UNGULATA
The systematic arrangement of the South American ungulates is of such
a nature that scarcely two students of these forms have agreed. I feel
that the Pyrotheridae are proboscideans as did Ameghino, but there
my agreement ends. The other varied groups I believe have a common
ancestry, their great divergencies being due to adaptations to the
greatly varied characters of the country they occupied. In spite of the
great variation they have certain features in common so that I agree
with those who have developed the term Notungulata to include them all.
From what source they originally came is not clear, but it seems to
me that these notungulates have more in common with what we know of
the African fauna of the Fayum than with any other fauna; so that my
feeling would be that these two faunas had a common ancestry at least,
and possibly the South American ungulates are derived from the African.
The lophiodont upper dentition, the bicrescentric lower molars with a
“pillar” in the posterior crescent, the development of the tympanic
bulla with the extension of the inflated cavity up into the squamosal
bone, the development of the post-tympanic portion of the squamosum,
and the general arrangement of the basicranial foramena indicate in my
mind that these notungulates have all risen from the same stock, and
that that stock had much in common with the hyracoids.
I should therefore arrange the various groups as follows.[11]
[Footnote 11: The following references discuss in detail the
arrangement of these forms. Ameghino, 1906, Formations Sedimentaires,
Anal. Museo Nac. de Buenos Aires, ser. 3, t. 8, p. 287-498: Roth, Los
Ungulados Sudamericanos, Anal. Mus. La Plata, t. 5, 1903, p. 1-36:
Scott, Princeton Patagonian Expeditions, vol. 6, p. 287-299, 1912:
Gregory, Bul. Amer. Mus. Nat. Hist., vol. 27, p. 273-285, 1910.]
NOTUNGULATA
Order I. Upper molars composed of an external longitudinal
crest and two transverse crests, the posterior the
less developed; lower molars composed of two joining crescents
with a “pillar” in the posterior crescent; structure
of the feet and limbs varying.
Suborder 1. =Litopterna=: teeth brachydont to
hypsodont; lower molars with the anterior and posterior
crescents subequal; squamoso-periotic region not
inflated; limbs elongated; pes unguligrade; digits 3-3
or 1-1.
Suborder 2. =Typotheria=: teeth hypsodont lower molars
with the anterior crescent shorter than the posterior;
squamoso-periotic region inflated; limbs elongated in
varying degrees; pes plantigrade to semi-plantigrade;
digits 5-4 or 4-4.
Suborder 3. =Toxodontia=: teeth brachydont to
hypsodont; lower molars with the anterior crescent
shorter than the posterior; squamoso-periotic region
inflated; limbs short; pes semidigitigrade to
digitigrade; digits 3-3.
Suborder 4. =Homalodontotheria=: teeth brachydont;
lower molars similar to those of Toxodontia; limbs
moderately elongate; pes semidigitigrade; digits with
large curved claws, 5-5.
Suborder 5. =Astrapotheria=: teeth brachydont to
moderately hypsodont; canines enlarged into tushes;
molars similar to those of Toxodontia; limbs greatly
elongated; feet unknown.
PROBOSCIDEA
Order II. (see page 68)
Suborder 1. =Pyrotheria=: incisors developed into
tushes; molars bilophodont; limbs short, especially the
lower element; feet digitigrade.
LITOPTERNA
This order of South American ungulates is less abundantly represented
in the Deseado formation than in the Santa Cruz, but most of the genera
of this latter formation have representatives in the Deseado so that
they seem to have diverged still earlier.
By Scott the order is divided into two families, the _Proterotheriidae_
and the _Macrauchenidae_, the less known Adiantidae being placed under
the latter family until better known. I feel that I should prefer to
retain the Adiantidae for the present, until they can be shown to be
subordinate to another family, so that in this paper the three families
are retained. The striking features of the two larger families may be
best brought out by a comparison of their chief features as follows.
Proterotheriidae Macrauchenidae
1 0 4 3 3 1 4 3
Formula --------- -------
2 0 4 3 3 1 4 3
Upper inc. 2 and lower inc. Incisors, canine, and premolar
3 enlarged and tush-like, 1 simple, compressed, subequal
growing from persistent pulps. in size, and rooted.
Nasals normal. Nasals shortened indicating
a proboscis.
