CHAPTER X. The Hip-Girdle and Hind Limb

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.

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.

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.

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.

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

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.

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

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.