AMER. ZOOLOGIST, 5:277-286 (1965).
MICROSAURS AND THE ORIGIN OF CAPTORHINOMORPH
REPTILES
JOSEPH T. GREGORY
Dept. of Paleontology, University of California,
Berkeley
SYNOPSIS. Microsaurs are Paleozoic lepospondylous Amphibia with slender bodies and
weak limbs. Their solidly roofed skulls lack otic notches, have large supratemporals
widely separating the squamosal from parietal, and double occipital condyles. The
stapes consists of a large footplate and extremely short columella. Vertebrae lack
intercentra. Originally based on a reptile, Hylonomus lyelli, by Dawson in 1863, the
Order Microsauria has long been restricted to these small amphibians (Romer, 1950).
Repeated confusion between primitive captorhinomorph reptiles and microsaurs steins
from superficial similarities between both skulls and vertebrae. This confusion and
occasional microsaur-like vertebrae in early Carboniferous deposits have led to suggestions that microsaurs are reptilian ancestors (cf. Vaughn, 1962).
Captorhinomorphs differ from microsaurs in their small supratemporal bone, single
occipital condyle, stapes with long columella reaching a pit in the quadrate and bearing a dorsal process, and dorsal intercentra. Captorhinomorph ancestors were probably
not labyrinthodonts, as Vaughn (1960) has pointed out, but they could not have had
the characteristic specializations of microsaurs. Their source must be sought in forms
much closer to crossopterygian fish.
Microsaurs resemble both urodeles and gymnophionans in their double occipital
joint and otic region. They differ from Lissamphibia in the absence of a non-calcified
zone in the teeth. At present, no criteria indicate decisively which structures developed
convergently. Microsaurs are possibly but not demonstrably related to the ancestry
of modern salamanders and caecilians.
Microsaurs are small animals of the late
Paleozoic, similar in size and habitus to
modern newts and salamanders. Their
skulls are solidly roofed over with numerous dermal bones like those of other Paleozoic amphibians; centra of their vertebrae
are single spool-shaped ossifications which
attach to neural arches by suture or may
be coossified; limbs are feebly developed.
The general similarity of their skulls and
vertebrae to those of certain primitive reptiles known as captorhinomorphs have led
several students—including me at one time
—to regard them as early reptiles or reptile
ancestors. I assume this is the reason I have
been asked to consider microsaur relationships in this symposium.
Questions of microsaur affinities have
further been complicated by nomenclatorial aberrations. The term Microsauria was
originally proposed in 1863 by Sir William
Dawson for certain small tetrapods found
in Pennsylvaniun deposits of Nova Scotia
which he legarded as reptiles. SubsequentContribution from Museum of Paleontology, University of California, Berkeley.
ly, this material has been shown to belong
to various labyrinthodonts and reptiles. In
particular Hylonomous lyelli, for which
Dawson had first proposed the Order Microsauria, proves to be a primitive captorhinomorph reptile close to Cephalerpeton
and Romeria, as Robert Carroll has most
recently shown. Various small tetrapods
from late Carboniferous deposits at Nyran
in Bohemia (Fritsch, 1879-1883), and the
early Permian Plauenschen Grund near
Dresden (Credner, 1885), were referred to
the Microsauria, and our present concept
is derived largely from this more complete
material. Thus by historical accident and
common usage the term Microsauria has
come to be applied to an order of lepospondylous Amphibia, and Hylonomus lyelli
explicitly excluded from it. This is nomenclatorially unsatisfactory, but a more legal
procedure would create endless confusion.
(See Romer, 1950, p. 647.) Professor Romer
summarized our present knowledge of the
content and osteology of the Microsauria
in 1950, and no significant advances have
been made since.
277
278
JOSEPH T. GREGORY
Captorhinomorphs are primitive reptiles
of the Carboniferous and Permian, with
solidly roofed skulls, for which reason they
are generally classified as a Suborder of the
Cotylosauria. They differ from the diadectormorphs in the straight posterior border
of the skull, which shows no trace of an
otic notch, and also in the way that the
stapes extends downward from the fenestra
ovalis to the quadrate bone. Several families of captorhinomorphs are recognized
on the basis of dental specialization; most
primitive are the Protorothyrididae, with
small, pointed, isodont teeth and unspecialized vertebrae. Typical captorhinids have
(usually) multiple rows of blunt teeth in
the rear of the jaws, and overhanging enlarged grasping teeth in front. Their vertebrae have massive neural arches and smaller
zygapophyses much like the diadectids.
