Brontosaurus

Brontosaurus is a sauropod dinosaur genus, a member of the family Diplodocidae, and it lived during the Late Jurassic epoch. It was a huge, quadrupedal herbivorous dinosaur, with a long neck and small head, a whip-like tail and with heavy body-built. All three Brontosaurus species which are currently considered to be valid was found in the Morrisson-formation, North-America. Brontosaurus is one of the best know dinosaurs by the greater public. The name Brontosaurus means "thunder lizard".

Depiction of Brontosaurus excelsus. By Tom Parker - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=40687300

Known occurrences of Brontosaurus fossils.

Discovery and history

The first Brontosaurus species, B. excelsus, was discovered and described by Otheniel Charles Marsh based on a nearly complete skeleton found in the Morrisson Formation, at Como Bluff, Wyoming, in 1979^[1]. In 1903 Elmer Riggs concluded that Brontosaurus was a junior synonym of Apatosaurus, and created the new combination Apatosaurus excelsus^[2], and the vast majority of 20^th century paleontologists agreed this view.

In contrast with this, the first ever mounted sauropod skeleton in the American Museum of Natural History was labeled as Brontosaurus by Henry Fairfield Osborn in 1905. Nevertheless, at this time the skull of Brontosaurus was not known, and the mounted one was largely based on the skull of Camarasaurus. The skull type of Brontosaurus remained question for long. Despite the former synonymisation, Robert T. Baker argued during the 1990s that Brontosaurus is different enough from Apatosaurus to be regarded as a separate genus^[3]. Later, in 2015, a detailed study based on statistical methods on diplodocid relationship also supported this view^[4].

Mounted skeleton of Brontosaurus parvus. By Rob DiCaterino - https://www.flickr.com/photos/goodrob13/24419721124/, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=47044512

Description

Brontosaurus was a large and robust quadrupedal dinosaur with long neck and a long and whip-like tail. Its forelegs were slightly shorter than its hindlegs. Brontosaurus excelsus weighed approximately 17 tons and was up to 22 meters long. We still do not know the skull of this dinosaur, but it was likely very similar to the skull of Apatosaurus, which was phylogenetically closely related to Brontosaurus. Based on the deeply bifurcated neck vertebrae, Brontosaurus probably had a deep and wide neck. The cervical vertebrae were quite stout compared to other diplodocid dinosaurs. Dorsal ribs jointed to the dorsal vertebrae with loose articulations^[5].

There were an extensive air-sack system in the muscular neck, which made it much lighter than it looked like. Inferred from the rapidly decreasing height of spines backwards from the hip, Brontosaurus probably had a tail which was even more slender than in other diplodocids, and its last proportion formed a whip-like structure. Their ribs were also unusual between diplodocids - these were longer, compared to other related species, which likely resulted a quite deep chest^[6]. Brontosaurus had robust, trunk-like legs, and possessed one large claw on each forelimbs, and smaller claws on the first three toes of each foot.

Valid species

Brontosaurus excelsus is the type species of the genus, described by marsh in 1979^[1]. An other taxon, B. amplus, has been found to be a junior synonymy of B. excelsus^[4]. As of 2021, these two skeletons are the only ones whic can be attributed doubtfully to Brontosaurus. Both of them have been found in Reed's Quarry 10 of the Morrison Formation, Brushy Basin, Wyoming. They are from the late Kimmeridgian age, and are approximately 152 million years old^[4].

Brontosaurus parvus was described by Peterson and Gilmore in 1902 as Elosaurus^[7]. This taxon was later assigned to Apatosaurus in 1994, and finally to Brontosaurus in 2015. Its type material was found in Sheep Creek Quarry 4 in Albany County, Wyoming. These fossils are of middle Kimmeridgian age. The weight and length of adult Brontosaurus parvus specimens were estimated up to 14 tonnes and up to 22 meters.

Brontosaurus yahnahpin, known only from Bertha Quarry in the lower Morrison Formation, Wyoming, is the oldest between the three species - it is dated to 155 million years ago^[8]. It was originally described as Apatosaurus yahnahpin by James Filla and Patrick Redman in 1994^[9]. Its length was estimated to 21 meters. The specific name is derived from the Lakota word "mah-koo yah-nah-pin", which means "breast necklace". This refers to the pairs of sternal ribs of the type specimen, that resemble the hair pipes which were traditionally worn by the tribe. Later, more fragmentary material also was assigned to this species.

Depiction of Brontosaurus excelsus. By Nobu Tamura email:nobu.tamura@yahoo.com http://spinops.blogspot.com/ - Own work[1], CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=50352921

Paleobiology

Early on after its discovery, Brontosaurus, just like other sauropods, was believed to be too heavy to keep its own body mass on dry land, therefore it was suspected that they might have lived at least partially submerged in water, which would have supported their enormous body weight. However, subsequent scientific result suggest that these dinosaurs were fully adapted to terrestrial lifestyle ^[10]. Originally, Brontosaurus and other diplodocids were depicted with their neck held high upwards, which made them possible to reach the foliage of high growing trees. Later, some studies have found that the neck of these dinosaurs were considerably less flexible than previously believed^[11].

