HISTORY OF THE PROBOSCIDEA
Utterly foreign as the elephant-tribe appears to be to present-day North
America, it was a very conspicuous element in the fauna of that continent
from the middle Miocene to the end of the Pleistocene, and in the latter
epoch it spread over South America also. Like so many others of the
mammals which have, from time to time, flourished in the Americas, the
elephants and their allies, the †mastodons, were immigrants from the
Old World, and, until comparatively lately, the region of their origin
was a complete mystery. They appeared suddenly and unheralded and at
approximately the same time in Europe and North America and nothing is
known from preceding geological formations of either continent which
could with any plausibility be regarded as ancestral to them. The mystery
was dispelled by the discoveries of Dr. C. W. Andrews in Egypt, which
demonstrated that these strange and huge beasts had originated in Africa
and had migrated thence through Asia to Europe, on the one side, and to
North America on the other.
The proboscideans occupy a very isolated position among the hoofed
mammals, and in structure they display a curious mingling of high
specialization with an extreme conservatism of primitive characters,
the specialization being exemplified in the teeth and head and the
conservatism in the body and limbs, very much as in the †oreodont family
of artiodactyls (p. 382). The most conspicuous of the external features
in the order is the long trunk, or proboscis, which gives its name to the
group, and is a great prolongation of the nose, with the nostrils at the
end and a finger-like tip, which can be used to pick up minute objects.
[Illustration: FIG. 223.—Molar of the African Elephant (_Loxodonta
africanus_) showing the oblique mode of wear. Heavy black lines indicate
enamel, enclosing areas of dentine, cement covering the whole tooth.]
In the true elephants the dental formula is: _i_ 1/0, _c_ 0/0, _p_ 0/0,
_m_ 3/3, × 2 = 14, though this formula is misleading, to the extent that
the milk premolars, three in number in each jaw, take the place and
perform the functions of the premolars, thus adding 12 to the effective
number of teeth. The single upper incisor on each side grows into an
immense tusk, which has enamel only on the tip, where it is speedily worn
away; the lower jaw is without incisors and there are no canines above
or below. The grinding teeth are very large and have a highly complex
structure and a most exceptional method of eruption on coming into use.
They are thoroughly hypsodont and each is composed of a large number of
high, broad and thin plates of dentine covered with enamel and the spaces
between the enamel ridges are filled with cement (see Fig. 47, p. 97);
indeed, the whole tooth is so thickly covered with cement that, when
unworn, it looks like a mere lump, with no ridges showing on the surface.
The teeth increase in size and in the number of component ridges from
before backward, and in the Indian species (_Elephas maximus_) the number
of ridges in the six grinding teeth, including the milk premolars, is:
4, 8, 12, 12, 16, 24. In the African Elephant (_Loxodonta africanus_)
the teeth are not so high and have fewer and thicker plates, the formula
being: 3, 6, 7, 7, 8, 10. The teeth do not succeed one another vertically
in the normal mammalian fashion, but come in successively from behind
and the series moves forward, so that the foremost tooth is pushed out,
when it is so worn down as to be of no further service. As these teeth
are very large and the jaws are relatively short, only one tooth on
each side, above and below, is in use at the same time, though part of
a second may also be involved. The movement of the successive teeth is
not directly forward, but oblique, an upper tooth coming forward and
downward and a lower tooth forward and upward. In consequence of this
arrangement the teeth are abraded obliquely, the anterior part first
coming into use, and, by the time a tooth is fully in place, the front
portion is worn down to less than half the height of the hinder part. All
of these peculiarities in the dental system imply a very high degree of
specialization and a notable difference from other mammals.
The skull is equally specialized, as is indeed required by the character
of the teeth and the development of the long and heavy proboscis. The
premaxillæ are converted into sheaths for the great tusks; the nasals
are extremely abbreviated and the anterior nasal opening is shifted to
the top, directly above the posterior opening, so that the nasal canal
passes vertically downward through the skull. All of the bones forming
the cranium are enormously thickened and at the same time lightened by
the formation of an extensive system of communicating sinuses, and thus
the brain-chamber is, as it were, hidden away in the middle of the huge
mass of the skull. This explains the difficulty of killing an elephant
by shooting it in the head; the shot must be so directed as to reach the
brain, which requires knowledge and skill.
