THE HIP-GIRDLE AND HIND LIMB
The bones of the hip-girdle form a basin which incloses and protects the
abdominal vital organs. It consists on each side of a composite bone,
the unnamed bones--_ossa innominata_ of the older anatomists--which are
each attached to the sacrum on their inner side, and on the outer side
give attachment to the hind limbs. As a rule three bones enter into the
borders of this cup, termed the acetabulum, in which the head of the
thigh bone, named the Femur, moves with a more or less rotary motion.
There are a few exceptions in this division of the cup between three
bones, chiefly among Salamanders and certain Frogs. In Crocodiles the
bone below the acetabular cup is not divided into two parts. And in
certain Plesiosaurs from the Oxford Clay--Muraenosaurus--the actual
articulation appears to be made by two bones--the ilium and ischium. The
three bones which form each side of the pelvis are known as the ilium,
or hip bone, sometimes termed the aitch bone; secondly, the ischium, or
sitz bone, being the bone by which the body is supported in a sitting
position; and thirdly the pubis, which is the bone in front of the
acetabulum. The pubic bones meet in the middle line of the body on the
under side of the pelvis in man, and on each side are partly separated
from the ischia by a foramen, spoken of as the obturator foramen, which
in Pterodactyles is minute and almost invisible, when it exists.
There is often a fourth bony element in the pelvis. In some Salamanders
a single cartilage is directed forward, and forked in front. According
to Professor Huxley something of this kind is seen in the Dog. The pair
of bones which extend forward in front of the pelvis in Crocodiles may
be of the same kind, in which case they should be called prepubic bones.
But among the lower mammals named marsupials a pouch is developed for
the protection of the young and supported by two slender bones attached
to the pubes, and these bones have long been known as marsupial bones.
In a still lower group of mammalia named monotremata, which lay eggs,
and in many ways approximate to reptiles and birds, stronger bones are
developed on the front edge of the pubes, and termed prepubic bones.
They do not support a marsupium.
Naturalists have been uncertain as to the number of bones in the pelvis
of Pterodactyles, because the bones blend together early in life, as in
birds. Some follow the Amphibian nomenclature, and unite the ischium and
pubis into one bone, which is then termed ischium, when the prepubis is
termed the pubis, and regarded as removed from the acetabulum. There is
no ground for this interpretation, for the sutures are clear between the
three pelvic bones in the acetabulum in some specimens, like
_Cycnorhamphus Fraasii_, from Solenhofen, and some examples of
Ornithocheirus from the Cambridge Greensand. Pterodactyles all have
prepubic bones, which are only known in Ornithorhynchus and Echidna
among mammals, and are absent from the higher mammals and birds. They
are unknown in any other existing animals, unless present in Crocodiles,
in which ischium and pubis are always undivided. Therefore it is
interesting to examine the characters of the Ornithosaurian pelvis.
The acetabulum for the head of the femur is imperforate, being a simple
oval basin, as in Chelonian reptiles and the higher Mammals. It never
shows the mark of the ligamentous attachment to the head of the femur,
which is seen in Mammals. In Birds the acetabulum is perforated, as in
many of the fossils named Dinosaurs, and in Monotremata.
[Illustration: FIG. 29. COMPARISON OF THE LEFT SIDE OF THE PELVIS IN
A BIRD AND A PTERODACTYLE]
Secondly, the ilium is elongated, and extends quite as much in front of
the acetabulum as behind it. The bone is not very deep in this front
process. Among existing animals this relation of the bone is nearer to
birds than to any other type, since birds alone have the ilium extended
from the acetabulum in both directions. The form of the Pterodactyle
ilium is usually that of the embryo bird, and its slender processes
compare in relative length better with those of the unhatched fowl and
Apteryx of New Zealand than with the plate-like form in adult birds.
In mammals the ilium is directed forward, and even in the Cape ant-eater
Orycteropus there is only an inappreciable production of the bone
backward behind the acetabulum. Among reptiles the general position of
the acetabulum is at the forward termination of the ilium, though the
Crocodile has some extension of the bone in both directions, without
forming distinct anterior and posterior processes. This anterior and
posterior extension of the ilium is seen in the Theriodont reptiles of
Russia and of South Africa, as well as in Dinosaurs.