Nevertheless, a more recent study, taking into consideration the effect of cartilage tissue between the neck's vertebrae, suggests that sauropod dinosaurs probably held their neck normally higher, at the maximum possible vertical extension^[12]. Studying sauropod trackways led to the result that these dinosaurs could walk about 20-40 kilometers in a day, and their top speed was probably between 20 and 30 km/h. The use and function of the single claw of the foreleg of sauropods is a matter of scientific debate. It was suggested to have been used for defence against predators, or in feeding, but these soultions seem to be unlikely. The most probable explanation for the use of these structures seems to be that they were for grasping tree trunks when they reared up onto their hindlegs^[13].

The metabolism of Brontosaurus, and that they were warm or cold-blooded, is still an open question. Some researchers suggest that the large body size of sauropods did not made possible for them to maintain high metabolic activity, nor producing enough heat^[14]. However, if these dinosaurs had an avian respiratory system, that would allow them a higher metabolic activity^[15]. A research based on a computer simulation model suggests that diplodocids like Brontosaurus were able to make a whip-like cracking sound with their quite long tail - the volumen of this sound could reach even over 200 decibels^[16]. The Morrison Formation, where Brontosaurus lived, probably was a semiarid environment, having distinct wet and dry seasons. Based on its fossil remains, Brontosaurus seems to have benn a more solitary dinosaur than other species of its era^[17].

Early depiction of Brontosaurus excelsus from 1896. By Biodiversity Heritage Library - n107_w1150, CC BY 2.0, https://commons.wikimedia.org/w/index.php?curid=38001290

Scientific references

[1] Marsh, O.C. (1879): Notice of new Jurassic reptiles. American Journal of Science. 18 (108): 501-505. doi: 10.2475/ajs.s3-18.108.501.

[2] Riggs, E.S. (1903): Structure and Relationships of Opisthocoelian Dinosaurs. Part I, Apatosaurus Marsh. Publications of the Field Columbian Museum Geographical Series. 2 (4): 165-196.

[3] Bakker, R.T. (1998): Dinosaur mid-life crisis: the Jurassic-Cretaceous transition in Wyoming and Colorado. In Lucas, Spencer G.; Kirkland, James I.; Estep, J. W. (eds.). Lower and Middle Cretaceous Terrestrial Ecosystems. 14. New Mexico Museum of Natural History and Science Bulletin. pp. 67-77.

[4] Tschopp, E.; Mateus, O.V.; Benson, R.B.J. (2015): A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ. 3: e857. doi: 10.7717/peerj.857.

[5] Gilmore, C.W. (1936): Osteology of Apatosaurus, with special references to specimens in the Carnegie Museum. Memoirs of the Carnegie Museum, 11 (4): 1-136.

[6] Lovelace, D.M.; Hartman, S.A.; Wahl, W.R. (2007): Morphology of a specimen of Supersaurus (Dinosauria, Sauropoda) from the Morrison Formation of Wyoming, and a re-evaluation of diplodocid phylogeny. Arquivos do Museu Nacional, 65 (4): 527-544.

[7] Peterson, O. and Gilmore, C. (1902): Elosaurus parvus, a new genus and species of Sauropoda. Annual Report Carnegie Museum, 1: 490 - 9.

[8] Turner, C.E. and Peterson, F., (1999): Biostratigraphy of dinosaurs in the Upper Jurassic Morrison Formation of the Western Interior, U.S.A. Pp. 77-114 in Gillette, D. D. (ed.), Vertebrate Paleontology in Utah. Utah Geological Survey Miscellaneous Publication 99-1.

[9] Filla, J.A., Redman, P.D. (1994): Apatosaurus yahnahpin: a preliminary description of a new species of diplodocid dinosaur from the Late Jurassic Morrison Formation of southern Wyoming, the first sauropod found with a complete set of "belly ribs". Wyoming Geological Association, 44th Annual Field Conference Guidebook, 159-178.

[10] Pierson, D.J. (2009): The Physiology of Dinosaurs: Circulatory and Respiratory Function in the Largest Animals Ever to Walk the Earth. Respiratory Care, 54 (7): 887-911. doi: 10.4187/002013209793800286.

[11] Stevens, Kent A.; Parrish, J. M. (1999). "Neck Posture and Feeding Habits of Two Jurassic Sauropod Dinosaurs". Science. 284 (5415): 798-800. doi: 10.1126/science.284.5415.798.

[12] Taylor, M.P. (2014): Quantifying the effect of intervertebral cartilage on neutral posture in the necks of sauropod dinosaurs. PeerJ. 2: e712. doi: 10.7717/peerj.712.

[13] Upchurch, P. (1994): Manus claw function in sauropod dinosaurs. Gaia, 10: 161-171.

[14] Spotila, J.R.; O'Connor, M.P.; Dodson, P. R.; Paladino, F.V. (1991): Hot and cold running dinosaurs. Metabolism, body temperature, and migration. Modern Geology, 16: 203-227.

[15] Wedel, M.J. (2003): Vertebral Pneumaticity, Air Sacs, and the Physiology of Sauropod Dinosaurs. Paleobiology, 29 (2): 243-255. doi: 10.1666/0094-8373(2003)029<0243:vpasat>2.0.co;2.

[16] Zimmer, C. (1997): Dinosaurs in Motion. Discover Magazine.

[17] Dodson, P.; Behrensmeyer, A.K.; Bakker, R.T.; McIntosh, J.S. (1980): Taphonomy and paleoecology of the dinosaur beds of the Jurassic Morrison Formation. Paleobiology, 6 (2): 208-232. doi: 10.1017/S009483730000676X.