THE PLAN OF THE SKELETON
While these animals are incontestably nearer to birds than to any other
animals in their plan of organisation, thus far no proof has been found
that they are birds, or can be included in the same division of
vertebrate life with feathered animals. It is one of the oldest and
soundest teachings of Linnaeus that a bird is known by its feathers; and
the record is a blank as to any covering to the skin in Pterodactyles.
There is the strongest probability against feathers having existed such
as are known in the Archaeopteryx, because every Solenhofen Ornithosaur
appears to have the body devoid of visible or preservable covering,
while the two birds known from the Solenhofen Slate deposit are well
clothed with feathers in perfect preservation. We turn from the skin to
the skeleton.
The plan on which the skeleton is constructed remains as evidence of the
animal's place in nature, which is capable of affording demonstration on
which absolute reliance would have been placed, if the brain and
pneumatic foramina had remained undiscovered. With the entire skeleton
before us, it is inconceivable that anatomical science should fail to
discover the true nature of the animal to which it belonged, by the
method of comparing one animal with another. There is no lack of this
kind of evidence of Pterodactyles in the three or four scores of
skeletons, and thousands of isolated or associated bones, preserved in
the public museums of Europe and America.
I may recall the circumstance that the discovery of skeletons of fossil
animals has occasionally followed upon the interpretation of a single
fragment, from which the animal has been well defined, and sometimes
accurately drawn, before it was ever seen. So I propose, before drawing
any conclusions from the skeletons in the entirety of their
construction, to examine them bone by bone, and region by region, for
evidence that will manifest the nature of this brood of Dragons. Their
living kindred, and perhaps their extinct allies, assembled as a jury,
may be able to determine whether resemblances exist between them, and
whether such similarity between the bones as exists is a common
inheritance, or is a common acquisition due to similar ways of life, and
no evidence of the grade of the organism among vertebrate animals.
The bones of these Ornithosaurs, when found isolated, first have to be
separated from the organisms with which they are associated and mixed in
the geological strata. This discrimination is accomplished in the first
instance by means of the texture of the surface. The density and polish
of the bones is even more marked than in the bones of birds, and is
usually associated with a peculiar thinness of substance of the bone,
which is comparable to the condition in a bird, though usually a little
stouter, so that the bones resist crushing better. Pterodactyle bones
in many instances are recognised by their straightness and comparatively
uniform dimensions, due to the exceptional number of long bones which
enter into the structure of the wing as compared with birds. When the
bones are unerringly determined as Ornithosaurian, they are placed side
by side with all the bones which are most like them, till, judged by the
standard of the structures of living animals, the fossil is found to
show a composite construction as though it were not one animal but many,
while its individual bones often show equally composite characters, as
though parts of the corresponding bone in several animals had been
cunningly fitted together and moulded into shape.
THE PLAN OF THE HEAD IN ORNITHOSAURS
The head is always the most instructive part of an animal. It is less
than an inch long in the small Solenhofen skeleton named _Pterodactylus
brevirostris_, and is said to be three feet nine inches long in the
toothless Pterodactyle Ornithostoma from the Chalk of Kansas. Most of
these animals have a long, slender, conical form of head, tapering to
the point like the beak of a Heron, forming a long triangle when seen
from above or from the side. Sometimes the head is depressed in front,
with the beak flattened or rounded as in a Duck or Goose, and
occasionally in some Wealden and Greensand species the jaws are
truncated in front in a massive snout quite unlike any bird. The back of
the head is sometimes rounded as among birds, showing a smooth
pear-shaped posterior convexity in the region of the brain. Sometimes
the back of the head is square and vertical or oblique. Occasionally a
great crest of cellular tissue is extended backward from above the
brain case over the spines of the neck bones.
There are always from two to four lateral openings in the skull. First,
the nostril is nearest to the extremity of the beak. Secondly, the
orbits of the eyes are placed far backward. These two openings are
always present. The nostril may incline upward. The orbits of the eyes
are usually lateral, though their upper borders sometimes closely
approximate, as in the woodpecker-like types from the Solenhofen Slate
named _Pterodactylus Kochi_, now separated as another genus. In most
genera there is an opening in the side of the head, between the eye hole
and the nostril, known as the antorbital vacuity; and another opening,
which is variable in size and known as the temporal vacuity, is placed
behind the eye. The former is common in the skulls of birds, the latter
is absent from all birds and found in many reptiles.
