| Apatosaurines | |
|---|---|
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| Skeleton of an apatosaurine (eitherApatosaurus orBrontosaurus),American Museum of Natural History 460 | |
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| Skeleton ofBrontosaurus | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | †Sauropodomorpha |
| Clade: | †Sauropoda |
| Superfamily: | †Diplodocoidea |
| Family: | †Diplodocidae |
| Subfamily: | †Apatosaurinae Janensch,1927 |
| Genera[1] | |
Apatosaurinae (the name deriving from the type genusApatosaurus, meaning "deceptive lizard") is asubfamily ofdiplodocidsauropods, an extinct group of large, quadrupedal dinosaurs, the other subfamily in Diplodocidae beingDiplodocinae. Apatosaurines are distinguished by their more robust, stocky builds and shorter necks proportionally to the rest of their bodies. Several fairly complete specimens are known, giving a comprehensive view of apatosaurine anatomy.
The subfamily includes only two genera for certain,Apatosaurus andBrontosaurus, according to a 2015 study, thoughAmphicoelias and a species ofAtlantosaurus may also belong to this group. All members are known from theLate Jurassic-agedMorrison Formation of theAmerican West, dating to between 157 and 146.8 million years ago. Fossils were first unearthed in 1877 by crews working for American paleontologistOthniel Charles Marsh in the midst of theBone Wars, a fossil-hunting rivalry with naturalistEdward Drinker Cope. Marsh went on to name both genera of apatosaurine;Apatosaurus in 1877 andBrontosaurus in 1879. For many years,Brontosaurus was considered ataxonomic synonym ofApatosaurus after a 1903 study by paleontologistElmer Riggs considered them synonyms. This was the prevalent notion until a 2015 paper led byEmmanuel Tschopp argued that the two were separate genera, however some authors still consider the two synonyms. According to Tschopp and colleagues, there are six valid species of apatosaurine; three inBrontosaurus and two inApatosaurus.
Apatosaurines were nonselective low-browsers, utilizing their peg-liketeeth to feed on plants likeferns,cycadoids, andhorsetails. They also had rapid growth rates, with studies finding that apatosaurines reached adult sizes in just ten years, possibly peaking in 5,000 kg (11,000 lb) gained in a single year. As in their relatives the diplodocines, bothApatosaurus andBrontosaurus bore long, whip-like tails which potentially were used for defense or as atactile organ, for touch. Of note is the robustly built and boxycervical (neck) vertebrae of apatosaurines, with a study hypothesizing that their necks were used forintraspecific fighting between individuals. Being from the Morrison Formation, apatosaurines coexisted with a menagerie of other taxa such as the sauropodsDiplodocus,Barosaurus, andBrachiosaurus; herbivorousornithischiansStegosaurus,Dryosaurus, andNanosaurus; as well as the carnivoroustheropodsAllosaurus,Marshosaurus andCeratosaurus. This formation was a hotspot of sauropod biodiversity, with over 16 recognized genera, which resulted in niche partitioning between different sauropods.
The first apatosaurine 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. Lakes wrote toOthniel Charles Marsh, Professor ofPaleontology atYale University, andEdward Drinker Cope, a paleontologist based inPhiladelphia, about the discovery until eventually collecting several fossils and sending them to both paleontologists. By this time, the Morrison Formation had become the center of theBone Wars, a fossil-collecting rivalry between Marsh and Cope. Because of this, the publications and descriptions of taxa by the two were rushed at the time.[2] Marsh namedAtlantosaurus montanus based on some of the fossils sent and hired Lakes, while Cope attempted to hire Lakes as well but was rejected.[3] One of the best specimens collected by Lakes in 1877 was a well preserved partial postcranial skeleton and a partialbraincase (YPM VP 1860), which was sent to Marsh and namedApatosaurus ajax in November 1877.[1][3] The composite termApatosaurus comes from theGreek wordsapatē (ἀπάτη)/apatēlos (ἀπατηλός) meaning "deception"/"deceptive", andsauros (σαῦρος) meaning "lizard"; thus, "deceptive lizard", the same meaning as Apatosaurinae's name. 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, most likely that of the representative speciesMosasaurus.[4] Marsh believed that both genera were part of a clade he namedAtlantosauridae in 1877.[5] Based on remains from the same quarry,Atlantosaurus immanis was described by Marsh in an 1878 article based on an incomplete postcranial skeleton, though it became mixed with the holotype ofA. ajax.[6][7] A 1906 paper describedgastroliths from theA. immanis holotype,[7] but gastrolith use in sauropods has since been disproven.[8] Later studies of theA. immanis holotype have determined that they are from an apatosaurine, potentiallyApatosaurus itself according several studies,[9][10][11] though Tschopp and colleagues stated it was adubious apatosaurine.[1]