Neck short. Neck long.
Feet with median digit enlarged, Feet with all three digits
lateral digits reduced. subequal in size.
=Proterotheriidae= Ameghino
In the Deseado, this family is scantily represented as compared
with the rich fauna, both as to species and numbers of individuals
in the Santa Cruz, but of the four chief genera of the Santa Cruz,
three have been found, though the remains are very fragmentary. They
are the genera _Eoprototherium_, belonging to the _Prototherium_
series, _Deuterotherium_ belonging to the _Thaotherium_ series, and
_Notodiaphorus_ representing the _Diadiaphorus_ series.
The following table will give what is known in comparing the two series.
PERIOD UPPER MOLARS NASALS PES
Proterotherium Santa Cruz metaconule present normal tridactyl
protoconule and
protocone separate
Eoproterotherium Deseado metaconule present
protoconule and
protocone separate
Licaphrium Santa Cruz metaconule present normal tridactyl
Diadiaphorus Santa Cruz metaconule present short tridactyl
protoconule and
protocone fused
Notodiaphorus Deseado tridactyl
Thaotherium Santa Cruz metaconule lacking normal monodactyl
protoconule and
protocone separate
Deuterotherium Deseado metaconule lacking
otoconule and
protocone separate
=Eoproterotherium= Ameghino
Eoproterotherium Amegh., 1904, Anal. Mus. Nac. B. A., ser. 3, t. 3, p.
441.
The genus is founded on single teeth of the upper molar series, which,
except for size, are very like those of _Proterotherium_. Limbs, etc.,
are unknown, so that this genus is simply a carrying back of the
_Proterotherium_ line into the Deseado. We found no teeth of this form,
but one species has been described, _E. inaequifacies_, of which I
reproduce Ameghino’s figure compared with _Proterotherium_, which shows
this species to have the metaconule better developed.
[Illustration: Fig. 5. _A_, Eoproterotherium inaequifacies, third
upper molar—natural size; _C_, Protherotherium karaikense, third upper
molar—natural size, after Ameghino.]
=Notodiaphorus= gen. nov.
The basis of this genus is particularly a hind limb found associated
which is much less developed than the Santa Cruz genus _Diadiaphorus_
to which it is most nearly related. These two genera are unique in
having the ectal facet on the astragulus developed in two planes so
that it appears as a deep notch. In the case of the new genus the toes
are almost equal in size, giving us a stage in the development of this
three-toed form which is much more primitive than the well-known Santa
Cruz genus.
=Notodiaphorus crassus= sp. nov.
The specimen selected as type is number 3287 of the Amherst Collection,
consisting of a complete pes, tarsus, lower end of the tibia, and the
femur, from the Deseado on the Chico del Chubut River, west of Puerto
Visser. Beside this, there are seven other specimens, mostly parts
of hind limbs, but others having also the lower end of the humerus,
the radius and ulna, metacarpals, and some phalanges. The species is
distinguished by its large size, being larger than the species of the
Santa Cruz, and, at the same time, the three toes of both the pes and
the manus are subequal in size.
[Illustration: Fig. 6. Distal end of right humerus—½ natural size.]
The distal end of the humerus associated indirectly with this species
is moderately heavy, with fair-sized epicondyles, and no entepicondylar
foramen. The supratrochlear fossa is moderately deep, the anconeal
very deep, the two being connected by a small foramen, as is typical
for this family. The trochlearis, slightly oblique to the long axis
of the shaft, has a simple pulley-like articular end without ridges
of division, the internal border being narrower and higher than the
external.
MEASUREMENTS, SPECIMEN 3201
Humerus, greatest diameter of the distal end 58 mm.
width of trochlea on the anterior side 37 mm.
width of trochlea on the posterior side 28 mm.
[Illustration: Fig. 7. Right radius and ulna, distal end of ulna from
specimen No. 3275—½ natural size.]
[Illustration: Fig. 8. Left femur posterior side—½ natural size.]
The radius and ulna were from another specimen which, however, was
associated with a typical astragulus. The two bones are long, slender,
strongly curved, and in contact with each other throughout their
entire length, so that there could have been no rotary movement of
the forearm. The radius is a slender bone with the proximal articular
facet relatively small, the facet being slightly concave, of ovoid
outline and with the transverse diameter the greater. There is but a
tiny band-like facet for the ulna situated on the posterior side near
the inner margin. Distally, the radius widens into a heavy end with a
rugose area on the outer side for contact with the ulna, and with two
distal facets, a larger for the scaphoid, and a smaller for the lunar,
the two being separated by a low ridge.
