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{{Use mdy dates|date=January 2024}}
{{Automatic taxobox
| fossil_range = [[Late Jurassic]] ([[Kimmeridgian]] to [[Tithonian]]), {{Geological range|152|
| image = Louisae.jpg
| image_caption = Mounted ''A. louisae'' (specimen CM 3018), [[Carnegie Museum of Natural History]]
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=== Initial discovery ===
[[File:Arthur Lakes illustration of Apatosaurus ajax and Atlantosaurus montanus at Morrison, Colorado.jpg|left|thumb|[[Arthur Lakes]]' painting of YPM crews excavating fossils of ''Apatosaurus ajax'' at Quarry 10 in Morrison.]]
The first ''Apatosaurus'' fossils were discovered by Arthur Lakes, a local miner, and his friend Henry C. Beckwith in the spring of 1877 in Morrison, a town in the eastern foothills of the [[Rocky Mountains]] in [[Jefferson County, Colorado]]. Arthur Lakes wrote to [[Othniel Charles Marsh]], Professor of [[Paleontology]] at [[Yale University]], and [[Edward Drinker Cope]], a paleontologist based in Philadelphia, about the discovery until eventually collecting several fossils and sending them to both paleontologists. Marsh named ''Atlantosaurus montanus'' based on some of the fossils sent and hired Lakes to collect the rest of the material at Morrison and send it to Yale, while Cope attempted to hire Lakes as well but was rejected.<ref name=":1">Kohl, M. F., & McIntosh, J. S. 1997, Discovering Dinosaurs in the Old West: The field journals of Arthur Lakes.</ref> One of the best specimens collected by Lakes in 1877 was a well preserved partial postcranial skeleton, including many vertebrae, and a partial braincase ([[Peabody Museum of Natural History|YPM]] VP 1860), which was sent to Marsh and named ''Apatosaurus ajax'' in November 1877.<ref name=":0" /><ref name=":1" /> The composite term ''Apatosaurus'' comes from the [[Ancient Greek|Greek]] words ''{{lang|grc-Latn|apatē}}'' ({{lang|grc|ἀπάτη}})/''{{lang|grc-Latn|apatēlos}}'' ({{lang|grc|ἀπατηλός}}) meaning "deception"/"deceptive", and ''{{lang|grc-Latn|sauros}}'' ({{lang|grc|[[wikt:σαῦρος|σαῦρος]]}}) meaning "lizard";<ref name="liddell" /> thus, "deceptive lizard". Marsh gave it this name based on the [[chevron (anatomy)|chevron]] bones, which are dissimilar to those of other dinosaurs; instead, the chevron bones of ''Apatosaurus'' showed similarities with those of [[mosasaur]]s,<ref name="marsh1877" /><ref name="Holtz2008" /> most likely that of the representative species ''[[Mosasaurus]]''. By the end of excavations at Lakes' quarry in Morrison, several partial specimens of ''Apatosaurus'' had been collected, but only the type specimen of ''A. ajax'' can be confidently referred to the species.<ref>{{Cite web |last=Marsh |first=O.T. |title=Apatosaurus ajax?; YPM VP 004833; North America; USA; Colorado; Jefferson County; Arthur Lakes |url=https://collections.peabody.yale.edu/search/Record/YPM-VP-004833 |access-date=March 11, 2022 |website=collections.peabody.yale.edu |language=en}}</ref><ref name=":0">{{Cite journal |last1=Tschopp |first1=Emanuel |last2=Mateus |first2=Octávio |last3=Benson |first3=Roger B. J. |date=April 7, 2015 |title=A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda) |journal=PeerJ |language=en |volume=3 |pages=e857 |doi=10.7717/peerj.857 |issn=2167-8359 |pmc=4393826 |pmid=25870766 |doi-access=free }}</ref>
During excavation and transportation, the bones of the holotype skeleton were mixed with those of another Apatosaurine individual originally described as ''Atlantosaurus immanis''; as a consequence, some elements cannot be ascribed to either specimen with confidence.<ref name="TMB2015" /> Marsh distinguished the new genus ''Apatosaurus'' from ''[[Atlantosaurus]]'' on the basis of the number of sacral vertebrae, with ''Apatosaurus'' possessing three and ''Atlantosaurus'' four. Recent research shows that traits usually used to distinguish taxa at this time were actually widespread across several taxa, causing many of the taxa named to be invalid, like ''Atlantosaurus''.<ref name=":0" /> Two years later, Marsh announced the discovery of a larger and more complete specimen (YPM VP 1980) from [[Como Bluff]], [[Wyoming]], he gave this specimen the name ''Brontosaurus excelsus''.<ref name="marsh1879" /> Also at Como Bluff, the Hubbell brothers working for Edward Drinker Cope collected a tibia, fibula, scapula, and several caudal vertebrae along with other fragments belonging to ''Apatosaurus'' in 1877–78 at Cope's Quarry 5 at the site.<ref>{{Cite web |date=2007 |title=Apatosaurus sp. AMNH FR 5755 |url=http://research.amnh.org/paleontology/search.php?action=detail&specimen_id=48563}}</ref> Later in 1884, Othniel Marsh named ''Diplodocus lacustris'' based on a chimeric partial dentary, snout, and several teeth collected by Lakes in 1877 at Morrison.