HISTORY OF THE EDENTATA
As here employed, excluding the so-called edentates of the Old World, the
Edentata form a highly variegated, but natural, assemblage of related
forms. The order is at present exclusively American and almost confined
to the Neotropical region, an armadillo which extends into Texas being
the sole exception. These animals are so peculiar and so isolated from
other mammals, that it has been proposed to treat them as a separate
subclass; and there is much to be said in favour of this procedure,
though it would perhaps be premature, until more is learned concerning
these most curious and exceptional animals. In the subjoined table only
the more important and better known genera are included.
Series PILOSA. Hairy Edentates
Suborder TARDIGRADA. Tree-Sloths
I. BRADYPODIDÆ.
_Bradypus_, Three-toed Sloth, Rec. _Cholœpus_,
Two-toed Sloth, Rec.
Suborder VERMILINGUA. Anteaters
II. MYRMECOPHAGIDÆ.
_Myrmecophaga_, Ant-Bear, Rec. _Tamandua_, Lesser
Anteater, do. _Cyclopes_, Tree Anteater, do.
Suborder †GRAVIGRADA. †Ground-Sloths
III. †MEGATHERIIDÆ.
_†Megatherium_, Plio. and Pleist., S. A.; Pleist., N.
A. _?†Prepotherium_, Santa Cruz. _?†Planops_, do.
IV. †MYLODONTIDÆ.
_†Mylodon_, Plio. and Pleist., S. A.; Pleist., N.
A. _†Paramylodon_, Pleist., N. A. _†Grypotherium_,
Pleist., S. A. _†Pseudolestodon_, Plio. and Pleist.,
S. A. _†Scelidotherium_, do. _†Nematherium_, Santa
Cruz. _†Analcitherium_, do.
V. †MEGALONYCHIDÆ.
_†Megalonyx_, Pleist., N.A. _†Nothrotherium_,
Pleist., S. A. _†Megalocnus_, Pleist., Cuba.
_†Hapalops_, Santa Cruz. _†Schismotherium_, do.
_†Pelecyodon_, do. _†Megalonychotherium_, do.
_†Protobradys_, Casa Mayor.
Series LORICATA. Armoured Edentates
Suborder DASYPODA. Armadillos
VI. DASYPODIDÆ.
_Dasypus_, 6-, 7- and 8-Banded Armadillos, Pleist.
and Rec., S. A. _Cabassous_, 11-Banded Armadillo,
do. _Priodontes_, Giant Armadillo, do. _Tolypeutes_,
Apar, Rec., S. A. _Zaëdyus_, Pygmy Armadillo, do.
_Scleropleura_, do. _Chlamydophorus_, Pichiciago, do.
_Tatu_, 9-Banded Armadillo, Pleist. and Rec., S. A.;
Rec., Texas. _†Eutatus_, Plio. and Pleist., S. A.
_†Chlamydotherium_, do. _†Proeutatus_, Santa Cruz.
_†Prozaëdius_, Deseado and Santa Cruz, _†Prodasypus_,
do. _†Stegotherium_, Santa Cruz. _†Meteutatus_,
Deseado. _†Sadypus_, do. _†Amblytatus_, do.
_†Prœuphractus_, do.
VII. PELTEPHILIDÆ.
_†Peltephilus_, Deseado and Santa Cruz.
VIII. INCERTÆ SEDIS.
_†Metacheiromys_, mid. Eoc., N. A.
Suborder †GLYPTODONTIA. †Glyptodonts
IX. †GLYPTODONTIDÆ.
_†Glyptodon_, Plio. and Pleist., N. and S. A.
_†Dœdicurus_, Pleist., S. A. _†Panochthus_,
do. _†Sclerocalyptus_, Plio. and Pleist.,
S. A. _†Glyptotherium_, mid. Plio., N. A.
_†Propalæohoplophorus_, Deseado and Santa Cruz.
_†Cochlops_, Santa Cruz. _†Eucinepeltus_, do.
_†Asterostemma_, do.
In the section Pilosa, which includes the sloths (Tardigrada), anteaters
(Vermilingua) and the extinct †ground-sloths (†Gravigrada), the skin
is thickly clothed with long hair, and in the Loricata, armadillos and
†glyptodonts, the head, body, tail and legs are more or less completely
encased in an armour of bony scutes covered with plates of horn, but with
some hairs also.
The name Edentata (toothless) is not very happily chosen, for only the
anteaters are quite toothless. Almost all the genera have no teeth in
the front of the mouth and the teeth are nearly always alike, so that
the distinction of regions among them is entirely a matter of position
in the jaws. In the tree-sloths and many †ground-sloths the foremost
tooth in each jaw is a more or less enlarged, canine-like tusk. The
teeth are always rootless, growing from permanent pulps, and are without
enamel, made up of dentine, which is sometimes homogeneous and sometimes
in layers of different hardness, and with a covering of cement, usually
thin and film-like. The number of teeth varies from 4/4 to 10/10 or more,
and their form usually approximates a simple cylinder, worn off flat at
the end, though the ends may be bevelled or grooved, differences which
are in no way due to pattern but simply to the mode of wear. In the
†glyptodonts the teeth were divided by deep vertical grooves into two
or three pillars, connected by narrow necks. In most of the edentates
there is no change of teeth, the milk-dentition having been completely
suppressed, but in the 9-Banded Armadillo (_Tatu_) each of the permanent
teeth is preceded by a two-rooted milk-tooth, and some other armadillos
have milk-teeth.
The skull varies much in form and proportions, according to the character
of the food and method of feeding. The tree-sloths and †ground-sloths
have short, rounded heads; in the †glyptodonts, the skull was short and
remarkably deep vertically; while the armadillos have long, shallow
heads, with tapering muzzle, the length and slenderness of which differ
in the various genera. In the anteaters the skull is extraordinarily
elongate and slender. The sagittal crest is seldom present at all and
never prominent. The zygomatic arch may be complete or interrupted;
in the tree-sloths, †ground-sloths, †glyptodonts and some extinct
armadillos, there is a descending, plate-like process given off beneath
the eye.
The backbone displays some of the most remarkable peculiarities of
the order. The neck in the tree-sloths has eight or nine vertebræ,
the only instances known among mammals in which the normal number of
seven is departed from. In the armadillos and †glyptodonts several of
the neck-vertebræ are coössified into a single piece, but the atlas is
always free, so as to permit the movements of the head. In the posterior
part of the dorsal and in the lumbar region the articulations between
the successive vertebræ are by far the most complex and intricate
known among mammals; in the tree-sloths these have degenerated, though
still plainly indicated. In the †glyptodonts, which were covered with
a huge, tortoise-like carapace, mobility of the backbone was needless,
and so all of the dorsal vertebræ were united into one long piece and
the lumbars were coössified with one another and with the sacrum.
