31 
Freshwater Fish Zoogeography 
In both cases, however, their ecological preferences do not appear to 
differ significantly from those of their closest respective relatives, 
Enneacanthus gloriosus and Elassoma evergladei, which are widely and 
uniformly distributed throughout the peninsula (Lee and Gilbert 1980a, 
Bohlke and Rohde 1980a). It seems clear that drainage divides in penin- 
sular Florida either are less easily breached than would first appear, or 
unknown ecological factors prevent species such as Enneacanthus obe- 
sus and Elassoma okefenokee from becoming established in the lower 
part of peninsular Florida. 
FACTORS CONTRIBUTING TO FAUNAL DISTINCTIVENESS 
There are several explanations for the pronounced faunal break 
between the Altamaha and Satilla river drainages, which show a Jac- 
card coefficient of association level of only about 0.42 (Fig. 2). The 
most obvious explanation is that much of the Altamaha drainage lies 
above the Fall Line, whereas the Satilla and adjacent drainages to the 
south are situated entirely below the Fall Line, on the Coastal Plain 
(Fig. 1). The Altamaha thus has a much greater diversity of habitat 
and a consequently more diverse fish fauna than the Satilla (62 vs. 44 
species). This is not the entire answer, however, since there are four 
species ( Notropis cummingsae , Ictalurus brunneus , Etheostoma olm- 
stedi and Percina nigrofasciata ) that are absent from both the Satilla 
and adjacent St. Marys rivers, but reappear in the St. Johns drainage 
farther south (Gilbert and Burgess 1980a,c; Lee and McAllister 1980; 
Burgess 1980b). This phenomenon appears to be related to the presence 
of Okefenokee Swamp, which is situated about 35 m above present sea 
level and forms the headwaters for the Suwannee, St. Marys and Satilla 
rivers. The swamp originated as a marine embayment (Cooke 1925), 
presumably during the Pliocene when sea levels were higher. It was sub- 
sequently isolated from the ocean by Trail Ridge, which has a maxi- 
mum elevation of nearly 50 m and which extends in a northeasterly 
direction toward Jesup, Georgia, from a point west of the north-flowing 
loop of the St. Marys River (between Georgia and Florida) (Fig. 3, in 
part). The time of formation of Trail Ridge has been the subject of 
debate, with estimates ranging from Miocene (Alt and Brooks 1965) to 
Pleistocene (Cooke 1939). Opdyke et al. (1984), however, have firmly 
placed its origin in the Pleistocene, a conclusion based partly on the 
presence, at elevations of 42-49 m, of a marine fossil invertebrate fauna 
consisting entirely of Recent species of gastropods, pelecypods, echi- 
noids, bryozoans, barnacles and others. Later in the Pleistocene, epeiro- 
genic uplift of the Florida peninsula raised Trail Ridge to its present 
elevation, after which the Okefenokee Swamp assumed its present char- 
acter. Since this provided an unsuitable habitat for most lotic fishes, 