The palate is usually imperfectly seen, but English and American
specimens have shown that it has much in common with the palate in
birds, though it varies greatly in form of the bones in representatives
from the Lias, Oolites, and Cretaceous rocks.
From the scientific aspect the relative size of the head, its form, and
the positions and dimensions of its apertures and processes, are of
little importance in comparison with its plan of construction, as
evidenced by the positions and relations to each other of the bones of
which it is formed. There usually is some difficulty in stating the
limits of the bones of the skull, because in Pterodactyles, as among
birds, they usually blend together, so that in the adult animal the
sutures between the bones are commonly obliterated.
Bones have relations to each other and places in the head which can only
change as the organs with which they are associated change their
positions. No matter what the position of a nostril may be--at the
extremity of a long snout, as in an ant-eater, or far back at the top of
the head in a porpoise, or at the side of the head in a bird--it is
always bordered by substantially the same bones, which vary in length
and size with the changing place of the nostril and the form of the
head. Every region of the head is defined by this method of
construction; so that eye holes and nose holes, brain case and jaw
bones, palate and teeth, beak, and back of the skull are all instructive
to those who seek out the life-history of these animals. We may briefly
examine the head of an Ornithosaurian.
BONES ABOUT THE NOSTRIL
No matter what its form may be, the head of an Ornithosaur always
terminates in front in a single bone called the intermaxillary. It sends
a bar of bone backward above the visible nostrils, between them; and a
bar on each side forms the margin of the jaw in which teeth are
implanted. The bone varies in depth, length, sharpness, bluntness,
slenderness, and massiveness. As the bone becomes long the jaw is
compressed from side to side, and the openings of the nostrils are
removed backward to an increasing distance from the extremity of the
beak.
The outer and hinder border of the nostril is made by another bone named
the maxillary bone, which is usually much shorter than the premaxillary.
It contains the hindermost teeth, which rarely differ from those in
front, except in sometimes being smaller.
The nasal bones, which always make the upper and hinder border of the
nostrils, meet each other above them, in the middle line of the beak.
[Illustration: FIG. 20
Showing that the extremity of the jaws in Rhamphorhynchus was
sheathed in horn as in the giant Kingfisher, since the jaws
similarly gape in front.
The hyoid bones are below the lower jaw in the Pterodactyle.]
The nostrils are unusually large in the Lias genus named Dimorphodon,
and small in species of the genus Rhamphorhynchus from Solenhofen. Such
differences result from the relative dimensions and proportions of these
three bones which margin the nasal vacuity, and by varying growth of
their front margins or of their hinder margins govern the form of the
snout.
The jaws are most massive in the genera known from the Wealden beds to
the Chalk. The palatal surface is commonly flat or convex, and often
marked by an elevated median ridge which corresponds to a groove in the
lower jaw, though the median ridge sometimes divides the palate into two
parallel concave channels. The jaw is margined with teeth which are
rarely fewer than ten or more than twenty on each side. They are sharp,
compressed from side to side, curved inward, and never have a saw-like
edge on the back and front margins. No teeth occur upon the bones of the
palate.
In most birds there is a large vacuity in the side of the head between
the nostril and the orbit of the eye, partly separated from it by the
bone which carries the duct for tears named the lachrymal bone. The same
preorbital vacuity is present in all long-tailed Pterodactyles, though
it is either less completely defined or absent in the group with short
tails. It affords excellent distinctive characters for defining the
genera. In the long-tailed genus Scaphognathus from Solenhofen this
preorbital opening is much larger than the nostril, while in Dimorphodon
these vacuities are of about equal size. Rhamphorhynchus is
distinguished by the small size of the antorbital vacuity, which is
placed lower than the nostril on the side of the face. The aperture is
always imperfectly defined in Pterodactylus, and is a relatively small
vacuity compared with the long nostril. In Ptenodracon the antorbital
vacuity appears to have no existence separate from the nostril which
adjoins the eye hole. And so far as is known at present there is no
lateral opening in advance of the eye in the skull in any Ornithosaur
from Cretaceous rocks, though the toothless Ornithostoma is the only
genus with the skull complete. When a separate antorbital vacuity
exists, it is bordered by the maxillary bone in front, and by the malar
bone behind. The prefrontal bone is at its upper angle. That bone is
known in a separate state in reptiles and, I think, in monotreme
mammals. Its identity is soon lost in the mammal, and its function in
the skull is different from the corresponding bone in Pterodactyles.
