FAMILY RELATIONS OF PTERODACTYLES TO ANIMALS WHICH LIVED WITH THEM
Enough has been said of the general structure of Pterodactyles and the
chief forms which they assumed while the Secondary rocks were
accumulating, to convey a clear idea of their relations to the types of
vertebrate animals which still survive on the earth. We may be unable to
explain the reasons for their existence, and for their departure from
the plan of organisation of Reptiles and Birds. But the evidence has not
been exhausted which may elucidate their existence. Sometimes, in
problems of this kind, which involve comparison of the details of the
skeleton in different animals, it is convenient to imagine the
possibility of changes and transitions which are not yet supported by
the discovery of fossil remains. If, for example, the Pterodactyle be
conceived of as divested of the wing finger, which is its most
distinctive character, or that finger is supposed to be replaced by an
ordinary digit, like the three-clawed digits of the hand which we have
regarded as applied to the ground, where, it may be asked, would the
animal type be found which approximates most closely to a Pterodactyle
which had been thus modified? There are two possible replies to such a
question, suggested by the form of the foot. For the old Bird
Archaeopteryx has three such clawed digits, but no wing finger. And some
Dinosaurs also have the hand with three digits terminating in claws,
which are quite comparable to the clawed digits of Pterodactyles.
The truth expressed in the saying that no man by taking thought can add
a cubit to his stature is of universal application in the animal world,
in relation to the result upon the skeleton of the exercise of a
function by the individual. Yet such is the relation in proportions of
the different parts of the animal to the work which it performs, so
marked is the evidence that growth has extended in direct relation to
use of organs and active life, and that structures have become dwarfed
from overwork, or have wasted away from disuse--seen throughout all
vertebrate animals, that we may fairly attribute to the wing finger some
correlated influence upon the proportions of the animal, as a
consequence of the dependence of the entire economy upon each of its
parts. Therefore if an allied animal did not possess a wing finger, and
did not fly, it might not have developed the lightness of bone, or the
length of limb which Pterodactyles possess.
The mere expansion of the parachute membrane seen in so-called flying
animals, both Mammals and Reptiles, which are devoid of wings, is
absolutely without effect in modifying the skeleton. But when in the Bat
a wing structure is met with which may be compared to a gigantic
extension of the web foot of the so-called Flying Frog, the bones of the
fingers and the back of the hand elongate and extend under the stimulus
of the function of flight in the same way as the legs elongate in the
more active hoofed animals, with the function of running. Therefore it
is not improbable that the limbs shared to some extent in growth under
stimulus of exercise which developed the wing finger. And if an animal
can be found among fossils so far allied as to indicate a possible
representative of the race from which these Flying Dragons arose, it
might be expected to be at least shorter legged, and possibly more
distinctly Reptilian in the bones of the shoulder-girdle which support
the muscles used in flight. It may readily be understood that the kinds
of life which were most nearly allied to Pterodactyles are likely to
have existed upon the earth with them, and that flight was only one of
the modes of progression which became developed in relation to their
conditions of existence. The principal assemblage of terrestrial animals
available for such comparison is the Dinosauria. They may differ from
Pterodactyles as widely as the Insectivora among Mammals differ from
Bats, but not in a more marked way. Comparisons will show that there are
resemblances between the two extinct groups which appeal to both reason
and imagination.
Dinosaurs are conveniently divided by characters of the pelvis first
into the order Saurischia, which includes the carnivorous Megalosaurus
and the Cetiosaurus, with the pelvis on the Reptile plan; and secondly
the order Ornithischia, represented by Iguanodon, with the pelvis on the
Bird plan. It may be only a coincidence, but nevertheless an interesting
one, that the characters of those two great groups of reptiles, which
also extend throughout the Secondary rocks, are to some extent
paralleled in parts of the skeleton of the two divisions of
Pterodactyles. This may be illustrated by reference to the skull,
pelvis, hind limb, and the pneumatic condition of the bones.
[Illustration: FIG. 77. COMPARISON OF THE SKULL OF THE DINOSAUR
ANCHISAURUS WITH THE ORNITHOSAUR DIMORPHODON]
The Saurischian Dinosauria have an antorbital vacuity in the side of the
skull between the nasal opening and the eye, as in the long-tailed
Ornithosaurs named Pterodermata. In some of the older genera of these
carnivorous Dinosaurs of the Trias, the lateral vacuities of the head
are as large as in Dimorphodon. But in some at least of the Iguanodont,
or Ornithischian Dinosaurs, there is no antorbital vacuity, and the side
of the face in that respect resembles the short-tailed Pterodactylia.
