Popular Science Monthly/Volume 24/January 1884/The Iguanodon
|←Religious Retrospect and Prospect||Popular Science Monthly Volume 24 January 1884 (1884)
THE iguanodon was discovered by Dr. Man tell, in the Weald en of England, in 1822, and has since figured in geological books as one of the largest and most remarkable of the animals whose former existence is revealed in the fossil beds of past ages. It is described in the second edition of Dana's "Geology" as "an herbivorous dinosaur of the Wealden. It was thirty feet long, and of great bulk, and had the habit of a hippopotamus. The femur, or thigh-bone, in a large individual, was about thirty-three inches long, and the humerus nineteen inches. The teeth were flat, and had a serrated cutting edge like the teeth of the iguana; and hence the name, signifying iguana-like teeth; many of them, from old animals, are worn off short." Le Conte's "Geology" also says that "the animal takes its name from the form of its teeth, which are much like those of the iguana, a living herbivorous reptile, although in other respects there is little affinity." Figs. 1 and 2 show respectively the tooth of an iguanodon, and a section of the jaw of the iguana, for comparison.
Fig. 1.—Tooth of an Iguanodon.
Fig. 2.—Section or Jaw of an Iguana. (After Buckland.)
Le Conte adds: "But the difference in size between the living and the extinct reptile is enormous. The iguana is from four to six feet long; the iguanodon was certainly thirty feet, perhaps fifty or sixty feet long, and of bulk several times greater than that of an elephant. A thigh-bone has been found fifty-six inches long, twenty-two inches in circumference at the shaft, and forty-two inches at the condyle. Its habits are supposed to have been something like those of a hippopotamus. Like this animal, it wallowed in the mud, and fed on the rank herbage of marshy grounds." The article "Iguanodon," in the "American Cyclopædia," in the course of its technical description of the bones of the animal that had been identified, suggests that the thighs "must have supported the heavy body in a manner like that of the large pachyderms," and states that the animal stood higher on its legs than any existing saurian, and was terrestrial in its habits. Dr. Mantell was of the opinion that the iguanodon had a nasal integumental horn. We reproduce in Fig. 3 a picture of the reptile restored, according to the ideas prevailing among geologists ten years ago, in contrast with a view of the actual skeleton set up in the museum at Brussels, as an illustration of the danger of making too hasty generalizations from too few or too imperfectly understood data.
A new and very considerable deposit of remains of iguanodons, from one of the nearly complete skeletons of which the present reconstruction of the animal has been made, was discovered in 1878 at the coal-mines of Bernissart, between Mons and Tournay, in Belgium, close to the French frontier. They occur there, like the English fossils, in the Wealden or lower cretaceous strata, or morts-terrains (dead layers), as the workmen call them, that overlie the coal-beds, and which have to be penetrated for about twelve hundred feet before the coal is reached. The discovery was made by M. Fages, director-general of the Bernissart Mining Company, and specimens of the bones were sent to Professor P. J. Van Beneden, who identified them as belonging to the iguanodon. The task of removing the fossils was attended with much difficulty, for they were charged with iron pyrites, the decomposition of which caused them to crumble as soon as they were exposed to the air. It was undertaken and accomplished successfully by M. Depauw, superintendent of the workshops of the museum at Brussels. He adopted the habits of the miners, and spent three years in the excavations, personally superintending the removal of every specimen. By subjecting them to a gelatine-bath and enveloping every piece, previous to removal, with a casing of plaster, he got them all out whole. The remains were then again examined by Professor Dupont, director of the museum, and again shown to be those of the iguanodon.
For the past two years the bones have been under the steady investigation of M. L. Dollo, a former pupil of Professor Giard, of Lille, who has published four papers giving accounts of his observations, and is expected, when he gets through with his work, to publish an exhaustive treatise on the subject. He thinks he has the skeletons, or parts of them, of twenty-three individuals, two of which belong to Mantell's species (Iguanodon Mantelli), and twenty-one to the species Iguanodon Bernissartensis. One of the specimens has been restored and mounted by M. Depauw, and set up in a glass chamber in the court of the museum. It is nearly complete, only a few phalanges and other minor details being wanting, while, on account of the impossibility of detaching the bones, most of them have been mounted still joined to one another, and fastened to the matrix as they were taken from the mine. The figure has, of course, for this reason a little stiffness, but not enough to attract the attention of the merely casual observer, and stands, in the natural attitude of progression of the animal on land, erect on its hind-limbs, with the top of its snout fourteen feet two inches from the ground, and covering, from the tip of the tail to a point immediately under the tip of the snout, a length of twenty-three feet nine inches.
