The Skeletal Characters of Leiopelma hamiltoni McCulloch, With Particular Reference to the Effects of Heterochrony on the Genus

By Elsie M. Stephenson,
Department of Zoology, School of Biological Sciences,
The University of New South Wales, Sydney, New South Wales

As far as could be judged from all the evidence available, no justification for a change in the specific status of the Maud. Is frogs existed, but the discovery of a new and more favourable locality than Stephens Is for L. hamiltoni was, of course, of very great importance. A key to the species and a discussion of the name for the family are to be found in a recent publication (E. M. Stephenson, 1960)

Acknowledgments

I wish to acknowledge with thanks the help afforded by the following: The Hon. Minister for Internal Affairs, Wellington, for permission to collect specimens of Leiopelma on various occasions, and particularly for permission to study specimens of L. hamiltoni.

Mr. B. Bell, Senior Field Officer of the Wild Life Branch of the Department of Internal Affairs, Wellington, for collecting three specimens of L. hamiltoni from Maud Is. and for carrying out a tedious series of measurements in the field at my request.

Mr. W. H. Dawbin, Department of Zoology, University of Sydney, for providing two specimens of L. hamiltoni from Stephens Is.

My husband, Dr. N. G. Stephenson, Department of Zoology, University of Sydney, for help in collecting and studying frogs in the Coromandel and other regions, and for reading the present manuscript.

Dr. J. Ruthven Lang and Mr. S. Lang, of Auckland, Mr. R. Bartley and Mrs. H. Smith, of Port Charles, for assistance during field work on Mt. Moehau.

Material and Methods

Details of the specimens illustrated or examined are provided below for each part of the skeleton described.

Five specimens of L. hamiltoni were available, two from Stephens Is. and three from Maud Is. One of the former had been partly sectioned for previous examination of the head (E. M. Stephenson, 1955). The other specimen from Stephens Is. and two of the specimens from Maud Is. were used to prepare alizarin transparencies, following examination with special reference to the condition of the gonads. The head of the third and obviously youngest specimen from Maud Is. was sectioned, and the hyoid reconstructed, the rest of the body being used for an alizarin transparency.

Specimens of L. archeyi and L. hochstetteri, collected under previous permits and prepared as alizain or toluidine blue transparencies, included as far as possible a representative series, although of course the actual age of any specimen was not known.

Where necessary, reference was made to previously prepared slides of heads of all three species (E. M. Stephenson, 1951, 1955).

The Skull (text–Fig. 1)

Descriptions or comments on the whole or parts of the adult skull of Leiopelma include those of Wagner (1934a, 1934b), de Beer (1937), E. M. Stephenson (1951), van Eeden (1951), E. M. Stephenson (1955).

My first investigations (1951) dealt with the heads of L. archeyi and L. hochstetteri. When a specimen of L. hamiltoni became available, a brief description of the head was published (1955), including photographs and reconstructions of certain regions, but a reconstruction of the entire skull was not provided. In the light of evidence arising from the present studies, it is probable that explanations for the differences between the head of the specimen of L. hamiltoni in question and those of L. archeyi can be provided on neotenic grounds. In the present discussion, however, attention is focused exclusively on skeletal structures.

The condition of the sphenethmoid in L. hochstetteri has previously been illustrated(E. M. Stephenson, 1951, 277). It approximates to the condition in L. hamiltoni, but again probably ossifies at a relatively faster rate. It is recognisable in at least one specimen of L. hochstetteri of 27 mm body length.

The Hyoid Apparatus (Text–Fig. 2)

In 1951, I recorded that certain differences in the hyoid skeleton appeared to have a specific basis, as follows:

In 1954, a head of L. hamiltoni was sectioned. The hyoid, unfortunately, was extremely buckled and a good reconstruction was impracticable. It could be seen, however, that an anterior process was absent, an alary process of simple type was present, and that the ceratohyal was relatively wider than in L. hochstetteri. Unfortunately, through an oversight, the reverse condition with regard to alary and anterior processes was recorded-i.e. it was stated that an anterior process was present and an alary process was absent (E. M. Stephenson, 1955, 799).

Fortunately, further material of L. hamiltoni has since become available. Figure 2F illustrates the hyoid of an old male specimen of L. hamiltoni from Stephens. Is An anterior process is absent and a slightly expanded alary process is present. The condition of the latter is not nearly so definite as in L. hochstetteri (Fig. 2B). The hyoid cornu is of no greater relative width than in L. hochstetteri.

