Amphicoelias

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. 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. 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. , 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" 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. ==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. }} In their 2021 phylogenetic analyses, Mannion, Tschopp and Whitlock recovered Amphicoelias within various positions among Diplodocoidea, and considered its placement as a non-diplodocimorph diplodocoid position to be the most parsimonious one. In 2025, Mannion and Moore also recovered similar results in their phylogenetic analyses, with various placements of Amphicoelias within the clade all being considered parsimonious. ==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. 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==