Part 10

Prof. Owen has described in a 'Palæontographical' monograph the proximal end of a mandible in which the sutures are obliterated. But there is one specimen of a young right ramus showing the inner and under part of the mandible to be the surangular bone which unites with the angular or outer bone by a longitudinal and vertical suture traversing on the inner side the great upper groove; and on the surangular the greater part of the articular bone rests. The articulation is strong and double, consisting of a deep transverse hollow, bounded by a strong over-locking ridge in front and a slight ridge behind; and this area is divided into two tapering furrows by a strong oblique and rounded crest, which passes from behind inward and forward. Just behind the articulation is a ?pneumatic aperture, and then the upper surface tapers to the under surface, forming a heel, of which one specimen measuring an inch deep on the inside of the articulation has 3/4 of an inch still left and is more than 1/4 inch thick at the fracture. In a specimen belonging to the Rev. T. G. Bonney the outside of the jaw is 11/16 of an inch deep, and under the articulation 5/16 of an inch deep. The articular area is 3/4 of an inch wide and 6/16 of an inch long.

Seven specimens indicate four species.

The proximal end of the lower jaw is entirely Avian. The pneumatic aperture, as in birds, is placed behind the articulation, which is shaped as in many birds. Commonly in Ornithosaurians the bones are anchylosed and all trace of sutures obliterated, as in most birds. In the Goose, however, the six elements of each side are sometimes as readily separated as in reptiles. And in some Pterodactyles the bones separate.

THE DENTARY BONE

Pl. 12.

The dentary bone consists of a single piece, as in birds and chelonians; and differs from both in being provided with teeth. It is described under the species O. machærorhynchus, page 113.

THE TEETH.

Pl. 12.

Case. Comp. Tablet. Specimen. =J= _c_ 17 1-39

The first three teeth are usually larger than those which are placed behind them, in this respect rather resembling some fossil reptiles than Dolphins, and presenting a character like that seen in the Dimorphodon. They are placed in oblique oval sockets. They have a single fang like Cetaceans, Edentates, Reptiles, and like the premaxillary teeth of Mammals. Cambridge specimens of jaws are not sufficiently perfect to show whether the teeth are limited to the premaxillary bone; but this appears to be the case in _Pterodactylus crassirostris_ (Goldf.), and probably in _Ornithocheirus compressirostris_ (Owen), [_Palæontographical Society_, 1851, Pl. 27], and is so regarded by Professor Owen in his later writings. Yet the significance of this fact seems to have been forgotten, and Cuvier's dictum about their teeth still has influence. He says, "The teeth, by which the examination of an animal ought always to be commenced, here present nothing equivocal. They are all simple, conical, and nearly alike, as in the crocodiles, monitors, and other lizards." But, on the one hand, the Dolphins demonstrate that a mammal might have similar teeth even in the maxillary bone; and, on the other hand, since teeth in the premaxillary bone always are single-fanged, and commonly have a simple sub-conical crown, there is absolutely no evidence in the teeth of the affinities of the animal, which, so far as this portion of its economy went, might as well have been a fish or a mammal as anything else. In the succession there is nothing very distinctive. In the Crocodile one tooth comes up under another, as is commonly the case with mammals; and in mammals the fangs of the old teeth are often partially absorbed so that the teeth drop out into the mouth. In the Pterodactyle the new teeth came up on the inner side, as in the Ichthyosauria--a tribe of animals as singular in their affinities as the Ornithosauria. Occasionally specimens show a small furrow on the inner side of the fang, indicating absorption, but there is nothing to show how many times the teeth were renewed: in the Dolphins there is but one set, and in Crocodiles the teeth are replaced many times. In form and size the teeth are very variable. They are directed obliquely forward, and are curved backward and inward. They taper in an elongate cone, compressed from side to side, flattened on the outside, moderately convex on the inside; rarely the sides meet in a ridge after the plan of Pliosaurus, Megalosaurus, Dakosaurus, &c.; more frequently the lateral margins round into each other. Usually the enamel is quite smooth, sometimes, as in No. 1, it is finely striated and wrinkled. Some teeth are nearly circular and some quite straight. The ovate fang contracts below, conically, and is closed, leaving a long hollow pulp-cavity in its interior. Nos. 9, 10 show the marks of the successional teeth on their inner sides. No. 11 appears to have had the crown slightly worn at the tip during the animal's lifetime. In transverse section of the crown the tooth structure resembles Ichthyosaurus, Cetaceans, and Bats. The dentine is filled with calciferous tubes which radiate as in Ichthyosaurus, and towards the centre of the tooth are seen in transverse section to present many angles, almost like radiated corpuscles. They are separated by interspaces of their own width, and run towards the circumference, sometimes straight and sometimes wavy, parallel to each other. They send off branches usually at right angles which anastomose with the adjoining tubes. The dentine is in concentric layers, and shows layers of sub-circular cells as in the teeth of Mammals. The enamel is a thin transparent layer with fewer and finer tubes than the dentine.

