AMER. ZOOI.CK.IST, 5:267-276 (1965).
THE RHIPIDISTIAN - AMPHIBIAN TRANSITION
BOBB SCHAEFFER
Dept. of Vertebrate Paleontology, The American Museum of Natural History and
Department of Zoology, Columbia University, Nezu York City
SYNOPSIS. The transition from the rhipidistian crossopterygian fishes to the amphibians involved many changes in morphology, physiology and behavior. The selection
pressures promoting terrestrial adaptations and concurrently preserving the aquatic
ones must have been responsible for extensive experimentation during the transitional
interval. A multiple origin of the amphibian level is therefore regarded as probable,
and this view is supported by the diversity of the earliest known amphibian groups.
The realization that the rhipidistian
crossopterygians gave rise to the amphibians, apparently first proposed by Cope in
1892 and by Baur in 1896, represents one
of the great advances in our understanding
of vertebrate history. This relationship was
confirmed during the first quarter of the
present century mainly by the work of E. S.
Goodrich, W. K. Gregory, and D. M. S.
Watson. Subsequent studies by many paleontologists and comparative anatomists on
the rhipidistian and early tetrapod skeletons have further verified these conclusions. The purpose of this paper is to review briefly current knowledge on the organization of the rhipidistians, and to consider certain aspects of the rhipidistianamphibian transition in relation to experimentation in evolution.
THE RHIPIDISTIANS
The strictly fresh-water rhipidistians
were primarily carnivorous fishes (Fig. 1).
Like the dipnoans, but unlike the early
actinopterygians, they presumably lived
most of the time on or close to the bottom.
I can imagine a rhipidistian stalking prey
by moving slowly along the bottom with
its lobed fins, and then making a sudden
powerful upward lunge to capture its victim. The feeding mechanism, although resembling that of the palaeoniscoids in several ways, was certainly more powerful, and
it seems probable that most rhipidistians
fed on other fishes which they seized and
gulped in much the same manner as pikes
feed today.
The extensive rhipidistian palate, which
in its post-orbital extension reached almost
to the top of the braincase, formed the medial side and floor of a chamber which contained a large and powerful adductor mandibulae musculature. The outer wall of
the chamber was made up of the dermal
cheek elements. As in the palaeoniscoids,
this muscle mass reached the Meckelian
fossa of the mandible through an elongated
slot situated immediately in front of the
jaw articulation.
Although the mechanics of the orobranchial region have not been studied in detail, it would seem that the forces and
movements involved in feeding and in gill
respiration were essentially similar to those
of a palaeoniscoid. One additional feature
not possessed by the primitive actinopterygians was an intracranial joint (Fig. 2) situated between the ethmosphenoid and the
otico-occipital moieties of the rhipidistian
braincase. This joint may have permitted
slight elevation and depression of the ethmosphenoid moiety through the intervention of the palate, as the mouth opened
and closed. The related coelacanths have
a nearly identical but more mobile intracranial joint. In Latimeria (Millot and
Anthony, 1958) a pair of subcephalic muscles extends along the basicranium from the
ethmoid to the occipital part of the skull.
When the mouth closes these muscles contract and the snout is brought forcefully
against the front of the mandible. Although motion in the rhipidistian intracranial joint was clearly more restricted, it
is probable that these fishes also had subcephalic muscles to increase the pressure
of the bite. Actually very little motion at
the joint would be required to provide
this pressure.
267
288
BOBB SCHAEFFER
In regard to swimming, it is evident that
two different groups of rhipidistians independently increased the flexibility of the
body by developing relatively thin, flexible
cycloidal scales. The paired fins, with their
well-developed internal skeleton and musculature, were undoubtedly capable of a
great range of movement. If the evidence
provided by a living specimen of Latimeria
(Millot, 1955) is indicative, the rhipidistian
paired appendages could rotate almost 180
degrees, and move forwards and backwards
through a considerable arc. These fins
would thus be able to not only propel the
body along the bottom or slowly through
the water above, but could also function
effectively during swimming for turning,
braking, changing level, and moving backwards, as Dean (1906) observed in a living
specimen of Neoceratodus.
