Mastozoologla Neotropical; 3(2)133-152
SAREM, 1996
ISSN 11321-930
LATE CRETACEOUS-RECENT LAND-MAMMALS.
AN APPROACH TO SOUTH
AMERICAN GEOBIOTIC EVOLUTION
Rosendo PASCUAL
Departamento Cientifico Paleontologia Vertebrados. Museo de La Plata. FCNyM. UNLP. Paseo
del Bosque, (1900) La Plata, Argentina.
ABSTRACT: The known late Cretaceous-Present South American land mammals, more than
any other land fossils, demonstrate that macroevolutionary processes can only be understood
if we have a prior knowledge of the historic biogeographical events. On this basis, the main
events marking the history of South American land mammals are analyzed in relation to the
geodynamic events that produced the successive geographical and climate-environmental
South American changes. It was found a close relationship between the abiotic and biotic
phenomena succeeded during the late Cretaceous-Present span. The mammals up to date
known representing the early and late Cretaceous time interval pertain only to non-tribosphenic
and pre-tribosphenic groups. Further, they are markedly endemic (to the familial level, and
even to higher levels, e.gr. Gondwanatheria), which are interpreted as related to the long
isolation of the Gondwanan Continent. This is recognized as the Gondwanan Stage, which
followed the still non-recorded Pangean Stage. It has been recognized that there exists a
long unrecorded span, representing at least most of the Maastrichtian and Daman. The next
record, estimated as spanning the time between ca. 63.2-61.8 Ma, indicated that there had
occurred a marked tumover, characterized by the replacement of the pre- and non-tnbosphenic
native mammals by the immigrant tribosphenid marsupials and placentals. Furthermore, the
Patagonian locality of this age records the last non-tribosphenic gondwanatherian (Sudamenca
ameghinot) together with the first non-Australian monotreme ( Monotrematum sudamericanum).
On the one hand, it is obvious that during the unrecorded Maastrichatian-Danian span there
existed connections both with North America and Australia (via Antarctica). On the other
hand, the so marked endemism of marsupials and placentals indicated that the isolation of
the South American continent had been acting from some time ago. This Isolation Stage
was broken up during the late Eocene, permitting the immigration of hystricognathan rodents
and platyrrhini monkeys —apparently from Africa—, and during the late Oligocene-early
Miocene, permitting that some megalonychoid edentates emigrated to the West Indies. The
final connection with North America occurred by the late Pliocene and through the Panamanian
Bridge. It led to the Great American Biotic Interchange. It was preceded by some premonitory
interchange, as island-hoppers, having the procyonid carnivores and the tardigrade edentates
as the main heralds, respectively from North America and South America. The Megafaunal
Extinction, succeeded around 10.000 years before Present, and the subsequent entrance of
the Sciuridae, Heteromyidae, and Geomyidae rodents, the lagomorph Lepondae, and the
Soricidae insectivores, built up the present scenario characterizing the Neotropical Region.
We estimate that at least 50% of the present genera descended from North American
immigrants. This paleobiogeographical history indicates that South America is the only continent
that preserves, as fossil or living, land mammals originated in any of the major continental
blocks that differentiated from the late Triassic on, when the oldest mammals was recorded.
Surprisingly, none of its extant land mammals represent any order proved to be originated
in the South American continent when differentiated as such.
Key words: South american geobiotic evolution, land mammals, late cretaceous-recent.
Recibido 17 Julio 1996. Aceptado 30 Seriembre 1996.
134
I INTRODUCTION
We recently qualified land mammals as the
paradigm for knowing the South American
geobiotic evolution throughout the Late Cretaceous-Recent span (Pascual, OrtizJaureguizar, & Prado, in press), i.e., approximately along the last 75 million years. There
is no other paleontological evidence, in space
and time, that likewise illustrates about the
terrestrial life throughout this span (Pascual
& Ortiz-Jaureguizar, 1990). Furthermore, the
succesive lithogenetic changes of the landmammal bearing formations, in conjunction
with changes in their geographical distribution, and in the ecological-types (ecomoprhs
and climate-sensitives) of their land-mammals,
led, on the one hand, to comprehend the evolution of climates and environments, and, on
the other hand, to realize how closely related
the latter were to the geodynamic processes
(Pascual, 1970, I984a, 1984b, 1992; Pascual
& Odreman Rivas, 1971,1973; Marshall et al.,
1 983; Marshall et al., 1984; Ortiz-Jaureguizar,
1 986; Ortiz-Jaureguizar & Pascual, 1989;
Pascual & Ortiz-Jaureguizar, 1990, 1991,
1 992; Legarreta, Uliana & Torres, 1990;
Legarreta y Uliana, 1994; Pascual, OrtizJaureguizar & Prado, in press; Bond et al., in
press). The use of other circumstantial biological evidence, corroborated the climatic and
environmental inferences attained by using
land-mammals in conjunction with geological
evidences.
Patterson & Pascual (1972) pointed out that
the record of mammals in South America has
one greater defficiency: it is largely restricted
to the southern part of the continent. But, we
presently realize that this defficiency is only
relatively valid if we intent to assess the precise diversity reached by land-mammals all
over the continent. On the contrary, this relatively rich southern record becomes an useful
tool for assessing the climatic and environmental changes succeeded throughout the "age
of mammals". It has to be taken into account
that because southern South America lies
downwind of that part of the Southern Ocean
that abuts West Antarctica, its climate has been
very sensitive to environmental changes in
Antarctica. On this account, let's remember
that the great diversity and dominance of
Rosen& Pascual
advanced therian mammals appear as related
to the global climatic and environmental
changes initiated by the end of the Mesozoic.
These changes particularly characterized the
Cenozoic, throughout which the trend to a
marked seasonatily increased. On purpose, this
lapse of the geological time is known as the
"age of mammals" since the most perfect endothermic mammals, the metatherian and eutherian, diversified and dominated. The geodynamic phenomena that in association with,
or consequent of, the astronomical ones, promoted changes of the climate and environments
are more ostensibly manifested in middle to
high latitudes. Consequently, the reciprocal
action of the biotic and abiotic factors is more
appropriately recognized in those latitudes. This
combination of particular geographical and
historical circumstances, added to the isolation of the continent during most of the Tertiary, converted the South American landmammals in paradigmatic for assesing the
whole South American geobiotic evolution
during the Late Cretaceous to Present span.
II. EVOLUTION AND GEOGRAPHY
11.1. Geographical changes and the origin
and diversification of mammals.