A possible genus of apatosaurine,Amphicoelias, was discovered in 1877 in Morrison Formation outcrops nearCañon City, Colorado by Aaron Ripley who was working for Cope. Ripley sent the remains, consisting ofdorsal vertebrae, afemur, and apubis, toCope's home Philadelphia on October 21, 1877.[12] In December 1877, Cope namedAmphicoelias, the genus name from the Greek termsamphi meaning "both" andkolios "concave" in reference to the anatomy of the dorsal vertebrae.[13] Three species were referred:A. altus, A. fragillimus, andA. latus.[13] Of these, onlyA. altus is still considered part of the genus whereasA. fragillimus is nowMaraapunisaurus[14] andA. latus is a synonym ofCamarasaurus supremus orC. grandis.[15][16]
The discovery of another large and fairly completesauropod skeleton was announced in 1879 by Marsh. The specimen was collected from Morrison Formation rocks atComo Bluff,Wyoming byWilliam Harlow Reed. Marsh identified it as belonging to an entirely new genus and species, which he namedBrontosaurus excelsus,[17] meaning "thunder lizard", from the Greekbrontē/βροντη meaning "thunder" andsauros/σαυρος meaning "lizard",[18] and from theLatinexcelsus, "noble" or "high".[19]Brontosaurus excelsus' type specimen (YPM 1980) was one of the most complete sauropod skeletons known at the time, preserving many of the characteristic but fragile cervical vertebrae.[5] A year later in 1880, another partial postcranialBrontosaurus skeleton was collected near Como Bluff by Reed,[20][21] including well-preserved limb elements.[1] Marsh named this second skeletonBrontosaurus amplus in 1881,[21] but it was considered a synonym ofB. excelsus in 2015.[1]
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. 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, bring it back to the home institution, and mount it in their fossil halls. The American Museum of Natural History was the first to launch an expedition, 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.[22][23] The specimen was found north ofMedicine Bow, Wyoming, in 1898 byWalter Granger, and took the entire summer to extract.[24] 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.[25][26][9] These skulls were likely those ofCamarasaurus, the only other sauropod for which good skull material was known at the time, causing workers 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).[27]
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. 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.[25] Despite Riggs' publication,Henry Fairfield Osborn, who was a strong opponent of Marsh and his taxa, labeled theApatosaurus mount of the American Museum of Natural HistoryBrontosaurus.[28][27] 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.[29]In August 1883,Marshall P. Felch collected a disarticulated partialskull (USNM V 5730) of a sauropod further south in the Felch Quarry atGarden Park,Colorado and sent the specimen to Yale.[30][31] Marsh referred the skull toB. excelsus,[30][32] later featuring it in a skeletal reconstruction of theB. excelsus type specimen in 1891[32] and the illustration was featured again in Marsh's landmark publication,The Dinosaurs of North America, in 1896.[5] At theYale Peabody Museum, the skeleton ofBrontosaurus excelsus was mounted in 1931 with a skull based on the Marsh reconstruction of the Felch Quarry skull.[27] While at the time most museums were usingCamarasaurus casts for skulls, the Peabody Museum sculpted a completely different skull based on Marsh's recon.[30][27] The mandible was based on aCamarasaurus'.[27] In 1998, the Felch Quarry skull that Marsh included in his 1896 skeletal restoration was suggested to belong toBrachiosaurus instead[30] and this was supported in 2020 with a redescription of thebrachiosaurid material found at the Felch Quarry.[31]
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 CM 3018) identified as the new speciesApatosaurus louisae. The skull was designated CM 11162; it was very similar to the skull ofDiplodocus.[28] The skull was accepted as belonging to theApatosaurus specimen by Douglass and Carnegie Museum directorWilliam J. Holland, although other scientists – most notably Osborn – rejected this identification. Holland defended his view in 1914 in an address, 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.[27] After Holland's death in 1934, museum staff placed a cast of aCamarasaurus skull on the mount.[28]
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. In 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.[27]
