Principal Characters of American Jurassic Dinosaurs; Part VI, Restoration of Brontosaurus
AMERICAN JOURNAL OF SCIENCE.
Art. IX.—Principal Characters of American Jurassic Dinosaurs. Part VI: Restoration of Brontosaurus, (with plate I); by Professor O. C. Marsh.
In the previous articles of this series, the writer has given the more important characters of the order Sauropoda. A volume on this group is now in preparation, and the illustrations (90 plates) are nearly completed. One of these is a restoration of Brontosaurus, which has so many points of interest that a reduced figure is here presented. Several new characters of this group are added, some of which will be of interest to comparative anatomists.
Nearly all the bones here represented belonged to a single individual, which when alive was nearly or quite fifty feet in length. The position here given was mainly determined by a careful adjustment of these remains. That the animal at times assumed a more erect position than here represented is probable, but locomotion on the posterior limbs alone was hardly possible. The head was remarkably small. The neck was long, and, considering its proportions, flexible, and was the lightest portion of the vertebral column. The body was quite short, and the abdominal cavity of moderate size. The legs and feet were massive, and the bones all solid. The feet were plantigrade, and each foot-print must have been about a square yard in extent. The tail was large, and nearly all the bones solid.
The diminutive head will first attract attention, as it is smaller in proportion to the body than in any vertebrate hitherto known. The entire skull is less in diameter or actual weight than the fourth or fifth cervical vertebra.
A careful estimate of the size of Brontosaurus, as here restored, shows that when living the animal must have weighed more than twenty tons. The very small head and brain, and slender neural cord, indicate a stupid, slow moving reptile. The beast was wholly without offensive or defensive weapons, or dermal armature.
In habits, Brontosaurus was more or less amphibious, and its food was probably aquatic plants or other succulent vegetation. The remains are usually found in localities where the animals had evidently become mired.
Among the new points in the skull of the Sauropoda recently determined are the following:
In Morosaurus, the pituitary fossa is comparatively shallow, much like that in the crocodile, and many birds, being connected with the under surface of the skull by the two usual divergent foramina for the passage of the internal carotid arteries. In Apatosaurus, however, it is remarkably different. Here the fossa becomes enlarged into a vertical canal, which, expanding below, communicates by a wide transverse orifice with the pharyngeal cavity. The arterial foramina are here canals thinly covered over with bone, and open just within the rim of the lower orifice. The pituitary cavity itself has a firm smooth wall throughout. The openings are both transverse, and oval in shape. The upper one is eighteen by six millimeters in its diameters; the lower opening thirty by twelve.
This remarkable connection of the cerebral cavity with the alimentary canal is an embryonic character, and corresponds to the condition observed in the chick at the fifth day of incubation. This peculiar feature appears to be a family character of the Atlantosauridæ.
In two genera of the Sauropoda, (Morosaurus and Brontosaurus), and probably in all members of this order, there is a pair of small bones connected with the skull which have not hitherto been observed in any vertebrates. These bones, which may be called the post-occipital bones, were found in position in one specimen, and with the skull in several others. When in place, they are attached to the occiput just above the foramen magnum, and extend backward and outward, overlapping the lateral pieces of the atlas, thus protecting the spinal cord at this point, which would otherwise be much exposed.
These bones are short, flattened, and slightly curved, resembling somewhat a riblet. The anterior end is thickened and rugose for attachment to a roughened surface on the exoccipital, just above and outside the foramen magnum. The shaft is flattened from above downward, and gradually converges to a thin posterior end. In Morosaurus grandis, these bones are about 65 mm. in length, and 30 along the surface which joins the occiput. They correspond in position to the muscle in mammals known as the rectus capitis posticus minor.
In the existing Cormorants (Graculus) a single slender bone is articulated to the occiput on the median line. This, however, does not correspond to the bones here described. To distinguish it from the post-occipitals, it may be called the nuchal bone.
In the skull of Morosaurus in which the post-occipital bones were found in position and the other bones at the base of the skull were undisturbed, a careful search was made for the stapes, but no indication of it was found. Its absence in this specimen, so well preserved, would indicate that it was wanting in this genus, if not in the other Sauropoda.
In a skull of Brontosaurus in which the bones, although displaced, were in very perfect preservation, a pair of bones were found which apparently are the columellæ. They are elongated, flattened bones, with the shaft somewhat constricted in the middle, and twisted. Their length corresponds to the elevated posterior part of the skull in this genus.
There are two pairs of hyoid bones in the Sauropoda. They are elongated, rodlike, and somewhat curved. In Brontosaurus excelsus, they are 210 and 130 mm. in length respectively.
