THE DESEADO FAUNA
The exposure of the Deseado, which the Amherst party worked, yielded
293 specimens, each presumably representing an individual. (There were
besides these a few that were indeterminate and are not therefore
included.) The consideration of the fauna as a whole suggests certain
ideas as to the country in which the animals lived, and also certain
comparisons with the fauna of the preceding and later faunas.
The first striking feature is the presence of so many excessively
large animals, as _Asmodeus_, _Parastrapotherium_, and _Pyrotherium_,
in each case forms larger than a rhinoceros. Further than that they
are in each case the largest members of their family, even larger than
the representatives in the later Santa Cruz. This would indicate a
period in which living conditions were at a high grade, suggesting both
abundance of food and a moderate climate.
The following table will give a good idea as to the range of species,
and their relative abundance in the fauna, also a suggestion as to
the class of food they used; and from that an idea as to what sort of
country they occupied:
PER NUMBER SPECIES FOOD COUNTRY
CENT
3 Hegetotherium shumwayi
7 Prosotherium garzoni
17 Prosotherium triangulidens
1 Eutrachytherus grandis
4 Eutrachytherus spegazzinius Grass, bark Plains
and browse
1 Isoproedrium solitarium
2 Phanophilus dorsatus
4 Argyrohyrax proavus
1 Plagiarthrus clivus
14% 40 TYPOTHERIA
1 Protheosodon coniferus Grass Plains
15 Notodiaphorus crassus
6% 16 LITOPTERNA
19 Rhynchippus equinus Grass Plains
7% 19 RHYNCHIPPIDAE
44 Leontinia gaudryi Browse Brush
15% 44 LEONTINIDAE plains
2 Proadinotherium leptognathus Grass or Plains
2 Coresodon scalpridens Browse
1% 4 NESODONTIDAE
7 Asmodeus osborni Browse ?
2% 7 HOMOLADONTIDAE
6 Parastrapotherium holmbergi Browse ?
2% 6 ASTRAPOTHERIDAE
11 Pyrotherium sorondoi Browse ?
4% 11 PYROTHERIA
55 Cephalomys arcidens
22 Cephalomys plexus
19 Cephalomys prorsus Hard Open
vegetation country
3 Asteromys prospicuus
1 Scotamys antiquus
1 Eosteiromys medianus
1 Litodontomys chubutensis
35% 102 RODENTS
9 Proeutatus lageniformis
1 Prozaedius planus Insects Open
and leaves country
2 Prozaedius depressus
3 Proeuphractus setiger
2 Peltephilus undulatus
1 Palaeopeltis inornatus
5 Indeterminate
8% 23 EDENTATA
1 Plichenia lucina
1 Epanorthus chubutensis
5 Callomenus praecursor Insects ?
and flesh
2 Pharsophorus tenax
1 Pharsophorus mitis
3% 10 MARSUPILAIA
3% 11 BIRDS Open
country
100% 293
In our collection, all from one point, there are thirty-nine different
species. Beside these Ameghino has described a considerable number of
species, some of which in time will probably turn up at our locality;
but others and I think the majority will be found to be representative
of other localities which he worked. It is to be expected that a
difference of locality will make a little difference in the fauna.
Further I expect that no two localities represent exactly the same
period of time, though they may do so approximately; but some of these
local deposits must have been begun earlier, and others probably lasted
to a later period. Thirty-nine species of mammals and land birds is
a fairly varied fauna for one spot; and the time element involved in
laying down the 50 feet which separated the bottom from the top of the
Deseado deposit is not probably very long; for the material of which
the deposit is composed is of a character which would have been laid
down fairly rapidly.
Of this fauna only 8 per cent belongs to the edentates; and if any
element were disproportionately represented it would be this one, for
the armadilloes have in addition to the skeleton the hundreds of tiny
plates of the carapace, and several of the forms are represented by
one or two plates only. When compared with the condition in the Santa
Cruz this 8 per cent is strikingly small, for in that later bed, fully
50 per cent of the finds represent edentates. Are the _Edentata_ just
originating? Or, was the country less favorable to their habitation?
The edentates which we did find are only slightly less advanced
in their development than those of the Santa Cruz. Also, though
infrequent, all of the families of the Santa Cruz are represented.
It would seem therefore that the origin of the edentates was much
earlier than the Deseado; and this relative paucity of edentates is
also characteristic of the Casamayor and Astraponotus beds; but they
are there, and in considerable variety, though small numbers. It
would seem then that the country for some reason was less adapted
to edentates, and that in some other part of South America they were
flourishing and evolving.
