AMER. ZOOLOGIST, 5:287-294 (1965).
PALEOZOIC LEPOSPONDYL AMPHIBIANS
DONALD BAIRD
Dept. of Geology, Princeton University, Princeton, New Jersey
SYNOPSIS. The lepospondyls are urodele-like amphibians of the Paleozoic Era, some
o£ which have been considered ancestral or closely related to reptiles or modern amphibians. Their origin is enigmatic, and the earliest occurrences reveal a long-established division into three contrasting orders: the serpentine aistopods, the newt-like
nectrideans and the deceptively reptilian microsaurs. The Subclasses Lepospondyli and
Labyrinthodontia are not so disparate as to indicate a polyphyletic origin for the
Amphibia. Which subclass gave rise to the present-day Lissamphibia is still disputed,
but superficial similarities between lepospondyls and lissamphibians cannot mask the
fundamental discrepancies in temporal range and morphology and (apparently) in
life history as well.
INTRODUCTION
During most of the latter third of Paleozoic time, a span of some 90 million years,
the swamps, streams, and ponds of the
Northern Hemisphere were inhabited by a
diverse assortment of amphibians which
resembled the modern urodeles in their size
range and adaptations. These amphibians
are distinguished from their contemporaries, the labyrinthodonts, by vertebral
centra which evidently ossified as cylinders
around the notochord. By virtue of these
"husk vertebrae" they are named Lepospondyli (or colloquially, lepospondyls),
and set apart as a subclass of the Amphibia.
Within the subclass three distinctive groups
are recognized as orders.
But classifying these ancient amphibians
only emphasizes the fundamental enigma
of their relationships. I may as well admit
here that the origins and antecedents of
the lepospondyls can only be guessed at,
the affiliations within the group are obscure, and the connections (if any) between
lepospondyls and present-day amphibians
are disputed. This paper offers a skimming
glimpse of the group in order to discuss
(rather than answer) the question of its
affinities.
In a very real sense this survey is premature, for few of the known genera are
adequately known, and several of the most
crucial specimens have been lying in museum drawers, unprepared and unstudied
or half-studied, for 80 to 100 years. Few
paleontologists have showed much interest
in lepospondyls as such. To put it bluntly,
the saving grace of the lepospondyls has
been that some of them look enough like
reptiles to have attracted the attention of
scientists interested in reptile origins: if
they had not been described under a misapprehension, many of these curious amphibians might never have been described
at all.
For an excellent review of the Lepospondyli we are indebted to Dechaseaux (1955),
who has judiciously sifted and weighed the
published accounts, though of course much
unpublished information was unavailable
to her. Three component orders of the subclass can be distinguished with confidence:
the limbless ATstopoda, the newt-like Nectridea, and the sometimes reptile-like
Microsauria.
All three orders share a basic characteristic in the form of their vertebrae, which
are composed of a single or twinned neural
arch and a one-piece, hourglass-shaped centrum which is very probably homologous
with that of the living lissamphibians
(urodeles, frogs, and apodans), as Williams
(1959) has argued. The centrum is thus a
pleurocentrum, and the trunk vertebrae
lack ossified intercentra, although chevron
bones which have been observed in the tails
of a few microsaurian genera may well be
intercentral in homology. This characteristic lepospondylous vertebra (Fig. 1) occurs in fully developed form in the earliest
known representatives of the group, and
nothing is known of intermediate stages
which might link it with the types of verte-
287
DONALD BAIRD
Having recently (1964) reviewed the
group, I will merely outline its peculiarities
here, using the best-known genus Phlegethontia (Figs. 1A, 2) as an example. The
skull with its bulbous braincase and temporal fenestrae is amazingly sophisticated
for so ancient a tetrapod; no comparable
degree of elaboration was attained by labyrinthodonts or reptiles during the Carboniferous. Similar specializations can be seen
in the skulls of apodans, lysorophid microsaurs, and amphisbaenid lizards; but Gregory (1948) has made it clear that these
similarities are the result of convergent
evolution. As comparisons between such
highly evolved skulls are more hindrance
than help in determining relationships, we
must seek more reliable guides in the relatively conservative structures of the postcranial skeleton.
The backbone is differentiated into neck,
trunk, and tail, and comprises about 230
vertebrae in adults, the number apparently
increasing with age. Neural spines are
blade-like and the transverse processes are
outgrowths of the centrum. The vertebrae
FIG. 1. Trunk vertebrae of representative lcpospondyls. A, the aistopod Phlegethontia (from
Gregory, retouched); B, the nectridean Diploceraspis (from Beerbower); C, the microsaur Cardiocephahis (from Gregory, Peabody and Price).
brae found in labyrinthodonts or crossopterygian fishes. It simply appears de novo
in the fossil record.