The sacrum consists, throughout the order, of a very large number of
vertebræ and is attached to the hip-bones at two different points,
instead of only one, as in other mammals. The tail varies much in
length and thickness; in the tree-sloths it is extremely short and in
the anteaters very long and bushy, prehensile in the arboreal members
of the group; in the †ground-sloths, especially the gigantic forms, it
was of immense thickness; while in most of the †glyptodonts a varying
number of the terminal vertebræ were fused together. The sternal ribs
are better developed than in any other mammals, and in the anteaters and
†ground-sloths they articulate with the breast-bone by regular synovial
joints, and each rib has head and tubercle like a vertebral rib.
In the limbs and feet there is great variety, according to the manner
of their employment. The shoulder-blade has a very long acromion and
very large coracoid, which long remains separate from the scapula;
collar-bones are very generally present, though often in much reduced
condition. The hip-bones have in the tree-sloths, †ground-sloths and
†glyptodonts a much expanded anterior element, which in the other groups
is narrow. The humerus usually has very prominent deltoid and supinator
ridges and epicondylar foramen; the fore-arm bones are always separate,
and there is generally much freedom of rotation of the manus. In the
wrist there is no distinct central and usually there are the ordinary
eight separate bones. The tibia and fibula are frequently coössified.
The tree-sloths, which lead most strictly arboreal lives and are almost
helpless on the ground, are unique among mammals in that the body is
habitually _suspended_ from the limbs, not carried upon them; the feet
are curved hooks, which fit over the tree-branches and support the weight
without muscular exertion. The limb-bones are very long and slender,
the claws long, curved and sharp, and the metapodials of each foot, two
or three in number, are fused into a single mass. In the †ground-sloths
there was much change in foot-structure during the course of their
recorded development; they were usually five-toed and the feet were armed
with one or more great claws; the later and larger representatives of the
suborder walked upon the outer edge of the feet.
The armadillos, which are largely burrowers, have five-toed feet and
long, heavy, pointed claws, but in some of them the pes has a varying
number of flat, hoof-like nails. The immense †glyptodonts had very short,
broad feet, shod with hoofs, which, in some of the genera, were longer
and more claw-like in the manus.
The recorded history of the edentates was developed almost entirely
in South America. In the Casa Mayor formation there were numerous
armadillos, but as only scutes of the carapace have been found, little is
known of them. The †ground-sloths (_†Protobradys_) have been reported,
but from such imperfect material that the reference is uncertain. The
first assuredly determinable members of this suborder were in the
Astraponotus beds and, associated with them, the most ancient known
†glyptodonts. In the Deseado stage were many armadillos, some of them
extremely peculiar, several †glyptodonts and †ground-sloths, some species
of the latter very large. Edentates were far more numerous and varied
in the Santa Cruz than in any of the preceding stages. Tree-sloths and
anteaters have both been reported, but the evidence is insufficient,
though there can be little doubt that these suborders had begun their
separate existence in some part of South America other than Patagonia.
The three families of †ground-sloths were already distinguishable,
though much less clearly separated than they afterwards became; none
of them were large animals, smaller even than some of the Deseado
species and veritable pygmies in comparison with the giants of the
Pliocene and Pleistocene. The †glyptodonts were likewise far smaller
than their Pliocene and Pleistocene successors and in several respects
more primitive, approximating the armadillos more closely; nor was
there any such variety of forms as in the later stages. The armadillos
were extremely numerous and varied; they all belonged to extinct genera
and most of them apparently have no descendants at the present day.
The tropical forests of Brazil and the Guianas must then, as now, have
swarmed with mammals which did not extend their range to Patagonia and
of which we consequently have no record. No doubt, it was in these
forests that the ancestors of most modern armadillos, as well as of the
tree-sloths and anteaters, lived in Miocene times.
Pliocene edentates were of the same suborders as those of the Santa
Cruz, but far larger in size. Most of them are known only from very
incomplete specimens, but the Pleistocene has yielded an enormous mass
of beautifully preserved material. Of the tree-sloths and anteaters,
only questionable remains have been found. That these tropical and
forest-loving animals should not have occurred in the open Pampas of
Argentina is not surprising, but it is difficult to account for their
absence from the extremely rich cave-faunas of Brazil. Nearly all the
existing genera of armadillos have been obtained, and with these were
associated several extinct genera, some of them (_†Chlamydotherium_,
_†Eutatus_) relatively huge, as large as tapirs. There was a wonderful
variety of †glyptodonts, most of them enormous creatures, of which no
less than five genera have been collected in Argentina and Brazil, and
the †ground-sloths were even more numerous and varied. Nine genera,
with many species, of these great beasts, which ranged in size from an
elephant to a tapir, are already known and no doubt the list is still
incomplete. These †glyptodonts and †ground-sloths must have been among
the most conspicuous elements of the Pleistocene fauna.
Aside from certain problematical Eocene forms, the first North American
edentates, which were immigrants from the southern continent, appeared
probably in the middle Miocene of Oregon in the form of †ground-sloths,
but the specimen, as well as a similar one from the lower Pliocene of
Nebraska, is not sufficiently complete for positive reference. In the
middle Pliocene the †ground-sloths and †glyptodonts were unquestionably
present, and in the Pleistocene these two suborders were numerously and
conspicuously represented. Three or four genera of the huge, elephantine
†ground-sloths coexisted in Pleistocene North America. _†Megalonyx_
was abundant in the forested regions east of the Mississippi, from
Pennsylvania southward, and on the Pacific coast; _†Mylodon_ was
transcontinental in distribution; while _†Megatherium_ was apparently
confined to the southern states. While all three genera undoubtedly
originated in South America, _†Megalonyx_ has not yet been found in that
continent.
This genus was originally named by President Jefferson in 1805 from an
ungual phalanx found in a cave in Virginia, and he imagined that it
belonged to a colossal lion which must still be living in the mountains
of western Virginia. This was deduced from the assumption that no species
could become extinct, and the passage is of interest as showing the
prevalent belief of the time, although Cuvier had already demonstrated
that many species had actually been extinguished. The passage is as
follows: “The movements of nature are in a never ending circle. The
animal species which has once been put into a train of motion is still
probably moving in that train. For, if one link in nature’s chain might
be lost, another and another might be lost, till this whole system of
things should evanish by piecemeal.”
The †glyptodonts were also southern in distribution, and only very
imperfect remains of them have yet been recovered from the North American
Pleistocene, quite sufficient, however, to make the identification
certain.
There were several genera of rather small †ground-sloths in the
Pleistocene of Cuba. The best known of these, _†Megalocnus_, had
several peculiarities of structure, but was plainly a member of the
†Megalonychidæ. The ancestors of this genus probably invaded Cuba in the
Pliocene, when the island was joined to Central America.
