In trying to determine the ancestral relationships of modern orders of
Amphibia it is not possible to select satisfactory structural ancestors
among a wealth of fossils, since very few of the known fossils could
even be considered possible, and scarcely any are satisfactory, for such
a selection. The nearest approach thus far to a solution of the problem
in this manner has been made with reference to the Anura. Watson's paper
(1940), with certain modifications made necessary by Gregory (1950),
provides the paleontological evidence so far available on the origin of
frogs. It shows that several features of the skeleton of frogs, such as
the enlargement of the interpterygoid spaces and orbits, reduction of
the more posterior dermal bones of the skull, and downward spread of the
neural arches lateral to the notochord, were already apparent in the
Pennsylvanian _Amphibamus_ (Fig. 1), with which Gregory synonymized
_Miobatrachus_ and _Mazonerpeton_. But between the Pennsylvanian and the
Triassic (the age of the earliest known frog, _Protobatrachus_) there
was a great lapse of time, and that which passed between any conceivable
Paleozoic ancestor of Urodela and the earliest satisfactory
representative of this order (in the Cretaceous) was much longer still.
The Apoda, so far as known, have no fossil record.
Nevertheless it should be possible, first, to survey those characters of
modern Amphibia that might afford some comparison with the early
fossils, and second, to discover among the known Paleozoic kinds those
which are sufficiently unspecialized to permit derivation of the modern
patterns. Further circumstantial evidence may be obtained by examining
some features of Recent Amphibia which could not readily be compared
with anything in the fossils; such are the embryonic development of the
soft structures, including cartilaginous stages of the skeleton, the
development and various specializations of the ear mechanism, adaptive
characters associated with aquatic and terrestrial life, and so on.
COMPARISON OF MODERN ORDERS WITH THE
LABYRINTHODONTS AND LEPOSPONDYLS
[Illustration: Fig. 1. _Saurerpeton_ (x 1/2, after Romer, 1930, fig. 6);
_Amphibamus_, the palatal view x 2-1/4, from Watson, 1940, fig. 4 (as
_Miobatrachus_), the dorsal view x 2-1/2, from Gregory's revised figure
of _Amphibamus_ (1950, Fig. 1); _Protobatrachus_, x 1, from Watson,
1940, fig. 18, 19.]
In both Anura and Urodela the skull is short, broad, relatively flat,
with reduced pterygoids that diverge laterally from the parasphenoids
leaving large interpterygoid vacuities, and with large orbits. (These
statements do not apply to certain larval or perennibranchiate forms.)
The skull in both orders has lost a number of primitive dermal bones in
the posterior part; these are: basioccipital, supraoccipital,
postparietal, intertemporal, supratemporal, and tabular. The
exoccipitals form the two condyles but there are no foramina for the
11th and 12th nerves, since these are not separate in modern Amphibia.
The opisthotic is missing in all except Proteidae (but see discussion
of the ear). Although the skull is normally autostylic, a movable
basipterygoid articulation is present among Hynobiid salamanders and in
at least the metamorphic stages of primitive frogs, and therefore should
be expected in their ancestors. The vertebrae are, of course, complete;
see discussion in later section. The quadratojugal, lost in salamanders,
is retained in frogs, and conversely the lacrimal, absent in frogs,
occurs in a few primitive salamanders. The situation in Apoda is
different, but postfrontal and jugal should be noted as bones retained
in this order while lost in the others.
Thus, in spite of minor differences, the above list shows that there are
numerous and detailed similarities between Anura and Urodela with
respect to the features in which they differ from the Paleozoic orders.
Pusey (1943) listed 26 characters which _Ascaphus_ shares with
salamanders but not with more advanced frogs; a few of these might be
coincidental, but most of them are of some complexity and must be taken
to indicate relationship. The main adaptive specializations of Anura,
however, including loss of the adult tail, extreme reduction in number
of vertebrae, formation of urostyle, elongation of the ilium and
lengthening of the hind legs, must have appeared at a later time than
the separation of that order from any possible common stem with Urodela,
although they are only partially developed in the Triassic
_Protobatrachus_.
Turning to the Paleozoic Amphibia, there are two groups in which some
likelihood of a relationship with modern order exists. In the
Pennsylvanian Trimerorhachoidea (Labyrinthodontia, order Temnospondyli)
some members, such as _Eugyrinus_, _Saurerpeton_, and notably
_Amphibamus_ (Fig. 1) had short, broad heads, an expansion of palatal
and orbital openings, posterior widening of the parasphenoid associated
with divergence of the pterygoids, a movable basipterygoid articulation,
and reduction in size (but not loss) of the more posterior dermal bones
of the skull. In recognition of Watson's (1940) evidence that these
animals make quite suitable structural ancestors of frogs, Romer (1945)
placed _Amphibamus_ in an order, Eoanura, but Gregory (1950) indicated
that it might better be left with the temnospondyls. Association of the
urodele stem with this group does not seem to have been proposed
hitherto.
