EVIDENCES OF THE ANIMAL'S HABITS FROM ITS REMAINS
Such are the more remarkable characters of the bones in a type of animal
life which was more anomalous than any other which peopled the earth in
the Secondary Epoch of geological time. Its skeleton in different parts
resembles Reptiles, Birds, and Mammals; with modifications and
combinations so singular that they might have been deemed impossible if
Nature's power of varying the skeleton could be limited. Since
Ornithosaurs were provided with wings, we may believe the animals to
some extent to have resembled birds in habit. Their modes of progression
were more varied, for the structures indicate an equal capacity for
movement on land as a biped, or as a quadruped, with movement in the
air. There is little evidence to support the idea that they were usually
aquatic animals. The majority of birds which frequent the water have
their bodies stored with fat and the bones of their extremities filled
with marrow. And a bird's marrow bones are stouter and stronger than
those which are filled with air. There are few, if any, bones of
Pterodactyles so thick as to suggest the conclusion that they contained
marrow, and the bones of the extremities appear to have been
constructed on the lightest type found among terrestrial birds. Their
thinness, except in a few specimens from the Wealden rocks, is
marvellous; and all the later Pterodactyles show the arrangement, as in
birds, by which air from the lungs is conveyed to the principal bones.
No Pterodactyle has shown any trace of the web-footed condition seen in
birds which swim on the water, unless the diverging bones of the hind
foot in Rhamphorhynchus supports that inference. The bones of the hind
foot are relatively small, and if it were not that a bird stands easily
upon one foot, might be considered scarcely adequate to support the
animal in the position which terrestrial birds usually occupy. Yet, as
compared with the length and breadth of the foot in an Ostrich, the toes
of an Ornithosaur are seen to be ample for support. These facts appear
to discourage the idea that the animals were equally at home on land and
water, and in air.
Some light may be thrown upon the animal's habits by the geological
circumstances under which the remains are found. The Pterodactyle named
Dimorphodon, from the Lias of the south of England, is associated with
evidences of terrestrial land animals, the best known of which is
Scelidosaurus, an armoured Dinosaur adapted by its limbs for progression
on land. And the Pterodactyle Campylognathus, from the Lias of Whitby,
is associated with trunks of coniferous trees and remains of Insects. So
that the occurrence of Pterodactyles in a marine stratum is not
inconsistent with their having been transported by streams from off the
old land surface of the Lias, on which coniferous trees grew and
Dinosaurs lived.
Similar considerations apply to the occurrence of the Rhamphocephalus in
the Stonesfield Slate of England. The deposit is not only formed in
shallow water, but contains terrestrial Insects, a variety of land
plants, and many Reptiles and other animals which lived upon land. The
specimens from the Purbeck beds, again, are in strata which yield a
multitude of the spoils of a nearly adjacent land surface; while the
numerous remains found in the marine Solenhofen Slate in Germany are
similarly associated with abundant evidences of varied types of
terrestrial life. The evidence grows in force from its cumulative
character. The Wealden beds, which yield many terrestrial reptiles and
so much evidence of terrestrial vegetation, and shallow-water conditions
of disposition, have afforded important Pterodactyle remains from the
Isle of Wight and Sussex.
The chief English deposit in which these fossils are found, the Upper
Greensand, has afforded thousands of bones, battered and broken on a
shore, where they have lain in little associated groups of remains,
often becoming overgrown with small marine shells. Side by side with
them are found bones of true terrestrial Lizards and Crocodiles of the
type of the Gavial of the Indian rivers, many terrestrial Dinosaurs, and
other evidences of land life, including fossil resins, such as are met
with in the form of amber or copal at the present day.
The great bones of Pterodactyles found in the Chalk of Kent, near
Rochester, became entombed, beyond question, far from a land surface.
There is nothing to show whether the animals died on land and were
drifted out to sea like the timber which is found water-logged and
sunken after being drilled by the ship-worm (Teredo) of that epoch.
Seeing the power of flight which the animal possessed, storms may have
struck down travellers from time to time, when far from land.
Evidence of habit of another kind may be found in their teeth. They are
brightly enamelled, sharp, formidable; and are frequently long,
overlapping the sides of the jaws. They are organs which are often
better adapted for grasping than for tearing, as may be seen in the
inclined teeth of Rhamphocephalus of the Stonesfield Slate; and better
adapted for killing than tearing, from their piercing forms and cutting
edges, in genera like Ornithocheirus of the Greensand. The manner in
which the teeth were implanted and carried is better paralleled by the
fish-eating crocodile of Indian rivers than by the flesh-eating
crocodiles, or Muggers, which live indifferently in rivers and the sea.
As the Kingfisher finds its food (see Fig. 20) from the surface of the
water without being in the common sense of the term a water bird, so
some Pterodactyles may have fed on fish, for which their teeth are well
adapted, both in the stream and by the shore.
A Pterodactyle's teeth vary a good deal in appearance. The few large
teeth in the front of the jaw in Dimorphodon, associated with the many
small vertical teeth placed further backward, suggest that the taking of
food may have been a process requiring leisure, since the hinder teeth
adapted to mincing the animal's meat are extremely small. The way in
which the teeth are shaped and arranged differs with the genera. In
Pterodactylus they are short and broad and few, placed for the most part
towards the front of the jaws. Their lancet-shaped form indicates a
shear-like action adapted to dividing flesh. In the associated genus
Rhamphorhynchus the teeth are absent from the extremity of the jaw, are
slender, pointed, spaced far apart, and extend far backward. When the
jaws of the Rhamphorhynchus are brought together there is always a gap
between them in front, which has led to belief that the teeth were
replaced by some kind of horny armature which has perished. In the
long-nosed English type of Ornithocheirus the jaws are compressed
together, so that the teeth of the opposite sides are parallel to each
other, with the margins well filled with teeth, which are never in close
contact, though occasionally closer and larger in front, in some of the
forms with thick truncated snouts.
It is not the least interesting circumstance of the dentition of
Pterodactyles that, associated in the same deposits with these most
recent genera with teeth powerfully developed, there is a genus named
Ornithostoma from the resemblance of its mouth to that of a bird in
being entirely devoid of teeth. It is scarcely possible to distinguish
the remains of the toothed and toothless skeletons except in the dentary
character of the jaws. There is no evidence that the toothless types
ever possessed a tooth of any sort. They were first found in fragments
in England in the Cambridge Greensand, but were afterwards met with in
great abundance in the Chalk of Kansas, where the same animals were
named Pteranodon. A jaw so entirely bird-like suggests that the
digestive organs of Pterodactyles may in such toothless forms at least
have been characterised by a gizzard, which is so distinctive of Birds.
The absence of teeth in the Great Ant-eater and some other allied
Mammals has transferred the function which teeth usually perform to the
stomach, one part of which becomes greatly thickened and muscular,
adapting itself to the work which it has to perform. It is probable that
the gizzard may be developed in relation to the necessities which food
creates, since even Trout, feeding on the shell-fish in some Irish
lochs, acquire such a thickened muscular stomach, and a like
modification is recorded in other fishes as produced by food.
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 _machaerorhynchus_ 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 striae, 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
Archaeopteryx 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 deg.
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.
[Illustration: 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)_]