Brachiosaurus altithorax
- 演化之聲
- 5 days ago
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Age
Jurassic（Kimmeridgian-Tithonian）
154-150 Ma
Taxonomy
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
Clade: Dinosauria
Order: Saurischia
Suborder: Sauropodomorpha
Family: Brachiosauridae
Genus: Brachiosaurus
Species: Brachiosaurus altithorax
Morphological description
Brachiosaurus altithorax was an enormous brachiosaurid sauropod dinosaur. Although its holotype specimen (FMNH P 25107) is incomplete, the preserved material is sufficient to show that it differed from many other sauropods in overall body proportions. Its forelimbs were proportionally long, and the preserved humerus, femur, pelvis, and dorsal vertebrae indicate that the front half of the body was elevated, giving the animal tall shoulders and a strongly sloping back profile. The holotype has been estimated at approximately 22 to 23 meters in total length, with a head height of roughly 11 to 12 meters.
The proportions of its dorsal vertebrae suggest that the trunk region was longer than that of its African close relative, Giraffatitan brancai, with which it has often been conflated. Brachiosaurus altithorax therefore probably did not have the shortened central body seen in some classic reconstructions, but instead possessed a longer and more massive torso. Although the tail is incompletely known, the available evidence suggests that it may also have been relatively longer and taller than is often assumed.
The mid-dorsal ribs of Brachiosaurus altithorax exceeded 2.7 meters in length, indicating an exceptionally broad and deep thoracic cavity. Some of the dorsal ribs preserve internal cavities and pneumatic foramina. At least two ribs exhibit different types of pneumatic features: one preserves a large opening in the shaft, whereas another has a pneumatic foramen near the tuberculum. These features indicate that the ribs were associated with the air-sac system of the respiratory apparatus.
In addition, the coracoid and scapula of the holotype are unfused. This usually suggests that the individual had not yet reached full skeletal maturity, implying that a fully grown adult may have been even larger.
In 2019, the skull specimen USNM 5730 was argued to most likely belong to Brachiosaurus altithorax. The skull was relatively tall and short, with the narial region positioned high on the skull. The upper and lower jaws bore regularly arranged teeth with relatively broad, spoon-shaped or spatulate crowns, rather than the narrow peg-like teeth seen in diplodocids.
Etymology
Genus name Brachiosaurus: from Greek, meaning "arm lizard," in reference to its proportionally elongated forelimbs.
Species name altithorax: meaning "high chest" or "high thorax," used by Elmer Riggs to describe the specimen's immense and elevated thoracic region.
Biological description
Brachiosaurus altithorax is the type species of the genus Brachiosaurus. It was discovered by Elmer Riggs in 1903 in the Colorado River Valley of Colorado, USA, and serves as the defining representative of the genus. However, the popular image of Brachiosaurus known to the public has, for a long time, not actually been based primarily on Brachiosaurus altithorax itself, but rather on the African species Giraffatitan brancai. Because the holotype of Brachiosaurus altithorax was too incomplete to provide a full picture of the animal, the much more complete African fossils discovered between 1909 and 1912 were initially assigned to Brachiosaurus as Brachiosaurus brancai. As a result, the classic image of Brachiosaurus—a towering dinosaur with a long neck and a high, elevated skull profile—came to be modeled largely after the African form. Later research concluded that this African species should be separated into its own genus, Giraffatitan, but by then the classic image of "Brachiosaurus" had already become deeply fixed in the public imagination.
The holotype of Brachiosaurus altithorax consists only of seven dorsal vertebrae, a sacrum, the first two caudal vertebrae, the left coracoid, the right humerus, an ilium, a femur, partial ribs, and some fragmentary additional bones. Although the species is therefore of great scientific importance, visual reconstructions have long depended on better-preserved close relatives to fill in the missing anatomy. This is why many historical restorations of Brachiosaurus incorporated features that may not actually have belonged to Brachiosaurus altithorax itself.
The skull specimen USNM 5730, redescribed in 2019, was originally discovered in Colorado in 1883. At the time of discovery, the skull was not associated with the rest of the skeleton, except for a single cervical vertebra that could not be collected. Because of this, the taxonomic identity of USNM 5730 remained uncertain for a very long time, and it was even incorrectly used at one point as the skull of Brontosaurus. Only after renewed preparation, reassembly, CT scanning, and comparative anatomical study did researchers conclude that this skull was most likely referable to Brachiosaurus altithorax. Based on this skull, they were able to estimate the tooth replacement rate of Brachiosaurus altithorax, which may have been slower than that of contemporaneous genera such as Camarasaurus and Diplodocus. Tooth replacement rates are often related to diet, feeding behavior, and tooth wear, and this broader tooth morphology has generally been interpreted as being better suited for processing relatively coarse and tougher plant material.
Brachiosaurus altithorax lived during the Late Jurassic, approximately from the Kimmeridgian to the Tithonian, a time when giant sauropods flourished in North America. In terms of geographic distribution, although brachiosaurids have traditionally been regarded as relatively rare within the Morrison Formation, later research has suggested that their remains may actually have been distributed across a fairly broad area. In addition to the holotype from Colorado, other material that may belong to brachiosaurids, and perhaps even to Brachiosaurus altithorax, has also been reported farther north in the same formation, including in Wyoming. A 2018 study of giant pedal material from Wyoming suggested that these fossils represent one of the northernmost known brachiosaurid occurrences in the Morrison Formation. If these remains are ultimately shown to belong to Brachiosaurus altithorax, then the species may have ranged across a considerable latitudinal span in western North America during the Late Jurassic. The relative scarcity of its fossils may also reflect its enormous body size and ecological role as a large, low-density herbivore.
References
D'Emic MD and Carrano MT. (2019). Redescription of Brachiosaurid Sauropod Dinosaur Material From the Upper Jurassic Morrison Formation, Colorado, USA. American Association for Anatomy.
Maltese A et al. (2018). The real Bigfoot: a pes from Wyoming, USA is the largest sauropod pes ever reported and the northern-most occurrence of brachiosaurids in the Upper Jurassic Morrison Formation. PeerJ.
Taylor MP. (2010). A re-evaluation of Brachiosaurus altithorax Riggs 1903 (Dinosauria, Sauropoda) and its generic separation from Giraffatitan brancai (Janensch 1914). Journal of Vertebrate Paleontology.
Taylor MP and Wedel MJ. (2023). Novel pneumatic features in the ribs of the sauropod dinosaur Brachiosaurus altithorax. Acta Palaeontologica Polonica.
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