This is the succession of Pterodactyles in geological time. Their history is like that of the human race. In the most ancient nations man's life comes upon us already fully organised. The Pterodactyles begin, so far as isolated bones are concerned, in the Rhætic strata; perhaps in the Muschelkalk or middle division of the Trias. And from the beginning of the Secondary time they live on with but little diversity in important and characteristic structures, and so far as habit goes, the great Pterodactyles of the Upper Chalk of England cannot be said to be more highly organised than the earlier stiff-tailed genera of the Lias or the Oolites. There is nothing like evolution. No modification such as that which derives the one-toed horse or the two-toed ox from ancestors with a larger number of digits. On the other hand, there is little, if any, evidence of degeneration. The later Pterodactyles do not appear to have lost much, although the tail in some of the Solenhofen genera may be degenerate when compared with the long tail of Dimorphodon; but the short-tailed types are found side by side with the long-tailed Rhamphorhynchus. The absence of teeth may be regarded as degeneration, for they have presumably become lost, in the same way that Birds now existing have lost the teeth which characterised the fossil birds—Ichthyornis of the American Greensand, and Archæopteryx of the Upper Oolites of Bavaria. But just as some of the earlier Pterodactyles have no teeth at the extremity of the jaw, such as Dorygnathus and Rhamphorhynchus, so the loss of teeth may have extended backward till the jaws became toothless. The specimens hitherto known give no evidence of such a change being in progress. But just as the division of Mammals termed Edentata usually wants only the teeth which characterise the front of the jaw, yet others, like the Great Ant-eater of South America named Myrmecophaga, have the jaws as free from teeth as the toothless Pterodactyles or living Birds, and show that in that order the teeth have no value in separating these animals into subordinate groups any more than they have among the Monotremata, where one type has teeth and the other is toothless.
The following table gives a summary of the Geological History and succession in the Secondary Rocks of the principal genera of Flying Reptiles.
| GEOLOGICAL FORMATIONS. | NAMES OF THE GENERA. | |
|---|---|---|
| British and European. | North American. | |
| Upper Chalk Lower Chalk Upper Greensand Gault | } Ornithocheirus } Ornithostoma | } Ornithostoma } (Pteranodon) } Nyctodactylus |
| Lower Greensand Wealden Purbeck | Ornithodesmus Doratorhynchus | |
| Portland Kimeridge Clay and Solenhofen Slate Coralline Oolite Oxford Clay | { Pterodactylus { Ptenodracon { Cycnorhamphus { Diopecephalus { Rhamphorhynchus { Scaphognathus | |
| Great Oolite and Stonesfield Slate Inferior Oolite | Rhamphocephalus | |
| Upper Lias Lower Lias | { Campylognathus { Dorygnathus Dimorphodon | |
| Rhætic Muschelkalk | bones ? bones | |
CHAPTER XVI
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]).