The neck is short, the body long and extremely massive, the tail of
moderate length. The shoulder-blade is very large and has a prominent
metacromion given off from the spine; the hip-bones are immensely
expanded in correlation with the breadth of the thorax and abdomen. The
limbs are long, massive and columnar, their upper segments, especially
the thigh, are very long, so that the knee-joint is brought below the
body and free from it to the position of the hock-joint in the Horse;
hence, the hind leg appears to bend in the opposite direction from the
bend in the legs of ordinary quadrupeds, in which the true knee-joint is
concealed. The fore-arm bones are separate and, for most of its length,
the ulna is far heavier than the radius, a wide departure from the
proportions usual in hoofed animals. The femur has no pit in its head for
the round ligament and no third trochanter; the shaft is broad and much
flattened, having quite lost the normal cylindrical shape. The bones of
the lower leg are also separate, but the fibula, though stout, is very
much more slender than the ulna. The long bones have no marrow-cavities,
but are filled with spongy bone. The feet are extremely short and broad
and of columnar shape, the weight resting upon a pad of elastic tissue
and the small, nail-like hoofs are mere excrescences upon the periphery.
There are five digits in manus and pes, but not all of them have hoofs;
in the Indian and West African species the number of hoofs is five in
the fore foot and four in the hind, in the East African four and three
respectively. In the adult the skin is quite hairless, though the young
calf has a considerable quantity of hair.
[Illustration: FIG. 224.—Right manus of the Indian Elephant (_E.
maximus_).]
At present, the Proboscidea are restricted to the warmer parts of Asia
and Africa, where five species, four of them African, are recognized.
This is a very great reduction in the number of species and in the area
inhabited during the Pleistocene epoch, when they ranged through every
continent, except Australia, and were adapted to every climate from
the tropics to the shores of the Arctic Sea. Four distinct species of
proboscideans existed in Pleistocene North America, three elephants and a
†mastodon, though not all in the same areas, nor probably all at the same
time, their ranges both in time and space overlapping to a greater or
less degree, but not exactly coinciding in either respect.
[Illustration: FIG. 225.—Vertical section through the manus of the Indian
Elephant. _U_, lower end of ulna. _L_, lunar. _M_, magnum. _III_, third
metacarpal. _1_, _2_, _3_, phalanges. _E_, pad of elastic tissue. (After
M. Weber.)]
The first species was an immigrant, the northern †Mammoth (_Elephas
†primigenius_), which extended over the greater part of the northern
hemisphere, both in the Old World and in the New. This is the species
of which complete carcasses with hide and hair have been found in the
frozen gravels of northern Siberia, its structure and appearance being
thus almost as well known as those of any modern elephant. That the
†Mammoth was perfectly adapted to life in a climate of severe cold is
shown not only by the contents of the stomach, which are comminuted
fragments of present-day Siberian vegetation, but also by the dense coat
of woolly hair, covered by long, coarse outer hair, which afforded full
protection against the cold. The tusks, with considerable variation of
form, had a tendency to spiral curvature, curving first downward and
outward, then upward and inward; the grinding teeth were characterized
by their relative breadth and the numerous thin enamel-ridges which
traversed them. The number of these ridges was very variable in different
individuals, but may be expressed for the six successive teeth as
follows: 3-4, 6-9, 9-12, 9-15, 14-16, 18-27. The skeleton was more
like that of the Indian Elephant than of the other species, though
with a number of small differences in the skull. In size, the †Mammoth
was comparatively small, standing about nine feet six inches at the
shoulders. In North America its range was from Alaska southeastward
across the continent to New England.
The second species, the †Columbian Elephant (_E. †columbi_ Fig. 114, p.
198), was eighteen inches or more taller than the †Mammoth and rivalled
the largest existing elephants in stature; its huge tusks curved first
downward and then upward and inward, their tips crossing when full-grown.
The grinding teeth had fewer and thicker enamel plates than those of
the †Mammoth. The range of the †Columbian Elephant overlapped the
southern border of that of the †Mammoth, but was, on the whole, much
more southern; it crossed the continent from ocean to ocean and covered
nearly the whole of the United States, extending down to the southern end
of the Mexican plateau. The two species were very closely related and in
some cases are so intergraded that it is difficult to distinguish them;
the †Mammoth was an undoubted immigrant and the †Columbian Elephant was
probably a local North American variant of it, adapted to a somewhat
warmer climate. Nothing is known of the skin or hair in the latter
animal, but, from the fact that it was not a tropical species and was
exposed to very cold winters, it may be inferred that it had a hairy
covering of some sort.