The skull of a carnivorous Dinosaur possesses teeth which, though easily
distinguished from those of Pterodactyles, can be best compared with
them. The most striking difference is in the fact that in the Dinosaur
the nostrils are nearly terminal, while in the Pterodactyle they are
removed some distance backward. This result is brought about by growth
taking place, in the one case at the front margin of the maxillary bone
so as to carry the nostril forward, and in the other case at the back
margin of the premaxillary bone. Thus an elongated part of the jaw is
extended in front of the nostril. Hence there is a different proportion
between the premaxillary and maxillary bones in the two groups of
animals, which corresponds to the presence of a beak in a bird, and its
absence in living reptiles. It is not known whether the extremity of the
Pterodactyle's beak is a single bone, the intermaxillary bone, such as
forms the corresponding toothless part of the jaw in the South African
reptile Dicynodon, or whether it is made by the pair of bones called
premaxillaries which form the extremity of the jaw in most Dinosaurs.
Too much importance may perhaps be attached to such differences which
are partly hypothetical, because the extinct Ichthyosaurus, which has an
exceptionally long snout, has the two premaxillary bones elongated so as
to extend backward to the nostrils. A similar elongation of those bones
is seen in Porpoises, which also have a long snout; and the bones are
carried back from the front of the head to the nostrils, which are
sometimes known as blowholes. But the Porpoise has those premaxillary
bones not so much in advance of the bones which carry teeth named
maxillary, as placed in the interspace between them. The nostrils,
however, are not limited to the extremity of the head in all Dinosaurs.
If this region of the beak in Dimorphodon be compared with the
corresponding part of a Dinosaur from the Permian rocks, or Trias, the
relation of the nostril to the bones forming the beak may be better
understood.
[Illustration: FIG. 78. COMPARISON OF THE SKULL OF THE DINOSAUR
ORNITHOSUCHUS WITH THE ORNITHOSAUR DIMORPHODON]
In the sandstone of Elgin, usually named Trias, a small Dinosaur is
found, which has been named Ornithosuchus, from the resemblance of its
head to that of a Bird. Seen from above, the head has a remarkable
resemblance to the condition in Rhamphorhynchus, in the sharp-pointed
beak and positions of the orbits and other openings. In side view the
orbits have the triangular form seen in Dimorphodon, and the preorbital
vacuities are large, as in that genus, while the lateral nostrils, which
are smaller, are further forward in the Dinosaur. The differences from
Dimorphodon are in the articulation for the jaw being carried a little
backward, instead of being vertical as in the Pterodactyle, and the bone
in front of the nose is smaller. Notwithstanding probable differences
in the palate, the approximation, which extends to the Crocodile-like
vacuity in the lower jaw, is such that by slight modification in the
skull the differences would be substantially obliterated by which the
skull of such an Ornithosaur is technically distinguished from such a
Dinosaur.
The back of the skull is clearly seen in the Whitby Pterodactyle, and
its structure is similar to the corresponding part of such Dinosaurs as
Anchisaurus or Atlantosaurus, without the resemblance quite amounting to
identity, but still far closer than is the resemblance between the same
region in the heads of Crocodiles, Lizards, Serpents, Chelonians. Few of
these fossil Dinosaur skulls are available for comparison, and those
differ among themselves. The coincidences rather suggest a close
collateral relation than prove the elaboration of one type from the
other. They may have had a common ancestor.
The Trias rocks near Stuttgart have yielded Dinosaurs as unlike
Pterodactyles as could be imagined, resembling heavily armoured
Crocodiles, in such types as the genus Belodon. Its jaws are compressed
from side to side, as in many Pterodactyles, and the nostrils are at
least as far backward as in Rhamphorhynchus. Belodon has preorbital
vacuities and postorbital vacuities, but the orbit of the eye is never
large, as in Pterodactyles. It might not be worth while dwelling on such
points in the skull if it were not that the pelvis in Belodon is a basin
formed by the blending of the expanded plates of the ischium and the
pubis, into a sheet of bone which more nearly resembles the same region
in Pterodactyles than does the ischio-pubic region in other Dinosaurian
animals like Cetiosaurus.
