Samuel Wendell Williston.

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which is certainly the fourth carpale. All the others disappeared as
bones but remained as cartilage, since space is left for them in many
specimens as they have been found in the rocks.



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^r



^s^



a CSS



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Es^ C3^ '^J^ _^c^^'




Fig. I46. Clidastes (Mosasaur), left front paddle: c, coracoid; h^ humerus; r, radius; sc,

scapula; u, ulna.

Fig. I47. Platecarpus (Mosasaur), right front paddle.

Fig, I48. Tylosaurus (Mosasaur), left front paddle.



iSi



l82



THE OSTEOLOGY OF THE REPTH^ES



In the swimming feet of Lariosaurus (Fig. 149) of the Nothosauria
the carpus is also reduced, the radiale apparently the most con-
spicuous for its absence.




' "(RrQoOQr-^




Fig. 149. Nothosaurian limbs: Lariosaurus. About four times natural size.



THE LIMBS



183



Very interesting are the modifications of the wrist and hand in the
marine CrocodiHa (Fig. 150). But two carpals remain, correspond-
ing to the elongated ossified bones of the terrestrial forms; the first
of them, the supposed radiale, is very broad and flat.

The carpus and hand of the
strictly aquatic or marine reptiles

are so like the ankle and foot ^*

that they may be discussed to-
gether (p. 193).

Tarsus

The earliest known tarsus is
that of Eosauravus (Fig. 151 b),
presumably a cotylosaur reptile,
though the skull is not known,
from the middle Pennsylvanian.
It has but two bones in the prox-
imal row, corresponding quite to
the astragalus and calcaneum of
mammals and the t3^ical reptiles.
Beyond these, six, and only six,
bones are visible, five of which
are undoubted tarsaha; one may
be a centrale. The whole num-
ber, eight, was the most known
in any reptile until recently.
Nine bones are present in the
tarsus of Ophiacodon (Fig. 152),
from the uppermost Pennsyl-
vanian or basal Permian of New
Mexico: two in the proximal
row, the astragalus and cal-
caneum, two centralia in the
middle row on the tibial side; and five tarsaha in the distal row,
one corresponding to each metatarsal. Since this discovery two
centralia have also been found by Watson in the genus Broomia
(Fig. 137 d), from the Permian of South Africa; and probably
also two in the cotylosaurian genus Lahidosaurus. The second




Fig. 150. Geosaurus (Thalattosuchia). Elon-
gate left hind leg, and paddle-like left front
leg. After Fraas.



i84



THE OSTEOLOGY OF THE REPTILES



centrale is usually retained in later reptiles, but the fifth tarsale is
absent in all reptiles since Triassic times, and a free centrale
is absent in all living reptiles, though present in most mammals.





Fig. 151. Limbs: A, Trematops (Temnospondyli). One half natural size. ^, Eosauravus
(Cotylosauria). About twice natural size.

Fig. 152. Ophiacodon (Theromorpha) : right hind leg, from mounted skeleton. A little
less than one third natural size, a, astragalus; c, cakaneum.



THE LIMBS 185

The mammalian foot, in this respect, is even more primitive than
that of the lizards, turtles, and crocodiles, the navicular corres-
ponding to the second centrale, the cuneiforms and cuboid to the
four tarsaHa. The fourth distale, primitively, as in the carpus and
as a general rule in all reptiles, is the largest of the series, corre-
sponding to the greater length and strength of the fourth toe.

The tarsus is known in but two temnospondylous amphibians,
both from later rocks than Eosauravus, Trematops (Fig. 151 a) , and
Archegosaurus. In the former, and according to Baur in the latter
also, there are three bones in the proximal row, the tibiale, inter-
medium, and fibulare; four centralia in the middle row; and five
tarsalia in the distal — twelve in all.