The other group of Paleozoic Amphibia that has been considered probably
ancestral to any modern type is the subclass Lepospondyli, containing
three orders, Aistopoda, Nectridia and Microsauria. In these the
vertebrae are complete (holospondylous), the centra presumably formed by
cylindrical ossification around the notochord, and there is no evidence
as to the contributions from embryonic cartilage units. It is important
to note at this point that precisely the same statement can be made
regarding the vertebrae of _adults_ of all three Recent orders, yet for
all of them, as shown in a later section, we have ample evidence of the
part played by cartilage elements in vertebral development. Therefore
(a) we cannot say that there were no such elements in embryonic stages
of lepospondyls, and (b) it would take more than the evidence from adult
vertebrae to relate a particular modern order (for example, Urodela) to
the Lepospondyli. Vague similarities to Urodela have been noted by many
authors in the Nectridia, Aistopoda and Microsauria, but these are not
detailed and refer mainly to the vertebrae. The skulls do not show,
either dorsally or in the palate, any striking resemblance to those of
generalized salamanders, and certainly most known lepospondyls are too
specialized to serve as the source of Urodela. It is true that the
elongate bodies, small limbs, and apparent aquatic habitus of some
lepospondyls accord well with our usual picture of a salamander, but
such a form and way of life have appeared in many early Amphibia,
including the labyrinthodonts. The family Lysorophidae (Fig. 2), usually
placed among microsaurs, is sufficiently close in skull structure to the
Apoda to be a possible ancestor of these, but it probably has nothing to
do with Urodela, by reason of the numerous morphological specializations
that were associated with its snakelike habitus.
[Illustration: Fig. 2. _Lysorophus tricarinatus_, lateral and posterior
views x 2-1/2, modified after Sollas, 1920, Figs. 8 and 12,
respectively; palatal view after Broom, 1918, x 1-1/2. For explanation
of abbreviations see Fig. 3.]
McDowell's (1958) suggestion that it would be profitable to look among
the Seymouriamorpha for the ancestors of frogs seems to be based upon a
few details of apparent resemblance rather than a comprehensive view of
the major characters of the animals. In most points which he mentions
(limb girdles, form of ear, pterygoid articulation) the present writer
does not see a closer similarity of frogs to Seymouriamorpha than to
Temnospondyli.
Still other opinions have been expressed. Herre (1935), for instance,
concludes "on anatomical, biological and paleontological grounds" that
the orders of Urodela, Anura, Apoda and Stegocephali were all
independently evolved from fish, but beyond citing the opinions of a
number of other authors he does not present tangible evidence for this
extreme polyphyletic interpretation.
More notable are the views of several Scandinavian workers
(Saeve-Soederbergh, 1934; Jarvik, 1942; Holmgren, 1933, 1939, 1949a, b),
of whom Jarvik, in a thorough analysis of the ethmoid region, would
derive the Urodela from Porolepid Crossopterygii, and all other
tetrapods from the Rhipidistia; Saeve-Soederbergh and Holmgren, the latter
using the structure of carpus and tarsus, see a relationship of Urodela
to Dipnoi, but accept the derivation of labyrinthodonts and other
tetrapods from Rhipidistia. All of this work is most detailed and
laborious, and has produced a great quantity of data useful to
morphologists, but the diphyletic theory is not widely adopted; the
evidence adduced for it seems to consist largely of minutiae which,
taken by themselves, are inconclusive, or lend themselves to other
interpretation. For instance Holmgren's numerous figures of embryonic
limbs of salamanders show patterns of cartilage elements that he would
trace to the Dipnoan type of fin, yet it is difficult to see that the
weight of evidence requires this, when the pattern does not differ in
any fundamental manner from those seen in other embryonic tetrapods, and
the differences that do appear may well be taken to have ontogenetic
rather than phylogenetic meaning. Further, the Dipnoan specialization of
dental plates and autostylic jaw suspension, already accomplished early
in the Devonian, would seem to exclude Dipnoi from possible ancestry of
the Urodela, an order unknown prior to the Mesozoic, in which the teeth
are essentially similar to those of late Paleozoic Amphibia, and the jaw
suspension is not yet in all members autostylic.