58 
Gary L. Paukstis and Lauren E. Brown 
reduced or wafer-shaped cartilage was found in the digits of the fore- 
limbs of P. ornata and P. streckeri (Fig. 1C). The intercalary cartilages 
of digits of the hind limbs are variable. They may be nearly wafer- 
shaped or cuboidal or of various intermediate shapes. Pseudacris ornata 
and P. streckeri show no vertical offsetting of the digital elements in the 
forelimb by the intercalary cartilage. This condition is not as well de- 
veloped in digits of the hind limbs in these species. 
DISCUSSION 
The intercalary cartilage of frogs that climb probably facilitates 
flexion of the end of the digit so that the toe pad may be more effi- 
ciently used for climbing and more readily applied to a vertical surface 
(Paukstis 1978). This type of flexion would seem disadvantageous and 
inefficient for forelimb burrowing because the terminal phalanx would 
move upwards when the digit is pushed against the substrate. In P. 
ornata and P. streckeri the problem of upward flexion has been resolved 
by the reduction in size of the intercalary cartilage and elimination of 
vertical offsetting of the terminal phalanx. These morphological charac- 
teristics have evolved to provide a relatively efficient mechanism of fore- 
limb burrowing in these species. It is probably appropriate to consider 
the thin, wafer-shaped intercalary cartilage of P. ornata and P. streckeri 
to be a degenerative condition, and further evolution in these species 
might result in loss of the cartilage. 
Paukstis (1978) noted that the nearly cuboidal intercalary cartilage 
characteristic of the species of Pseudacris that are not known to burrow 
is also found in some other semiterrestrial species in the family Hylidae, 
such as Acris crepitans, A. gryllus, Hyla crucifer, and Limnaeodus ocu- 
laris. It is possible that the semiterrestrial groups of frogs in all four of 
the aforementioned genera are independent lines that have separately 
evolved cuboidal intercalary cartilages. Thus, this may be an example of 
convergent evolution. On the other hand, the similar cuboidal interca- 
lary cartilages in these different genera may have descended relatively 
unchanged from an ancient common ancestor that was semiterrestrial in 
habits. 
The nearly cuboidal intercalary cartilage of the terrestrial species of 
Pseudacris that are not known to burrow may represent an intermediate 
evolutionary stage between those cartilages in the arboreal species of 
Hyla and those in the forelimbs of the fossorial P. ornata and P. streck- 
eri. The occasional occurrence of cuboidal intercalary cartilages in the 
hind limbs of the last two species supports this suggestion. However, the 
cuboidal intercalary cartilage is longer (relative to width) than either of 
the other two types of intercalary cartilages. Thus there would have had 
to have been selection for increase in length and then selection for 
marked decrease in length if the cuboidal intercalary cartilage is the 
intermediate stage. Another interpretation is also possible. The cuboidal 