The backbone in a few Dinosaurs is suggestive of Pterodactyles. In such
genera as have been named Coelurus and Calamospondylus, in which the
skeleton is only partially known, the neck vertebrae become elongated, so
as to compare with the long-necked Pterodactyles. The cervical rib is
often very similar to that type, and blended with the vertebra, as in
Pterodactyles and Birds. The early dorsal vertebrae of Pterodactyles
might almost be mistaken for those of Dinosaurs. The tail vertebrae of a
Pterodactyle are usually longer than in long-tailed Dinosauria.
In the limbs and the bony girdles which support them there is more
resemblance between Pterodactyles and Dinosaurs than might have been
anticipated, considering their manifest differences in habit. Thus all
Dinosaurs have the hip bone named ilium prolonged in front of the
articulation for the femur as well as behind it, almost exactly as in
Pterodactyles and Birds (see p. 95). There is some difference in the
pubis and ischium which is more conspicuous in form than in direction of
the bones. There is a Pterodactyle imperfectly preserved, named
_Pterodactylus dubius_, in which the ischium is directed backward and
the pubis downward, and the bones unite below the acetabular cavity for
the head of the femur to work in, but do not appear to be otherwise
connected. In Rhamphorhynchus the connexion between these two thickened
bars is made by a thin plate of bone. In such a Dinosaur as the American
carnivorous Ceratosaurus the two bars of the pubis and ischium remain
separate and diverging, and there is no film of bone extending over the
interspace between them. The development of such a bony condition would
make a close approximation between the Ornithosaurian pelvis and that
of those Dinosaurs which closely resemble Pterodactyles in skull and
teeth.
[Illustration: FIG. 79. LEFT SIDE OF PELVIS
A Pterodactyle is shown between a carnivorous Dinosaur above and a
herbivorous Dinosaur below]
Another pelvic character of some interest is the blending of the pubis
and ischium of the right and left sides in the middle line of the body.
There are some genera of Dinosaurs like the English Aristosuchus from
the Weald, and the American genera Coelurus, Ceratosaurus, and others,
in which the pubic bones, instead of uniting at their extremities, are
pinched together from side to side, and unite down the lower part of
their length, terminating in an expanded end like a shoe, which is seen
to be a separate ossification, and probably formed by a pair of
ossifications joined in the median line. This small bone, which is below
the pubes, and in these animals becomes blended with them, we may regard
as a pair of prepubic bones like those of Pterodactyles and Crocodiles,
except that they have lost the stalk-like portions, which in those
animals are developed to compensate for the diminished length of the
pubic bones. The prepubic bones may also be developed in Iguanodon, in
which a pair of bones of similar form remains throughout life in advance
of the pubes, as in Pterodactyles. In those Dinosauria with the
Bird-like type of pelvis the pubic bone is exceptionally developed,
sending one process backward and another process forward, so that there
is a great gap between these diverging limbs to the bone. In the region
behind the sternum to which the ribs were attached, and in front of the
pelvis, is a pair of bones in Iguanodon shaped like the prepubic bones
of Dimorphodon. They have sometimes been interpreted as a hinder part of
the sternum, but may more probably be regarded as a pair of prepubic
bones articulating each with the anterior process of the pubis (see Fig.
80). The small bones found at the extremities of the pubes in such
carnivorous Dinosaurs as Aristosuchus are blended by bony union with the
pubes. The bones in Iguanodon are placed behind the sternal region
without any attachment for sternal ribs, and the expanded processes
converge forwards from the stalk and unite exactly like the prepubic
bones of Ornithosaurs. While this character, on the one hand, may link
Pterodactyles with the Dinosaurs, on the other hand it may be a link
between both those groups and the Crocodiles, in which the front pair of
bones of the pelvis has also appeared to be representative of the
prepubic bones of Flying Reptiles (see Fig. 32, p. 98).
[Illustration: FIG. 80. DIAGRAM OF THE PELVIS SEEN FROM BELOW IN AN
ORNITHOSAUR AND A DINOSAUR]
The resemblances between Pterodactyles and Dinosaurs in the hind limb
are not of less interest, though it is rather in the older Pterodactyles
such as Dimorphodon, Pterodactylus, and Rhamphorhynchus that the
resemblance is closest with the slender carnivorous Dinosaurs. They
never have the head of the thigh bone, femur, separated from its shaft
by a constricted neck, as in the Pterodactyles from the Chalk. In many
ways the thigh bone of Dinosaurs tends towards being Avian; while that
of Pterodactyles inclines towards being Mammalian, but with a tendency
to be Bird-like in the older types, and to be Mammal-like in the most
recent representatives of the group in the Chalk.
