Apatosaurus (/əˌpætəˈsɔːrəs/;[3][4] meaning "deceptive lizard") is agenus ofherbivoroussauropoddinosaur that lived inNorth America during theLate Jurassicperiod.Othniel Charles Marsh described and named the first-known species,Apatosaurus ajax, in 1877, and a second species,Apatosaurus louisae, was discovered and named by William H. Holland in 1916.Apatosaurus lived about 152 to 151 million years ago (mya), during the lateKimmeridgian to earlyTithonian age, and are now known fromfossils in theMorrison Formation of modern-dayColorado,Oklahoma,New Mexico,Wyoming, andUtah in the United States.Apatosaurus had an average length of 21–23 m (69–75 ft), and an average mass of 16.4–22.4 t (16.1–22.0 long tons; 18.1–24.7 short tons). A few specimens indicate a maximum length of 11–30% greater than average and a mass of approximately 33 t (32 long tons; 36 short tons).
Apatosaurus is a member of thediplodocid family, differing fromDiplodocus proper in primarily robustness in thecervical vertebrae and the leg bones. Thecervical vertebrae ofApatosaurus are less elongated and more heavily constructed than those ofDiplodocus and the bones of the leg are much stockier despite being longer, implying thatApatosaurus was a more robust animal. The tail was held above the ground during normal locomotion.Apatosaurus had a singleclaw on each forelimb and three on each hindlimb. TheApatosaurus skull, long thought to be similar toCamarasaurus, is much more similar to that ofDiplodocus.Apatosaurus was a generalizedbrowser that likely held its head elevated. To lighten its vertebrae,Apatosaurus hadair sacs that made the bones internally full of holes. Like that of other diplodocids, its tail may have been used as a whip to create loud noises, or, as more recently suggested, as a sensory organ.
The skull ofApatosaurus was confused with that ofCamarasaurus andBrachiosaurus until 1909, when theholotype ofA. louisae was found, and a complete skull just a few meters away from the front of the neck.Henry Fairfield Osborn disagreed with this association, and went on to mount a skeleton ofApatosaurus with aCamarasaurus skull cast.Apatosaurus skeletons were mounted with speculative skull casts until 1970, when McIntosh showed that more robust skulls assigned toDiplodocus were more likely fromApatosaurus.
Apatosaurus is a genus in the family Diplodocidae. It is one of the morebasal genera, with onlyAmphicoelias and possibly a new, unnamed genus more primitive. Although the subfamilyApatosaurinae was named in 1929, the group was not usedvalidly until an extensive 2015 study. OnlyBrontosaurus is also in the subfamily, with the other genera being considered synonyms or reclassified asdiplodocines.Brontosaurus has long been considered ajunior synonym ofApatosaurus; its type species was reclassified asA.excelsus in 1903. A 2015 study concluded thatBrontosaurus is a valid genus of sauropod distinct fromApatosaurus, but not all paleontologists agree with this division. As it existed in North America during the late Jurassic,Apatosaurus would have lived alongside dinosaurs such asAllosaurus,Camarasaurus,Diplodocus, andStegosaurus.
