Editing Apatosaurus (section)

==Discovery and species==
=== 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]]

=== Second Dinosaur Rush and skull issue ===
After the end of the [[Bone Wars]], many major institutions in the eastern United States were inspired by the depictions and finds by Marsh and Cope to assemble their own dinosaur fossil collections.<ref name=":3">Brinkman, P. D. (2010). ''The second Jurassic dinosaur rush''. University of Chicago Press.</ref> The competition to mount the first sauropod skeleton specifically was the most intense, with the [[American Museum of Natural History]], [[Carnegie Museum of Natural History]], and [[Field Museum of Natural History]] all sending expeditions to the west to find the most complete sauropod specimen,<ref name=":3" /> bring it back to the home institution, and mount it in their fossil halls.<ref name=":3" /> The American Museum of Natural History was the first to launch an expedition,<ref name=":3" /> finding a well preserved skeleton (AMNH 460), which is occasionally assigned to ''Apatosaurus'', is considered nearly complete; only the head, feet, and sections of the tail are missing, and it was the first sauropod skeleton mounted.<ref name="Bakker" /> The specimen was found north of [[Medicine Bow, Wyoming]], in 1898 by [[Walter W. Granger|Walter Granger]], and took the entire summer to extract.<ref name="mathew1905">{{cite journal |last1=Matthew |first1=W.D. |date=1905 |title=The mounted skeleton of ''Brontosaurus'' |url=https://archive.org/stream/p2naturalhistory05ameruoft |journal=The American Museum Journal |volume=5 |issue=2 |pages=63–70}}</ref> To complete the mount, sauropod feet that were discovered at the same quarry and a tail fashioned to appear as Marsh believed it should{{snd}}but which had too few vertebrae{{snd}}were added. In addition, a sculpted model of what the museum thought the skull of this massive creature might look like was made. This was not a delicate skull like that of ''Diplodocus''{{snd}}which was later found to be more accurate{{snd}}but was based on "the biggest, thickest, strongest skull bones, lower jaws and tooth crowns from three different quarries".<ref name="gilmore36" /><ref name="riggs-1903" /><ref name="Bakker" /><ref name="mcintosh&berman1975" /> These skulls were likely those of ''Camarasaurus'', the only other sauropod for which good skull material was known at the time. The mount construction was overseen by Adam Hermann, who failed to find ''Apatosaurus'' skulls. Hermann was forced to sculpt a stand-in skull by hand. Osborn said in a publication that the skull was "largely conjectural and based on that of ''Morosaurus''" (now ''Camarasaurus'').<ref name="camarasaurusbully" />[[File:Sharp lull brontosaurus.jpg|thumb|left|Obsolete mount of an apatosaurine (possibly ''Apatosaurus'') specimen AMNH{{nbsp}}460 with sculpted skull, [[American Museum of Natural History]]]]
In 1903, [[Elmer S. Riggs|Elmer Riggs]] published a study that described a well-preserved skeleton of a diplodocid from the [[Grand Valley (Colorado-Utah)|Grand River Valley]] near [[Fruita, Colorado]], Field Museum of Natural History specimen P25112. Riggs thought that the deposits were similar in age to those of the Como Bluff in Wyoming from which Marsh had described ''Brontosaurus''. Most of the skeleton was found, and after comparison with both ''Brontosaurus'' and ''Apatosaurus ajax'', Riggs realized that the holotype of ''A.{{nbsp}}ajax'' was immature, and thus the features distinguishing the genera were not valid. Since ''Apatosaurus'' was the earlier name, ''Brontosaurus'' should be considered a junior synonym of ''Apatosaurus''. Because of this, Riggs recombined ''Brontosaurus excelsus'' as ''Apatosaurus excelsus''. Based on comparisons with other species proposed to belong to ''Apatosaurus'', Riggs also determined that the Field Columbian Museum specimen was likely most similar to ''A.{{nbsp}}excelsus''.<ref name="riggs-1903"/>