<ref name=":0" /><ref name=":2">Mossbrucker, M. T., & Bakker, R. T. (October 2013). Missing muzzle found: new skull material referrable to Apatosaurus ajax (Marsh 1877) from the Morrison Formation of Morrison, Colorado. In ''Geological Society of America Abstracts with Programs'' (Vol. 45, p. 111).</ref> In 2013, it was suggested that the dentary of ''D. lacustris'' and its teeth were actually from ''Apatosaurus ajax'' based on its proximity to the type braincase of ''A. ajax''.<ref name=":2" /> All specimens currently considered ''Apatosaurus'' were from the [[Morrison Formation]], the location of the excavations of Marsh and Cope.<ref name="OM06" />[[File:Apatosaurus ajax sacrum.jpg|thumb|''A. ajax'' sacrum, illustrated in 1879]]
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While most other museums were using cast or sculpted ''Camarasaurus'' skulls on ''Apatosaurus'' mounts, the Yale Peabody Museum decided to sculpt a skull based on the lower jaw of a ''Camarasaurus'', with the cranium based on Marsh's 1891 illustration of the skull. The skull also included forward-pointing nasals{{snd}}something unusual for any dinosaur{{snd}}and fenestrae differing from both the drawing and other skulls.<ref name="camarasaurusbully" />
[[File:Apatosaurus louisae side (Morrison Formation, Upper Jurassic; Carnegie Quarry, Dinosaur National Monument, northeastern Utah, USA).jpg|thumb|left|Side view of ''A. louisae'' CM{{nbsp}}3018 mounted with a cast of skull CM{{nbsp}}11162]]
No ''Apatosaurus'' skull was mentioned in literature until the 1970s when [[John Stanton McIntosh]] and David Berman redescribed the skulls of ''Diplodocus'' and ''Apatosaurus''. They found that though he never published his opinion, Holland was almost certainly correct, that ''Apatosaurus'' had a ''Diplodocus''-like skull. According to them, many skulls long thought to pertain to ''Diplodocus'' might instead be those of ''Apatosaurus''. They reassigned multiple skulls to ''Apatosaurus'' based on associated and closely associated vertebrae. Even though they supported Holland, it was noted that ''Apatosaurus'' might have possessed a ''Camarasaurus''-like skull, based on a disarticulated ''Camarasaurus''-like tooth found at the precise site where an ''Apatosaurus'' specimen was found years before.<ref name="mcintosh&berman1975" /> On October{{nbsp}}20, 1979, after the publications by McIntosh and Berman, the first true skull of ''Apatosaurus'' was mounted on a skeleton in a museum, that of the Carnegie.<ref name="parsons" /> In 1998, it was suggested that the Felch Quarry skull that Marsh had included in his 1896 skeletal restoration instead belonged to ''[[Brachiosaurus]]''.<ref name="carpenter98" /> This was supported in 2020 with a redescription of the [[Brachiosauridae|brachiosaurid]] material found at the Felch Quarry
=== Recent discoveries and reassessment ===
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[[File:Apatosaurus caudal vertebrae.png|thumb|Tail vertebrae of specimen FMNH P25112, showing pneumatic fossae (holes)]]
Given the large body mass and long neck of sauropods like ''Apatosaurus'', physiologists have encountered problems determining how these animals breathed. Beginning with the assumption that, like [[crocodilia]]ns, ''Apatosaurus'' did not have a [[thoracic diaphragm|diaphragm]], the [[dead space (physiology)|dead-space volume]] (the amount of unused air remaining in the mouth, trachea, and air tubes after each breath) has been estimated at
On this basis, its respiratory system would likely have been [[parabronchi]], with multiple pulmonary air sacs as in [[avian lungs]], and a flow-through lung. An avian respiratory system would need a lung volume of about {{convert|600|L|m3|order=flip|abbr=on}} compared with a mammalian requirement of {{convert|2,950|L|m3|order=flip|abbr=on}}, which would exceed the space available. The overall thoracic volume of ''Apatosaurus'' has been estimated at {{convert|1,700|L|m3|order=flip|abbr=on}}, allowing for a {{convert|500|L|m3|order=flip|abbr=on}}, four-chambered heart and a {{convert|900|L|m3|order=flip|abbr=on}} lung capacity. That would allow about {{convert|300|L|m3|order=flip|abbr=on}} for the necessary tissue.<ref name="paladinoetal1997"/> Evidence for the avian system in ''Apatosaurus'' and other sauropods is also present in the [[Skeletal pneumaticity|pneumaticity]] of the vertebrae. Though this plays a role in reducing the weight of the animal, Wedel (2003) states they are also likely connected to air sacs, as in birds.<ref name="Wedel 2003"/>
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[[Category:Paleontology in Wyoming]]
[[Category:Taxa named by Othniel Charles Marsh]]
[[Category:
[[Category:Multispecific
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