BONES ABOUT THE EYES
[Illustration: FIG. 21. UPPER SURFACE OF SKULL OF THE HERON
Compared with the same aspect of the skull of Rhamphorhynchus]
The third opening in the side of the head, counting from before
backward, is the orbit of the eye. In this vacuity is often seen the
sclerotic circle of overlapping bones formed in the external membrane of
the eye, like those in nocturnal birds and some reptiles. The eye hole
varies in form from an inverted pear-shape to an oblique or transverse
oval, or a nearly circular outline. It is margined by the frontal bone
above; the tear bone or lachrymal, and the malar or cheek bone in front;
while the bones behind appear to be the quadrato-jugal and post-frontal
bones, though the bones about the eye are somewhat differently arranged
in different genera.
The eyes were frequently, if not always, in contact with the anterior
walls of the brain case, as in many birds, and are always far back in
the side of the head. In Dimorphodon they are in front of the
articulation of the lower jaw; in Rhamphorhynchus, above that
articulation; while in Ornithostoma they are behind the articulation for
the jaw. This change is governed by the position of the quadrate bone,
which is vertical in the Lias genus, inclined obliquely forward in the
fossils from the Oolites, and so much inclined in the Chalk fossil that
the small orbit is thrown relatively further back.
Thus far the chief difference in the Pterodactyle skull from that of a
bird is in the way in which the malar arch is prolonged backward on each
side. It is a slender bar of bone in birds, without contributing
ascending processes to border vacuities in the side of the face, while
in these fossil animals the lateral openings are partly separated by the
ascending processes of these bones. This divergence from birds, in the
malar bone entering the orbit of the eye is approximated to among
reptiles and mammals, though the conditions, and perhaps the presence of
a bone like the post-orbital bone, are paralleled only among Reptiles.
The Pterodactyles differ among themselves enough for the head to make a
near approach to Reptiles in Dimorphodon, and to Birds in
Pterodactylus. In the Ground Hornbill and the Shoebill the lachrymal
bones in front of the orbits of the eyes grow down to meet the malar
bars without uniting with them. The post-frontal region also is
prolonged downward almost as far as the malar bar, as though to show
that a bird might have its orbital circle formed in the same way and by
the same bones as in Pterodactylus. Cretaceous Ornithosaurs sometimes
differ from birds apparently in admitting the quadrato-jugal bone into
the orbit. It then becomes an expanded plate, instead of a slender bar
as in all birds.
THE TEMPORAL FOSSA
A fourth vacuity is known as the temporal fossa. When the skull of such
a mammal as a Rabbit, or Sheep, is seen from above, there is a vacuity
behind the orbits for the eyes, which in life is occupied by the muscles
which work the lower jaw. It is made by the malar bone extending from
the back of the orbit and the process of bone, called the zygomatic
process, extending forward from the articulation of the jaw, which
arches out to meet the malar bone.
In birds there is no conspicuous temporal fossa, because the malar bar
is a slender rod of bone in a line with the lower end of the quadrate
bone.
Reptile skulls have sometimes one temporal vacuity on each side, as
among tortoises, formed by a single lateral bar. These vacuities, which
correspond to those of mammals in position, are seen from the top of the
head, as lateral vacuities behind the orbits of the eyes, and are termed
superior temporal vacuities. In addition to these there is often in
other reptiles a lateral opening behind the eye, termed the inferior
temporal vacuity, seen in Crocodiles, in Hatteria, and in Lizards; and
in such skulls there are two temporal bars seen in side view,
distinguished as superior and inferior. The superior arch always
includes the squamosal bone, which is at the back of the single bar in
mammals. The lower arch includes the malar bone, which is in front in
the single arch of mammals. The circumstance that both these arches are
connected with the quadrate bone makes the double temporal arch
eminently reptilian.
