determine the use of other structures, the meaning that terms like
Crocodilian and Chelonian really have is that they represent the
aspect of Reptilian organization when seen through the specialization
of respiration, or circulation of the blood. The soft parts thus
determining the nutrition and function of the muscles and skeleton,
anatomists in examining the bones of extinct animals are accustomed to
reverse the order of their inferences, and infer from modifications of
the skeleton what had been the characters of the soft and more vital
structures.
On the presumed accuracy of this method of research rest many results
of Comparative Anatomy. But since the shapes of bones are determined by
the muscles as well as by inheritance, it is always to be remembered
that a similar form of bone may obtain in different orders or classes
of animals, as the result of a similar function in a special region of
the body. Such resemblances are familiar to anatomists. Hence much
caution is required from the Palæontologist to distinguish between
the characteristics of a group, and the extent to which they may be
modified by function. This distinction is the first principle of
classification. But it is always difficult to estimate the importance
of characters in fragments of bones or parts of skeletons, and the
difficulty is increased by the fact that if what appears to be but a
functional modification should pervade all the species, it becomes
a characteristic of the group, and its power of modifying the other
organs in a peculiar way has to be considered.
Thus for all practical purposes birds may be said to be characterized
by wings, which almost acquire the dignity of class characters from
their influence on the respiratory function. But in some birds it has
been thought that no bone of the fore-limb was ever developed[V]; and
the difference between such a phenomenon and the wing of a Swift, for
example, is one almost of infinity, as compared with any other aspect
that the anterior limb might have assumed. Therefore, since a bird may
part with its fore-limbs and yet remain a bird, I infer that it might
apply its fore-limbs to the ground, become a quadruped, and be a bird
still. And if in this process the other structures remained unchanged,
no one would regard the modification as more than an ordinal one. But
should the vertebræ change also, or the pelvis, or the covering of
the integument, or the jaws become toothed, then, although the heart
and lungs and brain of the imaginary animal retained their class
characters, the functional differences being more than those of an
order would constitute it a sub-class.
[Footnote V: According to Prof. Owen, in Dinornis.]
In the same way it is conceivable that serpents may have existed with
well-developed limbs, and if they retained their other characters the
limbed forms would constitute a sub-order of serpents; but if to these
characters they added a closed palate united to the cranium, they would
constitute a new order of reptiles. A chelonian might be entirely
deprived of its bony covering, and it would still be a chelonian,
differing only as a separate family. So that structures which to the
eye appear fundamental may be lost without affecting an animal's
systematic position, just as animals while resembling each other in
form may possess dissimilar organization.
Even with the living or typical Reptilia, naturalists are divided as
to the number of ordinal groups into which they naturally fall. It is
however generally agreed that the Amphibia or Dipnoa of Fitzinger, have
no near affinity with the true reptiles. Milne-Edwards, Van der Hoeven
and Agassiz make the remainder into three orders, as did Cuvier:
Chelonia,
Sauria,
Ophidia.
Stannius, Gray, Owen and Huxley, on the other hand, by dividing the
Saurians make four orders, to which Dr Günther by his description of
Sphenodon has given evidence of a fifth:
Crocodilia,
Chelonia,
Sauria,
Ophidia,
(Rhynchocephalia.)
De Blainville in a remarkable classification (1816), made three orders,
Chelonians, Emydosaurians [crocodiles], and Saurophidians; the latter
group being subdivided into Saurians and Ophidians.
In his "Handbuch der Anatomie der Wirbelthiere" Stannius unites the
Crocodilia and Chelonia into a group called Monimostylica; while of
the Sauria and Ophidia he makes another group called Streptostylica.
Similar groups were made by Dr Gray, and named Cataphracta and
Squamata. They are identical with the "cuirassed" and "scaly" reptiles
of Dumeril and Bibron.
The _Astylica_ (Sphenodon) have no penis.
The _Streptostylica_ have a double penis, lungs simplified at the
distal end into a mere air-bladder, brains with a moderately elongated
cerebrum, the palate mesially open, scales, leathery shell to the egg
cut through by a tooth on the premaxillary bone.
