60 
Gary L. Paukstis and Lauren E. Brown 
intercalary cartilage may represent an ancestral condition from which 
both of the other types were independently derived. This can explain the 
occasional occurrence of cuboidal intercalary cartilages in the hind 
limbs of P. ornata and P. streckeri, while not requiring complex shifts 
in selection on this level of evolution. We do not mean to advocate that 
the genus Hyla was derived from Pseudacris with cuboidal intercalary 
cartilages (our comments here are restricted to the evolution of the 
intercalary cartilage). The cuboidal intercalary cartilages of the extant 
Pseudacris and the intercalary cartilages of the extant arboreal Hyla 
may both be derived from a cuboidal intercalary cartilage of a semiter- 
restrial ancestral form. There are at least seven other possible evolution- 
ary histories involving various combinations of the three types of inter- 
calary cartilages. However, these schemes lack a middle stage involving 
a cuboidal intercalary cartilage that would indicate an intermediate 
nonburrowing, terrestrial frog, or they involve a reversal of the direction 
of selection in regard to the length of the intercalary cartilage; some 
encounter both problems. Even if it is assumed that the cuboidal inter- 
calary is the ancestral stage, there are still problems of interpretation in 
regard to reversal in direction of selection. Presumably, the ancestral 
condition from which the cuboidal intercalary cartilage was derived 
could have been absence of the cartilage. Thus, the cuboidal intercalary 
cartilage would have passed through a stage that was shorter in length 
during the course of its evolutionary history. Consequently, both ances- 
tors and derivatives of the cuboidal intercalary cartilage would have 
been shorter, if this interpretation is followed. 
ACKNOWLEDGMENTS . — We thank J. Brown for preparing slides 
in the early phases of this study; W. Auffenberg (Florida State Museum), 
R. Funk, E. Keiser (University of Southeastern Louisiana), R. Martin 
(Texas Memorial Museum), A. Sanders (Charleston Museum), and R. 
Zweifel (American Museum of Natural History) for the loan of speci- 
mens and materials; R. Anderson, J. Brown, and E. Mockford for criti- 
cally reading the manuscript; and Jane Bucci for drawing Figure 1. 
LITERATURE CITED 
Axtell, Ralph W., and N. Haskell. 1977. An interhiatal population of Pseu- 
dacris streckeri from Illinois, with an assessment of its postglacial disper- 
sion history. Nat. Hist. Misc. No. 202:1-8. 
Brown, Lauren E. 1978. Subterranean feeding by the chorus frog, Pseudacris 
streckeri (Anura: Hylidae). Herpetologica 34(2):21 2-2 1 6. 
, and J. R. Brown. 1973. Notes on breeding choruses of two anurans 
( Scaphiopus holbrookii, Pseudacris streckeri) in southern Illinois. Nat. 
Hist. Misc. No. 192:1-3. 
