CLASSIFICATION OF REPTILES
There is very much doubt, very much uncertainty, among paleontologists
about the classification of reptiles. No two writers agree on the
number of orders, or the rank of many forms. Some recognize twenty or
more orders, others but eight or nine. And this doubt and uncertainty
are due chiefly to the many discoveries of early forms that have been
made during the past twenty years. The many strange and unclassifiable
types which have come to light in North America, South Africa, and
Europe have thrown doubt on all previous classificatory schemes,
have weakened our faith in all attempts to trace out the genealogies
of the reptilian orders; and classification is merely genealogy.
It is only the paleontologist who is competent to express opinions
concerning the larger principles of classification of organisms,
and especially of the classification of reptiles. The neozoölogist,
ignorant of extinct forms, can only hazard guesses and conjectures
as to the relationships of the larger groups, for he has only the
specialized or decadent remnants of past faunas upon which to base
his opinions. About some things we can be quite confident; about some
groups opinions have crystallized, and we all agree, except perhaps on
trifles. The dinosaurs, the pterodactyls, the crocodiles, for instance,
offer only minor problems to perplex the systematist, but the origin
and the relations, not only of these, but also of nearly all the
others, are still involved in obscurity. The question, whence came the
ichthyosaurs, the plesiosaurs, the turtles, etc., seems almost as far
from solution as it did fifty years ago. With every problem solved a
dozen more intrude themselves upon us. Hence, classification simply
represents the present condition of our knowledge, our present opinions
as to genealogies. It was the fashion a dozen years ago to draw all
sorts of genealogical trees on the slightest pretext, to trace in
beautifully clear lines the precise descent of all kinds of animals;
and very few have been worth the paper on which they were printed.
When facts are numerous enough, conclusions are patent even to the
novice; when facts are few and obscure, one can guess about as well as
another. In general, it may be said that the older a group of animals
is the more abstruse are the problems presented; first, because of the
lack of abundant material; second, because the forms speak to us in an
unfamiliar language that we cannot easily interpret. The classification
of the mammals approaches more nearly the ultimate truth than does that
of any other group of organisms, because we know more about the extinct
forms than we do of any other class, and also because we know more
about the living forms than we do about any other living animals.
Species of reptiles are, for the most part, vague quantities in
paleontology; they can be determined with assurance only by the
comparison of abundant material. Adult characters in mammals are
apparent in the ossification of the skeleton, and size can be used
within moderate limits in the determination of species; but size in
reptiles means but little; no one could possibly say that the skeleton
of an alligator six feet in length is not that of an adult animal if
he knew nothing else about the Crocodilia. So also the compression and
malformations of bones from the processes of fossilization obliterate
specific characters in great part. Nor are specific characters easily
distinguishable in the skeletons of living reptiles. The genus,
therefore, among fossil reptiles is practically the unit, and we may
be sure that for every well-defined genus we discover there existed
numerous minor variations, which, had we the living animals to study,
we should call species. We classify the living Crocodilia into two
families, about four well-defined genera—perhaps even five or six—and
about twenty-five species. Of the living lizards there are about
eighteen hundred species, twenty families, and four larger groups
or suborders. In all probability the lizards have never been more
abundant and more varied than they are at the present time. Possibly
these proportions of species, genera, families, and suborders may
represent approximately the proportions that have existed at some time
or other in most of the other groups which we call orders—approximately
only, for we can never be quite sure that we evaluate the structural
characters of different groups of organisms quite equally. The absence
of a molar tooth in a mammal would ordinarily indicate a genus, the
absence of a tooth in a reptile might not indicate even a variety or
a race. Whence it follows that classification of organisms is not
and never will be an exact science. The value of characters used in
classification is very unequal, as we have seen. No two persons see
these characters from the same viewpoints, and in consequence no
two persons whose opinions are worth while ever wholly agree as to
classification.
The following scheme differs only in minor details from the more
conservative of the generally accepted views, and those differences
are, for the most part, the writer’s own opinions, to be taken for
what they are worth. It may be said decisively that no classification
of the reptiles into major groups, into super-families or subclasses
that has so far been proposed is worthy of acceptance; there is no
such subclass as the Diapsida or Synapsida, for instance. And we have
very much more to learn about the early reptiles before any general
classification of the reptiles can be securely founded. It is very
probable that the primary radiation of the reptiles into the various
lines of descent, into its main branches, occurred much earlier than
we have been disposed to believe; that before the close of Paleozoic
time, perhaps before the close of the Carboniferous, all the great
groups of reptiles had gone off from the main stem, and that since then
only smaller and smaller branches have appeared. There have been no new
orders of reptiles in all probability since Triassic times, and perhaps
none since Permian.
Taxonomists are often disposed to cut the Gordian knots of
relationships by raising the ranks of the animals they study to
independent positions. More than thirty independent orders of reptiles
have been proposed by different students, and quite as many of mammals
and of birds; possibly after more forms have been discovered there will
be as many proposed for the amphibians. Sometimes, indeed, it is better
to make such independent groups than to unite lesser ones on doubtful
evidence. But the writer, for one, believes that it is more worthy of
the thoughtful scientific student to seek for relationships than for
differences. It is far easier to destroy than to construct, to make
new genera, families, and orders than to unite those already proposed.
To raise every proposed suborder of reptiles to an order, as has been
proposed by various writers, and the orders to subclasses, only leaves
classification where it was; nothing has been added to taxonomy save a
lot of new names to perplex and annoy the student.
In the following scheme of classification three groups provisionally
called orders are prefixed by an asterisk.
