Name: Amphicoelias ‭(both sides hollow‭)‬.
Phonetic: Am-fee-see-le-as.
Named By: Edward Drinker Cope‭ ‬-‭ ‬1878.
Classification: Chordata,‭ ‬Reptilia,‭ ‬Dinosauria,‭ ‬Saurischia,‭ ‬Sauropodomorpha,‭ ‬Sauropoda,‭ ‬Diplodocoidea.
Species: A.‭ ‬altus‭ (‬type‭)‬.
Diet: Herbivore.
Size: Roughly estimated to be about 25 meters long. Refer to main text for more details.
Known locations: USA,‭ ‬Colorado.
Time period: Kimmeridgian to Tithonian of the Jurassic.
Fossil representation: Fragmentary and isolated remains,‭ ‬original fossils are now lost.

       For one hundred and forty years Amphicoelias was regarded by some to have been one of the largest dinosaurs to ever walk the earth.‭ ‬First named in‭ ‬1878‭ ‬by Edward Drinker Cope,‭ ‬and based upon some truly large fossils of vertebrae and partial limbs and pubis bones.‭ ‬Three species were named,‭ ‬A.‭ ‬altus,‭ ‬A.‭ ‬fragillimus and A.‭ ‬latus.‭ ‬When the largest fossils belonging to A.‭ ‬fragillimus compared to fossils of the dinosaur Diplodocus and scaled up to match the size of the new vertebrae,‭ ‬Amphicoelias was regarded to being at least forty to maybe even sixty meters long.‭ ‬Then a bizarre thing happened,‭ ‬the‭ ‬large‭ ‬fossils of Amphicoelias disappeared.
       For the remainder of the nineteenth and throughout the twentieth century,‭ ‬Amphicoelias was spoken about as an almost mythical dinosaur genus.‭ ‬The only evidence that these fossils existed were the drawings included with the original description,‭ ‬and not from multiple sides and angles as they usually would have been‭ ‬rendered.‭ ‬It is‭ ‬also worth remembering that this happened during‭ ‬the Bone Wars,‭ ‬the great paleontological rivalry between Edward Drinker Cope and Othniel Charles Marsh as they raced against one another to catalogue as many fossils as possible.‭ ‬It could be that the missing fossils are somewhere in a museum storage facility,‭ ‬forgotten about for over a century.
       Throughout the twentieth century the scientific community steadily learned much more about dinosaurs,‭ ‬and Amphicoelias existed as a dubious genus,‭ ‬very difficult to fact check against.‭ ‬In‭ ‬1921‭ ‬one species,‭ ‬A.‭ ‬latus which was based upon the description of tail vertebrae and a femur‭ (‬thigh bone‭) ‬was actually found to actually be from‭ ‬the dinosaur Camarasaurus.‭ ‬This left Amphicoelias with the type species A.‭ ‬altus and the other species A.‭ ‬fragillimus.‭ ‬Several times it has been suggested that these two species were one and the same,‭ ‬just the fossils of A.‭ ‬fragillimus were of a larger individual.‭ ‬Most of the previous forty to sixty meter length estimates were because of the A.‭ ‬fragillimus fossils.
       Some researchers however began to note that the drawing of the fossil vertebra of A.‭ ‬fragillimus was actually less like a diplodocid sauropod,‭ ‬but more like a rebbachisaurid sauropod.‭ ‬The rebbachisaurid sauropods are usually treated as a sub group‭ ‬of diplodocids,‭ ‬related to them but morphologically different.‭ ‬No one knew about rebbachisaurid sauropods when Amphicoelias was named,‭ ‬it was only in the latter half of the twentieth century that they began to be discovered.‭ ‬In‭ ‬2018‭ ‬Kenneth Carpenter formalised the assessment,‭ ‬and moved A.‭ ‬fragillimus into its own genus,‭ ‬as the rebbachisaurid sauropod Maraapunisaurus fragillimus.‭ ‬Comparing the known material of Maraapunisaurus fragillimus to more complete rebbachisaurid sauropod genera such as Limaysaurus and adjusting for size has led to estimates of a little over thirty meters long,‭ ‬not forty to sixty for Maraapunisaurus fragillimus.
       This leaves A.‭ ‬altus as the only valid species,‭ ‬one that is definitively a diplodocid sauropod dinosaur.‭ ‬Comparing what we know about the size of the original fossils of A.‭ ‬altus,‭ ‬we know that the individual dinosaur that left these fossils was probably around twenty-five meters long.‭ ‬This means that while Amphicoelias was a big dinosaur,‭ ‬it was certainly within the known size scope of the diplodocid sauropod dinosaurs.‭

       Because of the lack of surviving fossil material we still don’t know‭ ‬much about the Amphicoelias genus.‭ ‬But as a diplodocid sauropod dinosaur we can infer that it would have been a large twenty-five long herbivore,‭ ‬using‭ ‬its long neck to either reach food growing up in trees or sweeping over wide expanses of low growing plants.‭ ‬Many other types of sauropod dinosaurs shared its‭ ‬environment as well as‭ ‬smaller ornithopod dinosaurs and‭ ‬also stegosaurs too.‭ ‬Some large predators such as Allosaurus would have also been present.

Further reading
-‭ ‬On the Vertebrata of the Dakota Epoch of Colorado,‭ ‬Edward Drinker Cope‭ ‬-‭ ‬1878a.
-‭ ‬Camarasaurus,‭ ‬Amphicoelias and other sauropods of Cope,‭ ‬H.‭ ‬F.‭ ‬Osborn‭ & ‬C.‭ ‬C.‭ ‬Mook‭ ‬-‭ ‬1921.
-‭ ‬New remains of Amphicoelias Cope‭ (‬Dinosauria:‭ ‬Sauropoda‭) ‬from the Upper Jurassic of Montana and diplodocoid phylogeny,‭ ‬J.‭ ‬A.‭ ‬Wilson‭ & ‬M.‭ ‬Smith‭ ‬-‭ ‬1996.
-‭ ‬Biggest of the big:‭ ‬a critical re-evaluation of the mega-sauropod Amphicoelias fragillimus.‭" ‬In Foster,‭ ‬J.R.‭ ‬and Lucas,‭ ‬S.G.,‭ ‬eds,‭ ‬K.‭ ‬Carpenter‭ ‬-‭ ‬2006.
-‭ ‬A new basal diplodocid species,‭ ‬Amphicoelias brontodiplodocus,‭ ‬from the Morrison Formation,‭ ‬Big Horn Basin,‭ ‬Wyoming,‭ ‬with taxonomic reevaluation of Diplodocus,‭ ‬Apatosaurus,‭ ‬and other genera,‭ ‬H.‭ ‬Galiano‭ & ‬R.‭ ‬Albersdorfer‭ ‬-‭ ‬2010.
- Maraapunisaurus fragillimus, N.G. (formerly Amphicoelias fragillimus), a basal Rebbachisaurid from the Morrison Formation (Upper Jurassic) of Colorado. - Geology of the Intermountain West. 5: 227–244. - K. Carpenter - 2018.


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