PROTOROSAURIA
PROTOROSAURUS
The genus _Protorosaurus_ is of peculiar interest, as one of the first,
if not the first, known fossil reptiles, described by Spener as long
ago as 1710 as a crocodile, from fragmentary remains found in 1706 in
the Permian deposits of Thuringia. Numerous other skeletons or parts
of skeletons attracted the attention of naturalists of the eighteenth
century, but were very imperfectly described. No name was given to the
animal represented by the various specimens until 1840, when Herman
von Meyer restudied all the known material and described it under the
name _Protorosaurus speneri_. The position of the genus among reptiles
always has been and yet is uncertain, for the reason that the structure
of the skull, and especially the structure of the temporal region, has
never been satisfactorily determined. Seeley, in 1887, described more
fully the original specimen of Spener, now preserved in the museum of
the College of Surgeons of London, and because of certain peculiarities
which it showed proposed for its reception the order Protorosauria.
He thought that he detected an upper temporal vacuity, like that of
lizards, but was very uncertain about the structure of the lower
part of the temporal region. The writer, who has examined this type
specimen, must admit that the structure of the region here is very
doubtful. Under the general assumption, however, that all old reptiles
must be related to _Sphenodon_, the Protorosauria have generally been
classified as a suborder of the Rhynchocephalia. It is merely another
instance of the proclivity we all have to propose hypotheses, and then,
speedily forgetting that they are hypotheses, to accept them as facts.
_Protorosaurus_ was long supposed to be an aquatic reptile, but we now
know that it was a strictly terrestrial one, probably with climbing
habits; and the genus concerns us only by reason of its possible
relationships to distinctly aquatic reptiles of a later age.
[Illustration: FIG. 62.—Life restoration of _Araeoscelis_.]
A few years ago the writer described a very slender, lizard-like
reptile about two feet in length from the Permian of Texas under
the name _Araeoscelis_, so named because of its slender legs. The
structure of both the skull and the skeleton of this reptile is now
quite satisfactorily known, so well known indeed that the accompanying
restoration (Fig. 62) has little that is conjectural about it, at
least so far as the form is concerned. The skull has a single, upper
temporal opening, quite like that of lizards, but the quadrate is
not loose below. And this is really what we should expect in the
ancestral lizards; and everything else of the skeleton, except perhaps
one character, is what would be expected. That one character is the
elongation of the cervical vertebrae, which are about twice the length
of the dorsal vertebrae following them. The cervical ribs are very
slender bones, articulating by a single head with the centrum only. In
these and other characters, so far as they are known, _Araeoscelis_
seems to agree with _Protorosaurus_, and both have very hollow bones.
[Illustration: FIG. 63.—Skeleton of _Pleurosaurus_. (After Lortet)]
PLEUROSAURUS
We may for the present be justified in maintaining the order
Protorosauria for those reptiles having a single, typically upper
temporal opening on each side, with a fixed quadrate, not including
the ichthyosaurs. It is not improbable, however, that when more is
known of the ancestors of the lizards, the whole group will find its
most natural place among the Squamata. This definition will include
a peculiar aquatic reptile that has been known for many years, but
which has been wrongly classed in the same family as _Sphenodon_, on
the purely gratuitous assumption that it has two temporal openings
on each side; we now know that it has but one. This reptile, known
scientifically as _Pleurosaurus_, was described originally by H. von
Meyer in 1843, but we are indebted to M. Lortet for a more precise
knowledge of the animal, and for the figure (Fig. 63) which is here
given of the skeleton. Not a few excellent skeletons are preserved in
the museums at Lyons and Munich. The specimen here figured, as actually
preserved, measures about three feet in length; a part of the tail is
missing, which is known from other specimens to have been remarkably
long.
The figures show clearly some of the remarkable aquatic adaptations of
the animal, especially the short neck, the very long and narrow body,
and the extraordinarily long and flattened tail. The head is elongate
triangular in shape, resembling very much that of the mosasaurs; and
the external nostrils are likewise situated remotely from the end
of the snout, as in the mosasaurs. The extremity of the snout has a
beak-like projection. The teeth are much longer, more pointed, and more
recurved than is the case with most land reptiles, indicating their use
for the capture and retention of slippery, quick-moving prey.
The single-headed ribs are short, proving that the body was slender
and doubtless cylindrical, more like that of a snake. The tail was
not only enormously elongated, but it was also compressed into a flat
and effective propelling organ in the water. This flattening of the
tail is apparent from the skeleton, with its elongated chevrons below
and spines above, and it is also proved by the fortunate preservation
of the extremity of the tail of one specimen, showing not only the
impressions of the scales in the matrix, but also the outlines that the
soft parts had in life. To quote from Lortet, in translation: “The tail
was covered wholly with small scales, regularly hexagonal in shape,
shining and nacreous, larger on the under side than above. The upper
border of the tail was surmounted by a broad crest, extending to its
extremity, and composed of large, oval scales.” The body doubtless was
wholly covered with scales, though it is not probable that the caudal
crest continued along the back.
[Illustration: FIG. 64.—Life restoration of _Pleurosaurus_.]
The limbs begin to show an aquatic adaptation, though not very
pronounced. They are much shorter and smaller than are those of
land-crawling reptiles; and the bones of the second series, that is,
the radius and ulna, tibia and fibula, are relatively short, the
beginning of adaptation to water habits. It is very probable that the
feet were webbed, though the fifth digit, as usual, is shorter than
the fourth. Doubtless on land the creature moved about in a serpentine
way, for it could not have progressed very rapidly by the aid of its
legs alone. The hind legs are longer than the front legs, and they were
connected firmly with the body by means of a sacrum. The number of
vertebrae in the neck is only five. The number of dorsal vertebrae is
forty-three, a larger number than is known in any other air-breathing
vertebrate with legs.
Upon the whole, these lizard-like, almost snake-like pleurosaurians
present some very curious adaptations to water life. In water they were
doubtless speedy, swimming in serpentine undulations, with the small
legs for the most part folded against the body and only of occasional
use. Doubtless, too, had the pleurosaurs lived longer in geological
history, they would have become quite snake-or eel-like, just as have
some modern salamanders.
In all probability the pleurosaurs lived habitually in fresh-water,
perhaps visiting the shores for refuge, or for the hatching of their
young. That they were not on the way toward a terrestrial snake-like
body is evident from the flattened tail, and especially the crest of
scales above; the tail was like that of the sea-snakes of the present
time. _Pleurosaurus_, then, affords the solitary instance among
reptiles of aquatic adaptation by the diminution of both front and hind
extremities and the acquisition of a snake-like body and snake-like
habits.