Among the other points of interest in the skull of the Sauropoda are the following:
The parietal bones are very short, and form but a small portion of the brain case. They are composed chiefly of the flattened arched processes, which meet with the squamosals at their outer ends. There is no parietal foramen. The squamosals lie upon the par-occipital processes. They have a short deep groove for the reception of the post-frontal. On their lower part, which descends in front of the par-occipital process, they expand into a thin spoon-shaped form, which fits over the head of the quadrate. The quadrate has an oval rounded head, and slender shaft. Below, it is firmly united to the pterygoids. On the outer side, the quadrato-jugals are attached. These bones are elongate, and slightly sigmoid in shape. The lower end is rodlike, and curved forward, descending below the articular surface of the quadrate. The pterygoids are tri-radiate bones, with the posterior ends cup-shaped, resembling the partially closed human hand. This cavity, somewhat restricted by a thumb-like process, receives the basi-pterygoid process.
There are twenty-seven precaudal vertebræ in Brontosaurus, of which the first twelve bear pleurapophyses, or hatchet bones, united to the centra, and may hence be called true cervicals. Of the remaining twelve which bear free ribs, the thirteenth, fourteenth, and fifteenth have the surface for the articulation of the head of the rib on the centrum, below the neural suture.
All the precaudal vertebræ have large cavities in the centrum, communicating exteriorly with the surface by means of large lateral foramina. This cavernous structure of the vertebræ gradually decreases posteriorly, until in the anterior caudal vertebræ it is confined to a small pocket above the transverse process. The neural arches of the presacral vertebræ contain numerous deep cavities. The pleurapophyses of the cervical vertebræ are also reticulate in their structure, and some of the anterior ribs have small but deep fossæ below the tubercle.
On the last two or three cervical vertebræ of Brontosaurus, there is a convoluted ridge of bone over the posterior zyga-pophyses. In the anterior dorsals, this ridge becomes stronger and more elongated, forming a distinct protuberance. These processes have not hitherto been described. As they are analogous to the processes in mammals known as meta-pophyses, they may bear the same general name, being distinguished as the post-metapophyses. The term pre-metapophyses should then be applied to the processes in mammals.
The post-metapophyses probably serve for the attachment of ligaments in the place of the neural spine, which is here wholly wanting. These processes, which are at first oblique in position, gradually become more vertical and stouter, and, coalescing at their bases, finally become united throughout, and are thus converted into the neural spine.
Remains of a very small dinosaur were found in immediate relation with the type specimen of Morosaurus grandis. These remains, which consist of a complete femur, the larger portion of both humeri, and several vertebræ, show no essential differences from the large specimens except in size, and indicate an animal of perhaps seven feet in length, and little more than two feet in height. The imperfect ossification of these bones indicates that the animal was very young, and it seems probable that it was fœtal. The only other similar case known in the Dinosauria is the apparent embryo observed by the writer in Compsognathus.
The various genera of the Sauropoda, and in fact of the Dinosauria in general, cannot at present be distinguished by the detached teeth. In one form, however, the teeth are quite peculiar, and the dentition appears to offer generic characters. The maxillary teeth of this form have been referred to Stegosaurus (this Journal, vol. xix, p. 255; pl. vi, figs 4 and 5), as they were first found in connection with the remains of that genus. Later investigations indicate that they belong to the Sauropoda, and there is some evidence that they are the teeth of Diplodocus.
The main characters of the order Sauropoda, and of the two families now known to belong to it, are as follows:
Order Sauropoda. Herbivorous.
Feet plantigrade, ungulate; five digits in manus and pes; second row of carpal and tarsal bones unossified. Pubes projecting in front, and united distally by cartilage; no post-pubis. Fore and hind limbs nearly equal; limb bones solid. Sternal bones parial. Premaxillary bones with teeth. Precaudal vertebræ hollow. Each sacral vertebra supports its own transverse processes.
Family Atlantosauridæ. Anterior vertebræ opisthocœlian. Ischia directed downward, with extremities meeting on median line. Anterior caudals with lateral cavities. A pituitary canal.
Family Morosauridæ. Anterior vertebræ opisthocœlian. Ischia directed backward, with sides meeting on median line. Anterior caudals solid. Pituitary fossa only.
Yale College, New Haven, July 12th, 1883.
- This Journal, xvi, 417, Nov. 1878; xvii, 86, Jan. 1879; xxi, 417, May, 1881; and xxiii, 81, Jan. 1882.
- This Journal, xxii, 340, November, 1881.