The third species of elephant (_E. †imperator_) was older geologically
than the others, as it was more characteristic of the lower Pleistocene
and uppermost Pliocene; its range coincided with the western half of
the region covered by _E. †columbi_, extending far into Mexico, but not
occurring east of the Mississippi River. It was an enormous creature, the
largest of known elephants, with an estimated height of thirteen and a
half feet at the shoulder (Osborn). The grinding teeth had thicker and
more crumpled enamel plates than in either of the other species.
[Illustration: FIG. 226.—The American †Mastodon (_†Mastodon
americanus_), Pleistocene. Restored from a skeleton in the museum of
Princeton University.]
The fourth of the Pleistocene proboscideans of North America was a member
of a different and much more ancient genus, _†Mastodon_, which in the
Old World became extinct before the end of the Pliocene. The American
†Mastodon (_†M. americanus_) was thus a belated survival of an ancient
type, seemingly out of place even in the strange Pleistocene world,
which had so many bizarre creatures. The distinguishing characteristic
of the genus was in the simple, low-crowned and comparatively small
grinding teeth, which had three or four prominent transverse ridges,
covered with heavy enamel, and, usually, with no cement on the crowns.
As these teeth were so much smaller than those of the elephants, as many
as three on each side of each jaw might be in simultaneous use. In this
species there was no vertical succession of teeth, but in some of the
Tertiary †mastodons such succession has been observed. The long tusks
were directed nearly straight forward and were almost parallel, with but
slight curvature, the convexity downward. In the males there was a short
single tusk or, less commonly, a pair of such tusks, in the lower jaw,
which were probably not visible externally; these were the vanishing
remnants of an earlier stage of development, when the †mastodons had a
fully developed pair of lower tusks, nearly as large as the superior pair.
[Illustration: FIG. 227.—Last lower molar of the American †Mastodon.]
The skull, while essentially proboscidean, was yet much lower and flatter
and less dome-like than in the elephants; the thickening of the cranial
bones was less extreme. The remainder of the skeleton differed so little
from that of the elephants as to require no description. In size, this
species about equalled the †Mammoth, the larger individuals measuring
nine feet six inches at the shoulder. Remains have been found which prove
that the American †Mastodon had a covering of long, coarse hair, and that
it fed upon the leaves, shoots and small branches of trees, especially of
conifers. There is much reason to believe that the species outlived the
elephants in this continent and persisted until after the establishment
here of the American Indian, and it may well have been human agency which
finally extinguished the dwindling race. The range of the species nearly
coincided with that of the †Columbian Elephant, but did not extend so
far into Mexico, and in the central part of the continent reached much
farther north, even into Alaska.
In the Pliocene of Texas, Nebraska and Idaho lived the American
representatives of a genus (_†Stegodon_) which was a connecting link
between the elephants and the †mastodons, and which was especially
characteristic of the Pliocene of India. The tusks, which were confined
to the upper jaw, had lost their enamel and the last molar, above and
below, had five or six enamel ridges, but the crowns, which in the
Asiatic species were buried in cement, had but a small amount of this
material. Several species of _†Mastodon_ occur in the same beds, but only
isolated teeth have been found.
The †mastodons, in a broad sense of the term, have been divided into
several genera and subgenera in accordance with different schemes; the
simplest perhaps is to group into a second genus those species which had
fully developed lower tusks. This four-tusked genus has received several
names, of which _†Tetrabelodon_ is most commonly used in this country,
but the term _†Gomphotherium_ is much older and, according to the law
of priority, must therefore be employed. The lower Pliocene species of
_†Gomphotherium_ had a pair of large lower tusks, of cylindrical shape,
and both upper and lower tusks had longitudinal bands of enamel, and
in order to support the weight of these great tusks the symphyseal, or
chin, region of the lower jaw was greatly elongated; the molars had four
cross-crests.
[Illustration: FIG. 228.—Head of upper Miocene †mastodon (_†Gomphotherium
productum_) showing the chisel-like lower tusks. Restored from a skull in
the American Museum of Natural History.]
