THE EAR

Fig. 3. Occipital region of skulls of Megalocephalus brevicornis (× 3/10, after Watson, 1926, as Orthosaurus), Dvinosaurus (× 1/4, modified after Bystrow, 1938; the lower figure after Sushkin, 1936), and Necturus maculosus (× 3, original, from K. U., No. 3471).

Abbreviations Used in Figures

In temnospondylous Amphibia the tympanum generally occupied an otic notch, at a high level on the skull, bordered dorsomedially by the tabular and ventrolaterally by the squamosal. In this position the tympanum could receive airborne sounds whether the animal were entirely on land or lying nearly submerged with only the upper part of its head exposed. Among those Anura in which the ear is not reduced the same is true, except that the tabular is lost. In Temnospondyli (Fig. 3) the posterior wall of the otic capsule was usually formed by the opisthotic, which extended up and outward as a buttress from the exoccipital to the tabular, and sometimes showed a paroccipital process for the insertion, presumably, of a slip or tendon of the anterior axial musculature. The stapes, in addition to its foot in the fenestra ovalis and its tympanic or extrastapedial process to the tympanum, bore a dorsal process (or ligament) to the tabular, an "internal" process (or ligament) to the quadrate or an adjacent part of the squamosal, and a ligament to the ceratohyal. Some of these attachments might be reduced or absent in special cases, but they seem to have been the ones originally present both phylogenetically and embryonically in Amphibia.

Among typical frogs (Fig. 4) the base, or otostapes, is present and bony, the extrastapedial process (extracolumella, or hyostapes) is usually cartilaginous, the dorsal process (processus paroticus) is of cartilage or ligament, but the other two attachments are absent in the adult. The exoccipital extends laterally, occupying the posterior face of the otic capsule. Between it and the otostapes is a small disc, usually ossified, the operculum, which normally fits loosely in a portion of the fenestral membrane, and is developed from the otic capsule. The opercularis muscle extends from this disc to the suprascapula, in many but by no means all families of Anura.

Fig. 4. Diagram of middle ear structures in Rana (upper figure, after Stadtmüller, 1936, and lower left after DeBeer, 1937), and Ambystoma (lower right, after DeBeer, 1937); all × 4. For explanation of abbreviations see Fig. 3.

Among Urodela (Fig. 4) the middle ear cavity and tympanum are lacking, and the stapes (columella) consists of no more than its footplate and the stylus, which is attached to the border of the squamosal, thus corresponding to the "internal" process. In families in which individuals metamorphose and become terrestrial (Hynobiidae, Ambystomidae, Salamandridae, Plethodontidae), an operculum and opercularis muscle appear in the adult, just as in frogs, except that in Plethodontidae, the most progressive family, the operculum fuses with the footplate of the stapes. Among neotenous or perennibranchiate urodeles there is no separate operculum or opercularis. The evidence given by Reed (1915) for fusion of the operculum with the columella in Necturus appears inconclusive, in spite of the great care with which his observations were made. On the other hand, Necturus and Proteus alone among living salamanders have a distinct opisthotic on the posterior wall of the otic capsule (Fig. 3), as do the Cretaceous Hylaeobatrachus and the Eocene Palaeoproteus. Probably these Proteidae should be regarded as primitive in this respect, although many other features may be attributed to neoteny.

There is a contrast between Anura and most Urodela in the relative positions of the stapes and facial nerve, as shown in DeBeer's (1937) diagrams. In the latter (Ambystoma) the nerve is beneath, and in the former (Rana) above, the stapes. Judging by figures of Neoceratodus, Hypogeophis, and several types of reptiles and mammals, the Urodela are exceptional. Necturus, however, has the nerve passing above its stapes, and this may be primitive in the same sense as the persistent opisthotic. There can be, of course, no question of the nerve having worked its way through or over the obstructing stapes in order to come below it in salamanders; rather, the peripheral growth of neuron fibers in the embryo must simply pursue a slightly different course among the partially differentiated mesenchyme in the two contrasting patterns.

Although DeBeer (1937) shows in his figure of Hypogeophis (one of the Apoda) an operculum, this is apparently a mistake. The stapes has a large footplate, and its stylus articulates with the quadrate, but no true operculum or opercularis has been described in the Apoda. The facial nerve passes above the stapes. It does not seem necessary to regard the conditions in this order as related directly to those of either salamanders or frogs, but a reduction of the stapes comparable to that in salamanders has occurred.

The presence in both frogs and terrestrial salamanders of a special mechanism involving the opercularis muscle and an operculum cut out in identical fashion from the wall of the otic capsule behind the stapes seems to require some other explanation than that of a chance convergence or parallelism. Although the stapes and otic region are readily visible in a number of labyrinthodonts and lepospondyls, no indication of an operculum seems to be reported among them. But in the Triassic Protobatrachus (Fig. 1), which is unmistakably a frog in its skull, pelvis and some other features, Piveteau (1937) has shown, immediately behind the foot of the stapes, a small bony tubercle, which he and Watson (1940) designated opisthotic. Very clearly it served for insertion of a muscle, and it is equally clear that the bone is a reduced opisthotic, carrying the paroccipital process already mentioned as characteristic of it in some temnospondyls. Since the remainder of the posterior wall of the otic capsule consists of cartilage, meeting the exoccipital, it may be that the opisthotic becomes the operculum in frogs. Protobatrachus was too far specialized in the Anuran direction, although it still had a tail, and the forelegs and hind legs were nearly the same size, to be considered a possible ancestor of the Urodeles. But at one stage in the general reduction of the skull in the ancestry of both groups, a condition similar to that in Protobatrachus may have characterized the otic region, long before the Triassic.

In the argument thus far we have considered terrestrial, adult amphibians, since it is only in these that either the normal middle ear and tympanum, or the opercular apparatus, is present. But among the urodeles several neotenic types occur (this term applies also to the perennibranchs). For most of these there is nothing about the otic region that would be inconsistent with derivation, by neoteny, from known families in which adults are terrestrial; for example, Cryptobranchus could have had a Hynobiid-like ancestor. But this, as mentioned above, does not hold for the Proteidae, which possess an opisthotic of relatively large size, distinctly separate from the exoccipital and prootic. Either this bone is a neomorph, which seems improbable, or there has not been in the ancestry of this particular family an episode of reduction comparable to that seen in the terrestrial families, where there is an operculum instead of a normal opisthotic. Therefore the Proteidae probably are not derived from the general stem of other salamanders, but diverged sufficiently long ago that the bones of the otic region were reduced on a different pattern. They need not be removed from the order, but, in this respect, recognized as more primitive than any other existing Urodela or Anura. A recent paper by Hecht (1957) discusses many features of Necturus and Proteus, and shows that they are remote from each other; his evidence does not seem to prove, however, that they were of independent origin or that they need be placed in separate families.