Figures 2C and 2D illustrate the hyoids of female specimens of L. hamiltoni from Maud Is. In C, a small but definite anterior process is present, but in D a process of this kind is completely lacking. In a third specimen (not illustrated), the ceratohyal is expanded in the region in question. All three specimens show a slight though variable degree of expansion to form what can be called an alary process, but again this does not approach the condition of the same process in L. hochstetteri.

The hyoids of two specimens of L. archeyi are also illustrated (Figs. 2A and 2E). Neither provides any indication of an anterior process, but A shows a slight expansion to form an alary process. In both, the hyoid cornu is relatively wider than in the specimens of L. hamiltoni examined, though in neither A nor E is the condition so apparent as in the specimen illustrated previously (E. M. Stephenson, 1951, Fig. 15B)

From the data provided above, an amended table can be drawn up as follows:

As far as the relative width of the ceratohyal is concerned, this is greater in L. archeyi than in L. hamiltoni If, in fact, L. archeyi is comparable to a juvenile stage of L. hamiltoni, this is to be expected. It is probable that the specimen of L. hamiltoni described previously (E. M. Stephenson, 1955) was younger than any of the members of the same species described in the present account.

From the data supplied above it is clear that the hyoids of L. archeyi and L. hamiltoni have a much closer resemblance to each other than either of them does to the hyoid of L. hochstetteri.

A parahyoid bone is normally present in all three species. Its absence has been recorded in one specimen of L. archeyi (Fig. 2E).

Ascaphus truei has an anterior process but lacks an alary process (de Villiers, 1934a, and personal observation)

Text–fig. 3.—Vertebral column. A—L. hamiltoni Specimen H2 Stephens Is Old male Ventral view. B—L. hochstetteri Specimen TH. Tokatea Mature male. Dorsal view of sacral region. C—L. hamiltoni. Specimen H2. Stephens Is. Old male. Dorsal view of sacral region. D—L. hochstetteri Specimen WHB. Warkworth. Very young female Ventral view. E—L. hochstetteri. Specimen WHB. Warkworth Very young female. Dorsal view of sacral region. F—L. hamiltoni Specimen H2. Stephens Is. Old male. Posterior view of sacral vertebra.

The Vertebral Column (Text–Fig. 3)

The vertebral column of L. hochstetteri has already been described in some detail (E. M. Stephenson, 1952). Further illustrations are included here for the sake of completeness as far as the genus is concerned. At the time of publication of the previous description, the vertebral column of L. hamiltoni had not been examined.

Ritland (1955) has provided a full description of the vertebral column of Ascaphus, with supplementary remarks concerning that of Leiopelma. He includes a discussion relating to the failure of the term “amphicoelous” alone to cover adequately the vertebral condition of Leiopelma and Ascaphus. However, as no completely satisfactory single term has yet been devised to replace “amphicoelous” it seems necessary to retain the latter in order to place the vertebrae of these two genera in a category separate from those of other anurans.

The vertebral column of L. hamiltoni (Fig. 3A) exhibits the essential characteristics already described for L. hochstetteri. The pattern of the vertical column of L. archeyi is also similar except that typically the whole series has a more delicate appearance, with a relatively greater amount of uncalcified cartilage in evidence.

In the vertebral column of the old male specimen of L. hamiltoni illustrated (Figs. 3A, 3C, and 3F), considerable calcification of cartilage and certain bony fusions have occurred. The ribs are partly fused with the transverse processes of the vertebrae concerned, while the characteristic forked and flattened expansions of the lateral extremities of the second pair of ribs, though visible dorsally, are largely fused together.

In all three species, nine presacral vertebrae are present except in occasional cases where fusion of two adjacent vertebrae is obvious. The urostyle has usually at least one pair of well-defined transverse processes developed on the first postsacral vertebra. Of these vertebrae, portions of three may sometimes be distinguished from the dorsal aspect of the urostyle.

The sacral vertebra, as already reported for L. hochstetteri (E. M. Stephenson, 1952), consists of two separate bony halves joined dorsally and ventrally by units of cartilage. This condition, which is not found in Ascaphus, is possibly unique among anurans. In the vertebral column of the specimen of L. hamiltoni illustrated in Figs. 3A, 3C and 3F, the ventral cartilage of the sacral region has remained uncalcified, but the dorsal portion, forming what is in effect the highest point of the neural arch, is seen to be heavily calcified, with possibly some development of bone. It still remains, however, as a discrete element and has been pushed slightly out of position by partial fusion of the two bony halves of the vertebra. Dorsal views of the same region in a mature specimen of L. archeyi and a young L. hochstetteri are provided (Figs. 3B and 3E).