SUBORDER †GRAVIGRADA. †GROUND-SLOTHS
As the †ground-sloths would appear to have had a more central position
within the order than any of the other groups, our study of development
may well begin with them. In the Pleistocene there were three families
of these gigantic brutes, which ranged through the western hemisphere
from Pennsylvania and California to Patagonia. Unfortunately our
knowledge of the developmental stages within the different families is
very unequal, and it is therefore impracticable to do more than sketch
the changes of the suborder as a whole and in a general way. In the
successive geological stages the proportionate representation of the
different phyla varied greatly; in the South American Pliocene and
Pleistocene the †Mylodontidæ and †Megatheriidæ were the abundant forms,
while the †Megalonychidæ were but scantily represented. In the Santa
Cruz Miocene, on the other hand, the overwhelming preponderance was with
the †Megalonychidæ, the other two families being comparatively rare and
incompletely known. From the still more ancient formations, the material
so far collected is so fragmentary that family distinctions have little
meaning. After all, there was no very wide range of variation among the
contemporary members of the three families, and the differences were
principally in size, in the form and number of the teeth, the shape of
the skull and the number of digits; in essentials they were all much
alike.
The genus _†Megatherium_ (Fig. 122, p. 220) included the largest and
most massive members of the suborder, _†M. americanum_ being as large as
an elephant, but very differently proportioned, as it was much longer
and lower in stature, owing to the shortness of the extraordinarily
heavy limbs; some of the skeletons measure 20 feet or more in length.
The teeth, which were 5/4 in number, formed an uninterrupted series on
each side; all had the same quadrate form and by abrasion were worn into
two transverse ridges, formed by the meeting of the harder dentine with
the thick coating of cement. The result was a form of tooth which much
resembled the lower molars of a tapir, but it was not a tooth-pattern in
any proper sense of the word, being due entirely to the mode of wear.
The skull was very small in proportion to the huge body and was low
and narrow in shape; the cranium had a broad, flat roof, without
sagittal crest; the orbit was completely encircled in bone, and the
descending process of the zygomatic arch beneath the eye was very long
and conspicuous. The nasals were short, and the slender, toothless
premaxillaries projected far in front of them, which makes the presence
of some sort of a proboscis likely. The lower jaw had a long, narrow,
spout-like symphysis, which was abruptly rounded at the free end,
not pointed; below the teeth, the lower margin of the jaw was very
strongly convex, descending in a great flange. The neck was short, the
body very long and enormously heavy, as was also the tail. The immense
shoulder-blade had a very long acromion, which curved forward and inward,
fusing with the coracoid and forming a bony loop or bridge. The hip-bones
had the anterior element (ilium) enormously expanded transversely, so as
to support the huge mass of viscera in the semi-erect position which the
animal, it is believed, frequently assumed in feeding. Collar-bones were
present.
The fore limb was very much more slender than the hind, but of nearly the
same length. The humerus had a comparatively slender upper portion and
extremely broad lower end, due to the great development of the internal
epicondyle and supinator ridge; there was no epicondylar foramen. The
radius evidently had the power of very free rotation upon the humerus.
The femur was short, flattened antero-posteriorly, but excessively
broad and heavy, and had no third trochanter. The tibia and fibula
were likewise short and very massive and were extensively coössified
at each end, leaving but a short interspace open between the bones.
The very peculiar feet were so connected with the limb-bones, that the
animal must have walked upon the outer edge of the foot, somewhat as the
existing Ant-Bear (_Myrmecophaga jubata_) uses the fore foot. The manus
had four functional digits, the first being a mere vestige; the fifth,
upon which the weight rested in walking, had two very small phalanges
and no claw, while the second, third and fourth had long, sharp claws.
The pes had but three functional digits, for the first and second were
reduced to rudiments; digit III had an enormous claw and of this digit
the metatarsal was short and very heavy and the first two phalanges were
fused together; the two external digits, Nos. IV and V, had no claws. The
astragalus had a very peculiar shape, made necessary by the application
of the external border of the foot to the ground and thus in both fore
and hind feet the great claws were turned inward and, in the case of the
pes, it must have been impossible to rest the sole upon the ground. The
heel-bone was enormous and club-shaped and formed the hinder portion of
the weight-carrying outer edge of the foot.
Almost all who have studied the structure of this extraordinary beast are
agreed as to its habits. That it fed principally, if not exclusively,
upon leaves, is indicated by the teeth. The general opinion as to its
manner of life is well summed up by von Zittel: “The hip-bones, hind legs
and tail are characterized by enormous strength. The entire structure
of the extremities proves that the gigantic sloth could move over the
ground but slowly and clumsily; on the other hand, the fore limbs
served as grasping organs and were presumably employed to bend down and
break off twigs and branches and even to uproot whole trees, while the
weight of the body was supported upon the hind legs and tail.”[19] It
would be quite absurd to suppose that such ponderous animals could have
been climbers or burrowers, hence the function of the enormous claws,
especially the single one of the pes, is not obvious, though they may
have been merely the weapons of the otherwise defenceless monsters. The
great claw in the fore foot of the Ant-Bear is a terrible weapon, with
which the creature vigorously and successfully defends itself against
dogs, and it may even be dangerous to men, if incautiously molested.
_†Megatherium_ had no bony scutes, or other ossifications in the skin, so
far as is known, and was probably covered with long and coarse hair, as
is known to have been the case in another †ground-sloth.
Less specialized in many respects than the †megatheres was _†Mylodon_,
type of a family which was numerously and variously represented in the
Pleistocene of South America, much less so in that of North America.
_†Mylodon_ was smaller and lighter, being from ⅓ to ¼ smaller in linear
dimensions than _†Megatherium_, and the contemporary _†Scelidotherium_
was no bigger than a tapir. The teeth numbered 5/4 and the anterior one
above and below had a somewhat tusk-like form; the others were worn off
evenly, with nearly horizontal grinding surface, but a vertical groove
on the inner side gave them a subtriangular, lobate form. The skull
was short and broad, with flat top, and orbit only partially enclosed
behind; the premaxillaries were very short and the muzzle very broad and
abruptly truncated, the nasal opening very large. The lower jaw had a
straight inferior border, a short, very wide and shovel-shaped symphysis
and square chin. Nothing indicates a proboscis, and the head must have
been very different from that of _†Megatherium_.
Within the family of the †mylodonts there was some variety in the
dentition and more in the shape of the skull. In _†Lestodon_, for
example, the first tooth in each jaw was a large, sharp-pointed tusk,
the muzzle was greatly broadened, and the whole animal was larger.
_†Scelidotherium_, the smallest Pleistocene member of the family,
had a much narrower and more elongate skull than the others. In
_†Glossotherium_, which also had an elongate skull, there was an arched
bony bridge connecting the anterior end of the nasal bones with the
premaxillaries and dividing the nasal opening into two parts.