The oldest remains recognized as pertaining
to a mammal comes from beds of about 225
million years in age (Lucas & Hunt, 1990;
Lucas & Zhexi Luo. 1993). Hence, the origin
of mammals appears related to the last stages
of Pangea II (Fig. la). Suggestively, the two
main post-pangeic continental fragmentations
were approximately coincident with the two
main subsequent phylogenetical radiation
known in mammals: (1) Early Jurassic-Early
Cretaceous, approximately between 180 and
130 million years ago. Along this span, Pangea breaked-up and finally differentiated two
main supercontinents, Laurasia in the northern
hemisphere, and Gondwanaland in the southern one (Fig. lb). According to some authors
(e.g., Storey, 1995), simultaneously with its
beginning, or somewhat later, the initial stages
of Gondwana break-up began. The phylogenetical radiation of non-tribosphenic (i.e., "prototherians". See McKenna, 1975; Clemens et
al, 1979) and pre-tribosphenic (i.e., "pan-
LATE CRETACEOUS-RECENT LAND-MAMMALS
totheres". See Bown Kraus, 1979) mammals
characterizes this part of the history; (2) Early
Cretaceous-Early Paleocene, approximately
between 130 and 63 million years before Present. Throughout this span, there occurred the
premonitory geographical stages of the present situation. Particularly, Gondwanaland was
opening of the Atlantic Ocean, and the isolation of the Antarctic continent are among the
most influential geographical phenomena succeeded during this lapse of the geological time.
Trascendental climatic changes appear as concomitantly related to this new geographical
na: (a) break-up of Western Gondwana, mak-
readjustment. From the late Cretaceous on we
assist to the progressive rupture of the climatic equability, with an increasing seasonality.
ing an African plate and a South American
concomitantly related to a more severe latitu-
one, which progressively drifted away giving
dinal differences in temperatures (Frakes.
1 979). The origin and phylogenetical radiation of tribosphenic mammals characterizes this
affected by the following two main phenome-
origin to the Southern Atlantic, and (b) desintegration and dispersion of Eastern Gondwana, shaping the present smaller continental
plates. The close of the Tethys Sea, the whole
last part of the history of mammals. The final
differentiation and diversification of the most
advanced endothermic mammals, the metatherian and eutherian, suggestively appear as related to these geographic and climatic events.
II.2. Evolutionary and paleobiogeographical pattern of the South American land
mammals
Simpson (1950) was the first paleontologist
who recognized that the Cenozoic history of
South American mammals was distincti.el ∎
characterized by few and clearl■ separable
episodes. In a figurative sense he distinguished
this episodic history as stratified. He recognized three main episodes, he named Faunal
Strata, each mainly distinguished b ∎ an
extracontinental immigratory event: 1. Ancient
I mmigrants; 2. Old Island-hoppers; and, 3. Late
(Island-hoppers and) Immigrants. In one of his
last books (Simpson, 1980), he particularly
emphasized that this stratified history resulted
from the prolonged geographical isolation of
the continent during the Tertiary, he qualified
as "splendid isolation", and from its sporadic
and geological brief interruptions, during which
arrived the immigrants. To him all the
i mmigratory groups came fron North America,
regarding Central America as part of it. Implicitly, he accepted that when the "ancient
i mmigrants" arrived in South America the
continent was deprived of mammals. At the
ti me, the oldest known South American mamFig. 1: a) World reconstruction for 190-200 Myr (Based
on Lillegraven et al.. 1979); b) World reconstruction for 130-140 Myr (Based on Lillegraven.
1979)
mals were not from the oldest Tertiary but from
the middle Paleocene, presently assigned to
the Itaboraian Land Mammal Age (Sec Bond
et al., in press). To Simpson, all of them were
136
immigrants from North America, or descendant of them, hence the name "ancient immigrants". Explicitly he never denied the existence of "Mesozoic Mammals" in South
America, but to him all the land mammals
known by then, extinct and extant, took origin
in the so called "ancient", "old island-hoppers",
and "late (island-hoppers) and immigrants", all
of them came from North America. Although
this conclusion was challenged by many foreign and local students, surprisingly the increased record of Cretaceous and earliest Paleocene land mammals suggests that in this
sense he was right. But, on the other hand,
this new late Cretaceous record of land mammals demonstrates that they represented higher
categories of pangeic groups, although quite
endemic, suggesting a long period of isolation
from the vicariant Laurasian groups. Furthermore, this record also demonstrates that one
of this gondwanan groups survived in Patagonian during the first part of the Paleocene.
Moreover, and surprisingly, this beds afforded
the first non-Australian monotreme. Among
others, this unexpected record demonstrates
that, contrary to Simpson (1953:63), the peculiar history of South American mammals cannot been explained without using the plate
tectonic rationale. South America is the only
continent that preserves, as fossil or living, land
mammals originated in any of the major continental blocks that differentiated from the late
Triassic on, when the oldest mammal was recorded: from Pangea, Laurasia (North America
+ Eurasia), East Gondwana (Antarctica + Tasmania + New Guinea + New Caledonia + New
Zealand + Madagascar + India), and Western
Gondwana (South America + Africa). But,
surprisingly, none of its extant land mammals
represent any order proved to be originated in
the South American continent when differentiated as such. This unique case of historical
biogeography can only be explained integrating the geological and biological systems (fide
Rosen, 1981).
11.3. Geographical history of South
America
Within the Pangea Supercontinent, the South
American block was situated in a central
Rosendo Pascual
occidental position, and in such a way that it
had direct contact with those blocks that were
to become North America, Africa, and Antarctica (Fig. la). Through these blocks it had
indirect contacts with the blocks that were to
become the remaining present continents. By
the Early Jurassic times (-180 Myr ago) Pangea
started the initial rifting and dispersion stages
leading to the differentiation of the Supercontinents Laurasia and Gondwanaland, respectively in the Northern and Southern Hemispheres (see Fig. lb). Passing through different stages (Paleo-Tethys and Neo-Tethys),
Laurasia and Gondwana Supercontinents became separated each other by a pan-ecuatorial
seaway, known as the Tethys Sea. According
to some authors (e.g., Storey et al., 1992;
Rapela & Pankhurst, 1992; Storey, 1995), simultaneously with the differentiation of
Laurasia and Gondwanaland there also began
the initial rifting stages of the latter, leading to
the formation of a seaway between West (South
America+Africa) and East Gondwanaland
(Antarctica+Australia+ India+New Zealand)
(Fig. 2a). The fossil record indicates that by
the late Cretaceous-Early Paleocene span the
southern tip of South America (Patagonia) had
distinctive geobiotic characters than those of
the remaining continents, having a biota more
closely related to that from East Gondwana
than to that from West Gondwana Apparently,
they were inherited from the independent geological history of the Patagonian territory. It is
curious that even today there still persist this
biogeographical peculiarities (see below, H.4.2)
The isolation of the South American continent, through the breakage of its connections
with the remaining Gondwanan continents, was
heterochronic. Its separation from Africa was
initiated about 130 million years ago (Storey,
op.cit.) (Fig. 2b), and culminated by 110 million years ago (Sclater et al., 1977; Parrish,
1993) (Fig. 2c). This last event initiated the
opening of the South Atlantic Ocean, which
was going to culminated with the whole separation of South America from Antarctica. This
latter event marks the second and last stages
of the continental isolation, and was relatively
recent. According to Veevers (1984), South
America kept some archipelagic connection
LATE CRETACEOUS-RECENT LAND-MAMMALS
137
with West Antarctica until about 35 millions
years ago, that is to say, up to the end of the
Eocene or the beginning of the Oligocene.