"UNSEEN FEET": THE AISTOPODS
Among lepospondyls, the most ancient
pedigree is that of the aistopods which first
appear in rocks of early Mississippian age,
about 330 million years old. Indeed, aside
from the late Devonian ichthyostegids the
oldest known aistopod is the oldest known
amphibian of any sort. Yet even at that
remote date these serpentine amphibians
were already highly specialized—about as
specialized, in fact, as any amphibians living or extinct. Only three genera have
been discovered, the last of which became FIG. 2. The aistopod Phlrgrthontia (skull from
extinct in early Permian time some '260 Gregory, skeleton from Dechaseaux). Abbreviations
million years ;igo.
following Fig. 5.
PALEOZOIC LEPOSPONDYL AMPHIBIANS
are pierced by the spinal nerves, a condition
found also in urodeles, but not in nectrideans or microsaurs. Ai'stopod ribs lie with
their straight shafts nearly parallel to the
body axis, their strange K-shaped proximal
ends articulating by the capitulum alone.
As in other lepospondyls, the belly is armored with a herringbone pattern of
slender bony scales; the dorsal scales (when
present) are distinctively pebble- or oatshaped.
But the chief peculiarity of the ai'stopods
is their total lack of limbs and girdles. Not
even a rudiment can be found in any of
the well-preserved skeletons from coalswamps of Pennsylvanian age, and the
unique early Mississippian skeleton seems
to be equally limbless. It might be tempting to speculate that limbs were never present in this lineage, and that ai'stopods originated directly from a crossopterygian fish
through the suppression of the fins. Common sense, however, suggests that the obvious dividing-points between neck, trunk,
and tail were once marked by girdles. This
consideration and the basic similarities between aistopods and the four-legged nectrideans and microsaurs convince me that
the forerunner of the aistopods had limbs.
If these serpent-amphibians had already
lost their appendicular skeletons by early
Mississippian time, we must search for their
four-legged ancestors among the mists of
the Devonian, a period from which very
few amphibians (and no lepospondyls) are
known at present. Until such an ancestor
is unearthed, the origins of the group must
remain a mystery.
NECTRIDEANS: NEWTS AND ARROWHEADS
Probably the lepospondyls most familiar
to non-specialists are the horned genera
Diplocaidus and Diploceraspis from the
early Permian. With their massive sculptured skulls shaped like broad arrows, these
bizarre forms readily catch the eye of the
museum visitor and student. As extreme
evolutionary end-products, however, they
offer little help toward an understanding
of the Order Xectridea and its affinities.
More representative of the order are the
289
FIG. 3. Skulls of the nectrideans Keraterpeton (left)
and Sauropleura (from Steen).
Pennsylvanian genera such as the jjrimitive
horn-bearer Keraterpeton (Figs. 3, 4B), and
the urocordylids Ptyonius and Urocordylns. Little about the latter genera will be
found in the literature, for Urocordyhis
(sensu stricto) is represented by a single
skeleton which has never been prepared,
while Ptyonius was mistakenly sunk into
synonymy in 1930, and has not yet struggled
back to the surface. So until paleontologists can catch up with their descriptive
chores, the common but somewhat specialized genus Sauropleura (Figs. 3, 4A) must
serve as an example.
Keraterpeton and Sauropleura, the most
newt-like and elegant of the lepospondyls,
seem to deserve an ordinal name meaning
"girl swimmers." Both clearly show the
basic nectridean trademark: a high-sided
swimming tail with fan-shaped neural and
haemal spines. Unlike those of most ancient tetrapods, the haemal arches are not
intercentral in origin, but form an integral
part of the pleurocentrum. Accessory articulations between vertebrae are another
nectridean characteristic. The relatively
short trunk consists of a fixed number of
vertebrae bearing two-headed ribs. Limbs
and girdles are diminutive but fully formed,
with four toes in the forefoot and five in
the hindfoot; the sculptured trio of thoracic
plates—clavicles and interclavicle—look very
much like those of labyrinthodonts. Belly
scales range from needle-like to rhomboidal
in different genera, and dorsal scales are
unknown.
