CHAPTER XVI. Classification of the Ornithosauria

CLASSIFICATION OF THE ORNITHOSAURIA

When an attempt is made to determine the place in nature of an extinct group of animals and the relation to each other of the different types included within its limits, so as to express those facts in a classification, attention is directed in the first place to characters which are constant, and persist through the whole of its constituent genera. We endeavour to find the structural parts of the skeleton which are not affected by variation in the dentition, or the proportions of the extremities, or length of the tail, which may define families or genera, or species.

It has already been shown that while in many ways the Ornithosaurian animals are like Birds, they have also important resemblances to Reptiles. They are often named Pterosauria. The wing finger gives a distinctive character which is found in neither one class of existing animals nor the other, and is common to all the Pterodactyles at present known. They have been named Ornithosauria as a distinct minor division of back-boned animals, which may be regarded as neither Reptiles nor Birds in the sense in which those terms are used to define a Lizard or Ostrich among animals which still exist. It is not so much that they mark a transition from Reptile to Bird, as that they are a group which is parallel to Birds, and more manifestly holds an intermediate place than Birds do between Reptiles and Mammals. In plan of structure Bird and Reptile have more in common than was at one time suspected. The late Professor Huxley went so far as to generalise on those coincidences in parts of the skeleton, and united Birds and Reptiles into one group, which he named Sauropsida, to express the coincidences of structure between the Lizard and the Bird tribes. The idea is of more value than the term in which it is expressed, because Reptiles are not, as we have seen, a group of animals which can be defined by any set of characters as comprehensive as those which express the distinctive features of Birds. From the anatomist's point of view Birds are a smaller group, and while some Reptiles have affinity with them, it is rather the extinct than the living groups which indicate that relation. Other Reptiles have affinities of a more marked kind with Mammals, and there are points in the Ornithosaurian skeleton which are distinctly Mammalian. So that when the Monotreme Mammals are united with South African reptiles known as Theriodontia, which resemble them, in a group termed Theropsida to express their mammalian resemblances, it is evident that there is no one continuous chain of life or gradation in complexity of structure of animals.

We have to determine whether the Ornithosauria incline towards the Sauropsidan or Bird-Reptile alliance, or to the Mammal-Reptile or Theropsidan alliance. There can be no doubt that the predominant tendency is to the former, with a minor affinity towards the latter.

The Ornithosauria are one of a series of groups of animals, living and extinct, which have been combined in an alliance named the Ornithomorpha. That group includes at least five great divisions of animals, which circle about birds, known as Ornithosauria, Crocodilia, Saurischia, Aves, Ornithischia, and Aristosuchia. Their relations to each other are not evident in an enumeration, but may be shown in some degree in a diagram (see p. 190).

THE ORNITHOMORPHA

The Ornithomorpha arranged in this way show that the three middle groups--carnivorous Saurischia, Aristosuchia, herbivorous Ornithischia--which are usually united as Dinosauria, intervene between Birds and Ornithosaurs; and that the Crocodilia and Ornithosauria are parallel groups which are connected with Birds, by the group of Dinosaurs, which resembles Birds most closely.

The Ornithomorpha is only one of a series of large natural groups of animals into which living and extinct terrestrial vertebrata may be arranged. And the succeeding diagram may contribute to make evident the relations of Ornithosauria to the other terrestrial vertebrata (see p. 191).

Herein it is seen that while the Ornithomorpha approach towards Mammalia through the Ornithosauria, and less distinctly through the Crocodilia, they approach more directly to the Sauromorpha, through the Plesiosaurs and Hatteria; while that group also approaches more directly to the Mammals through the Plesiosaurs and Anomodonts.

After a diagram in the Philosophical Transactions of the Royal Society, 1892.]

The Aristosuchia is imperfectly known, and therefore to some extent a provisional group. It is a small group of animals.

After a diagram in the Philosophical Transactions of the Royal Society, 1892.]

Cordylomorpha are Ichthyosaurs and the Labyrinthodont group. Herpetomorpha include Lacertilia, Homoeosauria, Dolichosauria, Chameleonoidea, Ophidia, Pythonomorpha.

The Sauromorpha comprises the groups of extinct and living Reptiles named Chelonia, Rhynchocephala, Sauropterygia, Anomodontia, Nothosauria, and Protorosauria. These details may help to explain the place which has been given to the Ornithosauria in the classification of animals.