The neck, body and tail of _†Mylodon_ did not differ materially from
those of _†Megatherium_, except in being smaller and less massive. The
fore limb was relatively somewhat shorter and much stouter, but otherwise
similar; the humerus had no epicondylar foramen and the femur no third
trochanter; the tibia and fibula were separate. The manus had five
digits, Nos. I, II and III carrying claws, that of III being especially
large; IV and V had no claws and the outer edge of the manus rested on
the ground in walking, the sole turned inward. The pes had lost the first
digit, the second and third had claws, but not the fourth and fifth; the
weight rested on the outer edge.
The skin is definitely known from large pieces belonging to the allied
genus _†Grypotherium_, found in a cavern near Last Hope Inlet, Patagonia,
where it had been preserved by burial in dry dust. Externally, the skin
was thickly covered with coarse hair and in the deeper layers was a
continuous armour of small ossicles, which were close set and in the
Last Hope specimens show like a cobble-stone pavement on the inner side
of the skin, the innermost layers of which have been destroyed; in life,
these small bones were not visible. Similar ossicles have been found
in association with several skeletons of _†Mylodon_. The habits, diet
and mode of feeding of the latter were no doubt essentially similar to
those of _†Megatherium_, but _†Scelidotherium_, which had a much shorter
and lighter tail, was probably more quadrupedal and browsed upon low
shrubbery.
[Illustration: FIG. 288.—Gigantic †ground-sloth (_†Mylodon robustus_),
Pampean. Restored from Owen’s figure of the skeleton.]
The third family, the †Megalonychidæ, was scantily represented in the
Pleistocene of South America, but relatively common in North America.
_†Megalonyx_ was, on the whole, less specialized than _†Mylodon_ or
_†Megatherium_, but had a strong resemblance to both of them. The
teeth, 5/4 in number, had the foremost one in each jaw separated by a
considerable space from the others and more or less tusk-like in form;
the grinding teeth were worn smooth, without ridges, and of somewhat
trihedral shape. The skull was short, broad and deep, resembling in shape
that of the tree-sloths; there was a long, but feebly developed sagittal
crest, and the orbits were widely open behind, with hardly a trace of
any posterior boundary. The muzzle was very short and broad and abruptly
truncated and the premaxillary bones were extremely small. The lower
jaw was short, thick and massive, with very broad symphysis and almost
vertical chin. Neck, body, tail, shoulder and hip-bones did not differ
sufficiently from those of _†Megatherium_ to require particular notice.
The fore limb was shorter and more slender than the hind; the humerus had
the epicondylar foramen and the very massive femur retained the third
trochanter; the tibia and fibula were separate. The feet had five digits,
three of which carried claws; the calcaneum was very peculiar, not at
all like the massive, club-shaped bone of _†Megatherium_ and _†Mylodon_,
but long, comparatively thin and sickle-shaped. Nothing in the skeleton
suggests that the creature’s habits differed in any important way from
those of the genera last named.
_†Megalocnus_, of the Cuban Pleistocene, a member of this family,
was apparently peculiar to the island and was probably derived from
ancestors which in the Pliocene migrated from Central America. Aside
from certain remarkable peculiarities of the teeth, this animal was more
primitive, as well as smaller, than any other of the Pleistocene genera.
Although remains of †Gravigrada are comparatively common in all of the
fossiliferous formations between the Pampean and the Santa Cruz, the
material is too imperfect to throw any useful light upon the development
of the various families. From the Santa Cruz beds, on the other hand, a
great wealth of specimens has been obtained, and it is possible to give
some fairly adequate account of the †ground-sloths of that time. These
animals were then extremely abundant individually and of extraordinary
variety; evidently, they were in a state of rapid expansion and divergent
evolution along many lines, for hardly any two specimens are alike
and therefore the satisfactory discrimination of species is well-nigh
impossible. Yet, with all this remarkable variability, the range of
structural differences was not great; the group was a very homogeneous
and natural one, and separation into families was not obvious. Two of the
three families were, however, unequivocally present in this fauna and
the third somewhat doubtfully so. The †Megalonychidæ, which in the South
American Pleistocene had dwindled to such insignificant proportions,
formed the overwhelming majority of the Santa Cruz †Gravigrada;
the †Mylodontidæ were quite rare in comparison and are still very
incompletely known; while the †Megatheriidæ, though probably present,
have not been identified beyond all doubt.
All of the Santa Cruz †ground-sloths were small animals, the largest not
approximating the smallest Pleistocene species, those of Cuba excepted,
and many of them were no larger than the modern tree-sloths. This was
a wonderful difference between the Santa Cruz and the Pampean, but a
difference which involved nearly all other groups of mammals. So far
as the skeleton is concerned, this is known with completeness only for
the †Megalonychidæ, especially the genus _†Hapalops_; but enough has
been learned of the others to show that there was far less difference
between the families than had arisen in the later epochs. This backward
convergence of the three groups towards a common term plainly indicates
their common origin, being exactly what might have been predicted in
advance of experience.
[Illustration: FIG. 289.—Santa Cruz †ground-sloth (_†Hapalops
longiceps_) and †glyptodont (_†Propalæohoplophorus australis_). Restored
by Knight from skeletons in Princeton University and the museum of La
Plata.]
In all the genera the teeth number 5/4; the teeth on each side were
sometimes in continuous series, sometimes the first one was isolated
and almost always more or less tusk-like, most so in _†Eucholœops_. The
other teeth were usually of transversely elliptical shape and worn into
two ridges, with a hollow between; the †mylodonts (_†Nematherium_, etc.)
already had the triangular, or lozenge-shaped, lobate form of teeth,
characteristic of the family.
The skull varied considerably in its proportions; generally, it was long
and narrow, with shortened face and elongate cranium; the sagittal crest
was seldom present, never prominent, and the orbit was always widely open
behind, without postorbital processes. The premaxillaries were always
short and toothless and in most of the genera they were slender rods, in
others (_e.g._ _†Hyperleptus_) broad and plate-like. The lower jaw had an
elongate spout-like symphysis, in which the two halves were coössified,
tapering forward to a blunt point and, though the length of this spout
differed greatly in the various genera, in none was there a broad, abrupt
chin such as _†Mylodon_ and _†Megalonyx_ had. In _†Prepotherium_, which
is believed to be referable to the †Megatheriidæ, the lower jaw had the
extremely convex inferior border, in less exaggerated degree, of its huge
Pampean successor; it would be premature to say descendant.
While the long, slender skull was the prevailing type among the Santa
Cruz †Gravigrada, there was a group of small animals in which the skull
was shorter and more rounded and had a very suggestive likeness to that
of the modern tree-sloths, as was likewise true of the teeth.