The _Monimostylica_ have a single penis, lungs well subdivided,
ventricle of heart partly [turtles] or entirely divided [crocodiles],
brains having the cerebrum broad or high, a closed palate, scutes, a
calcareous shell to the egg.
Thus the chief differences between Turtles and Crocodiles on the one
hand, and Lizards and Serpents on the other hand, are not so much
in the fundamental vital structures, though these undergo changes
even in the families, as in the different ways in which the muscles
and skeleton are modified. The typical lizards diverge widely from
the crocodiles, and in those osteological features which admit of
comparison they make at least as near an approach to the Chelonians.
But leaving the limbs and pectoral and pelvic girdles out of
consideration, lizards find their natural place side by side with the
serpents.
Attempts have been made by Palæontologists to incorporate the new
ordinal groups which they have been compelled to create for some
fossils, along with the true Reptilia; but such a proceeding destroys
the value of the term Reptile as a measure of a known organization. In
the absence of knowledge of the brains of Dinosaurs, Ichthyosaurs, and
Dicynodonts, their union with the Reptilia can only have a stagnating
effect on Palæontology, for there is no proof that they are Reptiles
in the same sense as are Crocodiles or Chameleons: while their bones
being used as standards of Reptilian structure in comparisons, they
adjudicate the place in nature of other animals by an authority which
has never been established.
Before any inference can be drawn from the forms of bones in extinct
animals, their relations to vital structures and to way of life must
be known in animals which still live. This may give some clue both
to their functional significance and to the extent to which they are
inherited and not directly attributable to function. But an idea of
the morphological value of the bones of living animals is only gained
by comparing them with the remains of their extinct allies, tracing
the now imitative structure back to its origin in a function which has
ceased to be displayed.
Professor Owen in his "Comparative Anatomy of the Vertebrates" (1866)
admits nine orders of Reptiles, five of which are extinct, some of the
extinct orders being supposed to rank lower, while others are higher
than the living types. They are arranged in this way,
* Pterosauria,
* Dinosauria,
Crocodilia,
Ophidia,
Lacertilia,
Chelonia
* Anomodontia,
* Sauropterygia,
* Ichthyopterygia.+
* Extinct.
+ Prof. Owen, _Comp. Anat._ Vol. I. p. 7-9, defines his
sub-classes. At p. 15, in the details of the orders, he puts
Ichthyosaurus in the 5th sub-class _Monopnoa_. But at p. 50,
treating of the vertebral column of Ichthyosaurus, it is written of
as an extinct order of _Dipnoal_ reptiles. The Dipnoa then
would include
Ichthyosauria,
Batrachia,
Labyrinthodontia,
Ganocephala.
But Ichthyosaurus obviously belongs to Haeckel's group Monocondylia.
In what characters the Ichthyosaurs are lower than living reptiles I
have been unable to discover. The palate may be better compared with
a struthious bird than with a reptile; and the pectoral girdle may be
better compared with the Ornithodelphia than with a reptile, while all
the trunk-vertebræ have ribs such as are associated in living animals
with a four-celled heart. But if it is a lower animal type than living
reptilia, the student will ask, how much lower? does it descend to
the Dipnoa, and prove to be the missing link between the Amphibia and
Reptilia? and wherein is the evidence? Or does it not with Dicynodonts
and Dinosaurs rather form an outlying class uniting the reptiles with
the mammals.
In the same way, when Pterosauria and Dinosauria are placed above
living reptiles, we are compelled to ask how much are they above, or
what are the characters which bind them to the Reptilia at all? No
satisfactory evidence has ever been adduced to show that the Dinosauria
are Reptiles. And of the claim of the Pterodactyles to such a position,
the facts detailed and now summarised will be the best evidence.
The highest structure shown in these remains is the brain-case. The
cavity for the brain is in every respect like that in the skull of a
bird. It resembles brains of a high type in having the cerebral lobes
convex in front; since, in the lower mammals, there is a resemblance
to reptiles in the conical form of the cerebrum; while the brains even
of some of the placental mammals are not well distinguished from those
of reptiles. Although the brain of the Ornithorhynchus is entirely
mammalian, it is more like the brain of a reptile than is the brain
of the Pterodactyle. No evidence of affinities could be adduced which
would outweigh this. Taken by itself it would lead us to anticipate for
the Pterodactyle those vital structures which birds have in common.
