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The ilia are represented by the right bone, which has lost its. greater peduncle, but is otherwise quite well preserved, and a badly weathered left, which, fortunately, has the greater peduncle preserved. The right bone has been modified by a distortion which has depressed the whole posterior half, straightened the superior border, distorted the acetabulum, and reduced the depth in the supraacetabular region. It is figured from a lateral view without correction, other than the restoration of the greater peduncle from a comparison with that of the left. (Fig. 3, Plate LXXV.)
The important feature of the ilium is the unusual development of the pre-acetabular portion. The greater peduncle is near the middle; the anterior half is expanded into a broad but rather thin plate, most of which stands anterior to the attachment of the first sacral rib. At the point of greatest eminence the crest is thickened for the attachment of the first diapophysis. The anterior border is much more rounded than it is in any of the other large dinosaurs. At the anterio-inferior angle the border is quite thick and roughened for muscular attachment. This roughening continues one-third of the way from the angle to the point of attachment for the first sacral diapophysis, where it disappears and the border becomes thin and rounded. The crest continues very thin to a point over the middle of the acetabulum, where it again becomes stronger and rugose. This continues to the posterior angle, which is much thickened, and offers the usual broad surface for the origin of lateral caudal muscles.
The acetabulum is distorted by the depression of its arch. It presents to the head of the femur a broad and rounded surface, which terminates at either end in the peduncles. Its superior surface takes the form of an arch which juts far beyond the iliac wall on the mesial side, and presents a concave surface for the support of the sacricostal yoke. The attachment for the first diapophysis is a triangular fossa bounded by two ridges which descend from the crest and converge at a point seven or eight inches below. From this point a rounded ridge passes obliquely downward and backward to the base of the greater peduncle. The points of attachment of the second and third diapophyses are very slight; no trace of a fourth appears. The fifth has a strong attachment at the thickened crest above the lesser peduncle. The plates connecting the diapophyses with the sacral ribs do not attach to the iliac wall as in Apatosaurus.
THORACIC RIBS.
The unusual length of the ribs, as well as the breadth of the head and tubercle and the strength of the shaft, bears evidence of the immense thorax of this animal. One of the snore slender ribs from the mid-thoracic region measures fully nine feet (2.745 m.) in length. Another has a shaft eight inches (.204 m.) in breadth. The head and TUBERCLE are almost equally developed and widely separated to give the firm attachments rendered necessary by the great length of the ribs. (Fig. 5, Plate LXXV.) In some instances the attachment is strengthened by a second tubercle on the inferior surface of the head similar to that figured by Marsh on the cervical vertebrae of Apatosaurus. The anterior surface of the shaft below the head is perforated by a large foramen which leads to an internal cavity. On account of the elevation of the capitular facet on the vertebrae, the head and tubercle are borne almost on a level. By reason of this the flattened surface of the proximal end passes insensibly into the lateral surface of the shaft without that twist common to the ribs in animals of this group.
LEG BONES.
TABLE OF MEASUREMENTS OF LEG BONES
The humerus is somewhat crushed antero-posteriorly and twisted so that the head and distal end are brought into the same plane. The surface of the distal end has flaked away in the process of weathering to a firm chalcedony core. (Fig. 1.) In relative slenderness the humerus is approached more nearly by that of Diplodocus than that of any other well-known American dinosaur. The head is considerably expanded, forming a rounded prominence especially conspicuous on the posterior surface. The great tuberosity is stout and rugose; its proximal surface meets the lateral margin of the shaft in a pronounced angle. This angle is not produced posteriorly to inclose a fossa as in Morosaurus. The inner border below the head is drawn out into a rather thin margin, though roughened for muscular attachment. The deltoid crest is partially broken away, but was evidently prominent. (Fig. 4, Plate LXXIV.) Its base forms with the anterior surface of the shaft a broad and shallow concavity.
Midway between the deltoid crest and the great tuberosity is a second rugose surface evidently for the insertion of other shoulder muscles. The ectepicondylar ridge is entirely lost, owing to the weathering to which the distal end has been subjected. The direction of the bone fibres on the lateral margin indicates that it may have been quite prominent. All traces of rugosity have likewise disappeared from the distal end, indicating that the humerus was probably some inches longer than it now stands.
