INVESTIGADORES
ORTIZ JAUREGUIZAR Edgardo
artículos
Título:
The Gondwanan and South American Episodes: Two major and unrelated moments in the history of the South American mammals
Autor/es:
PASCUAL, ROSENDO; ORTIZ JAUREGUIZAR, EDGARDO
Revista:
Journal of Mammalian Evolution
Editorial:
Springer Netherlands
Referencias:
Año: 2007 vol. 14 p. 75 - 137
ISSN:
1064-7554
Resumen:
The first steps in the history of South American
mammals took place ca. 130 Ma., when the South American plate, still
connected to the Antarctic Peninsula, began to drift away from the
African-Indian plate. Most of the Mesozoic history of South American
mammals is still unknown, and we only have a few enigmatic taxa (i.e.,
a Jurassic Australosphenida and an Early Cretaceous Prototribosphenida)
that pose more evolutionary and biogeographic questions than answers.
The best-known Mesozoic, South American land-mammal fossils are from
Late Cretaceous Patagonian beds. These fossils represent the last
survivors of non- and pre-tribosphenic Pangaean lineages, all of them
with varying endemic features: some with few advanced features (e.g.,
?Eutriconodonta and Symmetrodonta), some very diversified as endemic
groups (e.g., ?Docodonta Reigitheriidae), and others representing
vicariant types of well known Laurasian Mesozoic lineages (e.g.,
Gondwanatheria as vicariant of Multituberculata). These endemic mammals
lived as relicts (although advanced) of pangeic lineages when a
primordial South American continent was still connected to the
Antarctic Peninsula and, at the northern extreme, near the North
American Plate. By the beginning of the Late Cretaceous, the volcanic
and diastrophic processes that finally led to the differentiation of
the Caribbean region and Central America built up transient geographic
connections that permitted the initiation of an overland inter-American
exchange that included, for example, dinosaurian titanosaurs from South
America and hadrosaurs from North America. The immigration of other
vertebrates followed the same route, for example, polydolopimorphian
marsupials. These marsupials were assumed to have differentiated in
South America prior to new discoveries from the North American Late
Cretaceous. The complete extinction of endemic South American Mesozoic
mammals by the Late Cretaceous-Early Paleocene, and the subsequent and
in part coetaneous immigration of North American therians,
respectively, represent two major moments in the history of South
American mammals: a Gondwanan Episode and a South American Episode. The
Gondwanan Episode was characterized by non- and pre-tribosphenic mammal
lineages that descended from the Pangeic South American stage (but
already with a pronounced Gondwanan accent, and wholly extinguished
during the Late Cretaceous-Early Paleocene span). The South American
Episode, in turn, was characterized only by therian mammals, mostly
emigrated from the North American continent and already with a South
American accent obtained through isolation. The southernmost extreme of
South America (Patagonia) remained connected to the present Antarctic
Peninsula at least up until about 30 Ma., and both provided the
substratum where the primordial cladogenesis of South American
mammals occurred. The resulting cladogenesis of South American therian
mammals followed Gould's motto:
early experimentation, later standardization. That is to say, early
cladogenesis engendered a great variety of taxa with scarce
morphological differentiation. After this early cladogenesis (Late
Eocene-Early Oligocene), the variety of taxa became reduced, but each
lineage became clearly recognizable distinctive by a constant
morphologic pattern. At the same time, those mammals that underwent the
early experimentation were part of communities dominated by archaic
lineages (e.g., brachydont types among the native ungulates), whereas
the subsequent communities were dominated by mammals of markedly
modern stamp (e.g., protohypsodont types among the native
ungulates). The Gondwanan and South American Episodes were separated
by a critical latest Cretaceous-earliest Paleocene hiatus, it is as
unknown as it is important in which South American land-mammal
communities must have experienced extinction of the Gondwanan mammals
and the arrival and radiation of the North American marsupials and
placentals (with the probable exception of the xenarthrans, whose
biogeographic origin is still unclear).
Keywords Mesozoic - Cenozoic - Gondwana - South America - Patagonia - Antarctica - Paleobiogeography - Paleoecology