Another specimen of an apatosaurine was discovered in 1993 by theTate Geological Museum, also from the Morrison Formation of central Wyoming. The specimen consisted of a partial postcranial skeleton, including a completemanus and multiple vertebrae, and was described by James Filla and Pat Redman a year later. Filla and Redman named the specimenApatosaurus yahnahpin,[33] butRobert T. Bakker gave it the genus nameEobrontosaurus in 1998. Bakker believed thatEobrontosaurus was the direct predecessor toBrontosaurus,[34] although Tschopp and colleague's phylogenetic analysis placedB. yahnahpin as the basalmost species ofBrontosaurus.[1]
In 2011, the first specimen ofApatosaurus where a skull was found articulated with its cervical vertebrae was described. This specimen,CMC VP 7180, was found to differ in both skull and neck features fromA. louisae, but shared many features of the cervical vertebrae withA. ajax.[35] 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.[36] 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.[37] A skull from Como Bluff, this specimen associated with several cervical vertebrae, was described in 2022.[38]
Almost all 20th-century paleontologists agreed with Riggs that allApatosaurus andBrontosaurus species should be classified in a single genus. According to the rules of theICZN, which governs the scientific names of animals, the nameApatosaurus, having been published first, had priority;Brontosaurus was considered a junior synonym and was therefore discarded from formal use.[39][40][41][42] Despite this, at least one paleontologist—Robert T. Bakker—argued in the 1990s thatA. ajax andA. excelsus are sufficiently distinct that the latter continues to merit a separate genus.[34] In 2015, an extensive study of diplodocid relationships by Emanuel Tschopp and colleagues concluded thatBrontosaurus was indeed a valid genus of sauropod distinct fromApatosaurus. The scientists developed a statistical method to more objectively assess differences between fossil genera and species and concluded thatBrontosaurus could be "resurrected" as a valid name. They assigned two formerApatosaurus species,A. parvus, andA. yahnahpin, toBrontosaurus, as well as the type speciesB. excelsus.[1] The publication was met with some criticism from other paleontologists, including Michael D'Emic,[43]Donald Prothero, who criticized the mass media reaction to this study as superficial and premature,[44] and many others. Some paleontologists likeJohn andRebecca Hunt-Foster continue to considerBrontosaurus as a synonym ofApatosaurus.[45][46]
Apatosaurines are large, long-necked,quadrupedal animals with long, whip-like tails, and forelimbs that were slightly shorter than its hindlimbs. The size of apatosaurines varies, likeA. louisae which measures 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 mass andB. excelsus which is 21–22 m (69–72 ft) long and 15–17 t (17–19 short tons).[47] Some specimens ofA. ajax (such asOMNH 1670) represent individuals 11–30% longer, suggesting masses twice that of CM 3018 or 32.7–72.6 t (32.2–71.5 long tons; 36.0–80.0 short tons), potentially rivaling the largesttitanosaurs.[48] However, the upper size estimate of OMNH 1670 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.[49]
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. The snout ofApatosaurus and similardiplodocoids is squared, with onlyNigersaurus having a squarer skull.[50] The braincase ofApatosaurus is well preserved in specimen BYU 17096, which also preserved much of the skeleton. A phylogenetic analysis found that the braincase had a morphology similar to those of other diplodocoids.[36] Some skulls ofApatosaurus have been found still in articulation with their teeth. Those teeth that have thedentine surface exposed bear many small, subcircular features, but no scratches or features on the enamel.[50]
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Like those of other diplodocids, the vertebrae of the neck were deeply bifurcated on the dorsal side; that is, they carried paired spines, resulting in a wide and deep neck.[51] Their spines and tails consists of 15 cervicals, ten dorsals, five sacrals, and about 82 caudals inApatosaurus and likelyBrontosaurus. However, the number of caudal vertebrae has been noted to vary, even within a species. Vertebrae in the neck, torso, andsacrum of sauropods bore large pneumaticforamen on their lateral sides. These are used to lighten the bones which aided in keeping the animal lighter. Within the vertebrae as well, smooth bone walls in addition todiverticula would make pockets of air to keep the bones light.[52] Similar structures are observable in birds and large mammals.