Early Permian Vertebrates from the Cutler Formation of the ...

C24 CONTRIBUTIONS TO PALEONTOLOGY
depression on the anterior face of the conjoin.ed internal
ridges of the pubes in Limnoscelops longifernur makes
this face more nearly vertical than it is in Li1nnoscelis
or Lim;nosceloides. In this, Lhnnoscelops is more like
the captorhinids than are the other two genera.
The external opening of the obturator foramen is
similarly placed in Li1nnoscelops, Limnoscelis, and Limnosceloides.
In Limnoscelis, as in Labidosaurus, the
internal opening of the foramen lies anterior to the
internal ridge of the pubis, but in Lirn/Jwrweloides, the
opening lies at the upper end of the ridge (Romer, 1952,
p. 90). Limnoscelopslongifemu,r shows a somewhat intermediate
condition in that the opening lies immediately
anterior to a crest above the greatest depth of
the ridge, but the position of the opening is more like
that in Lim;nosceloides than it is like that in Limnosceiis.
The proximal fragment of an ulna is about fivesevenths
the size of the corresponding part of Limnoscelis
paludis figured by Williston (1912, fig. 27), but
the two are morphologically indistinguishable.
The hind led of Limnoscelops longifemur is represented
in the type by only the proximal and distal ends
of the left femur; they are very similar to those of
Labidosaurus, but what is left of its stumps seems to
indicate that the shaft was more slender and more like
that of Oaptorhinus. The relative slenderness in Limnoscelops
is a significant difference from Labidosaurus,
a much smaller reptile by comparison of the proximal
and distal ends. (See Case, 1911a, fig. 48.)
The complete femur may have been at least 130 mm
long from the most proximal point on the head to the
end of the posterior condyle, and at least 122 mm long
from the most proximal point on the head to the center
of the intercondylar notch. This calculated distance
from head to intercondylar notch is about one and onefourth
times the comparable length in Li'lnnosceloides
dunkardensis (USNM 12166). The pelves are about
the same size in these two; therefore, Limnoscelops
longifemur had proportionally much longer femora,
and probably also had proportionally longer hind legs
and slenderer shaft than Limtnosceloides dunkardensis:
The stump of the shaft on the proximal fragment of
Limnoscelops longifernrur is about 17 mm thick ( measured
in the plane of the head of the femur) as compared
to about 25 mm in Limnosceloides dunkardensis
(calculated from Romer, 1952, fig. 12). The proximal
and distal ends of the femur in Limnosceloides are not
so wide in comparison with the shaft as in Limnoscelis
whose femur is short, stoutly built, and in general comparable
to the femur in Seymouria. (See Romer, 1952,
fig.12; 1956, figs.171A., B, 0, F.)
The trochanteric crest is set off from the head by a
distinct notch as in Labidosaurus (Romer, 1956, fig.
171G), but in contrast to Lhnnoscelis, makes almost
a right angle with the head, and does not flare widely
at the front. The trochanteric crest in Labidosaurus
and L,imnoscelis is directed anteriorly as well as ventrally,
so that it makes an obtuse ventral angle with the
head; only the base of the crest is preserved in Limnosceloides
dunkardensis (USNM 12166). There is a
prominent excavation on the posterior side of the posterior
condyle as in Labidosaurus and Lim;noscelis, but
in contrast to Limnosceloides.
To sum up the significant morphologic features of
Limnoscelops longifemur: The vertebrae are typically
limnoscelid, the preserved parts of the pelvis are much
like those in Lirrunosceloides except for the thicker symphysis,
and for the almost vertical, captorhinomorphlike
face of the conjoined internal ridges of the pubes.
Romer (1956) recognizes three families of captorhinmnorph
cotylosaurs: the Limnoscelidae, the Romeriidae,
and the Captorhinidae. The consensus seems to be
that vertebral structure is more conservative than the
structure of the appendages, as illustrated by the currently
accepted classification of labyrinthodont amphibians;
thus, the classification of Limnoscelops must depend
more on its vertebrae than on the other materials
available to us. We therefore tentatively assign Limnoscelops
to the family Limnoscelidae. Romer's assignment
of Limnosceloides to this same family was also
only tentative. Limnoscelis, known only from the Cutler
(El Cobre Canyon) of New Mexico, has heretofore
been the only unquestioned representative of this family.
Limnoscelops, for the time being, is best thought
of as a limnoscelid advanced in the direction of the
captorhinids.
The romeriids and captorhinids are closely related,
the captorhinids having apparently been derived from
the romeriids. The postcranial skeleton of the romeriids
is, unfortunately, still unknown. Protorothyris and
Romeria of the romeriids are known from the lower
part of the Wichita Group in Texas, M elanothyris is
known from the lower part of the Dunkard Group in
the eastern United States, and Oephalerpeton may be
a romeriid from the Allegheny Formation of Illinois.
The Wichita Group has yielded only a few primitive
captorhinids, but the advanced Oaptorhinus and Labidosaurus
are common in the over-lying Clear Fork
Group.
The known limnoscelids could not have been the actual
ancestors of the romeriids or captorhinids if we
consider their respective ages. The phylogenetic divergence
must therefore have occurred no later than
Middle Pennsylvanian time.