Three of these have been lost in all known reptiles, the inter-
medium, or tibiale, and the third and fourth centralia. Nine bones,
then, we may assume was the primitive number of tarsal bones in
the reptiles. A separate intermedium has been accredited to certain
reptiles, Howesia of the Rhynchocephalia, Oudenodon (Dicynodon)
of the Anomodontia, and the ichthyosaurs and plesiosaurs. But,
unless such forms have enjoyed an uninterrupted and independent
descent of which we have no knowledge from the Amphibia, it is
altogether improbable that both intermedium and tibiale have ever
been present as separate bones in reptiles since early Pennsylvanian
times. Otherwise we must assume that there has been a reversion
from the specialized to the generalized condition of the Amphibia
in these animals, a seeming impossibility in evolution. Moreover,
there are but two bones in the proximal row of the tarsus of the
Nothosauria (Fig. 149), and these reptiles are generally supposed
to have a real genetic relationship with the plesiosaurs.

There have been various theories as to what has become of the
additional bones of the amphibian tarsus.^ Since Gegenbaur, it is
generally believed that the intermedium is fused with the tibiale to
form the astragalus. This is denied by Baur, who says there is no
evidence of such union. Others have thought that the intermedium
alone forms the astragalus, the tibiale represented by the tibial sesa-
moid, which occurs in certain mammals but is unknown as such in
lower animals. In this uncertainty it is better to use the two mam-

1 [For an excellent review of this subject see Broom, 1921, in Froc. Zool. Soc,
London, pp. 143-155.— Ed.]



i86



THE OSTEOLOGY OF THE REPTILES



malian names astragalus and calcaneum and abandon the names
tibiale, fibulare, and intermedium for the reptihan tarsus. Of the




Fig. 153. Limbs: A, Mesosaurus (Proganosauria). Modified from McGregor. Natural size.
B, Sauranodon (Protorosauria). Modified from Lortet. Three fourths natural size.



centraha the most probable theory is that the fourth of the amphib-
ian tarsus has united with the astragalus, the third with the fourth
tarsale. The second is known to fuse with the astragalus in the



THE LIMBS 187

modern Chelonia (Fig. 1 54 c) ; perhaps at other times it is lost. And
it is very probable that the first centrale of the amphibian and rep-
tilian tarsus ceased very soon to be ossified, and is not represented,
even in a fused condition, in any later reptilian tarsus. It has been
shown by Baur and others that the fifth tarsale is not fused with the
fourth, but has disappeared.

Among the Cotylosauria there are usually eight tarsal bones. ^ In
Pariasaurus the centrale and fifth tarsale are not known with cer-
tainty. In the Theromorpha eight are present in all known forms
except Ophiacodon (Fig. 152), which has nine. The centrale has not
been recognized in the Proganosauria (Fig. 153 a), but there are five
tarsalia; until their discovery four were the most known in any rep-
tile. Indeed, Baur based the order Proganosauria chiefly upon this
character. All other known reptiles, except certain Therapsida (like
the mammals), have not more than seven tarsal bones, the fifth
tarsale being invariably absent.

In Pariasaurus, Sclerosaurus, and Telerpeton of the later Cotylo-
sauria, Sphenodon (Fig. 139 a) of the Rhynchocephalia, and most
Lacertilia (Fig. 140 d) and Chelonia (Figs. 145 c, 154 d, g), the
astragalus and calcaneum are fused into a single bone, and the cal-
caneum is either fused or lost in the Pterosauria (Fig. 155 d) and
some Dinosauria (Fig. 156 1). A free centrale is absent in all modern
reptiles, though sometimes suturally fused with the astragalus in
the Chelonia (Fig. 154 c).

In the Chelonia the small calcaneum is sometimes free (Fig. 154 c).
The centrale is never free. Four tarsalia are usually present, the
third sometimes suturally united with the fourth. The fourth tarsale
is always large.

In the kionocrane Lacertilia (Fig. 143 b) there is a similar condi-
tion, the small calcaneum either indistinguishably fused with the
astragalus, or suturally attached in the adult. There is no centrale
or fifth tarsale, and the first and second tarsalia are either vestigial
or lost. The tarsus of the chameleons (Rhiptoglossa), like the wrist,.
is very curiously modified (Fig. 143 b). But two bones remain in the

1 [Watson {Proc. Zool. Soc, 1919) reports the presence of three bones in the proxi-
mal row of the tarsus of the very primitive Seymouria, and adopts the view that the
true tibiale has disappeared in later reptiles, the astragalus representing the inter-
medium only. — Ed.]