In the upper Miocene is found another and more primitive stage of
proboscidean development. In these species the grinding teeth were
three-ridged; the upper tusks were quite short and curved downward,
diverging somewhat from each other, and they had enamel bands. The
lower tusks were still shorter and of depressed, flattened and somewhat
chisel-like form and so worn as to show that they were regularly
employed in cropping and browsing. The skull was low and broad and the
symphysis of the lower jaw was greatly prolonged to carry the tusks.
A very important fact concerning these early †mastodons is that they
had the normal method of tooth-succession, permanent premolars forming
beneath (in the lower jaw, above in the upper) the milk-teeth and pushing
them out at maturity.
Of the middle Miocene proboscideans not much is known beyond the mere
fact of their presence in North America at that time and indeed little of
the skeleton, other than the skull, has yet been found in the American
Miocene; but well-nigh complete skeletons have been obtained from the
middle Miocene of Europe, and these bring out the surprising fact that
the body and limbs of these species did not differ in any noteworthy
manner from those of the existing elephants; the modern skeletal
structure of these animals had been attained at a time when the dentition
and skull were still in a far less advanced stage of development. In
size, however, there was a decided difference, the species of the
American Miocene rarely attaining a height of six feet.
Proboscidea have been reported from the lower Miocene of the Great
Plains, but the material is insufficient for a definitive judgment.
There is no doubt as to their presence in Europe at that time, but in
neither continent can the history be traced any farther and we must
turn to Africa for a backward continuation of the story. In the lower
Oligocene of the Fayûm, southwest of Cairo in Egypt, occurs the highly
interesting genus _†Palæomastodon_, which was much more primitive than
any of the genera described above, though it was an unmistakable member
of the order and even of the family Elephantidæ. The dentition was
already much reduced, giving the formula: _i_ 1/1, _c_ 0/0, _p_ 3/2,
_m_ 3/3. The upper tusks were short, compressed, directed downward, and
slightly divergent, and had a broad band of enamel on the outer side;
the lower tusks were still shorter and procumbent, pointing straight
forward, and were covered with enamel, which was very thick on the
lower side and thin or wanting on the upper. All of the grinding teeth
were in place and function at the same time, which was not true of any
of the genera previously considered, and each of the premolars had its
predecessor in the milk-series, which it succeeded and displaced in the
normal vertical manner. The premolars were smaller and simpler than the
molars, which were made up of three pairs of conical tubercles arranged
to form a three-crested crown. The skull, as compared with that of the
elephants, was long and narrow, the premaxillaries extending into a long
snout; the nasals were shortened, though not so much as in the succeeding
genera, and there was probably rather a long and flexible snout than a
true proboscis. The skull had a long and well-defined sagittal crest,
which none of the later genera had, and the development of sinuses in the
cranial bones, though considerable, was much less than in the elephants.
The occiput was relatively high and the thickened parietals did not tower
above it to any such degree as they do in the elephants. The symphysis
of the lower jaw was greatly prolonged, extending out beyond the ends of
the upper tusks, and this implies that the lower lip had a corresponding
prolongation.
The skeleton is still incompletely known, though it may be said that the
neck was probably longer than in the subsequent genera of the family.
The limb-bones were already proboscidean in character, differing only
in details from those of the more typical members of the order, but the
animal was more lightly built and had less massive limbs. The presence
of the third trochanter on the femur, which is lacking in all of the
succeeding forms, is an interesting approximation to other and still more
primitive groups of ungulates. The several species of _†Palæomastodon_
represent a considerable range in size, from animals which were not much
larger than a tapir to those which equalled a half-grown Indian Elephant.
It is possible to take another and very long step back from
_†Palæomastodon_, so long, indeed, as to make it apparent that one or
more links in the chain are still missing. The genus _†Mœritherium_ is
found together with _†Palæomastodon_ in the lower Oligocene, but also
occurs separately in the upper Eocene. It seems likely that it is a
persistent middle Eocene type and that the known species of it were
somewhat aside from the main line of descent, but that it very closely
represents, nevertheless, a very early stage in the elephant genealogy.
These known species were quite small animals, about the size of a tapir,
and therefore not much less than the smaller members of _†Palæomastodon_.