Comparison ofA. ajax (orange) andA.louisae (red) with a human (blue) andBrontosaurus parvus (green)
Apatosaurus was a large, long-necked,quadrupedal animal with a long, whip-like tail. Itsforelimbs were slightly shorter than itshindlimbs. Most size estimates are based on specimenCM3018, thetype specimen ofA.louisae, reaching 21–23 m (69–75 ft) in length and 16.4–22.4 t (16.1–22.0 long tons; 18.1–24.7 short tons) in body mass.[5][6][7][8] A 2015 study that estimated the mass of volumetric models ofDreadnoughtus,Apatosaurus, andGiraffatitan estimates CM3018 at 21.8–38.2 t (21.5–37.6 long tons; 24.0–42.1 short tons), similar in mass toDreadnoughtus.[9] Some specimens ofA.ajax (such asOMNH1670) represent individuals 11–30% longer, suggesting masses twice that of CM3018 or 32.7–72.6 t (32.2–71.5 long tons; 36.0–80.0 short tons), potentially rivaling the largesttitanosaurs.[10] However, the upper size estimate of OMNH1670 is likely an exaggeration, with the size estimates revised in 2020 at 30 m (98 ft) in length and 33 t (36 short tons) in body mass based on volumetric analysis.[11]
A. ajax skull, specimen CMC VP 7180
The skull is small in relation to the size of the animal. The jaws are lined with spatulate (chisel-like) teeth suited to anherbivorous diet.[12] The snout ofApatosaurus and similardiplodocoids is squared, with onlyNigersaurus having a squarer skull.[13] The braincase ofApatosaurus is well preserved in specimen BYU17096, which also preserved much of the skeleton. A phylogenetic analysis found that the braincase had a morphology similar to those of other diplodocoids.[14] Some skulls ofApatosaurus have been found still in articulation with their teeth. Those teeth that have theenamel surface exposed do not show any scratches on the surface; instead, they display a sugary texture and little wear.[13]
Cervical vertebra ofA. ajax (holotype,YPM 1860) in side and anterior view
Like those of other sauropods, the neck vertebrae are deeply bifurcated; they carried neural spines with a large trough in the middle, resulting in a wide, deep neck.[12] The vertebral formula for the holotype ofA.louisae is 15cervicals, 10dorsals, 5sacrals, and 82caudals. The caudal vertebra number may vary, even within species.[15] The cervical vertebrae ofApatosaurus andBrontosaurus are stouter and more robust than those of otherdiplodocids and were found to be most similar toCamarasaurus byCharles Whitney Gilmore.[15][16] In addition, they supportcervical ribs that extend farther towards the ground than in diplodocines, and have vertebrae and ribs that are narrower towards the top of the neck, making the neck nearly triangular in cross-section.[16] InApatosaurus louisae, theatlas-axis complex of the first cervicals is nearly fused. The dorsal ribs are not fused or tightly attached to their vertebrae and are instead loosely articulated.[15]Apatosaurus has ten dorsal ribs on either side of the body.[17] The large neck was filled with an extensive system of weight-saving air sacs.Apatosaurus, like its close relativeSupersaurus, has tall neural spines, which make up more than half the height of the individual bones of its vertebrae. The shape of the tail is unusual for a diplodocid; it is comparatively slender because of the rapidly decreasing height of the vertebral spines with increasing distance from the hips.Apatosaurus also had very long ribs compared to most other diplodocids, giving it an unusually deep chest.[18] As in other diplodocids, the tail transformed into a whip-like structure towards the end.[15]
The limb bones are also very robust.[18] WithinApatosaurinae, thescapula ofApatosaurus louisae is intermediate in morphology between those ofA.ajax andBrontosaurus excelsus. The arm bones are stout, so thehumerus ofApatosaurus resembles that ofCamarasaurus, as well asBrontosaurus. However, the humeri ofBrontosaurus andA.ajax are more similar to each other than they are toA.louisae. In 1936,Charles Gilmore noted that previous reconstructions ofApatosaurus forelimbs erroneously proposed that theradius andulna could cross; in life they would have remained parallel.[15]Apatosaurus had a single large claw on each forelimb, a feature shared by all sauropods more derived thanShunosaurus.[15][19] The first three toes had claws on each hindlimb. The phalangeal formula is 2-1-1-1-1, meaning the innermost finger (phalanx) on the forelimb has two bones and the next has one.[20] The single manual claw bone (ungual) is slightly curved and squarelytruncated on the anterior end. The pelvic girdle includes the robustilia, and the fused (co-ossified)pubes andischia. The femora ofApatosaurus are very stout and represent some of the most robust femora of any member of Sauropoda. The tibia and fibula bones are different from the slender bones ofDiplodocus but are nearly indistinguishable from those ofCamarasaurus. The fibula is longer and slenderer than the tibia. The foot ofApatosaurus has three claws on the innermost digits; the digit formula is 3-4-5-3-2. The first metatarsal is the stoutest, a feature shared among diplodocids.[15][21]
Arthur Lakes' painting of YPM crews excavating fossils ofApatosaurus ajax at Quarry 10 in Morrison.
