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opment of associations through geological time” (Reig 1962:133). Reig
analyzed Simpson's (1950) contribution, which, closely following Matthew's
(1915) theses, postulated the origin of all South American mammals in the
Holarctic realm. Reig did not invalidate it totally, but he restricted its ap-
plication to Cenozoic mammals, allowing other tetrapods to show relation-
ships with the austral continents. As part of his critique, Reig questioned the
concept of faunistic strata, which Simpson used to explain the fact that dif-
ferent taxa have occupied some areas during determined periods of time of
different duration. He replaced it with the term
cenocron,
synthesizing in its
etymology the notions of community and time.
Reig (1981) published a more extensive work that represents the ma-
turity of his conceptions (Morrone 2003a), in which his differences with
Simpson's approach are more evident. He took an “equidistant position
between the Holarcticism of the Matthewian school and the
ad hoc
Aus-
tralism of other biogeographers, trying to formulate an integrative theory of
the biogeographic history of South American vertebrates” (Reig 1981:13).
Holarcticism, formulated by Matthew, Simpson, and Darlington, emphasizes
the origin of all South American vertebrates in the Northern Hemisphere.
Australism, represented by Ameghino, postulates that all mammals that in-
habited the world in the Cretaceous originated in Patagonia. Reig's (1981)
synthetic theory, which he considered analogous to the theory developed
by Halffter (1964, 1965) for the Scarabaeidae (Coleoptera) of the Mexican
Transition Zone, rescued some postulates of Simpson but incorporated
alternative interpretations to explain pre-Cenozoic faunistic connections,
based on vicariance due to continental drift. Thus Reig combined both dis-
persal and vicariance into a single model. Reig (1981) concluded his pro-
posal by postulating the reconstruction of “horofaunas,” or main patterns of
change in the fauna of South American mammals. Horofaunas imply suc-
cessive events of adaptive evolution of different lineages that interact, max-
imize the exploitation of available resources, and reach certain stability, cov-
ering all possible ecological niches.
Erwin (1979, 1981) expanded the “taxon cycle” concept, introduced by Dar-
lington (1943) and Wilson (1961), into a model that accounts for habitat
shifts along certain pathways (
fig. 3.9
). It postulates the existence of a
unidirectional progression along a sequence of habitats from tropical wet-
