Evolution of the Intercalary Cartilage 
in Chorus Frogs, Genus Pseudacris 
(Salientia: Hylidae) 
Gary L. Paukstis 
Department of Zoology, 
Colorado State University, Fort Collins, Colorado 80523 
AND 
Lauren E. Brown 
Department of Biological Sciences, 
Illinois State University, Normal, Illinois 61761 
ABSTRACT. — A comparison was made of the structure of interca- 
lary cartilages of the digits of three groups of hylid frogs: (1) arboreal 
Hyla; (2) terrestrial Pseudacris not known to burrow; and (3) forelimb 
burrowing Pseudacris. In the arboreal Hyla the biconcave intercalary 
cartilage has an apex shorter than the base and a height greater than 
the length. A nearly cuboidal intercalary cartilage characterizes terres- 
trial Pseudacris not known to burrow, whereas a reduced, wafer- 
shaped intercalary is found in the forelimbs of forward-burrowing 
Pseudacris. In the arboreal Hyla and Pseudacris not known to bur- 
row, the cartilage offsets the positions of the terminal and subterminal 
phalanges so that the terminal phalanx is displaced ventrally relative to 
the other phalanges. Forward-burrowing Pseudacris do not exhibit 
this offsetting of digital elements in the forelimb. In the arboreal Hyla 
the shape and spatial relationship of the intercalary cartilage to the 
other phalanges probably facilitate flexion of the end of the digit and 
enhance arborealism. In contrast, the wafer-shaped intercalary carti- 
lage in forward-burrowing Pseudacris could be considered a degenera- 
tive condition resulting from the evolution of forelimb burrowing. 
In spite of the predominance of arborealism in the frog family 
Hylidae, several lines have evolved terrestrial habits. Of considerable 
interest among the terrestrial frogs of the Hylidae are the seven chorus 
frog species of the genus Pseudacris, of which five species are found in 
the southeastern United States. Two species, Pseudacris ornata and P. 
streckeri, are highly unusual in that they burrow forward into the sub- 
strate using their forelimbs (Brown et al. 1972, Brown and Means 1984). 
Important morphological adaptations for this type of burrowing include 
thick forelimbs, enlarged tubercles on the palms of the hands, stout fin- 
gers, and the absence (or great reduction) of toe pads. In contrast, the 
arboreal hylids have long, narrow limbs and digits, and enlarged, adhe- 
sive toe pads. The vast majority of other fossorial species of salientians 
Brimleyana No. 13:55-61, July 1987 
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