Limnoscelis is a rather isolated large form
with the pointed teeth of a carnivore, yet
with typical massive vertebrae. It has been
recognized since the studies of Watson
(1917) and Sushkin (1927) that the Captorhinomorpha and Diadectomorpha were
not closely related; Watson abandoned his
early view that captorhinomorphs were ancestral to most later reptiles except the
turtles (1917), and sought to derive the
sauropsid branch of the Reptilia from diadectomorphs (1955) or seymouriamorphs
(1957), in contrast to the therapsid or mammal-like branch whose roots in the captorhinomorphs are well established. Parrington, on the contrary, has offered strong
arguments (1958, p. 110) for a common
origin of both the synapsid and diapsid
reptiles from captorhinomorph cotylosaurs.
This controversy lies outside the present
discussion; suffice it to say that the origin
of the Captorhinomorpha is a major problem in the origin of the Reptilia as a whole.
COMPARISON OF MICROSACRS AND
C:APTORHINOMORPHS
A "central" microsaur such as Microbrae his pelikmii resembles the early captolhinomorph Romeriei in general skull outline and proportions, including the shapes
ot many indhidual roofing bones. The
posterior margin of the cheek region is
straight, and there is no trace of an otic
notch. Close comparison reveals the following differences.
1) Cardiocephalus, Hyloplesion and most
other microsaurs have well developed postparietals at the rear of their skull table,
whereas these elements are either extremely
reduced or entirely confined to the occipital surface of the skull in the captorhinomorphs. Microhrachis and Tuditanus are
unusual microsaurs in the small size of
their postparietals. This difference is not
particularly significant, as early tetrapods
show a trend toward reduction of bones
from the rear of the skull roof.
2) Microbrachis and all other microsaurs
have a well developed supratemporal bone
separating the parietal from the squamosal.
In the protorothyridids, on the contrary,
the supratemporal is a small vestige inserted in the posterior edge of a broad
parietal which reaches a relatively large
squamosal. The captorhinomorph squamosal (Fig. 1C) occupies much of the space
filled by the microsaur supratemporal
(Fig. 1A, B), so these bones might be confused. Microsaurs have two bones lateral
to the supratemporal, reptiles only one, the
quadratojugal, lateral to their large squamosal.
Unfortunately not all taxa conform to
this sharp distinction. Limnoscelis, a large
captorhinomorph, has a relatively large
supratemporal (Fig. ID) which meets the
postorbital to separate parietal from squamosal in a very microsaurlike fashion. But
its reptilian palate with strong transverse
pterygoid crests and narrow interpterygoid
vacuities, its thoroughly reptilian postcranial skeleton, and the characteristic captorhinomorph occiput which indicates a reptilian type middle ear, although the stapes
has not been found, all proclaim it to be a
captorhinomorph reptile. The slender posterior prolongation of the Limnoscelis supratemporal is more suggestive of Captorliimis than of the wide bone at the rear of
the microsaur skull.
.") Kaily captorhinoinorphs, especially
the Protoroth\rididae but also Limnoscelis,
lia\e a weak /one between the skull deck
MicRosAims
279
and cheek. Watson (1954, p. 343) has shown
how the skull becomes strengthened during
the evolution of the Captorhinidae by formation of a suture between parietal and
squamosal. Microsaurs lack this zone of
weakness, with the exception of the early
Westphalian Asaphestera in which it has
recently been reported by Carroll (1963,
p. 4). One may not say, therefore, that its
absence from later microsaurs is a distinc-
1-'1G. 1. Dorsal views of skulls of microsaurs (A, B)
and captorhinomorph cotylosaurs (C, D). A. Cardiocephalus sternbergi Broili (after Gregory, Peabody, and Price). B. Microbrachis pelikani Fritsch
(after Steen). C. Captorhinus sp. (after Romer).
D. Limnoscelis paludis Williston (after Romer).
Xot to scale: Limnocelis four times size o£ Captorhimis which is twice size of the microsaurs. PP,
postparietal; QJ, quadratojugal; SQ, squamosal;
ST, supratemporal; T, tabular.