1 88 THE OSTEOLOGY OF THE REPTILES

highly speciaHzed species, the astragalo-calcaneum and the large,
hemispherical fourth tarsale, articulating together enarthrodially,
around which all the short metatarsals closely articulate in two
groups of two and three.




G





Fig. 154. Limbs and feet of Chelonia. Natural size. A, Chrysemys, hind leg from post-
axial side. B, Chrysemys, front foot, dorsal side. C, Chely'dra, hind foot, dorsal side.
D, Cistudo, hind foot, dorsal side. E, Podocnemys, left hind leg, postaxial side. F,
Podocnemys, right front forearm and foot, dorsal side. G, Trionyx, left hind foot,
dorsal side.



THE LIMBS 189

The hind foot is poorly known in the Therapsida. In Galeckirus
(Fig. 137 b) of the Dromasauria the fifth tarsale is lost, but a small
one has been recognized in the related genus Galesphyrus (Fig. 137 c).
The Anomodontia have the astragalus and calcaneum, four tarsalia,
and a small, frequently unossified centrale; the fifth tarsale is ab-
sent. The tarsus is unknown in other groups.

The tarsus of the modern Sphenodon (Fig. 139 a), unlike the
carpus, is highly specialized. In addition to the fused calcaneum
and astragalus, the centrale and fifth tarsale have disappeared and
the first three tarsalia are fused in the adult.

The tarsus of the Pterosauria (Fig. 1550), like the carpus, is
highly specialized. In the early forms the astragalus is suturally
united with the tibia, the calcaneum fused with the astragalus. In
the later forms the astragalus is indistinguishably united with the
end of the tibia, the calcaneum fused or lost as in birds, forming a
large, pulley-like articulation. In the early pterodactyls there were
at least three other tarsals; in the later ones, like Pteranodon or
Nyctosaurus, there are but two free tarsalia, probably the fourth and
the fused second and third, or fused first, second, and third. Cen-
tralia are unknown in all.

The tarsus of the dinosaurs (Fig. 156), like the carpus, has been
much modified in adaptation to upright-walking habits. There is a
tendency in all for the two proximal bones, the astragalus and cal-
caneum, to articulate closely with the leg bones. The astragalus of
the Theropoda (Fig. 156 b, c, e) fits more or less closely in a depres-
sion or groove on the under and anterior side of the tibia; in the later
forms {e. g., Ornithomimus , Fig. 156 e) developing a high ascending
process in front, as in the young of birds — a parallel character
which has no genetic value. In the Sauropoda (Fig. 156 i) there is a
less close union, perhaps due to the larger amount of cartilage in the
joints of these animals. The centrale and first and fifth tarsalia are
always absent. The second and third tarsalia are often fused, ap-
parently; the fourth is always single when present. The tarsalia, like
the carpalia, are absent in the Trachodontidae (Fig. 156 g) ; even the
fourth is said to be wanting • — possibly a vestige yet remains. If
really absent it is the only known example among reptiles of the
absence of all the bones of the distal row.




Fig. 155. l^imhs: A, ^raeoscelis (Protorosauria). Three fourths natural size. B, Hal/opus (Dino-
sauria). After Marsh. One half natural size. C, Pteranodon (Pterosauria). About one third
natural size. D, Pteranodon. About five sixths natural size.



190




Fig. 156. Dinosaur pedes: A., Plateosaurus (Saurischia). After Huene. One twelfth natural
size. B, Anchisaurus (Saurischia). After Marsh. One eighth natural size. C, Allosaurus
(Saurischia). After Osborn. One seventeenth natural size. D, Struthiomimus (Saurischia).
After Osborn. A little more than one sixth natural size. ^, Ornithomimus (Saurischia).
After Marsh. One sixth natural size. F, Thescelosaurus (Ornithischia). After Gilmore.
One fifth natural size. G, Trachodon (Ornithischia). After Brown. About one nineteenth
natural size. H, Monoclonius (Ornithischia). After Brown. One sixteenth natural size.
I, Morosaurus (Saurischia). After Hatcher. About one eighth natural size. [Brontosaurus.]