[53] The cervical vertebrae were stouter than those of other diplodocids, being wide and rectangular in anterior view. On the lateral sides of the cervicals, apatosaurines had well-developed and thickparapophyses (extensions on the lateral sides of the vertebrae that attached to cervical ribs) which would point ventrally under thecentrum. These parapophyses in conjunction with densediapophyses andcervical ribs were strong anchors for neck muscles, which could sustain extreme force.[54] The cervicals were also more boxy than in other sauropods due to their truncatedzygapophyses and tall build.[55][1] These vertebrae are triangular in anterior view, whereas they most often are rounded or square in genera likeCamarasaurus. Despite its pneumaticy, the necks ofBrontosaurus andApatosaurus are thought to have been double the mass of that of other diplodocids due to the former's sturdiness.[54]Brontosaurus differs fromApatosaurus in several regards, such as their posterior dorsal vertebrae'sneural spines are longer than they are wide andApatosaurus' posterior cervicals bear a detached horizontal lamina in the fossae of theirdiapophyseal laminae in the mid-posterior cervicals.[1]
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Their dorsal vertebrae have short centra with largefossae (shallow excavations) on their lateral sides, though not as extensively as the cervicals'.[56] Neural canals, which contain thespinal cord of the vertebral column, are ovate and large in the dorsals. The diapophyses protrude outward and curve downward in a hook-shape. Neural spines are thick in anterior-posterior view with a bifurcate top.[1] The neural spines of the dorsals would increase in height further towards the tail, creating an arched back. Apatosaurine neural spines compose more than half the height of the vertebrae. The dorsal ribs are not fused or tightly attached to their vertebrae, instead being loosely articulated.[26] Tendorsal ribs are on either side of the body of apatosaurines.[25] There are widened excavations within the sacrum, giving it a hollow cylindrical shape. The posteriormost caudal vertebra was lightly fused to the sacral vertebrae, becoming part of the plate. Internally, theneural canal was enlarged.[57][58][25] The shape of the tail was typical for diplodocids, being comparatively slender due to the vertebral spines rapidly decreasing in height the farther they are from the hips. As in other diplodocids, the tail ended in a strongly elongated, whip-like structure.[26] The tail also had an extensive air-sac system as observed in specimens ofB. parvus.[59][60]
Severalscapulae are known from apatosaurines, all of which are long and thin with relatively elongated shafts.[57] One of traits that distinguishesBrontosaurus andApatosaurus is the presence of a depression on the posterior face of the scapula, which the latter lacks. The scapula ofBrontosaurus also has a rounded extension off of its edge, a characteristic unique toBrontosaurus among Apatosaurinae. Thecoracoids of apatosaurines have a quadratic outline in dorsal view.Sterna have been preserved in some specimens ofApatosaurus andBrontosaurus, which display an oval outline.[1] The hip bones include robustilia and the fusedpubes andischia. The limb bones are also very robust,[61] with thehumerus resembling that ofCamarasaurus. The humerus had a thinbone shaft and larger transverse ends. Its anterior end bears a largedeltopectoral crest, which was on the extremities of the bone.[62] Charles Gilmore in 1936 noted that previous reconstructions erroneously proposed that theradius andulna could cross, when in life they would have remained parallel.[26] Apatosaurines have a single large claw on each forelimb which faces towards the body, whereas the rest of thephalanges lackedunguals.[63] Even by 1936, it was recognized that no sauropod had more than one hand claw preserved, and this one claw is now accepted as the maximum number throughout the entire group.[26][64] The metacarpals are elongated and thinner than the phalanges, bearing boxy articular ends on its proximal and distal faces.[5] The single front claw bone is slightly curved and squarely shortened on the front end. 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. The single manual claw bone (ungual) is slightly curved and squarely truncated on the anterior end. Proportions of the manus bones vary within Apatosaurinae as well, withB. yahnahpin's ratio of longest metacarpal to radius length around 0.40 or greater compared to a lower value inApatosaurus louisae.[1] The femora of apatosaurines are very stout and represent some of the most robust femora of any member of Sauropoda. Thetibia andfibula bones are different from the slender bones ofDiplodocus but are nearly indistinguishable from those ofCamarasaurus. The fibula is longer and slenderer than the tibia. Apatosaurine feet have 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 all diplodocids.[26]