Despite Riggs' publication, [[Henry Fairfield Osborn]], who was a strong opponent of Marsh and his taxa, labeled the ''Apatosaurus'' mount of the [[American Museum of Natural History]] ''Brontosaurus''.<ref name="camarasaurusbully"/><ref name="parsons"/> Because of this decision the name ''Brontosaurus'' was commonly used outside of scientific literature for what Riggs considered ''Apatosaurus'', and the museum's popularity meant that ''Brontosaurus'' became one of the best known dinosaurs, even though it was invalid throughout nearly all of the 20th and early 21st centuries.<ref name="crafton82">{{cite book|last1=Crafton|first1=D.C.|title=Before Mickey: The Animated Film 1898–1928|publisher=MIT Press|year=1982|isbn=978-0-262-03083-0|url=http://www.micheleleigh.net/wp-content/uploads/2014/01/Crafton.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://www.micheleleigh.net/wp-content/uploads/2014/01/Crafton.pdf |archive-date=October 9, 2022 |url-status=live|pages=1–57}}</ref>
[[File:Field Museum Apatosaurus with original incorrect skull.jpg|thumb|Apatosaurine mount (FMNH P25112) in the [[Field Museum of Natural History]] in the 1950s, with its original, inaccurately reconstructed skull]]
It was not until 1909 that an ''Apatosaurus'' skull was found during the first expedition, led by [[Earl Douglass]], to what would become known as the Carnegie Quarry at [[Dinosaur National Monument]]. The skull was found a short distance from a skeleton (specimen CM{{nbsp}}3018) identified as the new species ''Apatosaurus louisae'', named after [[Louise Whitfield Carnegie|Louise Carnegie]], wife of [[Andrew Carnegie]], who funded field research to find complete dinosaur skeletons in the American West. The skull was designated CM{{nbsp}}11162; it was very similar to the skull of ''Diplodocus''.<ref name="parsons" /> Another smaller skeleton of ''A.{{nbsp}}louisae'' was found nearby CM{{nbsp}}11162 and CM{{nbsp}}3018.<ref name="glut1977" /> The skull was accepted as belonging to the ''Apatosaurus'' specimen by Douglass and Carnegie Museum director William H. Holland, although other scientists{{snd}}most notably Osborn{{snd}}rejected this identification. Holland defended his view in 1914 in an address to the Paleontological Society of America, yet he left the Carnegie Museum mount headless. While some thought Holland was attempting to avoid conflict with Osborn, others suspected Holland was waiting until an articulated skull and neck were found to confirm the association of the skull and skeleton.<ref name="camarasaurusbully" /> After Holland's death in 1934, museum staff placed a cast of a ''Camarasaurus'' skull on the mount.<ref name="parsons" />

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.<ref name="D'Emic-2019">{{Cite journal |last1=D'Emic |first1=Michael D. |last2=Carrano |first2=Matthew T. |date=June 28, 2019 |title=Redescription of Brachiosaurid Sauropod Dinosaur Material From the Upper Jurassic Morrison Formation, Colorado, USA |journal=The Anatomical Record |volume=303 |issue=4 |pages=732–758 |doi=10.1002/ar.24198 |pmid=31254331 |s2cid=195765189 |issn=1932-8486|doi-access=free}}</ref>

=== Recent discoveries and reassessment ===
In 2011, the first specimen of ''Apatosaurus'' where a skull was found articulated with its cervical vertebrae was described. This specimen, [[Cincinnati Museum Center|CMC]]{{nbsp}}VP{{nbsp}}7180, was found to differ in both skull and neck features from ''A.{{nbsp}}louisae'', but shared many features of the cervical vertebrae with ''A.{{nbsp}}ajax''.<ref name="barrettetal11" /> Another well-preserved skull is [[Brigham Young University]] specimen 17096, a well-preserved skull and skeleton, with a preserved braincase. The specimen was found in Cactus Park Quarry in western [[Colorado]].<ref name="balanoff2010" /> In 2013, Matthew Mossbrucker and several other authors published an abstract that described a premaxilla and maxilla from Lakes' original quarry in Morrison and referred the material to ''Apatosaurus ajax.''<ref name=":2" /> 
[[File:Brontosaurus infographic.svg|thumb|left|261x261px|Infographic explaining the history of ''Brontosaurus'' and ''Apatosaurus'' according to Tschopp ''et{{nbsp}}al.'' 2015]]
Almost all modern paleontologists agreed with Riggs that the two dinosaurs should be classified together in a single genus. According to the rules of the [[International Code of Zoological Nomenclature|ICZN]] (which governs the scientific names of animals), the name ''Apatosaurus'', having been published first, has priority as the official name; ''Brontosaurus'' was considered a [[synonym (taxonomy)|junior synonym]] and was therefore long discarded from formal use.<ref name="taylor10"/><ref name="endeavour"/><ref name="Dinosauria04"/><ref name="Macintosh"/> Despite this, at least one paleontologist{{snd}}[[Robert T. Bakker]]{{snd}}argued in the 1990s that ''A.{{nbsp}}ajax'' and ''A.{{nbsp}}excelsus'' were in fact sufficiently distinct for the latter to merit a separate genus.<ref name="Bakker98"/>