Figure 3D illustrates the condition found in an immature female L. hochstetteri. This should be compared with the photograph of the vertebral column of an adult specimen of L. hochstetteri previously published (E. M. Stephenson, 1955, 610). It is of some interest to note that the uncinate processes typically seen on the most anterior pair of ribs are seen on the second pair in Fig. 3D.

As has already been reported (E. M. Stephenson, 1952, 606), fusion of two adjacent vertebrae is occasionally seen. It seems unlikely that this feature, as far as Leiopelma is concerned, is of any particular phylogenetic significance or that it provides evidence of a tendency towards specialisation. More probably, it indicates the fact that in the populations concerned, such occurrences are of little selective importance.

The Pectoral Girdle (Text–Fig. 4)

An outline diagram of the pectoral girdle of L. hamiltoni was provided with McCulloch's first description of the species (1919). De Vos (1938a) was the first.

to provide a description and a reconstruction of the pectoral girdle of any species of Leiopelma A further outline description (E. M. Stephenson, 1952) was discussed by Ritland (1955) and two corrections, involving the position of the cleithrum and the shape of the suprascapula, were made by him. Ritland also described in more detail than previous authors the pectoral girdle of Ascaphus.

Pectoral girdles of the three species of Leiopelma are illustrated in Fig. 4. Comparing these with the diagrams of the pectoral girdle of Ascaphus (Ritland, 1955, 146), it is clear that in Leiopelma the clavicle does not extend so far laterally, relative to the scapula, as it does in Ascaphus.

The extent to which a procoracoid element can be identified varies with the species and from specimen to specimen within a species. It appears to develop and to ossify at a relatively faster rate in L. hochstetteri than it does in the other two species, in fact it seems probable that in L. archeyi and L. hamiltoni it does not ossify at all.

Fig. 4C illustrates the pectoral girdle of a mature male specimen of L. hochstetteri, in which the procoracoid is seen as an obvious bony element A similar condition, seen in external view, is illustrated in Fig. 3 in a previous publication (E. M. Stephenson, 1952, 607). No specimen of L. archeyi so far examined by the author has shown any trace of a procoracoid ossification and alizarin transparencies show no differential staining of the area in question (Fig. 4D) In specimens of L. hamiltoni examined, alizarin transparencies of two mature females and one old male show pink staining in the region where a procoracoid would be expected, but it has not been found as a discrete bony element.

The Abdominal Ribs

These structures have previously been described and illustrated (E. M. Stephenson, 1952, 608). In general, these elements and the styles of the sternum remain cartilaginous and uncalcified in L. archeyi but tend to become heavily calcified and even ossified in mature specimens of L. hochstetteri. In mature specimens of L. hamiltoni, it is quite common to find completely cartilaginous abdominal ribs, but calcification and ossification have been noted in this species.

The Pelvic Girdle (Text–Fig. 5)

Description or discussions of the pelvic girdle of Leiopelma include those of de Villiers (1934b), E. M. Stephenson (1952), Ritland (1955), and van Dijk (1955).

In general, in L. archeyi, the pubis remains cartilaginous throughout life. It is typically unstained in alizarin transparencies (Fig. 5A).

In L. hochstetteri, at least in some of the more mature specimens examined, the pubis shows a lateral and somewhat superficial development of bone and/or calcified cartilage which tends to extend as a strip towards the ilium or ischium or both. The whole arrangement is somewhat irregular, but the general position is constant. The epipubis typically shows extensive calcification or ossification (Fig. 5E).

The old male specimen of L. hochstetteri illustrated in Fig. 5B shows essentially the condition described above for L. hochstetteri, but the public ossification is complicated by irregular development of calcified cartilage, producing fusion with the ischium. The epipubis also shows irregular calcification. Mature female specimens of L. hamiltoni (Fig. 5C and another specimen not illustrated) show deep pink staining in the lateral region of the pubis, but the latter is quite clearly cartilaginous. The epipubis is unstained in both cases.

Comparing the pelvic girdle of Leiopelma with that of Ascaphus, it is found that the condition of L. hochstetteri resembles that of Ascaphus more closely than does that of the other two species. However, examination of a male specimen of.

