WILLIAMS: CHASMODES TAXONOMY


Sound. LOUISIANA: TU 12832 (1), 67392 (17), 67648 (2), 68315 (1), 69569
(1), 76615 (1), 76601 (10), 77611 (1), 77736 (2), 77751 (1), 77880 (5), 77918
(1), 77939 (5), 77995 (1), 78008 (1), 78298 (3).
 ZOOGEOGRAPHY
 Springer (1959) presented a hypothetical scenario of the evolutionary
history of the genus Chasmodes. This hypothesis suggests that a single
Pliocene species of Chasmodes, inhabiting the present range of the genus,
was divided into continental and insular populations by rising sea level in
the Pleistocene. The geographic isolation of the insular population eventu-
ally led to genetic divergence from the continental population. By the end
of the Pleistocene, the insular population presumably had reached the
specific level. The islands were then reunited with the mainland as the sea
level receded to its present height, resulting in the disjunction of the
mainland species, C. bosquianus. After the islands were reunited with the
mainland, C. saburrae has excluded C. bosquianus from the peninsula.
Springer (1959) further suggested that the disjunct populations of C. bos-
quianus have been diverging from each other since the emergence of the
Florida peninsula.
 This scenario has been used to describe the evolutionary history of a
broad range of terrestrial and aquatic organisms in the southeastern U.S.
(Ginsburg 1952, Relyea 1965, Zug 1968, Shipp and Yerger 1969, Shipp 1974,
and Burgess and Franz 1978). In view of the number of different forms
exhibiting this distributional pattern this pattern can be considered a gen-
eralized track sensu Croizat et al. (1974).
 Only two aspects of this hypothetical scenario have been seriously
criticized, the time frame for the events and the nature of the highest
terrace (i.e. whether or not it is due to high sea level). Deevey (1950)
questioned the marine origin of the high terraces of Cooke (1939) and
suggested that they may be constructional rather than erosional. Zug (1968)
chose to recognize five Pleistocene marine terraces, including the high
terraces. Robertson (1976) confirmed the marine nature of the "90-100
foot" terrace of Alt and Brooks (1965), but suggested a late Pliocene age.
This leads to the controversy over the timing of the events. Cooke (1935,
1939, 1954) and MacNeil (1950) suggest a Pleistocene age while other
geologists prefer a Pliocene or even Miocene age for the terraces higher
than the 10 m terrace (Alt and Brooks 1965; Osmond et al. 1965, 1970;
Bender et al. 1979). Bender et al. (1979, and references therein) provide
data that argue strongly against sea level rising higher than 10 m above the
present level in approximately the last 500,000 years before present (BP),
and probably not in the last one to two million years BP.
 Although the timing of the hypothesized events has little effect on the
mechanism, proposed by Springer (1959), for the evolution of Chasmodes