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Amphicoelias

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Amphicoelias
Temporal range:Late Jurassic,
150Ma
↓
Holotype vertebrae ofA. altus,AMNH5764
Scientific classificationEdit this classification
Domain: Eukaryota
Kingdom: Animalia
Phylum: Chordata
Clade: Dinosauria
Clade: Saurischia
Clade: †Sauropodomorpha
Clade: †Sauropoda
Superfamily: †Diplodocoidea
Family: †Diplodocidae
Genus: †Amphicoelias
Cope,1878[1]
Species:
†A. altus
Binomial name
†Amphicoelias altus
Cope, 1878[1]

Amphicoelias(/ˌæmfɪˈsiːliəs/,meaning "biconcave", from theGreekἀμφί,amphi:"on both sides", and κοῖλος,koilos:"hollow, concave" ) is agenusof herbivoroussauropoddinosaurthat lived approximately 150 million years ago during theTithonian(Late JurassicPeriod) of what is nowColorado,United States.Amphicoeliaswas moderately sized at about 18 metres (59 ft) in length and 15 metric tons (17 short tons) in body mass, shorter than its close relativeDiplodocus.Its hindlimbs were very long and thin, and its forelimbs were proportionally longer than in relatives.

The namesake fossil of thetype speciesAmphicoelias altus,American Museum of Natural History5764, is uncertain in included material. When described byEdward Drinker Copeshortly after its discovery in1877,Amphicoeliaswas noted to include many backvertebrae,a singlepubis,and afemur.However, after purchase and cataloging of the material by the AMNH,Henry Fairfield Osbornand Charles Mook described that the specimen had only two vertebrae, a pubis, femur,tooth,scapula,coracoid,ulnaand a second femur. The additional material, not mentioned by Cope, was found close in proximity to theholotypeand was similar toDiplodocus,a relative ofAmphicoelias.Their assignment was questioned by subsequent authors Emanuel Tschoppet al.in an analysis ofDiplodocidae.

During the description ofAmphicoelias altusin 1877, Cope additionally namedA. latus,for a femur and tail vertebrae. Following its description, Osborn and Mook in1921reidentified the material as a specimen ofCamarasaurus,an assignment followed by other authors who reviewed the material. A year later1878,Cope named the third species ofAmphicoelias,A. fragillimusfor a gigantic dorsal vertebra that was subsequently lost. Measuring approximately 2.7 m (8.9 ft) if reconstructed based onDiplodocus,early estimates for the length of the animal in life were between 40 and 60 m (130 and 200 ft) long. Due to the incomplete nature, such lengths–the longest of any known dinosaur and sauropod–were largely ignored. In2018,Kenneth CarpenterrenamedAmphicoelias fragillimusas the new genusMaraapunisaurus,and reclassified it fromDiplodocidaetoRebbachisauridae.

Discovery and history

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E.D. Cope's original sketch ofAmphicoelias altusas an "amphibious lizard", c. 1897

In1877,a number of fossils were discovered by Oramel William Lucas in rock outcrops inColoradoclose toCañon City.These bones, including some namedLaelaps trihedrodon,Camarasaurus supremusandCaulodon,were assumed to be from theCretaceousDakota FormationbypaleontologistEdward Drinker Cope,from the same age as otherLaelapsfossils.[1]Among the assortment of fossils discovered, a specimen including multipledorsal vertebrae,apubis,and afemurwas found by Aaron Ripley, Lucas' brother-in-law, in Quarry XII. They were sent to Cope on October 21, 1877.[2]Cope concluded they belonged to a new taxon withinSauropoda,which Cope namedAmphicoelias altusin December 1877.[3]Cope determined that the taxon would have been asauropodrelated toCamarasaurusfound nearby, both being large animals with lightly built vertebrae and solid limbs.[1]Cope sold his collection of fossils to theAmerican Museum of Natural History(AMNH) in 1895. Shortly before his death in 1897, he worked with the museum's artistCharles R. Knightto produce illustrations and paintings depicting some of his species in life, includingAmphicoelias.Based on notes accompanying his sketches provided to Knight as reference material, Cope evidently considered bothAmphicoeliasandBrontosaurusjunior synonyms ofCamarasaurus supremusat this time. The specimen was given theaccession numberAMNH 5764, then including two dorsals, a pubis, a femur, atooth,ascapula,coracoidand anulna.While the forelimb material was not originally assigned toAmphicoelias,paleontologistsHenry Fairfield Osbornand Charles Mook in1921added them to the holotype specimen on the basis that they were found in the same strata not far away, and bore differences from theCamarasaurusthey were found alongside.[4]Because of the uncertainty about the referral of these remains, in2015Emanuel Tschopp and colleagues hesitantly rejected the referral of the tooth, scapula and coracoid, but accepted the ulna referral.[5]Based on collection data, the specimen was determined to have come from theLate JurassicMorrison Formation,in the youngest layers, separate from those that held theCamarasaurusfossils also described by Cope.[4]

