Amphicoelias

''Amphicoelias'' (, meaning "biconcave", from the Greek ἀμφί, ''amphi'': "on both sides", and κοῖλος, ''koilos'': "hollow, concave") is a genus of herbivorous sauropod dinosaur that lived approximately 150 million years ago during the Tithonian (Late Jurassic Period) of what is now Colorado, United States. ''Amphicoelias'' was moderately sized at about in length and in body mass, shorter than its close relative '' Diplodocus''. Its hindlimbs were very long and thin, and its forelimbs were proportionally longer than in relatives.

The namesake fossil of the type species ''Amphicoelias altus'', American Museum of Natural History 5764, is uncertain in included material. When described by Edward Drinker Cope shortly after its discovery in 1877, ''Amphicoelias'' was noted to include many back vertebrae, a single pubis, and a femur. However, after purchase and cataloging of the material by the AMNH, Henry Fairfield Osborn and Charles Mook described that the specimen had only two vertebrae, a pubis, femur, tooth, scapula, coracoid, ulna and a second femur. The additional material, not mentioned by Cope, was found close in proximity to the holotype and was similar to ''Diplodocus'', a relative of ''Amphicoelias''. Their assignment was questioned by subsequent authors Emanuel Tschopp ''et al.'' in an analysis of Diplodocidae.

During the description of ''Amphicoelias altus'' in 1877, Cope additionally named ''A. latus'', for a femur and tail vertebrae. Following its description, Osborn and Mook in 1921 reidentified the material as a specimen of '' Camarasaurus'', an assignment followed by other authors who reviewed the material. A year later 1878, Cope named the third species of ''Amphicoelias'', ''A. fragillimus'' for a gigantic dorsal vertebra that was subsequently lost. Measuring approximately if reconstructed based on ''Diplodocus'', early estimates for the length of the animal in life were between long. Due to the incomplete nature, such lengths–the longest of any known dinosaur and sauropod–were largely ignored. In 2018, Kenneth Carpenter renamed ''Amphicoelias fragillimus'' as the new genus ''Maraapunisaurus'', and reclassified it from Diplodocidae to Rebbachisauridae.

Discovery and history

In 1877, a number of fossils were discovered by Oramel William Lucas in rock outcrops in Colorado close to Cañon City. These bones, including some named '' Laelaps trihedrodon'', '' Camarasaurus supremus'' and '' Caulodon'', were assumed to be from the Cretaceous Dakota Formation by paleontologist Edward Drinker Cope, from the same age as other '' Laelaps'' fossils. Among the assortment of fossils discovered, a specimen including multiple dorsal vertebrae, a pubis, and a femur was found by Aaron Ripley, Lucas' brother-in-law, in Quarry XII. They were sent to Cope on October 21, 1877. Cope concluded they belonged to a new taxon within Sauropoda, which Cope named ''Amphicoelias altus'' in December 1877. Cope determined that the taxon would have been a sauropod related to ''Camarasaurus'' found nearby, both being large animals with lightly built vertebrae and solid limbs. Cope sold his collection of fossils to the American Museum of Natural History (AMNH) in 1895. Shortly before his death in 1897, he worked with the museum's artist Charles R. Knight to produce illustrations and paintings depicting some of his species in life, including ''Amphicoelias''. Based on notes accompanying his sketches provided to Knight as reference material, Cope evidently considered both ''Amphicoelias'' and ''Brontosaurus'' junior synonyms of ''Camarasaurus supremus'' at this time. The specimen was given the accession number AMNH 5764, then including two dorsals, a pubis, a femur, a tooth, a scapula, coracoid and an ulna. While the forelimb material was not originally assigned to ''Amphicoelias'', paleontologists Henry Fairfield Osborn and Charles Mook in 1921 added them to the holotype specimen on the basis that they were found in the same strata not far away, and bore differences from the ''Camarasaurus'' they were found alongside. Because of the uncertainty about the referral of these remains, in 2015 Emanuel Tschopp and colleagues hesitantly rejected the referral of the tooth, scapula and coracoid, but accepted the ulna referral. Based on collection data, the specimen was determined to have come from the Late Jurassic Morrison Formation, in the youngest layers, separate from those that held the ''Camarasaurus'' fossils also described by Cope.