The firstApatosaurus 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 theRocky Mountains inJefferson County, Colorado. Arthur Lakes wrote toOthniel Charles Marsh, Professor ofPaleontology atYale University, andEdward Drinker Cope, a paleontologist based in Philadelphia, about the discovery until eventually collecting several fossils and sending them to both paleontologists. Marsh namedAtlantosaurus 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.[22] One of the best specimens collected by Lakes in 1877 was a well preserved partial postcranial skeleton, including many vertebrae, and a partial braincase (YPM VP 1860), which was sent to Marsh and namedApatosaurus ajax in November 1877.[23][22] The composite termApatosaurus comes from theGreek wordsapatē (ἀπάτη)/apatēlos (ἀπατηλός) meaning "deception"/"deceptive", andsauros (σαῦρος) meaning "lizard";[24] thus, "deceptive lizard". Marsh gave it this name based on thechevron bones, which are dissimilar to those of other dinosaurs; instead, the chevron bones ofApatosaurus showed similarities with those ofmosasaurs,[25][26] most likely that of the representative speciesMosasaurus. By the end of excavations at Lakes' quarry in Morrison, several partial specimens ofApatosaurus had been collected, but only the type specimen ofA. ajax can be confidently referred to the species.[27][23]
During excavation and transportation, the bones of the holotype skeleton were mixed with those of another Apatosaurine individual originally described asAtlantosaurus immanis; as a consequence, some elements cannot be ascribed to either specimen with confidence.[23] Marsh distinguished the new genusApatosaurus fromAtlantosaurus on the basis of the number of sacral vertebrae, withApatosaurus possessing three andAtlantosaurus 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, likeAtlantosaurus.[23] Two years later, Marsh announced the discovery of a larger and more complete specimen (YPM VP 1980) fromComo Bluff,Wyoming, he gave this specimen the nameBrontosaurus excelsus.[28] 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 toApatosaurus in 1877–78 at Cope's Quarry 5 at the site.[29] Later in 1884, Othniel Marsh namedDiplodocus lacustris based on a chimeric partial dentary, snout, and several teeth collected by Lakes in 1877 at Morrison.[23][30] In 2013, it was suggested that the dentary ofD. lacustris and its teeth were actually fromApatosaurus ajax based on its proximity to the type braincase ofA. ajax.[30] All specimens currently consideredApatosaurus were from theMorrison Formation, the location of the excavations of Marsh and Cope.[31]
After the end of theBone 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.[32] The competition to mount the first sauropod skeleton specifically was the most intense, with theAmerican Museum of Natural History,Carnegie Museum of Natural History, andField Museum of Natural History all sending expeditions to the west to find the most complete sauropod specimen,[32] bring it back to the home institution, and mount it in their fossil halls.[32] The American Museum of Natural History was the first to launch an expedition,[32] finding a well preserved skeleton (AMNH 460), which is occasionally assigned toApatosaurus, is considered nearly complete; only the head, feet, and sections of the tail are missing, and it was the first sauropod skeleton mounted.[33] The specimen was found north ofMedicine Bow, Wyoming, in 1898 byWalter Granger, and took the entire summer to extract.[34] To complete the mount, sauropod feet that were discovered at the same quarry and a tail fashioned to appear as Marsh believed it should – but which had too few vertebrae – 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 ofDiplodocus – which was later found to be more accurate – but was based on "the biggest, thickest, strongest skull bones, lower jaws and tooth crowns from three different quarries".[15][17][33][35] These skulls were likely those ofCamarasaurus, 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 findApatosaurus 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 ofMorosaurus" (nowCamarasaurus).[36]
In 1903,Elmer Riggs published a study that described a well-preserved skeleton of a diplodocid from theGrand River Valley nearFruita, 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 describedBrontosaurus. Most of the skeleton was found, and after comparison with bothBrontosaurus andApatosaurus ajax, Riggs realized that the holotype ofA.ajax was immature, and thus the features distinguishing the genera were not valid. SinceApatosaurus was the earlier name,Brontosaurus should be considered a junior synonym ofApatosaurus. Because of this, Riggs recombinedBrontosaurus excelsus asApatosaurus excelsus. Based on comparisons with other species proposed to belong toApatosaurus, Riggs also determined that the Field Columbian Museum specimen was likely most similar toA.excelsus.[17]