C
D
FIG. 2. Lateral views of skulls. A. Cardiocephalus, B. Microbrachis, C. Captorhinus, as in Fig. 1;
D. Komeria iexana Price (after Watson). Xot to scale. Lettering as in Fig. 1.
280
JOSEPH T . GREGORY
A
C
FIG. 3. Palates of: A. Michobrachis pelikani Fritsch after Steen. B. Ricnodon Hmnophyes Steen.
C. Caplorhinus sp. (after Romer). PT-pterygoid. Not to scale.
tion from the most primitive captorhino- in early Carboniferous microsaurs ("adelomorphs. One should expect such a loose spondyls").
contact between skull deck and cheek in
5) Captorhinomorphs have a single octhe earliest members of every tetrapod line, cipital condyle (Fig. 4A) formed by the
as it represents the separation between
skull table and opercular apparatus of the
crossopterygian fishes (Watson, 1954, p.
341). It should be emphasized that this
does not imply an ancestral otic notch.
4) Both groups have a movable basipterygoid articulation, a primitive tetrapod
feature. But here the resemblance of the
palates ends. Captorhinomorph pterygoids
have prominent transverse flanges (Fig.
ST
3C) adjacent to the basal articulation, at
the rear of the palatal vault; behind this
the quadrate ramus is vertical in position,
permitting a large adductor muscle mass to
lie between it and the side of the skull.
The interpterygoid vacuity is short and
narrow.
Microsaurs lack the transverse pterygoid
flange, and the quadrate ramus of the pterygoid appears (at least in gymnarthrids, Fig.
3B) to lie largely in the plane of the palate. Interpterygoid vacuities are moderate- FIG. 4. A. Occipital view of skull of Captorhinus
(after Romer) showing relationship of stapes to
ly wide and amphibianlike in Hyloplesion quadrate.
B. Occipital view of Cardiocephalus (after
and Microbrachis (Fig. 3A). In the gym- Gregory, Peabody, and Price). C. Lateral view of
narthrids they are relatively narrow and Cardiocephalus skull with portion of temporal roof
more reptilian; apparently this was also cut away to show quadrate (Q) and Stapes (S).
true of Sparodus. Seeleya (Fritsch pi. 41) Opening in floor of braincase medioventral to stapes
have been cartilaginous but is more probably
has a wide dentigerous parasphenoid sepa- may
an artifact of preparation. Based on A.M.N.H.
rated by narrow slitlike vacuities from 4763a. OP, opisthotic; other abbreviations as in
broad pterygoids. The palate is unknown Jig. 1.
MlCROSAURS
281
basioccipital bone; in this they resemble cervical or dorsal intercentra at least marks
other primitive reptiles, labyrinthodonts, them as more specialized than one would
and even fishes. Microsaurs, on the-con- expect to find captorhinomorph ancestors,
trary, lack an ossified basioccipital and have if not completely divergent in vertebral
paired condyles (Fig. 4B) at the side of the structure.
foramen magnum on the exoccipitals. In
8) Primitive labyrinthodonts have 24
this respect they resemble Triassic and later presacral vertebrae, according to Romer
amphibians. Correlated with this shift of (1947, p. 63). Captorhinomorphs likewise
the occipital joint the atlas vertebra has have this presumably primitive tetrapod
paired articular facets like those of uro- number of presacrals (23-26 according to
deles and gymnophionans.
Romer 1956, p. 229). Microsaurs appear
6) The stapes of Captorhinus (Fig. 4A) divergent in having appreciably longer preis a heavy rod which extends outward and sacral columns; Microbrachis has 38, other
slightly downward from a large footplate genera appear to have in excess of 30.
which fills the fenestra ovalis to a facet at
9) Finally microsaurs are not known to
the rear of the inner quadrate condyle. A have the full tetrapod complement of toes;
foramen for the stapedial artery perforates no specimen has been found with more
its shaft near the base. Slightly distal to than three fingers in the manus. In this
this foramen, a dorsal process extends up- respect also they either are too specialized
ward-to the paroccipital process. The mor- to be reptile ancestors or else are even more
phological relations of the captorhino- divergent.
morph stapes are among the most primitive found among tetrapods, recalling those
THE PROBLEM OF CAPTORHINOMORPH
of rhipidistians.