In 2015, Emanuel Tschopp, [[Octávio Mateus]], and Roger Benson released a paper on diplodocoid systematics, and proposed that genera could be diagnosed by thirteen differing characters, and species separated based on six. The minimum number for generic separation was chosen based on the fact that ''A.{{nbsp}}ajax'' and ''A.{{nbsp}}louisae'' differ in twelve characters, and ''Diplodocus carnegiei'' and ''D.{{nbsp}}hallorum'' differ in eleven characters. Thus, thirteen characters were chosen to validate the separation of genera. The six differing features for specific separation were chosen by counting the number of differing features in separate specimens generally agreed to represent one species, with only one differing character in ''D.{{nbsp}}carnegiei'' and ''A.{{nbsp}}louisae'', but five differing features in ''B.{{nbsp}}excelsus''. Therefore, Tschopp et{{nbsp}}al. argued that ''Apatosaurus excelsus'', originally classified as ''Brontosaurus excelsus'', had enough morphological differences from other species of ''Apatosaurus'' that it warranted being reclassified as a separate genus again. The conclusion was based on a comparison of 477 morphological characteristics across 81 different dinosaur individuals. Among the many notable differences are the wider{{snd}}and presumably stronger{{snd}}neck of ''Apatosaurus'' species compared to ''B.{{nbsp}}excelsus''. Other species previously assigned to ''Apatosaurus'', such as ''Elosaurus parvus'' and ''Eobrontosaurus yahnahpin'' were also reclassified as ''Brontosaurus''. Some features proposed to separate ''Brontosaurus'' from ''Apatosaurus'' include: posterior dorsal vertebrae with the centrum longer than wide; the scapula rear to the [[acromion|acromial edge]] and the distal blade being excavated; the acromial edge of the distal scapular blade bearing a rounded expansion; and the ratio of the proximodistal length to transverse breadth of the [[Talus bone|astragalus]] 0.55 or greater.<ref name=TMB2015/> Sauropod expert [[Michael D'Emic]] pointed out that the criteria chosen were to an extent arbitrary and that they would require abandoning the name ''Brontosaurus'' again if newer analyzes obtained different results.<ref name="d'emic 2015">D'Emic, M. 2015. [https://time.com/3810104/not-so-fast-brontosaurus/ "Not so fast, Brontosaurus"]. Time.com</ref> Mammal paleontologist [[Donald Prothero]] criticized the mass media reaction to this study as superficial and premature, concluding that he would keep "Brontosaurus" in quotes and not treat the name as a valid genus.<ref name="prothero 2015">Prothero, D. 2015. [http://www.skeptic.com/insight/is-brontosaurus-back-not-so-fast/ "Is "Brontosaurus" Back? Not So Fast!"]. Skeptic.com.</ref>