According to common geologic features, from
as early as the Late Cretaceous and up to the
late Eocene or earliest Oligocene, both regions
were part of the same geologic province. The
record of late Cretaceous-earliest Paleocene
marine and terrestrial biota indicates that there
exists then a southern Gondwanan biogeographical region, known as Weddellian Province (See below 11.4.2., and Fig. 3). To the
•
former separation of the continental microplate
of the South Tasmanian Rise, occurred at least
by 50 Ma, followed the northward drifting of
the South American Plate. As summarized by
Ehrmann & Mackensen (1992), and Lawyer
et al. (1992), the Drake Passage opened about
36 Ma, which led to the complete isolation of
the South American continent. In turn, this
event led to the differentiation of Antarctica
and the influential Circum Antarctic Current,
which established the premonitory physical
conditions leading to the Present global cooler
climate.
Between those continental ruptures that led
to the South American isolation, there occurred
another geographical event that quite trascendentally affected the history of its biota. According to Donnelly (1985), the first steps leading to the closing of the Caribbean part of the
Tethyan Sea appears as related to the compressional environment that by 85 Ma
suplanted the formerly tensional one. To him,
and as a consequence, the movement of South
America relative to North America changes to
northward. There occurred then the inception
of explosive island arc vulcanism, which could
•
70 Ms
Fig. 2: a) Gondwanaland reconstruction for 160 Myr
(Based on Storey. 1995); b) Gondwanaland reconstruction for 130 Myt; c) Gondwanaland reconstruction for 110 Myr;
Fig. 3: The paleobiogeographical Weddellian Province
about the latest Cretaceous (Based on
Zinsmeister, 1982). I. Southern South America;
2. Western Antarctica; 3. Antarctic craton; 4.
Australia; 5. New Zealand; 6. Southern Africa.
I 38
build up some kind of selective connection
between both Americas. This could explain the
likewise selective North American immigrants
recorded in southern South America by that
ti me (Bonaparte, 1984, 1986; Parrish, 1993).
Some mammals immigrants from North
America, recorded in early to middle Paleocene
beds from both central (Muizon, 1991) and
southern of South America (Pascual y OrtizJaureguizar, 1991; Bonaparte, Van Valen y
Kramarz, 1993), indicate that probably this
connection remained active until this time.
According to Donnelly (op.cit.), the late Paleocene-late Eocene span was a time of tectonic
relaxation. South America slowly moved southwestward, relieving the compressive environment that has dominated late Cretaceous tectonics and magmatism. He qualified the early
Tertiary as a period of vastly diminished volcanic activity and local rifting.
Those geodynamic events explain the isolation attested by mammals of the late Paleocene-Eocene span (See below, 11.4.3.). This
was the fundamental evidence that led to the
interpretation that during this time South
America was a vast island continent Notwithstanding, the record in the Antarctic Peninsula
of early Eocene South American mammals
appear to indicate that either the Antarctic
Peninsula was part of the huge South American island, or that this island continent
mantained southern geographical connections
up to a more recent time.
According to Smith (1985), and based on
many paleogeographical reconstructions of the
Caribbean region, a complete link of southern
Central America with South America probably
did not occur at least until the period 10-5 Ma
ago. To him, most models appear to favor a
closure at the Miocene-Pliocene boundary
around 5 Ma ago. To Keigwin (1982), comparisons of planktonic foraminiferal oxygen
isotopic data for the Caribbean and the Pacific
suggest that the salinity of Caribbean surface
waters began increasing 4 million years ago,
possibly in response to shoaling of the Panama
isthmus. To him, it is a suggestion that modern circulation patterns in the Caribbean and
eastern Pacific developed by 3 million years
Rosendo Pascual
ago in concert with changing tectonics, climatic, and biogeographic patterns. From that
moment on the present basic geographic conditions were established, allowing the advent
of the so called The Great American Biotic
Interchange (See below, 11.4.4) However, the
differentiation of the climatic-environmental
conditions characterizing the present Neotropical Region is a much recent event, resulting
from geologic and related climatic events occurred by the very end of the Pleistocene (See
below, and Webb. 1985; Marshall et al., 1984;
Pascual, 1984a, 1984b, 1986; Rancy, 1989).
11.4. Non-flying land mammals as a testimony of the South American biogeographic
history.
Non-flying South American land mammals
from the Cretaceous-Cenozoic span very
appropiately illustrate about the global geographic-climatic events sketched above. Specifically, they provide direct or indirect evidence on the regional biotic and abiotic variants that drove the peculiar history of South
American mammals. Besides, the only known
early Cretaceous mammal is both a testimony
and a suggestion of the mammals that inhabited South America, or at least in what was
going to be southern South America, during
the Pangeic and Gondwanic geographical
stages.
11.4.1. The Gondwanic Stage: testimonies
and suggestions of the Pangeic Stage.
From late Cretaceous beds (CampanianMaastrichtian?) of northern Patagonia (Rio
Negro) come the only significative South
American Mesozoic mammal remains. They
pertain only to non-tribosphenic groups,
Eotheria, Allotheria, and to primitive Theria
(Symmetrodonta and Dryolestida). Their high
endemism (Bonaparte, 1990, 1994) is compatible with the prolonged geographical isolation
of Gondwana, recognized by other evidences.
Ratifying the insinuation that in the South
American sector of Gondwanaland during the
Mesozoic the tribosphenic mammals were not
present, from likewise Patagonian (Neuquën)
but early Cretaceous beds were unearthed very
LATE CRETACEOUS-RECENT LAND-MAMMALS
well preserved remains of another advanced
Dryolestida. Its quite advanced features suggest that by that time Gondwana had been
isolated from long time ago (Bonaparte, 1986,
1988; Bonaparte y Rougier, 1987; Rougier,
Wible and Hopson, 1992).