CLASS REPTILIA
Order =COTYLOSAURIA=
Primitive reptiles with notochordal vertebrae, imperforate
temporal region, persistent intercentra; two coracoids;
plate-like pelvis, with all or most of the amphibian skull
elements; short legs and short neck; phalangeal formula
primarily 2, 3, 4, 5, 3(4).
Suborder =Diadectosauria= Permocarboniferous, North America.
=Pantylosauria= Permocarboniferous, North America.
=Labidosauria= Lower Permian, North America.
=Pareiasauria= Upper Permian, Europe, Africa.
=Procolophonia= Triassic, Europe, Africa.
Order =CHELONIA=
Temporal region imperforate. Head and limbs more or less
retractile within a box formed chiefly by the exoskeleton.
Suborder =Pleurodira= Triassic to recent.
=Cryptodira= Jurassic to recent.
=Trionychoidea= Cretaceous to recent.
Order =THEROMORPHA=
Primitive reptiles with notochordal vertebrae, perforate
temporal region, persistent intercentra; two coracoids;
plate-like pelvis with median vacuity; no free
dermosupraoccipitals in skull; longer legs and neck;
phalangeal formula 2, 3, 4, 5, 3(4).
Suborder =Pelycosauria= (_sens. lat._) Permocarboniferous,
North America, Europe.
=Dromasauria= Upper Permian, Africa.
=Dinocephalia= Middle and Upper
Permian, Africa.
Order =THERAPSIDA=
Reptiles with a single temporal perforation on each side;
vertebrae not notochordal; intercentra not persistent;
pelvis with vacuity; skull bones reduced; teeth heterodont;
phalangeal formula, 2, 3, 3, 3, 3.
Suborder =Anomodontia= Permo-Trias, Africa, North America.
=Therocephalia= Upper Permian, Africa.
=Theriodontia= Trias, Africa.
Order =SAUROPTERYGIA=
Aquatic reptiles with a single temporal vacuity; no
supratemporal bone, or quadratojugal; ribs single-headed,
diapophysial; coracoids large, meeting in middle line,
single; neck long, tail short.
Suborder =Nothosauria= Triassic, Europe.
=Plesiosauria= Triassic to close of Cretaceous,
cosmopolitan.
⃰Order =PROGANOSAURIA=
Primitive aquatic reptiles; single (? upper) temporal
perforation; neck elongate; nares posterior; vertebrae
notochordal; intercentra persistent; pelvis plate-like;
phalangeal formula 2, 3, 4, 5, 4(6). Permocarboniferous,
Africa, South America.
Order =ICHTHYOSAURIA=
Reptiles with all aquatic adaptations; a single, upper
temporal perforation; both supratemporal and squamosal
present; a single coracoid. Middle Triassic to Benton
Cretaceous, cosmopolitan.
⃰Order =PROTOROSAURIA=
A single, upper temporal vacuity, quadrate fixed (neck
vertebrae elongate); bones hollow; cervical ribs
single-headed, articulating with centrum; pelvis plate-like.
Permian, North America, Europe.
Order =SQUAMATA=
A single, upper temporal vacuity, or, secondarily none;
quadrate loosely articulated with cranium; teeth on palate;
intercentra more or less persistent; a single coracoid; ribs
single-headed, central.
Suborder =Lacertilia= Trias to recent.
=Mosasauria= Upper Cretaceous, cosmopolitan.
=Ophidia= Upper Cretaceous to recent.
⃰Order =THALATTOSAURIA=
Aquatic reptiles; two (?) temporal vacuities; ribs
single-headed, attached to centrum; single coracoid; no
intercentra. Trias, California.
Order =RHYNCHOCEPHALIA=
Two temporal vacuities on each side; palate with teeth;
intercentra persistent; a single coracoid; teeth acrodont;
ribs articulating with centrum and arch.
Suborder =Rhynchosauria= Triassic, Europe.
=Sphenodontia= Triassic to recent.
=Choristodera= Uppermost Cretaceous, lowermost
Eocene, North America, Europe.
Order =PARASUCHIA=
Subaquatic reptiles, with two temporal vacuities; an
antorbital vacuity; no false palate; pubis entering
acetabulum; ribs double-headed, diapophysial.
Suborder =Phytosauria= Upper Trias, cosmopolitan.
=Pelycosimia= Trias, Africa.
=Pseudosuchia= Trias, Europe, North America.
Order =CROCODILIA=
Two temporal vacuities; teeth thecodont; a false palate;
pubis excluded from acetabulum; single coracoid; ribs
double-headed, diapophysial; subaquatic or aquatic.
Suborder =Eusuchia= Jurassic to recent.
=Thalattosuchia= Upper Jurassic, Europe.
Order =DINOSAURIA=
Ambulatory reptiles, with two temporal vacuities; no false
palate; pubis entering acetabulum; ribs double-headed,
diapophysial.
Suborder =Theropoda= Upper Trias to close of Cretaceous,
cosmopolitan.
=Orthopoda= Close of Trias to close of Cretaceous,
cosmopolitan.
=Sauropoda= Upper Jurassic, Lower Cretaceous,
cosmopolitan.
Order =PTEROSAURIA=
Volant reptiles; fourth finger greatly elongated to support
patagium; neck vertebrae elongated; bones hollow; ribs
double-headed, diapophysial; a single coracoid; no clavicles
or interclavicle; two temporal vacuities.
Suborder =Pterodermata= Jurassic, Europe.
=Pterodactyloidea= Upper Jurassic to Upper Cretaceous,
Europe, North America.