Text–fig. 5.—Pelvic Girdle, lateral view. (Epipubis incomplete in most cases.) A—L. archeyi. Specimen MA3. Mt. Moehau. Mature female. Pubis and epipubis cartilaginous and uncalcified. B—L. hamiltoni. Specimen H2. Stephens Is. Old male Ossification in lateral region of pubis partly obscured by irregular calcification of cartilage in adjacent regions, producing fusion with ischium. Irregular calcification also in epipubis C—L. hamiltoni. Specimen LM2. Maud Is. Mature female. Pubis cartilaginous but stained deep pink in region indicated by dotted lines Epipubis unstained. D—L. hochstetteri Specimen WHB. Warkworth. Very young female. Pubis and epipubis cartilaginous and unstained. E—L. hochstetteri. Specimen TH. Tokatea. Mature male. Lateral and fairly superficial ossification and calcification in lateral region of pubis, extending into ilium. Epipubis ossified.

Ascaphus prepared as an alizarin transparency shows that this specimen at least possesses a far more rigid and fused condition of the pelvic elements as a whole and a more definite lateral public ossification than any specimen of L. hochstetteri so far examined.

Limb Bones

The limb bones are not here described in detail but the same sort of comparative picture exists as for other parts of the skeleton L. archeyi has a relatively greater degree of uncalcified cartilage and less fusion of elements in general than either of the other two species.

The Carpus (Text–Fig. 6)

It will be seen from the representative series of distal carpal bones of all the species and both sexes as illustrated in Figure 6 that ossification and/or calcification is well advanced in mature specimens of L. hamiltoni and L. hochstetteri (Figs. 6A,

6E, 6F and 6G). In L. archeyi, however, even in specimens which are apparently sexually mature, only a relatively slight degree of ossification occurs (Figs. 6B, 6C, 6D and 6H).

Individual variation in fusion of the distal elements is very pronounced Typically, eight free carpal elements are observable, while the metacarpal of the prepollex is usually distinguishable In Fig. 6C, however, this metacarpal is fused with the adjacent carpal element In some cases, evidence has been noted of two apparent centres of ossification in the metacarpal of the prepollex, but in only one case, a specimen of L. hochstetteri, has a clear separation of a metacarpal element and a phalanx of the prepollex been noted Unfortunately, this condition was illustrated by the author in a previous paper (E. M. Stephenson, 1952, 610) and gives the impression of being the normal condition for the genus. It would appear from all specimens examined recently that a condition involving a separate phalanx of the prepollex is an uncommon individual variation of little significance. If this is the case, Bombina alone (Ridewood and Howes, 1888) among the Discoglossidae and Ascaphidae commonly possess a phalangeal element of the prepollex.

The simple, relatively small, and usually cartilaginous metacarpal of the prepollex of Leiopelma is far removed from the corresponding enlarged and specialised bony element of Ascaphus (Figures of Ritland, 1955, and personal observation).

The Tarsus (Text–Fig. 7)

The tarsal elements of six different specimens representing the three species of Leiopelma are illustrated.

Even in mature specimens of L. archeyi of both sexes (Figs. 7B, 7C and 7F) no alizarin staining of the distal tarsal elements occurs. Various ossification centres can be seen in the carpal elements figured for L. hamiltoni and L. hochstetteri, but it is difficult to generalize from these In several specimens of L. hamiltoni from Maud Is. including two fully mature females, very few traces of staining of the distal tarsal elements could be seen. It would seem that the process of ossification of these elements occurs relatively more quickly in L. hochstetteri than in the other two species. The elements do not appear to ossify in any observable fixed order.

As with the carpus, variations occur in the fusions of the distal elements. In L. archeyi, in addition to the tibiale and fibulare, the most common pattern of tarsal elements appears to include five free distal elements. A small metatarsal of the prehallux is usually present. The specimen illustrated in Fig. 7B has an additional element on both sides. In Fig. 7F, the metatarsal of the prehallux is fused with the adjoining tarsal element although an indication of demarcation between them can be seen.

In one specimen of L. hamiltoni (Figs. 7D and 7G), an additional small element is present distally to the metatarsal of the prehallux. This presumably represents a phalanx of the prehallux, an element not represented in any adult Discoglossid or in Ascaphus It has not been found in any other specimen of L. hamiltoni so far examined and probably it is merely an individual variation of little phylogenetic significance.

Discussion

The main fact emerging from a comparison of L. archeyi and L. hamiltoni is that a sexually mature specimen of L. archeyi is typically comparable with an immature form of L. hamiltoni as far as size and skeletal characters are concerned. Externally, the two species resemble each other closely, except for the maximum sizes attained (47 mm for L. hamiltoni and 41 mm, recorded once, for L. archeyi), and the fact that a green colour pattern, sometimes seen in L. archeyi, has not been.