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.
SUMMARY
The Anura probably originated among temnospondylous labyrinthodonts, through a line represented approximately by Eugyrinus, Amphibamus, and the Triassic frog Protobatrachus, as shown by Watson, Piveteau and others. The known Paleozoic lepospondyls do not show clear indications of a relationship with Urodela, but Lysorophus may well be on the ancestral stem of the Apoda.
Between Urodela and Anura there are numerous resemblances which seem to indicate direct relationship through a common stock: (1) a similar reduction of dermal bones of the skull and expansion of palatal vacuities; (2) movable basipterygoid articulation in primitive members of both orders; (3) an operculum formed in the otic capsule, with opercularis muscle; (4) many details of cranial development, cranial muscles, and thigh muscles, especially between Ascaphus and the Urodela, as shown by Pusey and Noble; (5) essentially similar manner of vertebral development, quite consistent with derivation of both orders from Temnospondyli; (6) presence in the larva of Leiopelma of a salamander-like gular fold, four limbs, and no suggestion of modification from a tadpole (Stephenson).
LITERATURE CITED
Broom, R.