The skulls are seen on the left side in the order of the names below them]

Turning to the Pterodactyles themselves, Von Meyer divided them naturally into short-tailed and long-tailed. The short-tailed indicated by the name Pterodactylus he further divided into long-nosed and short-nosed. The short-nosed genus has since been named Ptenodracon (Fig. 59, p. 167). The long-tailed group was divided into two types--the Rhamphorhynchus of the Solenhofen Slate (Fig. 56, p. 161) and the English form now known as Dimorphodon (Fig. 52, p. 150), which had been described from the Lias.

The Cretaceous Pterodactyles form a distinct family. So that, believing the tail to have been short in that group (Fig. 58), there are two long-tailed as well as two short-tailed families, which were defined from their typical genera Pterodactylus, Ornithocheirus, Rhamphorhynchus, and Dimorphodon.

The differences in structure which these animals present are, first: the big-headed forms from the Lias like Dimorphodon, agree with the Rhamphorhynchus type from Solenhofen in having a vacuity in the skull defined by bone, placed between the orbit of the eye and the nostril. With those characters are correlated the comparatively short bones which correspond to the back of the hand termed metacarpals, and the tail is long, and stiffened down its length with ossified tendons. These characters separate Ornithosaurs with long tails from those with short tails.

The short-tailed types represented by Pterodactylus and Ornithocheirus have no distinct antorbital vacuity in the skull defined by bone. The metacarpal bones of the middle hand are exceptionally elongated, and the tail, which was flexible in both, appears to have been short. These differences in the skeleton warrant a primary division of flying reptiles into two principal groups.

The short-tailed group, which was recognised by De Blainville as intermediate between Birds and Reptiles, may take the name Pterodactylia, which he suggested as a convenient, distinctive name. It may probably be inconvenient to enlarge its significance to comprise not only the true Pterodactyles originally defined as Pterosauria, but the newer Ornithostoma and Ornithocheirus which have been grouped as Ornithocheiroidea.

The second order, in which the wing membrane appears to have had a much greater extent, in being carried down the hind limbs, where the outermost digit and metatarsal are modified for its support, has been named Pterodermata, to include the types which are arranged around Rhamphorhynchus and Dimorphodon.

Both these principal groups admit of subdivision by many characters in the skeleton, the most remarkable of which is afforded by the pair of bones carried in front of the pubes, and termed prepubic bones. In the Pterodactyle family the bones in front of the pubes are always separate from each other, always directed forward, and have a peculiar fan-shaped form with concave sides like the bone which holds a similar position in a Crocodile. In the Ornithocheirus family the prepubic bones appear to have been originally triangular, but were afterwards united so as to form a strong continuous bar which extends transversely across the abdomen in advance of the pubic bones. This at least is the distinctive character in the genus Ornithostoma according to Professor Williston, which in many ways closely resembles Ornithocheirus.

The two families in the long-tailed order named Pterodermata are separated from each other by a similar difference in their prepubic bones. In Dimorphodon those bones are separate from each other, and remain distinct through life, meeting in the middle line of the body in a wide plate. On the other hand, in Rhamphorhynchus the prepubic bones, which are at first triangular and always slender, become blended together into a slight transverse bar, which only differs from that attributed to Ornithostoma in its more slender bow-shaped form.

Thus if other characters of the skeleton are ignored and a classification based upon the structure of the pelvis and prepubic bones, there would be some ground for associating the long-tailed Rhamphorhynchus from the Upper Oolites which is losing the teeth in the front of its jaw with the Cretaceous Ornithostoma, which has the teeth completely wanting; while the long-tailed Dimorphodon would come into closer association with the short-tailed Pterodactylus. The drum-stick bone or tibia in Dimorphodon, with its slender fibula, like that of a Bird, also resembles a Bird in the rounded and pulley-shaped terminal end which makes the joint corresponding to the middle of the ankle bones in man. The same condition of a terminal pulley joint is found in the Cretaceous Pterodactyles. But in the true Pterodactyles and in Rhamphorhynchus there usually is no pulley-shaped termination to the lower end of the drum-stick, for the tarsal bones remain separate from each other, and form two rows of ossifications, showing the same differences as separate Dinosaurs into the divisions which have been referred to, from their Bird-like pelvis and tibio-tarsus, as Ornithischia in the one case, and Saurischia in the other from their bones being more like those of living Lizards.