PROGANOSAURIA
MESOSAURUS
There is some doubt whether those little creatures of Paleozoic times,
to which some years ago the late Professor Baur gave the ordinal
name Proganosauria, are really entitled to so much distinction among
reptiles. The question of their rank has been much disputed for the
past twenty years without any positive conclusion. Nor were they wholly
aquatic in habit, though they did possess many aquatic adaptations.
That they were skilful and fleet swimmers, and capable of rapid
evolutions in the water is quite certain, and, as the oldest known
water reptiles, they are of more than passing interest.
[Illustration: FIG. 60.—_Mesosaurus_; life restoration, after McGregor,
the posture of hind leg slightly modified.]
But two genera and three or four species of the group are known, and
of them, even, our knowledge in some respects is not as complete as
one could desire. The first description of any member of the group
was by the late Professor Gervais of Paris in 1867. He had only the
anterior part of a single skeleton, from the Karoo beds of South
Africa, to which he gave the name _Mesosaurus_, a rather meaningless
term signifying “middle” or “intermediate” saurian. Nothing more
was learned about any form till 1885, when the late Professor Cope
described a specimen from the supposed Carboniferous of Brazil, which
he believed to be closely related to _Mesosaurus_, though he had
only a very imperfect specimen. He called it _Stereosternum_, also a
meaningless term, since none of the animals has a “solid sternum,”
nor any sternum at all, in fact! A few years later, in 1888 and 1892,
the late Professor Seeley of England studied a number of specimens of
_Mesosaurus_, adding not a little to our knowledge of the animals. More
recently Dr. Woodward of England and Professor Osborn of America have
given us still further information concerning them, and within the past
few years Dr. McGregor of Columbia University has figured and described
excellent specimens of a new species from Brazil, which he calls
_Mesosaurus brasiliensis_. Not only were Dr. McGregor’s discoveries
of great interest as settling many doubtful points in their structure,
but they were still more so from the fact that he found his species
so nearly like that from Africa that he placed it in the same genus.
Since the proganosaurs were purely fresh-water or terrestrial animals,
one can only wonder how they crossed from Africa to America, or, what
is more probable, how they migrated from America to Africa, across the
broad Atlantic Ocean, so long ago. The geologists tell us that the
Atlantic and Pacific, in the main, have always been oceans since the
beginning of terrestrial life upon the earth. Possibly the tribe of
proganosaurs migrated by the very circuitous route of Europe and North
America, or Asia and the Northwest; but that is very improbable, since
nothing whatever resembling them has ever been found in the Northern
Hemisphere, and it is quite certain that in the many thousands of
years it must have taken them to travel from southern Africa to South
America many of the reptiles must have perished on the way and left
their remains in the rocks. The only conclusion that seems probable is
that there was a direct land communication in those olden times between
Africa, or at least India, and South America across what is now the
Atlantic Ocean. Of course this route will be very difficult to prove,
since we can never get to the bottom of the ocean to hunt for fossil
proganosaurs. Were this peculiar distribution of the proganosaurs an
isolated example, one might perhaps ascribe our lack of knowledge of
any fossil proganosaurs in the Northern Hemisphere to the meagerness of
the fossil records, but there are many other examples of similar import
among other early animals.
[Illustration: FIG. 61.—_Mesosaurus_; restoration of skeleton. (After
McGregor)]
The age of the South American proganosaurs is now believed to be lower
or lowermost Permian, like that of the African _Mesosaurus_; possibly,
however, the age first described to _Stereosternum_ (Carboniferous) may
be correct.
The known skeletons are all small, none exceeding a few feet in length.
The skull, as shown in the figure by Dr. McGregor, is elongate, and
its teeth are extraordinarily so, and very slender. The external
nostrils are situated close to the eyes; and no sclerotic bones have
been discovered. There are small teeth in the bones of the palate.
The neck is elongate, composed of ten or twelve vertebrae. The trunk
also is long and slender, and the tail is not only long, but also
much flattened or compressed. All these are very characteristic of
water life. The limbs, however, show a much less complete adaptation
for swimming—not much more so in fact than do those of the living
Crocodilia. The upper arm and the thigh bones are relatively long,
while those of the forearm and the leg are shorter than among
terrestrial reptiles, the first indication of swimming habits to appear
in crawling animals. The digits are not much elongated, and they
have no additional finger bones, save perhaps in a lately discovered
form in Africa, in which Dr. Broom reports supernumerary bones in
the fifth or “little” toe.[3] The fingers and toes have only blunt
terminal bones, that is, they were not distinctly clawed, and they
were probably connected with each other by a membrane, as in a frog’s
foot. This webbing of the feet is probable, not only because of the
positions in which the bones have been found, but also because of the
great length of the “little” toe, which is the longest in the foot, a
character quite abnormal for a land reptile and quite characteristic
of certain aquatic mammals, like the seals and sea-otters. There is
a strong sacrum of two vertebrae, however, the pelvis and hind legs
being connected with the spinal column firmly, clearly proving that,
like the crocodiles, the proganosaurs had by no means lost their land
proclivities.