Despite Riggs' publication,Henry Fairfield Osborn, who was a strong opponent of Marsh and his taxa, labeled theApatosaurus mount of theAmerican Museum of Natural HistoryBrontosaurus.[36][37] Because of this decision the nameBrontosaurus was commonly used outside of scientific literature for what Riggs consideredApatosaurus, and the museum's popularity meant thatBrontosaurus became one of the best known dinosaurs, even though it was invalid throughout nearly all of the 20th and early 21st centuries.[38]
Composite apatosaurine mount (P25112 and P27021) with skull ofCamarasaurus (Cast ofCM12020) at theField Museum of Natural History in the 1950s
It was not until 1909 that anApatosaurus skull was found during the first expedition, led byEarl Douglass, to what would become known as the Carnegie Quarry atDinosaur National Monument. The skull was found a short distance from a skeleton (specimen CM3018) identified as the new speciesApatosaurus louisae, named afterLouise Carnegie, wife ofAndrew Carnegie, who funded field research to find complete dinosaur skeletons in the American West. The skull was designated CM11162; it was very similar to the skull ofDiplodocus.[37] Another smaller skeleton ofA.louisae was found nearby CM11162 and CM3018.[39] The skull was accepted as belonging to theApatosaurus specimen by Douglass and Carnegie Museum director William H. Holland, although other scientists – most notably Osborn – 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.[36] After Holland's death in 1934, museum staff placed a cast of aCamarasaurus skull on the mount.[37]
While most other museums were using cast or sculptedCamarasaurus skulls onApatosaurus mounts, the Yale Peabody Museum decided to sculpt a skull based on the lower jaw of aCamarasaurus, with the cranium based on Marsh's 1891 illustration of the skull. The skull also included forward-pointing nasals – something unusual for any dinosaur – and fenestrae differing from both the drawing and other skulls.[36]
Side view ofA. louisae CM3018 mounted with a cast of skull CM11162
NoApatosaurus skull was mentioned in literature until the 1970s whenJohn Stanton McIntosh and David Berman redescribed the skulls ofDiplodocus andApatosaurus. They found that though he never published his opinion, Holland was almost certainly correct, thatApatosaurus had aDiplodocus-like skull. According to them, many skulls long thought to pertain toDiplodocus might instead be those ofApatosaurus. They reassigned multiple skulls toApatosaurus based on associated and closely associated vertebrae. Even though they supported Holland, it was noted thatApatosaurus might have possessed aCamarasaurus-like skull, based on a disarticulatedCamarasaurus-like tooth found at the precise site where anApatosaurus specimen was found years before.[35] On October20, 1979, after the publications by McIntosh and Berman, the first true skull ofApatosaurus was mounted on a skeleton in a museum, that of the Carnegie.[37] In 1998, it was suggested that the Felch Quarry skull that Marsh had included in his 1896 skeletal restoration instead belonged toBrachiosaurus.[40] This was supported in 2020 with a redescription of thebrachiosaurid material found at the Felch Quarry.[41]
In 2011, the first specimen ofApatosaurus where a skull was found articulated with its cervical vertebrae was described. This specimen,CMCVP7180, was found to differ in both skull and neck features fromA.louisae, but shared many features of the cervical vertebrae withA.ajax.[42] Another well-preserved skull isBrigham Young University specimen 17096, a well-preserved skull and skeleton, with a preserved braincase. The specimen was found in Cactus Park Quarry in westernColorado.[14] 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 toApatosaurus ajax.[30]
Infographic explaining the history ofBrontosaurus andApatosaurus according to Tschoppetal. 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 theICZN (which governs the scientific names of animals), the nameApatosaurus, having been published first, has priority as the official name;Brontosaurus was considered ajunior synonym and was therefore long discarded from formal use.[43][44][45][46] Despite this, at least one paleontologist – Robert T. Bakker – argued in the 1990s thatA.ajax andA.excelsus were in fact sufficiently distinct for the latter to merit a separate genus.[47]