The ulna is heavier above, with a strong backwardly directed olecranon
process. The sigmoid notch makes almost a semicircle, the articular
surface being broad and extending well onto either side of the bone.
The facets for the radius are tiny. The distal end of this bone is
wanting.
MEASUREMENTS, SPECIMEN NO. 3275
Radius, length 251 mm.
greatest width at proximal end 28 mm.
greatest width at distal end 36 mm.
least diameter of shaft 16 mm.
[Illustration: Fig. 9. Distal end of left tibia—½ natural size.]
The femur belongs to the type specimen which is about 5% larger than
the other specimens. This bone is long and rather slender, with the
greater trochanter rising well above the head, which is rounded, on a
short neck, and has the ligamentary pit on the posterior margin. The
thick, rugose, greater trochanter bends in over the head at its upper
end. The lesser trochanter is relatively small, and prolonged into a
ridge. Unfortunately the third trochanter is broken off in my specimen.
The digital fossa is extremely large and deep. Proximally the shaft is
flattened, but becomes rounded distally. Just above the condyles there
is a deep rugose pit for the plantaris muscle, and on the anterior
side the suprapatellar fossa is well marked. The condyles are placed
a trifle obliquely; the internal one being shorter and with a rounded
articular face, the external condyle being longer, and with a flattened
articular face which slopes obliquely inward.
Of the tibia, only the distal end is preserved. This indicates a rather
slender bone, with a shallow, fairly wide concavity for the external
astragular trochlea, and a narrower and deeper concavity for the
internal astragular trochlea. On the internal side of the tibia there
is a rugose surface for the fibula.
An isolated lower end of a fibula indicates a slender bone, enlarged
distally where it comes in contact with the tibia. The fibula carries
on its inner face a moderately large facet for the external side of
the astragulus, and on the distal end a wider one for contact with the
calcaneum.
The tarsus is compactly built, wider than that of Diadiaphorus, because
the external digits are not as much reduced. This especially shows in
the greater development of the cuboid and the mesocuneiform, but in
other features it is similar to that of its descendant.
[Illustration: Fig. 10. Left pes, dorsal side, ungual phalanx from
specimen No. 3275—½ natural size.]
The astragulus is a very characteristic bone. The trochlea is
asymetrical, the external condyle rising higher than the internal, and
the median groove being wide and shallow. On the nearly vertical outer
face of the astragulus, there is a semicircular band-like facet for the
fibula. The trochlea extends well around the top of the bone, allowing
a wide movement of the foot. The neck of the astragulus is long and
wide, carrying a broad flattened head, with its convex facet for the
navicular, covering the entire end. On the plantar side are the most
marked features. The ectal facet is in two planes, the anterior portion
being bent down to nearly right angles with the posterior, which seems
to be characteristic of this _Diadiaphorus_ series. The sustentacular
facet also is characteristic, being gently rounded and extending
clear to the navicular facet on the head, in _Diadiaphorus_ becoming
actually confluent with the navicular facet. Just at the edge of this
sustentacular facet is a tiny surface where the astragulus rubs on the
cuboid, the only case, as far as I am aware, where this occurs in any
_Litopterna_.
[Illustration: Fig. 11. Left astragulus plantar side: _a_, ectal
facet—½ natural size; _b_, sustentacular facet; _c_, facet for cuboid.]
The calcaneum is long and slender, the tuber being but slightly
enlarged, its sustentacular facet being a broad oval surface, while the
ectal facet is in two planes to correspond to that on the astragulus.
The facet for the cuboid is at the distal end, but is unusually
oblique, its inner margin sloping up almost to the sustentacular facet.
It is this slope which brings the cuboid in contact with the astragulus.
The navicular is broad and low, with a prominent hook behind. On its
upper face there is only the broad facet for the astragulus head; on
the lower face are three facets, externally, a large, more or less
triangular area, for the ectocuneiform; medianly a smaller similar
facet for the mesocuneiform; and on the internal side, sloping up onto
the internal face, a small facet for the reduced endocuneiform. On the
external face of this bone there is a tiny beveled facet for the cuboid.
