LIZARDS
Popularly a lizard is any four-legged reptile covered with scales, but such a definition is not strictly correct, since some lizards are legless and some other four-legged reptiles are covered with horny scales, notably the tuatera or Sphenodon of New Zealand, a reptile long classed with lizards, but now known to belong to quite a different order. Bearing in mind those characters given as characters of the order, it will be necessary to mention only those distinguishing the lizards from the snakes.
Fig. 65.—Iguana.
(By permission of the New York Zoölogical Society)
It is true that the great majority of lizards have four legs, while the snakes are always functionally legless, but there are some lizards, like the glass snakes and the amphisbaenas, or slow lizards, which are quite legless and there are some snakes which have small but functionless hind legs. As usual, more important differences are found in the skull. The brain-case in all snakes is surrounded on all sides by bone, for the better protection of the brain, with the head resting quite prone on the ground. The brain of the lizards, for the most part, is protected on the sides and in front by a simple membrane. Nearly all lizards have movable eyelids, while snakes do not; snakes have a single lung, and a protrusible tongue, which very few lizards possess; and the lower jaws in front are united in the snakes by a ligament only. Notwithstanding these differences, the snakes and lizards are closely related animals, and must have come from a common ancestry; among all reptiles the known geological history of the snakes is shortest.
Lizards, on the other hand, have a very high antiquity, beginning, as we now know, at least as long ago as early Triassic times. They still have many primitive characters in their structure and are the least advanced type of reptiles now living, with the exception of the tuatera. Their remains are seldom found in the rocks, probably because they have always been so strictly terrestrial in habit, for the most part seldom frequenting even the vicinity of the water. The true lizards now living number about eighteen hundred kinds, classified into about twenty families, divided among four chief groups, of which the chameleons, the amphisbaenas, our common lizards, and the monitors are representatives.
Most living lizards are inhabitants of warm climates, though some extend rather far north in the temperate zone. With the exception of New Zealand, and the polar and subpolar regions, lizards are found in all parts of the world. The great majority live only in high and dry places, though some are denizens of low and marshy places, a few even not being averse to the water. They are, for the most part, spry in their movements, some little ones scarcely six inches in length taxing a vigorous man’s speed to capture; and many are expert climbers of cliffs, trees, and even the ceilings of residences. Some, the remarkable little flying dragons of Ceylon, have an extraordinary development of the skin on the sides of the body, supported by the expanded ribs, forming a sort of parachute whereby the creatures can sail considerable distances through the air. Nearly all are carnivorous, feeding upon small mammals, birds, other reptiles, frogs, and insects; a few only are herbivorous, such as the iguanas, which are often used for human food. Nearly all lizards are oviparous, laying from two to thirty eggs. In size the great majority are small, less than a foot in length; but some, such as the monitors and iguanas, reach a length of from four to six feet, or even more, and certain extinct monitors of India are known to have attained a length of thirty feet. They are, for the most part, slender, graceful, prettily marked, and quite inoffensive creatures. A few are short, flat, or stumpy in shape, such as the so-called horned toad. One or two species only, the “Gila monsters,” are reputed to be venomous.
Fig. 66.—Amblyrhynchus cristatus, the Galapagos sea-lizard.