ANCESTRY
Microsaur stapes are known mainly in
the specialized gymnarthrids. This family
It should be clear from this enumeration
has forward-sloping quadrates which carry that microsaurs are so divergent from capthe articulations of shortened jaws well torhinomorph reptiles that ancestors of the
ahead of the occipital region. The stapes latter cannot be sought among known mem(Fig. 4C) has a greatly enlarged footplate bers of the former group. Could they have
which covers the lower lateral portion of had a common parentage later than the
the braincase, but the columnella is re- separation of the labyrinthodont and diaduced to a tubercle that lies close behind dectomorph lines from their ancestry?
the rodlike quadrate. A well developed
Early labyrinthodonts such as Edops or
opisthotic completes the rear of the skull, Macrerpeton differ from captorhinids in
and there is no trace of a lissamphibian having flatter larger skulls. Their palates
"operculum." The form and position of lack transverse pterygoid flanges and tend
the bony stapes is much like that of uro- to develop wider interpterygoid vacuities.
dcles, gymnophionans, and various frogs, Crossopterygian-like sets of tusks and pits
lizards, and other tetrapods which have lost are retained on the inner palatal bones.
their tympanic apparatus (Barry, 1963). Labyrinthodonts have large and dorsally
The gymnarthrid otic structure is closest to placed otic notches into which the stapes
such perennibranchiate urodeles as Nec- projects upwardly from the fenestra ovalis,
turus, which lack the operculum and retain thereby losing its primitive contact with
a well developed opisthotic. Gymnothio- the quadrate. They have divided (temnonan ears are similarly constructed.
spondylous) centra instead of holospondy7) Although individual microsaur verte- lous vertebrae with vestigial intercentral
brae so closely resemble those of the Pro- crescents. These characteristic labyrinthotorthyridids—captorhinomorphs which have doni and captorhinomorph reptile patterns
not developed massive neural arches typi- must have diverged from an ancestral struccal of most cotylosaurs—that discrimination ture extremely similar to that of a crossopis difficult, the absence of any trace of terygian.
282
JOSEPH T. GREGORY
In the rhipidistian crossopterygian Eusthenopteron the hyomandibular extends
outward, downward, and backward from
the otic region of the braincase to the quadrate bone at the rear of the palate. On its
posterior border is a facet for articulation
with the opercular bone, which is regarded
as homologous to the tympanic process of
the tetrapod stapes (Eaton, 1939). The otic
notch of labyrinthodonts appears to be an
enlargement of the spiracular cleft of the
crossopterygian fishes. Evolution of the
labyrinthodont middle ear involved a dorsal shift of this opercular-tympanic process,
and at the same time loss of the primitive
connection of the stapes to the quadrate.
Sushkin (1927) first pointed out that this
dorsal position of the stapes and related
structure of the occiput in labyrinthodonts
was divergent from that of early reptiles.
Parrington (1958, p. 107), following a suggestion by Westoll (1943), has suggested
that these relations mean that labyrinthodonts cannot be ancestral to amniotes—
here let us say to the captorhinomorph
reptiles—but must be looked upon as a
divergent evolutionary line.
The captorhinomorph palate retains the
movable basal articulation, and narrow interpterygoid vacuities of the ancestral crossopterygian. The anterior portion of the
parasphenoid becomes slender and loses its
dentition except in a few rare forms. Palatal tusks are lost, though various arrangements of palatal teeth are retained in reptiles. On the contrary, the typical labyrinthodont palate evolved by widening the interpterygoid vacuities and frequently by
broadening the parasphenoid. Crossopterygian-like tusks are retained in many families.
These and many other features indicate
the early divergence of the ancestors of
labyrinthodonts and captorhinomorphs.
Peter Vaughn (1960) considered Parrington's idea as favoring the polyphyletic origin of the reptiles, and later (1962) suggested that microsaurs might be the nonlabyrinthodont group ancestral to captorhinomorphs. Abundant reasons have already been given for considering this unlikely; the known microsaurs became spe-
cialized at an early time much more in the
direction of modern urodeles and/or caecilians rather than of reptiles.