Family variants in the nectridean skull
pattern can be seen in Figure 3. Both
290
DONALD BAIRD
FIG. 4. Reconstructions of lepospondyl skeletons. A, the urocordylid nectridean Sauropleura;
B, the horned nectridean Keraterpeton; C, the microsaur Microbrachis (from Steen).
families are typically short-snouted and
have the usual amphibian pattern of bones
in the skull roof. Urocordylids retain a
small intertemporal bone, while keraterpetontids have traded it for a horn-shaped
enlargement of the tabular which, in primitive genera, apparently connected the skull
to the T-shaped upper bone of the shoulder
girdle. In the earliest keraterpetontids the
palate is closed, with only slit-like openings
between the pterygoids and the parasphenoid, but these palatal vacuities widen in
the course of time and become broad windows in the terminal genera Diplocaulus
and Diploceraspis.
Nectrideans make their first appearance,
already full-blown and diversified, in lower
Pennsylvanian deposits about 300 million
years old; like the other lepospondyl orders,
they disappear from the scene in early Permian time. Some similarities in vertebral
structure which appear to ally nectrideans
more closely to ai'stopods than to microsaurs have been pointed out by Gregory,
Peabody, and Price (1956), but these resemblances are counterbalanced by the fact
that the spinal nerves perforate the vertebrae in aistopods but pass between the
vertebrae in nectrideans. Thus the relationship between these orders must be remote at best.
To the extent that they are the least specialized and the most labuinthodont-like
of the lepospondyls, the Carboniferous nectrideans may be thought to represent the
basic stem of the subclass. In all but the
most basic features, however, they are so
far removed from the other orders that any
attempt to reconstruct a composite common
ancestor, an eo-lepospondyl, would be
largely an exercise in science fiction.
MICROSAURS: LITTLE BOCUS REPTILES
As the Microsauria form the subject of
Dr. Gregory's contribution to this symposium, a brief review for the sake of comparison is sufficient here. Microsaurs, which
range from late Mississippian to early Permian time, form a natural but diversified
group which has yet to be fully sorted into
families (Romer, 1950). They are longtrunked and (for the most part) feeblelimbed, with either three or four toes in
the forefoot (Fig. 4C). Their vertebrae, a
trademark of the order, are three-piece
affairs with one suture dividing the neural
arch and another separating arch from centrum. Since ossification may obliterate
either suture or both, however, too much
importance should not be attached to their
condition. Separate caudal chevrons which
are presumably intercentral in origin have
been reported in Microbrachis and Pantylus. The characteristic dorsal scales are
o\al with radial striations superimposed on
PALEOZOIC LEPOSPONDVL AMPHIBIANS
291
pasph _JM_ _^_
FIG. 5. Microsaur skulls. A, Dolichopareias; B, Euryoclus; C, Ostodolepis; D, Microbrachis; E,
Hyloplesion (with palate); V, Tuditanus; G. Lysoroplws. Right postparietal stippled, supratemporal hatched. (A-D from Romer; E modified from Dechaseaux; F original, from the newly
prepared type specimen; G original, based on Sollas.)
a—angular
d—dentary
eo—exoccipital
ep—epipterygoid
t—frontal
1, la—lacrimal
in, mx—maxilla
n—nasal
ABBREVIATIONS FOR FIG. 2, 5, AND 6.
p—parietal
q—quadrate
pal—palatine
qj—quadra tojugal
pasph—parasphenoid
sa—surangular
pC—posttrontal
so—supraoccipital
pro, pmx—premaxilla
sp—splenial
po-—postorbital
sq—squamosal
pp—postparietal
st—supratemporal
prf—prefrontal
v—vomer
pt—pterygoid
a concentric growth pattern; the belly
scales are rod-like or rhomboidal.
Microsaur skulls (Fig. 5) are characterized by the absence of an otic notch and by
the large size of the supratemporal bone.
The palate is closed in gymnarthrids such
as Euryoclus but conspicuous palatal vacuities occur in members of other groups, for
example Microbrachis, Hyloplesion and
Tuditanus.
The genus Tuditanus from the middle of
Pennsylvanian of Ohio requires special and
apologetic mention because it has erroneously been called a reptile by Baird (1958)
and "the oldest known reptile, Eosauravus"
by Peabody (1959). Since those papers were
published, however, preparation of the type
specimens has put matters in a different
light: Eosauravus is a synonym of Tuditanus, trunk intercentra are not present,
the scales are typically microsaurian, the
forefoot has only four toes, and the supposedly reptilian tarsus is not irreconcilably
different from that of the microsaur Hyloplesion.