The dental formula of _†Mœritherium_ was: _i_ 3/2, _c_ 1/0, _p_ 3/3, _m_
3/3, × 2 = 36. The first or median upper incisor was a relatively small
and simple tooth, but the second was quite a large, downwardly directed
tusk, which was much smaller and less curved than in _†Palæomastodon_,
and was not capable of indefinite growth. The third incisor and the
canine were small, spike-like teeth of no functional importance, but
their presence is significant as approximating the primitive, unreduced
dentition of the ungulates. The lower incisors were nearly procumbent,
with a slight upward inclination; the first one was long and the second a
thick, enamel-covered tusk, with a chisel-like edge, which was produced
by wear. The premolars were smaller and simpler than the molars, which
were quadritubercular, the four conical cusps arranged so as to form
two transverse crests, giving a pattern like that of the early pigs and
peccaries and of precisely the kind that might have been predicted from
the teeth of _†Palæomastodon_.
The skull had an utterly different appearance from that of
_†Palæomastodon_, the difference being much greater than between the
latter and the Miocene _†Gomphotherium_. It was long and narrow, and,
except for the very prominent zygomatic arches, of nearly uniform,
tubular shape, the brain-case being of small capacity, though, as
compared with other Eocene mammals, the brain was proportionately large.
“It is possible that the early tendency toward a considerable cerebral
development shown in these primitive Proboscidea is one of the causes
why the group has survived and flourished through so long a period”
(Andrews). The cranium was very long and the facial region extremely
short, the premaxillaries not being prolonged into a snout, as they
were in _†Palæomastodon_; the occipital bones formed nearly the entire
posterior surface of the cranium and even encroached slightly upon the
roof. There was a long, but not very prominent, sagittal crest, and some
of the cranial bones were much thickened; in one species the hinder part
of the cranial walls was distinctly inflated, a beginning of the enormous
thickening which has culminated in the true elephants. The nasal bones
were already much shortened, though they were twice as long as those of
_†Palæomastodon_, and the animal would appear to have had an incipient
proboscis.
The neck was of moderate length and the body very long, with at least
twenty pairs of ribs, and there was probably a long tail. The hip-bone
differed remarkably in its extreme narrowness from that of the later
Proboscidea and the limb-bones were much more slender, though not
dissimilar in shape.
At a very early period the order became divided into two main branches,
one of which includes all the forms so far considered, and the other the
very strange _†Dinotherium_. The †dinotheres entered Europe together
with the †mastodons in the lower Miocene and continued into the Pliocene
without much change and then died out, leaving no descendants. They
never invaded North America, probably because they were of more or
less aquatic habit, like the hippopotamuses, and therefore less likely
to find suitable conditions in the narrow and unstable land-bridges
which connected the Old World with the New, than were animals of purely
terrestrial habitat. The †dinotheres were of huge size, equalling the
larger elephants in this respect and closely resembling them in the
skeleton of the body and limbs. As usual in this order, the generic
peculiarities were to be found in the teeth and skull. There were no
superior tusks, all the upper incisors and canines being lost, but there
was a pair of large lower tusks, which were directed downward, with a
strong backward curvature. The dental formula then was: _i_ 0/1, _c_ 0/0,
_p_ 2/2, _m_ 3/3, × 2 = 22. The grinding teeth were relatively quite
small and had, except the first molar, two transverse crests, giving a
pattern singularly like that seen in the tapirs. The skull was remarkably
long, low and flat, and no doubt these animals had a proboscis of some
sort. That the †dinotheres were derived from the same ancestral stock as
the †mastodons and elephants is perfectly obvious and is not questioned
by any one, but it is not yet possible to trace the connection.
The proboscideans were late immigrants into South America, being known
there only in the Pleistocene and late Pliocene times, and only the
†mastodons entered the southern continent, where they gave rise to
several peculiar local species in Argentina, Bolivia, Chili and Brazil;
one of these (_†Mastodon andium_) had a deposit of cement on the
crowns of the grinding teeth. Why the elephants, which extended to the
northern border of the Neotropical region, should have failed to reach
South America and maintain themselves there, is but one of many similar
questions to which no assured answer can be given.