[3] An additional phalange has also been observed in the fifth toe of a
South American species.
Their vertebrae, as would be expected in such old reptiles, are quite
primitive in structure, that is, they are deeply concave in each end,
probably being perforated for the remains of the notochord. The pelvis
also is of the old-fashioned type, that is, without an opening or
vacuity between the bones below. The shoulder bones are old fashioned
too. The shoulder-blade, especially, shows a decided adaptation to
water life in its short, fan-like shape, very much like those of the
mosasaurs, ichthyosaurs, whales, etc. Just why swimming animals should
have short and broad shoulder-blades has not yet been explained, but
doubtless they afforded better attachment for those muscles used more
especially in swimming. The ribs are remarkably flat and heavy, and
were not very firmly attached to the vertebrae. Heavy ribs are unusual
among free swimming animals, but do occur in the modern sirenians,
which live on the bottoms of shallow bays, etc., feeding upon plants.
We may perhaps infer from this peculiar structure of the ribs that
the proganosaurs lived more on the bottoms of shallow waters, feeding
upon such fishes or invertebrates as they could capture, coming to the
surface to breathe from time to time. Possibly they sought the shores
for safety from their enemies, as do the Galapagos lizards, figured
on p. 142; and doubtless they laid and hatched their eggs on land.
A character which suggests that the proganosaurs lived only in the
shallow waters is the elongated neck, reminding one of those two other
groups of swimming reptiles, the dolichosaur lizards and the nothosaurs
of the Sauropterygia, the only known reptiles besides the plesiosaurs
having an abnormal number of neck bones. Still more suggestive of
shallow, fresh-water habits is the absence of eye bones, as in the
modern crocodiles.
The long snout, with the long and slender teeth, and the position
of the external nostrils far back near the eyes, together with the
flattened and long tail and the webbed feet, are sufficient proof of
expert swimming habits. The legs still functioned more or less for the
support and propulsion of the body on the land, and they probably were
only of slight service in the water. The alligator swims sinuously
with its front legs collapsed and extended by the side of the body;
its hind legs are used more as propellers, with the knee flexed and
the feet turned outward and expanded. The legs of the proganosaurs
doubtless were used in the same way, as shown in the restoration, which
has been modified from the original of Dr. McGregor in accordance with
this probable use of the legs.
There seems to be an incongruity between the posterior nostrils and the
heavy flat ribs, the former suggesting free swimming and diving habits,
the latter shallow water and bottom habits. Possibly the position of
the nostrils has been the result of the great elongation of the face in
front of the nostrils; and we know that their posterior position in the
phytosaurs (Figs. 95 and 96) has not been due to swimming habits only.
Nothing has been discovered to indicate the nature of the external
covering of the body. Possibly, even probably, the skin was more or
less covered by horny scales or plates, though it may have been quite
bare, as in the salamanders.
To which other reptiles the proganosaurs are nearest related has long
been a subject of dispute, and still is. The more probable view,
however, is that they were a very early branch of the most primitive
stock of reptiles, the Cotylosauria, one that soon perished, leaving
no descendants, unless possibly the ichthyosaurs were their progeny.
Some writers have thought that they were the early ancestral stock of
the plesiosaurs, and they are often classified with the Sauropterygia.
Still others have believed that they were an early side-branch of the
great group of Rhynchocephalia. And this doubt has been chiefly due
to our imperfect knowledge of the bones of the cranium. As has been
explained, very much stress in the classification of reptiles has been
laid by students on the possession of one, two, or no openings on the
side of the skull back of the eyes. And this part of the skull of the
Proganosauria has not yet been satisfactorily made out. Dr. McGregor
thought that there are two openings in the temporal region, allying the
group with the Rhynchocephalia. Dr. Huene is more positive that there
is but one, like that of the ichthyosaurs. In this state of indecision,
the proganosaurs may be dignified by giving them an ordinal position by
themselves.