191



192



THE OSTEOLOGY OF THE REPTILES



The calcaneum of the Crocodilia (Fig. 157 a, b) is produced into a
heel-like process; the first and fifth tarsalia and the centrale are ab-
sent, the second tarsale is small. Hallopus (Fig. 155 b), usually re-




FiG. 157. Crocodilian limbs: A, B, left hind limb of Alligator, dorsal and postaxial; C, left
femur dorsal; D, Alligator ellus. After Lortet. About three fourths natural size.

ferred to the Dinosauria, also has a heel-like calcaneum, as is the
case in Scleromochlus, and other genera of the Pseudosuchia, Arae-
oscelis (Fig. 155 a), Broomia (Fig. 137 d), and other leaping or
springing reptiles.



THE LIMBS



193



In the web-footed Mosasauria the tarsus, like the carpus (Figs.
146-148), progressively became more cartilaginous. In Platecarpus
(Fig. 158 a) and Clidastes (Fig. 158 b) the astragalus, calcaneum, and
fourth tarsale alone remain, with the divaricated fifth metatarsal, as
in land lizards. In Tylosaurus, the most specialized of mosasaurs,
but one, or at most two, small bones remain. Other tarsal bones
remained unossified, though represented by cartilage in the adult.




Fig. 158. Limbs of aquatic reptiles: A, Platecarpus (Mosasauria), right hind leg. About
one sixth natural size. B, Clidastes (Mosasauria), right hind leg and tarsus. One third
natural size. C, Ophthalmosaurus (Ichthyosauria), left front paddle. One eighth natural
size. D, Ichthyosaurus platydactylus (Ichthyosauria), left front paddle. One sixth
natural size.

Not more than six bones of the plesiosaurs can be called' tarsals,
and their homologies are doubtful (Fig. 159 b, c). They have the
same shapes and relations as the carpal bones and cannot be dis-
tinguished from them except by their smaller size. The three in the
first row are usually called the tibiale, intermedium, and fibulare; a
fourth, on the postaxial side, has sometimes been called the pisiform
in both front and hind limbs, but as there never was in any terrestrial
reptiles a pisiform in the tarsus, that name is of course incorrect.



194



THE OSTEOLOGY OF THE REPTH^ES



There are valid reasons for doubting the reappearance of the inter-
medium after its loss in the terrestrial ancestors of the plesiosaurs.
It may be the enlarged centrale. The bones in the distal row may be
the first, fused second and third, and the fourth tarsaha. The homol-
ogies of the mesopodial bones of the Ichthyosauria (Fig. 158 c, d),
where a like similarity between the front and hind Hmbs exists, are




iff



Fig. 159. Paddles of Plesiosaurs: A, right hind paddle of Thaumatosaurus , after Fraas.
B, right hind paddle of Trinacromerum. C, right front paddle of same individual. /,
femur; fb, fibula; /, tibia; h, humerus; r, radius; u, ulna.



even more doubtful. There is the same objection to the recognition
of an intermedium tarsi in this order as in the plesiosaurs, whatever
may be the corresponding bone in the carpus.



THE LIMBS 195

Metapodials and Phalanges

The most primitive hand or manus known is that of the Cotylo-
sauria, from the Permocarboniferous (Figs. 128, 133). The five meta-
carpals increase in length to the fourth ; the fifth is shorter, but is not
markedly divaricated. There are two phalanges in the thumb or
pollex, three in the second digit, four in the third, five in the fourth,
and but three in the fifth. The first and fifth metacarpals are more
freely movable on the wrist than are the other three.

Of the Temnospondylous amphibians no complete hand is known.
That there were five functional digits is certain,^ since there are five
functional carpaha in both Eryops (Fig. 136) and Trematops. It is
often assumed that all amphibians of the past, as of the present, had
but four fingers, as is known to be the case in some of the ancient
Stegocephaha. The phalangeal formula was either 2, 3, 4, 4, 3 or 2,
3, 3, 4, 3, in the rhachitomous temnospondyls. It must be remem-
bered, however, that we know nothing whatever of the hands or feet
of the earHest amphibians, and it is purely an assumption that the
reptiHan hands and feet were evolved from forms like the later ones
of Permocarboniferous times. In all probability the embolomerous
ancestors of the reptiles had the phalangeal formulae of both front
and hind feet like those of the known earliest reptiles. We can hardly
conceive of an increase either of the number of digits or number of
phalanges in the earHest reptiles.