The endocuneiform is a large scale-like ossicle articulating on the
lateral internal face of the navicular, and overlapping markedly the
inner surface of Metatarsus II.
The mesocuneiform is considerably reduced in size, carrying a broad
flat facet on the upper surface for the navicular, and a shallow
saddle-like one below for Mt. II, which is entirely carried by this
bone.
The ectocuneiform is considerably larger than the mesocuneiform,
resting above on the navicular, and carrying below the whole of Mt.
III. On its inner side are two facets which rub against the upper end
of Mt. II.
The cuboid is a nodular bone, its upper surface occupied by the facet
for the calcaneum, the lower face occupied by the facet for Mt. IV,
while on the external side there is a tiny beveled facet for the
vestige of Mt. V, and with a small boss on the inner surface which
carries two tiny facets, the upper one for the ectocuneiform and the
lower for the navicular. On this same inner side, near the top there
is a second small boss, which carries a tiny facet to rub on the
astragulus, and below that a second tiny facet for the navicular.
[Illustration: Fig. 12. Cuboid internal side to show: _a_, facet for
astragulus; _b_, upper facet for navicular; _c_, facet for calcaveum;
_d_, lower facet for navicular; _e_, facet for mesocunieforms—½ natural
size.]
The pes consists of three digits, with a vestige of Mt. V. Of the
developed digits, the median one is the largest, but the two lateral
digits are only a little smaller and were functional, so that this
form was truly three-toed, comparable in the digital reduction to
_Mesohippus_.
Mt. II is flattened above but soon broadens into a rounded shaft of
considerable length, on the end of which is the articular trochlea,
with the carina extending onto both the upper and lower surface,
being, however, higher on the lower surface. Proximally this bone is
overlapped by the endocuneiform, is carried by the small mesocuneiform,
and also articulates on the inner side of the ectocuneiform. Mt. III
is also compressed at the upper end, broadens below, and carries an
articular trochlea similar to that of Mt. II, except that the carina
does not extend so far onto the upper surface. Like Mt. II, Mt. IV is
carried high on the tarsus, and therefore, though nearly as long as
Mt. III, it does not have the same effective length. Proximally it
articulates entirely on the cuboid; distally it has a trochlea similar
to that of Mt. II, the carina extending onto the dorsal surface. While
Mt. V is lacking, it is clearly indicated that a vestige of it should
have been present, as there is a tiny articular surface for it on the
cuboid, and a rugose surface on the outside of Mt. IV.
The phalanges are long and have the articular ends swollen somewhat as
in camels. The phalanges of the first row are nearly equal in size,
each with the proximal trochlea deeply notched for the carina of the
metatarsus; and with the distal trochlea simple, though slightly
concave from side to side, and reflexed well onto the dorsal surface.
The phalanges of the second row are shorter and simpler, and somewhat
depressed distally. The ungual phalanges are flattened from top to
bottom, of moderate size, somewhat longer than wide, and without any
indications of a cleft.
MEASUREMENTS, SPECIMEN NO. 3287 NO. 3275
Femur, length 289 mm.
diameter across gr. trochanter 80 mm.
diameter of middle of shaft 32 mm.
diameter of distal end 70 mm.
Tibia, diameter of shaft 28 mm., 24 mm.
diameter at distal end 38 mm., 36 mm.
Calcaneum, length 103 mm., 96 mm.
width 36 mm., 35 mm.
Astragulus, length 48 mm., 44 mm.
width 38 mm., 35 mm.
Metatarsus II, length 114 mm., 105 mm.
Metatarsus III, length 122 mm., 114 mm.
Metatarsus IV, length 110 mm., 101 mm.
Phalanx 1 of digit III, length 49 mm.
Phalanx 2 of digit III, length 27 mm.
Phalanx 3 of digit III, length 29 mm.
=Deuterotherium= Ameghino
Deuterotherium Amegh., 1895, Bol. Inst. Geog. Argen., t. 15, p. 633.
Deuterotherium Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 452.