Despite innumerable variations of detail, the skeleton of the Santa Cruz
†ground-sloths may be described without distinction of genera, though
it should be added that the skeleton is but partially known in many of
the genera, and fuller knowledge might require modification of some of
the statements. The neck was of moderate length, the body long, the
tail long and heavy and, in some instances, very massive. The sternal
ribs were completely ossified and already had the same elaborate mode
of articulation with the breast-bone as in the great Pampean forms, and
the vertebræ the same intricate connections. The shoulder-blade also had
the same characteristics as in the latter, but the hip-bones had but a
moderate transverse expansion, having no huge mass of viscera to support.
The limbs were stout and short, fore and hind legs of nearly equal
length; the humerus had the epicondylar foramen and the broad, flattened
femur retained the third trochanter. The radius had a discoidal upper
end, which rotated freely upon the humerus; the tibia and fibula were
always separate. The feet were five-toed, all the digits complete and
functional and all provided with claws; there was no coössification
between the phalanges. The astragalus was little different from
the normal form, but in some genera (_e.g._ _†Prepotherium_) the
highly peculiar form of this bone characteristic of _†Mylodon_ and
_†Megatherium_ was distantly foreshadowed. The gait must have been simply
plantigrade, though some of the forms had probably begun to throw the
weight upon the outer edge of the foot.
[Illustration: FIG. 290.—Left pes of _†Mylodon_, Pampean (after Owen).
_Cal._, calcaneum. _As._, astragalus. _N._, navicular. _Cn. 2_, _Cn. 3_,
middle and external cuneiforms. _Cb._, cuboid.]
No dermal armour has yet been found in association with any of the
genera, and, so far as the predominant †Megalonychidæ are concerned, of
which so many skeletons have been collected, this negative evidence must
be allowed great weight. But the material of the other two families is
so rare and incomplete, that the failure to find dermal ossicles is of
no value in determining the question; probably, the †mylodonts possessed
them.
These small Santa Cruz †ground-sloths were not so clumsy and slow-moving
as their gigantic successors of the Pampean, and must have been
inoffensive plant-eaters, some of them perhaps more or less arboreal in
habits, but they could defend themselves with their long, sharp claws.
[Illustration: FIG. 291.—Left pes of _†Hapalops_, Santa Cruz. Princeton
University Museum. Letters as in Fig. 290 and scale of reduction the
same.]
It would require far too much space and lead us into a labyrinth of
anatomical technicalities to point out all the many resemblances to
other edentate suborders which are to be noted in the skeleton of the
Santa Cruz †Gravigrada, which thus justified their position as the most
nearly central group of the entire order. Not only was the skeleton of
these Miocene †ground-sloths very much less specialized than in their
Pleistocene successors, but they were much closer to the anteaters
than were the latter. Aside from the skull, all parts of the skeleton
displayed this resemblance in so marked a manner that the common
derivation of the two suborders seems hardly open to question. Different
as was the skull in the two groups, the differences were not such as
to preclude the origin of both from the same type. Even more closely
connected were the †ground-sloths and the tree-sloths; the resemblance
was most clear in the teeth and skull, but there were also many points
of likeness throughout the skeleton. In the tree-sloths the entire
bony structure has been profoundly modified in adaptation to their
altogether exceptional mode of life, in hanging _suspended_ from the
branches of trees; but, despite this modification, there are so many
notable resemblances between the Santa Cruz †Gravigrada and the existing
Tardigrada as irresistibly to suggest their community of origin, and thus
the former served to connect the anteaters, on the one hand, with the
tree-sloths, on the other. This must not be construed as meaning that
the Miocene †ground-sloths were the ancestors of the other suborders,
which were probably already in existence as distinct groups, but merely
that all three suborders had a common origin, from which the Santa Cruz
†Gravigrada had departed less than have the sloths and anteaters.
There is evidence that at least two of the †ground-sloth families, the
†Megalonychidæ and the †Mylodontidæ, were distinguishable in the Deseado
stage, but materials are still lacking to give us any real knowledge of
the suborder in that or the more ancient stages.
SECTION LORICATA. ARMOURED EDENTATES
SUBORDER DASYPODA. ARMADILLOS
Armadillos are still an important and characteristic element of the
Neotropical fauna, ranging from Texas to Patagonia and showing a
considerable variety of structure and appearance. Existing species are
all of small or moderate size, and the one which is by far the largest
(_Priodontes gigas_) may somewhat exceed three feet in length, exclusive
of the tail, and the smallest (_Chlamydophorus truncatus_) is hardly more
than five inches long. In most armadillos the hair is greatly diminished
in quantity and the animal is sheathed in a conspicuous armour of bony
scutes, covered with horny plates. There is a head-shield which covers
the top of the skull, and the tail is enclosed in a sheath; the back and
sides are protected by the great carapace and the limbs by irregular
scutes and scales, leaving only the under side of the body and the inside
of the legs uncovered. In most existing genera, the carapace is in
three parts, an anterior and posterior buckler, in which the plates are
immovably fixed together by their edges, and between a varying number
of transverse, overlapping bands, from 3 to 13, which permit sufficient
flexibility of the body. The tail-sheath is made up of a series of rings.
One genus (_Tolypeutes_) has the power of rolling itself into a ball, the
head-shield exactly closing the anterior notch of the carapace and the
tail-sheath filling the posterior notch. The animal is thus perfectly
protected against attack and does not seek refuge by digging, as other
armadillos do and with astonishing rapidity. In the little Pichiciago
(_Chlamydophorus_) the dermal ossifications are very thin and the
carapace is composed of twenty transverse bands of horny plates, without
bucklers; the rump is covered with a broad and heavy shield of bone,
overlaid with thin plates of horn, which is attached to the hip-bones
and notched below for the short tail. In certain rare and little known
genera there is a greater development of hair; in one (_Praopus_) the
whole carapace is covered with a dense coat of hair, and in another
(_Scleropleura_) the middle of the back has only a hairy skin and the
carapace is restricted to the sides.
The teeth vary in number and size in the different genera; in some (_e.g.
Dasypus_) there is one upper incisor on each side; the teeth are all
simple and of nearly cylindrical form. The skull is low and flattened,
with long tapering snout and orbits widely open behind; the zygomatic
arches are uninterrupted. Most of the vertebræ of the neck are fused into
a single piece; in the lumbar and posterior dorsal regions there are not
only the usual highly intricate articulations between the vertebræ, but
also high processes on each side for the support of the carapace. The
fully ossified sternal ribs have movable joints with the breast-bone, but
not the double articulations found in the anteaters and †ground-sloths.
The shoulder-blade has a very long acromion, which does not form a bony
loop with the coracoid, and the clavicles are complete. The anterior
element (ilium) of the hip-bone is narrow, very different from the
broad plate of the †Gravigrada. The humerus has prominent deltoid and
supinator ridges and an epicondylar foramen, and the femur has the third
trochanter. Though the fore-arm bones are separate, the radius has no
freedom of rotation; tibia and fibula are coössified at both ends.
In the hind foot there is no great variety of character; it is
five-toed and usually has claws, but may have broad, flat nails (_e.g.