Synthetically, the most remarkable feature
of the known pre-Maastrichtian record in
Patagonia is the absence of tribosphenic mammals. In turn, the early Cretaceous record from
northwestern Patagonia insinuates that only
non-tribosphenic and pre-tribosphenic mammals lived in Gondwanaland during the Mesozoic. This appears to be an inheritance of the
pangeic stage of the continents, and its endemism an effect of its prolonged isolation within
the southern hemisphere. These records reinforce the conclusion got from the extra-South
American record: Metatheria and Eutheria
originated in Laurasian territories. Consequently, marsupials and placentals are immigrants in South America, which ratify
Simpson's conclusion got from the Cenozoic
record. Only one of the endemic groups of
the Patagonian late Cretaceous record,
Gondwanatheria, survived in the early Paleocene of Patagonia, up to date represented by
Sudamerica ameghinoi
only one species,
(Scillato Yan6 y Pascual, 1984,1985). A recently collected specimen of this species is so
complete and eloquent that led us to the following phylogenetic and paleobiogeographic
hipothesis: Gondwanatheria is the sister-group
of the Laurasian Multituberculata, both representing vicariant groups differentiated from a
common ancestor living in Pangea before it
became separated in Laurasia in the Northern
Hemisphere, and Gondwanaland in the Southern Hemisphere (Pascual, Goin, OrtizJaureguizar, Carlini, and Prado, 1993).
11.4.2. The Isolation Stage of South
America: testimonies and suggestions
The oldest Cenozoic record of land mammals are from central Patagonia (Scillato Yan6
y Pascual, 1984, 1985; Pascual y OrtizJaureguizar, 1991; Pascual et al., 1992a,1992b;
Bonaparte, Van Valen y Kramarz, 1993), and
from Cochabamba, Bolivia (Gayet, Marshall,
and Sempere, 1991; de Muizon, 1991). Al-
139
though both represents the first part of the
Paleocene, the Bolivian one has been regarded
as somewhat older (Bonaparte, Van Valen, and
Kramarz, op.cit.). Anyway, none of them appear to represent the earliest Paleocene.
Although there are no records of South
American mammal faunas representing the
crucial latest Cretaceous-earliest Paleocene
span, the known late Cretaceous (Campanian)
and early Paleocene (late Danian or early
Thanetian) record suggests that during that span
there occurred quite abrupt compositional
changes. Differently, the known late Cretaceous-Paleocene land-mammal faunas of Norh
America show no such abrupt changes (Pascual
& Ortiz-Jaureguizar, 1992). Except one
relictual taxon (Sudamerica ameghinoi) and the
first non-Australian monotreme (Pascual et al.,
1992a, 1992b; Pascual, 1996), and only in
Patagonia (see below), the known early Paleocene record marks the end of the nontribosphenic and pre-tribosphenic mammals.
Thus, the whole Bolivian community and most
of the Patagonian one were composed by very
advanced Metatheria and Eutheria. We estimate that between those Patagonian Campanian
and early Paleocene record there elapsed about
10 Ma, up to date without record. Throughout
this lapse of time, there had to occurred very
trascendental events, as the whole extinction
of the pre-tribosphenic mammals, the extinction of the majority of the non-tribosphenic
ones, and the immigration of monotremes from
Australia and metatherian and eutherian from
North America. From a biogeographical point
of view, these immigratory events indicate both
that the long Gondwanan Isolation Period was
over (estimated as occurred between the early
Jurassic and the early late Cretaceous), and
that the long Tertiary Isolation Period (late
early Paleocene-Pliocene) was already on the
way (Ortiz-Jaureguizar and Pascual, 1989;
Pascual and Ortiz-Jaureguizar, 1990, 1991,
1992). Obviouly, during this span the South
American continent had some kind of connection, on the one hand, with North America,
and, on the other hand, with East Gondwana
(via Antarctica). The definition of South
America as a discrete island-continent had to
have occurred by then.
Rosendo Pascual
140
The comparison of the Patagonian Campanian record with both the late early Paleocene
Patagonian and Bolivian records, permitted to
infer the above trascendental intercontinental
exchanges. But, the comparison of both Paleocene records permitted to reach likewise trascendental intracontinental conclusions. From
the same Patagonian Paleocene beds that provided the last non-tribosphenic mammal (Sudamerica ameghinoi) are coming just those
mammal taxa that demonstrate that during that
late Cretaceous-early Paleoceno span, South
America had connections both with North
America and East Gondwana. The first ones,
are remains of the extinct placental order
Condylarthra, and another one of a quite advanced indeterminate order (Pascual y OrtizJaureguizar, 1991). The second ones, pertain
to a monotreme Ornithorhynchidae (Monotremarum sudamericanum. See Pascual et al.,
1992a, 1992b; Pascual, 1996). In turn, those
Paleocene mammals from Bolivia (de Muizon,
1991), correlative or somewhat older (fide
Bonaparte, Van Valen y Kramarz, op.cit.) than
the Patagonian ones, apparently were deprived
of both the monotreme and the non-tribosphenic mammals. All of the mammals recorded in
this Bolivian locality are North American immigrants, or descendants of them, composed
by distinct orders of marsupials, and placentals. The latter represented by the orders
+Condylarthra, +Leptictida and +Pantodonta.
Together with these, were the first native
Notoungulata. This latter, and the marked
endemism of both the marsupials and placentals, attest the already established continental
isolation. But, more importantly, the compositional difference with the supposedly younger
Patagonian communities suggest that both regions integrate distinct biogeographical units.
This is in accordance with the long known
distinct geological history of Patagonia respect
the remaining South American continent To
Ramos (1989), Patagonia was a quite distinct
geotectonic unit, accreted to the South American Plate by the end of the Paleozoic, and
recognized by him as the "Patagonia Terrane".
Thus, geologically, the history of the "Patagonia Terrane" was more closely related to the
Eastern Gondwana continental blocks than to
the Western Gondwana ones. Likewise, Rape.
la and Pankhurst (1992) regarded Patagonia as
a discrete geotectonic unit, but to them it is
not necessary to invoke a relationship to deep
mantle structure, neither to a plume activity.
Further, to them the geological boundary of
the allochtonous Patagonian block was the transcurrent Gastre Fault System in central Patagonia, known as the Southern Patagonian
Block. The Gastre Fault System is recognized
as a major dextral shear-zone which allowed
dextral displacement of this block relative to
the rest of South America during the earliest
rifting phase of Gondwana break-up, i.e., about
the late Triassic-early Jurassic span (Fig. 4).