This genus was first founded on a clacaneum and a bit of the
mandibular symphysis, to which were added, later, both the upper
and lower premolar and molar teeth. As far as it is known, it is
distinguished by the upper molars lacking the metaconule entirely, and
being approximately like those of Thaotherium. The dental formula is
given by Ameghino as
1 0 4 3
-------,
2 0 4 3
the same as Thaotherium. But one species has been described.
=Deuterotherium distichum= Ameghino
We did not find this species, but the teeth assigned to it are very
characteristic, and so I reproduce Ameghino’s figure of them. The
species is distinguished by its size primarily. The following are the
chief measurements given.
[Illustration: Fig. 13. Upper pm. 3-m. 3 of the left side—natural size,
after Ameghino.]
Upper dentition, pm. 3 to m. 3, length 50 mm.
Lower dentition, inc. 1 to m. 3, length 80 mm.
Macrauchenidae
(= Mesorhinidae Amegh.)
This family is distinguished, first, by the complete dental series in
which none of the anterior teeth are developed into tushes; by the
nasals being shortened, apparently in connection with the development
of a proboscis; by its long neck; and by its feet being permanently
tridactyl, all the three toes being equally developed. In the Deseado
it is infrequent, but to it Ameghino has assigned two genera;
_Protheosodon_, which he describes as similar to _Theosodon_, but which
I find much nearer to the Casamayor representatives of this family,
such as _Lambdaconus_, though it doubtless belongs to the series which
is represented in the Santa Cruz by _Theosodon_. He has also made a
second genus, _Conioptotherium_, which represents a large Macrauchenid,
equal in size to _Theosodon_. This genus is based on the calcaleum and
astragulus and seems to be rare.
=Protheosodon= Ameghino
Protheosodon, Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 453.
Protheosodon, Amegh., 1904, Anal. Mus. Nac. B.A., ser. 3, t. 3, p. 421.
This genus was founded on an upper second molar and the fourth
premolar. I figure m. 2, and it will be seen that they represent a
form little specialized, resembling in the low crowns, plump cusps,
and presence of both protoconule and metaconule, the Casamayor types,
such as _Lambdaconus_ or _Didolodus_, rather than the advanced type
like the Santa Cruz genus, _Theosodon_. We found a specimen with the
lower jaws complete and with the hind limb complete, which, I am
confident, is the same form, though I can not duplicate any tooth, for
we found no upper teeth; but in size they agree with _Protheosodon_,
also in the primitive character; and, were one from the lower teeth to
postulate the upper, they would be just such as Ameghino has described
under the name _Protheosodon_. Therefore I have assigned my material
to this genus and species. It adds to the genus characters the fact
that this form had a shorter back, relatively as well as actually,
than _Theosodon_; that the hind limb, at least, was much heavier and
also shorter than that of _Theosodon_, especially in the metatarsal
region where relatively the elements are only about half as long. The
pes is of the same character as in _Theosodon_, but again relatively
much shorter. I believe in _Protheosodon_ we have to do with a form
intermediate between _Lambdaconus_ and _Theosodon_, and nearer to the
former.
=Protheosodon coniferus= Ameghino.
P. coniferus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 453.
Ameghino has described two upper teeth. Specimen No. 3001 of the
Amherst Collection from the Chico del Chubut River, west of Puerto
Visser, adds to this the knowledge of twelve vertebrae (seven dorsal
and five lumbar), the lower dentition complete, the left hind limb
complete, and the right hind limb complete except for the femur. In
general, the animal is about ⅗ the size of _Theosodon garrettorum_, but
in parts varies from this as follows. The lower jaw is ⅗, the vertebrae
are ⅖ in length, the hind leg is ⅗ in length but ¾ in diameter of
bones, while the metatarsus is only ⅓ in length. This makes an almost
plantigrade form of heavy, clumsy proportions.
[Illustration: Fig. 14. Upper molar. 2 of the right side—natural size,
after Ameghino.]
Of the upper dentition we know only what Ameghino has given us. The
molar is distinguished by the presence of both the protoconule and
metaconule, by the development of the posterior cingulum and by the
presence of three external styles.
MEASUREMENTS
Upper premolar 4, length 12 mm., width 15 mm.
Upper molar 2, length 14 mm., width 17 mm.
[Illustration: Fig. 15. Right lower dentition—natural size.]