The coracoid is a larger, but less massive, bone than that of Apatosaurus. (Fig. 4, Plate LXXV.) It is elongate antero-posteriorly, rounded on its inferior margin, and straight at the scapular articulation. The glenoid articular surface is directed outward as well as backward, a feature observed in no other opisthocoelian genus. The postero-inferior surface is thick and rugose for epicoracoidal attachment near the glenoid cavity', from which it is separated by a narrow notch only. The posterior border becomes gradually thinner, and its rugose character disappears midway between the glenoid cavity and the anterior scapular angle. The emargination noticeable in the specimen at this point is due to crushing from contact with the head of the humerus while in the matrix.
The femur is well preserved, though somewhat compressed anteroposteriorly. (Figs. I and 2, Plate LX XIV.) Regardless of its great length, this bone is almost as stout in the shaft as that of Apatosaurus, though the articular ends are proportionally less expanded. 'The lateral surface of the shaft has a prominent convexity one-fourth of its entire length below the great trochanter. A marked rugosity probably for the insertion of one of the gluteal muscles, extends down-ward from the great trochanter to this point. As in all opisthocoelians the fourth trochanter forms a rugose prominence on the posterior internal margin of the shaft above the middle.
RELATIONSHIPS
In proposing the genus Brachiosaurus from the characters presented by the leg bones and sacral centra of this specimen the writer had some doubt as to the possible relations with the imperfectly described genus Camarasaurus. However, a recent examination, through the courtesy of Dr. Osborn, of the type specimen as it is being prepared for exhibition in the American Museum, at once set at rest all such doubts. The massive structure of the vertebrae and the distinctly Morosaur-like spines of that genus have nothing in common with the slenderly constructed and elongate centrum and the single median spines of Brachiosaurus. Relationship with the tall and slender posterior dorsal vertebra which constitutes. the type of Amphicoelias is equally impossible.
Of all the American opisthocoelia the only known form which may be regarded as closely related to this genus is Haplocanthosaurus, recently described* by Hatcher. These two genera display certain structural affinities common to no other North American group.
* Memoirs of the Carnegie Museum vol. ii. No.1.
The most important of these are: (I) the single median spines which persist throughout the vertebral column in the above named genus and which appear to have been equally persistent in Brachiosaurus; (2) the unusual breadth of the sacrum in comparison with its length and the height of its component vertebrae; (3)(4) the great expansion of the preacetabular portion of the ilium. In addition to these, are to be considered the presence of fourteen dorsal vertebrae as described by Hatcher in Haplocanthosaurus, and the unusual length of the humerus in Brachiosaurus. Recent developments with regard to the number of vertebrae in Diplodocus+and in ApatosaurusY might throw some doubt upon this point were it not that the genera under consideration evidently belong to a distinct phylogenetic line, and that Hatcher has made a strong case in favor of fourteen dorsal vertebrae in his genus. The vertebral formula in Brachiosaurus cannot, of course, be determined from the interrupted series preserved in the type specimen. It is most probable that the number will prove to be the same as in Haplocanthosaurus, although the elongation of the dorsal centra might suggest that a reduction had taken place in this form.
Of the two genera, it will be observed that Haplocanthosaurus is much the smaller and comparatively primitive, while Brachiosaurus is a long-limbed and highly specialized type. Points of generic difference are abundant. The first and most evident of these is one of size, which is well indicated by the comparative length of femora. That of Haplocanthosaurus measures about fifty inches, while in Brachiosaurus the same bone measures fully eighty. More important differences are to be found in the relative height of the neural arches, the development of the hyposphene-hypantrum articulation, and the length of the centra in the dorsal vertebrae.
In the smaller genus the vertebral pedicles are peculiarly attenuate, giving to the neural arch the extraordinary elevation pointed out by Hatcher, and to the neural canal the unusually great vertical ~ diameter. In the larger and more highly specialized Brachiosaurus there is no evidence whatever of this characteristic, The pedicles ~ are broad antero-posteriorly, narrow and rounded on the posterior ~ margin instead of being produced into an angle, as in the smaller form, and the neural canal is but little deeper than wide.