In the Deseado the rodents appear for the first time in South America.
They are all _Hystricomorpha_ and in a relatively primitive stage of
development, but they are typically developed already. Did they migrate
in from some other locality, or were they evolved on the spot? Ameghino
believed that they were developed from some such form as _Promysops_ or
_Propolymastodon_ of the Casamayor, and that these forms were ancestral
to rodents all over the world. If my interpretation of the age of these
beds is anywhere near correct, this last at least is impossible, for
in North America and Europe typical rodents are present in the Eocene.
Then as to even the hystricomorphs being developed in Patagonia, I am
very skeptical, for the material offered in evidence of this is very
insufficient, especially in the region of the incisors; and may be
interpreted in other more probable ways. I am confident that either
just before the beginning of the Deseado, or at the beginning, the
rodents of these beds migrated, either from some other continent, or
at least from some other section of South America into this Patagonian
region.
Some idea of the type of country and the climate of the Deseado period
in Patagonia may be obtained by analyzing the fauna as to the character
of its teeth as indicative of the food; and by studying the feet as
indicative of the ground on which they were used.
The _Typotheria_ with their chisel-like front teeth, lack of canines,
and their permanently growing grinders evidently ate a hard type of
vegetation. Deep and permanently growing molars are characteristic of
the eaters of grass, a form of vegetation which is especially hard on
the grinding teeth, on account of the silica in the stems and leaves.
This however would scarcely necessitate the development of permanently
growing incisors. They are typical of gnawing animals, eaters of bark,
twigs, and possibly also leaves, the wood and bark being also a hard
type of vegetation to grind. In the case of these forms I believe they
were feeders on grass and bark. Their feet are developed either for
running or hopping and would suggest hard ground for their habitat.
The _Litopterna_ are typically plains animals, paralleling in their
development the horses. The cropping teeth and the grinding molars
become progressively longer. The limbs are progressively elongated,
the animals walking more and more on the tips of the toes. With this,
the metapodials especially and the other limb bones to a less degree,
are progressively lengthened. At the same time the side toes are
progressively reduced. The teeth indicate grass eating; the limbs life
on the plains.
The _Rhynchippidae_, while not as advanced as the _Litopterna_,
show cropping front teeth, and the molars developing in depth. The
locomotion is semidigitigrade, the feet small, and the number of toes
reduced to three. They too must be interpreted as grazing or grazing
and browsing animals, living on hard ground.
The _Leontinidae_ are heavier forms, but with much the same features as
Rynchippidae, though less specialized. On account of the broad upper
molars and the less specialization of the dentition, I should feel that
these forms were browsers and lived among bushes, but the feet were
three-toed and semidigitigrade and they seem to have walked on hard
ground.
The _Nesodontidae_ belong to the same type of adaptation as the
foregoing family, but have the grinding teeth more complicated,
indicative of a more advanced adaptation to hard vegetation. The feet
were also adapted to hard ground.
The _Homalodontotheria_, the _Astrapotheria_, and the _Pyrotheria_
were all very large animals, known mostly by their dentition,
which is adapted to browse. Whether they lived on soft or hard
ground is not known, as the feet are not known in any case but the
_Homalodontotheridae_, where they are five-toed and adapted to soft
ground. Such large animals were probably inhabitants of some river bank.
The rodents do not contribute much in the determination as to the type
of the country, for they could have lived in the open or in the wooded
country, but their relative abundance is rather typical of open country.
The birds are all running birds, and indicative of the country having
been an open one.
Of our fauna 11 per cent were flesh or insect eating, and for the
purpose of determining the type of country may best be omitted.
The rodents could have been either forest or open country forms.
Of the remaining 54 per cent, the typotheres, the litopternas, the
_Rhynchippidae_, the _Leontinidae_, the nesodonts and the birds (46
per cent) were distinctly adapted to live on hard ground; the other
8 per cent being evidently suited to living near a river. All 54 per
cent ate either grass or browse. The litopternas are grass eaters;
the typotheres were specialized to eat grass or bark; nesodonts,
_Leontinidae_, and _Rhynchippidae_ are grass and browse eaters. Even
the _Pyrotherium_ has a pair of gnawing tushes. The picture arising
from these considerations is a bush covered prairie, a country not
unlike the upland bush pampas of Patagonia today.