In the lower dentition, none of the teeth are reduced, and all are in
a continuous series, except that there is a small diastema either side
of pm. 1. The incisors are simple, compressed teeth, with but a trace
of a cingulum. The canine is incisiform and a trifle larger than the
incisors. Premolar 1 is also incisiform, and is isolated by a small
diastema on either side. The second premolar is longer and wider than
the first, and begins to show molariform characters, the anterior
portion being composed of a high compressed cusp, the posterior portion
by a low crescent on which but one cusp is fully developed. The third
premolar is composed of two complete crescents, and has the “pillar”
already developed opposite the posterior end of the back crescent. In
fact, the tooth is molariform, except as to the tiny extra cusp found
on the molars. Premolar 4 is more completely molariform consisting of
the same parts as the preceding tooth.
The molars may be distinguished by the presence of a tiny median
cusp on the rear of the tooth, behind the crescent, which, when the
tooth is worn, makes a median spur to the rear. In both the premolars
and molars, the teeth are characterized by their plumpness, and the
isolation and lowness of the cusps.
[Illustration: Fig. 16. Right mandible—½ natural size.]
The two halves of the lower jaw are completely fused at the symphysis.
The horizontal ramus is thick, but low dorso-ventrally, giving the
appearance of a slender jaw. The posterior angle is prolonged backward
and bent inward. The fossa for the masseter muscle, while large, is
but faintly outlined. The ascending ramus hardly rises above the level
of the teeth, except as the slender coronoid projects to a good height
above the articular condyle and curves backward over it.
MEASUREMENTS
Lower dentition, total length 114 mm.
incisors, length 20 mm.
canine, length 8 mm.
premolar 2 to 4 35 mm.
molar 1 to 3 42 mm.
Mandible, total length 188 mm.
height under molar 1 24 mm.
height to top of coronoid 95 mm.
The dorsal vertebrae have short, wide, and somewhat depressed centra
(in this individual the epiphyses are free, though this is the only
indication of youth). The lower rib facets are small, that on the
posterior margin of the centrum being a mere streak, while the one
on the anterior margin is narrow. The upper rib facet is a rounded
convex surface on the end of a short stout transverse process. The
prezygapophyses are convex surfaces, wide transversely, but narrow
in the antero-posterior direction, while the postzygapophyses are
correspondingly narrow concave facets under the rear of the spines. The
spines are thin and high, and the neural canal is nearly circular in
section.
The lumbar vertebrae have laterally compressed, deep centra, with very
long transverse processes, shorting spines, and zygapophyses of the
subcylindrical interlocking type. In all their features the vertebrae
resemble those of _Theosodon_, being nearly as highly specialized and
in the same manner.
MEASUREMENTS OF TYPICAL VERTEBRAE
Dorsal vertebra No. 7, length 23 mm.
width of centrum 22 mm.
Dorsal vertebra No. 9, length 28 mm.
Lumbar vertebra No. 2, length 29 mm.
Lumbar vertebra No. 2, width of centrum 24 mm.
Lumbar vertebra No. 2, width across transverse processes 160 mm.
The femur is short and very stocky. The rounded head is carried
on a short neck, and does not rise nearly as high as the greater
trochanter, the sulcus for the round ligament being a broad, deep
notch on the posterior margin. The greater trochanter is rugose, heavy,
and high, but not incurved at the top. The lesser trochanter is a
small, thin ridge well below the head. The third trochanter is a large,
thin process, projecting almost directly backward, though curved inward
at the end, and is situated well below the middle of the bone. The
shaft of the femur is flattened above, but thick, and changes in the
lower part to subcylindrical. The condyles are small, subequal in size,
and widely separated, while the rotular trochlea is relatively wide and
shallow.
[Illustration: Fig. 17. Left femur anterior side—½ natural size.]
[Illustration: Fig. 18. Tibia and fibula (right)—½ natural size.]
The tibia is about three-fourths the length of the femur, very stocky
and heavily built. On the proximal end, the convex external condyle is
much narrower antero-posteriorly than the larger and slightly concave
internal condyle. The low spine is bifid. A cnemidial crest extends
to the middle of the bone. On the distal end, the broad and shallow
external articular facet is separated from the narrow and deeper
internal facet by a low intercondylar ridge. The fibula is fused to the
tibia at the upper end, but is free below, being approximated to the
tibia along a rugose surface nearly an inch long. This bone is rather
slender and strongly bowed outward. Distally, there is a large facet
for the outside of the astragulus, the back part of which rests on
the calcaneum. This is peculiarly developed so that the articulation
represents what is two separate facets, the one for the outside of the
astragulus the other for the calcaneum. Here, however, they are blended.