Whichever the geotectonic limit of Patagonia,
either as Ramos's Patagonia Terrane, or as
Fig. 4: Drifting of the "Southern Patagonian Block"
throughout the Late Triassic-Early Cretaceous
span (Based on Rapela and Pankhurst's hypothesis, 1992). AFZ = Aghulas Fracture Zone; AP
= Antarctic Peninsula; BCP = Batholith of Central Patagonia ; EA = Eastern Antarctica; EWM
= Ellsworth-Whitemore Mountains Crustal
Block; GFS = Gastre Fault System; MFI =
Malvinas/Falkland Islands; MFP = Malvinas/
Falkland Plateau; SPB = Southern Patagonia
Block; TI = Thurston Island.
LAM CRETACEOUS-RECENT LAND-MAMMALS
Rapela and Pankhurst's Southern Patagonian
Block, the extra-Patagonian sector of South
America was connected with Africa, having a
quite distinct geological history than the Patagonian sector. The subsequent biota of both
sectors, whether extinct (Pascual y Bondesio,
1975; Pascual, Bond y Vucetich, 1981: Pascual et al., 1992a, 1992b; Crisci et al., 1993;
Pascual, Ortiz-Jaureguizar and Prado, in press;
Pascual, 1996) or extant (Crisci and Morrone,
1990: Crisci et al., 1991), show distinctive
peculiarities. It is remarkable that this peculiarities still persist in both biotas.
Unfortunatelly, we still don't know likewise
eloquent Mesozoic mammals from the remaining South American continent. Consequently,
we cannot evaluate the backward extension and
grade of the differences between extra- and
Patagonian territories. However, there exists
other biotic evidence suggesting that during
the late Cretaceous, and quite possibly the
earliest Paleocene, Patagonia was a distinct
biogeographical unit. For instance, half of the
late Cretaceous ammonites fauna of northern
Antarctica (Seymour Island) has Indo-Pacific
relationships, and the other half is composed
by mostly endemic taxa of the Weddellian
Province (sensu Zinsmeister, 1982). To
Zinsmesiter, the Weddellian Province was integrated by present southern Patagonia, Antarctica, New Zealand, New Caledonia, and
Australia (Fig. 3). Many other late CretaceousPaleocene land faunistic elements from
Patagonia show closer relationships with those
of East Gondwana. In turn, those correlative
from northern Sourh America show closer
relationships with the African ones. On this
account, two eloquent examples are the dipnoan fishes (Pascual y Bondesio, 1976) and
the tortoises (Broin, 1991). According to Broin
et de la Fuente (1993), the latter indicate that
by then there exists "...deux provinces
gondwaniennes..." Still the marine middle
Eocene fauna from central Patagonia (San
Julian Formation from San Jorge Gulf) shows
a extraordinary high percentage of genera and
subgenera (39%) only known from the Southern Hemisphere (Zinsmeister, 1981), that is to
say, typical from the Weddellian Province. To
Case (1988), the land biota from the late
Eocene of the Antarctic Peninsula (Seymour
141
Island) indicates that still persisted the
Weddellian Province. However, land mammals from La Meseta Formation of the Antarctic Peninsula (Seymour Island), indicate that
up to approximately the middle Eocene there
existed a land connection just between southern South America and West Gondwana
(Marenssi et al., 1994). There is no Eocene
land mammals indicating a continuous connection between those territories that integrated
the Weddellian Province.
The probably latest Cretaceous-early Paleocene inmigratory event to Patagonia of the
above mentioned Australian monotreme, support our hypothesis (Pascual y OrtizJaureguizar, 1991) that marsupials emigrated
from Patagonia to Australia by the same time
(See Flannery et al., 1995). If that isolated
molar tooth from the early Eocene of northeastern Australia (Tingamarra porterorum)
really represents a placental (?Condylarthra),
as sustained by Godthelp et al. (1992), the
placentals were also part of the AustralianSouth American interchange occurred by that
time, contrary to what was long thought. The
success of the marsupials over the placentals
remains to be explained. These facts and the
apparent restriction to the Antarctic Peninsula
of Eocene South American land mammals,
suggest that the Paleogene migratory events
of land mammals between Patagonia and the
remaining continents of East Gondwana were
heterochronic. The latest Cretaceous to early
Paleocene one along most of the Weddellian
Province (except New Zealand, where apparently mammals never lived). The Eocene one
just between southern South America
(Patagonia) and West Gondwana. This model
is compatible with the geographical history
inferred by the geophysical evidence: Australia lost its connection with Antarctica before
South America did it (See 11.3, and Zinsmeister,
1982). Deep marine conditions in the Drake
Passage occurred about 23,5 million years ago,
but apparently long before (ca. 38 million years
ago) there already existed effective marine
barriers to land mammal migration between
South America and Antarctica.
Summing up, those early Paleocene records
from central Patagonia (Chubut) and Bolivia
(Tiupampa, Cochabamba), including the first
142
non-Australian monotreme (up to date only in
Patagonia), and marsupials (up to date only
one in Patagonia) and placentals related to
North American taxa, are testimonies indicating that by that time, or somewhat earlier, the
South American continent had a southern connection with Australia (via Antarctica), and a
northern one with North America (including
Central America). If those Paleocene mammals
from Bolivia were older than the Patagonian
ones, and representing the earliest Paleocene
as sustained by Bonaparte, Van Valen and
Kramarz (1993), the so marked endemism
shown by marsupials, and in a lesser grade by
placentals, suggest that the connection with
North America was broken up as late as the
earliest Paleocene. Whatever the chronological difference between the Paleocene mammals
from central Patagonia and Cochabamba (Bolivia), from a paleobiogeographical point of
view both records indicate that by the end of
the Paleocene (1) South America was already
an isolated continent, and (2) the southern tip
of South America (Patagonia) was receiving
land mammals from the northern South American sector. This latter inference is supported
by the record of other biotic elements. For
example, late Paleocene and middle Eocene
beds from the Antarctic Peninsula (Seymour
Island and 25 de Mayo Island), Magellan
Basin, and central Patagonia have afforded the
older remains of cold-temperate flora "associated" to the first remains of the subtropical
flora, that is to say, the first record of what
Romero (1978) distinguished as the "Flora
Mixta". Even some of the elements of the
"Flora Mixta" of nordwestern Patagonia (Laguna del Hunco and Confluencia) are associated to Pipidae frogs, which presently characterize subtropical South American and African regions (See Casamiquela, 1961,1965:
Baez, 1986; Arag6n y Romero, 1984; Baez,
Zamaloa y Romero, 1990). Quite suggestively
these records are from continental beds succeeding those beds representing the Maastrichtian-Danian marine transgression, so wide
ranged that like a seaway separated a Nordeastern sector from a Southwestern one (Fig.