===Valid species===
[[File:AMNH Apatosaurus.jpg|thumb|Apatosaurine specimen AMNH 460 at the AMNH as re-mounted in 1995]]
[[File:FMNH Apatosaurus Pano.jpg|thumb|Apatosaurine mount (FMNH P25112) in the FMNH]]
[[File:Skeleton of Apatosaurus ajax.jpg|thumb|upright|Specimen NSMT-PV 20375, [[National Museum of Nature and Science]], which may be ''A.{{nbsp}}ajax'' or a new species]]
Many species of ''Apatosaurus'' have been designated from scant material. Marsh named as many species as he could, which resulted in many being based upon fragmentary and indistinguishable remains. In 2005, Paul Upchurch and colleagues published a study that analyzed the species and specimen relationships of ''Apatosaurus''. They found that ''A.{{nbsp}}louisae'' was the most basal species, followed by FMNH{{nbsp}}P25112, and then a [[polytomy]] of ''A.{{nbsp}}ajax'', ''A.{{nbsp}}parvus'', and ''A.{{nbsp}}excelsus''.<ref name="Upchurch05"/> Their analysis was revised and expanded with many additional diplodocid specimens in 2015, which resolved the relationships of ''Apatosaurus'' slightly differently, and also supported separating ''Brontosaurus'' from ''Apatosaurus''.<ref name=TMB2015/>

*'''''Apatosaurus ajax''''' was named by Marsh in 1877 after [[Ajax the Great|Ajax]], a hero from Greek mythology.<ref name="carpenter&currie92"/> Marsh designated the incomplete, juvenile skeleton YPM{{nbsp}}1860 as its [[holotype]]. The species is less studied than ''Brontosaurus'' and ''A.{{nbsp}}louisae'', especially because of the incomplete nature of the holotype. In 2005, many specimens in addition to the holotype were found assignable to ''A.{{nbsp}}ajax'', YPM{{nbsp}}1840, NSMT-PV 20375, YPM{{nbsp}}1861, and AMNH{{nbsp}}460. The specimens date from the late [[Kimmeridgian]] to the early [[Tithonian]] ages.<ref name="Upchurch05"/> In 2015, only the ''A.{{nbsp}}ajax'' holotype YPM{{nbsp}}1860 assigned to the species, with AMNH{{nbsp}}460 found either to be within ''Brontosaurus'', or potentially its own taxon. However, YPM{{nbsp}}1861 and NSMT-PV 20375 only differed in a few characteristics, and cannot be distinguished specifically or generically from ''A.{{nbsp}}ajax''. YPM{{nbsp}}1861 is the holotype of ''"Atlantosaurus" immanis'', which means it might be a junior synonym of ''A.{{nbsp}}ajax''.<ref name="TMB2015"/>
*'''''Apatosaurus louisae''''' was named by Holland in 1916, being first known from a partial skeleton that was found in Utah.<ref name="CM94"/> The holotype is CM{{nbsp}}3018, with referred specimens including CM{{nbsp}}3378, CM{{nbsp}}11162, and LACM{{nbsp}}52844. The former two consist of a vertebral column; the latter two consist of a skull and a nearly complete skeleton, respectively. ''Apatosaurus louisae'' specimens all come from the late Kimmeridgian of [[Dinosaur National Monument]].<ref name="Upchurch05"/> In 2015, Tschopp et{{nbsp}}al. found the type specimen of ''Apatosaurus laticollis'' to nest closely with CM{{nbsp}}3018, meaning the former is likely a junior synonym of ''A.{{nbsp}}louisae''.<ref name="TMB2015"/>

The [[cladogram]] below is the result of an analysis by Tschopp, Mateus, and Benson (2015). The authors analyzed most diplodocid type specimens separately to deduce which specimen belonged to which species and genus.<ref name="TMB2015"/>

{{clade| style=font-size:85%; line-height:85%
|label1=[[Apatosaurinae]]
|1={{clade
 |1={{clade
   |1=YPM 1840 ("''Atlantosaurus''" ''[[Atlantosaurus immanis|immanis]]'' type)
   |2=NSMT-PV 20375 }}
 |2={{clade
  |1=AMNH 460
  |2={{clade
   |label1='''''Apatosaurus'''''
   |1={{clade
    |label1='''''Apatosaurus ajax'''''
    |1=YPM 1860 (''Apatosaurus ajax'' type)
    |label2='''''Apatosaurus louisae'''''
    |2={{clade
     |1=CM 3018 (''Apatosaurus louisae'' type)
     |2=YPM 1861 (''Apatosaurus laticollis'' type) }} }}
   |label2=''[[Brontosaurus]]''
   |2={{clade
    |label1=''[[Brontosaurus excelsus]]''
    |1={{clade
      |1=YPM 1980 (''Brontosaurus excelsus'' type)
      |2=YPM 1981 (''Brontosaurus amplus'' type) }}
    |2={{clade
     |1=AMNH 5764 (''[[Amphicoelias altus]]'' type)
     |2={{clade
       |1=FMNH P25112
       |2={{clade
         |label1=''[[Brontosaurus yahnahpin]]''
         |1=Tate-001 (''Eobrontosaurus yahnahpin'' type)
         |label2=''[[Brontosaurus parvus]]''
         |2={{clade
           |1=CM 566 (''Elosaurus parvus'' type)
           |2={{clade
             |1=UM 15556
             |2=BYU 1252-18531
}} }} }} }} }} }} }} }} }} }}

