Considering the postembryonic developmental stages of modern Amphibia,
there can be no doubt that a gill-bearing, four-legged larva of a
salamander, in which lateral line pores and a gular fold are present,
represents much more closely the type of larva found in labyrinthodonts
than does the limbless, plant-nibbling tadpole of the Anura.
Salamander-like larvae of labyrinthodonts are well known, especially
those formerly supposed to comprise the order Branchiosauria. Many,
perhaps the majority of, labyrinthodonts show some features associated
with aquatic life even when full-grown, as do the lepospondyls. These
features may include impressions of sensory canals on the dermal bones
of the skull, persistence of visceral arches, reduction in size of
appendages, and failure of tarsal and carpal elements to ossify. In
fact, it appears that very few of the Paleozoic Amphibia were successful
in establishing themselves as terrestrial animals even as adults.
Nevertheless, in the ancestry of Anura, and that of at least the
Hynobiid, Ambystomid, Salamandrid and Plethodontid salamanders, there
must certainly have been a terrestrial adult, transforming from an
aquatic larva. The leaping mechanism of Anura, shown in so many features
of their anatomy, is perhaps to be explained as a device for sudden
escape from land into the water, but it was not yet perfected in the
Triassic _Protobatrachus_ or the Jurassic _Notobatrachus_.
The middle ear, its sound-transmitting mechanism, and the tympanum, well
developed in most Anura, are readily derived from those of early
labyrinthodonts, and are presumably effective for hearing airborne
sounds whether on land or while floating in the water. Reduction of
these organs in Urodela may be correlated with their customary
restriction to subsurface habitats and inability to maintain a floating
position while in water.
Some light may be shed on the significance of the tadpole of Anura by
considering the early stages of the ribbed frogs, Liopelmidae.
_Leiopelma_ and _Ascaphus_ are so closely similar in the adult that
there is no doubt that they belong in one family, primitive in some
respects (including articulated ribs; pyriformis and
caudalipuboischiotibialis muscles) but not in others (absence of
tympanum and middle ear). In both genera the eggs are large, 5 mm. in
_Leiopelma_, 4.5 mm. in _Ascaphus_, and unpigmented; but at this point
the resemblance ends.
[Illustration: Fig. 10. _Leiopelma hochstetteri_ larva, lateral and
ventral (after Stephenson, 1955), x4.]
Stephenson (1955) showed that embryos of _L. hochstetteri_ develop
equally well on land (in damp places) or in the water, and that embryos
prematurely released from egg capsules develop successfully in the
water. The larvae possess both pairs of legs (Fig. 10) and a broad gular
fold similar to that of larval salamanders. In _L. hochstetteri_ the
fold grows back over the forelegs temporarily, but remains free from the
body and presently the legs reappear, whereas in _L. archeyi_ the
forelegs are not covered at any time. No branchial chamber or spiracle
is formed. Of course direct development, without a tadpole, occurs in
several other groups of Anura, but in each case terrestrial adaptations
are obvious. This is not true of _Leiopelma_, which Stephenson regards
as more nearly comparable with Urodela in its development than with
other Anura, and he sees in it a "primary and amphibious" mode instead
of a terrestrial specialization.
The _Ascaphus_ tadpole bears no outward resemblance to the larva of
_Leiopelma_, but is a normal tadpole in form, although sluggish in
activity. Its greatly expanded labial folds bear numerous rows of horny
epidermal "teeth," which, with the lips, serve to anchor the tadpole to
stones in the swift water of mountain brooks. Noble (1927) noticed that
particles of food were taken in through the external nares, and that a
current of water passed through these openings and out by way of the
median spiracle. It appears that any action by the teeth and jaws in
scraping algae from the rocks (which were bare in the stream where I
have collected _Ascaphus_) would be quite incidental, and that the lips
and teeth must be primarily a clinging mechanism. Certain other mountain
brook tadpoles (for example, _Borborocoetes_) show similar devices, but
these are developed independently, as specializations from the usual
sort of tadpole.
May it not be that closure of the gill-chamber by the opercular (=gular)
fold, retardation of limb development, expansion of the lips, growth of
parallel rows of horny teeth, and other correlated features that make a
tadpole, were brought about as an adaptation of the primitive Anuran
larva to a swift-stream habitat, and that this "basic patent" then later
served to admit the tadpoles of descendant types to an alga-scraping
habit in quiet water as well? The tadpole, as a unique larval type among
vertebrates, bears the hallmarks of an abrupt adaptive shift, such as
might have occurred within the limits of a single family, and it seems
difficult to imagine the enclosed branchial chamber, the tooth-rows, and
lips of a familiar tadpole as having evolved without some kind of
suctorial function along the way.