The deceptive nature of these little tetrapods is doubly illustrated in Fig. 6. Tuditanus, once thought to be a reptile, is
now known to be a microsaur; while Cephalerpeton, once thought to be a microsaur,
is now known to be a stem-reptile of the
family Romeriidae, a structural ancestor
292
DONALD BAIRD
FIG. 6. Deceptive similarity in skulls of the microsaur Tuditanus (above) and the true reptile Cephalerpeton (original reconstructions from the type
specimens). Abbreviations following Fig. 5.
for most of the Eureptilia and their offspring the birds and mammals. Besides
looking superficially alike, Tuditanus and
Cephalerpeton inhabited the same coalswamp and shared the trait of elusiveness,
for only two skeletons of each have ever
been found.
Two problematical groups are mentioned
last because not everyone is willing to include them with the microsaurs. Noteworthy for their antiquity are the adelospondyls (Fig. 5A) from the late Mississippian of Scotland, very rare and insufficiently
studied. Their skulls are essentially microsaurian, although exceptionally long behind
the orbits; the elongate body has microsaur-like vertebrae and a shoulder girdle
of sculptured dermal bones; minute but
well-developed limbs are present, and three
toes can be seen in the forefoot; the belly
scales are slender and rhomboidal. Watson
(1929), the only previous commentator on
adelospondyl taxonomy who has actually
examined an adelospondyl, points out the
strong similarities between these Mississippian forms and the Pennsylvanian and Permian lysorophids (discussed below). Fur-
ther research, I predict, will only strengthen
the existing evidence that the adelospondyls
are an early-divergent family of microsaurs.
The lysorophids, or molgophids, range
from middle Pennsylvanian to early Permian,1 and are best known from a Permian
species assigned to Lysorophus (Fig. 5G).
Like the microsaurs, they have extremely
long bodies with relatively tiny limbs, a
stemmed interclavicle, and either three or
four toes in the forefoot. The skulls are
highly specialized with a bulbous braincase
(in most genera), a closed palate, and freefloating maxillae and premaxillae. Despite
their atypical features, the skulls share the
microsaur characteristics of a long postorbital region and a large supratemporal
bone; the supraoccipital bone is usually
wedged in between the postparietals as in
Ostodolepis (Fig. 5C).
Molgophid vertebrae are clearly microsaurian in pattern, with or without a divided neural arch, with or without a persistent neurocentral suture. Obscure structures which appear to be paired haemal
arches are present in the tail of Molgophis.
The scales are feebly ossified, but seem to
be of microsaurian type. These peculiar
amphibians form a closely knit family
which, like so many others mentioned in
this paper, is much in need of study. Lysorophids have been assigned hither and
yon by various classifiers, but are best interpreted, I believe, as specialized microsaurs.
LEPOSPONDYL AFFINITIES
The three orders just described are sufficiently clear-cut that their status as natural
groups seems beyond question. Despite
their peculiarities, these orders share
enough basic features to justify brigading
them as a subclass of the Amphibia. Their
relationships to other amphibian groups,
however, are by no means clear.
l A supposed lysorophid from the late Triassic
has been described by Huene and Bock (1954)
under the name Lysoroceplialus. Their specimen is
not a lysorophid and probably not an amphibian at
all, but appears instead to be the incomplete and
ihimaged skull roof of a palaeoniscoid fish.
PALEOZOIC LEPOSPONDYL AMPHIBIANS
Except in the most specialized forms, the
skulls, girdles, and limbs of lepospondyls
are essentially similar to those of labyrinthodont amphibians and primitive reptiles.
Though the lepospondyl skull and the labyrinthodont skull might conceivably have
originated independently of each other at
the crossopterygian fish level, a duplicate
origin of the tetrapod appendicular skeleton is simply incredible. Thus common
sense assures us that the Amphibia must
be a monophyletic class, Jarvik (1960) to
the contrary notwithstanding. The available evidence—which is only sketchily summarized in this paper—indicates that the
Lepospondyli, Labyrinthodontia, and Eureptilia had a tetrapod ancestor in common,
and that this ancestor must have lived well
back in Devonian time, some 380 million
years ago.
Until more ancient tetrapod fossils have
been discovered, and until those already
discovered have been fully described, there
will be little point in further speculation
on amphibian origins. Devonian ancestors
are to be found not in hypothetical reconstructions but somewhere under a rock.
Evidence for and against the lepospondyls as ancestors of the present-day amphibians is presented in the accompanying paper
by Dr. Estes, who views the ancient amphibians in the light of the modern groups.