5). We regarded this seaway as a decisive
physical barrier that perpetuated the biogeographical differences that, distinguished —and
Rosendo Pascual
still distinguishes— the southern South American region (Crisci et al., 1993; Pascual, OrtizJaureguizar, and Prado, in press). The "subtropical" elements of the "Flora Mixta", and
the mammal "ungulates" of North American
origin, for the first time recorded in the Paleocene beds of Patagonia, appear as the "invaders" that succeeded the MaastrichtianDanian marirne regression.
II.43. Premonitory interruptions of the
isolation.
The "curious history" of the South American mammals resulted mainly from the so long
continental isolation, succeeded during most
of the Tertiary. But, even if this sustained isolation was quite decisive, the uniqueness of
this history was likewise related to sporadic
ruptures and to the associated transcontinental
migratory processes (See II.2.). From a geological point of view these ruptures were
circunstancial, and selective from a chorological point of view. The most severe and influential period of isolation was that elapsed be-
Fig. 5: Late Cretaceous-earliest Paleocene South Amencan seaways (Based on Zambrano, 1987, and
Legarreta et al., 1989)
LATE CRETACEOUS-RECENT LAND-MAMMALS
tween the ealy Paleocene (approximately 63
million years ago) and the late Eocene (approximately 35 million years ago. See Wyss et
al., 1993). We qualified this span as the most
autochthonous part of the history (Pascual,
Vucetich, Scillato-Yand, and Bond, 1985,
p.230) since the most autochthonous South
American mammals took origin by then. We
distinguish this period as the First Stage of the
Isolation. A multivariate analysis, using the
SALMAs (South American Land Mammals)
plus the Recent Epoch as "characters", and as
OTUs the land mammal families (See Pascual
and Ortiz-Jaureguizar, 1990), distinguished this
span as the most distinctive cycle of the Cenozoic history of South American mammals. It
was separated as the Paleocenozoic Megacycle.
Most of the mammals characterizing this cycle
are coming from Patagonia, and this record is
the one that afforded the most trascendental
evidence on the first steps of the history of
Sourh American mammals. With the exception of the Gondwanatheria Sudamerica
ameghinoi and the monotreme Monotrematum
sudamericanum from the Patagonian Early
Paleocene, all the coetaneous and later mammals were marsupials and placentals, and all
of them descendants of the Ancient Immigrants. For a long time one probable exception were the edentates, regarded as the only
placentals that could originated in South
America. The recent finding of earliest Paleocene Patagonian (Pascual and OrtizJaureguizar, 1991; Bonaparte, Van Valen and
Kramarz, 1993) and Bolivian (de Muizon,
1991) mammals failed to record them. This
apparent absence become very suggestive if
we take into account that they also were not
recorded in the only representative South
American Mesozoic mammal-bearing locality
(Patagonia. See II.4.1.). Likewise suggestive
is that the oldest record of an edentate is from
middle Paleocene beds of eastern Brazil
(Itaboraian SALMA), and pertaining to a
dasypodid (See Scillato-Yand, 1986). Curiously
enough, correlative mammal-bearing beds from
central Patagonia (likewise assigned to the
Itaboraian SALMA) didn't record dasypodids
nor any other edentate (Bond et al., 1995). To
make even more suggestive this apparent absence of edentates during the late Cretaceous
143
and the earliest part of the "age of mammals",
they were scarce in late Paleocene and Eocene
beds, and almost restricted to the Cingulata
(Pascual, 1970; Pascual y Odreman Rivas,
1971; Marshall, Hoffstetter and Pascual, 1983;
Pascual, Vucetich, Scillato-Yan6, and Bond,
1985; Scillato-Yane, 1986; Bondesio, 1986).
According to Storch (1984;1986) the edentates
were in West Gondwana before the South
American and African continents drifted apart,
and already diversified. The final separation
of both continents succeeded about 110 miilion
years ago (See 11.3.). On this account, the
record of a Myrmecophagidae edentate
(Eurotamandua joresi Storch, 1981) in middle
Eocene beds of Germany has trascendental
implications. Storch regarded this edentate as
an immigrant from Africa through the Tethys
Sea. If so, the myrmecophagids were in Africa at least by the middle Eocene, that is to
say, earlier than in South America since the
first record was in Patagonian beds of late
Oligocene or early Miocene in age (Carlini,
Scillato-Yane, Vizcaino y Dozo, 1992). This
circumstantial evidence suiggests that the edentates didn't originated in South America, as
long thought. If this were the case, none of
the land mammals recorded in the South
American continent from the middle Paleocene
on are strictly native. In such a case, they were
descendants of immigrants from other continents. Thus, the allocthonous origin of most
of the Cenozoic mammals sustained by
Simpson (See 11.2) appears reinforced by the
late findings.
The long First Stage of Isolation was interrupted by the entrance of rodents, whose first
record was in Chilean beds dated as about 35
million years in age (Wyss et al., 1993). On
the one hand, this record indicates that there
existed then an intercontinental coonection.
There exists consensus that it was with Africa
(See Lavocat, 1993; Wyss et al., op.cit.). On
the other hand, the remaining mammals associated with this first rodent represent a quite
distinct community, indicating that a severe
turnover had occurred. This change is compatible with likewise severe environmental
changes, which appear as related to trascendental geodynamic phenomena. These were
associated to the complex diastrophic "Fase
144
Inca", which appears responsable of the circunstancial and selective connections that permitted that immigration. Apparently, the same
connection permitted the immigration of the
primates, either those ancestral to the Platyrrhini or already differentiated as such (See
Aiello, 1993). It has to be taken into account
that its record is later respect to that of rodents
(Hoffstetter, 1969; Marshall and Sempere,
1991), suggesting that probably both represent
heterochronic immigratory events.
The immigration of rodents and primates
marks Simpson's second faunal strata, that he
named "Old Island Hoppers" because of the
supposed way they used to disperse, to him
from North America. This immigratory event
is the most ostensible phenomenon that marked
the rupture of the isolation, but otherwise it is
one more phenomenon of the turnover succeeded by then, initiating in such a way the
Second Stage of the Isolation. The same
multitivariate analysis mentioned above, distinguished this cycle as the Mesocenozoic
Supercycle. The taxonomical and ecological
changes in the composition of mammal communities are quite drastic. Among others, it
records the absence of most low crowned herbivorous taxa, which dominated those faunas
representing the First Stage of the Isolation.
For the first time, there began the dominance
of the high crowned (hypsodont) herbivores,
the appearance of which has been associated
to climatic shifts, the establishment of seasonally drier and more open habitats, and more
abrasive diets ( Pascual, 1984a; Pascual,
Vucetich, Scillato Yand, and Bond, op.cit.;
Pascual and Ortiz-Jaureguizar, 1990). This is
a climatic-environmental trend that, with some
fluctuations (e.g., during the early Miocene
Pan-Santacruzian Cycle), increased throughout
the Cenozoic.