Cope listed multiple features to separate ''Amphicoelias'' and ''A. altus'' from their relatives. Unlike in ''Camarasaurus'', the of dorsals did not have an , where the front end has a ball that fits into the socket of the preceding vertebra. Instead, ''Amphicoelias'' had vertebrae, 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 being Camarasauridae and the other being Amphicoeliidae. As well, ''Camarasaurus'' possessed (openings in the side of the centrum) that were taller than long-contrasting to the condition in ''Amphicoelias''- and a more robust pubis and femur. However, both families also had accessory articulations in the , which Cope termed the , limiting the motion of vertebrae respective to one another. Amphicoeliidae and ''Amphicoelias'' were both found to be intermediate and poorly defined by paleontologist Othniel Charles Marsh in 1881, wherein the primary feature-two mildly concave articular faces-was shown to be widespread and also found in sauropods like '' Brontosaurus excelsus''.

Previously assigned species

''Amphicoelias latus'' was named in the same description as the type species, ''Amphicoelias altus''. It was named for a series of four caudal vertebrae and a femur (AMNH 5765), in relatively good preservation. The caudals are bi-concave like the dorsals of ''A. 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 long, but the very end is not fully preserved. Due to the robusticity of the femur Osborn & Mook in 1921 referred ''Amphicoelias latus'' to ''Camarasaurus supremus'', making ''C. supremus'' the valid name for the material once called ''A. latus''.

The third named ''Amphicoelias'' species, ''A. fragillimus'', was known only from a single, incomplete tall neural 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 tall in life. 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 tall. In addition to this vertebra, Cope's field notes contain an entry for an " mense 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. In 2018, ''A. fragillimus'' was given its own genus, ''Maraapunisaurus'', and reclassified as a primitive rebbachisaurid.

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" to ''Amphicoelias'', 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 genus ''Amphicoelias'', 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".

Osborn and Mook, in 1921, provisionally synonymized ''A. fragillimus'' with ''A. altus'', while sinking ''A. latus'' into ''Camarasaurus supremus'', and suggesting also that ''A. fragillimus'' is just a very large individual of ''A. altus'', a position that most subsequent studies, including McIntosh 1998, Foster (2007), and Woodruff and Foster (2015) have agreed with. Carpenter (2006) disagreed about the synonymy of ''A. altus'' and ''A. 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 for ''A. fragillimus''. However, he went on to caution that the validity of ''A. fragillimus'' as a separate species is nearly impossible to determine without the original specimen to study. Although ''Amphicoelias latus'' is clearly not ''Amphicoelias'', it is probably synonymous with ''Camarasaurus grandis'' rather than ''C. supremus'' because it was found lower in the Morrison Formation and the deeply concave articular faces on the caudal vertebrae are more consistent with ''C. grandis''.

Description

Henry Fairfield Osborn and Charles Craig Mook noted the overall close similarity between ''Amphicoelias'' and '' Diplodocus'', as well as a few key differences, such as proportionally longer forelimbs in ''Amphicoelias'' than in ''Diplodocus''. The femur of ''Amphicoelias'' is unusually long, slender, and round in cross section; while this roundness was once thought to be another distinguishing characteristic of ''Amphicoelias'', it has since been found in some specimens of ''Diplodocus'' as well. Gregory S. Paul initially estimated ''A. altus'' to be similar in size to ''Diplodocus'' at in length, but later moderated its size at in length and in body mass.