[Illustration: FIG. 30. LEFT PELVIC BONES WITH PREPUBIC BONE IN
_PTERODACTYLUS LONGIROSTRIS_]
Thirdly, in all pterodactyles the ischium and pubis are more or less
completely blended into a sheet of bone, unbroken by perforation, though
there is usually a minute vascular foramen; or the lower border may be
notched between the ischium and the pubis, as in some of the Solenhofen
species, and the pubis does not reach the median line of the body. But
in Dimorphodon the pelvic sheet of bone is unbroken by any notch or
perforation. The notch between the ischium and pubis is well marked in
_Pterodactylus longirostris_, and better marked in _Pterodactylus
dubius_, _Cycnorhamphus Fraasii_, and Rhamphorhynchus. The fossil
animals which appear to come nearest to the Pterodactyles in the
structure of the pelvis are Theriodonts from the Permian rocks of
Russia. The type known as Rhopalodon has the ilium less prolonged front
and back, and is much deeper than in any Pterodactyle; but the
acetabulum is imperforate, and the ischium and pubis are not always
completely separated from each other by suture. In the pelvis referred
to the Theriodont Deuterosaurus there is some approximation to the
pelvis of Rhamphorhynchus and of _Pterodactylus dubius_ in the depth of
the division between the pubis and ischium.
[Illustration: FIG. 31 PELVIS AND PREPUBIC BONES OF RHAMPHORHYNCHUS
On the left-hand side the two prepubic bones are separate. On the
right-hand they are united into a transverse bar which overlaps the
front of pelvis seen from the under side]
There are three modifications of the Ornithosaurian pelvis. First, the
type of Rhamphorhynchus, in which the pubis and ischium are inclined
somewhat backward, and in which the two prepubic bones are triangular,
and are often united together to form a transverse bow in front of the
pubic region.
Secondly, there is the ordinary form of pelvis in which the pubis and
ischium usually unite with each other down their length, as in
Dimorphodon, but sometimes, as in _Pterodactylus dubius_, divide
immediately below the acetabulum. All these types possess the
paddle-shaped prepubic bones, which are never united in the median line.
Thirdly, there is the cretaceous form indicated by Ornithocheirus and
Ornithostoma, in which the posterior half of the ilium is modified in a
singular way, since it is more elevated towards the sacrum than the
anterior half, suggesting the contour of the upper border of the ilium
in a lizard. Without being reptilian--the anterior prolongation of the
bone makes that impossible--it suggests the lizards. This type also
possesses prepubic bones. They appear, according to Professor Williston,
to be more like the paddle-shaped bones of Pterodactylus than like the
angular bones in Rhamphorhynchus. The prepubic bones are united in the
median line as in Rhamphorhynchus. But their median union in that genus
favours the conclusion that the bones were united in the median line in
all species, though they are only co-ossified in these two families.
[Illustration: FIG. 32. THE PELVIC BONES OF AN ALLIGATOR SEEN FROM
BELOW
The bones in front are here regarded as prepubic, but are commonly
named pubic]
This median union of the prepubic bones is a difference from those
mammals like the Ornithorhynchus and Echidna, which approach nearest to
the Reptilia. In them the prepubic bones have a long attachment to the
front margin of the pubis, and extend their points forward without any
tendency for the anterior extremities to approximate or unite. The
marsupial mammals have the same character, keeping the marsupial bones
completely distinct from each other at their free extremities. The only
existing animals in which an approximation is found to the prepubic
bones in Pterodactyles are Crocodiles, in bones which most writers term
the pubic bones. This resemblance, without showing any strong affinity
with the Crocodilia, indicates that Crocodiles have more in common with
the fossil flying animals than any other group of existing reptiles; for
other reptiles all want prepubic bones, or bones in front of the pubic
region.