The so initiated Second Stage of the Isolation was also interrupted during its last steps,
first by an emigratory event, and later by the
first American interchange (Simpson's "Late
Island Hoppers"), both premonitory of the
Great American Biotic Interchange succeeded
with the outcome of the Panamanian Bridge.
The emigratory event had the megalonychid
edentates and the caviomorph rodents as ac-
Rosendo Pascual
tors, and the West Indies as destination. Their
remains had originally been recorded in late
Pleistocene and Recent strata (See Paula Couto,
1979). However, recently, a megalonychid
edentate was recorded in Cuba, in beds dated
as 17,3-18,5 million years in age (MacPhee
and Iturralde-Vinent, 1994), that is to say,
representing the early Miocene. However,
based on paleogeographical reconstructions,
those workers admitted that these edentates
could immigrated earlier, during the Oligocne
(Fig. 6). This date coincide with our inference
based on the phylogenetical relationships of
the Pleistocene West Indian representatives
with the extinct South American ones (Pascual,
Vucetich, and Scillato-Yane, 1990). In turn,
those mammals that initiated the American
interchange, known by Simpson as Late Island Hoppers, and recently by Webb (1985)
as "heralds", were tardigrade edentates
(Mylodontidae and Megalonychidae) from
South America, and raccoons (Procyonidae)
from North America. This interchange was
dated as succeeded betweem 8 and 9 million
years ago (See Marshall et al., 1979; Marshall,
1 985; Webb, 1985). These reciprocal migratory events appear to be concomitantly related
to global geodynamic phenomena and climaticenvironmental changes. Actually, the regressive phase of that late Miocene marine transgression that covered wide regions to the east
of the Andean Cordillera, between the incipient Caribbean Sea and northern Patagonia
(Rasãnen et al., 1995) (Fig. 7), built up a wide
denudation plain on which there began a new
Fig. 6:Paleogeographic reconstruction of the Caribbean
area for the Oligocene (Based on MacPhee and
lturralde-Vinent, 1994). AR = Aver Ridge; BR
= Beata Rise; C = Cuba; CY = Cozumel-Yucatan
Ridge; H = Hispaniola; LSA = Lesser Antilles;
PR = Puerto Rico; VI = Virgin Islands.
145
LATE CRETACEOUS-RECENT LAND-MAMMALS
continental sedimentary cycle (Pascual, 1984b;
Pascual, Vucetich, Scillato-Yane and Bond,
op.cit., Pascual and Ortiz-Jaureguizar, 1990;
Alberdi, Ortiz-Jaureguizar and Prado, 1995).
We distinguished the second half of this cycle
as "edad de las planicies australes" (Pascual y
Bondesio, 1982). The sediments representing
this cycle just record the first "heralds", and
so distinctive mammal communities that for
long time the Argentinian paleontologists recognize their mammals as "araucanian mammals". They characterize a well defined cycle
of the South American land mammal history,
distinguished as Eopampian Cycle by
Kraglievich (1930, 1934). Araucanian Faunal
Type by Simpson (1940), and Araucanian
Subcycle of the Pan-Araucanian Cycle by
• Ortiz-Jaureguizar (1986. See also Pascual and
Ortiz Jaureguizar, 1990, and Alberdi, OrtizJaureguizar and Prado, op.cit.). Beds bearing
these mammals are well exposed on the eastern slopes of the northwestern Cordillera. From
low to up, these beds show an increase
Fig. 7: Estimated range of the Late Miocene-Early
Pliocene marine transgression (marine and
semimarine beds) (Based on Räsäen et al., 1995)
frequence of torrential deposits, compatible
with an increased seasonality. Suggestively
these lithogenetic trend appears as concomitantly related with the entrance of the Late
Island Hoppers, and on the whole with a notable ecological turnover. This is compatible
with the onset of mesic up to arid habitats,
apparently not very different to those characterizing the climax of the later Great American Biotic Interchange (Webb, 1985). Progressively, during this span the grazer herbivorous increased, and the tree dwellers and sylvan mammals disappeared from the southern
regions. On this account, the platyrrhine monkeys offer a very eloquent example since they
have been recorded anymore from the late
Miocene up to the latest Pleistocene, not even
in those regions where they presently live.
Importantly enough, this ecological turnover
was closely related to a likewise trascendental
taxonomical turnover. The native "ungulates"
were not more the predominant cursorial grazers since they were "replaced" by huge ground
sloth and cingulate edentates, and by gigantic
cursorial rodents, these latter the biggers ever
known (Pascual, Vucetich, Scillato-Yane, and
Bond, op. cit.; Vucetich, 1986,1989). In apparent concomitance, the depredator marsupials changed a lot. The most carnivorous
borhyaenids were "replaced" by huge didelphoid marsupials, and by a new lineage of
saber-toothed marsupials, the Thylacosmilidae
(See Goin y Pascual, 1987). This latter success has been distinguished by Goin (1989) as
"endoreplacement", and followed by the decrease, and even extinction of many lineages
of native "ungulates". This fact demonstrates
that the original hypothesis of the "competitive displacement" of the carnivorous marsupials and most of the native ungulates by the
i mmigrants holarctic counterparts cannot be
sustained anymore. This supposed competitive
displacement during the late Pliocene-Pleistocene was based on Arthur & Wilson's "Island Biogeography Theory (Marshall, Webb,
Sepkoski, and Raup, 1982). The record indicates that the extinction of the carnivorous
marsupials (Goin, 1989) and the majority of
the native "ungulates" (Ortiz-Jaureguizar,1989;
Bond, 1989) happened well before the entrance
of the holarctic ungulates and carnivores.
-
146
11.4.4. End of the isolation: outcome and
effects of the interamerican connection.
According to the geophysical and non-mammalian biotic evidence, about 4 million years
ago there existed a geological interamerican
connection (See 11.3). However, the first
mammal record has been dated as somewhat
less than 3 million before Present (Webb.