Priodontes_), but the manus, which is a burrowing organ, displays
different degrees of specialization, which is carried farthest in the
Giant Armadillo (_Priodontes_). _Tatu_ has the fore foot of quite
different type. The armadillos feed chiefly upon insects and worms, but
they are omnivorous and eat roots and carrion and sometimes even capture
and devour small rodents and lizards.
As in the case of the †ground-sloths, the fossil armadillos so far
available are insufficient for tracing the history of the various phyla,
or for doing more than making a very brief sketch of the development
of the suborder as a whole. Nearly all of the modern genera have been
found in the Pleistocene together with several that are extinct, some of
the latter of very large size. One of these, _†Eutatus_, had a carapace
without bucklers and made up of 33 movable, transverse bands. Another,
_†Chlamydotherium_, as large as a rhinoceros and the largest known
armadillo, had anterior and posterior bucklers, with several movable
bands between; it was especially characterized by the teeth, which were
divided by a vertical groove into pillars or lobes, thus approximating
the teeth of the †glyptodonts. The genus went far back into the Pliocene,
and the more ancient species were successively smaller.
Though remains of armadillos abound in the formations between the
Pampean and the Santa Cruz, they are for the most part so fragmentary
as to be of no service in deciphering the history of the group. In the
Santa Cruz beds also they are very abundant and varied, and several of
the genera are very completely known. As a whole, this assemblage of
armadillos was very different from that of the Pleistocene, and only a
few direct ancestors of the latter have been found in the Miocene of
Patagonia; no doubt, like the ancestral tree-sloths and anteaters, they
were then living in the warmer regions of the north. Most of the Santa
Cruz armadillos belonged to aberrant types, of which no descendants
have survived; but, nevertheless, they throw welcome light upon the
developmental stages of the suborder.
[Illustration: FIG. 292.—Skull of _†Peltephilus_, Santa Cruz. Ameghino
collection.]
These armadillos had the complete armour of head-shield, carapace and
tail-sheath, but the carapace had no anterior buckler in any of the Santa
Cruz genera, and in some there was no posterior buckler, the carapace
consisting entirely of transverse, movable bands, as in the Pleistocene
_†Eutatus_. In one especially peculiar genus, _†Peltephilus_, the
head-shield was remarkable; it was made up of large, polygonal plates,
the two anterior pairs of which were elevated into high, sharp points,
which must have supported horns, that were quite large in proportion to
the size of the animal. A 4-horned armadillo, like a tiny rhinoceros in
armour, must have been a sufficiently bizarre object.
As a rule, the teeth of the Santa Cruz armadillos were of the same
simple, cylindrical form as in the modern genera and arranged in the same
way, but there were some exceptions. In the horned _†Peltephilus_, the
teeth of each jaw were so inserted as to form a continuous series around
the sides and front of the mouth; and, at first sight, it would seem that
this genus differed from all other known edentates in having a full set
of incisors, but actually it had but one on each side above and below, as
has the modern _Dasypus_, with the difference that, in the latter, the
incisors of the opposite sides are widely separated and in _†Peltephilus_
were brought close together. The anterior upper teeth were long and sharp
and passed outside of the lower ones, when the jaws were closed, and
all the teeth had an external layer of hard and shining dentine, which
had almost the appearance of enamel. Another variant in dentition was
_†Proeutatus_, which was the largest of Santa Cruz armadillos and larger
than any existing forms except _Priodontes_ and _Cabassous_. It had teeth
like those of the huge Pliocene and Pleistocene _†Chlamydotherium_, of
which it was a probable ancestor; the five posterior ones in each jaw
were of trihedral shape, and the two kinds of dentine, of which they were
composed, were so arranged as to form a rough grinding surface. Probably
this animal subsisted largely upon vegetable food; at all events, the
food was of such a nature as to keep the teeth worn down more than in
any of the associated genera. A fourth type of dentition was displayed
by _†Stegotherium_ (Fig. 243, p. 480); the teeth were so few and small
that they can have had no functional value and were merely minute points
almost level with the gums. In all probability, _†Stegotherium_ was more
exclusively insectivorous than the other genera.
Among the Santa Cruz armadillos may be distinguished four well-marked
types of skull. (1) That which agrees closely with the modern form,
especially as exemplified by the genus _Dasypus_. (2) _†Proeutatus_ had
a higher and less flattened cranium and a very long, cylindrical muzzle.
(3) In the horned _†Peltephilus_ the face was very short and broad,
and the lower jaw was horseshoe-shaped, the two halves coössified at
the symphysis, which is not true of any other armadillo. (4) Quite the
opposite extreme was displayed by _†Stegotherium_, in which the face was
drawn out into a very long, slender and tapering muzzle; the lower jaw
was extremely weak and thin, the posterior, ascending portion low and
ill-defined, the condyle and coronoid process much reduced. No other
known armadillo has such fragile jaws, and there was a distinct likeness
in the skull to that of the Ant-Bear.
[Illustration: FIG. 293.—Skull of _†Proeutatus_, Santa Cruz. Princeton
University Museum.]
[Illustration: FIG. 294.—Skull of _†Stegotherium_, Santa Cruz. Princeton
University Museum.]
Aside from carapace and skull, the skeleton of the Santa Cruz armadillos
was surprisingly modern. The vertebræ of the neck were coössified,
those of the lumbar and posterior dorsal regions had the extremely
complex articulations and the high processes for the support of the
carapace, just as in the Recent genera. The limb-bones did not differ
in any significant way from those of the latter, and the feet closely
resembled those of the modern _Dasypus_; none of the genera displayed
the specialization of the manus seen in _Cabassous_, _Priodontes_ or
_Tolypeutes_. Whether these specializations have all been acquired since
Santa Cruz times, or whether they had already appeared in some other
region of the continent, is a question that remains to be determined.
Little can yet be done in the way of tracing the history of the
armadillos through the stages preceding the Santa Cruz times, because of
the fragmentary character of the material. The suborder was abundantly
represented in the Deseado stage, in which some of the Santa Cruz genera
existed. Even in the most ancient of the Patagonian Tertiary formations
are found scutes of the carapace essentially like those of the modern
armadillos. The group is thus of very high antiquity, older than any
other of the suborders is known to be.