THE HIND LIMB
The hind limb is exceptionally long in proportion to the back. This is
conspicuous in the skeletons of the short-tailed Pterodactyles, and is
also seen in Dimorphodon. In Rhamphorhynchus the hind limb is relatively
much shorter, so that the animal, when on all fours, may have had an
appearance not unlike a Bat in similar position. The limb is
exceptionally short in the little _Ptenodracon brevirostris_. The bones
of the hind limb are exceptionally interesting. One remarkable feature
common to all the specimens is the great elongation of the shin bones
relatively to the thigh bones. The femur is sometimes little more than
half the length of the tibia, and always shorter than that bone. The
proportions are those of mammals and birds. Some mammals have the leg
shorter than the thigh, but mammals and birds alone, among existing
animals, have the proportions which characterise Pterodactyles. The
foot appears to have been applied to the ground not always as in a bird,
but more often in the manner of reptiles, or mammals in which the digits
terminate in claws.
THE FEMUR
[Illustration: FIG. 33. THE FEMUR
On the right is a front view of femur of a bear. In the middle are
front and side views of the femur of Ornithocheirus. On the left is
the femur of Echidna. These comparisons illustrate the mammalian
characters of the Pterodactyle thigh bone]
The thigh bone, on account of the small size of many of the specimens,
is not always quite clear evidence as an indication of technical
resemblance to other animals. The bone is always a little curved, has
always a rounded, articular head, and rounded distal condyles. Its most
remarkable features are shown in the large, well-preserved specimens
from the Cambridge Greensand. The rounded, articular head is associated
with a constricted neck to the bone, followed by a comparatively
straight shaft with distal condyles, less thickened than in mammals. No
bird is known, much less any reptile, with a femur like Ornithocheirus.
Only among Mammals is a similar bone known with a distinct neck; and
only a few mammals have the exceptional characters of the rounded head
and constricted neck at all similar to the Cretaceous Pterodactyles. A
few types, such as the higher apes, the Hyrax, and animals especially
active in the hind limb, have a femur at all resembling the Pterodactyle
in the pit for the obturator externus muscle, behind the trochanter
major, such as is seen in a small femur from Ashwell. The femur varies
in different genera, so as to suggest a number of mammalia rather than
any particular animal for comparison. These approximations may be
consequences of the ways in which the bones are used. When functional
modifications of the skeleton are developed, so as to produce similar
forms of bones, the muscles to which they give attachment, which act
upon the bones, and determine their growth, are substantially the same.
In the _Pterodactylus longirostris_ the femur corresponds in length to
about eleven dorsal vertebrae. The end next the shin bone is less
expanded than is usual among Mammals, and rather suggests an approach to
the condition in Crocodiles, in the moderate thickness and breadth of
the articular end, and the slight development of the terminal
pulley-joint. One striking feature of the femur is the circumstance that
the articular head, as compared with the distal end, is directed forward
and very slightly inward and upward. So that allowing for the outward
divergence of the pelvic bones, as they extend forward, there must have
been a tendency to a knock-kneed approximation of the lower ends of the
thigh bones, as in Mammals and Birds, rather than the outward divergence
seen in Reptiles.
Apparently the swing of the leg and foot, as it hung on the distal end
of the femur, must have tended rather to an inward than to an outward
direction, so that the feet might be put down upon the same straight
line; this arrangement suggests rapid movement.
[Illustration: FIG. 34. COMPARISON OF THE TIBIA AND FIBULA IN
ORNITHOSAUR AND VULTURE]
TIBIA AND FIBULA
In _Pterodactylus longirostris_ the tibia is slender, more than a fifth
longer than the femur. A crest is never developed at the proximal end,
like that seen in the Guillemot and Diver and other water birds. The
bone is of comparatively uniform thickness down the shaft in most of the
Solenhofen specimens, as in most birds. At the distal end the shin bone
commonly has a rounded, articular termination, like that seen in birds.
This is conspicuous in the _Pterodactylus grandis_. In other specimens
the tarsal bones, which form this pulley, remain distinct from the
tibia; and the upper row of these bones appears to consist of two
bones, like those which in many Dinosaurs combine to form the
pulley-like end of the tibia which represents the bird's drum-stick
bone. They correspond with the ankle bones in man named astragalus and
os calcis.