The osteichthyans probably arose from
an unknown acanthodian stock in the Silurian, and were differentiated into the
crossopterygians, dipnoans, and actinopterygians by the beginning of the Devonian.
A basic osteichthyan structure, the airbladder, was undoubtedly present in all
these groups. It presumably arose as a
lung, and secondarily became a hydrostatic
organ. The combination of a lung and a
well-developed gill system in the rhipidistians reinforces the idea that these fishes
periodically inhabited water deficient in
oxygen, and that they could survive at
least for short intervals in a terrestrial environment. Like the dipnoans, the rhipidistians must have gulped air through the
mouth, and the nasopharyngeal duct with
its internal nostril was used at this stage
only for olfaction. In water, the air bubble
TlO. 1. Knipidistian cro«opter\gians. A. Osteolepid (Osteolepis macrolepidutus). B. Rhizoduntid
iF.itsthetiopteron
fiinxli).
C. Holo|)[\i hud
iHutuf/txchun
ijuel>firn\ii).
K. Svenska Vctenskapsacad. Handl.; B and C after Jarvik, 1960.)
( A ,attei
J.uvik,
\l.HH,
THE RHIPIDISTIAN-AMPHIBIAN TRANSITION
in the lung also provided for neutral or
positive buoyancy, which was certainly a
major factor in osteichthyan maneuverability.
The detailed studies by Jarvik (1942,
1944, 1948, 1954) on the Devonian rhipidistians, and on Eusthenopteron in particular, have greatly increased our knowledge
269
of these fishes, but at the same time have
introduced some very controversial ideas
about their relationships with the living
Amphibia. Jarvik has divided the rhipidistians into two groups which he calls the
Porolepiformes and the Osteolepiformes.
The former, in his opinion, gave rise to
the urodeles, and the latter to the anurans.
FIG. 2. The skull of Eusthenopteron. A. Lateral view of braincase. B. Braincase plus palate and
hyomandibular. C. Diagrammatic transparency of entire skull. Arrow indicates position of intracranial joint. (A and li after Janik, 1954, K. Svenska Vetenskapsacad. Handl.; C. based on Jarvik,
1954.)
270
BOBB SCHAEFFER
Jarvik believes that the porolepiforms
and osteolepiforms can be separated on the
basis of numerous anatomical differences,
especially in the snout region. He has emphasized differences in snout width, in the
anterior palatal recesses, in the nature of
the internasal wall, in the number of external narial openings, in the details of the
nasal chamber, and in the cranial nerves
and blood vessels related to this region.
Mainly from the work of Kulczynski (1960),
Vorobjeva (1962), and Thomson (1962, 1964
a,b,c) it is evident that a number of these
presumed differences occur in both groups,
and that they cannot have the systematic
or phylogenetic significance Jarvik has
given them.
Although the differences cannot be discussed in detail here, a few points are worth
consideration. Current investigation supports the recognition of four rhipidistian
families. The Porolepidae undoubtedly
gave rise to the Holoptychidae, and the
Osteolepidae to the Rhizodontidae. These
families are, however, not as diverse as
Jarvik's limited systematic survey indicates.
The porolepids and holoptychiids are
broad-snouted, but the osteolepids and
rhizodontids also include broad as well as
narrow-snouted forms. The anterior extension of the cranial cavity (pars ethmoidalis
cranialis) beyond the branching of the olfactory canals is not confined to the porolepiforms (Fig. 3). This extension may be
related in part to the width of the snout
or to the relative width of the internasal
wall; Thomson (1964) believes it is associated with the relative size of the nasal
capsules. The anterior palatal fenestrae,
thought by Jarvik to contain an intermaxillary gland in the porolepiforms as in
some of the urodeles, are generally present
to house the front teeth of the lower jaw—
which in the porolepiforms consist of
paired tooth whorls. These and other characters indicate that the rhipidistians are a
closely related group with no trends leading
specifically to the urodeles or to the anurans—from which they were separated by
well over 200 million years.
One of the particularly interesting aspects
ot the rhipidistian stoi) is the increasing
FIG. 3. Diagrammatic horizontal sections of the
rhipidistian snout region. Note laterally situated
nasal capsules and median cranial cavity. Median
internasal wall is present in D and E. A. Porolepid
(Porolepis). B. Osteolepid (Panderichthys). C.