In addition to the typical armadillos of South America, there were, in
other continents, certain more or less doubtful forms, concerning which a
word should be said. In the Bridger Eocene of North America was a genus
(_†Metacheiromys_) of armadillo-like animals, the true relationships
of which are far from clear. The teeth were mostly lost, leaving but
one on each side of each jaw, and this _was covered with enamel_, which
is not true of any unquestioned edentate. However, this is not an
insuperable objection to the inclusion of these animals in the edentates,
for there can be no doubt that these were derived from ancestors with
enamel-covered teeth. Even in modern armadillos the enamel-organ is
formed in the embryo, though it does not perform its functions. The skull
of _†Metacheiromys_ had something of the armadillo-shape, but was not
especially characteristic. The vertebræ of the neck were all separate,
and those of the dorsal and lumbar regions did not have the complex
articulations common to all known edentates, fossil and Recent; the
sacrum had on each side but one point of contact with the hip-bones, and
the sternal ribs were not ossified. The shoulder-blade, hip-bones and
humerus were all armadillo-like. The plantigrade feet were five-toed
and the metapodials were very edentate in form. No indication of bony
armour has been found. While these curious animals may very possibly have
been referable to the Edentata and, at all events, had several features
suggestive of relationship to that order, it can hardly be maintained
that they were unequivocal members of it. In the Oligocene of France have
been obtained some very fragmentary fossils which were classified and
described as armadillos, but their character is quite problematical. It
is thus possible, though far from certain, that in the early Tertiary,
armadillo-like edentates were spread all over the northern hemisphere.
SUBORDER †GLYPTODONTIA. †GLYPTODONTS
In the Pliocene and Pleistocene these huge armoured creatures ranged
from the southern United States to Patagonia. That they were nearly
related to the armadillos is clear, but they were so greatly modified and
specialized as to demand recognition as a distinct suborder.
Aside from their enormous size, the most striking feature of the
†Glyptodontia is the extraordinary development of their defensive
armour, which was far more complete and massive than in the armadillos.
The top of the head was protected by a thick head-shield, or _casque_,
composed of several coössified plates; the body and much of the limbs
were enclosed in the immense carapace of elongate-oval, domed shape,
which covered the neck and trunk and on the sides almost reached to
the ground. This tortoise-like carapace was composed of very thick,
polygonal plates of bone (no doubt covered externally with horny plates)
immovably fixed together by their rough edges, and ornamented with an
elaborate pattern of sculpture, which varied according to the genus.
With one or two exceptions, the plates of the carapace were not arranged
in transverse rows, but formed a mosaic without discernible banding. In
the exceptions noted, the sides of the carapace were made up of bands,
and near the margins were two or three overlapping transverse bands which
permitted a minimal degree of flexibility. The tail-sheath was remarkable
and differed much in appearance and make-up in the various genera. In
_†Glyptodon_ the tail was comparatively short and the tail-sheath was
made up of a series of overlapping rings, each ring consisting of two
rows of plates; those of the second row were ornamented, on the top and
sides of the tail, with very prominent, conical projections, capped, in
the living animal, with still longer and sharper spines of horn, so that
the tail must have bristled with spikes. A more usual type of tail-sheath
was exemplified by _†Sclerocalyptus_, in which there were several
overlapping rings at the root of the tail, but for much the greater part
of its length the plates of the sheath were fused together into a long,
transversely oval tube, tapering very gently to the free end, where
it was bluntly rounded. A modification of this type was the very long
tail-sheath of _†Panochthus_, in which there were seven overlapping rings
at the root, followed by a long, massive tube, the sides of which were
set with three or more large and heavy, horn-like spines. In _†Dœdicurus_
was reached the maximum specialization of this type; the very long tube
had its free end greatly expanded and thickened into a huge, club-shaped
mass, on the top and sides of which were fixed long and sharp horns.
The teeth, which in all the known genera numbered 8/8, were all very much
alike; each was divided by two broad and deep vertical grooves on each
side into three pillars, connected by narrow necks. Harder dentine in
the centre and on the periphery of the tooth, with a softer intermediate
layer, kept the grinding surface rough through differential wear. Teeth
of this character are indicative of a vegetable diet and these great
creatures were, no doubt, as harmless and inoffensive as possible.
[Illustration: FIG. 295.—Pampean †glyptodonts, _†Dœdicurus
clavicaudatus_ and _†Glyptodon clavipes_. Restored from skeletons in the
museums of La Plata and Buenos Aires.]
The skull was remarkably short, broad and high, the facial region being
especially abbreviated; the cranium, though forming the greater part
of the skull, was yet small in comparison with the size of the animal;
it had a distinct, though not prominent, sagittal crest. The occipital
surface was inclined forward and had a very elevated position, the
condyles being near the top of the head and raised very far above the
level of the teeth. The orbits were relatively small, more or less
completely encircled with bone and as near to the top of the head as
they could be brought; this was to make room for the extremely high
teeth, which required a great depth of jaw; the elevation of the whole
cranium left unlimited space for the jaws beneath it. The zygomatic
arches were strong and curved out widely from the sides of the skull;
beneath each eye was given off a very long descending process which
projected downward, outside of the lower jaw. In most of the species the
upper profile of the skull was nearly straight, but in _†Panochthus_
it descended very steeply from the forehead to the nose. The forehead
was dome-like and the nasals extremely short. Sinuses were extensively
developed, especially in the frontals, and in _†Sclerocalyptus_ the
bones around the nostrils were grotesquely inflated. The two halves
of the lower jaw were fused together, and the symphysis was prolonged
into a short, wide spout, which projected considerably in advance of
the upper jaw, showing that the soft parts of the muzzle must have had
a corresponding extension. The horizontal portion of the lower jaw,
carrying the teeth, was short and very deep; the posterior, ascending
portion had a forward inclination and was very high.
The skeleton of the Pleistocene †glyptodonts was unique among mammals,
though evidently a modification of the armadillo type. The extreme
modification was conditioned by the enormous weight of the carapace,
which the skeleton had to support. The neck was very short, made up of
short vertebræ, which were extensively coössified; the atlas was always
free, but the axis was fused with a varying number of the succeeding
vertebræ; usually, the axis and the third to the sixth formed one mass,
while the seventh was fused with the dorsals. The joint between the sixth
and seventh vertebræ was such as to permit at least a partial downward
bending of the head beneath the carapace, closing its anterior opening
with the head-shield. The seventh neck vertebra and all the dorsals,
except the last one, were coössified into a heavy curved rod, the “dorsal
tube”; the conjoined neural arches formed a tunnel for the spinal cord
and the spines made a continuous ridge. As the hind legs were very much
longer than the fore, the back was strongly arched upward from the neck
to the hips. The last dorsal, the lumbars and the sacrum were all fused
together to form the “lumbo-sacral tube,” of which the coössified neural
spines made a very prominent ridge, the principal support of the carapace
in the median line; the anterior half of the trunk skeleton, comprising
the short, deep thorax, was free from the carapace, which in that region
must have rested upon the muscles of the back and shoulders. The number
of neck and trunk vertebræ combined varied in the different genera from
26 to 28, but fusion had reduced the number of separate parts to 4, or
at most 5. Such greatly diminished flexibility of the back was rather
an advantage. The tail differed much in length in the various genera,
but was always massive; the anterior vertebræ, usually 7 in number, were
free, the others were fused into a heavy, tapering rod; but for nearly
its whole length the processes of the vertebræ were very prominent, each
vertebra touching the tail-sheath at five points and thus giving it very
effective support. In _†Glyptodon_ the tail-vertebræ were all free.