The bones of the leg in Ornithosaurs, known as tibia and fibula, are
remarkable for the circumstance first that they resemble Birds in the
fibula being slender and only developed in its upper part towards the
femur, and secondly that in a genus like Dimorphodon this drum-stick
bone has the two upper bones of the ankle blended with the tibia, so as
to form a rounded pulley joint which is indistinguishable from that of a
Bird (see p. 102). There is a large number of Dinosaurs in which this
remarkable distinctive character of Birds is also found. Only, Dinosaurs
like Iguanodon, for instance, have the slender fibula as long as the
tibia, and contributing to unite with the separate ankle bones of the
similarly rounded pulley at the lower end. There are no Birds in which
the tarsal bones remain separated and distinct throughout life. But in
Pterodactylus from Solenhofen, as in a number of Dinosaurs, especially
the carnivorous genera, the bones of the tarsus remain distinct
throughout life, and never acquired such forms as would have enabled the
ankle bone, termed astragalus, to embrace the extremity of the tibia, as
it does in Iguanodon. Thus the resemblance of the Ornithosaur drum-stick
is almost as close to Dinosaurs as to Birds.
There is great similarity between Dinosaurs and Pterodactyles seen in
the region of the instep, known as the metatarsus. These bones are
usually four in number, parallel to each other, and similar in form.
They are commonly longer than in Dinosaurs; but among some of the
carnivorous Dinosaurs their length approximates to that seen in
Pterodactyles. In neither group are the bones blended together by bony
union, while they are always united in Birds, as in Oxen and similar
even-hoofed mammals. Dinosaurs agree with Pterodactyles in maintaining
the metatarsal bones separate, but they differ from them and agree with
Birds frequently, in having the number of metatarsal bones reduced to
three, as in Iguanodon, though Dinosaurs often have as many as five
digits developed.
The toe bones, the phalanges of these digits of the hind limb, are
usually longer in Pterodactyles than in Dinosaurs, but they resemble
carnivorous Dinosaurs in the forms of their sharp terminal bones for the
claws, which are similarly compressed from side to side.
So diverse are the functions of the fore limb in Dinosaurs and
Pterodactyles, and so remarkably does the length of the metacarpal
region of the back of the hand vary in the long-tailed and short-tailed
Ornithosaurs, that there is necessarily a less close correspondence in
that region of the skeleton between these two groups of animals; for the
Pterodactyle fore limb is modified in relation to a function which can
only be paralleled among Birds and Bats; and yet neither of those groups
of animals approximates closely in this region of the skeleton to the
Flying Reptile. Under all the modifications of structure which may be
attributed to differences of function, some resemblance to Dinosaurs may
be detected, which is best evident in the upper arm bone, humerus; is
slight in the fore-arm bones, ulna and radius; and becomes lost towards
the extremity of the limb.
If the tendency of the thigh bone to resemble a Mammalian type of femur
(p. 100) is a fundamental, deep-seated character of the skeleton, it
might be anticipated that a trace of Mammalian character would also be
found in the humerus. For what the character is worth, the head of the
humerus does show a closer approximation to a Monotreme Mammal than is
seen in Birds, and is to some extent paralleled in those South African
reptiles which approximate to Mammals most closely. Not the least
remarkable of the many astonishing resemblances of these light aerial
creatures to the more heavy bodied Dinosaurs is the circumstance that
the humerus in both groups makes a not dissimilar approach to that of
certain Mammals.
