ANOMODONTIA
LYSTROSAURUS
Over a large area of South Africa, chiefly along the Orange River and
its tributaries, there is an extensive series of deposits many hundreds
of feet in thickness, usually called the Karoo beds, which, for more
than fifty years, have been widely famous among scientific men for the
many and remarkable vertebrate fossils which they have yielded. These
deposits seem to represent the whole of the vast interval of time from
the Carboniferous to the Jurassic, that is, the whole of the Permian
and Triassic, though not many fossils have been found in the lowermost
strata. Among the fossils of the lower strata are those of the strange
creatures described in the following pages as _Mesosaurus_. From the
deposits representing the Upper Permian and the Triassic the fossils
that have been obtained are both abundant and diverse. Unfortunately,
however, of the scores of forms that have been discovered few are known
completely, and still fewer are known sufficiently well to enable us to
picture the living animals.
From the Upper Permian Karoo rocks two orders of reptiles have been
recognized, the Cotylosauria, represented by more specialized forms
than those from the Lower Permian of North America; and the order or
group called by Broom the Therapsida. While the forms of this latter
group have certain definite structural relationships with each other,
they show so great a diversity among themselves that, when they shall
be better known, it will be found necessary perhaps to separate them
into several distinct orders.
At least five groups of the Therapsida are now recognized by Broom,
the Dromasauria, Dinocephalia, Anomodontia, Therocephalia, and
Theriodontia. Of all these the members of the last-mentioned group
have attracted the greatest interest among geologists and naturalists,
because of their intimate relationships to the mammals—so intimate,
indeed, that they seem almost to bridge over the interval between the
two classes. From higher Karoo beds primitive representatives of the
more crocodilian types have been discovered, forms which seem to be the
beginning of that order described on later pages as the Parasuchia.
It would lead us too far astray to mention even, let alone describe,
the many forms of reptiles that have been discovered in the Karoo beds;
nor indeed is it possible for anyone who has not attentively studied
their remains to get a very clear conception of many of them, so
incompletely have they been made known.
Doubtless from among all these diverse forms there have been not a few
which sought wider opportunities in the water, but, if so, we have
as yet very little knowledge of them. One form only, so far as the
writer is aware, has been credited with aquatic habits, a remarkable
reptile belonging to the group originally called by Sir Richard Owen,
the Anomodontia, a word meaning “lawless teeth,” and to the genus
_Lystrosaurus_, also described by the same noted paleontologist.
A restoration of the skeleton of _Lystrosaurus_ has recently been
published by Watson. This restoration the writer has reproduced in
the present pages, though he has taken the liberty of making some
minor changes, to accord better with what he believes must have been
the position of the shoulder-blades and the hind legs. And he would
also suggest that the tail in life did not turn down so much at its
extremity as depicted by Watson.
Both Broom and Watson believe that this animal was a powerful swimmer,
and thoroughly aquatic in habit. To the present writer, however, this
does not seem so evident. He is rather inclined to believe that the
creature was chiefly terrestrial in habit, living probably in marshy
regions, and perhaps seeking its food in shallow waters and in the mud.
Aside from the position of the nostrils, which it will be observed are
rather close to the eyes, a position so characteristic of many swimming
reptiles and mammals, there is but little indication of aquatic
adaptations elsewhere in the skeleton.
[Illustration: FIG. 50.—Skeleton of _Lystrosaurus_, as restored by
Watson, slightly modified.]
The skull is of most extraordinary form. The face is turned downward,
leaving the nostrils high up, in front of the eyes. The jaws were
doubtless covered with a horny shield, like that of the turtles, having
a cutting edge. There is a single pair of elongated canine teeth,
possibly a sexual character. The lower jaws are heavy and stout, and
Watson has said that the animal doubtless had the ability to open its
mouth very widely. The quadrate, the bone with which the lower jaws
articulate, is firmly fixed to the skull, and there is a single opening
on the side of the skull posteriorly, a character common to all the
Therapsida.
The vertebrae are stout, and they have stout spines. The tail is
remarkably short, stout, and stumpy; it could have been of no use
whatever in the water for propulsion or even for steering. The front
legs are short and stout; the forearm bones are short, suggesting
either swimming or digging habits, and the foot is short and broad.
The pelvis or hip bones are massive and were very firmly connected
with the backbone by the aid of six vertebrae, a very unusual number
in reptiles. The hind legs, as figured, show no indications whatever
of aquatic adaptation, unless possibly the very slight shortening of
the shin may be so construed. Watson believes that the bones of the
pelvis, indicate, aside from its strong union with the backbones,
strong swimming powers, but of this again the present writer is very
skeptical. The very strong ischia and the flatness of the pelvis are
both characters found among American Permian reptiles, which do not
show otherwise the slightest indications of water habits.
If then _Lystrosaurus_ was a powerful swimmer, as has been maintained,
it is very evident that the hind legs must have been used as the seals
or sea-otters use them, to propel and to guide; but they in nowise
resemble the legs of these swimming mammals. It seems altogether
more reasonable to suppose that _Lystrosaurus_ lived in the marshes,
feeding upon vegetable food obtained by aid of its strong jaws
and tusks—if the tusks were possessed by both sexes; and that the
position of the nostrils may be ascribed to causes like those which
brought about their recession in the Phytosauria, and not to strictly
aquatic habits. Possibly the animal had habits somewhat similar to
those of the hippopotamus; that it was an expert swimmer appears,
to the present writer, improbable. The powerful front legs may be
indicative of digging habits; the animal may have used them as an aid
to its powerful jaws and tusks in uprooting marsh and water plants.
However, _Lystrosaurus_, whatever may have been its habits, was a
curious reptile. It was about three feet in length, massive in all its
structure, and doubtless of slow and sluggish gait.