1897 restoration of aquaticA. altus,byCharles R. Knight,based on Cope's original.

Cope listed multiple features to separateAmphicoeliasandA. altusfrom their relatives. Unlike inCamarasaurus,thecentraof dorsals did not have anopisthocoely,where the front end has a ball that fits into the socket of the preceding vertebra. Instead,Amphicoeliashadamphicoelousvertebrae, where both ends are mildly to moderately concave. Because of this difference, Cope named two separate families to contain the different morphs of vertebral centra, one beingCamarasauridaeand the other beingAmphicoeliidae.As well,Camarasauruspossessedpleurocoels(openings in the side of the centrum) that were taller than long-contrasting to the condition inAmphicoelias- and a more robust pubis and femur. However, both families also had accessory articulations in theneural arch,which Cope termed thehyposphene,limiting the motion of vertebrae respective to one another.[1]Amphicoeliidae andAmphicoeliaswere both found to be intermediate and poorly defined by paleontologistOthniel Charles Marshin1881,wherein the primary feature-two mildly concave articular faces-was shown to be widespread and also found in sauropods likeBrontosaurus excelsus.[6][7]

Previously assigned species

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Amphicoelias latuswas named in the same description as the type species,Amphicoelias altus.It was named for a series of fourcaudal vertebraeand a femur (AMNH 5765), in relatively good preservation. The caudals are bi-concave like the dorsals ofA. altus,with short centra, long prezygapophyses and shallow pleurocoels. The femur is extremely robust, as well as being wide but short front-to-back. The femur is 1.4 m (4.6 ft) long, but the veryproximalend is not fully preserved.[1]Due to the robusticity of the femur Osborn & Mook in 1921 referredAmphicoelias latustoCamarasaurus supremus,makingC. supremusthe valid name for the material once calledA. latus.[4]

The third namedAmphicoeliasspecies,A. fragillimus,was known only from a single, incomplete 1.5 m (4.9 ft) tallneural arch,either last or second to last in the series of back vertebrae. Based only on an illustration published in 1878, this vertebra would have measured 2.7 meters (8.9 ft) tall in life.[8]However, it has been argued that the scale bar in the published description contained a typographical error, and the fossil vertebra was in fact only 1.38 meters (4.5 ft) tall.[9]In addition to this vertebra, Cope's field notes contain an entry for an "[i]mmense distal end of femur”, located only a few tens of meters away from the giant vertebra. It is likely that this undescribed leg bone belonged to the same individual animal as the neural spine, but it was never collected or described.[8]In 2018,A. fragillimuswas given its own genus,Maraapunisaurus,and reclassified as a primitive rebbachisaurid.[10]

Three skeletons informally assigned to "A. brontodiplodocus" in 2010, now labelled as diplodocids inLee Kong Chian Natural History Museum,Singapore

In 2010, an article was made available, but not formally published, by Henry Galiano and Raimund Albersdorfer in which they referred to the new species "A. brontodiplodocus" toAmphicoelias,based on several complete specimens found in the Dana Quarry of Big Horn Basin, Wyoming and held in a private collection. The specific name referred to their hypothesis based on these specimens that nearly all Morrison diplodocid species are either growth stages or represent sexual dimorphism among members of the genusAmphicoelias,[11]but this analysis has been met with skepticism and the publication itself has been disclaimed by its lead author, explaining that it is "obviously a drafted manuscript complete with typos, etc., and not a final paper. In fact, no printing or distribution has been attempted".[12]