The iguanodon belongs to the sub-class of dinosaurians and the order Ornithopoda, or bird-footed. Among the special characteristics of the family of the iguanodons are a single row of teeth, three functional digits on the foot, and two symmetrical sternal plates. The last, which Professor Marsh, from his studies of specimens in the British Museum, regarded as clavicles, and traced in them a point of structural resemblance with birds, are declared by M. Dollo, from specimens at Bernissart, in which they are preserved in their natural relations, to be sternal, while no clavicles are found. There are, however, says Mr. H. N. Moseley, in "Nature," abundance of other points in the skeleton of the iguanodon "in which the remarkable resemblances between the Ornithopoda and birds indicated by Professor Huxley, more than twelve years ago, are borne out in a most remarkable manner.... First of all, there seems to be little doubt possible that the iguanodons walked, as he pointed out, on their hind-limbs erect, like birds, in somewhat the attitude of the accompanying figure (see Fig. 4). Several different lines of coincidence, as M. Dollo points out, tend to prove this. Firstly, the remarkable resemblances be- tween the structure of the pelvis and the posterior limbs of birds, and the corresponding parts in the iguanodons. The points of resemblance of the ilium and ischium, pointed out by Professor Huxley, are fully confirmed by the Bernissart specimens.... The actual pubis is very large in the iguanodon, as will be seen in the figure, and projects forward and outward, forming an obtuse angle with the pubis.... The post-pubis is long and slender, and directed backward alongside the ischium, as in birds, for a considerable distance beyond the ischial tuberosity.... M. Dollo is inclined to follow Professor Marsh in identifying the dinosaurian pubis with the pectineal process of birds, a conclusion which receives most interesting support in the valuable memoir recently published by Miss Alice Johnson, of Cambridge, on "The Development of the Pelvic Girdle in the Chick," in which it is shown that in the embryo fowl the cartilaginous representative of the pectineal process is at first much larger and more prominent in proportion to the dimensions of the pelvis than subsequently, and becomes gradually reduced as development proceeds. The peculiar form of the pelvis is, no doubt, directly connected with the muscular arrangements concerned in the erect posture, originated probably in the dinosaurians and transmitted to birds, in which it has been improved upon by the elimination, almost complete, of the original pubis through disuse."
The fore-limbs are considerably shorter than the hinder ones, and are massive and strong; and this difference in structure is cited as further though not conclusive evidence of the animal's having maintained an erect position. As further evidence in the same direction, and of the approach of the type of structure to that of birds, are mentioned the reduction of the volume of the head and thorax as compared with that of reptiles and the position of a large mass of the viscera behind the hip-joint, as in birds, whereby, with the aid of the long tail, the balancing of the head and fore-part of the body was more easily secured. The dorsal spines of the vertebrae are connected with a set of ossified ligaments binding the whole dorso-lumbar region into a rigid mass—another peculiarity in which the structure is strikingly like that of birds. The fore-limbs of the animal have five and the hind-limbs four claws, or toes, leaving a three-toed track. Here, again, is another and probably the most decisive proof that the iguanodon walked on its hind-limbs only. The feet have been compared by M. Dollo with the tridactyl Wealden foot-prints—which the iguanodon only among known Wealden dinosaurians could have made—and have been found to fit accurately. "If the animal had walked on all-fours," Mr. Moseley remarks, "it is impossible but that pentadactyl impressions should have occurred with the tridactyl, but such is not the case. Long series of the tridactyl prints are found without a trace of pentadactyl marks. The arrangement of the tridactyl tracks shows that the iguanodon walked on its hind-feet, and did not spring, like a kangaroo, with the aid of its tail. This merely dragged lightly behind, and has left no impression in connection with the foot-tracks." The first finger, or thumb, constitutes a large horny spur, the remains of which when first found were supposed to be the nose-horns of Mantell's ideal. According to M. Dollo's description, the head is relatively small, and very much compressed from side to side. The nostrils are spacious, and chambered in their anterior region; the orbits are of moderate size and elongated along the vertical. The temporal fossa is limited above and below by a bony arch, a disposition which is otherwise found among living lizards only in the Hatteria. The
Fig. 4.—Iguanodon Bernissartensis.—(At the Brussels Royal Museum of Natural History. Restored and mounted by M. L. F. Depauw.)
Head: a, left nostril; b, left orbit; c, left temporal fossa. Vertebral column: d, cervical region; e, dorso-lumbar region; f, sacral region: g, caudal region; h, left scapula; i, left coracoid; k, left humerus; l, left ulna; m, left radius; n, sternum; o, left ilium; p, left pubis; q, left post-pubis; r, left ischium; s, left femur; t, left tibia; u, left fibula; v, third trochanter. I, II, III, IV, V, digits; X, diagrammatic transverse section of the body between the fore and hind limbs.
A multitude of other treasures besides the iguanodons were found at Bernissart, and are awaiting careful examination. Among them are crocodiles, in which two new genera have been defined; turtles, which have given one new genus; and "a vast quantity of fishes."