Complete English specimens of tibia and fibula are found in the genus
Dimorphodon from the Lias, in which the terminal pulley of the distal
end has some expansion, and is placed forward towards the front of the
tibia, as in some birds. The rounded surface of the pulley is rather
better marked than in birds. The proximal end of the shaft is relatively
stout, and is modified by the well-developed fibula, which is a short
external splint bone limited to the upper half of the tibia, as in
birds; but contributing with it to form the articular surface for the
support of the lower end of the femur, taking a larger share in that
work than in birds. Frequently there is no trace of the fibula visible
in Solenhofen specimens as preserved; or it is extremely slender and
bird-like, as in _Pterodactylus longirostris_. In Rhamphorhynchus it
appears to extend the entire length of the tibia, as in Dinosaurs. In
the specimens from the Cambridge Greensand there is indication of a
small proximal crest to the tibia with a slight ridge, but no evidence
that this is due to a separate ossification. The patella, or knee-cap,
is not recognised in any fossil of the group. There is no indication of
a fibula in the specimens thus far known from the Chalk rocks either of
Kansas in America, or in England.
The region of the tarsus varies from the circumstance that in many
specimens the tibia terminates downward in a rounded pulley, like the
drum-stick of a bird; while in other specimens this union of the
proximal row of the tarsal bones with the tibia does not take place, and
then there are two rows of separate tarsal bones, usually with two bones
in each row. When the upper row is united with the tibia the lower row
remains distinct from the metatarsus, though no one has examined these
separate tarsal bones so as to define them.
THE FOOT
[Illustration: FIG. 35. METATARSUS AND DIGITS IN THREE TYPES OF
ORNITHOSAURS]
The foot sometimes has four toes, and sometimes five. There are four
somewhat elongated, slender metatarsal bones, which are separate from
each other and never blended together, as in birds. There has been a
suspicion that the metatarsal bones were separate in the young
Archaeopteryx. In the young of many birds the row of tarsal bones at the
proximal end of the metatarsus comes away, and there is a partial
division between the metatarsal bones, though they remain united in the
middle. And among Penguins, in which the foot bones are applied to the
ground instead of being carried in the erect position of ordinary birds,
there is always a partial separation between the metatarsal bones,
though they become blended together. The Pterodactyle is therefore
different from birds in preserving the bones distinct through life, and
this character is more like Reptiles than Mammals. The individual bones
are not like those of Dinosaurs, and diverge in Rhamphorhynchus as
though the animals were web-footed. There is commonly a rudimentary
fifth metatarsal. It is sometimes only a claw-shaped appendage, like
that seen in the Crocodile. It is sometimes a short bone, completely
formed, and carrying two phalanges in Solenhofen specimens: though no
trace of these phalanges is seen in the large toothless Pterodactyles
from the Cretaceous rocks of North America. In the _Pterodactylus
longirostris_ the number of foot bones on the ordinary digits is two,
three, four, five, as in lizards; but the short fifth metatarsal has
only two toe bones. In Dimorphodon the fifth digit was bent upward, and
supported a membrane for flight. There are slight variations in the
number of foot bones. In the species _Pterodactylus scolopaciceps_ the
number of bones in the toes follows the formula two, three, three, four.
In _Pterodactylus micronyx_ the number is two, three, three, three. The
terminal claws are much less developed than is usual with Birds; and
there is a difference from Bats in the unequal length of the digits.
Taken as a whole, the foot is perhaps more reptilian than avian, and in
some genera is crocodilian.
The foot is the light foot of an active animal. Von Meyer thought that
the hind legs were too slender to enable the animal to walk on land; and
Professor Williston, of the University of Kansas, remarks that the
rudimentary claws and weak toes indicate that the animal could not have
used the feet effectively for grasping, while the exceedingly free
movement of the femur indicates great freedom of movement of the hind
legs; and he concludes that the function of the legs was chiefly for
guidance in flight through their control over the movements, and
expresses his belief that the animal could not have stood upon the
ground with its feet. There may be evidence to sustain other views. If
the limb bones are reconstructed, they form limbs not wanting in
elegance or length. If it is true, as Professor Williston suggests, that
the weight of his largest animals with the head three feet long, and a
stretch of wing of eighteen or nineteen feet, did not exceed twenty
pounds, there can be no objection to regarding these animals as
quadrupeds, or even as bipeds, on the ground of the limbs lacking the
strength necessary to support the body. The slender toes of many birds,
and even the two toes of the ostrich, may be thought to give less
adequate support for those animals than the metatarsals and digits of
Pterodactyles.