Apatosaurinae forms, together with Diplodocinae, the clade Diplodocidae. Diplodocids have been found inAfrica,Europe, andNorth America, the group originating in theMiddle Jurassic and going extinct in theEarly Cretaceous.[65][66] Apatosaurinae is definitively represented by only two genera;Apatosaurus andBrontosaurus, according to a comprehensive 2015 study by researcher Emmanuel Tschopp and colleagues which found the two to be separate genera.[1] It isstem-based, being defined as all taxa more closely related toApatosaurus thanDiplodocus by paleontologistsMike Taylor andDarren Naish in 2005.[67] Two other sauropods,Amphicoelias and a species of the dubious genusAtlantosaurus,A. immanis, have been suggested to be part of the group as well. However, some studies have placedAmphicoelias as a basal member of Diplodocidae or Diplodocoidea,[1] though a 2021 paper found it to be an apatosaurine in three phylogenetic analyses.[68] A 2007 paper foundSupersaurus andSuuwassea to be within Apatosaurinae and sister toApatosaurus, though the reason for perceived similarities betweenApatosaurus andSupersaurus may have been due to size-coupled increases in robustness in sauropods.[61] This analysis also foundSuuwassea to be the basalmost member of Apatosaurinae,[61] however more detailed descriptions of its remains suggest it was a basaldicraeosaurid.[69][70] Recent studies have found little support for includingSupersaurus orSuuwassea in Apatosaurinae,[71][1] limiting the group toApatosaurus,Brontosaurus, and possiblyAmphicoelias.[1][68] Several indeterminate apatosaurine skeletons have also been found and likely belong to eitherApatosaurus orBrontosaurus, such as the specimens AMNH 460 and FMNH P25112, though their statuses are inconclusive.[1] Dozens of isolated or fragmentary specimens have also been assigned to the genera, but were not reviewed by Tschopp and colleague's study.[45]
Thecladogram below is the result of a 2015 analysis by Tschopp, Mateus, and Benson. The authors analyzed most diplodocid type specimens separately to deduce which specimen belonged to which species and genus.[1]
| Apatosaurinae |
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Marsh was the first to recognize the differences betweenApatosaurus andBrontosaurus versus other sauropods and researcherOtto Jaekel erected Brontosauridae in 1911.[72] However, the name Apatosaurinae is attributed to a 1927 paper byFriedrich von Huene, who included onlyApatosaurus and the invalidUintasaurus in the subfamily, according to the ICZN'sprinciple of coordination (publishing a new zoological name simultaneously applies the name to all usableranks). He considered Apatosauridae, Diplodocidae, Dicraeosauridae,Morosauridae,Cardiodontidae, andBrachiosauridae to be part ofSauropoda.[73] Its first explicit use as a subfamily was in a 1929 paper byWerner Janensch, who classified it, including onlyApatosaurus, within the family Homalosauropodidae.[74] Homalosauropodidae was thought to contain the other subfamilies Titanosaurinae, Dicraeosaurinae, and Diplodocinae. Later sauropod specialists considered Apatosaurinae and Diplodocinae to be together in Diplodocidae, which excluded Titanosauria and Dicraeosauridae.[39][1]
Below is a cladogram of apatosaurinae interrelationships based on Tschoppet al., 2015.[1]
WhenBrontosaurus andApatosaurus were described in the late 1870s, the widespread notion in the scientific community was that sauropods weresemi-aquatic, lathargic reptiles that were inactive.[39][5][17] In Othniel Marsh's publicationThe Dinosaurs of North America, he describedB. excelsus as "more or less amphibious, and its food was probably aquatic plants or other succulent vegetation" (174).[5] This is unsupported by fossil evidence. Instead, sauropods were active and had adaptations for dwelling on land.[39] Marsh also noted the animal's supposed lack of intellect based on the smallbraincase of the Felch Quarry skull and slenderneural cord. A 2009 study has found that the brain of the related sauropodBrachiosaurus was small, even for a reptile, suggesting a low intellect.[75]
Trackways of sauropods show that they could potentially reach a top speed of 12–19 km/h (7.5–11.8 mph).[76] The slow locomotion of sauropods may be due to the minimal muscling or recoil after strides, a predictable quality due to the gigantism and graviportal limb posture of sauropods.[77][78]
Being diplodocids, apatosaurines were herbivorous and fed onferns,cycadeoids,seed ferns, andhorsetails.[50] Unlike in diplodocines, the entire tooth row ofApatosaurus was replaced at once. The teeth ofApatosaurus are thick, lack denticles, and are strongly cylindrical in cross-section whereas they are long, slender, and elliptical in cross-section inDiplodocus. These characteristics imply thatApatosaurus consumed tougher vegetation thanDiplodocus.[38] Diplodocids in general also have shorter necks than the long-necked, vertically inclined macronarians.[79][80] This would result inniche partitioning, the various taxa thus avoiding direct competition with each other due to feeding on different plants and at different heights.[68]
