The Ornithoscelida

The very remarkable extinct reptiles which constitute this group, present a large series of modifications intermediate in structure between existing Reptilia and Aves.

This transitional character of the Ornithoscelidan skeleton is most marked in the pelvis and hind-limbs.

If the pelvis of any existing reptile be compared with that of any existing bird, the following points of difference will be observed:

1. In the reptile (Fig. 78, C), the ilium is not prolonged in front of the acetabulum; and the acetabulum is either wholly closed by bone, or presents only a moderate-sized fontanelle, as in the Crocodilia.

In the bird (Fig. 78, A.), the ilium is greatly prolonged iu front of the acetabulum, and the roof of the acetabular cavity is a wide arch, the inner wall of that cavity remaining membranous. The anterior pier of the arch, or prae-acetabular process, extends farther downward than the posterior pier, or post-acetabular process.

But, in all the Ornithoscelida, the ilium extends far in front of the acetabulum, and furnishes only a widely-arched roof to that cavity, as in birds. It retains a reptilian character in the further proportional extension of the post-acetabular process downward (Fig. 78, B.).

2. The ischium, in the reptile (Fig. 78, C), is a moderately elongated bone, which becomes connected with the pubis in the acetabulum, and extends downward, inward, and somewhat backward, to unite with its fellow in a median ventral symphysis. The obturator space is not interrupted by any forward process of the outer and anterior half of the ischium.

In all birds (Fig. 78, A.), the ischium is elongated and inclined backward, the backward direction being least marked in Apteryx, and most in Rhea. The ischia never come together directly in a median ventral symphysis, though they unite dorsally in Rhea. The anterior edge of the external, or acetabular, half of the ischium very generally sends off a process which unites with the pubis, thus dividing the obturator space.

In all the Ornithoscelida (Fig. 78, B.), in which I have been able to identify the bone (Thecodontosaurus, Teratesaurus, Megalosaurus, Iguanodon, Stenopelyx, Hadrosaurus, Hypsilophodon), the ischium is greatly elongated. In Iguanadon it has the obturator process characteristic of the same bone in Birds; and I imagine that the same process is seen in Compsognathus. In Hypsilophodon there can be no mistake about the matter, and the remarkable slenderness and prolongation of the ischium give it a wonderfully ornithic character. In Iguanodon the slenderness and prolongation are even carried beyond what are to be seen in Birds. I am disposed to think, however, that, as was certainly the case in Hypsilophodon, the ischia united in a median ventral symphysis in all the Ornithoscelida.

3. In all reptiles the pubis is inclined forward, as well as downward, toward the ventral median line. In all, except the Crocodile, it takes a considerable share in the formation of the acetabulum; and the ossified pubis unites directly with its fellow in the middle line.

The pubes of Compsognathus are, unfortunately, obscured by the femora. They seem to have been very slender; and to have been directed forward and downward, like those of Lizards. Some lizards, in fact, have pubes which, if the animal were fossilized in the same position as Compsognathus, would be very similar in form and direction. Hypsilophodon, however, affords unequivocal evidences of a further step toward the bird. The pubes are not only as slender and elongated as in the most typical bird, but they are directed downward and backward parallel with the ischia, thus leaving only a very narrow and elongated obturator foramen, which is divided by the obturator process.

It remains to be seen how far the hypsilophodont modification extended among the Ornithoscelida. The remains of Compsognathus and of Stenopelyx tend to show that it was by no means universal.

As to the hind-limb, in existing reptiles-

1. The proximal end of the tibia has but a very small, or quite rudimentary, cnemial crest, and it presents no ridge for the fibula on its outer side.
   
2. The flattened sides of the distal end of the tibia look, the one directly forward, or forward and inward; and the other backward, or backward and outward. And when the posterior edges of the two condyles of the proximal end of the tibia rest on a flat surface which looks forward, the long axis of the distal end is either nearly parallel with that surface, or is inclined obliquely from in front and without, backward and inward.
   
3. There is no depression on the anterior face of the tibia for the reception of an ascending process of the astragalus.
   
4. The distal end of the fibula is as large as, or larger than, the proximal end, and articulates largely with a facet on the outer part of the astragalus.
   
5. The astragalus is not depressed and flattened from above downward, nor does it send a process upward in front of the tibia.
   
6. The astragalus remains quite free from the tibia.

In all these respects, the leg of any existing bird (see Fig. 78) is very strikingly contrasted with that of the reptile:

1. The proximal end of the tibia is produced forward and outward into an enormous cnemial crest, in all walking and swimming birds (Fig. 78, A.); and, on the outer side, there is a strong ridge for the fibula.
   