Fossil vertebrates are extremely rare in
the early Carboniferous or Mississippian
Period when the first tetrapod radiation
took place. No remains are known of an
animal that would stand intermediate between crossopterygian fish and captorhinomorph reptile. This does not mean that
reptiles arose directly from a fish ancestor;
it means we have no record of whatever
intermediate stages may have occurred. But
perhaps the hypothesis is not unreasonable.
Professor Romer has suggested that the
amniote egg was developed by aquatic animals.
Seymouriamorphs appear to be labyrinthodonts in the broad sense. Their skull
structure in particular links them more
closely to typical labyrinthodonts than to
captorhinomorphs, and their temnospondylous vertebrae likewise favor this view. If
diadectomorphs and chelonians are related
to Seymouria, and are also reptiles, one
cannot avoid the conclusion that the Reptilia have arisen polyphyletically from
rather diverse sources. Placing the Seymouriamorpha and their diadectomorph
derivatives in a separate Class Batrachosauria solves the problem semantically at
the expense of proliferating higher categories. Moreover, if by any chance the
Chelonia are diadectomorph descendants,
they logically would have to be removed
from the Reptilia, which is contrary to
both time-honored custom and what we
know of their physiology and anatomy.
The major obstacle to including descendants of both diadectomorph and captorhinomorph stocks in the Reptilia is the
improbability that the complex developmental pattern of the amniote egg could
have arisen independently in the parareptilian (diadectomorph) and eureptilian
(captorhinomorph) lines. For some reason
biologists seem more loath to consider the
possibility of parallel development of biochemical and physiological mechanisms
than of complex anatomical details. I see
no real distinction between these, nor do I
know a wa\ to decide which features of the
MlCROSAL'RS
mosaic were established in the common
ancestor and which were added later by
parallel evolution. Parrington (1958, p.
101) suggests that the similarities between
Seymouria and Diadectes are convergent,
and implies that the diadectomorph and
captorhinomoiph reptiles may have had an
unknown common ancestor. He has sought
to support this view with evidence of a
connection between stapes and quadrate in
various diadectomorphs (1962). More recently Vaughn (1964) has described Tseajaia, which combines captorhinomorph,
diadectomorph, and seymourian features.
As its similarity to captorhinomorphs is
limited to retention of a primitive palate,
it does little to strengthen the case for close
relationship of these Suborders. Other possible solutions to the problem of reptilian
ancestry would include attributing the assemblage of amniote characteristics to the
Anthracosauroid branch of the Labyrinthodonts, which includes the Seymouriamorpha and conceivably could also include the
unknown captorhinomorph ancestry. But
little evidence supports either of these
points. However it is worth speculating on
the possibility that the amniote egg developed concurrently with the tetrapod limb,
and that modern amphibians have simplified their development in the course of readaptation to depositing their eggs in
water. In other words, all tetrapods might
be phylogcnetically amniotes, but amphibians have retrogressed in their development.
Returning to the question of microsaurreptile relationships, the microsaurs seem
far more divergent from reptiles than
primitive labyrinthodonts. The earlier microsaurs— Dolichopareias and Adelogyrinus
of the early Pennsylvanian, differ from
later genera most obviously in the great
elongation of the postorbital skull region.
In this they show less resemblance to captorhinomorphs than do the more typical
Hyloplesion or even the gymnarthrids.
There is no suggestion of approach to a
common reptile-microsaur ancestor. Hence
it is most unlikely that they had a closer
common ancestor with captorhinomorphs
than other amphibians.
283
MICROSACR-LISSAMPHIBIAN RELATIONSHIPS
The prevalent view that lepospondylous
amphibians form a homogeneous phyletic
group implies that modern urodeles and
caecilians were derived from some Paleozoic lepospondylous ancestor, generally conceded to be as yet unrecognized. Anurans,
on the contrary, have been regarded as
descendants of the labyrinthodonts. The
recent critical examinations of the many
similarities between the living amphibian
orders by Eaton (1959) and by Parsons and
Williams (1963) have raised questions as
to the correctness of this view. In particular, the occurrence of a non-calcified zone
in the teeth of all three modern orders, and
similarities in the middle ear (especially
the conversion of the opisthotic to an "operculum" and its attachment to the footplate of the stapes) suggest a common ancestry of these forms later than rather than
prior to the Paleozoic stegocephalians.
