b'd.c.--basidorsal cartilage (neural arch)
b'oc.--basioccipital
ce._{1-4}--centrale_{1-4}
ch.--ceratohyal
clav.--clavicle
clei.--cleithrum
cor.--coracoid
d.c._{1-4}--distal carpal_{1-4}
diap.--diapophysis
exoc.--exoccipital
ep.--episternum
hyost.--hyostapes
i.--intermedium
Mk.--Meckel's cartilage
n.--notochord
om.--omosternum
op.--operculum
opis.--opisthotic
par.--parietal
par. proc.--paroccipital process
peri. cent.--perichordal centrum
p'p.--postparietal
prep.--prepollex
pro.--prootic
p'sp.--parasphenoid
pt.--pterygoid
p.t.f.--post-temporal fossa
postzyg.--postzygapophysis
qj.--quadratojugal
qu.--quadrate
ra.--radiale
r.hy.--hyomandibular ramus of VII
rib-b.--rib-bearer
r.md.--mandibular ramus of VII
sc.--scapula
sc'cor.--scapulocoracoid
s'd.--supradorsal cartilage
s'd.(postzyg.)--supradorsal (postzygapophysis)
soc.--supraoccipital
sp.c.--spinal cord
sq.--squamosal
s'sc.--suprascapula
s't.--supratemporal
sta.--stapes
ster.--sternum
tab.--tabular
uln.--ulnare
v.a.--vertebral artery
xiph.--xiphisternum
I,IV--digits I and IV
V, VII, X, XII--foramina for cranial nerves of these numbers (in
Fig. 4, VII is the facial nerve)
]
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.
[Illustration: Fig. 4. Diagram of middle ear structures in _Rana_ (upper
figure, after Stadtmueller, 1936, and lower left after DeBeer, 1937), and
_Ambystoma_ (lower right, after DeBeer, 1937); all x 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.