Rhizodontid (Platycephalichthys). D. Rhizodontid
(Euslhenopleron). E. Rhizodontid (Eusthenodon).
(After Vorobjeva, 1962, Trans. Paleont. Inst., Acad.
Xauk, U.S.S.R.)
evidence for the development of essentially
parallel modifications in distantly related
genera. This is apparent in the snout region, and it may be true for other parts of
the skeleton as well. For instance, the trigeminal nerve exit can be either within the
posterior braincase moiety or through a
gap at the intracranial joint. The structure
of the vertebral column is poorly known,
but within the Rhizodontidae the centra
may consist of wedge-shaped, half-ring intercentra and small pleurocentra (Eusthenopteron), or of intercentra forming complete rings (Strepsodus). The osteolepid
Ectosteorhachis also has ring-like intercentra, but pleurocentra are apparently
absent. The holoptychiids and the rhizodontids independently evolved c\cloidal
scales with loss of the enamel and dentine
THE RHIPIDISTIAN-AMPHIBIAN TRANSITION
FIG. 4. Dorsal and lateral views of rhipidistian and early amphibian skulls. Parietal bones
stippled. A. Rhipidislian (Eusthenopteron). B. Apsidospondyl (tchthyostega). C. Lepospondyl
(Microbrachis). (A after Jarvik, 1944, K. Svenska Vetenskapsacad. Handl.; B after Jarvik (1952);
C after Steen, 1938, Proc. Zool. Soc, London.)
layers. The picture that is emerging, not
an unexpected one, indicates a combination
of parallelism and of divergence in the
several rhipidistian lineages.
THE TRANSITION IN MORPHOLOGY AND
FUNCTION
We still do not know which group of
rhipidistians gave rise to the Amphibia,
and no intermediate stages have been discovered. Nevertheless, by comparing rhipidistians and early amphibians we can arrive
at some conclusions about the nature of the
transition. It is evident that this involved
a number of interrelated modifications in
the feeding, respiratory, sound detection,
and locomotor mechanisms. These observed alterations were surely associated
with many unknown changes in the soft
anatomy, physiology, and behavior.
In regard to the feeding mechanism, the
intracranial joint was lost, and with it the
subcephalic muscles. Otherwise, as Olson
(1961) has pointed out, this mechanism was
carried over into the amphibians almost
unchanged. Elongation of the ethmosphenoid region was initiated, and the relative
position of the jaw articulation was ele-
vated. The loss of internal gills, the transformation of the gill arches into a hyobranchial apparatus, and the development of a
tongue must have represented a series of
interdependent changes. The closer association of the palate with the braincase
released the hyomandibular from its suspensory function to assume a new role as
the stapes. Also involved here was the
reduction and disappearance of the opercular-branchiostegal series. The otic notch
was emphasized in the apsidospondyls, but
it was lost in the lepospondyls. Changes in
the dermal roof pattern were related to
these modifications. Incidentally, most paleoichthyologists, including Jarvik (1960,
Figs. 17 and 19), agree that the dermal elements surrounding the pineal opening in
the osteolepiform rhipidistians and the
early amphibians are homologous. Since
these elements are clearly the parietals in
tetrapods (Fig. 4), they should be similarly
labeled in the rhipidistians (Romer, 1962).
Alterations in the occipital part of the
skull were probably related to modifications
in the appendicular skeleton. The latter
included freeing of the skull from the pectoral girdle, dorsal expansion of the pelvis
to meet the vertebral column via the sacral
272
BOBB ScHAEFFER
rib and, of course, the change from fins to
limbs. A recent study by Westoll (1963)
indicates that most major features of the
tetrapod humerus can be identified in its
rhipidistian homologue. Otherwise we have
no new information on the fin to limb transition—with the possible exception of the
curious forelimb of Hesperoherpeton
(Eaton and Stewart, 1960). The loss of the
lepidotrichia and the reorganization of the
mesenchyme in the more distal part of the
limb bud to bring about differentiation of
the complex carpus or tarsus and five digits
represent a dramatic example of change in
a canalized morphogenetic system.