Most of the numerous other similarities between the modern orders cited by these
authors serve largely to set the Amphibia
apart from other classes of vertebrates, and
are inconclusive as to phyletic relationships
within the class itself. Many of them, such
as the double occipital condyle and loss of
hypoglossal foramen, can be demonstrated
to have occurred independently in separate
lines of descent. Discussion of relationships
of the modern orders is the responsibility
of others in this symposium, but as microsaurs are the central and most conservative
group of Paleozoic lepospondyls, I may be
permitted to comment on a few points.
Many features of the microsaurs suggest
the modern lepospondylous salamanders
and caecilians: microsaur vertebrae most
closely resemble those of the Gymnophiona; their occipital articulation and atlas
vertebra resemble both urodeles and gymnophionans; the auditory region consisting
of a stapes with short plectrum or columella
and large footplate together with a normal
opisthotic bone is like that of the gymnophionans1 and perennibranchiate urol Parsons and Williams (1963, p. 30) cite evidence
that suggests that gymnophionans possess an oper(ii In MI Cused to the columella.
284
JOSEPH T. GREGORY
deles such as Nee turns. The well ossified
skull roof particularly resembles that of the
Gymnophiona, in which Marcus had demonstrated that an essentially primitive tetrapod assemblage of dermal bone rudiments unite to form the considerably
smaller number of adult bones which form
the solid skulls of these burrowing animals.
The extreme forward position of the quadrate articulation throughout these groups
is likewise to be noted. For these reasons
urodeles and gymnophionans have been
classified together with various Paleozoic
orders as a separate subclass Lepospondyli,
defined primarily by their "hour glass
shaped" vertebrae which lack intercentra.
Neither labyrinthodonts nor any of the
Paleozoic lepospondyls show any trace of
the pedicellate teeth with non-calcified
zones, which are so characteristic of all living orders of Amphibia. Other features
must be relied upon in determining their
ancestry. Of these the structure of the
middle ear in certain Anura, for example
Rana and Bufo, which have a bony stapes
extending from the fenestra ovalis to a
tympanic membrane situated in an otic
notch, is so similar to that of the labyrinthodonts that it forms one of the strongest
arguments for deriving the Anura from the
Labyrinthodontia. Many other genera of
anurans lack tympanic membrane and cavity (T. H. Barry, 1963). In these the stapes
(plectrum) is lost and the operculum—the
peculiarly amphibian auditory elementcovers the entire fenestra ovalis. The resulting structures resemble the reduced
middle ears of urodeles and gymnophionans. Similar reduction of the stapes is
known in various lizards. It would not
therefore be surprising to find convergence
in this feature within the Amphibia.
It is reasonable to assume that the loss
of tympanum and associated structures was
an irreversible evolutionary event. Therefore those anurans with a fully developed
tympanic ear must be primitive. Their
structures can be derived from those of the
Labyrinthodontia, but not from any known
Paleozoic lepospondyl. All known lepospondyl ears are of the reduced type associated with loss of the tympanic cavity.
If the Lissamphibia form a monophyletic
subclass including the Anura, and are derived from labyrinthodont ancestry, then
the similarities mentioned above among
living urodeles and caecilians and the
niicrosaurs and other Paleozoic lepospondyls must all be the result of convergence.
Indeed one cannot account for the morphologic patterns of the various orders of recent and fossil amphibians without postulating convergence or parallelism. Either
teeth with non-calcified zones or lepospondylous vertebrae have evolved more than
once, and the reduced stapes and accessory
operculum of the ear must have arisen in
parallel or convergent fashion several times
regardless of whether teeth or vertebrae
are the most conservative unit. Until some
more surely primitive feature is recognized,
it may be impossible to decide which of the
similar structures are convergent and which
inherited from a common ancestor.
Modern Amphibia are successful survivors of the earliest terrestrial radiation of
tetrapods. That radiation produced highly
diverse adaptations, many of which would
not have been in direct competition with
known later reptiles, birds, and mammals.
It seems reasonable that more than one
such stock might persist. Structurally
primitive anurans resemble labyrinthodonts; urodeles are more like certain Paleozoic lepospondyls, especially nectrideans;
and gymnophionans retain many features
of the microsaurs. Perhaps the possibility
that non-calcified pedicellate teeth developed independently and convergently in
three discrete lines of small predominantly
insectivorous animals, is less unlikely than
the manifold convergence required to derive animals otherwise so similar to ancient
lepospondyls from the labyrinthodonts.
