stem of the clavicle, and is about as wide as
the dorsal L-shaped tip of the clavicle. The posterior end of the
cleithrum presumably met the tip of the clavicle, while the rest of it
was directed anteriorly and a little dorsally. There seems to be a
small articular surface near the anterior extremity which suggests the
presence of a supracleithrum. The upper border of the cleithrum is
slightly convex and the lower concave.


FORELIMB (Fig. 11)

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, × 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.

TABLE 2.--APPROXIMATE MEASUREMENTS OF THE FORELIMB (in mm.)

--------------------------+-------------------------------------------
                          |                 Dimensions
                          +----------+--------------------------------
 ELEMENT                  |          |              Width
                          |  Length  +----------+----------+----------
                          |          | Proximal |  Midway  |  Distal
--------------------------+----------+----------+----------+----------
Humerus                   |   16.0   |    5.0   |    2.0   |    7.5?
Radius                    |    9.0   |    4.0   |    1.5   |    3.5
Ulna                      |    8.5   |    4.5   |    1.5   |    3.5
Radiale                   |    3.0   |    2.0   |    1.5   |    2.0
Intermedium               |    1.5   |     --   |    2.0   |     --
Ulnare                    |    3.5   |    2.0   |    2.0   |    2.5
Metacarpal        A       |    4.5   |    2.5   |    1.0   |    2.0
Metacarpal        B       |    4.5   |    3.0?  |    1.5   |    2.5
Metacarpal        C       |    4.0   |    2.0   |    1.5   |    2.0
Metacarpal        D       |    3.5   |    2.5   |    1.0   |    1.5
Proximal Phalanx  A       |    2.0   |    1.5   |    1.0   |    1.5
Proximal Phalanx  B       |    2.0   |    1.5   |    1.0   |    1.5
Terminal Phalanx  A       |    1.5   |    1.5   |    1.0   |    1.0
Terminal Phalanx  B       |    1.5   |    1.5   |    1.0   |    1.0
--------------------------+----------+----------+----------+----------


COMPARISONS AND DISCUSSION

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. × 0.5.]


TAXONOMY

Evidently _Hesperoherpeton_ is a small, lagoon-dwelling survivor of
the Devonian forms that initiated the change from Crossopterygii to
Amphibia (Jarvik, 1955). It shows, however, that this transition did
not affect all structures at the same time, for some, as the braincase
with its notochordal canal, the mandibular bones and axial limb bones,
are unchanged from the condition normal for the Rhipidistia, but most
other characters are of amphibian grade. To express these facts
taxonomically requires that _Hesperoherpeton_ be removed from the
family Cricotidae, suborder Embolomeri, order Anthracosauria, and
placed in a new order and family of labyrinthodont Amphibia.

     Order PLESIOPODA

     (_plesios_, Gr., near, almost; _podos_, Gr., foot)

     Labyrinthodontia having limbs provided with digits, but
     retaining posterior flanges on axial bones as in
     Rhipidistia, without joint-structure at elbow and wrist
     essential for terrestrial locomotion; neurocranium having
     separate otico-occipital section, large notochordal canal,
     no occipital condyle, as in Rhipidistia; nares separate from
     rim of mouth; pectoral girdle anthracosaurian; vertebrae
     having U-shaped intercentrum and paired, but large,
     pleurocentra.

Probably associated with the characters of the order, as given above,
are the connection of pectoral girdle with skull, and the presence of a
tympanic membrane, the stapes functioning in both sound-transmission
and palatoquadrate suspension.

     Family HESPEROHERPETONIDAE

     Orbits and foramen magnum unusually large in correlation
     with reduced size of animal; squamosal forming posterior
     margin of orbit; circumorbital series absent (except for
     postorbital); sensory pits on squamosal and frontal.

Characters defining the family are evidently the more specialized
cranial features, which probably evolved during Mississippian and early
Pennsylvanian times.

The definition of the genus and species may be left to rest upon
Peabody's (1958) original description and the present account, until
the discovery of other members of the family gives reason for making
further distinctions.


SUMMARY

_Hesperoherpeton garnettense_ Peabody (1958), based on a
scapulocoracoid and part of a vertebra, was originally placed in the
order Anthracosauria, suborder Embolomeri, family Cricotidae. A new
skeleton from the type locality near Garnett, Kansas (Rock Lake shale,
Stanton formation, Upper Pennsylvanian), shows that the animal has the
following rhipidistian characters: Large notochordal canal below
foramen magnum, otico-occipital block separate from ethmosphenoid,
postaxial processes on three axial bones of forelimb, pectoral girdle
(probably) articulated with tabular. Nevertheless, _Hesperoherpeton_
has short digits, an anthracosaurian type of pectoral girdle, an otic
rather than spiracular notch, nostrils separate from the mouth, and
vertebrae in which the intercentrum is U-shaped and the pleurocentra
large but paired. The stapes reaches the quadrate.

_Hesperoherpeton_ is placed in a new order, PLESIOPODA, on the basis of
the characters stated above, and a new family, HESPEROHERPETONIDAE.
Specialized characters of the family include: Reduction of
circumorbital bones, bringing the squamosal to the edge of the orbit,
loss of certain bones of the temporal region, and relative enlargement
of the orbits and foramen magnum, in correlation with the diminutive
size of the animal. The structural characters of _Hesperoherpeton_
suggest to us that it lived in the shallow, weedy margins of lagoons,
rested with its head partly out of water, and normally did not walk on
land.


LITERATURE CITED

EATON, T. H., JR.
    1951. Origin of tetrapod limbs. Amer. Midl. Nat., 46: 245-251.

JARVIK, E.
    1952. On the fish-like tail in the ichthyostegid stegocephalians.
          Meddel. om Grønland, 114: 1-90.
    1954. On the visceral skeleton in _Eusthenopteron_ with a discussion
          of the parasphenoid and palatoquadrate in fishes. Kgl. Svenska
          Vetenskapsakad. Handl., 5: 1-104.
    1955. The oldest tetrapods and their forerunners. Sci. Monthly,
          80: 141-154.

MOORE, R. C., FRYE, J. C., and JEWETT, J. M.
    1944. Tabular description of outcropping rocks in Kansas. Kansas
          State Geol. Surv. Bull., 52: 137-212.

PEABODY, F. E.
    1952. _Petrolacosaurus kansensis_ Lane, a Pennsylvanian reptile
          from Kansas. Univ. Kansas Paleont. Contrib., Vertebrata,
          Art. 1: 1-41.
    1958. An embolomerous amphibian in the Garnett fauna (Pennsylvanian)
          of Kansas. Jour. Paleont., 32: 571-573.

ROMER, A. S.
    1937. The braincase of the Carboniferous crossopterygian
          _Megalichthys nitidus_. Mus. Comp. Zool. Bull., 82: 1-73.
    1947. Review of the Labyrinthodontia. Mus. Comp. Zool. Bull.,
          99: 1-368.
    1957. The appendicular skeleton of the Permian embolomerous
          amphibian _Archeria_. Univ. Michigan Contrib. Mus. Paleont.,
          13: 103-159.

WATSON, D. M. S.
    1926. The evolution and origin of the Amphibia. Phil. Trans. Roy.
          Soc. London, (B) 214: 189-257.

_Transmitted January 13, 1960._


28-2495





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