===Reassigned species===
[[File:Einstein Apatosaurus.jpg|thumb|The most complete specimen known to date, ''A.'' sp. BYU 17096 nicknamed "Einstein"]]
* ''Apatosaurus grandis'' was named in 1877 by Marsh in the article that described ''A.{{nbsp}}ajax''. It was briefly described, figured, and diagnosed.<ref name="gilmore36"/> Marsh later mentioned it was only provisionally assigned to ''Apatosaurus'' when he reassigned it to his new genus ''Morosaurus'' in 1878.<ref name="marsh1878"/> Since ''Morosaurus'' has been considered a synonym of ''Camarasaurus'', ''C.{{nbsp}}grandis'' is the oldest-named species of the latter genus.<ref name="foster-2007"/>
* ''Apatosaurus excelsus'' was the original type species of ''Brontosaurus'', first named by Marsh in 1879. Elmer Riggs reclassified ''Brontosaurus'' as a synonym of ''Apatosaurus'' in 1903, transferring the species ''B.{{nbsp}}excelsus'' to ''A.{{nbsp}}excelsus''. In 2015, Tschopp, Mateus, and Benson argued that the species was distinct enough to be placed in its own genus, so they reclassified it back into ''Brontosaurus''.<ref name="TMB2015"/>
* ''Apatosaurus parvus'', first described from a juvenile specimen as ''Elosaurus'' in 1902 by Peterson and Gilmore, was reassigned to ''Apatosaurus'' in 1994, and then to ''Brontosaurus'' in 2015. Many other, more mature specimens were assigned to it following the 2015 study.<ref name="TMB2015"/>
* ''Apatosaurus minimus'' was originally described as a specimen of ''Brontosaurus'' sp. in 1904 by Osborn. In 1917, Henry Mook named it as its own species, ''A.{{nbsp}}minimus'', for a pair of ilia and their sacrum.<ref name="gilmore36"/><ref name="taylor2012"/><ref name="taylor&wedel2012"/> In 2012, [[Mike P. Taylor]] and [[Matt J. Wedel]] published a short abstract describing the material of ''A. minimus'', finding it hard to place among either Diplodocoidea or Macronaria. While it was placed with ''[[Saltasaurus]]'' in a phylogenetic analysis, it was thought to represent instead some form with convergent features from many groups.<ref name="taylor&wedel2012"/> The study of Tschopp et{{nbsp}}al. did find that a camarasaurid position for the taxon was supported, but noted that the position of the taxon was found to be highly variable and there was no clearly more likely position.<ref name=TMB2015/>
* ''Apatosaurus alenquerensis'' was named in 1957 by [[Albert-Félix de Lapparent]] and Georges Zbyweski. It was based on post cranial material from Portugal. In 1990, this material was reassigned to ''Camarasaurus'', but in 1998 it was given its own genus, ''[[Lourinhasaurus]]''.<ref name="Upchurch05"/> This was further supported by the findings of Tschopp et{{nbsp}}al. in 2015, where ''Lourinhasaurus'' was found to be sister to ''Camarasaurus'' and other camarasaurids.<ref name=TMB2015/>
* ''Apatosaurus yahnahpin'' was named by James Filla and Patrick Redman in 1994. Bakker made ''A.{{nbsp}}yahnahpin'' the [[holotype|type]] species of a new genus, ''[[Eobrontosaurus]]'' in 1998,<ref name="Bakker98"/> and Tschopp reclassified it as ''[[Brontosaurus yahnahpin]]'' in 2015.<ref name="TMB2015"/>