The evolution of the Proboscidea was, in a certain sense, very similar
to that of the †oreodont family (p. 381) among the Artiodactyla, in
that the developmental changes affected chiefly the dentition and the
skull, the skeleton of the body and limbs having very early acquired a
character which was afterward but little modified. Were the skull and
teeth of the lower Miocene _†Gomphotherium_ not known, we should hardly
hesitate to refer the skeleton to the genus _Elephas_, and even in the
Oligocene _†Palæomastodon_ all the bones of the skeleton, other than the
skull, were characteristically and unmistakably proboscidean. On the
other hand, the transformations of the teeth and skull were very profound
and far-reaching, very much more so than those which took place in the
†oreodonts.
[Illustration: FIG. 229.—Evolution of the Proboscidea: on the right, a
series of skulls; on the left, last lower molar. (After Lull, modified by
Sinclair.) N.B. _†Tetrabelodon_ should read _†Gomphotherium_.]
In the dentition we may consider separately the development of the
tusks and of the grinding teeth. The first step in the known series, as
exemplified by _†Mœritherium_, was the enlargement of the second incisor
in each jaw to form a tusk which, though actually quite long, was very
small when judged by the proboscidean standard. The upper tusk was
directed vertically downward and the lower one was procumbent, pointing
almost directly forward; the third incisor and the canine were small and
in the lower jaw already lost. In the next known stage, _†Palæomastodon_,
all of the anterior teeth, except the tusks, had been suppressed; the
upper tusks were longer and more curved and of an oval cross-section;
they extended less directly downward and more forward, while the enamel
was restricted to the outer side of the tusk; the lower tusks were more
fully procumbent than in the preceding genus. The third stage, that
of the lower Miocene _†Gomphotherium_, showed the upper tusks greatly
elongated and directed more forward than downward, while the lower
tusks were but little larger than before. From the middle Miocene two
phyla may be distinguished by the tusks alone; in one, which was not
destined to long life, the lower pair increased greatly both in length
and in diameter, while in the other series they rapidly diminished and
eventually disappeared. Even in the Pleistocene, however, the American
†Mastodon had remnants of these tusks in the males. In the later
†mastodons, the †stegodonts, and true elephants, the upper tusks, which
alone remained, lost the enamel bands and attained enormous proportions,
differing in the various genera and species in the extent and direction
of curvature. An aberrant mode of tusk development was to be seen in the
†dinotheres, in which the upper pair was suppressed and the lower pair
enlarged and so curved that the points were directed backward.
The grinding teeth underwent much more radical and striking changes.
At first (_†Mœritherium_) they were small, very low-crowned and of
simple pig-like or quadritubercular pattern, making two interrupted
cross-crests; all were in use simultaneously and the succession of
milk-teeth and premolars was by vertical replacement, as in normal
mammals generally. In _†Palæomastodon_ there were three pairs of
tubercles on the molars and in _†Gomphotherium_ these coalesced into
ridges, but in all the †mastodons there was more or less distinctness
of the conical tubercles. In one or more phyla the three-ridged plan
persisted for a long time, one such phylum terminating in the Pleistocene
_†Mastodon americanus_. In the other series the number of ridges
increased, first to four, then to five, six and more (_†Stegodon_); the
crowns of the teeth became much larger and higher, and the ridges, as
their number increased, became much thinner, and the valleys between
them were filled with cement, and finally, in the true elephants, with
their fully hypsodont, many-crested teeth, were thickly covered all over
with cement. The vertical succession of milk-teeth and premolars was
retained in _†Gomphotherium_, at least in some species, but the large
molars, which could not find room to be exposed while the premolars were
in place, came in successively from behind. This horizontal mode of
succession is the only one to be seen in the true elephants, in which but
one tooth, or parts of two, on each side of each jaw are in simultaneous
use and the premolars have entirely disappeared, but the milk-teeth are
retained.
The changes in the skull, which amounted to a reconstruction, were
very largely conditioned by the great increase in the length and
consequent weight of the tusks, in the size of the grinding teeth and the
development of the proboscis. In the earliest known type (_†Mœritherium_)
the skull had little about it that would, at first sight, suggest
proboscidean affinities; it was long and narrow, with sagittal crest
and occiput of normal type, very long cranial and very short facial
region. The nasal opening was directed forward and the nasal canal was
relatively long and horizontal in direction, but the nasal bones were
already much shortened, indicating that the proboscis was probably in
an incipient stage. The symphysis of the lower jaw was procumbent and
somewhat elongated, but to only a comparatively slight degree.