Looking backward in time, he concludes
that the living Lissamphibia are more
plausibly derived from the Labyrinthodontia than from the Lepospondyli. As a student of Paleozoic amphibians I look forward in time to seek their descendants
among the Cenozoic forms—and come to
the same conclusion. The following evidence impresses me most:
Lepospondyls disappear from the fossil
record in the early Permian, but lissamphibians do not appear until the early Triassic; and no connecting forms are known.
The vestigial dorsal scales of apodans resemble those of microsaurs, but are equally
similar to the dorsal scales of labyrinthodonts, as Noble (1931) has pointed out.
Lissamphibians (aside from sirenids) have
pedicillate teeth with a distinct separation
between pedicel and crown; lepospondyls
293
do not. Urodeles have spinal nerves which
pierce the vertebrae; lepospondyls (aside
from aistopods) do not—and the totally
limbless aistopods can hardly be made into
urodele ancestors on the basis of their intravertebral spinal nerves. The open palate
of modern amphibians may be derived as
easily from that of labyrinthodonts as from
that of some microsaurs or nectrideans.
Similarities between highly specialized
skulls of lepospondyls and lissamphibians
should be discounted, for structures which
are so obviously subject to convergent evolution make most unreliable guides to
kinship.
In summary, then, the lissamphibians are
remote from the lepospondyls both in time
and in structure. Still another line of evidence, however, remains to be considered.
THE PROBLEM OF METAMORPHOSIS
Juvenile labyrinthodont amphibians, the
"branchiosaurs" or "phyllospondyls" of
classic authors, are well known for their
highly cartilaginous skeletons and filamentous external gills. The smallest individuals differ so much from adult labyrinthodonts, and show so few diagnostic characters, that despairing taxonomists have
lumped most of them into the catch-all
genus Branchiosaurus (where few of them
properly belong, certainly none of the Permian forms). Although limbless tadpoles
of labyrinthodonts have not been found,
the gill-bearing and semi-ossified "branchiosaurs" are generally accepted as being unmetamorphosed larvae. Both of the main
labyrinthodont subdivisions, the rhachitome-stereospondyl group and the anthracosaur group, had larvae of this sort. Furthermore, large, gilled labyrinthodonts
which are obviously neotenous are also
known. Thus there can be no doubt that
metamorphosis (or its alternative, neoteny)
was characteristic of the labyrinthodont life
cycle.
Quite a different situation is found in the
Paleozoic lepospondyls. Here, examples
from each of the orders show that juvenile
individuals are essentially miniature adults,
readily assignable to adult-based species
294
DONALD BAIRD
wherever the specimens are well-enough
preserved to be identified at all, and differing from adults only in proportions which
are subject to allometric growth. No juvenile lepospondyl shows traces of external
gills.
Although the matter needs further investigation before positive statements can
be made, at present I know of no evidence
for metamorphosis in Paleozoic lepospondyls.
If the lepospondylous amphibians were
characterized by direct development, as
seems to be the case, it is difficult to conceive of them as the ancestors of metamorphosing amphibians such as frogs and salamanders. For this additional reason I concur with Estes and other authors in believing that lissamphibian origins are to
be sought among the Labyrinthodontia
rather than among the Lepospondyli.
Parenthetically, let us hope that this observation on the apparent absence of metamorphosis in lepospondyls will not provoke
still another attempt to link the microsaurs
with the reptiles!
Now that the Lissamphibia are accepted
as constituting a natural group, the taxonomic status of that group needs to be put
into perspective. Under Romer's 1945
classification, the Class Amphibia had been
bisected into the Subclass Lepospondyli
(the lepospondyls as defined here plus the
urodeles and apodans) and the Subclass
Apsidospondyli (the labyrinthodonts plus
the frogs). When the lissamphibian orders
have been subtracted from this system and
combined, three alternative placements are
possible: to include them as a superorder
in the Lepospondyli (where I feel confident
they do not belong), to place them in the
Apsidospondyli as a superorder equivalent
in rank to the Labyrinthodontia (which is
more plausible), or to recognize their dis-
tinctness in morphology and temporal
range by treating them as a third major
component of the Amphibia, the Subclass
Lissamphibia. The last alternative seems
best to me. In that case the categories Apsidospondyli and Labyrinthodontia become
conterminous, and most students will
doubtless prefer to designate this taxon by
the more familiar name. We may thus consider the Class Amphibia as comprising the
Subclasses Labyrinthodontia, Lepospondyli,
and Lissamphibia.
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