There is not an aquatic form (fish or turtle) in the whole list, so
it is evident that the stream which deposited these Deseado beds was
not abundantly inhabited. To me it looks like so many of the streams
in an arid country, dry through a considerable part of the year, and
so uninhabited. In the whole list I see nothing to indicate forests
or swamps. The arid bush covered plain alone seems to suit the
requirements.
As I see this fauna it is composed of several distinct elements,
representing different invasions and an element which arose in situ.
The reasons for the affinities expressed in the different groups will
be found in the introductory paragraphs of the systematic discussion of
each group.
The _Notungulata_, including the _Typotheria_, the _Toxodontia_, the
_Litopterna_, the _Homalodontotheria_, and the _Astrapotheria_ are
a group with apparently a common ancestry. In Patagonia they have
specialized into the various subdivisions as we find them in the
Deseado. This group was in Patagonia as early or earlier than the
Casamayor. Their relationships appear to me to be with the _Hyracoidea_
which are generally credited with originating in Africa.
The _Pyrotheria_ are related to the early elephants which also arose in
Africa, but it seems to me that this form came to Patagonia at least at
a later period, making its first appearance in the upper part of the
Astraponotus period. Ultimately the elephants and _Hyracoidea_ had a
common origin in Africa.
The _Rodentia_ are all hystricomorphs and appear in South America for
the first time in the Deseado. They also occur in the Oligocene of
Europe and the Fayum of north Africa. They never reached North America
so must have come to South America by some southern route.
The Edentata are an element of the Casamayor fauna and as there is no
evidence of their originating anywhere else it would seem that they
were indigenous to South America, where they later flourished and
developed the greatest variety and profusion of numbers.
The group of marsupials is an element the origin of which presents
a most difficult problem. Some belong to the opossum series which
could well have been developed from some remnant of the Mesozoic
marsupial fauna that had a world wide distribution; but the presence
of diprotodonts, which are characteristic of Australia, and of the
_Borhyaenidae_ which are closely related to the _Thylacinidae_ of
Australia, suggests a migration from that continent as late as
Tertiary times; but to my mind this involves a connection which is
most too difficult to postulate. There is no evidence that they came
to South America in company with other faunas, for they have not been
found associated with any other fauna outside of Southern Patagonia.
The explanation of the affinities of the Patagonian marsupials with the
Australian marsupials is a problem which is not yet in position to be
settled.
The birds probably came from Africa with the invasion of the ancestors
of the Notungulates.
The idea of an invasion from Africa in Upper Cretaceous times, and
possibly another at a later time is correlated with the other evidence
of a land bridge between these two continents, as deduced by students
of other groups.
Eigenmann, working on the freshwater fishes,[6]
Lydekker, studying the hystricomorphs,[7]
Von Ihering, studying the freshwater mussels,[8]
Ortmann, studying the freshwater crabs,[9]
not to mention several others studying mullocks, insects, plants, etc.,
have all postulated a land connection from Brazil to northern Africa
during Cretaceous time to explain the distribution of their various
groups. The divergence is in the time when this land bridge sank,
some believing it to have lasted into Tertiary times, most feeling
that it sank in Upper Cretaceous times. Another body of evidence is
presented to show that a land bridge connected the West Indies with
the Mediterranean regions.[10] There was presumably but one such
transatlantic connection. Its position further to the south would seem
to me to explain the distributional facts found in the West Indies,
but the striking resemblances between the faunas of Africa and South
America require a connection from the South Upper American Continent
and Africa.
[Footnote 6: Princeton Expeditions to Patagonia, vol. 3, p. 310,
1905-11.]
[Footnote 7: History of Mammals, p. 127, 1896.]
[Footnote 8: Archhelenis and Archinotis, p. 125-145, 1907.]
[Footnote 9: Proc. Amer. Philos. Soc. Philadelphia, vol. 41, p. 350,
1902.]
[Footnote 10: See Scharff, Distribution and Origin of Life in America,
Ch. 11, 1912.]
It was along this land bridge which the ancestors of the _Notungulata_
traveled, and when in South America, due to their isolation, developed
all the peculiarities of the group. This must have been not later than
the latter part of the Cretaceous.
Either this bridge remained until into the early Tertiary; so the
_Pyrotheria_ and _Hystricomorpha_ made their migration later, or these
two groups did not reach the isolated Patagonian section until later
than the first invasion. I am inclined to believe in the migration
being at a later period. This bridge does not explain the presence
of the edentates, for which there is every reason to believe that
they developed in situ. The Marsupial invasion must have been from
some other direction, or their presence in Africa has not yet been
discovered.