While the skull of _†Mœritherium_ was not obviously proboscidean, that
of its successor, _†Palæomastodon_, was unmistakably so, yet retained
several primitive features, which were lost in all of the subsequent
genera, such as the sagittal crest, the relatively low cranium and
moderate thickening of the cranial bones, the forward direction of the
nasal opening, etc.; the symphysis of the lower jaw was very greatly
prolonged.
As the tusks enlarged and the proboscis grew longer, the weight of
the head and its appendages necessitated a largely increased area
of attachment for the neck-muscles, and this was attained by a very
great thickening of the cranial roof, the occiput not increasing
proportionately; at the same time, the thickened bones were honeycombed
with sinuses, so as to reduce their weight without sacrifice of strength.
In those species of the Miocene _†Gomphotherium_ which had large and
heavy tusks, this thickening was not very much less than in the true
elephants. The enlargement of the tusks had other consequences, as, for
example, in lengthening and broadening the premaxillaries and, in the
elephants, in their downward bending, so as to shorten still further the
facial region of the skull. With the development of the proboscis, the
nasal bones were reduced to a minimum and the anterior nasal opening was
no longer directed forward, but obliquely upward, while the nasal passage
lost its horizontal direction and became almost vertical. The lower
jaw continued to elongate the symphysis, reaching a maximum in certain
species of _†Gomphotherium_; but the reverse process of shortening this
anterior region of the jaw began with the reduction of the lower tusks,
and, when these had disappeared, nothing remained of the immensely
elongated symphysis, except the short spout of the elephant’s jaw. As
the grinding teeth increased in height, there was a concomitant increase
in the vertical depth of the jaws for their lodgment.
It was an obvious advantage in the mechanical problem of supporting the
enormous weight of head, tusks and trunk to shorten the neck and thus
bring the weight nearer to the point of support at the withers, the
lengthening proboscis rendering it unnecessary for the mouth to reach the
ground in feeding or drinking. The other parts of the skeleton underwent
comparatively little change, the degree of modification being greatest
between _†Mœritherium_ and _†Palæomastodon_. Throughout the series the
bones of the fore-arm and lower leg remained separate, and the feet very
short and five-toed. In size also the great stature and massiveness were
attained early. After the great migration of the Proboscidea to the
northern continents, we find considerable differences of size between the
various phyla, though all were very large, and even as early as the lower
Miocene of France, there were species which rivalled the modern elephants
in bulk. It was this rapid attainment of great size and weight which
appears to have been the determining factor in the conservatism of the
skeleton. After the skeleton had become fully adjusted to the mechanical
necessities imposed by immense weight, and that adjustment, as we have
seen, was effected at a comparatively early period in the history of the
order, then no further modification of importance would seem to have
been called for. No doubt the habits and mode of life of these massive,
sedate and slow-moving animals underwent but little change from the
lower Oligocene onward. There is reason to think that _†Mœritherium_ was
semi-aquatic and a haunter of marshes and streams, but, if so, the change
to a life on dry ground was complete in the lower Oligocene, for the
structure of _†Palæomastodon_ gives no reason for supposing that it was
anything but a dweller on solid land and a denizen of forests.
Although this book does not undertake to deal with the obscure problems
connected with the marine mammals, it may be noted in passing that one
of these problems has been brought near to solution, if not actually
solved, by the discoveries in Egypt and that is the question concerning
the origin of the Sirenia. The order includes the existing Manatee or
Sea-Cow (_Manatus_) of the coast of Florida, northeastern South America
and western Africa, and the Dugong (_Halicore_) of the Indian Ocean.
These are mammals which are adapted to a strictly marine habitat and are
incapable of existence on land, having lost the hind limbs and converted
the fore limbs into swimming paddles. Unlike the whales, porpoises
and other Cetacea, the Sirenia are herbivorous and feed upon seaweed
and eel-grass and the aquatic plants of large rivers. The Egyptian
discoveries tend very strongly to the conclusion that the Sirenia and
Proboscidea were both derived from a common stock and that the genus
_†Mœritherium_ was not very far removed from the probable ancestor from
which both of the orders descended.