[Illustration: Fig. 19. Right foot, the phalanges in outline from the
left foot—½ natural size.]
[Illustration: Fig. 20. Astragulus plantar side—_a_, external facet;
_b_, sustentacular facet—½ natural size.]
While in general the tarsus is similar to that of _Theosodon_, there
are some marked contrasts. The astragulus has an asymetrical trochlea
with a shallow groove, the external condyle being higher and narrower
than the internal. The head is depressed in the dorso-plantar plane,
is carried on a moderately long neck, and has a broad convex facet for
the navicular on which alone it articulates. On the plantar side, the
ectal facet is broadly oval and slightly concave, differing from that
of _Theosodon_ in having no sulcus dividing it into lobes. The broad
sustentacular facet is slightly convex, and widely separated from
the ectal. On the external side the astragulus carries an expanded
articular facet for the inner side of the fibula, which, instead of
being vertical, is expanded below, making an oblique face which is
continuous with the fibular facet on the calcaneum. In this feature
_Protheosodon_ is, as far as the feet are known, unique.
The calcaneum is a long bone with a club-shaped expansion of the upper
end. The fibular facet is small, being continuous, as above described,
with that on the outer side of the astragulus. On the face toward
the astragulus, the ectal facet is broadly convex (not divided as in
_Theosodon_), while the sustentacular facet is slightly concave. The
distal end is occupied by the large concave facet for the cuboid.
The navicular is of moderate height, with a prominent hook behind. On
the upper surface is only the broad, deep facet for the astragulus;
while the lower surface is divided into facets for the three
cuneiforms, and the external distal margin is beveled to make a narrow
facet for the cuboid. This navicular differs from that of Theosodon
in that the facet for the ectocuneiform is not cut step-like into its
external face.
The endocuneiform is a small scale-like bone with a narrow facet on the
navicular, and overlapping the inner side of Mt. II. The mesocuneiform
is small, with a flat facet above for the navicular, and a convex
one below for Mt. II, which is carried wholly on this bone. The
ectocuneiform is far the largest of these three bones, and carries a
broad facet above for the navicular, a similar one below for Mt. III, a
small facet on the internal side for the mesocuneiform and a second one
below that for the side of Mt. II, while externally there are facets
for the cuboid and for the side of Mt. IV.
The cuboid is large, the external side being longer than the internal.
The upper surface is entirely occupied by the facet for the calcaneum,
while the lower face is mostly devoted to the facet for Mt. IV, with
a narrow streak on the external margin for the vestige of Mt. V.
The internal face carries a boss beveled above by the facet for the
navicular, and below by the facet for the ectocuneiform.
All the metatarsals are short and heavy as compared with those of
_Theosodon_. Mt. II is compressed above, but enlarges below into a
subcylindrical bone, ending in an extensive articular trochlea for
the phalanx, the trochlea carrying a carina which extends into the
upper surface of the articular area. Proximally, it is so closely
approximated to the adjacent metatarsus that these could have had very
little independent movement. On the upper internal surface, there is
a roughened area, where the endocuneiform overlaps this bone. Mt. III
is slightly heavier than the others. On the distal end, its articular
trochlea extends well onto the dorsal surface, as does also the carina.
Mt. IV is a trifle shorter than the others and stouter. Mt. V is absent
but its former presence is indicated by the beveled facet on the
cuboid, and by the small roughened surface on Mt. IV.
The phalanges of the third digit are a trifle heavier than those of the
other two digits, but of approximately the same lengths. The ungual
phalanges were broad compressed hoofs, without traces of clefts.
MEASUREMENTS OF THE HIND LIMB
Femur, length from the head 177 mm.
greatest proximal width 70 mm.
greatest distal width 56 mm.
Tibia, total length 149 mm.
greatest proximal width 52 mm.
greatest distal width 56 mm.
Fibula diameter of shaft 9 mm.
Astragulus, length 23 mm.
width 24 mm.