1985; Tonni et al., 1992). In terms of the
Present biogeographical situation, this event,
known as the Great American Biotic Interchange (Stehli and Webb, 1985), is of most
importance. The interchange so initiated was
by "walking" along the newborn "Panamanian Bridge". The participant mammals were
recognized by Webb & Marshall (1982) and
Webb (1985) as "legions". According to Webb
(op.cit.), consequently related to the great
Cordilleran orogenies of the late Tertiary (we
correlated to the Diaguita phase of Andean
Orogeny, Pascual, 1984b; Pascual, Vucetich,
Scillato-Yane, and Bond, 1985), and to the
rain-shadows and subhumid savanna corridors
extended northward through South America,
and southward through North and Central
America, there began an active interchange,
which involved groups representing predominantly savanna-adapted mammals. A continuous corridor of such vegetational formations
as thorn scrub or seasonally arid woodland
extended throughout the present humid tropical forest. Such present vegetational formations in both North and South America are
now separated by at least 1.700 km of humid
tropical forest. The abiotic and biotic evidence
indicate that the final link between American
mesic equatorial biotas was established by
latest Pleistocene. To explain the known success of the American mammal interchange, it
is necessary to postulated that the habitats set
on that continuous corridor had to be composed by a homogeneous floristic assemblage.
Obviously, there had to occurred a previous
American floristic interchange. If we accept
this as a sine qua non condition, it is quite
probable that this floristic interchange was
initiated already by the end of the Tertiary,
when the Late Island Hoppers announced the
beginning of the interchange. The continuous
corridor was the final link of such habitats.
Notwithstading, the habitats set on between
Rosendo Pascual
both Americas were not totally uniform. As
Patterson and Pascual (1972:257) pointed out,
"...the North American tropical peninsula was
evidently a fact of ancient geography, and
there is convincing Recent and suggestive
Tertiary evidence that its mammals arc and
were distinctive to some degree within the
continental fauna as a whole" (See also Webb,
1989). Anyhow, a moving mosaic of opencountry habitats, as that postulated by Webb
(1985:378), accords with the fluctuating global climatic changes recognized for the Late
Pliocene-Pleistocene span (Frakes, 1979).
During the Great American Mammalian Interchange, there occurred a rapid diversification in relation to the preceding times, having
the North American immigrants groups as the
primary contributors (Patterson and Pascual,
1972). For the first time, the immigrants reverted the long standing trend towards
hypsodonty, establishing an absolute predominance of the low-crowned ungulates. Except
the native and impoverished Notoungulata and
Litopterna, all the remaining ungulates pertained to immigrant families, and all of them
were brachydont to mesodont mammals
(Gomphotheriidae, Tapiridae, Equidae, Tayassuidae, Camelidae and Cervidae). This trend
is presently increased because the putative
latest Pleistocene Megafaunal extinction led
to the disappearance of the last native "ungulates": 1-2 genera of Macraucheniidae
Litopterna, and 1 genus of the Toxodontidae
Notoungulata (Bond, 1989). That is to say that,
all the ungulates and carnivorous (with the
exception of some few exclusively carnivorous marsupials) that overcome the Megafaunal Extinction and are presently living in South
America are descendant of North American
immigrants, and most of them descendants of
those arrived during the Great American Biotic Interchange.
Obviously, the notable extinction of the
native mammals succeeded by the end of the
Tertiary, and its subsequent "replacement" by
the North American immigrants, drastically
had to change the economy of the South
American land mammal communities. A new
scenario was onset. For example, the strategy
of the newcomer placental predators had to
LATE CRETACEOUS-RECENT LAND-MAMMALS
147
influence the behavioral habits and evolutionary trend of the relictual native preys, among
them the notoungulates and litoptperns, and
the varied xenarthran edentates, e.g., tardi-
acterizing the Neotropical Region. A late Pleistocene and Holocene moving mosaic of open
habitats, as that postulated by Webb (op.cit.)
to explain the onset of the late Pliocene-late
grades, cingulates and anteaters. However,
we have not be able yet to detect such influence.
Pleistocene Great American Biotic Interchange, could also account for the relatively
recent entrance of the Sciuridac, Heteromyidae and Geomyidae rodents, the lagomorph
Leporidae, and the Soricidae insectivores.
Actually, arid periods alternated with warmer
and wetter ones have been recognized by the
latest Pleistocene, Holocene, and even quite
recent, record of mammals (Tonni, 1985).
These late climatic swings probably built up
transitory habitats that permitted such late
i mmigratory events.
Two very important events subsequent, or
contemporary with, the late Pleistocene
"Megafaunal Extinction" input the final cachet to the uniqueness of the present Neotropical Region: (1) entrance of the Sciuridae,
Heteromyidae, and Geomyidac rodents,
Leporidae lagomorph, and Soricidae insectivores; and (2) onset of the intertropical rainy
forest. It has to he taken into account that
within the occidental region of the Amazonas
basin were recorded megamammals compatible with open-country habitats, contrasting
with the present intertropical rainy forest
(Rancy, 1989). The present Panamanian region also became a rainy forest by the end of
the Pleistocene (Webb, 1978, 1985). That it
to say that the present wide-ranged intertropical rainy forests are vegetational structures
differentiated quite recently.
Synthetically, both the Great American Biotic Interchange, initiated by the end of the
Pliocene, and the latest Pleistocene Megafaunal Extinction built up the present scenario
characterizing the Neotropical Region. Before
these trascendental events, the sustained isolation of the South American continent had
led to the differentiation of mammal communities quite distinct to any other communities
living then all over the world. The extinction
of most of the native taxa differentiated in
such a manner, followed by the Great American Biotic Interchange, mark the end of such
so long sustained distinction. Consequently,
both American mammal communities became
relatively alike, much more than at the present time. This Pleistocene-Present change was
already noted by Darwin (1839). Curiously,
the survival in South America of many mammal taxa that didn't overcome the Megafaunal Extinction in North America accentuated
the differences, establishing the present differences between both Americas. Among them
are the Tapiridae and Camelidae, which represent two of the most peculiar mammals char-
Webb and Marshall (1982) estimated that at
the beginning of the Great American Biotic
Interchange only 5% of the genera living in
South America were immigrants from North
America. We estimate that at least 50% of the
present genera descended from North American immigrants. On this account, it has to be
pointed out (Patterson and Pascual, 1968, 1972;
Hershkovitz, 1972) that the present record of
land mammals indicate that Central America
acted no only as an "stepping stone" between
both Americas but as a "staging area", or "centre of origin", for many taxa that emigrated to
South America (Webb,l989). This is particularly true for the placental carnivores and ungulates. Those climatic and related enrironmental changes mentioned above, together with the
role of "staging area" of Central America,
appear to count for the recognized asymmetry
of the Great American Biotic Interchange
(Webb, 1985).
The study of South American fossil land
mammals affords eloquent examples indicating how distinct could be the meaning of "autochthonous" and "allochthonous" under a
historical perspective. Besides, it demonstrates
that the understanding of any macroevolutionay process requires, as a sine qua non condition, the previous knowledge of the related
historical biogeographic events. As Rosen
(1981) stated, "...the study of cladistic congruence between the earth and its life will be
the next revolution of the earth science —an
integrated natural history of geological and
biological systems"
148
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