Endotherms (mammals) andectotherms (reptiles) require a specific amount ofnutrition to survive which correlates with theirmetabolism as well as body size. Estimations of the dietary necessities ofBrontosaurus were made in 2010, with an estimate of 2•10^4 to 50•10^4kilojoules needed daily. This led to hypotheses on the distributions ofBrontosaurus to meet this requirement, though whether it was an ectotherm or endotherm is debated. IfBrontosaurus was an endotherm, fewer adult individuals could be sustained than if it were an ectotherm, which could have tens of animals per square kilometer.[81][82] Due to this, it has been theorized that sauropods living within the arid environment of the Morrison Formation participated in migrations between feeding sites.[83] James Farlow in a 1987 paper calculated that aBrontosaurus-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 gut contents.[84] AssumingApatosaurus had an avian respiratory system and a reptilian resting-metabolism, Frank Paladino and colleagues estimated in 1987 that the animal would have needed to consume only about 262 liters (58 imp gal; 69 U.S. gal) of water per day.[85]
Historically, apatosaurines and other sauropods were believed to have been too massive to support their weight on land, and were therefore assumed to have lived partly submerged in water, perhaps in swamps. Recent findings do not support this, and sauropods are now thought to have been fully terrestrial.[86] Diplodocids are often portrayed with their necks held high up in the air, allowing them to browse on tall trees. Though some studies have suggested that diplodocid necks were less flexible than previously believed,[87] other studies have found that alltetrapods appear to hold their necks at the maximum possible vertical extension when in a normal, alert posture, and argue that 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.[88]
James Spotila and colleagues suggested in 1991 that the large body size of sauropods would have made them unable to maintain high metabolic rates, as they would not be able to release enough heat. However, temperatures in the Jurassic were 3 degrees Celsius higher than present.[89] Furthermore, they assumed that the animals had a reptilian respiratory system. Matt Wedel found that an avian system would have allowed them to dump more heat.[90] Some scientists have also argued that the heart would have had trouble sustaining sufficient blood pressure to oxygenate the brain.[86]
Given the large body mass and long neck of sauropods, physiologists have encountered problems determining how these animals breathed. Beginning with the assumption that, likecrocodilians,Brontosaurus andApatosaurus 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.[85]
Based on this, its respiratory system would likely have consisted ofparabronchi, 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 of the same-sizedApatosaurus has been estimated at 1.7 m3 (1,700 L), allowing for a 0.50 m3 (500 L), a 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.[85] Evidence for the avian system in apatosaurines and other sauropods is also present in the pneumaticity of the vertebrae. Though this plays a role in reducing the weight of the animal, a 2003 publication by Matt Wedel states they are also likely connected to air sacs, as in birds.[90]
A 1999 microscopic study ofApatosaurus andBrontosaurus bones concluded the animals grew rapidly when young and reached near-adult sizes in about 10 years.[91] In 2008, a study on the growth rates of sauropods was published by biologists Thomas Lehman and Holly Woodward. They said that by using growth lines and length-to-mass ratios,Apatosaurus would have grown to 25 t (25 long tons; 28 short tons) in 15 years, with growth peaking at 5,000 kg (11,000 lb) in a single year. An alternative method, using limb length and body mass, foundBrontosaurus andApatosaurus grew 520 kg (1,150 lb) per year, and reached their full mass before it was about 70 years old.[92] These estimates have been called unreliable because the calculation methods are not sound; old growth lines would have been obliterated by bone remodeling.[93] 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.[26][25] JuvenileBrontosaurus material is known based on the type specimen ofB. parvus. The material of this specimen, CM 566, includes vertebrae from various regions, one pelvic bone, and some bones of the hindlimb.[94] When describingB. parvus, Peterson and Gilmore noted that the neural spines were sutured, the sacral vertebrae were unfused, features that are signs of immaturity in other archosaurs, showing that sauropods had these traits too.[95]