Next in importance to the brain are the pneumatic perforations in
the bones. They are seen in the lower jaw, the quadrate bone, in the
whole of the vertebral column, in all the bones of the fore-limb,
excepting one or two fragments, in the scapula and coracoid, in the
os innominatum, in the femur and in the tibia. In such of the bones
as can be compared, the pneumatic perforation is usually situated in
Birds as it is in Pterodactyles. In Birds the bones are filled with
air through these perforations, and as a principle the greater the
motion of the animal, the greater is the number of bones filled with
air. This air is received from the air-sacs which receive it from the
lungs, and return it through the lungs again. There is thus in birds
a sort of supplemental lung-system, which circulates air through the
body. Nothing of the kind exists in any other class of animals. The
respiratory system in birds is more perfect and complex than in the
other vertebrata, and, as a result, the temperature of the blood on the
whole is hotter.
In Pterodactyles the reticulate character of the perforations proves
that they were pneumatic, and supplied the bones with air. The fact
that the bones were supplied with air, necessitates an elaborate system
of air-sacs to furnish the supply. And the existence of these air-sacs
speaks incontestably to bronchial tubes opening on the surface of the
lungs to supply them, and to the existence of lungs essentially like
those of birds. The outward and backward direction of the coracoid
bones may indicate that the lungs were larger than in a bird.
The circulation of air through the bird's body has relation to rapid
motion through the atmosphere, which necessarily produces more rapid
respiration than would comparative quiescence. The same inference must
be applied to the Pterodactyles. But rapid respiration only means more
rapid oxidation of the blood, and conversion of the purple cruorine
into scarlet cruorine,--that is, the conversion of venous blood into
arterial blood. And if venous blood is converted into arterial blood by
a lung-apparatus like that of a bird, and with a rapidity like that in
a bird, there must be a circulation of the blood as rapid as in birds.
Such a circulation is only maintained by a heart with two auricles and
two ventricles. Therefore Pterodactyles had the heart like that of
birds and mammals.
Now, since the temperature of the blood is chiefly dependent on
respiration and circulation, and Pterodactyles had respiratory and
circulatory organs which in living animals produce hot blood, it
results that they were hot-blooded animals.
Thus the heart and lungs are exactly such as would have been inferred
from the brain, and, like it, they are avian. And so important are
these vital structures all taken together, that the inference from them
upon an animal's affinities would overbear all other evidence that
could be adduced except reproduction; for they demonstrate the plan on
which an animal was built, and are the motor power which enabled it to
use its skeleton in a way that stamped upon it a peculiar form.
In the head such structures as are preserved conform with slight
variations to the avian plan. Other Ornithosaurians show in the parts
which are not preserved in Cambridge specimens some characters which
are not avian; they are in part as much mammalian as reptilian, and
in a few points entirely reptilian. But it might be misleading to
take German specimens into consideration in forming an estimate of
the Pterodactyles of the Cambridge Greensand, which were probably a
different ordinal group, and may have had material differences in
structure.
The vertebral column as a whole is distinctive.
The neck and sacrum are mammalian, and the tail reptilian. The
procœlous vertebræ are characteristic of reptiles, but in some animals,
as Chelonians, they vary in different regions of the body; and among
amphibians the character is inconstant in genera nearly allied.
The hind-limb is in part mammalian and in part avian; if there be any
reptilian characters in the foot, they are not less mammalian.
The os innominatum is avian and mammalian.
The pectoral girdle is avian.
The fore-limb is avian and mammalian.
The wing-finger is distinctive, though formed on the avian plan.
Thus, if with an avian basis some parts of the skeleton present points
of agreement with reptiles, in other points there are resemblances
with mammals not less characteristic. These phænomena do not show that
in so far the animal is a mammal or a reptile, but only that mammals,
ornithosaurians, and reptiles have had a common origin, and that while
they have been differentiated so as to form separate classes they have
severally retained characters which formerly were united in one class.