In crawHng reptiles the structure of the digits, it is seen, has not
changed much to the present time, the modern Sphenodon (Fig.
139 A, b) as well as most modern Hzards (Fig. 140 c, d) having the
same number of bones arranged in the same ways. This primitive
phalangeal formula is that of the Cotylosauria, TherocephaHa, The-
romorpha, Phytosauria, Pseudosuchia, Rhynchocephalia, Notho-
sauria, or at least some members of the group, and the group called
by the author the Acrosauria, that is, the early Araeoscelis (Fig.
155 a), Protorosaurus (Fig. 138 d), Pleurosaurus (Fig. 139 d), and
Sauranodon (Fig. 139 c). In the Crocodilia (Figs. 140 A, 157 A, b),
the postaxial fingers are in all cases shorter and weaker, with fewer
phalanges.

1 [For a different interpretation, however, see Gregory, Miner, and Noble in
Bulletin, Amer. Mus. Nat. Hist., 1923, vol. XLvni. — Ed.]



196 THE OSTEOLOGY OF THE REPTn.ES

In no other reptiles has there been as great modification of the
fingers as in the Pterosauria (Fig. 142), so great indeed that there is
dispute as to the homologies of the ones that remained. The maxi-
mum of changes was reached in the latest forms, especially Nycto-
saurus and Pteranodon, where there are three very short and weak
fingers on the preaxial side, with two, three, and four phalanges, the
terminal ones in the shape of strong claws. On the postaxial side the
fourth finger is very long and strong, with four phalanges for the sup-
port of the patagium. This wing finger has generally been supposed
to be the fifth, the first finger or pollex represented by a slender bone
turned backward from the wrist toward the humerus and known as
the pteroid. It seems more probable that the wing finger is the
fourth, as originally so called by Cuvier, the fifth being absent. In
the development of the patagium the claw of the wing finger would
in all probabihty disappear, as in the bats, leaving the normal num-
ber for the fourth digit. If it is really the fifth, not only has the claw
been converted into a long membrane-supporting phalange, but
an additional phalange has been added; while each of the preceding
three digits has lost one phalange. We can conceive of no cause for
such hypo- and hyper-phalangy in the hand in these volant reptiles.
One of the phalanges of the third finger is short, as in the third digit
of the foot.

The first three metacarpals of the early pterodactyls articulated
normally with the carpus (Fig. 142) ; in the later ones they were mere
splints lodged loosely in the flesh at the distal end of the fourth meta-
carpal, only the first of them retaining a very slender connection with
the wrist. The fourth metacarpal, on the other hand, progressively
increased in length till it much exceeded the length of the forearm.
Its distal articulation is a very perfect pulley-like joint, permitting
flexion of the first phalange through almost one hundred and eighty
degrees.

A general reduction of the postaxial digits of both front and hind
feet is characteristic of the Dinosauria (Figs. 141, 156). Only in the
primitive Anchisaurus and Plateosaurus (Fig. 141 a) is a nearly com-
plete hand recognized, and even in these, two phalanges of the fifth
finger are gone. The fifth finger is absent in all Theropoda since the
early Jurassic, the fourth usually, the third sometimes. In Gorgo-
saurus (Fig. 141 c),from the uppermost Cretaceous, the hand is func-



THE LIMBS 197

tionally didactyl, the extreme of specialization among reptiles. In
the Theropoda (Fig. 141 a-e) the thumb is the stoutest digit, its
claw the largest. In the herbivorous dinosaurs (Fig. 141 r, h, i, l, m)
the hand is less preaxial, the first and second fingers being the larger.
In the Trachodontidae (Fig. 141 h), indeed, the first finger is ab-
sent. In all herbivorous forms the outer fingers are reduced, though
the fifth is seldom entirely absent, the phalangeal formula never ex-
ceeding 2, 3, 4, 3, 2, the claws lacking in the two postaxial digits. In
Trachodon a greater reduction has occurred, almost the maximum
among reptiles, the formula, according to Lambe, being o, 3, 3, 2, 2.
The ungual phalanges of both front and hind feet are characteristic,
curved and sharply pointed in the Theropoda (Fig. 141 a-d), more
obtuse in the Sauropoda (Fig. 141 f, g), for the most part hoof-
like in the Predentata (Fig. 141 h-m).