A 1997 study byNathan Myhrvold, acomputer scientist fromMicrosoft, carried out acomputer simulation of the tail, which in diplodocids, including apatosaurines, was a very long, tapering structure resembling abullwhip. This computer modeling suggested that sauropods were capable of producing a whip-like cracking sound of over 200decibels, comparable to the volume of acannon.[96] There is some circumstantial evidence supporting this as well: a number of diplodocids have been found with fused or damaged tail vertebrae, which may be a symptom of cracking their tails: these are particularly common between the 18th and the 25th caudal vertebra, a region the authors consider a transitional zone between the stiff muscular base and the flexible whiplike section.[97] However, a 2012 article notes thatCamarasaurus while lacking a tailwhip, displays a similar level of caudal co-ossification and thatMamenchisaurus while having the same pattern of vertebral metrics, lacks a tailwhip and does not display fusion in any "transitional region". Also, the crush fractures which would be expected if the tail was used as a whip have never been found in diplodocids.[98] More recently in 2021, Matthew Baron has considered 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 to the sides of the animal in a group, 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 others behind, or communicating vocally.[99] A 2022 study by Simone Conti and colleagues proved that the whip-like cracking would not be possible through the use of more accurate 3D models and mathematical data.[100]
The cervical vertebrae ofBrontosaurus andApatosaurus are robust, which has led to speculation on the purpose of the neck's robusticity. The neck had expensive energy requirements, so the reason for their evolution must have been important to the animal. notable features include: dense cervical ribs and diapophyses, ribs that are angled ventrally, and an overall subtriangular cross-section.[101] Cervical ribs acted as anchors for thelongus colli ventralis andflexer colli lateralis muscles, which are used in the ventral motion of the neck. Stronger muscles for ventral motions allowed more force to be exerted ventrally. The cervical ribs formed a "V"-shape in anterior view, which could be used to shelter the softer underlying tissues of the neck from damage. The bottoms of these cervical ribs were capped by round, protrudingprocesses (extenstions of bone). These have been suggested to have been attachment points forbosses orkeratinous spikes.[102][54] In a 2015preprint, Taylor and colleagues argued that due to the combination of these traits, apatosaurines would use their necks for combat between individuals by striking each other with their necks, with this hypothesis being nicknamed "Brontosmash".[103][54][104] Behavior like this has been observed in other animals like giraffes.[105][106]
All known apatosaurine material has been reported fromoutcrops in the US states ofArizona,[107] Colorado,[4]Montana,[108][109]New Mexico,[110] Oklahoma,[48]South Dakota,[111]Utah,[1] and Wyoming.[37][17] The Morrison Formation is a sequence of shallow marine and alluvial sediments which, according toradiometric dating, ranges between 156.3 million years old (Mya) at its base,[112] and 146.8 Mya at the top,[113] which places it in the lateOxfordian, Kimmeridgian, and earlyTithonianstages of the Late Jurassic period. This formation is interpreted as asemiarid environment with distinctwet anddry seasons.[46] The Morrison Formation was a hotspot of sauropod evolution, with the presence of at least 6 different families of sauropods present.[114][14][81]
Fossils ofBrontosaurus are relatively uncommon whereasApatosaurus is the second most common sauropod in the formation, afterCamarasaurus.Apatosaurus andBrontosaurus may have been more solitary animals than other Morrison Formation dinosaurs.[115] Both genera existed for a long interval, and were found in most levels of the Morrison.B. excelsus,A. ajax, andA. louisae fossils have been reported from only the Brushy Basin Member, dating to the late Kimmeridgian age, about 151 Mya.[109] OlderBrontosaurus remains have also been identified from the middle Kimmeridgian, and are assigned toB. parvus.[94] Fossils of these animals have been found inNine Mile Quarry andBone Cabin Quarry in Wyoming and at sites in Colorado, Oklahoma, and Utah, present in stratigraphic zones 2–6 according to John Foster's model.[79]Amphicoelias has been reported from a single site in Garden Park, Colorado, which bears layers dating to the lower Tithonian and come from the Brushy Basin Member.[68]
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