Diplodocus
Diplodocus is one of the best known sauropod dinosaur genus, which lived in North America in the Late Jurassic Period. Due to its relatively abundant fossil remains, it is one of the most studied dinosaurs. Its fossils were first discovered in the Morrison Formation in 1877 by S. W. Williston and B. Mudge, and was scientifically described as Diplodocus longus by Othniel Charles Marsh in 1878[1]. However, this species is considered as a Nomen dubium, due to its fragmentary fossil remains.
The name Diplodocus refers to the double-beamed chevron bones, which are located in the underside of its tail - originally it was thought that these structures are present only in this dinosaur, but later it was discovered in further diplodocid and in non-diplodocid sauropoda. The most completely known species of the genus is Diplodocus carnegii, named after Andrew Carnegie. Besides D. carnegii, there is an other valid species of the genus, D. hallorum.
Modern depiction of Diplodocus sp. Creator:Dmitry Bogdanov - dmitrchel@mail.ru, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=5358923
Known occurrences of Diplodocus fossils.
Description and anatomy
Diplodocus was a large quadrupedal dinosaur, with very long neck, and long, whip-like tail. It had stronger and larger hind legs than its forelegs, and this resulted a horizontal posture for the animal. As of 2021, Diplodocus carnegii belongs to the longest known dinosaurs with its 24 meters length. The weight of the animal was estimated to be about 11-14.8 tonnes[2]. The other species of the genus, Diplodocus hallorum, discovered in 1991 by David Gillette[3], may have reached even 32 meters in length, and it was the longest known dinosaur[4].
The tail of Diplodocus was quite long, and its skeletal structure was built by approximately 80 caudal vertebrae[5] - this is almost twice as much as in case of some other earlier sauropods. Such long tail was needed to counterbalance the long neck of the animal, which was composed of about 15 vertebrae. The tail might have further functions, e.g. in the defence against predators[6], or making noise like cracking a whip[7]. Diplodocus had only one claw on each of its forelegs. This claw was detached from the bones, flattened on its sides, and relatively large compared to other sauropods, but its exact function is still unknown[8].
The skull of Diplodocus is not known, but we have a perception about it based on the known skulls of other diplodocid dinosaurs which are closely related to Diplodocus. These dinosaurs had relatively small skull compared to their body size, and the braincase of the animal was also small. They had teeth only in the anterior part of their jaws, and these theeth were small and peg-like in their form[9]. The animal probably held its neck parallel to the ground surface, and due to its structure, it was not possible to elevate the neck much from horizontal line[10]. In 1990 skin impressions of diplodocid dinosaurs were discovered, which showed that these dinosaurs had narrow, long and pinted keratinous spines, just like modern iguanas, on the dorsal side of their tail and neck, and also on their back[11].
Diplodocus sp. skeleton. Creator: Valdiney Pimenta - Source: https://www.flickr.com/photos/valdiney/4086241221 - License: CC BY 2.0, https://creativecommons.org/licenses/by/2.0/
Paleobiology
Their scleral rings suggest that Diplodocus was probably a cathemeral animal, which means that it was active for short certain intervals throughout the day[12]. Earlier it was supposed that Diplodocus and other large sauropods spent most of their time in water - this hipothesys was based on the fact that their nasal openings were positioned at the apex of the cranium[13]. However, later it was indicated that these dinosaurs probably could not breath when their body was submerged in water, due to the enormous water pressure on their chest wall[14]. Since the 1970s there is a general consensus in science that these large sauropods were terrestrial beings[15]. Presumably, Diplodocus and other sauropods had an avian-like respiratory system, with great pneumacity, which allowed a much more efficient respiration than in case of reptiles and mammals[16].
Diplodocus was first thought to have a sprawling posture, with lizard-like legs, as its first depiction by Oliver P. Hay shows[17]. Later our perception regarding to the posture of the animal went trough several changes. After the rejection of Hay's theory, Diplodocus was depicted with its neck held upwards, making possible for the animal to graze the foliage of trees. However, subsequent studies concluded that the basic position of the neck must have been close to horizontal[18]. An other recent study, which infers the neck posture of dinosaurs from extant animals, suggests that sauropods could have used their neck in a much more flexible way, and probably held it at a more elevated position than previous studies suggested[19]. Nevertheless, a near-horizontal posture would have been more favourable considering the blood-supply of the head[20].