Calcaneum, length 62 mm.
Metatarsus II, length 45 mm.
Metatarsus III, length 48 mm.
Metatarsus IV, length 42 mm.
Phalanx 1 of digit III, length 24 mm.
Phalanx 2 of digit III, length 16 mm.
Phalanx 3 of digit III, length 17 mm.
[Illustration: Fig. 21. (Half tone.) Restoration of Protheosodon
coniferus to show the relatively short limbs, and heavy build as
compared with Theosodon—⅕ natural size.]
RESTORATION
In order to get a comparison of what is known of this form with
_Theosodon_ I have outlined a restoration of the animal as a whole,
realizing that some essential parts are lacking, but the general
proportions can hardly vary greatly from those given. It appears,
first, that this form has an unusually short back. Though the limbs and
lower jaw are ⅗ the length of those of _Theosodon garrettorum_, the
vertebrae are ½ as long. I have assumed that the number of vertebrae
would prove to be the same as in _Theosodon_. While the limb bones
are ⅗ as long as in the _Theosodon_, they are relatively half again
as heavy and with the processes much more developed. The greatest
difference is found in the tarsus which is only ⅓ as long as that of
_Theosodon_, though relatively as heavy, and the foot was carried in
a nearly plantigrade position the heel raised but a little from the
ground, though the anticular ends of the metatarsals and the phalanges
indicate that there was a considerable freedom of movement of the
various elements. The form seems to be fairly close to the ancestral
types such as _Lambdaconus_ of the Casamayor, the limbs of which,
however, are entirely unknown, but I should expect that when found
these earlier forms would prove to be approximately plantigrade.
=Coniopternium= Ameghino
Coniopternium Amegh., 1895 Bol. Inst. Geog. Argen., t. 15, p. 632.
Coniopternium Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 453.
The genus is based on a calcaneum and astragulus of the macrauchenid
type, but of unusually large size. The real generic characters are
not evident in the description, but the presence of these bones,
and of three cervical vertebrae, which we also found, indicating a
macrauchenid of about the same size, are evidence that a form larger
than the Santa Cruz representatives will turn up in the Deseado beds,
for which this name may be reserved. The material is described under
the specific name _C. andinum_.
=Adianthidae= Ameghino
This family is based primarily on the genus Adianthus of the Santa
Cruz to contain some macrauchenid-like forms which, however, are of
much smaller size, and differentiated by the narrow character of the
teeth and their early tendency to hypsodonty. It seems to be a valid
series of dwarf types, which are all scarce and known only by the
most fragmentary remains. Two genera are described from the Deseado,
_Tricoelodus_, peculiar in having the posterior lobe of the lower
molars somewhat subdivided so that the tooth appears three-lobed; and
_Proadianthus_, known only by premolars which however show an unusual
development of the styles on the inner side of the teeth.
=Tricoelodus= Ameghino
Tricoelodus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 454.
The genus is based primarily on the three-lobed character of the
molars, which is a secondary effect of an infolding on the inner side
of the posterior lobe. They are rooted, but strongly hypsodont. The
margins of the crescents are well developed and the “pillar” is a
prominent feature in the posterior crescent.
=Tricoelodus bicuspidatus= Ameghino
T. tricuspidatus Amegh., loc. cit. above.
[Illustration: Fig. 22. Lower right pm. 3-m. 1—natural size, after
Ameghino.]
The species is the only one known of the genus, and its features are
those of the genus. The following measurements indicate the size, lower
pm. 3 to m. 1, 25 mm.; height of the mandible under molar 1 is 12 mm.
=Proadianthus= Ameghino
Proadianthus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 455.
This genus is known only by the last two premolars of the lower jaw,
which are compressed and moderately high. The two crescents are
markedly separated by a cleft from the external side of the tooth,
opposite to which is a high denticle, made by the fusion of the two
ends of the crescents where they come together.
=Proadianthus excavatus= Ameghino
P. excavatus Amegh., loc. cit. above.
The species is based on the two lower premolars described above. I
reproduce the figure given by Ameghino. The measurements are: length of
pm. 3 and 4, 10 mm.; height of mandible under pm. 4, 8 mm.
[Illustration: Fig. 23. Lower right pm. 3 and 4—natural size, after
Ameghino.]