   
2. When the posterior edges of the condyles of the tibia rest upon a flat surface, the one flat face of the distal end of the bone looks outward as well as forward, and the other inward as well as backward. Further, the long axis of the distal end is inclined, at an angle of 45° to the flat surface, from within and in front, backward and outward, thus exactly reversing the direction in the reptile.
   
   
3. There is a deep longitudinal depression on the anterior face of the distal end of the tibia, which receives an ascending process of the astragalus.
   
   
4. The distal end of the fibula is a mere style, and does not articulate with the astragalus.
   
   
5. The astragalus is a much-depressed bone, with a concave proximal, and a convex, pulley-like, distal, surface. A process ascends from its front margin in the groove on the front face of the tibia. This process is comparatively short, and perforated by two canals for the tibialis anticus and extensor communis, in the Fowl; while in the Ostrich and Emeu it is extremely long and not so perforated.
   
   
6. The astragalus becomes anchylosed with the tibia (though it remains distinct for a long time in the Ostrich and Rhea, and in some breeds of fowls).

In the Ornithoscelida:

1. There is a great cnemial crest and a ridge for the fibula.
   
2. The disposition of the distal end of the tibia is literally that observed in the Bird.
   
3. There is a fossa for the reception of the ascending process of the astragalus.
   
4. The distal end of the fibula is much smaller than the pioximal, though not so slender as in Aves.
   
5. The astragalus is altogether similar to that of a bird, with a short ascending process.
   
6. The astragalus appears to have remained distinct from the tibia throughout life in Iguanodon, Megalosaurus, and many other genera; but it seems to have become anchylosed in Compsognathus, Ornithotarsus, and Euskelosaurus.

The reptiles belonging to this group are for the most part of very large size, and some of them, as the Iguanodon, are among the largest of known terrestrial animals. They occur throughout the whole range of the Mesozoic formations, being represented by Thecodontosaurus, Palaeosaurus, Teratosaurus, Plataeosaurus, and other genera in the Trias; by Scelidosaurus in the Lias; by Megalosaurus, Poikilopleuron, Euskelosaurus, Hylaeosaurus, Polacanthus, Acanthopholis, Iguanodon, Hadrosaurus, Trachodon, and Laelaps in the middle and upper Mesozoic strata.

There is no evidence that Megalosaurus, or Iguanodon, possessed any dermal armor; but several genera (e. g., Scelidosaurus, Hyloeosaurus, and Acanthopholis) had osseous dermal scutes, sometimes produced into prodigious spines.

The faces of the centra of the vertebrae are slightly amphicoelous, or nearly flat; but those of the anterior dorsal and cervical regions seem, in some cases, to have been opisthocoelous. The sacrum seems to have consisted of at fewest four vertebrae, which in some (Scelidosaurus) are crocodilian, in others (Megalosaurus) take on a somewhat ornithic character. The caudal region had many and long vertebrae, between which the chevron-bones are attached. The rami of the chevron-bones have their vertebral ends united by bone.

The thoracic vertebral ribs are very strong; but the sternal ribs and sternum are unknown. However, there is some reason to think that the sternum was broad and expanded. Abdominal dermal ribs are developed in some species, if not in all.

The structure of the skull seems to have been intermediate, in many respects, between the crocodilian and the lacertilian types. In Iguanodon and Hypsilophodon, the extremities of the premaxillae appear to have been edentulous and beak-like; and the symphysis of the mandible is excavated to receive the beak, almost as in the mandible of a Parrot.

The teeth vary extremely, from the sharp, recurved, serrated fangs of Megalosaurus, to the broad grinders, wearing down by mutual attrition, of Iguanodon. Their mode of implantation varies, but they are not anchylosed to the jaws.

The scapula is vertically elongated, narrow, and devoid of any acromial process; the coracoid rounded and without fontanellos or processes.

No Ornithosoelidan is known to have possessed a clavicle. The fore-limb is shorter, and often much shorter, than the hind-limb. The structure of the manus is not certainly known.

The femur usually has a strong inner trochanter; and its distal end is particularly bird-like, in the development of a strong ridge, which plays between the tibia and the fibula.

The metatarsals are elongated, and fit together in such a way that they can hardly, if at all, move on one another. The inner and outer digits are either shorter than the rest, or quite rudimentary; and the third digit is the longest, as in birds in general.

The Ornithoscelida are divisible into two sub-orders, the Dinosauria and the Compsognatha. The type of the latter division is the wonderful little extinct reptile, Compsognathus, which differs from the Dinosauria in the great length of the centra of the cervical vertebrae, and in the femur being shorter than the tibia. It has a light bird-like head (provided with numerous teeth), a very long neck, small anterior limbs, and very long posterior limbs. The astragalus appears to have been anchylosed with the tibia, as in birds. A single specimen only of this reptile has been obtained, in the Solen hofen slates.