It will be observed that in Haplocanthosaurus the zygapophyses are about as strong as those of Morosaurus, and extend a like distance from the median line. Their articular surfaces are plane and directed slightly upward as well as laterally. The hyposphene has a moderate development. In Brachiosaurus on the other hand, there is a marked crowding together of the zygapophyses at the median line, especially in the mid-dorsal region, and an unusual development of the hyposphene-hypantrum articulation. The articular surfaces of the zygapophyses are directed laterally and are slightly curved, so that the postzygapophyses embrace the anterior pair. '[he centra of the dorsal vertebrae in the larger genus are, with the exception of the last, much more elongate and more thinly walled than in the smaller form. The sacral ribs also show a much higher degree of specialization. In Haplocanthosaurus they arise uniformly from the anterior end of the centra and show but little variation in strength throughout the series. In the larger animal, however, the second pair are greatly expanded, as. has been described above, while the third pair are correspondingly reduced in size. There is also a much greater relative expansion of the thoracic cavity, as is shown by the comparative length of the anterior ribs.
The neural spines of Brachiosaurus, so far as preserved, present the same type of structure as do those of Haplocanthosaurus, but while those of the latter genus reach their greatest length at the third presacral and begin to diminish with he sixth, those of the former increase regularly to the seventh. There is also a more pronounced shortening of the neural Spines and transverse processes of the anterior caudals in Brachiosaurus, indicating a reduction of the caudal series, consistent with the cephalad specialization of this form.
In most of the above noted differences, Haplocanthosaurus presents a more primitive structure, from which Brachiosaurus might well be derived. However, the elevation of the neural arches and the upward direction of the diapophyses in the smaller form present a degree of specialization which precludes this possibility. A very similar condition exists in Stegosaurus, probably as an adaptation to the support of its dermal armor.
+Memoirs the Carnegie Museum. vol. i, No.1, p. 19. YThis publication, GEOL. SER., vol. ii, No.4. p.196.
While it is hardly possible that such an armor could have existed in this form without having been preserved in one or the other of the two Specimens described by Hatcher, the elongation may have resulted from a like muscular development, or it may be due to acquired aquatic habits, as such a tendency is found in certain cetaceans. To assume that the dorsal pedicles had attained such an elevation and were again reduced to the low, broad structure found in Brachiosaurus, would be inconsistent with all laws of organic development. We must, therefore, conclude that the two forms have arisen from a common ancestry, probably somewhere in the lowermost Jurassic, and that while the Haplocanthosaurus phylum remained in general conservative, it reached a high degree of specialization in this one particular. Brachiosaurus, on the other hand, is a somewhat later representative of the more highly specialized line which had taken to purely terrestrial habits.
These conclusions are in accord with the evidence to be derived from the relative age of the horizons in which these fossils are found. While there is little reliable stratigraphic data for comparison between the eastern and western Colorado localities, the evidence which has been adduced would indicate that Brachiosaurus comes from a horizon at least one hundred feet above the Canon City quarry. However, various species of Apatosaurus and Morosaurus are found in the same horizons with both forms, so that the difference in time alone is not sufficient to account for the differences between these two related forms. We must, therefore, v attribute these differences chiefly to digressive development.
PROBABLE HABITS.
The habits of the Opisthocoelia have been regarded as semi aquatic or at least marsh-dwelling. This' conception was based largely upon the structure of the teeth, which are fitted for masticating soft, succulent vegetation, and upon the ponderous bulk of the animals which seemed best suited for aquatic locomotion. It is true that the massive structure of the vertebrae in such forms as Apatosaurus ajax bears some resemblance's to the vertebrae in cetaceans. However, the writer has failed to discover in the skeletal structure of aquatic or semi-aquatic animals, either reptilian or mammalian, any of that fluting and hollowing of the vertebrae which have been interpreted as evidence of aquatic habits in the Opisthocoelia. This same effort at lightening the skeleton was regarded by an earlier English writer as evidence of aerial habits!
That which appears to the present writer to afford most reliable evidence as to habits is the structure of foot and limb. As pointed out by Hatcher, there no evidence among opisthocoelians of that shortening or angulation of limb, or the broadening of foot, which is common to amphibious animals. Nor is there anything in the structure of the opisthocoelians which is not found in some terrestrial forms. The straight hind leg occurs in quadrupeds only among forms which inhabit the uplands. Familiar instances of development of this character are found in the proboscidea, dinocerata, etc. The short, stout metapodials and blunted phlanges characteristic of the Opisthocoelia would be as ill adapted for propulsion in water or upon marsh lands as are those of the elephant. The reduction in the number of claws offers further evidence along this line, and finds a parallel in the great ground sloths. In short, if the foot structure of these animals indicates anything, it indicates specialization for terrestrial locomotion.