In most of the genera the scapula was very broad and had the very long
acromion common to all the edentates; there were no clavicles. The
hip-bones were very peculiar; the anterior element (ilium) stood almost
vertically, at right angles to the backbone, and formed a broad plate,
facing forward, the top of which was roughened and thickened to support
the carapace. The posterior element (ischium) was also much expanded,
but faced outward, and its hinder end, curved upward and thickened,
was another point of strong support for the carapace. The two elements
together formed an inverted arch, the crown of which rested on the head
of the femur.
Though less massive than those of the hind leg, the bones of the fore
limb were yet very heavy. The humerus was short and had reduced deltoid
and supinator ridges and no epicondylar foramen; the short fore-arm bones
were separate and heavy, the ulna especially so. The femur was much the
longest of the limb-bones and was extremely strong, especially in its
great breadth, the antero-posterior flattening, common to nearly all very
heavy mammals, being well marked. A very unusual feature was the position
of the third trochanter near the lower end of the shaft. The tibia and
fibula were much shorter than the femur, extremely heavy and coössified
at both ends. The very short and broad feet retained five digits; in the
manus the claws were sometimes comparatively long and sharp, sometimes
blunt and hoof-like; those of the hind foot were always broad hoofs.
Among all the many strange and grotesque mammals which the study of
fossils has brought to light, none can have been more remarkable than the
Pleistocene †glyptodonts; slow-moving hillocks they must have seemed, the
larger species 12 to 14 feet long and 5 feet or more in height. Those
that had claws on the fore feet probably used them to dig for roots and
tubers, but all were plant-feeders. When attacked by the †sabre-tooth
tigers (_†Smilodon_) or the great bears (_†Arctotherium_) they needed
only to squat down, bringing the edges of the carapace to the ground, and
draw in the head, to be perfectly protected, while a sweep of the spiny
or club-like and horned tail would have been fatal to anything in its
path.
As in the case of so many other groups, little has yet been learned
regarding the history of the †glyptodonts during the interval between
the later Pliocene and the Santa Cruz; the intermediate formations have
yielded many †glyptodonts, but not in such preservation as to be of any
service in this connection. We find, as might be expected, many and very
great differences between the Pampean and the Santa Cruz representatives
of the suborder, the latter being in all respects less modified and less
widely removed from the armadillos.
(1) The most obvious and striking distinction was in size, the Santa Cruz
forms being all small and some of them very small.
(2) In all cases the carapace was made up of transverse bands, which
permitted a slight degree of flexibility, and near the anterior end, at
the margins of the shell, were two or three overlapping bands. The plates
were thin and were but rarely coössified; the ornamentation was made by
shallow grooves.
(3) The tail-sheath, which was of very uniform character, consisted
of two quite distinct portions; the anterior region consisted of 5 or
more freely movable, overlapping rings, each of two rows of plates, and
in the posterior region the rings were closely fitted together, less
distinctly marked and not movable. This posterior portion was sometimes
thick and ended abruptly, sometimes slender and tapering and in one genus
(_†Asterostemma_) it was very armadillo-like. In none of the genera
were there any spines or horns, nor were the separate plates ever fused
together to form a tube.
(4) There was considerable variety in the head-shield, which was usually
made up of many separate plates, but in one genus (_†Eucinepeltus_) they
were coössified into a single heavy casque.
(5) The teeth had a less extreme height and the four anterior ones of
each jaw were much simpler than in the Pampean forms. An interesting
survival was the retention of two minute incisors in each premaxillary
bone, in one genus (_†Propalæohoplophorus_), but these were of no
functional value and were early lost.
(6) The skull was much longer, narrower and lower and had a relatively
longer facial portion; the occiput was higher and more erect, and the
condyles had no such elevation above the level of the teeth; the orbit
was widely open behind and the descending process given off from the
zygomatic arch beneath the eye had no such exaggerated length; the bones
were not conspicuously inflated by sinuses. The lower jaw was shallower,
the symphysis and anterior spout shorter and the ascending portion far
lower.
(7) The backbone had a greater number of separate parts; the atlas, as
always, was free, the axis was fused with two or three of the following
vertebræ; the sixth was free and the seventh fused with the first and
second dorsals to form one piece, which was succeeded by two or three
separate vertebræ: the other dorsals, except the last one, were united in
the dorsal tube, and the lumbo-sacral tube was already complete. Thus,
instead of four or five, there were eight or nine distinct parts. None of
the tail-vertebræ were fused together.
(8) There was the same disparity in the length of the fore and hind
limbs, but the bones were far more slender and armadillo-like; this
was especially true of the radius and humerus, the latter having
well-developed deltoid and supinator ridges and epicondylar foramen; the
ulna was more massive and glyptodont-like. The femur was very much more
slender and rounded and the third trochanter was placed higher up the
shaft; tibia and fibula were coössified at both ends and resembled those
of the Pampean genera, except for their much greater slenderness.
(9) The feet were much as in the latter, but relatively narrower, and the
manus had longer claws.
In short, the Santa Cruz †glyptodonts departed much less widely from
the armadillos than did the Pliocene and Pleistocene genera, and, to a
certain extent, bridged over the gap between the two suborders. Such
backward convergence in time is very strong evidence for the community of
origin of the two groups.
The †glyptodonts of the more ancient formations, so far as they are
known, teach us little concerning the stages of modification in these
extraordinary animals, because of their fragmentary condition. The oldest
stage in which representatives of the suborder have been detected is the
Astraponotus beds, which may be Oligocene or upper Eocene. On the face
of the records, therefore, the †glyptodonts had no such antiquity as the
armadillos.
* * * * *
It has long been recognized that the Edentata occupy a very isolated
position among the placental mammals; their relationships to other orders
and their point of departure from the main stem are unsolved problems.
The South American fossils have so far thrown little light into these
dark places, but they bear very cogent witness to the unity of origin of
the five suborders, which were most probably all derived from a single
early Eocene or Paleocene group.
In the Paleocene and through most of the Eocene of North America there
lived an order of mammals called the †Tæniodontia (or †Ganodonta)
which many of the foremost palæontologists regard as an ancestral type
of the Edentata, and Dr. Schlosser actually includes them in that
order. That the †tæniodonts had certain striking resemblances to the
edentates, especially to the †ground-sloths, is not to be denied, but
the interpretation of these resemblances is a very complex and difficult
question. Unfortunately, no member of the order is known from an even
approximately complete skeleton, and therefore a discussion of the matter
here would be unprofitable. My own conclusion, however, may be stated,
to the effect that the supposed relationship of the †tæniodonts to the
edentates is illusory and not real. Definite decision must await the
finding of more complete material both of the †tæniodonts and the most
ancient South American edentates.