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

A partial forelimb, provisionally referred to ''Amphicoelias'' by Osborn & Mook in 1921, resembles '' Diplodocus'' but is more robust overall. The distal end of the scapula, while only partially preserved, show that the expansion of the was smaller than ''Camarasaurus'' but larger than ''Apatosaurus''. long as preserved, the bone is noticeably thicker than in ''Diplodocus'', but not quite as thick as in ''Camarasaurus''. The coracoid found alongside the scapula is far more similar to ''Diplodocus'' than ''Camarasaurus'', being round and longer than tall. However, it is also thicker than in ''Diplocodus''. The in the coracoid is large and centered on the short axis of the bone. The ulna is more elongate than any comparable bone known from ''Diplocodus''. It has prominent articular faces for the humerus and radius, and narrows toward the incomplete distal end.

The pubis of ''Amphicoelias'' is very fragmentary, and the only discernible characteristics are that it is long but thick, and has a small surface for articulation with the . A single, slender right femur of ''Amphicoelias'' is known from the holotype, long. It is slightly longer than the femur of ''Camarasaurus supremus'', but significantly less robust, being approximately round in cross-section and only wide. The shaft of the femur is gently curved towards the hip articulation, and straight close to the prominent articular . 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.

Classification

In 2007, John Foster suggested that the differences usually cited to differentiate ''Amphicoelias altus'' from the more well known '' Diplodocus'' are not significant and may be due to individual variation. Foster argued that ''Amphicoelias'' is probably the senior synonym of ''Diplodocus'', and that if further research bears this out, the familiar name ''Diplodocus'' would need to be abandoned in favor of ''Amphicoelias'', as was the case with ''Brontosaurus'' and its senior synonym ''Apatosaurus''. In 2015, Woodruff and Foster reiterated this conclusion, stating that there is only one species of ''Amphicoelias'' and that it could be referred to ''Diplodocus'' as ''Diplodocus altus''. They considered the name ''Amphicoelias'' to be a ''nomen oblitum''. This conclusion was disputed by Emanuel Tschopp ''et al.'' in 2015, where ''Amphicoelias'' was analysed as part of an extensive specimen-level phylogenetic analysis of Diplodocidae. There were two strongly supported placements found for ''Amphicoelias'', as the basalmost genus within Apatosaurinae, and as an apatosaurine nested between '' Brontosaurus excelsus'' and '' Brontosaurus yahnahpin''. There were no unambiguous characters found to support an assignment within Apatosaurinae, so the authors instead opted to classify ''Amphicoelias'' as 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 following cladogram of the Diplodocidae follows the species-level agreement tree of Tschopp ''et al.'' (2015) showing ''A. altus'' as the basalmost diplodocid.

Paleoecology

In his 2006 re-evaluation, Carpenter examined the paleobiology of giant sauropods, including ''Amphicoelias'', 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 the late Triassic period, and concluded that whatever evolutionary pressure caused large size was present from the early origins of the group. Carpenter cited several studies of giant mammalian herbivores, such as elephants and rhinoceros, which showed that larger size in plant-eating animals leads to greater efficiency in digesting food. Since larger animals have longer digestive 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 of ''Amphicoelias'' was essentially a savanna, 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.

The Morrison Formation environment in which ''Amphicoelias'' lived would have resembled a modern savanna, though since grasses did not appear until the Late Cretaceous, ferns were probably the dominant plant and main food source for ''Amphicoelias''. Though Engelmann ''et al.'' (2004) dismissed ferns as a sauropod food source due to their relatively low caloric content, 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. Carpenter also noted that the occasional presence of large petrified logs indicate the presence of 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 like ''Amphicoelias'' may 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.

References

External links

* Discussion on the Dinosaur Mailing List:
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Re: Bruhathkayosaurus

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{{Portal bar, Dinosaurs, Colorado

Diplodocidae
Dinosaur genera
Tithonian dinosaurs
Morrison Formation
Dinosaurs of the United States
Fossil taxa described in 1878
Taxa named by Edward Drinker Cope