Geography Reference
In-Depth Information
terns may need to be treated within a multiple time plane approach (Rosen
and Smith 1988) or time slicing (Upchurch et al. 2002). The latter term cor-
respondstothe analysisofbiotic distributional data accordingtoasequence
of individual stratigraphic intervals (time slices).
Let us consider an example. Originally (time t
0
), we have a biotic com-
ponent A. Then a vicariance event at time t
1
produces two biotic compon-
ents endemic to areasB andC. At time t
2
these biotic components converge
in a single biotic component in area D because of geographic coalescence,
but a trace of the previous endemism may be still recognizable. Finally, at
time t
3
two new endemic biotic components occur in areas E and F after
another vicariance event that subdivided the former component inhabiting
area D. How does one recognize the vicariance event at t
1
if the effects
of biogeographic convergence that formed the biotic component of area D
overwhelmed the biotic fingerprint on the two original biotic components B
and C? How does one recognize what happened at times t
1
and t
2
if one
studies only the latest time plane (t
3
) and the lineages occurring in areas E
and F?
Events of biogeographic convergence such as those described for
Mesozoic ammonites (Cecca 2002) or for plant and animal taxa from the
Mexican Transition Zone (Halffter 1987) produce a sort of overprinting of
past biogeographic histories by more recent patterns (e.g., reticulated area
histories). This overprinting lowers the chances of establishing area relation-
ships through congruence, which is the final goal of cladistic biogeography.
The solution to problems posed by instances of biogeographic convergence
is time slicing (Grande 1985; Upchurch and Hunn 2002). Although assess-
ments of faunal similarity usually are undertaken with faunas of successive
geological ages, traditional cladistic biogeography has used data on organ-
ism relationships and spatial distributions only on a single time plane (usu-
ally the present). Time slicing may reconcile the use of time and a synchron-
icapproach.Ideally,paleobiogeographersshouldbeabletouseasynchron-
ic approach for each time slice they identify. This is difficult because of the
limits imposed by geological constraints (e.g., insufficient precision or resol-
ution of chronological correlations, incompleteness of the fossil record).
In order to apply time slicing, three methods are available. Two of them,
parsimony analysis of endemicity and area cladistics, have been already
