CLASSIFICATION OF THE ORNITHOSAURIA
When an attempt is made to determine the place in nature of an extinct
group of animals and the relation to each other of the different types
included within its limits, so as to express those facts in a
classification, attention is directed in the first place to characters
which are constant, and persist through the whole of its constituent
genera. We endeavour to find the structural parts of the skeleton which
are not affected by variation in the dentition, or the proportions of
the extremities, or length of the tail, which may define families or
genera, or species.
It has already been shown that while in many ways the Ornithosaurian
animals are like Birds, they have also important resemblances to
Reptiles. They are often named Pterosauria. The wing finger gives a
distinctive character which is found in neither one class of existing
animals nor the other, and is common to all the Pterodactyles at present
known. They have been named Ornithosauria as a distinct minor division
of back-boned animals, which may be regarded as neither Reptiles nor
Birds in the sense in which those terms are used to define a Lizard or
Ostrich among animals which still exist. It is not so much that they
mark a transition from Reptile to Bird, as that they are a group which
is parallel to Birds, and more manifestly holds an intermediate place
than Birds do between Reptiles and Mammals. In plan of structure Bird
and Reptile have more in common than was at one time suspected. The late
Professor Huxley went so far as to generalise on those coincidences in
parts of the skeleton, and united Birds and Reptiles into one group,
which he named Sauropsida, to express the coincidences of structure
between the Lizard and the Bird tribes. The idea is of more value than
the term in which it is expressed, because Reptiles are not, as we have
seen, a group of animals which can be defined by any set of characters
as comprehensive as those which express the distinctive features of
Birds. From the anatomist's point of view Birds are a smaller group, and
while some Reptiles have affinity with them, it is rather the extinct
than the living groups which indicate that relation. Other Reptiles have
affinities of a more marked kind with Mammals, and there are points in
the Ornithosaurian skeleton which are distinctly Mammalian. So that when
the Monotreme Mammals are united with South African reptiles known as
Theriodontia, which resemble them, in a group termed Theropsida to
express their mammalian resemblances, it is evident that there is no one
continuous chain of life or gradation in complexity of structure of
animals.
We have to determine whether the Ornithosauria incline towards the
Sauropsidan or Bird-Reptile alliance, or to the Mammal-Reptile or
Theropsidan alliance. There can be no doubt that the predominant
tendency is to the former, with a minor affinity towards the latter.
The Ornithosauria are one of a series of groups of animals, living and
extinct, which have been combined in an alliance named the
Ornithomorpha. That group includes at least five great divisions of
animals, which circle about birds, known as Ornithosauria, Crocodilia,
Saurischia, Aves, Ornithischia, and Aristosuchia. Their relations to
each other are not evident in an enumeration, but may be shown in some
degree in a diagram (see p. 190).
THE ORNITHOMORPHA
The Ornithomorpha arranged in this way show that the three middle
groups--carnivorous Saurischia, Aristosuchia, herbivorous
Ornithischia--which are usually united as Dinosauria, intervene between
Birds and Ornithosaurs; and that the Crocodilia and Ornithosauria are
parallel groups which are connected with Birds, by the group of
Dinosaurs, which resembles Birds most closely.
The Ornithomorpha is only one of a series of large natural groups of
animals into which living and extinct terrestrial vertebrata may be
arranged. And the succeeding diagram may contribute to make evident the
relations of Ornithosauria to the other terrestrial vertebrata (see p.
191).
Herein it is seen that while the Ornithomorpha approach towards Mammalia
through the Ornithosauria, and less distinctly through the Crocodilia,
they approach more directly to the Sauromorpha, through the Plesiosaurs
and Hatteria; while that group also approaches more directly to the
Mammals through the Plesiosaurs and Anomodonts.
[Illustration: DIAGRAM OF THE AFFINITIES OF THE ORDERS OF ANIMALS
COMPRISED IN THE ORNITHOMORPHA.
After a diagram in the _Philosophical Transactions of the Royal
Society_, 1892.]
The Aristosuchia is imperfectly known, and therefore to some extent a
provisional group. It is a small group of animals.
[Illustration: DIAGRAM SHOWING THE RELATIONS OF THE ORNITHOMORPHA
TO THE CHIEF LARGE GROUPS OF TERRESTRIAL VERTEBRATA, AND THEIR
AFFINITIES WITH EACH OTHER.
After a diagram in the _Philosophical Transactions of the Royal
Society_, 1892.]
Cordylomorpha are Ichthyosaurs and the Labyrinthodont group.
Herpetomorpha include Lacertilia, Homoeosauria, Dolichosauria,
Chameleonoidea, Ophidia, Pythonomorpha.
The Sauromorpha comprises the groups of extinct and living Reptiles
named Chelonia, Rhynchocephala, Sauropterygia, Anomodontia, Nothosauria,
and Protorosauria. These details may help to explain the place which has
been given to the Ornithosauria in the classification of animals.