(From Brehm)
There is but a single species of lizard now living which is in any true sense aquatic in habit, the well-known sea-lizard of the Galapagos Islands, scientifically known as Amblyrhynchus cristatus. It is a large lizard, with a short rounded head, a flat tail, and webbed feet. Its specific name is derived from the erect fringed crest along its back and tail. Its habits are best given in Darwin’s words:
It is extremely common on all the islands throughout the group, and lives exclusively on the rocky sea-beaches, being never found, at least I never saw one, even ten yards inshore. It is a hideous looking creature, of a dirty black color, stupid and sluggish in its movements. The usual length of a full grown one is about a yard, but there are some even four feet in length; a large one weighed twenty pounds. The tails are flattened sideways, and all four feet are partially webbed. They are occasionally seen some hundred yards from the shore swimming about. When in the water this lizard swims with perfect ease and quickness, by a serpentine movement of the body and flattened tail—the legs being motionless and closely collapsed to the sides. A seaman on board sank one, with a heavy weight attached to it, thinking thus to kill it directly; but when an hour afterward he drew up the line it was quite active. Their limbs and strong claws are admirably adapted for crawling over the rugged and fissured masses of lava, which everywhere forms the coast. The nature of this lizard’s food (seaweed) as well as the structure of the tail and feet, and the fact of its having been seen voluntarily swimming out at sea, absolutely proves its aquatic habits; yet there is in this respect one strange anomaly, namely, that when frightened it will not leave the island. Hence it is easy to drive these lizards down to any little point overhanging the sea, where they will sooner allow a person to catch hold of their tails than jump into the water. They do not seem to have any notion of biting; but when much frightened they squirt a drop of fluid from each nostril. I threw one several times as far as I could into a deep pool left by the retreating tide, but it invariably returned by a direct line to the spot where I stood. It swam near the bottom, with a very graceful and rapid movement, and occasionally aided itself over the uneven ground with its feet. As soon as it arrived near the edge, but still being under water, it tried to conceal itself under the tufts of seaweed, or it entered some crevice. I several times caught the same lizard by driving it to a point, and, though possessed of such perfect powers of diving and swimming, nothing could induce it to enter the water; and as often as I threw it in it returned in the manner described above. Perhaps this singular piece of apparent stupidity may be accounted for by the circumstance that this reptile has no enemies whatever on shore, whereas at sea it must often fall a prey to the numerous sharks.
These lizards are of much interest as indicating one of the ways in which true land reptiles have become aquatic in their habits. Tempted by the abundance of food growing in shallow water a little beyond their reach, the reptiles ventured farther and farther to obtain it. The tail gradually became a propelling organ, though the lizard still retained in large measure its land habits and land feet, because of the dangers from its water enemies. It is not at all improbable that, in course of time, were these Galapagos lizards left unmolested, they would become fleeter swimmers by the development of a terminal caudal fin and paddle-like legs, thus competing with their aquatic enemies and no longer needing recourse to the land for protection. They also serve to indicate that long-tailed aquatic reptiles never used their legs to an appreciable extent as organs of propulsion in the water.
Fig. 67.—Varanus, Australian monitor lizard.
(By permission of the New York Zoölogical Society.)
Flat-headed lizards.—Among the living lizards there is one group, called the monitors, which have so many characters peculiar to themselves that they seem rightfully entitled to an isolated place among the lizards of the present time. The group includes about thirty species, all belonging in the one genus Varanus, and all living in India, Africa, and Australia. In size, some of the species of Varanus are the largest of all terrestrial lizards known in the past or present; in other ways also they have reached the maximum of specialization among lizards. The head is pointed, broad, and flat, and the body and tail are long. They have nine vertebrae in the neck, a larger number than is to be found in any other terrestrial lizard. Unlike other lizards they have a protrusible tongue like that of the snakes. All are carnivorous in habit, feeding upon small backboned animals, insects, and especially upon eggs, which they crush between their teeth while holding them aloft. Most species live wholly upon the land, and some are arboreal. Others, especially those of the Nile, live about water and are excellent swimmers. The terrestrial species have a round tail and small external nostrils, but the water species have the tail much flattened, and the nostrils have large cavities, which, when closed under water, are said to serve as reservoirs of air for respiration. Of one of these swimming species Annandale writes:
Varanus salvator is common in Lower Siam where it is equally at home on land, in water, and among the branches of trees. The eggs are laid in hollow tree trunks. When in the water the lizard swims beneath the surface, the legs being closely applied to the sides, and the tail functioning both as oar and rudder.