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 thatA.ajax andA.louisae differ in twelve characters, andDiplodocus carnegiei andD.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 inD.carnegiei andA.louisae, but five differing features inB.excelsus. Therefore, Tschopp etal. argued thatApatosaurus excelsus, originally classified asBrontosaurus excelsus, had enough morphological differences from other species ofApatosaurus 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 – and presumably stronger – neck ofApatosaurus species compared toB.excelsus. Other species previously assigned toApatosaurus, such asElosaurus parvus andEobrontosaurus yahnahpin were also reclassified asBrontosaurus. Some features proposed to separateBrontosaurus fromApatosaurus include: posterior dorsal vertebrae with the centrum longer than wide; the scapula rear to theacromial 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 theastragalus 0.55 or greater.[23] Sauropod expertMichael D'Emic pointed out that the criteria chosen were to an extent arbitrary and that they would require abandoning the nameBrontosaurus again if newer analyzes obtained different results.[48] Mammal paleontologistDonald 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.[49]
Apatosaurine specimen AMNH 460 at the AMNH as re-mounted in 1995Apatosaurine mount (FMNH P25112) in the FMNHSpecimen NSMT-PV 20375,National Museum of Nature and Science, which may beA.ajax or a new species
Many species ofApatosaurus 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 ofApatosaurus. They found thatA.louisae was the most basal species, followed by FMNHP25112, and then apolytomy ofA.ajax,A.parvus, andA.excelsus.[21] Their analysis was revised and expanded with many additional diplodocid specimens in 2015, which resolved the relationships ofApatosaurus slightly differently, and also supported separatingBrontosaurus fromApatosaurus.[23]
Apatosaurus ajax was named by Marsh in 1877 afterAjax, a hero from Greek mythology.[50] Marsh designated the incomplete, juvenile skeleton YPM1860 as itsholotype. The species is less studied thanBrontosaurus andA.louisae, especially because of the incomplete nature of the holotype. In 2005, many specimens in addition to the holotype were found assignable toA.ajax, YPM1840, NSMT-PV 20375, YPM1861, and AMNH460. The specimens date from the lateKimmeridgian to the earlyTithonian ages.[21] In 2015, only theA.ajax holotype YPM1860 assigned to the species, with AMNH460 found either to be withinBrontosaurus, or potentially its own taxon. However, YPM1861 and NSMT-PV 20375 only differed in a few characteristics, and cannot be distinguished specifically or generically fromA.ajax. YPM1861 is the holotype of"Atlantosaurus" immanis, which means it might be a junior synonym ofA.ajax.[23]
Apatosaurus louisae was named by Holland in 1916, being first known from a partial skeleton that was found in Utah.[51] The holotype is CM3018, with referred specimens including CM3378, CM11162, and LACM52844. 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 ofDinosaur National Monument.[21] In 2015, Tschopp etal. found the type specimen ofApatosaurus laticollis to nest closely with CM3018, meaning the former is likely a junior synonym ofA.louisae.[23]
Thecladogram 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.[23]
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 describedA.ajax. It was briefly described, figured, and diagnosed.[15] Marsh later mentioned it was only provisionally assigned toApatosaurus when he reassigned it to his new genusMorosaurus in 1878.[52] SinceMorosaurus has been considered a synonym ofCamarasaurus,C.grandis is the oldest-named species of the latter genus.[53]
Apatosaurus excelsus was the original type species ofBrontosaurus, first named by Marsh in 1879. Elmer Riggs reclassifiedBrontosaurus as a synonym ofApatosaurus in 1903, transferring the speciesB.excelsus toA.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 intoBrontosaurus.[23]
Apatosaurus parvus, first described from a juvenile specimen asElosaurus in 1902 by Peterson and Gilmore, was reassigned toApatosaurus in 1994, and then toBrontosaurus in 2015. Many other, more mature specimens were assigned to it following the 2015 study.[23]
Apatosaurus minimus was originally described as a specimen ofBrontosaurus sp. in 1904 by Osborn. In 1917, Henry Mook named it as its own species,A.minimus, for a pair of ilia and their sacrum.[15][54][55] In 2012,Mike P. Taylor andMatt J. Wedel published a short abstract describing the material ofA. minimus, finding it hard to place among either Diplodocoidea or Macronaria. While it was placed withSaltasaurus in a phylogenetic analysis, it was thought to represent instead some form with convergent features from many groups.[55] The study of Tschopp etal. 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.[23]
