The only known, significant, fossil _Proechimys_ comes from deposits in
the limestone caves of Lagoa Santa, Minas Gerais, Brazil. These
deposits, of Late Pleistocene or Recent age, were extensively studied by
P. W. Lund and the results published in a series of French and Danish
papers. F. Ameghino (1934:110) studied another fauna from a deposit of
similar age in the cave of Iporanga, São Paulo, Brazil. _Proechimys_ is
recorded in his account under the inclusive specific name _fuliginosus_.
The molariform teeth of the fossil described by Lund (1841:pl. 21, fig.
14) shows its close relationship to the living form _P. s._ _elegans_
(Lund) which still inhabits the same region. It belongs in the more
specialized subgenus _Trinomys_ which seems to have been derived from
_Proechimys_. _Trinomys_ has the main fold in the molars always greatly
developed and the fold tends to set apart one lamina in the occlusal
surface. The Lagoa Santa fossil, like some specimens of the living
subspecies, has a small main fold in P4. However, the main fold is large
in all upper molars and in the lower molariform teeth which are notably
specialized in the extreme reduction of the number of counterfolds to
only one.
One hypothesis concerning the evolution of the genus is that a more
primitive group of _Proechimys_ lived in all of the Central Plateau of
Brazil in the Pleistocene Time. The climatic conditions at that time
might have been such as to support large forests but, since the
Pleistocene, these climatic conditions may have changed from humid to
the present drier conditions, which support the dominant, savanna,
floral climax. Actually the extinct fauna from the caves includes
animals which have disappeared from the area and now live only in more
humid areas, as for example _Myocastor_, which has shifted to the
lowlands to the west and south.
Possibly climatic changes were responsible for the faunal shift from the
region that is now a plateau in Central Brazil. This climatic change may
have resulted from the gradual uplift of the eastern part of the
continent. This uplift prevents part of the trade winds which come from
the east from carrying the same amount of moisture inland as they did
previously. In fact, the Andean revolution, even if it occurred as late
as Late Tertiary, would have had no perceptible influence on the
amount of water precipitated on the more eastern parts of the continent.
Oliveira and Leonardos (1943:617) point out that after a Cretaceous
submersion of the central part of Brazil, there was a general uplift.
The authors (_op. cit._:689) mention the presence of continental
Cretaceous deposits in the Central Plateau of Brazil, in support of
these changes, and state that "pelo menos em certas zonas do litoral a
elevação do continente prolongou-se até o Pleistoceno."
Berry (1942:373) concluded, among other things, that there was a
southward extension "in South America of equatorial floras in the lower
Miocene," and (_op. cit._:372) that ... "east of the Andean Axis in
the south temperate zone there was a normal mesophytic flora ... instead
... of present day large steppes."
My idea is that a tropical forest still covered the Central Plateau of
Brazil in (early?) Pleistocene times and that populations of
_Proechimys_ of a primitive type, similar to _P. g. steerei_, for
example, lived in that extensive forest-climax. The gradual uplift of
the plateau, however, gradually brought about drier conditions in this
region. As a result a large cliseral change was initiated, which shifted
the forest-climax to the more humid eastern escarpments and lowlands
that were gradually being developed, while the savanna climax was being
established on the plateau. Eventually the effect of the decreasing
moisture was locally accentuated by the erosion of the sandstones
(Oliveira and Leonardos, 1943:690) in northeastern Brazil, thus
depriving it of a natural reservoir of rain water. An arid belt was
developed which now constitutes an efficient geographic barrier to the
distribution of many kinds of animals.
One marginal species may have shifted eastward with the forest-climax
to effect the Recent distribution. The eastern species became completely
isolated from the main group, accumulated mutations, and evolved into
the subgeneric type _Trinomys_. The generic trend that gave rise to
_Trinomys_ probably remained more stable as far as supraspecific changes
are concerned. The lack of barriers in the distributional area of the
original group favored the dispersal and submergence of mutations and,
therefore, there was but little further supraspecific evolution. The
speciation in both subgenera finally resulted from gradual
differentiation of varying populations since they show combinations of
the generic biotypes and possess few truly qualitative characters.
The cliseral changes in the Central Plateau, which developed the dry
belt, a barrier, might explain the evolution of a few more supraspecific
groups of mammals, as indicated by the presence of similar forms in the
Amazonian region and in Southeastern Brazil. Among these _Echimys_ and
_Phyllomys_, in the same family with _Proechimys_, show differences that
are parallel to those observed in _Proechimys_. One of these parallel
changes is the increased lamination of the cheekteeth. Although
_Echimys_, from the Amazonian region, has upper molariform teeth with
the four laminae fused, _Phyllomys_ has the four laminae completely
separated.
None of the genera known from the Upper Oligocene and Miocene of
Argentine deposits seems to be directly ancestral to _Proechimys_.