The foot or pes of reptiles is similar in structure to the hand, the
reduction of the toes being usually anticipatory of the fingers in the
terrestrial forms. There was one more phalange in the fifth toe than
in the fifth finger primitively. In Pariasaurus, only, of the Cotylo-
sauria, the phalangeal formula is slightly reduced, though primitive
in Propappus, a related genus.

The loss of the fifth toe is rare among reptiles, aside from the Dino-
sauria. The crocodiles (Fig. 157 a, b, d) have only the fifth meta-
tarsal left, and the fourth toe has but four phalanges. A very few
lizards also have lost the fifth toe. It is often reduced among the
Chelonia (Fig. 1 54 c, d) ; usually one, sometimes two, of the normal
phalanges are lost. The greater strength of the foot in this order as
in the dinosaurs is more to the preaxial side, unlike most other
reptiles.

The foot of dinosaurs (Fig. 156), so far as the reduction of pha-
langes is concerned, is less specialized than the hand, the Theropoda
(Fig. 156 a-e) retaining the original formula, except in the fifth toe.
Plateosaurus (a) and Anchisaurus (b), from the Trias, have the for-
mula 2,3,4, 5, 1 ; Alios aurus (c), from the lower Cretaceous, and Stru-
thiomimus (d), from the uppermost Cretaceous, 2, 3, 4, 5, o, the fifth
metatarsal a vestige. The known Sauropoda (i) have 2, 3, 4, 3, i
phalanges. Among the Predentata (e-h) the phalanges of the fifth
toe are invariably absent in known forms, the formula, 2,3,4, 5, o
being the usual one, and in Trachodon (g), o, 3, 4, 5, o. Among the



198 THE OSTEOLOGY OF THE REPTILES

quadrupedal Sauropoda (i) the axis of the foot is more to the preaxial
side; in other dinosaurs it is the third toe that is the stoutest, though
less so in the oldest theropods (a, b), this arguing perhaps a more
sauropod-like mode of progression.

The earliest pterodactyls had two or three phalanges in the fifth
toe; the later ones (Fig. 155 d) have only the hook-shaped meta-
tarsal left. The greatly elongated feet were adapted more for perch-
ing or clinging than for locomotion. A striking peculiarity is seen in
the greatly reduced second phalange of the third toe and the second
and third of the fourth toes, singularly identical with the correspond-
ing phalanges of the hand of the therocephalian Scymnognathus (Fig.
138) . Similar reduced phalanges are seen in the hand of the theropod
Struthiomimus and the tree sloths among mammals, in all cases
doubtless to be ascribed to the grasping or clinging habits.

A peculiarity of the fifth metatarsal among the Diapsida (Figs.
139 A, 153 b), or many of them, and the Sauria (Fig. 140 d) and
Chelonia (Figs. 144 b, 145 c, 154) is the more or less hook-like shape,
proximally, a character which has been adduced in proof of their
phylogenetic relationships. In all such cases the metatarsal articu-
lates with the fourth tarsale, and the fifth tarsale is absent. In those
reptiles which have a fifth tarsale, either ossified or cartilaginous,
the metatarsal is straight, and perhaps also in those reptiles in which
the foot had become more or less erect or digitigrade before its
entire loss.

Hypophalangy. In the Chelonia (Figs. 144, 145, 154), Droma-
sauria (Fig. 137 a, b), Anomodontia, Cynodontia, as in the mam-
mals, the primitive phalangeal formula suffered a reduction to 2, 3,
3, 3, 3 in both front and hind feet, with a further reduction to 2, 2,
2, 2, 2 (i) (Fig. 145 a) in many tortoises. The river turtles (Triony-


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