In the genus Brachiosaurus there are very different proportions of fore and hind leg, of pelvis and thorax and tail , from those of the usual type of opisthocoelian dinosaurs. To harmonize with them we may expect that decided differences existed in the head and neck. The foot structure in Apatosaurus and Diplodocus may, as before stated, be regarded as highly specialized, but the length and slenderness of limb, the deep thorax, the broad sacrum, the expanded ilium. and the abbreviated tail of Brachiosaurus all point to a greater agility and a much better adaptation to terrestrial habits than is found in any other representative of the Opisthocoelia.
The varying structure of the dorsal vertebrae in the larger dinosaurs offers some interesting problems as to the habits of the animals. In some genera a relation between the length of the dorsal spines and the breadth of the zygapophyses is noticeable. We may well assume, with other writers, that the heavier forms, such as Apatosaurus and Diplodocus, which are provided with long spines in the sacral and posterior dorsal region, were adapted to rearing up on the hind legs as is represented in the conventional mounted skeleton of Megatherium. In these forms we find that the body is short and therefore well adapted to this habit. Morosaurus and Camarasaurus are less perfectly adapted to such a habit by reason of their shorter spines, but might well have been capable of assuming the upright position. The slender shaft of the ischium in the former genus could have hardly afforded sufficient support in the sitting posture. In all of these, however, it will be observed that the zygapophyses are strong and placed far apart, especially in the anterior dorsal region of Apatosaurus. Their separation is doubtless directly due to the bifurcate spines, but that arrangement was a concomitant of the development of a median set of dorsal muscles whose function it was to elevate the anterior portion of the body. The unusually strong zygapophyses served to support and control the ponderous shoulders and neck when the upright position was assumed. All of this bespeaks great mobility of the anterior portion of the body.
In Haplocanthosaurus we have noticed that the spines are short posteriorly, but reach their greatest length at the middle of the dorsal series. Together with this arrangement we find a more generalized structure of the zygapophyses and hyposphene, such as was noticed in Morosaurus. The former are moderately broad, but are nowhere placed far apart; the latter has a slightly greater development than in the above mentioned genus. In Brachiosaurus, the zygapophyses reach the extreme reduction and the hyposphene-hypantrum articulation reaches the extreme development. The zygapophyses are crowded together near the median line in a way which would render impossible any considerable lateral movement of the body. The hyposphene articulation is well constructed to prevent lateral displacement. The strong thoracic ribs with widely divergent head and tubercle firmly anchored in the same horizontal plane doubtless added greatly to the strength' of the vertebral column in this region. In this we recognize the rigid quadrupedal structure of the vertebral column.
From the above considerations it will be seen that Brachiosaurus is the culminating type of a phylum, distinct from anything hitherto known in America. Its length of limb and bodily proportions show that it was fitted fur purely quadrupedal movements. Though the structure of the feet and lower legs is unknown, there is every reason for believing that the animal was specialized in terrestrial habits. This fact would restrict its range to grounds where conditions were less favorable for its preservation and so account for its remaining so long undiscovered.
SUMMARY.
The following conclusions are reached in this paper: 1. The genus Brachiosaurus, together with the smaller related form, Haplocanthosaurus. should be placed in a new family of the optisthocoelia for which the name Brachiosauridae is here proposed.
2. The distinguishing characters of this family, so far as known, are: Humerus as long as femur; neural spines of the vertebrae simple; dorsal vertebrae numbering more than ten.
3. Brachiosaurus is a highly specialized form, related to, but not directly descending from, Haplocanthosaurus.
4. The known characters which distinguish Brachiosaurus are: Its large size, immense thorax, highly specialized hyposphene-hypantrum articulation of the vertebrae, low, broad structure of the vertebral pedicles. and the unusual breadth of the sacrum
5. While Morosaurus, Apatosaurus, and Diplodocus are, by reason of their elevated sacral spines and strong dorsal vertebrae, fitted for assuming the upright position, the short sacral spines and heavy fore legs of Brachiosaurus indicate that its habits of locomotion were entirely quadrupedal.
6. The foot structure in most opisthocoelians furnishes unmistakable evidence of terrestrial habits. The length of fore limb and bodily proportions of Brachiosaurus show that this form was more highly specialized for terrestrial habits than any other known member of this order.
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