These illustrations may be accepted as demonstrating a relationship
between the Ornithosaurs and Dinosaurs now compared, which can only be
explained as results of influence of a common parentage upon the forms
of the bones. But more interesting than resemblances of that kind is the
similarity that may be traced in the way in which air is introduced into
cavities in the bones in both groups. In some of the imperfectly known
Dinosaurs, like Aristosuchus, Coelurus, and Thecospondylus, the bone
texture is as thin as in Pterodactyles, and the vertebrae are excavated
by pneumatic cavities, which are amazing in size when compared with the
corresponding structures in birds, for the vertebra is often hollowed
out so that nothing remains but a thin external film like paper for its
thickness. In the Dinosaurian genus Coelurus this condition is as well
marked in the tail and back as it is in the neck. The essential
difference from Birds appears to be that in the larger carnivorous
Dinosaurs the pneumatic condition of the bones is confined to the
vertebral column; while Birds and Pterodactyles have the pneumatic
condition more conspicuously developed in the limb bones. The pneumatic
skeleton, however, appears to be absent from the herbivorous types like
Iguanodon and all Dinosaurs which have the Bird-like form of pelvis, and
are most Bird-like in the forms of bones of the hind limb. It is
possible that some of the carnivorous Dinosaurs also possessed limb
bones with pneumatic cavities. Many of those bones are hollow with very
thin walls. If their cavities were connected with the lungs the foramina
are inconspicuous and unlike the immense holes seen in the sides of the
vertebrae.
According to the late Professor Marsh, the limbs of Coelurus and its
allies, which at present are imperfectly known, are in some cases
pneumatic. Therefore there is a closer fundamental resemblance between
some carnivorous Dinosaurs and Pterodactyles than might have been
anticipated. But the skull of Coelurus is unknown, and the fragments of
the skeleton hitherto published are insufficient to do more than show
that the two types were near in kindred, though distinct in habit. Each
has elaborated a skeleton which owes much to the common stock which
transmitted the vital organs, and the tendency of the bones to take
special forms; but which also owes more than can be accurately measured
to the action of muscles in shaping the bones and the influence of the
mechanical conditions of daily life upon the growth of the bones in both
of these orders of animals. Enough is known to prove that all Dinosaurs
cannot be regarded as Ornithosaurs which have not acquired the power of
flight; though the evidence would lead us to believe that the primitive
Ornithosaur was a four-footed animal, before the wing finger became
developed in the fore limb as a means of extending a patagial membrane,
like the membrane which in the hind limb of Dimorphodon has bent the
outermost digit of the foot upward and outward to support the
corresponding organ of flight extending down the hind legs.
It may thus be seen that the characters of Ornithosaurs which have
already been spoken of as Reptilian, as distinguished from the
resemblances to Birds, may now with more accuracy be regarded as
Dinosaurian. The Dinosaurs, like Pterodactyles, must be regarded as
intermediate in some respects between Reptiles and Birds. The
resemblances enumerated would alone constitute a partial transition from
the Reptile to the Bird, although no Dinosaurs have organs of flight;
many are heavily armoured with plates of bone, and few, if any,
approximate in the technical parts of the skeleton to the Bird class,
except in the hind limbs. Yet Dinosaurs have sometimes been regarded as
standing to Birds in the relation of ancestors, or as parallel to an
ancestral stock.
Before an attempt can be made to estimate the mutual relation of the
Flying Reptiles to Dinosaurs on the one hand, and to Birds on the other,
it may be well to remember that the resemblance of such a Dinosaur as
Iguanodon to a Bird in its pelvis and hind limb is not more remarkable
than that of Pterodactyles to Birds in the shoulder-girdle and bones of
the fore limb. The keeled sternum, the long, slender coracoid bones and
scapulae, are absolutely Bird-like in most Ornithosaurs; and that region
of the skeleton only differs from Birds in the absence of a furculum
which represents the clavicles, and is commonly named the
"merry-thought." The elongated bones of the fore-arm and the hand,
terminating in three sharp claws, are characters in which the fossil
bird Archaeopteryx resembles the Pterodactyle Rhamphorhynchus, a
resemblance which extends to a similar elongation of the tail. It is
remarkable that the resemblance should be so close, since Archaeopteryx
affords the only bird's skeleton known to be contemporary which can be
compared with the Solenhofen Flying Reptiles. The resemblance may
possibly be closer than has been imagined. The back of the head of
Archaeopteryx is imperfectly preserved in the region of the quadrate
bone, malar arch, and temporal vacuity. And till these are better known
it cannot be affirmed that the back of the head is more Reptilian in
Pterodactyles than in the oldest Birds. The side of the head in
Archaeopteryx is distinguished by the nostril being far forward, the
vacuity in front of the orbit being as large as in the Pterodactyle
Scaphognathus from Solenhofen and other long-tailed Pterodactyles.