In Ornithosaurs the lateral temporal vacuity varies from a typically
reptilian condition to one which, without becoming avian, approaches the
bird type. In skulls from the Lias, Dimorphodon and Campylognathus,
there is a close parallel to the living New Zealand reptile Hatteria, in
the vertical position of the quadrate bone and in the large size of the
vacuity behind and below the eye, which extends nearly the height of the
skull. In the species of the genus Pterodactylus, the forward
inclination of the quadrate bone recalls the Curlew, Snipe, and other
birds. The back of the head is rounded, and the squamosal bone, which
appears to enter into the wall of the brain case as in birds and
mammals, is produced more outward than in birds, but less than in
mammals, so as to contribute a little to the arch which is in the
position of the post-frontal bone of reptiles. It is triangular, and
stretches from the outer angle of the frontal bone at the back of the
orbit to the squamosal behind, where it also meets the quadrate bone.
Its third lower branch meets the quadratojugal, which rests upon the
front of the quadrate bone, as in Iguanodon, and is unlike Dimorphodon
in its connexions. In that genus the supra-temporal bone, or
post-orbital bone, appears to rest upon the post-frontal and connect it
with the quadrato-jugal. In Dimorphodon the malar bone is entirely
removed from the quadrate, but in Pterodactylus it meets its articular
end. Between the post-frontal bone above and the quadrato-jugal bone
below is a small lunate opening, which represents the lateral temporal
vacuity; and so far, this is a reptilian character. But if the thin
post-frontal bone were absorbed, Pterodactylus would resemble birds.
There is no evidence that the quadrate bone is free in any Ornithosaurs,
as it is in all birds, while in Dimorphodon it unites by suture with the
squamosal bone. In Ornithostoma the lateral temporal vacuity is little
more than a slit between the quadrate bone below, the quadrato-jugal in
front, and what may be the post-frontal bone behind (see Fig. 2, p. 12).
BONES ABOUT THE BRAIN
The bones containing the brain appear to be the same as form the brain
case in birds. The form of the back of the skull varies in two ways.
First it may be flat above and flat at the back, when the back of the
head appears to be square. This condition is seen in all the long-tailed
genera, such as Campylognathus from the Lias and Rhamphorhynchus, and is
associated with a high position for the upper temporal bar. Secondly,
the back of the head may be rounded convexly, both above and behind.
That condition is seen in the short-tailed genera, such as
Pterodactylus. But in the large Cretaceous types, such as Ornithocheirus
and Ornithostoma, the superior longitudinal ridge which runs back in
the middle line of the face becomes elevated and compressed from side to
side at the back of the head as a narrow deep crest, prolonged backward
over the neck vertebrae for some inches of length. All these three types
are paralleled more or less in birds which have the back of the head
square like the Heron, or rounded like the Woodpecker; or crested,
though the crest of the Cormorant is not quite identical with
Ornithocheirus, being a distinct bone at the back of the head in the
bird which never blends with the skull. In so far as the crest is
reptilian it suggests the remarkable crest of the Chameleon. In the
structure of the back of the skull the bones are a modification of the
reptilian type of Hatteria in the Lias genus Campylognathus, but the
reptilian characters appear to be lost in the less perfectly preserved
skulls of Cretaceous genera.
The palate is well known in the chief groups of Ornithosaurs, such as
Campylognathus, Scaphognathus, and Cycnorhamphus.
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.
[Illustration: 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 space 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 crushing. 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 HORNY BEAK
Sometimes a horny 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 interspace had
originally been occupied by organic substance like a horny 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.
[Illustration: 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 astonishing as was the discovery of teeth in
extinct birds. These characters of the head are also found in fossil
animals named Dinosaurs, in association 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.