It is a skeleton intermediate between reptiles and mammals, and well
distinguished by mammalian, reptilian, and peculiar characters, from
birds. It therefore forms a parallel group with birds, displaying
the ornithic organization in a differently modified skeleton. Yet it
differs more from existing birds than they differ among themselves,
for the discrepancies are in points of structure in which all existing
birds agree: they are in having teeth, in the procœlous centrum, in the
separate condition of the carpal and metacarpal (and of the tarsal and
metatarsal) bones; in having more than two bones in the fore-arm, in
the sacrum formed of few vertebræ, in the expanded pubic (and prepubic)
bones, in a long neck to the femur, and in the modification of the wing
by the great development of the phalanges of one finger.
I therefore regard the Pterodactyles as forming a group of equal value
with birds, for which group the name Ornithosauria is here used.
It cannot form a separate class, because they have a fundamental
organization in common; and it cannot form an order of birds, because
its differences from birds are greater than those of an order. It
is a group which itself probably includes several orders, and must
constitute a sub-class, which finds its place in nature side by side
with birds and between mammals and reptiles, thus:--
Mammalia. \
|
| Ornithosauria. Aves
|
Reptilia. /
Restoration.
Of the form and size[W] of the animals from the Cambridge Greensand, an
idea will best be given by a few measurements.
[Footnote W: There are Ornithosaurians hereafter to be described
compared with which the largest at present known will seem diminutive.
A vertebra of one such, from the Wealden, is contained in the British
Museum (numbered 28632). The centrum alone is between 9 and 10 inches
long and 8 inches deep. It is named Streptospondylus, but constitutes
a new group of Ornithosaurians. Nothing so gigantic exists in the
Woodwardian Museum. Another vertebra of the same or an allied genus has
been figured by Prof. Owen as the tympanic bone of ?Iguonodon (Fossil
Reptilia of the Wealden, Part 2, pl. 10).]
In the species Ornithocheirus nasutus (Seeley), =J=._c_.2.11.1:
The premaxillary extends for 6 inches without reaching the nares.
The lower jaw is 3/4 of an inch deep at the articulation.
The four cervical vertebræ are each 1-1/2 inch long.
The sternum measures 1-1/2 inch over the facets for the coracoids.
The humerus is 2-1/16 inches over the proximal end, the radial crest
not being preserved.
The coracoid is 1-1/4 inch over the proximal end.
The scapula is about 3-1/2 inches long.
The proximal carpal (imperfect) is 1-5/8 inch wide.
The distal carpal is 1-1/2 inch wide.
The lateral carpal is 1-1/4 inch long.
The wing-metacarpal is 1-1/4 inch wide at the proximal end, and 7/8
inch wide at the distal end.
The proximal end of the first phalange is about 1-5/8 inch wide.
The proximal end of the second phalange is less than an inch wide.
The claw-phalange (imperfect) is about 1-1/4 inch long.
The femur is 4 inches long.
Putting the animal together, the bones give this size :
Head 1 ft. 3 in. long.
Neck 9 "
(_Back and sacrum_) ? 8 "
(_Tail_) ? 10 "
With the hypothetical parts, this would give a length of about 3 ft. 6
in. from the tip of the snout to the tip of the tail. Then
Humerus 8 in. long.
(_Fore-arm_) ?1 ft. 0 "
Carpus 2 "
Metacarpus 10 "
Which, if the fore-limbs were kept together as in ordinary quadrupeds,
would give a height to the body of about 2 ft. 6 in., but as the limbs
probably spread in walking as among the bats, the hind-limb would give
a better idea of the height of the animal.
(Flesh, sacrum, os innominatum). 2 in.
Femur 4 "
(_Tibia_) 6 "
(Metatarsus, &c.) 1 "
Which would give a height of about 13 inches; and, standing in the
position of a bird, the height to the crown of the head would be about
2 feet. The majority of the Ornithosaurians of the Cambridge Greensand
are of this size.
The spread of the wings, if there were 4 phalanges, would be
Body 10 in. wide.