A SUMMING UP.

The story of the structure of the Ornithosaurians of the Cambridge Greensand has now been told, and it only remains to gather up the threads of their affinities and determine the Pterodactyle's place in nature. But before doing so, so various in importance are the characters enumerated, that I would first offer a few remarks on the classificational value of characters among the Reptilia, with which Pterodactyles have been most commonly grouped.

The naturalist who only examines organisms now living on the earth, symbolizes to himself, by the term Reptile, a definite sum of characters, with definite subdivisions and subordinate grouping, to which the extinct types of life extricated from the rocks cannot entirely be adapted. When the fragmentary, and often isolated or ill-associated, bones of fossilized animals are contrasted with corresponding bones in the skeletons of Serpents, Crocodiles, Lizards and Turtles, not infrequently it is found that the characters attributed to different Ordinal groups are interlaced in a single individual with a type of organization peculiar to itself, and important as are the modifications of existing orders. These characters occasionally are grouped with others which in living animals had been deemed characteristic of Fishes, Amphibia, Birds, and Mammals.

The Reptilia of the Palæontologist is therefore a vast and provisional group, ever acquiring new characters, to which no diagnosis can be applied. And although certain empirical characters have served to allocate the specimens in their several orders, in general with sufficient accuracy, yet from the imperfect preservation of some of the remains, or the imperfect extent to which their structures are known, and the want of recognised canons by which to measure their relative values, it has not been possible to discuss the relations of the several orders to each other or with the larger groups on which some of them impinge.

Classifications represent more or less faithfully the gradational increase in the sum of the characters of an organism, as well as the increase in importance that those character severally attain. Thus gathering, so far as may be, from the chaos of individuals, _a common plan of structures_ on which the genus, order, or class is moulded from a less specialized group of organs. The fundamental structures of a vertebrate animal, so far as their persistent importance can be measured, are, those connected with

I. Perpetuating the race.

II. Construction of the brain.

III. Circulation and oxidation of the blood.

IV. Locomotion, i.e. skeleton, muscles, &c.

And these characteristics are for the most part so interlinked, that it becomes difficult to assign to one order of animals a relative superiority over another order; since when a single set of organs is prominently developed in one group it often happens that another set of organs has a like pre-eminence in an allied group. Thus among reptiles it might be considered that

_Crocodiles_ have the best hearts, and _Turtles_ the best lungs.

And since these structures in their functions severally modify and determine the use of other structures, the meaning that terms like Crocodilian and Chelonian really have is that they represent the aspect of Reptilian organization when seen through the specialization of respiration, or circulation of the blood. The soft parts thus determining the nutrition and function of the muscles and skeleton, anatomists in examining the bones of extinct animals are accustomed to reverse the order of their inferences, and infer from modifications of the skeleton what had been the characters of the soft and more vital structures.

On the presumed accuracy of this method of research rest many results of Comparative Anatomy. But since the shapes of bones are determined by the muscles as well as by inheritance, it is always to be remembered that a similar form of bone may obtain in different orders or classes of animals, as the result of a similar function in a special region of the body. Such resemblances are familiar to anatomists. Hence much caution is required from the Palæontologist to distinguish between the characteristics of a group, and the extent to which they may be modified by function. This distinction is the first principle of classification. But it is always difficult to estimate the importance of characters in fragments of bones or parts of skeletons, and the difficulty is increased by the fact that if what appears to be but a functional modification should pervade all the species, it becomes a characteristic of the group, and its power of modifying the other organs in a peculiar way has to be considered.

Thus for all practical purposes birds may be said to be characterized by wings, which almost acquire the dignity of class characters from their influence on the respiratory function. But in some birds it has been thought that no bone of the fore-limb was ever developed[V]; and the difference between such a phenomenon and the wing of a Swift, for example, is one almost of infinity, as compared with any other aspect that the anterior limb might have assumed. Therefore, since a bird may part with its fore-limbs and yet remain a bird, I infer that it might apply its fore-limbs to the ground, become a quadruped, and be a bird still. And if in this process the other structures remained unchanged, no one would regard the modification as more than an ordinal one. But should the vertebræ change also, or the pelvis, or the covering of the integument, or the jaws become toothed, then, although the heart and lungs and brain of the imaginary animal retained their class characters, the functional differences being more than those of an order would constitute it a sub-class.

[Footnote V: According to Prof. Owen, in Dinornis.]