The transition from rhipidistian to amphibian thus involved the reduction and
disappearance of some structures, and more
importantly, various degrees of transformation for others. The mosaic nature of this
change is clearly suggested by the ichthyostegids, as pointed out by de Beer (1954).
Internal gills were lost but lungs were retained. The neuromast organs and the
inner ear were carried over relatively unchanged, but the new tympanum-stapes
combination amplified vibrations for hearing in the air. With the disappearance of
the opercular elements and modification of
the gill arches, the head and the throat
became more mobile, and the newly evolved
tongue aided in the acquisition and manipulation of terrestrial food. Scales were
retained on at least part of the body. Respiration in the rhipidistians would be difficult when out of water without an adequate
mechanism for aerating the lungs. Szarski
(1962) has therefore postulated rapid reduction of the scale cover and development of
cutaneous respiration. But the earliest amphibians had elongated robust ribs which
would seemingly facilitate costal breathing.
The blood presumably became more insensitive to carbon dioxide, and this increased the absorption of oxygen by the
lungs.
Locomotion in water and on land was
accomplished mostly by lateral undulations
of the body. On land the newly evolved
limbs moved in a particular sequence which
held the body off the ground. This terrrstiial gait picsuniabh required a different
pattern of neuromuscular coordination
than the fin movements, and the architecture of the cerebellum was correspondingly
modified.
CIRCUMSTANCES OF THE TRANSITION
All the available evidence indicates that
the sensory canals of the earliest amphibians were functional in sexually mature
adults regardless of whether these forms
were in some ways neotenic. This can mean
only that they were primarily aquatic. The
question then is, why did the early tetrapods become tetrapods at all, when they
apparently remained most of the time in
the same aquatic environment as their
rhipidistian ancestors? Numerous ingenious
hypotheses have been proposed to explain
this apparent enigma (see Romer, 1958;
Eaton and Stewart, 1960). Perhaps none
can be regarded as definitive, but all agree
that some group of rhipidistians, for one
reason or another, could and did move
about on the land, probably to find another
body of water. The motivation for these
terrestrial excursions must have existed for
many millions of years—otherwise it seems
impossible to explain the rise of terrestrial
adaptations in the emerging amphibians.
The selection pressures promoting the
terrestrial adaptations and concurrently
preserving the aquatic ones were surely
responsible for extensive experimentation
during the transitional interval. Since experimentation is usually associated with
parallelism in lineages of common ancestry,
it is reasonable to ask if there is any evidence favoring a multiple origin of the
amphibians. Szarski (1962) in his detailed
review on amphibian origin says no—the
amphibian character suite could only have
arisen once. There is admittedly no evidence for polyphyly at the rhipidistian end,
but there is at least a suspicion at the amphibian level.
The early amphibians were sharply divided into the Apsidospondyli and the
Lepospondyli at the time of their first
known appearance in the fossil record (Fig.
5). They obviously had different ways of
Hie. reflecting di\eii>ence during the De\o-
THE RHIPIDISTIAN-AMPHIBIAN TRANSITION
273
FIG. 5. Lateral views oE rhipidistian and early amphibian skeletons. A. Rhipidistian (Eusthenopteron). B. Apsidospondyl (temnospondyl—Ichthyostega). C. Lepospondyl (microsaur—Microbrachis). D. Lepospondyl (nectridian—Sauropleura). (A after Gregory, 1941, and Jarvik, 1954;
B after Jarvik, 1960; C after Steen, 1938; D after Steen, 1931.)
nian. Within the Apsidospondyli, the ichthyostegids were separated from the main
line of temnospondyl evolution by the late
Devonian, and the temnospondyls differed •
in numerous ways from the anthracosaurs
(Romer, 1947). The various groups of lepospondyls, some of which are known to have
extended back into the Mississippian, are
clearly diverse in many parts of the skeleton
(Gregory et al., 1956; Baird, 1964). The
apsidospondyls and the lepospondyls are
most obviously separated by the structure
of their vertebrae (Fig. 6). Even the temnospondyls and the anthracosaurs are divergent enough in this and other regards to
project their lines back into the Devonian.
Important questions in regard to polyphyly are how we should define the Amphibia, and how we might separate an
"almost-amphibian" from a form we would
definitely regard as belonging to this class.