Apatosaurus alenquerensis was named in 1957 byAlbert-Félix de Lapparent and Georges Zbyweski. It was based on post cranial material from Portugal. In 1990, this material was reassigned toCamarasaurus, but in 1998 it was given its own genus,Lourinhasaurus.[21] This was further supported by the findings of Tschopp etal. in 2015, whereLourinhasaurus was found to be sister toCamarasaurus and other camarasaurids.[23]
Apatosaurus yahnahpin was named by James Filla and Patrick Redman in 1994. Bakker madeA.yahnahpin thetype species of a new genus,Eobrontosaurus in 1998,[47] and Tschopp reclassified it asBrontosaurus yahnahpin in 2015.[23]
Apatosaurus is a member of thefamilyDiplodocidae, aclade of giganticsauropoddinosaurs. The family includes some of the longest creatures ever to walk the earth, includingDiplodocus,Supersaurus, andBarosaurus.Apatosaurus is sometimes classified in the subfamilyApatosaurinae, which may also includeSuuwassea,Supersaurus, andBrontosaurus.[18][56][57]Othniel Charles Marsh describedApatosaurus as allied toAtlantosaurus within the now-defunct groupAtlantosauridae.[17][25] In 1878, Marsh raised his family to the rank of suborder, includingApatosaurus,Atlantosaurus,Morosaurus (=Camarasaurus) andDiplodocus. He classified this group within Sauropoda, a group he erected in the same study. In 1903,Elmer S. Riggs said the name Sauropoda would be a junior synonym of earlier names; he groupedApatosaurus withinOpisthocoelia.[17] Sauropoda is still used as the group name.[21] In 2011, John Whitlock published a study that placedApatosaurus a morebasal diplodocid, sometimes less basal thanSupersaurus.[58][59]
Cladogram of the Diplodocidae after Tschopp, Mateus, and Benson (2015).[23]
It was believed throughout the 19th and early 20th centuries that sauropods likeApatosaurus were too massive to support their own weight on dry land. It was theorized that they lived partly submerged in water, perhaps in swamps. More recent findings do not support this; sauropods are now thought to have been fully terrestrial animals.[60] A study of diplodocid snouts showed that the squaresnout, large proportion of pits, and fine, subparallel scratches of the teeth ofApatosaurus suggests it was a ground-height, nonselectivebrowser.[13] It may have eatenferns,cycadeoids,seed ferns,horsetails, andalgae.[61] Stevens and Parish (2005) speculate that these sauropods fed from riverbanks on submerged water plants.[62]
A 2015 study of the necks ofApatosaurus andBrontosaurus found many differences between them and other diplodocids, and that these variations may have shown that the necks ofApatosaurus andBrontosaurus were used forintraspecific combat.[16] Various uses for the single claw on the forelimb of sauropods have been proposed. One suggestion is that they were used for defense, but their shape and size make this unlikely. It was also possible they were for feeding, but the most probable use for the claw was grasping objects such as tree trunks when rearing.[19]
Trackways of sauropods likeApatosaurus show that they may have had a range of around 25–40 km (16–25 miles) per day, and that they could potentially have reached a top speed of 20–30 km (12–19 miles) per hour.[12] The slow locomotion of sauropods may be due to their minimal muscling, or to recoil after strides.[63] A trackway of a juvenile has led some to believe that they were capable of bipedalism, though this is disputed.[64][65]
Skeletal diagram ofA. louisae, showing the neck held high
Diplodocids likeApatosaurus are often portrayed with their necks held high up in the air, allowing them to browse on tall trees. Some studies state diplodocid necks were less flexible than previously believed, because the structure of the neck vertebrae would not have allowed the neck to bend far upward, and that sauropods likeApatosaurus were adapted to low browsing or ground feeding.[61][62][66]
Other studies by Taylor find that alltetrapods appear to hold their necks at the maximum possible vertical extension when in a normal, alert posture; they argue the same would hold true for sauropods barring any unknown, unique characteristics that set the soft tissue anatomy of their necks apart from that of other animals.Apatosaurus, likeDiplodocus, would have held its neck angled upward with the head pointing downward in a resting posture.[67][68] Kent Stevens and Michael Parrish (1999 and 2005) stateApatosaurus had a great feeding range; its neck could bend into a U-shape laterally.[61] The neck's range of movement would have also allowed the head to feed at the level of the feet.[62]
