Many Dinosaurs possess hollow instead of solid bones. The vertebrae have large cavities in the Sauropoda, notably in Brontosaurus; in many Theropoda, e.g. Coelurus, Anchisaurus, Compsognathus, the limb-bones and the vertebrae are hollow, the latter being reduced to thin-walled shells with a few inner partitions, the bones being at the same time much swollen and enlarged. In the Ornithopoda the vertebrae are solid, but the limb-bones are hollow. The reason of this hollowing out is not easily found. Undoubtedly it results in a saving of material and weight, whilst at the same time, without loss of strength, the surfaces for the attachment of the necessarily powerful muscles are increased. But Compsognathus is a small, Brontozoum a gigantic, creature. On the other hand, the bones of the huge Stegosauri are solid. Most probably these cavities were, as in birds, filled with air-sacs ultimately in communication with the lungs; and it is by no means a baseless suggestion of Haeckel's that the Dinosaurs were warm-blooded. Their mode of propagation can only be guessed at from the circumstance that a rather well-preserved specimen of Compsognathus contains in its abdomen what may possibly be an embryo. There is nothing against the assumption that the Dinosaurs were viviparous; on the contrary, it seems more natural than that, for instance, an Atlantosaurus of more than 100 feet in length and many tons in weight, should have laid eggs.
Some of the herbivorous Dinosaurs, namely, the Stegosauri and the Ceratopsia, had a dermal armour of variable extent; the plates were loosely imbedded in the skin, and reached their greatest size along the middle of the back and tail, and these crested plates were probably covered with horny scutes, obviously weapons of defence. The Ceratopsia were armed with a pair of huge pointed horns on the head, and a smaller one on the nose (see Fig. 102, p. [430]). It is difficult to guess the use of the weapons of these terrestrial monsters, unless they were employed against the equally large carnivorous Dinosaurs or in the combats for the possession of their charming mates.
About the ancestry of the Dinosaurs we know nothing except that their affinities lie with the Crocodilia; but it is impossible to derive either from the other. The oldest forms, in the present state of our knowledge–those which have left their three-toed spoors in the Trias of Connecticut–were already much specialised by having attained to an upright bipedal gait, while the Sauropoda, which except for their gigantic size are the most generalised, are of comparatively recent date, none of them being known from strata older than the Upper Jurassic. Twenty years ago, until the discoveries of numerous kinds in the United States, our knowledge of the whole group was very limited. There is a widely spread notion that the birds have sprung from some Dinosaurian stock. Huxley was the first to show clearly that birds were an offshoot of the reptiles, and he said of the Dinosaurs, especially his Ornithoscelida (Iguanodon, Scelidosaurus, Megalosaurus, Compsognathus, and others), that they "present a large series of modifications intermediate in structure between existing reptiles and Aves." Baur proved to his own satisfaction that we have to look for the ancestors of the Ratitae among the herbivorous Dinosaurs, especially the Ornithopodous forms, whilst the Carinatae are descendants of the Ratitae. However, even he had to give up this absolutely unwarrantable view.
It is easy to select a considerable number of characters amongst the various Dinosaurs which also occur in birds, and some of these have until a recent date been considered as peculiar to birds. For instance, the double, bifurcated pubic bones of the Orthopoda; the increased number of vertebrae to which the horizontally elongated ilia are attached, especially in the forms with an upright gait, and the bipedal feature itself; the possession of an ascending process of the astragalus and its fusion with the tibia in Compsognathus and Ceratosaurus among the Theropoda, and in Ornithomimus; the attachment of the distal tarsalia to the metatarsalia, e.g. in Compsognathus,–in fact, the formation of an intertarsal joint, a feature otherwise characteristic of, and peculiar to, birds; the frequent reduction of the fifth metatarsal bone; the backward position of the hallux and the proximal reduction of its metatarsal in Compsognathus; the elongation and partial fusion of the functional metatarsals in the latter genus and in Ceratosaurus; the regular increase of the phalangeal numbers of the first four toes from two to five in many of the Ornithopoda;–in short, the great resemblance between the feet of some of the Dinosaurs and those of the birds. However striking these arguments are, they are instances of convergent analogies. The upright walk, which has been assumed and improved upon independently by members of both Theropoda and Orthopoda, has produced the same, or nearly the same modifications in them as in the birds.
It is easy to show that these features are mere coincidences. The oldest bird known is Archaeopteryx from the Upper Oolite of Bavaria. Consequently all those Dinosaurs, which are of the same and of later date, have to be excluded from the supposed ancestry, and they happen to be those in which (as in Ceratosaurus, Compsognathus, Ornithomimus, Iguanodon) the resemblances are greatest. There remains only Anchisaurus of the Upper Trias, more or less contemporary with the Brontozoum, which left its three-toed footprints (Archaeopteryx has four well-developed toes) with Zanclodon. Moreover, the most bird-like foot is either that of the Theropoda, which, like Anchisaurus and Zanclodon, differ from birds by the formation of the pelvis, or of some of the latest Ornithopoda. What, then, is the good of selecting a number of bird-like features from members of Dinosaurs which we are bound to class in different groups, and which existed, some in the lower, others in the middle, or even in the latest Mesozoic periods?
Lastly, the advocates of the Dinosaurian ancestry of birds cannot have fully appreciated the enormous differences between the wing of Archaeopteryx and the fore-limb of any Dinosaur with the most avian resemblances in the hind-limbs. The fore-limbs of these reptiles are modified in a direction diametrically opposed to that from which a bird-like wing could be developed. The skull presents another difficulty,and here again Compsognathus, a contemporary of Archaeopteryx, comes perhaps nearest to that of a generalised bird's skull. The ancestors of the birds must have combined the following characters:–Of not later than Mid-Oolitic age, with bifurcated pubic bones, four functional toes, elongated metatarsals, complete clavicles, premaxillary teeth, and free, not firmly fixed quadrate bones. But such creatures are not Dinosaurs.
We divide the enormous number of Dinosaurs according to the formation of the pelvis, that of the hind-limbs, and the dentition, into four orders.
Order I. SAUROPODA.
Pubes simple, with symphysis. Premaxillae with teeth. Plantigrade.
The teeth are mostly spatulate, laterally compressed, with sharp edges, but without serrations. Skull with a pair of large pre-orbital fossae. The centra of the vertebrae of the trunk have large lateral cavities. The fore- and hind-limbs are pentadactyle, plantigrade, and hoofed, of the typical walking type; the bones of the limbs are stout and solid; the femur is devoid of an inner distal or fourth trochanter. The carpal and tarsal bones are free. Herbivorous. The Sauropoda comprise some of the most gigantic terrestrial creatures which have ever existed, compared with some of which the bulk of an elephant appears almost insignificant. Their range in time extends from the Lower Oolite into the Cretaceous, with a perhaps world-wide distribution, namely, Western Europe, North America, Patagonia, Madagascar, and India. Although they are, except for their size, the least specialised of all Dinosaurs, none of the Sauropoda hitherto discovered are old enough to claim to be the ancestors of the other Dinosaurs.