[Illustration: FIG. 75. COMPARISON OF SIX GENERA
The skulls are seen on the left side in the order of the names below
them]
Turning to the Pterodactyles themselves, Von Meyer divided them
naturally into short-tailed and long-tailed. The short-tailed indicated
by the name Pterodactylus he further divided into long-nosed and
short-nosed. The short-nosed genus has since been named Ptenodracon
(Fig. 59, p. 167). The long-tailed group was divided into two types--the
Rhamphorhynchus of the Solenhofen Slate (Fig. 56, p. 161) and the
English form now known as Dimorphodon (Fig. 52, p. 150), which had been
described from the Lias.
The Cretaceous Pterodactyles form a distinct family. So that, believing
the tail to have been short in that group (Fig. 58), there are two
long-tailed as well as two short-tailed families, which were defined
from their typical genera Pterodactylus, Ornithocheirus,
Rhamphorhynchus, and Dimorphodon.
The differences in structure which these animals present are, first: the
big-headed forms from the Lias like Dimorphodon, agree with the
Rhamphorhynchus type from Solenhofen in having a vacuity in the skull
defined by bone, placed between the orbit of the eye and the nostril.
With those characters are correlated the comparatively short bones which
correspond to the back of the hand termed metacarpals, and the tail is
long, and stiffened down its length with ossified tendons. These
characters separate Ornithosaurs with long tails from those with short
tails.
The short-tailed types represented by Pterodactylus and Ornithocheirus
have no distinct antorbital vacuity in the skull defined by bone. The
metacarpal bones of the middle hand are exceptionally elongated, and the
tail, which was flexible in both, appears to have been short. These
differences in the skeleton warrant a primary division of flying
reptiles into two principal groups.
The short-tailed group, which was recognised by De Blainville as
intermediate between Birds and Reptiles, may take the name
Pterodactylia, which he suggested as a convenient, distinctive name. It
may probably be inconvenient to enlarge its significance to comprise not
only the true Pterodactyles originally defined as Pterosauria, but the
newer Ornithostoma and Ornithocheirus which have been grouped as
Ornithocheiroidea.
The second order, in which the wing membrane appears to have had a much
greater extent, in being carried down the hind limbs, where the
outermost digit and metatarsal are modified for its support, has been
named Pterodermata, to include the types which are arranged around
Rhamphorhynchus and Dimorphodon.
Both these principal groups admit of subdivision by many characters in
the skeleton, the most remarkable of which is afforded by the pair of
bones carried in front of the pubes, and termed prepubic bones. In the
Pterodactyle family the bones in front of the pubes are always separate
from each other, always directed forward, and have a peculiar fan-shaped
form with concave sides like the bone which holds a similar position in
a Crocodile. In the Ornithocheirus family the prepubic bones appear to
have been originally triangular, but were afterwards united so as to
form a strong continuous bar which extends transversely across the
abdomen in advance of the pubic bones. This at least is the distinctive
character in the genus Ornithostoma according to Professor Williston,
which in many ways closely resembles Ornithocheirus.
The two families in the long-tailed order named Pterodermata are
separated from each other by a similar difference in their prepubic
bones. In Dimorphodon those bones are separate from each other, and
remain distinct through life, meeting in the middle line of the body in
a wide plate. On the other hand, in Rhamphorhynchus the prepubic bones,
which are at first triangular and always slender, become blended
together into a slight transverse bar, which only differs from that
attributed to Ornithostoma in its more slender bow-shaped form.
[Illustration: FIG. 76. LEFT SIDE OF PELVIS OF ORNITHOSTOMA
(After Williston)]
Thus if other characters of the skeleton are ignored and a
classification based upon the structure of the pelvis and prepubic
bones, there would be some ground for associating the long-tailed
Rhamphorhynchus from the Upper Oolites which is losing the teeth in the
front of its jaw with the Cretaceous Ornithostoma, which has the teeth
completely wanting; while the long-tailed Dimorphodon would come into
closer association with the short-tailed Pterodactylus. The drum-stick
bone or tibia in Dimorphodon, with its slender fibula, like that of a
Bird, also resembles a Bird in the rounded and pulley-shaped terminal
end which makes the joint corresponding to the middle of the ankle bones
in man. The same condition of a terminal pulley joint is found in the
Cretaceous Pterodactyles. But in the true Pterodactyles and in
Rhamphorhynchus there usually is no pulley-shaped termination to the
lower end of the drum-stick, for the tarsal bones remain separate from
each other, and form two rows of ossifications, showing the same
differences as separate Dinosaurs into the divisions which have been
referred to, from their Bird-like pelvis and tibio-tarsus, as
Ornithischia in the one case, and Saurischia in the other from their
bones being more like those of living Lizards.