Earlier depiction of Diplodocus, with elevated neck posture. Creator: James St. John - Source: https://www.flickr.com/photos/jsjgeology/15396161892 - License: CC BY 2.0, https://creativecommons.org/licenses/by/2.0/
Inferred from their highly unusual teeth and teeth arrangement, Diplodocus and other diplodocines probably had quite different way of feeding than other sauropods[21]. Presumably, this mechanism was unilateral branch-stripping, which would be supported by the teeth arrangement[22]. During this process the teeth of one jaw were to strip the foliage from the branch, while the teeth of the other jaw were to support the branch. The elongated anterior part of the skull also could have contributed to this. Diplodocine sauropods had several replacement teeth for all teeth sockets, which means that they probably replaced their teeth after deteriorating.
Having its center of mass near to the hip, Diplodocus was probably able to chage to bipedal posture (which could have been actually tripodal, if the animal used its tail for support against the ground), which would allow this dinosaur to feed tree foliage from even higher[23]. Some researchers hypothesize that the length and flexibility of its neck allowed Diplodocus to graze plants from ground-level, and maybe even aquatic plants from low underwater areas[18]. The skull of a young Diplodocus was found in 2010, which differs in several morphological traits from the skull of adult diplodocid dinosaurs. This sugeests that the adult and juvenile diplodocids might have had different feeding strategies[24].
Skull of diplodocid dinosaur. By Ryan Somma - Diplodocus longusUploaded by FunkMonk, CC BY-SA 2.0, https://commons.wikimedia.org/w/index.php?curid=8671979
Diplodocus had very small eggs compared to its body size, which required shorter incubation time and this was more favourable against predation risk[25]. Just like other sauropods, Diplodocus probably had a very fast growing rate, and could reach adulthood just in some more than ten years, and then grow continuously farther[26].
The paleoenvironment in which Diplodocus lived probably was a semiarid biotope having at least two (wet and dry) seasons. Inferred from geological data, that environment consisted of lakes, swampy lowlands, floodplains and river channels, covering the eastern drainage basins of the young Rocky Mountains. The vegetation was also very diverse, including cycades, gingkoes and conifers - there were gallery forests composed mostly by tree ferns and fern savannas with a few straggling trees just as Araucaria-like conifers.
Scientific references
[1] Marsh, O.C. (1878): Principal characters of American Jurassic dinosaurs. Part I. American Journal of Science. 3 (95): 411-416. doi:10.2475/ajs.s3-16.95.411.
[2] Paul, Gregory S. (2016): Princeton Field Guide to Dinosaurs. Princeton University Press. ISBN 978-0-691-13720-9.
[3] Gillette, D.D. (1994): Seismosaurus: The Earth Shaker. New York, Columbia University Press, 205 pp
[4] Herne, Matthew C.; Lucas, Spencer G. (2006): Seismosaurus hallorum: Osteological reconstruction from the holotype. New Mexico Museum of Natural History and Science Bulletin. 36.
[5] Wilson, J.A. (2005): Overview of Sauropod Phylogeny and Evolution. In Rogers KA, Wilson JA (eds.). The Sauropods:Evolution and Paleobiology. Indiana University Press. pp. 15-49. ISBN 978-0-520-24623-2.
[6] Holland, W.J. (1915): Heads and Tails: a few notes relating to the structure of sauropod dinosaurs. Annals of the Carnegie Museum. 9: 273-278.
[7] Myhrvold, N.P.; Currie, P.J. (1997): Supersonic sauropods? Tail dynamics in the diplodocids. Paleobiology. 23 (4): 393-409. doi:10.1017/s0094837300019801.
[8] Bonnan, M. F. (2003): The evolution of manus shape in sauropod dinosaurs: implications for functional morphology, forelimb orientation, and phylogeny. Journal of Vertebrate Paleontology. 23 (3): 595-613. doi:10.1671/A1108.
