Amphicoelias

*Amphicoelias* is a possibly dubious genus of large sauropod dinosaur, that lived in the United States during the Late Jurassic.

## History

Many fossils were discovered by Oramel William Licas (1877) in rocky outcrops close to Canyon City, with some named *Laelaps trihedrodon*, *Camarasaurus supremus* and *Caulodon* by Edward Drinker Cope, and assumed to be of the Cretaceous since other Laelaps occur there. Of these, some dorsal vertebrae, a pubis and a femur was found by Aaron Rply in Quarry XII. Cope received these on October 21, 1877, concluding they were sauropod in origin and named them *A. altus* in December 1877. He found it was related to *Camarasaurus* since they were both solid limbed, pneumatized vertebral behemoths. He then sold his collection to the American Museum of Natural History. Before he died, he worked with Charles R. Knight to create paleoart, based on his field notes. Based on Knight's reconstructions, he considered *Amphicoelias* and *Brontosaurus* synonyms with *Camarasaurus supremus*. It was then assigned AMNH 5764, and assigned more material. Osborn and Mook (1921) referred a forelimb to the holotype since it was found in the same strata not far from the type locality and differs from *Camarasaurus*. Due to uncertainty on this, Emanuel Tschopp et al. (2015) rejected the tooth, scapula and coracoid referred to AMNH 5764, but accepted the ulna.

Cope lists amphicoelous vertebrae as a reason to erect amphicoeliidae, which was found to be intermediate and poorly-defined by Othniel Charles Marsh (1881); he used two mildly-concave articular faces to differentiate it, which was later found to be widespread in related sauropods. Thus, it was merged with diplodocidae.

## Description

Henry Fairfield Osborn and Charles Craig Mook note close similarity between *Amphicoelias* and *Diplodocus*, but are differentiated from longer forelimbs, a long and slender femur with a round cross-section (though this has also been seen in some *Diplodocus*. They were of similar size. *Amphicoelias* vertebrae are incomplete, but anatomy is discernible. Centra are compressed in the middle of all dimensions, having a large lateral pleurocoel inside the large lateral fossa. The neural arch is tall, with a prominent lamina from the posterior centrum to the prezygapophyses. Behind the latter sits the diapophyes, which slightly project up and out, which is surrounded shallow fossae and a large lamina that reaches to the neural spine, which is thin. A pair of ridges sit along each edge, with the distal wider than the main spine; it is approximately subequal in length and width. Osborn and Mook (1921) referred a partial forelimb that is more robust than but overall resembles *Diplodocus*. The distal of the scapula is incomplete, but has a scapular blade expansion smaller than *Camarasaurus* but larger than *Apatosaurus*. The preserved element is 161 centimeters (63 inches) long, thicker than *Diplodocus* but not as thick as *Camarasaurus*. The coracoid is more similar to the former, being round and long rather than tall, also being thicker. The foramen is large and centered on the short axis. The ulna, more elongate than any *Diplodocus*, has prominent articular faces for the humerus and radius, tapering to the incomplete distal. The pubis is fragmentary, and all that is known is that it is long, thick and has a small surface where the ilium articulates. The femur is slender, with only the right known of the holotype; it is 1.524 meters (5 feet) long. This is slightly longer than *Camarasaurus supremus*, but less robust, being somewhat round in the cross section and 22 centimeters (8.7 inches) wide. Its shaft curves towards the hip articulation, straightening at the prominent articular condyles. A partial left feur was found in the American Museum of Natural History's collections, assigning it to the holotype (though it was not mentioned by Cope). Most of the upper segment is lost, but the shaft and end are similar to Cope's femur, possibly being of the same individual. A 2021 study found *A. altus* is not likely dubious, and thus, the genus is preserved.

## Classification

John Foster (2007) suggested the differences in *A. altus* may be due to individual variation, finding them insignificant ; Foster thus uses *Amphicoelias* as a senior synonym of *Diplodocus*, that further research would need to abandon *Diplodocus*. Woodruff and Foster (2015) reiterate this, finding *Amphicoelias* is monotypic and only species is referable to *Diplodocus* as *D. altus*, with *A. altus* as a nomen oblitum. Tschopp et al. (2015) dispute this in an extensive specimen-level phylogenetic analysis; they found A. altus as either the basalmost apatosaurine or between *Brontosaurus excelsus* and *Brontosaurus yahnahpin*. No unambiguous characters supported the apatosaurine placement, so they placed it as a basalmost diplodocid outside apato and diplodocinae since it has primitive features, a slightly more parsimonious apatosaurine phylogeny with slightly more diplodocine morphology and minimal differences from either subfamily. This is the cladogram they recovered:

A 2021 analysis reassured that *Amphicoelias* was distinct, noting a much higher sauropod diversity in the Morrison than historically expected.

## Paleoecology

The Morrison Formation that Amphicoelias lived in resembled a savanna, but grass had not yet evolved. Since, ferns were likely the dominant plants and a food source for *A. altus*; however, Engelmann et al. (2014) dismiss this since it has a relatively low caloric content. Carpenter, however, argued that sauropods were adapted to feeding on low caloric plants, allowing for ferns to be largely ingested. Carpenter also argues petrified logs 20-30 meters tall suggested tall trees were indeed present in the Morrison. However, such trees are rare, since they require more water not found in such a dry environment. Thus, they likely existed in narrow gallery forests beside the rivers and gulleys where water accumulated. Carpenter speculates large sauropods like *A. altus* may have used shade to stay cool, feeding in the savanna at night.

## References

1. ↑ https://en.wikipedia.org/wiki/List\_of\_informally\_named\_dinosaurs
2. ↑ https://royalsocietypublishing.org/doi/pdf/10.1098/rsos.210377
3. ↑ 3.0 3.1 3.2 3.3 https://en.wikipedia.org/wiki/Amphicoelias
4. ↑ https://www.amnh.org/explore/news-blogs/research-posts/amphicoelias-sauropod-fossil?utm\_source=twitter&utm\_medium=social&utm\_campaign=share-from-amnh-org