The limited information we have on transitions from one higher level of organization
to another indicates that the boundaries
between levels are never sharp, and that
lineages approaching or entering a new
level may do so at different rates, at different times, and in somewhat different ways.
All the organisms involved in a transition
are constantly adapted to the environments
in which they live. In retrospect, however,
we can see certain successful adaptations
evolving more or less in parallel among
related lineages. These are the broad or
FIG. 6. Lateral views of rhipidistian and early amphibian vertebrae. A. Rhipidistian (Etisthenopteron). B. Apsidospondyl (temnospondyl—Ichthyostega). C. Apsidospondyl (temnospondyl—Eryops). D.
Apsidospondyl (anthracosaur—Archeria). E. Lepospondyl (microsaur—Cardiocephalus). (After Williams, 1959, Quart. Rev. Biol.)
274
Bonn SCHAEFFER
general adaptations (Simpson, 1959), common to several lineages, that are associated
with the attainment of a new level. If we
had as many facts about the rhipidistianamphibian transition as we have for the
therapsid-mammal one, the boundary between "almost-amphibian" and "true" amphibian would surely be a fuzzy one.
The concept of mosaic evolution as developed by de Beer (1954) emphasizes that
all organisms are in a real sense integrated
combinations of primitive and advanced
characters. During a transition, however,
the advanced characters which may represent broad adaptations assume a greater
significance than the retained ancestral
characters. The process is entirely opportunistic, representing a complicated interplay among such factors as the ancestral
genotype, canalization, directional selection,
and the demands of functional specificity.
If we define success in terms of the amphibian level, the change from fin to limb
must have involved many more unsuccessful than successful experiments. It is conceivable, however, that definitive tetrapod
appendages arose only once, mainly because
a radical modification of a morphogenetic
pattern was involved, and because the limits of functional specificity were apparently
very narrow. The vertebrae, however, indicate a different picture. The ichthyostegids attained amphibian status with a rhipidistian type of centrum, but each of the
other two major apsidospondylous groups
evolved distinctive modifications of this
pattern, as did the lepospondyls. Although
lungs persisted relatively unchanged to the
amphibian level, they could function effectively on land only through the development of a new sort of breathing mechanism. There was also experimentation in
skull proportions, in the dermal bone pattern, and in the middle ear region.
The available evidence suggests that one
of the first steps in an amphibian direction
involved relatively rapid evolution of the
tetrapod locomotor mechanism. Experimentation with water conservation, which
was never solved by the amphibians, and
with respiratory mechanisms also must have
begun at an e;irh stugc in the transition.
APSIDOSPONDYLI
MPHIBIAN
LEVEL
TRANSITIONAL
PHASE
RHIPIDISTIAN
LEVEL
1'IG. 7. Diagram showing possible origin and relationships of early amphibian groups in relation to
levels of organization. Crosses represent hypothetical "unsuccessful" experiments in an amphibian
direction. (1) "Successful" fin to limb experiment;
(2) Apsidospondyl radiation with double centrum,
well-developed limbs, and "normal" body proportions; (3) Lepospondyl radiat-ion with single centrum, small to absent limbs, and trend towards
trunk elongation.
along with reduction of the internal gills
and the scale cover. It is conceivable that
costal breathing preceded the other modes
of terrestrial respiration, but the generally
small size of the lepospondyls also suggests
that cutaneous and/or buccopharyngeal
pump breathing may have evolved near the
beginning of the lepospondyl diversification. Experimentation with the structure
of the vertebrae and possibly with the feeding mechanism could have continued for a
long interval. Nevertheless, the demands
of functional specificity would, and did,
promote various degrees of parallelism in
the evolving lineages.
The change from rhipidistian to amphibian represents a continuum, and the
boundaries separating levels on the accompanying chart (Fig. 7) are therefore not
real. In terms of their origin, the problem
of defining the Amphibia could become as
arbitrary and semantic as defining a mammal—if we had as much evidence on the
transition. Certainly all the character complexes that we associate with the Amphibia
must have arisen through experimentation;
and this, in my opinion, greatly increases
the probability of a pohphyletic: origin—as
this term is now defined In Simpson (1961).