Matthew Cobleyet al. (2013) dispute this, finding that large muscles and cartilage would have limited movement of the neck. They state the feeding ranges for sauropods likeDiplodocus were smaller than previously believed, and the animals may have had to move their whole bodies around to better access areas where they could browse vegetation. As such, they might have spent more time foraging to meet their minimum energy needs.[69][70] The conclusions of Cobleyetal. are disputed by Taylor, who analyzed the amount and positioning of intervertebral cartilage to determine the flexibility of the neck ofApatosaurus andDiplodocus. He found that the neck ofApatosaurus was very flexible.[67]
Tail vertebrae of specimen FMNH P25112, showing pneumatic fossae (holes)
Given the large body mass and long neck of sauropods likeApatosaurus, physiologists have encountered problems determining how these animals breathed. Beginning with the assumption that, likecrocodilians,Apatosaurus did not have adiaphragm, thedead-space volume (the amount of unused air remaining in the mouth, trachea, and air tubes after each breath) has been estimated at 0.184 m3 (184 L) for a 30 t (30 long tons; 33 short tons) specimen. Paladino calculates itstidal volume (the amount of air moved in or out during a single breath) at 0.904 m3 (904 L) with an avian respiratory system, 0.225 m3 (225 L) if mammalian, and 0.019 m3 (19 L) if reptilian.[71]
On this basis, its respiratory system would likely have beenparabronchi, with multiple pulmonary air sacs as inavian lungs, and a flow-through lung. An avian respiratory system would need a lung volume of about 0.60 m3 (600 L) compared with a mammalian requirement of 2.95 m3 (2,950 L), which would exceed the space available. The overall thoracic volume ofApatosaurus has been estimated at 1.7 m3 (1,700 L), allowing for a 0.50 m3 (500 L), four-chambered heart and a 0.90 m3 (900 L) lung capacity. That would allow about 0.30 m3 (300 L) for the necessary tissue.[71] Evidence for the avian system inApatosaurus and other sauropods is also present in thepneumaticity 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.[72]
James Spotilaet al. (1991) concludes that the large body size of sauropods would have made them unable to maintain high metabolic rates because they would not have been able to release enough heat.[73] They assumed sauropods had a reptilian respiratory system. Wedel says that an avian system would have allowed it to dump more heat.[72] Some scientists state that the heart would have had trouble sustaining sufficient blood pressure to oxygenate the brain.[60] Others suggest that the near-horizontal posture of the head and neck would have eliminated the problem of supplying blood to the brain because it would not have been elevated.[61]
James Farlow (1987) calculates that anApatosaurus-sized dinosaur about 35 t (34 long tons; 39 short tons) would have possessed 5.7 t (5.6 long tons; 6.3 short tons) of fermentation contents, though he cautions that the regression equation being used is based on living mammals which are much smaller and physiologically different.[74] AssumingApatosaurus had an avian respiratory system and a reptilian resting-metabolism, Frank Paladino etal. (1997) estimate the animal would have needed to consume only about 262 litres (58 imp gal; 69 US gal) of water per day.[71]
JuvenileA. sp. mount, Sam Noble Oklahoma Museum of Natural History
A 1999 microscopic study ofApatosaurus andBrontosaurus bones concluded the animals grew rapidly when young and reached near-adult sizes in about 10years.[75] In 2008, a study on the growth rates of sauropods was published by Thomas Lehman and Holly Woodward. They said that by using growth lines and length-to-mass ratios,Apatosaurus would have grown to 25t (25 long tons; 28 short tons) in 15years, with growth peaking at 5,000 kg (11,000 lb) in a single year. An alternative method, using limb length and body mass, foundApatosaurus grew 520 kg (1,150 lb) per year, and reached its full mass before it was about 70years old.[76] These estimates have been called unreliable because the calculation methods are not sound; old growth lines would have been obliterated by bone remodeling.[77] One of the first identified growth factors ofApatosaurus was the number of sacral vertebrae, which increased to five by the time of the creature's maturity. This was first noted in 1903 and again in 1936.[15]
