Part 5 (1/2)
Mr. E. T. Newton, F.R.S., has shown that in the English skull from the Lias of Whitby, the forms of the bones are similar to the palate in birds and unlike the conditions in reptiles. There is one feature, however, which may indicate a resemblance to Dicynodon and other fossil reptiles from South Africa. A slender bone extends from the base of the brain case, named the basi-sphenoid bone, outward and forward to the inner margin of the quadrate bone (Fig. 22). A bone is found thus placed in those South African Reptiles, which show many resemblances to the Monotreme and Marsupial Mammals. It is not an ordinary element of the skeleton and is unknown in living animals of any kind in that position.
It has been thought possible that it may represent one of the bones which among mammals are diminutive and are included in the internal ear.
The resemblance may have some interest hereafter, as helping to show that certain affinities of the Ornithosaurs may lie outside the groups of existing reptiles. Instead of being directed transversely outward, as in the palatal region of _Dicynodon lacerticeps_, they diverge outward and forward to the inner border of the articulation for the lower jaw which is upon the quadrate bone.
[Ill.u.s.tration: FIG. 22]
BONES OF THE PALATE
There is a pair of bones which extend forward from these inner articular borders of the quadrate bones, and converge in a long #V#-shape till they merge in the hard palate formed by the bones of the front of the beak, named intermaxillary and maxillary bones. The limits of the bones of the palate are not distinct, but there can be no doubt that the front of the #V# is the bone named vomer, that the palatine bones are at its sides, and that its hinder parts are the pterygoid bones as in birds. There is a long, wide, four-sided, open s.p.a.ce in the middle of the palate, between the vomer and the basi-sphenoid bone, unlike anything in birds or other animals.
Professor Marsh, in a figure of the palate in the great skull of the toothless Pterodactyle named Ornithostoma (Pteranodon), from the Chalk of Kansas, found a large oval vacuity in this region of the palate. In that genus the pterygoid bones meet each other between the quadrate bones as in Dicynodon (Fig. 73, p. 182). Hence the great palatal vacuity here seen in the Ornithosaur is paralleled by the small vacuity in the South African reptile, which is sometimes distinct and sometimes partly separated from the anterior part of the vacuity which forms the openings of the nostrils on the palate.
The Solenhofen skulls which give any evidence of the palate are exposed in side view only, and the bones, imperfectly seen through the lateral vacuities, are displaced by crus.h.i.+ng. They include long strips like the vomerine bones in the Lias fossil, and they diverge in the same way as they extend back to the quadrate bones. The oblique division into vomer in front and pterygoid bone behind is shown by Goldfuss in his original figure of Scaphognathus. Thus there is some reason for believing that all Ornithosaurs have the palate formed upon the same general plan, which is on the whole peculiar to the group, especially in not having the palatal openings of the nares divided in the middle line. It would appear probable that the short-tailed animals have the pterygoid bones meeting in the middle line and triangular; and that they are slender rods entirely separate from each other in the long-tailed genera.
THE TEETH
The teeth are all of pointed, elongated shape, without distinction into the kinds seen in most mammals and named incisors, canines, and grinders. They are organs for grasping, like the teeth of the fish-eating Crocodile of India, and are not unlike the simple teeth of some Porpoises. They are often implanted in oblique oval sockets with raised borders, usually at some distance apart from each other, and have the crown pointed, flattened more on the outer side than on the inner side, usually directed forward and curved inward. As in many extinct animals allied to existing reptiles, the teeth are reproduced by germs, which originate on the inner side of the root and grow till they gradually absorb the substance of the old tooth, forming a new one in its place. Frequently in Solenhofen genera, like Scaphognathus and Pterodactylus, the successional tooth is seen in the jaw on the hinder border of the tooth in use. There is some variation in the character of bluntness or sharpness of the crowns in the different genera, and in their size.
The name Dimorphodon, given to the animal from the Lias of Lyme Regis, expresses the fact that the teeth are of two kinds. In the front of the jaw three or four large long teeth are found in the intermaxillary bone on each side, as in some Plesiosaurs, while the teeth found further back in the maxillary bone are smaller, and directed more vertically downward. This difference is more marked in the lower jaw than in the upper jaw. In Rhamphorhynchus the teeth are all relatively long and large, and directed obliquely forward, but absent from the extremities of the beak, as in the German genus from the Lias named Dorygnathus, in which the bone of the lower jaw (which alone is known) terminates in a compressed spear. In Scaphognathus the teeth are few, more vertical, and do not extend backward so far as in Rhamphorhynchus, but are carried forward to the extremity of the blunt, deep jaw.
