Dragons of the Air: An Account of Extinct Flying Reptiles

by H. G. Seeley

Closely connected with an animal's habits is the protection to the body which is afforded by the skin. In Pterodactyles the evidence of the condition of the skin is scanty, and mostly negative. Sometimes the dense, smooth texture of the jaw bones indicates a covering like the skin of a Lizard or the hinder part of the jaw of a Bird. Some jaws from the Cambridge Greensand have the bone channeled over its surface by minute blood vessels which have impressed themselves into the bone more easily than into its covering. Thus in the species of Ornithocheirus distinguished as microdon the palate is absolutely smooth, while in the species named machærorhynchus it is marked by parallel impressed vascular grooves which diverge from the median line. This condition clearly indicates a difference in the covering of the bone, and that in the latter species the covering had fewer blood vessels and more horny protection than in the other. The tissue may not have been of firmer consistence than in the palate of Mammals. The extremity of the beak is often as full of blood vessels as the jaw of a Turtle or Crocodile.

COVERING OF THE BODY

There is no trace even in specimens from the Solenhofen or Stonesfield Slate of any covering to the body. There are no specimens preserved like mummies, and although the substance of the wings is found there is no trace of fur or feathers, bones, or scales on the skin. The only example in which there is even an appearance suggesting feathers is in the beautiful Scaphognathus at Bonn, and upon portions of the wing membrane of that specimen are preserved a very few small short and apparently tubular bodies, which have a suggestive resemblance to the quills of small undeveloped feathers. Such evidences have been diligently sought for. Professor Marsh, after examining the wing membranes of his specimen of Rhamphorhynchus from Solenhofen, stated that the wings were partially folded and naturally contracted into folds, and that the surface of the tissue is marked by delicate striæ, which might easily be taken at first sight for a thin coating of hair. Closer investigation proved the markings to be minute wrinkles on the under surface of the wing membrane. This negative evidence has considerable value, because the Solenhofen Slate has preserved in the two known examples of the bird Archæopteryx beautiful details of the structure of the larger feathers concerned in flight. It has preserved many structures far more delicate. There is, therefore, reason for believing that if the skin had possessed any covering like one of those found in existing vertebrate animals, it could scarcely have escaped detection in the numerous undisturbed skeletons of Pterodactyles which have been examined.

The absence of a recognisable covering to the skin in a fossil state cannot be accepted as conclusive evidence of the temperature, habits, or affinities of the animal. Although Mammalia are almost entirely clothed with dense hair, which has never been found in a recognisable condition in a fossil state in any specimen of Tertiary age, one entire order, the Cetacea, show in the smooth hairless skins of Whales and Porpoises that the class may part with the typical characteristic covering without loss of temperature and without intelligible cause. That the absence of hair is not due to the aquatic conditions of rivers or sea is proved by other marine Mammals, like Seals, having the skin clothed with a dense growth of hair, which is not surpassed in any other order. The fineness of the growth of hair in Man gives a superficial appearance of the skin being imperfectly clothed, and a similar skin in a fossil state might give the impression that it was devoid of hair. There are many Mammals in which the skin is scantily clothed with hair as the animal grows old. Neither the Elephant nor the Armadillo in a fossil state would be likely to have the hair preserved, for the growth is thin on the bony shields of the living Armadilloes. Yet the difficulty need be no more inherent in the nature of hair than in that of feathers, since the hair of the Mammoth and Rhinoceros has been completely preserved upon their skins in the tundras of Siberia, densely clothing the body. This may go to show that the Pterodactyle possessed a thin covering of hair, or, more probably, that hair was absent. Since Reptiles are equally variable in the clothing of the skin with bony or horny plates, and in sometimes having no such protection, it may not appear singular that the skin in Ornithosaurs has hitherto given no evidence of a covering. From analogy a covering might have been expected; feathers of Birds and hair of Mammals are non-conducting coverings suited to arrest the loss of heat.

With the evidence, such as it is, of resemblance of Ornithosaurs to Birds in some features of respiration and flight, a covering to the skin might have been expected. Yet the covering may not be necessary to a high temperature of the blood. Since Dr. John Davy made his observations it has been known that the temperature of the Tunny, above 90° Fahrenheit, is as warm as the African scaly ant-eater named the Pangolin, which has the body more amply protected by its covering. This illustration also shows that hot blood may be produced without a four-celled heart, with which it is usually associated, and that even if the skin in Pterodactyles was absolutely naked an active life and an abundant supply of blood could have given the animal a high temperature.

The circumstance that in several individuals the substance of the wing membrane is preserved would appear to indicate either that it was exceptionally stout when there would have been small chance of resisting decomposition, or that its preservation is due to a covering which once existed of fur or down or other clothing substance, which has proved more durable than the skin itself.

FIG. 48. REMAINS OF DIMORPHODON FROM THE LIAS OF LYME REGIS
SHOWING THE SKULL, NECK, BACK AND SOME OF THE LONGER BONES OF THE SKELETON

From a slab in the British Museum (Natural History)