Biology Reference
In-Depth Information
There are also amphibians and reptiles that show the same pattern. What
better time for them to have arrived than during the cold intervals of the
Pleistocene, only to be left stranded when climates warmed and the desert
was reconstituted during the last interglacial? From the present perspec-
tive, this implies that potentially eighteen to twenty interchanges occurred
in the recent geologic past, and that this continuity, or near continuity, ex-
isted for some 90 percent of the time.
Four studies later provided amendments to these early interpretations of
Quaternary biogeography in the south-central United States, with broader
implications for the presence of temperate elements in eastern Mexico and
farther south. First, it was observed that affi nities among the amphibians,
reptiles, and to some extent the trees and shrubs between southeastern
United States and eastern Mexico exist mostly at the generic level (Mar-
tin 1958; Martin and Harrell 1957), implying a long period of geographic
and reproductive separation. Second, a reexamination of the Texas mate-
rial showed an absence of
Abies
and a much smaller presence of
Picea
than
previous calculations, only 1.5 percent rather than 11 percent (Graham and
Heimsch 1960). Three pollen grains of spruce out of every 200 would eas-
ily be within wind transport of stands growing further south than at pres-
ent during the colder intervals of the Quaternary without requiring a full-
fl edged boreal forest along the Gulf Coast. Moreover, macrofossils of boreal
trees like larch and spruce frequently drift down the Mississippi River and
are deposited in backswamp sediments during fl oods. Third, Delcourt and
Delcourt (1977) further showed that the macrofossils in Louisiana iden-
tifi ed as
Thuja occidentalis
(northern white cedar) were actually the local
Chamaecyparis thyoides
(southern yellow cedar). Fourth, my study on the
Paraje Solo Formation of southeastern Mexico (Graham 1976), mentioned
in chapter 7, documents that
Abies
,
Picea
,
Alnus
,
Celtis
,
Ilex
,
Juglans
,
Liquid-
ambar
,
Populus
,
Quercus
, and
Ulmus
were already established in eastern
Mexico by at least the middle Pliocene. The most likely time for their prin-
cipal early introduction was around the middle Miocene, that is, with the
third of the major paleotemperature declines (see fi g. 9.3 below).
Mexico
Periodic displacement of the jet streams and the Hadley/Bermuda-Azores
convection cells southward during the Quaternary brought alternating peri-
ods of warm-dry and cool-moist conditions to the arid southwest. The latest
of these sequences is revealed by cave speleothems in Carlsbad Caverns,
New Mexico (Brook et al. 2006), and by packrat middens (Van Devender