REFERENCES
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Camp, C. L., et. al. 1940-65. Bibliography of Fossil
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,1944-48), 84 (1949-53), 92 (1954-58).
MlCROSAURS
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. 1962. The paleozoic microsaurs as close
relatives of reptiles again. Amer. Midi. Nat. 67:
79-84.
. 1964. Vertebrates from the Organ Rock
shale of the Cutler Group, Permian of. Monument
Valley and vicinity, Utah and Arizona. J. Paleont. 38:567-583.
COMMENTS
Robert L. Carroll, Redpath Museum,
McGill University, Montreal, Canada
Current work on the microsaurs from
Linton, Ohio, the Joggins formation, and
the Czechoslovakian gas-coal substantiates
Dr. Gregory's conclusion that the microsaurs are not related to the ancestry of reptiles. The structure of the otic region, the
occipital condyle, and the atlas differs fundamentally in the two groups, while the
configuration of the palate and skull roof
helps to differentiate them.
It should be noted, however, that the
elongated, salamander-like proportions of
Microbrachis and the gymnarthrids are not
representative of all microsaurs. Other
genera, particularly Tuditanus and Pan-
285
tylus, have large, sturdy limbs, and girdles
of reptilian proportions and configuration.
These genera also have a reptile-like astragalus and calcaneum (also present in Ricnodon copei), and at least four front toes.
Tuditanus has 29-30 presacral vertebrae,
compared with 38 in Microbrachis and 2326 in early reptiles. Although an accurate
vertebral count is difficult from the material available, Pantylus also appears to have
had a short, essentially reptilian vertebral
column. It is genera of these proportions
that have been mistaken for reptiles, and
it remains difficult to distinguish the two
groups without a thorough knowledge of
the skull. Despite the fundamental difference in vertebral and cranial morphology,
indicating a distinct ancestry, the degree of
convergence between certain microsaurs
and the captorhinomorph reptiles is extensive.
If, as now seems certain, the microsaurs
are not ancestral to captorhinomorph reptiles, what group of amphibians is? The
presence of an otic notch and a dorsally
directed stapes in the labyrinthodonts is
difficult to reconcile with the ventrally
directed stapes and absence of a notch in
the captorhinomorphs, as has been pointed
out by Gregory, as well as by Parrington,
Westoll, and Vaughn. Yet it is difficult to
escape the conclusion that captorhinomorphs did evolve from labyrinthodonts
if we consider the total lack of any possible
non-labyrinthodont predecessors, the labyrinthodont features retained in some of
the captorhinomorphs, and the numerous
reptilian features in certain seymouriamorphs.
If there were a group leading to the captorhinomorphs independent of the labyrinthodonts, it is not surprising that no
fossils of this group are known prior to
the earliest captorhinomorphs at Joggins,
since this is the earliest fauna of truly terrestrial vertebrates. However, one would
expect to find relicts of such intermediate
forms surviving into the later Pennsylvanian or Permian. There are no such forms,
aside from the labyrinthodont seymouriamorphs. Since the seymouriamorphs, or
more specifically the gephyrostegids (dis-
286
JOSEPH T. GREGORY
cussed in the comments on Dr. Olson's
paper), do show many reptilian features,
particularly in the palate and vertebral
column, while retaining the labyrinthodont
otic notch, it seems probable that they are
relicts of the forms transitional between
primitive anthracosaurs and captorhinomorph reptiles. This would mean that the
otic notch has become closed from a previously open condition and that the stapes
has become redirected.
Among the captorhinomorphs, Limnoscelis still retains a number of labyrinthodont features: labyrinthine teeth, a weakness in the area of the otic notch, an occiput
very similar to that of Seymonria, and an
independent centrale, intermedium, and
tibiale, which further suggest that captorhinomorphs have evolved from labyrinthodonts. A remnant of an otic notch has also
recently been reported in the pelycosaurs
by Langston.
As yet, no additional features have been
found in my study of the microsaurs to
either support or refute relationship with
any of the Lissamphibia. Except for differences in limb and skull proportions, and
some minor morphological details, other
microsaurs vary relatively little from the
pattern of the gymnarthrids determined
by Gregory, Peabody, and Price.