Two arms 5 ft. 2 "
Two wing fingers 7 0 "
Giving a total expanse of about 13 feet. But, from the indications
of the wing-finger, I should incline to think an expanse of 10 feet
a truer estimate. The largest species attained to twice this size,
and the smallest was a fourth as large. Another memoir will present
descriptions and restorations of the Greensand species.
Habits.
The varying organization of different Ornithosaurians probably depends
on the different habits of the tribes. That they could all fly is
probable from the enormous radial crest to the humerus and the great
development of the wing-bones, to which a wing-membrane was stretched,
comparable to that of a Bat in texture, but more comparable to a
Bird in its extent. The groups with long hind-legs probably had the
membrane limited to the bones of the arm, while in the species with
small hind-legs it may have attained even as great a development as in
Bats, though there is no reason for suspecting that it extended to
the tail. A Pterodactyle cannot be supposed to have hung itself up by
the hind-legs as does a Bat, because the hind-claws appear invariably
to be directed forward. A Bat walks upon four legs with considerable
elegance and speed; the wing is folded in, close to the side, so as to
be scarcely noticed; and the outer claw is free to climb with. There
can be little doubt but that Pterodactyles walked in a similar way. The
thickened mammilate knob at the proximal end of the first phalange is
well calculated for contact with the ground. And if it were supposed
that the large wing-metacarpal bone were only used to support the wing,
and the small metacarpals only used to support the claws by which
the creature has sometimes been pictured suspending itself, it would
be difficult to believe that the forces of pressure and tension in
flying so exactly corresponded to the forces manifested in suspension
as to cause the large and the small metacarpals invariably to attain
the same length. A correspondence of this kind may be presumed to
indicate a correspondence in function; and since the animal did not
fly by means of its claws, the inference is that it walked by means of
the metacarpal bones. In no other way could the bones have been used
equally. The avian ilium would suggest a probability that they also at
times stood erect like birds, from which position they could with more
ease expand their wings; nor is such an idea opposed by the resemblance
of some bones of the hind-limb to what obtains in birds, and of the
neck of the femur to what is seen in mammals of great power in the
hind-legs.
That they lived exclusively upon land and in air is improbable,
considering the circumstances under which their remains are found.
It is likely that they haunted the sea-shores, and, while sometimes
rowing themselves over the water with their powerful wings, used the
wing-membrane as does the Bat to enclose their prey and bring it to the
mouth. But the superior development of the pneumatic foramina suggest
that their activity was greater than in ordinary sea-birds.
The large Cambridge Pterodactyles probably pursued a more substantial
prey than dragon-flies. Their teeth are well suited for fish, but
probably fowl and small mammal, and even fruits, made a variety in
their food. As the lord of the cliff, it may be presumed to have taken
toll of all animals that could be conquered with tooth and nail. From
its brain it might be regarded as an intelligent animal. The jaws
present indications of having been sheathed with a horny covering, and
some of the species show a rugose anterior termination of the snout
suggestive of fleshy lips like those of the Bat, and which may have
been similarly used to stretch and clean the wing-membrane.
The high temperature, coupled with the sub-aerial life, are opposed to
the idea of the animal having been naked. The undisturbed condition of
the skeleton and some points of structure are opposed to the idea of
their having had large feathers. The absence of such remains does not
favour the hypothesis of their having been covered with scales, though
in the legs of birds a scaly covering is met with. I should anticipate
for them a filamentous downy feather, or hair, like a Bat's. The Bat
combs its hair with its claws, and the Ornithosaurians may have used
their claws in a similar way.
They cannot be supposed to have been gregarious, from the large number
of species relatively to specimens. The reproduction may have been
much the same as in birds; and the young were probably reared with
affectionate care[X].
[Footnote X: Mr Carruthers has shown me crushed Turtle-like eggs from
the Stonesfield slate, which in the external pitting of the egg-shell
are not so different from some birds as to preclude a suspicion that
they might possibly be Ornithosaurian.]
* * * * *
_The following notes indicate structures in perfect specimens from the
Lithographic slate which supplement the fragmentary remains from the
Cambridge Greensand_[Y].
[Footnote Y: The German animals form different family groups. And
it cannot be inferred that the structures seen in them pertained to
Cambridge specimens.]