In the short-tailed Pterodactyles the teeth are smaller, shorter, wider at the base of the crown, closer together, and do not extend so far backward in the jaw. In Ornithocheirus two teeth always project forward from the front of the jaw. Ornithostoma is toothless.
SUPPOSED h.o.r.n.y BEAK
Sometimes a h.o.r.n.y covering has been suggested for the beak, like that seen in birds or turtles, but no such structure has been preserved, even in the Solenhofen Slate, in which such a structure would seem as likely to be preserved as a wing membrane, though there is one doubtful exception. There are marks of fine blood vessels on some of the jaws, indicating a tough covering to the bone. In Rhamphorhynchus the jaws appear to gape towards their extremities as though the inters.p.a.ce had originally been occupied by organic substance like a h.o.r.n.y beak.
LOWER JAW
The lower jaw varies in relative length with the vertical or horizontal position of the quadrate bone in the skull. In Dimorphodon the jaw is as long as the skull; but in the genera from the Oolitic rocks the mandible is somewhat shorter, and in Ornithostoma the discrepancy reaches its maximum. The hinder part of the jaw is never prolonged backward much beyond the articulation, differing in this respect from Crocodiles and Plesiosaurs.
The depth of the jaw varies. It is slender in Pterodactylus, and is probably stronger relatively to the skull in Scaphognathus than in any other form. It fits between the teeth and bones of the alveolar border in the skull, in all the genera. In Dimorphodon its hinder border is partly covered by the descending edge of the malar process which these animals develop in common with some Dinosaurs, and some Anomodont reptiles, and many of the lower mammals. In this hinder region the lower jaw is sometimes perforated, in the same way as in Crocodiles. That condition is observed in Dimorphodon, but is not found in Pterodactylus.
The lower jaw is always composite, being formed by several bones, as among reptiles and birds. The teeth are in the dentary bone or bones, and these bones are almost always blended as in most birds and Turtles, and not separate from each other as among Crocodiles, Lizards, and Serpents.
An interesting contour for the lower border of the jaw is seen in Ornithostoma, as made known in figures of American examples by Professors Marsh and Williston. It deepens as it extends backwards for two-thirds its length, stops at an angle, and then the depth diminishes to the articulation with the skull. This angle of the lower jaw is a characteristic feature of the jaws of Mammals. It is seen in the monotreme Echidna, and is characteristic of some Theriodont Reptiles from South Africa, which in many ways resemble Mammals. The character is not seen in the jaws of specimens from the Oolitic rocks, but is developed in the toothed Ornithocheirus from the Cambridge Greensand, and is absent from the jaws of existing reptiles and birds.
[Ill.u.s.tration: FIG. 23 COMPARISON OF THE LOWER JAW IN ECHIDNA AND ORNITHOSTOMA]
SUMMARY OF CHARACTERS OF THE HEAD
Taken as a whole, the head differs from other types of animals in a blending of characters which at the present day are found among Birds and Reptiles, with some structures which occur in extinct groups of animals with similar affinities, and perhaps a slight indication of features common to the lowest mammals. It is chiefly upon the head that the diverse views of earlier writers have been based. Cuvier was impressed with the reptilian aspect of the teeth; but in later times discoveries were made of Birds with teeth--Archaeopteryx, Ichthyornis, Hesperornis. The teeth are quite reptilian, being not unlike miniature teeth of Mosasaurus. If those birds had been found prior to the discovery of Pterodactyles, the teeth might have been regarded as a link with the more ancient birds, rather than a crucial difference between birds and reptiles.
All the specimens show a lateral temporal hole in the bones behind the eye, and this is found in no bird or mammal, and is typical of such reptiles as Hatteria. The quadrate bone may not be so decisive as Cuvier thought it to be, for its form is not unlike the quadrate of a bird, and different, so far as I have seen, from that of living reptiles. This region of the head is reptilian, and if it occurred in a bird the character would be as astonis.h.i.+ng as was the discovery of teeth in extinct birds. These characters of the head are also found in fossil animals named Dinosaurs, in a.s.sociation with many resemblances to birds in their bones.
The palate might conceivably be derived from that of Hatteria by enlarging the small opening in the middle line in that reptile till it extended forward between the vomera; but it is more easily compared with a bird, which the animal resembles in its beak, and in the position of the nares. Excepting certain Lizards, all true existing Reptiles have the nostrils far forward and bordered by two premaxillary bones instead of one intermaxillary, as in Birds and Ornithosaurs. If nothing were known of the animal but its head bones, it would be placed between Reptiles and Birds.
CHAPTER IX