Long-bone histology enables researchers to estimate the age that a specific individual reached. A study by Eva Griebeler etal. (2013) examined long-bone histological data and concluded theApatosaurus sp.SMA0014 weighed 20,206 kg (22.3 short tons), reached sexual maturity at 21years, and died aged 28. The same growth model indicatedApatosaurus sp.BYU 601–17328 weighed 18,178 kg (20.0 short tons), reached sexual maturity at 19years, and died aged 31.[77]
Compared with most sauropods, a relatively large amount of juvenile material is known fromApatosaurus. Multiple specimens in the OMNH are from juveniles of an undetermined species ofApatosaurus; this material includes partial shoulder and pelvic girdles, some vertebrae, and limb bones. OMNH juvenile material is from at least two different age groups and based on overlapping bones likely comes from more than three individuals. The specimens exhibit features that distinguishApatosaurus from its relatives, and thus likely belong to the genus.[21][78] Juvenile sauropods tend to have proportionally shorter necks and tails, and a more pronounced forelimb-hindlimb disparity than found in adult sauropods.[79]
CarnegieA. louisae mount (upper right), showing the long, tapering tail
The estimated tail length ofApatosaurus is approximately 54% of the total body length, with the tail sometimes hypothesized to be capable of functioning like a very long, taperingbullwhip.[80] An article published in 1997 reported research of the mechanics ofApatosaurus tails through computer simulation byNathan Myhrvold and paleontologistPhilip J. Currie. This computer modeling suggested diplodocids were capable of producing a whiplike cracking sound of over 200decibels, comparable to the volume of a cannon being fired.[81]
A pathology has been identified on the tail ofApatosaurus, caused by a growth defect. Two caudal vertebrae are seamlessly fused along the entire articulating surface of the bone, including the arches of the neural spines. This defect might have been caused by the lack or inhibition of the substance that forms intervertebral disks or joints.[82] It has been proposed that the whips could have been used in combat and defense, but the tails of diplodocids were quite light and narrow compared toShunosaurus andmamenchisaurids, and thus to injure another animal with the tail would severely injure the tail itself.[81] More recently, Baron (2020) considers the use of the tail as a bullwhip unlikely because of the potentially catastrophic muscle and skeletal damage such speeds could cause on the large and heavy tail. Instead, he proposes that the tails might have been used as a tactile organ to keep in touch with the individuals behind and on the sides in a group while migrating, which could have augmented cohesion and allowed communication among individuals while limiting more energetically demanding activities like stopping to search for dispersed individuals, turning to visually check on individuals behind, or communicating vocally.[80]
TheMorrison Formation is a sequence of shallow marine and alluvial sediments which, according toradiometric dating, dates from between 156.3mya at its base,[83] and 146.8mya at the top,[84] placing it in the lateOxfordian,Kimmeridgian, and earlyTithonianstages of the Late Jurassic period. This formation is interpreted as originating in a locallysemiarid environment with distinct wet and dry seasons. The Morrison Basin, where dinosaurs lived, stretched from New Mexico to Alberta and Saskatchewan; it was formed when the precursors to theFront Range of the Rocky Mountains started pushing up to the west. The deposits from their east-facing drainage basins were carried by streams and rivers and deposited in swampy lowlands, lakes, river channels, and floodplains.[85] This formation is similar in age to theLourinhã Formation in Portugal and theTendaguru Formation in Tanzania.[31]
Apatosaurus was the second most common sauropod in the Morrison Formation ecosystem, afterCamarasaurus.[53][86]Apatosaurus may have been more solitary than other Morrison Formation dinosaurs.[87] Fossils of the genus have only been found in the upper levels of the formation. Those ofApatosaurus ajax are known exclusively from the upperBrushy Basin Member, about 152–151 mya.A.louisae fossils are rare, known only from one site in the upper Brushy Basin Member; they date to the late Kimmeridgian stage, about 151mya. AdditionalApatosaurus remains are known from similarly aged or slightly younger rocks, but they have not been identified as any particular species,[88] and thus may instead belong toBrontosaurus.[23]
Ischium of anApatosaurus showing bite marks from a large theropod
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