Osborn and Mook, in 1921, provisionally synonymizedA. fragillimuswithA. altus,while sinkingA. latusintoCamarasaurus supremus,and suggesting also thatA. fragillimusis just a very large individual ofA. altus,a position that most subsequent studies, including McIntosh 1998, Foster (2007), and Woodruff and Foster (2015) have agreed with.[13][4]Carpenter (2006) disagreed about the synonymy ofA. altusandA. fragillimus,however, citing numerous differences in the construction of the vertebra also noted by Cope, and suggested these differences are enough to warrant a separate species or even a separate genus forA. fragillimus.However, he went on to caution that the validity ofA. fragillimusas a separate species is nearly impossible to determine without the original specimen to study.[8]AlthoughAmphicoelias latusis clearly notAmphicoelias,it is probably synonymous withCamarasaurus grandisrather thanC. supremusbecause it was found lower in the Morrison Formation and the deeply concave articular faces on the caudal vertebrae are more consistent withC. grandis.[14]

Description

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Restoration ofA. altus

Henry Fairfield OsbornandCharles Craig Mooknoted the overall close similarity betweenAmphicoeliasandDiplodocus,as well as a few key differences, such as proportionally longer forelimbs inAmphicoeliasthan inDiplodocus.The femur ofAmphicoeliasis unusually long, slender, and round in cross section; while this roundness was once thought to be another distinguishing characteristic ofAmphicoelias,it has since been found in some specimens ofDiplodocusas well.[8]Gregory S. Paulinitially estimatedA. altusto be similar in size toDiplodocusat 25 metres (82 ft) in length,[15]but later moderated its size at 18 metres (59 ft) in length and 15 metric tons (17 short tons) in body mass.[16]

The dorsal vertebrae ofAmphicoeliasare partly incomplete, but their anatomy is discernible from the known remains. The centrum is very compressed in the middle in all dimensions, with a largelateralpleurocoel set inside a large lateralfossa.InAmphicoeliastheneural archis very tall, and along its side there is a prominent lamina extending from the posterior centrum to theprezygapophyses(articular surfaces with the neural arch of the preceding vertebra). From directly behind the prezygapophyses, thediapophyes(lateral processes for rib articulation) project slightly upwards and outwards, surrounded by shallow fossae and a large lamina extending up theneural spine.The neural spine is thin, with a pair of ridges going up along either side on the edges. Thedistalend is wide compared to the main spine, but approximately subequal in length and width.[1]

A partial forelimb, provisionally referred toAmphicoeliasby Osborn & Mook in 1921, resemblesDiplodocusbut is more robust overall. The distal end of the scapula, while only partially preserved, show that the expansion of thescapula bladewas smaller thanCamarasaurusbut larger thanApatosaurus.161 cm (63 in) long as preserved, the bone is noticeably thicker than inDiplodocus,but not quite as thick as inCamarasaurus.The coracoid found alongside the scapula is far more similar toDiplodocusthanCamarasaurus,being round and longer than tall. However, it is also thicker than inDiplocodus.Theforamenin the coracoid is large and centered on the short axis of the bone. The ulna is more elongate than any comparable bone known fromDiplocodus.It has prominent articular faces for the humerus and radius, and narrows toward the incomplete distal end.[4]

The pubis ofAmphicoeliasis very fragmentary, and the only discernible characteristics are that it is long but thick, and has a small surface for articulation with theilium.[4]A single, slender right femur ofAmphicoeliasis known from the holotype, 1.524 m (5 ft) long. It is slightly longer than the femur ofCamarasaurus supremus,but significantly lessrobust,being approximately round in cross-section and only 22 cm (8.7 in) wide. The shaft of the femur is gently curved towards the hip articulation, and straight close to the prominent articularcondyles.[1]A second, partial left femur was also found in the collections of the AMNH, and was assigned to the holotype specimen, while it was not mentioned by Cope. It lacks a majority of the upper bone, but the known shaft and end are very similar to the one figured by Cope, and it may be the left femur of the same individual.[4]