[9] Upchurch, P.; Barrett, P.M. (2000): The evolution of sauropod feeding mechanism. In Sues, Hans Dieter (ed.). Evolution of Herbivory in Terrestrial Vertebrates. Cambridge University Press. ISBN 978-0-521-59449-3.
[10] Tschopp, E.; Mateus, O. V. (2012): The skull and neck of a new flagellicaudatan sauropod from the Morrison Formation and its implication for the evolution and ontogeny of diplodocid dinosaurs. Journal of Systematic Palaeontology. 11 (7): 1. doi:10.1080/14772019.2012.746589.
[11] Czerkas, S. A. (1993): Discovery of dermal spines reveals a new look for sauropod dinosaurs. Geology. 20 (12): 1068-1070. doi:10.1130/0091-7613(1992)020<1068:dodsra>2.3.co;2.
[12] Schmitz, L.; Motani, R. (2011): Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology. Science. 332 (6030): 705-8. doi:10.1126/science.1200043.
[13] Hatcher, J.B. (1901): Diplodocus (Marsh): Its osteology, taxonomy, and probable habits, with a restoration of the skeleton. Memoirs of the Carnegie Museum, vol. 1, pp. 1-63
[14] Kermack, Kenneth A. (1951): A note on the habits of sauropods. Annals and Magazine of Natural Historyä. 12 (4): 830-832. doi:10.1080/00222935108654213.
[15] Gangewere, J.R. (1999): Diplodocus carnegii Archived 12 January 2012 at the Wayback Machine. Carnegie Magazine.
[16] 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.
[17] Hay, Dr. Oliver P. (1908): On the Habits and Pose of the Sauropod Dinosaurs, especially of Diplodocus. The American Naturalist, Vol. XLII, Oct. 1908.
[18] Stevens K.A., Parrish J.M. (2005): Neck Posture, Dentition and Feeding Strategies in Jurassic Sauropod Dinosaurs. In Carpenter, Kenneth, Tidswell, Virginia (eds.). Thunder Lizards: The Sauropodomorph Dinosaurs. Indiana University Press. pp. 212-232. ISBN 978-0-253-34542-4.
[19] Taylor, M.P.; Wedel, M.J.; Naish, D. (2009): Head and neck posture in sauropod dinosaurs inferred from extant animals. Acta Palaeontologica Polonica. 54 (2): 213-220. doi:10.4202/app.2009.0007.
[20] Stevens, K.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.
[21] Upchurch, P.; Barrett, P.M. (2000): The evolution of sauropod feeding mechanism. In Sues, Hans Dieter (ed.). Evolution of Herbivory in Terrestrial Vertebrates. Cambridge University Press. ISBN 978-0-521-59449-3.
[22] Barrett, P.M.; Upchurch, P. (1994): Feeding mechanisms of Diplodocus. Gaia. 10: 195-204.
[23] Mallison, H. (2011): Rearing Giants - kinetic-dynamic modeling of sauropod bipedal and tripodal poses. In Farlow, J.; Klein, N.; Remes, K.; Gee, C.; Snader, M. (eds.). Biology of the Sauropod Dinosaurs: Understanding the life of giants. Life of the Past. Indiana University Press. ISBN 978-0-253-35508-9.
[24] Whitlock, John A.; Wilson, Jeffrey A.; Lamanna, Matthew C. (2010): Description of a Nearly Complete Juvenile Skull of Diplodocus (Sauropoda: Diplodocoidea) from the Late Jurassic of North America. Journal of Vertebrate Paleontology. 30 (2): 442-457. doi:10.1080/02724631003617647.
[25] Ruxton, Graeme D.; Birchard, Geoffrey F.; Deeming, D Charles (2014): Incubation time as an important influence on egg production and distribution into clutches for sauropod dinosaurs. Paleobiology. 40 (3): 323-330. doi:10.1666/13028.
[26] Sander, P. M.; N. Klein (2005): Developmental plasticity in the life history of a prosauropod dinosaur. Science. 310 (5755): 1800-1802. doi:10.1126/science.1120125.