THE RHIIMDISTIAN-AMPHIBIAN TRANSITION'
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. 1961. Principles of animal taxonomy. Columbia Univ. Press, New York. 247 p.
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COMMENTS
Theodore H. Eaton, Dept. of Zoology,
University of Kansas, Lawrence
In our present state of gradually waning
ignorance of the earliest Amphibia, it is
still possible to conceive of various polyphyletic patterns of the origin of this class.
But to do so is to indulge in conjecture not
adequately sustained by evidence, as several
recent authors have shown (Szarski, 1962;
Parsons and Williams, 1963; and especially
Thomson, 1964 a, b). I think that this statement is also valid in reference to the origins
of other classes of tetrapods, for present
evidence is not sufficient to designate clearly, beyond reasonable doubt, two or more
ancestral sources of Reptilia, or even of
Mammalia, although this may eventually
276
BOBB SCHAEFFER
become possible. In Amphibia it seems to
me probable that the few (albeit distinct)
differences between Apsidospondyli and
Lepospondyli, or between labyrinthodonts
and "Lissamphibia," or even between Urodela and Anura, will appear to diminish in
importance as our knowledge increases, and
that a firm application of Occam's Razor is
the most useful of all approaches to problems of supposed polyphyly.
Commenting more directly upon Dr.
Schaeffer's important paper in this symposium, 1 am glad that he reminded us of
our perspective; we look back across a vast
stretch of time and evolution, and attempt
to picture the factors that led to the rise of
Amphibia from rhipidistian fishes. In doing so we may easily confuse two different
kinds of change: the change to amphibian
structure and the change to terrestrial life.
These are not necessarily the same, or concomitant; indeed it is probable that the
former far preceded the latter (Eaton,
1960).
In animals that were primitively aquatic
and lived their entire lives in water, as
early amphibians evidently did, there were
no terrestrial adaptations at all, even if
these animals were tetrapods, lacked opercular bones, had flexible necks, gulped air
at the surface, and heard air-borne sounds
when they put the tops of their heads out
of water. These features can, I think, be
seen as special adaptations to life in shallow
fresh water, where prolonged rapid swimming was neither necessary nor convenient,
where insects and other arthropods as well
as small fishes provided food, where aquatic
vegetation was plentiful, and where a variety of ecological niches lay open to exploitation. The Ichthyostegalia, early temnospondyls, embolomeres, and at least the
majority of Paleozoic Lepospondyli lacked
mechanical competence for locomotion on
land or support of the body out of water.
This is shown in such characters as the
persistent notochord not completely bolstered by vertebrae in some, and the inadequacy of feet or limbs in others. The
same is clearly true in the late Pennsylvanian Hesperoherpeton, which retained
somewhat fishlike paired limbs, a divided
braincase, and a notochordal canal in the
floor of the cranium.
It would be strange if this radiation during late Devonian and Carboniferous times
had not led to exploitation of the shoreline,
perhaps first as a refuge from aquatic predators, then also as a new food-source, by
more than one stock of primarily aquatic
Amphibia. There can be little doubt, for
example, that this happened more than
once among rhachitomous labyrinthodonts,
again in the ancestry of frogs, perhaps also
separately in that of salamanders, and definitely in the origin of reptiles from aquatic
anthracosaurs. Each of these groups in its
own way developed mechanical support for
the body at the stage when it left the water,
with or without metamorphosis. Thus terrestrial adaptations were attained through
evolutionary opportunism, not once but
several times.
REFERENCES
Eaton, T. H., Jr. 1960. The aquatic origin o£ tetrapods. Trans. Kansas Acad. Sci. 63:115-120.
Parsons, T. S., and E. E. Williams. 1963. The relationships of the modern Amphibia: a re-examination. Quart. Rev. Biol. 38:26-53.
Szarski, H. 1962. The origin of the Amphibia.
Quart. Rev. Biol. 37:189-241.
Thomson, K. S. 1964a. The ancestry of the tetrapods. Sci. Progress 52:451-459.
. 1964b. The comparative anatomy of the
snout in rhipidistian fishes. Bull. Mus. Comp.
Zool. 131:313-357.