Classification

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In 2007, John Foster suggested that the differences usually cited to differentiateAmphicoelias altusfrom the more well knownDiplodocusare not significant and may be due to individual variation. Foster argued thatAmphicoeliasis probably thesenior synonymofDiplodocus,and that if further research bears this out, the familiar nameDiplodocuswould need to be abandoned in favor ofAmphicoelias,as was the case withBrontosaurusand its senior synonymApatosaurus.[17]In 2015, Woodruff and Foster reiterated this conclusion, stating that there is only one species ofAmphicoeliasand that it could be referred toDiplodocusasDiplodocus altus.They considered the nameAmphicoeliasto be anomen oblitum.[9]This conclusion was disputed by Emanuel Tschoppet al.in 2015, whereAmphicoeliaswas analysed as part of an extensive specimen-level phylogenetic analysis of Diplodocidae. There were two strongly supported placements found forAmphicoelias,as the basalmost genus withinApatosaurinae,and as an apatosaurine nested betweenBrontosaurus excelsusandBrontosaurus yahnahpin.There were no unambiguous characters found to support an assignment within Apatosaurinae, so the authors instead opted to classifyAmphicoeliasas the basalmost diplodocid outside Apatosaurinae and Diplodocinae, having primitive features, minimal significant differences from either subfamily, a slightly more parsimonious apatosaurine phylogeny, but a slightly more diplodocine morphology. The followingcladogramof the Diplodocidae follows the species-level agreement tree of Tschoppet al.(2015) showingA. altusas the basalmost diplodocid.[5]

Skeletal mount of potentially closely relatedBrontosaurus excelsusholotype YPM 1980
Diplodocidae

Paleoecology

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In his 2006 re-evaluation, Carpenter examined the paleobiology of giant sauropods, includingAmphicoelias,and addressed the question of why this group attained such a huge size. He pointed out that gigantic sizes were reached early in sauropod evolution, with very large sized species present as early as thelate Triassicperiod, and concluded that whatever evolutionary pressure caused large size was present from the early origins of the group. Carpenter cited several studies of giantmammalianherbivores, such aselephantsandrhinoceros,which showed that larger size in plant-eating animals leads to greater efficiency in digesting food. Since larger animals have longerdigestive systems,food is kept in digestion for significantly longer periods of time, allowing large animals to survive on lower-quality food sources. This is especially true of animals with a large number of 'fermentation chambers' along the intestine, which allow microbes to accumulate and ferment plant material, aiding digestion. Throughout their evolutionary history, sauropod dinosaurs were found primarily in semi-arid, seasonally dry environments, with a corresponding seasonal drop in the quality of food during the dry season. The environment ofAmphicoeliaswas essentially asavanna,similar to the arid environments in which modern giant herbivores are found, supporting the idea that poor-quality food in an arid environment promotes the evolution of giant herbivores. Carpenter argued that other benefits of large size, such as relative immunity from predators, lower energy expenditure, and longer life span, are probably secondary advantages.[8]

Paleoenvironment restoration of a young diplodocid feeding on ferns

TheMorrison Formationenvironment in whichAmphicoeliaslived would have resembled a modern savanna, though sincegrassesdid not appear until theLate Cretaceous,fernswere probably the dominant plant and main food source forAmphicoelias.Though Engelmannet al.(2004) dismissed ferns as a sauropod food source due to their relatively low caloric content,[18]Carpenter argued that the sauropod digestive system, well adapted to handle low-quality food, allows for the consumption of ferns as a large part of the sauropod diet.[8]Carpenter also noted that the occasional presence of large petrified logs indicate the presence of 20–30 m (66–98 ft) tall trees, which would seem to conflict with the savanna comparison. However, the trees are rare, and since tall trees require more water than the savanna environment could generally provide, they probably existed in narrow tracts or "gallery forests" along rivers and gulleys where water could accumulate. Carpenter speculated that giant herbivores likeAmphicoeliasmay have used the shade of the gallery forests to stay cool during the day, and done most of their feeding on the open savanna at night.[8]