In the same way it is conceivable that serpents may have existed with well-developed limbs, and if they retained their other characters the limbed forms would constitute a sub-order of serpents; but if to these characters they added a closed palate united to the cranium, they would constitute a new order of reptiles. A chelonian might be entirely deprived of its bony covering, and it would still be a chelonian, differing only as a separate family. So that structures which to the eye appear fundamental may be lost without affecting an animal's systematic position, just as animals while resembling each other in form may possess dissimilar organization.

Even with the living or typical Reptilia, naturalists are divided as to the number of ordinal groups into which they naturally fall. It is however generally agreed that the Amphibia or Dipnoa of Fitzinger, have no near affinity with the true reptiles. Milne-Edwards, Van der Hoeven and Agassiz make the remainder into three orders, as did Cuvier:

Chelonia, Sauria, Ophidia.

Stannius, Gray, Owen and Huxley, on the other hand, by dividing the Saurians make four orders, to which Dr Günther by his description of Sphenodon has given evidence of a fifth:

Crocodilia, Chelonia, Sauria, Ophidia, (Rhynchocephalia.)

De Blainville in a remarkable classification (1816), made three orders, Chelonians, Emydosaurians [crocodiles], and Saurophidians; the latter group being subdivided into Saurians and Ophidians.

In his "Handbuch der Anatomie der Wirbelthiere" Stannius unites the Crocodilia and Chelonia into a group called Monimostylica; while of the Sauria and Ophidia he makes another group called Streptostylica. Similar groups were made by Dr Gray, and named Cataphracta and Squamata. They are identical with the "cuirassed" and "scaly" reptiles of Dumeril and Bibron.

The _Astylica_ (Sphenodon) have no penis.

The _Streptostylica_ have a double penis, lungs simplified at the distal end into a mere air-bladder, brains with a moderately elongated cerebrum, the palate mesially open, scales, leathery shell to the egg cut through by a tooth on the premaxillary bone.

The _Monimostylica_ have a single penis, lungs well subdivided, ventricle of heart partly [turtles] or entirely divided [crocodiles], brains having the cerebrum broad or high, a closed palate, scutes, a calcareous shell to the egg.

Thus the chief differences between Turtles and Crocodiles on the one hand, and Lizards and Serpents on the other hand, are not so much in the fundamental vital structures, though these undergo changes even in the families, as in the different ways in which the muscles and skeleton are modified. The typical lizards diverge widely from the crocodiles, and in those osteological features which admit of comparison they make at least as near an approach to the Chelonians. But leaving the limbs and pectoral and pelvic girdles out of consideration, lizards find their natural place side by side with the serpents.

Attempts have been made by Palæontologists to incorporate the new ordinal groups which they have been compelled to create for some fossils, along with the true Reptilia; but such a proceeding destroys the value of the term Reptile as a measure of a known organization. In the absence of knowledge of the brains of Dinosaurs, Ichthyosaurs, and Dicynodonts, their union with the Reptilia can only have a stagnating effect on Palæontology, for there is no proof that they are Reptiles in the same sense as are Crocodiles or Chameleons: while their bones being used as standards of Reptilian structure in comparisons, they adjudicate the place in nature of other animals by an authority which has never been established.

Before any inference can be drawn from the forms of bones in extinct animals, their relations to vital structures and to way of life must be known in animals which still live. This may give some clue both to their functional significance and to the extent to which they are inherited and not directly attributable to function. But an idea of the morphological value of the bones of living animals is only gained by comparing them with the remains of their extinct allies, tracing the now imitative structure back to its origin in a function which has ceased to be displayed.

Professor Owen in his "Comparative Anatomy of the Vertebrates" (1866) admits nine orders of Reptiles, five of which are extinct, some of the extinct orders being supposed to rank lower, while others are higher than the living types. They are arranged in this way,

* Pterosauria, * Dinosauria, Crocodilia, Ophidia, Lacertilia, Chelonia * Anomodontia, * Sauropterygia, * Ichthyopterygia.+

* Extinct.

+ Prof. Owen, _Comp. Anat._ Vol. I. p. 7-9, defines his sub-classes. At p. 15, in the details of the orders, he puts Ichthyosaurus in the 5th sub-class _Monopnoa_. But at p. 50, treating of the vertebral column of Ichthyosaurus, it is written of as an extinct order of _Dipnoal_ reptiles. The Dipnoa then would include

Ichthyosauria, Batrachia, Labyrinthodontia, Ganocephala.

But Ichthyosaurus obviously belongs to Haeckel's group Monocondylia.