These lizards take to the water to escape from their land enemies and not for food, a habit also known among certain other lizards, and one precisely the reverse of that of the Galapagos lizards. It would seem very probable that animals of such carnivorous habits as are the monitors might easily learn to capture water animals for food and thus eventually become aquatic in habit. This inclination toward, and partial adaptation to, water habits in the monitors is of much interest because in all probability the instinct is one of long inheritance from those remote ancestors which gave origin to the truly aquatic members of the order. Though the known geological history of the monitors does not extend far back, they are so intimately allied in their anatomical structure to the aquatic and semiaquatic lizards of Cretaceous times that there could seem to be no doubt of the common ancestry.
Dolichosaurs.—About fifty years ago Professor Owen, the famous English paleontologist, described a peculiar semiaquatic lizard from the Cretaceous rocks of England to which he gave the name Dolichosaurus, in allusion to the slender form of the body. Just what relations these slender lizards have to modern lizards has long been a problem; some have thought that they were their progenitors, but there are very good reasons for doubting this. No modern lizards, save the monitors, have more than eight vertebrae in the neck, while these dolichosaurs had as many as seventeen, a remarkable specialization for aquatic life that could hardly have been lost by their descendants. For this reason the dolichosaurs have usually been considered as representing a distinct suborder. But they have many resemblances otherwise to the monitors. They were semiaquatic in habit, and never more than six feet in length. They are yet imperfectly known, and no restoration of any form has hitherto been attempted. Their peculiar interest lies in the elongation of the neck, quite like that of the wholly unrelated nothosaurs and proganosaurs, which have been described in the foregoing pages. Doubtless similar habits in each had like results, but just what these habits were in the slender lizards we do not yet know.
Aigialosaurs.—Within recent years a number of other lizards have been made known from the Lower Cretaceous rocks of Dalmatia which present most remarkable intermediate characters between the monitors, dolichosaurs, and the mosasaurs, the famous sea-lizards of Upper Cretaceous age. Some of these lizards had twelve or thirteen vertebrae in the neck, while others had but seven—an unusually short neck characteristic of the mosasaurs. These latter kinds, belonging to two or three genera, are included in a distinct group. They were long and slender, the head long and pointed. The teeth, conical and sharp, were attached in shallow pits, quite as in the mosasaurs. The lower jaws had a hinge just back of the teeth, as in the mosasaurs, of which the only trace in modern lizards is found among the monitors. Still more remarkable, though perhaps not so easily appreciated, is the shape of the quadrate bone, with a broad flaring rim for the ear cavity, quite unlike that of land lizards, but quite like that of the mosasaurs. In fact, the very peculiar skull is almost identical with that of the true sea-lizards. The body and tail also resemble those of the mosasaurs more than those of the monitors, but there is a firm attachment of the pelvis to the backbone, and the legs are long and lizard-like, though not as long as those of land lizards. The feet were webbed in life, and the toes have no claws, conclusively demonstrating their water habits. The vertebrae indeed have the same peculiar articulations, called zygosphenes, as in most of the mosasaurs. The largest aigialosaurs were about six feet in length, that is, of about the size of the smallest known mosasaurs.
Fig. 68.—Clidastes, an American mosasaur.
Life restoration.
We have then in the aigialosaurs nearly every known intermediate character that we could wish for in a connecting link between the mosasaurs and the monitors, lizards that were equally at home on land or in the water, and there can be scarcely a doubt that they were either the direct ancestors or closely akin to the direct ancestors of the strictly marine mosasaurs; and scarcely a doubt that they were the descendants of the actual forbears of the modern monitors, which, as we have seen, have acquired partial aquatic habits in escaping from their enemies. The dolichosaurs we can now understand were a side branch from these semiaquatic aigialosaurs which, specializing in another direction, quickly came to grief, perhaps in competition with their more agile and skilful short-necked kin.
Taking all these facts into consideration it seems best to unite the monitors, dolichosaurs, and aigialosaurs into one group of the Lacertilia, the Platynota, intermediate in place between the true land lizards and the truly aquatic mosasaurs.