References

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  1. ^abcdefghCope, E.D. (1878a). "On the Vertebrata of the Dakota Epoch of Colorado".Proceedings of the American Philosophical Society.17:233–247.
  2. ^J.S. McIntosh, 1998, "New information about the Cope collection of sauropods from the Garden Park, Colorado", In: Carpenter, K., Chure, D. and Kirkland, J.I. (eds.)The Morrison Formation: An Interdisciplinary Study. Part 2. Modern Geology23(1–4): 481–506
  3. ^Cope, E.D. (1877)."OnAmphicoelias,a genus of Saurians from the Dakota epoch of Colorado "(PDF).Proceedings of the American Philosophical Society.17:242–246.
  4. ^abcdefgOsborn, H.F.; Mook, C. C. (1921). "Camarasaurus,Amphicoeliasand other sauropods of Cope ".Memoirs of the American Museum of Natural History.New Series.3(3): 249–387.hdl:2027/mdp.39015042532476.
  5. ^abTschopp, E.; Mateus, O. V.; Benson, R. B. J. (2015)."A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda)".PeerJ.3:e857.doi:10.7717/peerj.857.PMC4393826.PMID25870766.
  6. ^Marsh, O.C. (1881)."Principal Characters of American Jurassic Dinosauria: Part IV"(PDF).American Journal of Science.21(125): 417–423.Bibcode:1881AmJS...21..417M.doi:10.2475/ajs.s3-21.125.417.S2CID219234316.
  7. ^Wilson, J.A.; Smith, M. (1996). "New remains ofAmphicoeliasCope (Dinosauria: Sauropoda) from the Upper Jurassic of Montana and diplodocoid phylogeny ".Journal of Vertebrate Paleontology.16(Supplement 3): 73A.
  8. ^abcdefgCarpenter, K. (2006). "Biggest of the big: a critical re-evaluation of the mega-sauropodAmphicoelias fragillimus".In Foster, J.R.; Lucas, S.G. (eds.).Paleontology and Geology of the Upper Jurassic Morrison Formation.Vol. 36. New Mexico Museum of Natural History and Science Bulletin. pp. 131–138.
  9. ^abWoodruff, C; Foster, JR (2015)."The fragile legacy of Amphicoelias fragillimus (Dinosauria: Sauropoda; Morrison Formation – Latest Jurassic)".PeerJ PrePrints.doi:10.7287/peerj.preprints.838v1.
  10. ^Carpenter, K. (2018)."Maraapunisaurus fragillimus,N.G. (formerlyAmphicoelias fragillimus), a basal Rebbachisaurid from the Morrison Formation (Upper Jurassic) of Colorado ".Geology of the Intermountain West.5:227–244.doi:10.31711/giw.v5i0.28.
  11. ^Galiano, H.; Albersdorfer, R (2011)."A new basal diplodocid species,Amphicoelias brontodiplodocus,from the Morrison Formation, Big Horn Basin, Wyoming, with taxonomic reevaluation ofDiplodocus,Apatosaurus,and other genera "(PDF).Dinosauria International, LLC. pp. 1–44. Archived fromthe original(PDF)on July 10, 2011.
  12. ^Taylor, M. (October 7, 2010)."The elephant in the living room:Amphicoelias brontodiplodocus".Sauropod Vertebra Picture of the Week.
  13. ^McIntosh, J.S. (1998). "New information about the Cope collection of sauropods fromGarden Park, Colorado".In Carpenter, K.; Chure, D.; Kirkland, J.I. (eds.).The Upper Jurassic Morrison Formation: an interdisciplinary study: Modern Geology.Vol. 23. pp. 481–506.
  14. ^Carpenter, K. (1998). "Vertebrate biostratigraphy of the Morrison Formation near Canon City, Colorado: In: The Upper Jurassic Morrison Formation: an interdisciplinary study". In Carpenter, K.; Chure, D.; Kirkland, J.I. (eds.).The Upper Jurassic Morrison Formation: an interdisciplinary study: Modern Geology.Vol. 23. pp. 407–426.
  15. ^Paul, G.S. (1994a).Big sauropods — really, really big sauropods.The Dinosaur Society. pp. 12–13.
  16. ^Paul, Gregory S. (2016).The Princeton Field Guide to Dinosaurs.Princeton University Press. p. 212.ISBN978-1-78684-190-2.OCLC985402380.
  17. ^Foster, J. (2007).Jurassic West: The Dinosaurs of the Morrison Formation and Their World.Indiana University Press.
  18. ^Engelmann, G.F.; Chure, D.J.; Fiorillo, A.R. (2004). "The implications of a dry climate for the paleoecology of the fauna of the Upper Jurassic Morrison Formation". In Turner, C.E.; Peterson, F.; Dunagan, S.P. (eds.).Reconstruction of the extinct ecosystem of the Upper Jurassic Morrison Formation: Sedimentary Geology.Vol. 167. pp. 297–308.
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