In what characters the Ichthyosaurs are lower than living reptiles I have been unable to discover. The palate may be better compared with a struthious bird than with a reptile; and the pectoral girdle may be better compared with the Ornithodelphia than with a reptile, while all the trunk-vertebræ have ribs such as are associated in living animals with a four-celled heart. But if it is a lower animal type than living reptilia, the student will ask, how much lower? does it descend to the Dipnoa, and prove to be the missing link between the Amphibia and Reptilia? and wherein is the evidence? Or does it not with Dicynodonts and Dinosaurs rather form an outlying class uniting the reptiles with the mammals.

In the same way, when Pterosauria and Dinosauria are placed above living reptiles, we are compelled to ask how much are they above, or what are the characters which bind them to the Reptilia at all? No satisfactory evidence has ever been adduced to show that the Dinosauria are Reptiles. And of the claim of the Pterodactyles to such a position, the facts detailed and now summarised will be the best evidence.

The highest structure shown in these remains is the brain-case. The cavity for the brain is in every respect like that in the skull of a bird. It resembles brains of a high type in having the cerebral lobes convex in front; since, in the lower mammals, there is a resemblance to reptiles in the conical form of the cerebrum; while the brains even of some of the placental mammals are not well distinguished from those of reptiles. Although the brain of the Ornithorhynchus is entirely mammalian, it is more like the brain of a reptile than is the brain of the Pterodactyle. No evidence of affinities could be adduced which would outweigh this. Taken by itself it would lead us to anticipate for the Pterodactyle those vital structures which birds have in common.

Next in importance to the brain are the pneumatic perforations in the bones. They are seen in the lower jaw, the quadrate bone, in the whole of the vertebral column, in all the bones of the fore-limb, excepting one or two fragments, in the scapula and coracoid, in the os innominatum, in the femur and in the tibia. In such of the bones as can be compared, the pneumatic perforation is usually situated in Birds as it is in Pterodactyles. In Birds the bones are filled with air through these perforations, and as a principle the greater the motion of the animal, the greater is the number of bones filled with air. This air is received from the air-sacs which receive it from the lungs, and return it through the lungs again. There is thus in birds a sort of supplemental lung-system, which circulates air through the body. Nothing of the kind exists in any other class of animals. The respiratory system in birds is more perfect and complex than in the other vertebrata, and, as a result, the temperature of the blood on the whole is hotter.

In Pterodactyles the reticulate character of the perforations proves that they were pneumatic, and supplied the bones with air. The fact that the bones were supplied with air, necessitates an elaborate system of air-sacs to furnish the supply. And the existence of these air-sacs speaks incontestably to bronchial tubes opening on the surface of the lungs to supply them, and to the existence of lungs essentially like those of birds. The outward and backward direction of the coracoid bones may indicate that the lungs were larger than in a bird.

The circulation of air through the bird's body has relation to rapid motion through the atmosphere, which necessarily produces more rapid respiration than would comparative quiescence. The same inference must be applied to the Pterodactyles. But rapid respiration only means more rapid oxidation of the blood, and conversion of the purple cruorine into scarlet cruorine,--that is, the conversion of venous blood into arterial blood. And if venous blood is converted into arterial blood by a lung-apparatus like that of a bird, and with a rapidity like that in a bird, there must be a circulation of the blood as rapid as in birds. Such a circulation is only maintained by a heart with two auricles and two ventricles. Therefore Pterodactyles had the heart like that of birds and mammals.

Now, since the temperature of the blood is chiefly dependent on respiration and circulation, and Pterodactyles had respiratory and circulatory organs which in living animals produce hot blood, it results that they were hot-blooded animals.

Thus the heart and lungs are exactly such as would have been inferred from the brain, and, like it, they are avian. And so important are these vital structures all taken together, that the inference from them upon an animal's affinities would overbear all other evidence that could be adduced except reproduction; for they demonstrate the plan on which an animal was built, and are the motor power which enabled it to use its skeleton in a way that stamped upon it a peculiar form.

In the head such structures as are preserved conform with slight variations to the avian plan. Other Ornithosaurians show in the parts which are not preserved in Cambridge specimens some characters which are not avian; they are in part as much mammalian as reptilian, and in a few points entirely reptilian. But it might be misleading to take German specimens into consideration in forming an estimate of the Pterodactyles of the Cambridge Greensand, which were probably a different ordinal group, and may have had material differences in structure.

The vertebral column as a whole is distinctive.

The neck and sacrum are mammalian, and the tail reptilian. The procœlous vertebræ are characteristic of reptiles, but in some animals, as Chelonians, they vary in different regions of the body; and among amphibians the character is inconstant in genera nearly allied.

The hind-limb is in part mammalian and in part avian; if there be any reptilian characters in the foot, they are not less mammalian.

The os innominatum is avian and mammalian.

The pectoral girdle is avian.

The fore-limb is avian and mammalian.

The wing-finger is distinctive, though formed on the avian plan.