During the Bone Wars in the late 1800s, Edward Drinker Cope (1840-1897) and Othniel Charles Marsh (1831-1899) described several sauropod taxa from the Morrison Formation of western North America, with Marsh erecting the most sauropod species from the Morrison. Although several sauropod genera erected by Marsh have stood the taxonomic test of time, like Apatosaurus, Barosaurus, Brontosaurus, and Diplodocus, the only sauropod genus from the Morrison Formation named by Cope whose validity has been upheld is Camarasaurus, while Caulodon has been synonymized with Camarasaurus (but see here). One sauropod genus described from the Morrison Formation by E.D. Cope whose taxonomic status has fluctuated over time, however, is Amphicoelias Cope, 1877. Although Amphicoelias is poorly known in terms of the holotype of its type species, A. altus, being representing by a few elements, one nominal species of Amphicoelias, A. fragillimus, enjoyed conjectural fame as a super-giant diplodocid until Carpenter (2018) drastically reduced the size estimates for this taxon to 102 feet (31 meters) and reclassified it as a rebbachisaurid, erecting the new genus Maraapunisaurus for it. On the other hand, the validity and precise systematic position of Amphicoelias has been debated, with some studies placing it as a basal diplodocoid and others recovering it as diplodocid. Recently, a new paper on the anatomy and systematics of Amphicoelias was published by Mannion et al. (2021), and while it reaffirms the validity of Amphicoelias as upheld by several authors, I have taken the liberty of expressing some thoughts about the Mannion et al. paper regarding Amphicoelias with respect to diagnostic characters, phylogenetic position, and the bearing of studies on Morrison diplodocoid ontogeny upon Morrison sauropod diversity.
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| Selected elements of the holotype of Amphicoelias altus (AMNH 5764): posterior dorsal vertebra (top) and right femur (bottom) (from Mannion et al. 2021) |
In the section of their paper in which they redescribe the holotype of Amphicoelias altus (AMNH 5764), Mannion et al. list the "femoral shaft with subcircular cross-section" as one of three autapomorphies for Amphicoelias in the revised diagnosis for this taxon, noting that the femur of AMNH 5764 differs from described Morrison diplodocoid taxa in having a ratio of the mediolateral to anteroposterior diameter of the femur being 0.99 to 1.1 (despite a few signs of taphonomic crushing). However, they also note that the dicraeosaurid specimen MOR 592 found in Montana also has a femur whose cross-section is subcircular at the midshaft; in fact, the subcircular cross-section of the femur was used by Wilson and Smith (1996) to justify referring MOR 592 to Amphicoelias and conclude that Amphicoelias was a basal diplodocoid based on cladistic results that were never published. However, Whitlock (2011) assigned MOR 592 to the family Dicraeosauridae due to the presence of a sharp supraoccipital crest and a symphyseal tuberosity on the dentary, although Woodruff & Fowler (2012) and Woodruff et al. (2017) regarded MOR 592 as an immature diplodocine specimen, but nevertheless recovered Amphicoelias as a basal diplodocoid more derived than Haplocanthosaurus and Amazonsaurus. According to Mannion et al. (2021), the ratio of the mediolateral to anteroposterior diameter of the femur of MOR 592 is approximately 1.3, slightly greater than that for Amphicoelias altus, and MOR 592 has a femur with a slightly beveled distal end in contrast to the more pronounced beveling of the distal femur of AMNH 5764. On the other hand, Tschopp et al. (2015) note that the holotype of Brontosaurus parvus (CM 566) also has a subcircular femoral cross-section, while Wilhite (2005) reports that the subcircular femoral cross-section observed in Amphicoelias, the Brontosaurus parvus holotype, and MOR 592 also occurs in a few diplodocid femora from the Dry Mesa Quarry in Colorado. Since Amphicoelias is recovered as either a basal diplodocid by Tschopp et al. (2015) or an apatosaurine diplodocid by Tschopp and Mateus (2017), whereas Amphicoelias is variously recovered as a stem diplodocoid more derived than Haplocanthosaurus or a diplodocid by Mannion et al. (2021), the subcircular femoral cross-section described for Amphicoelias most likely evolved convergently among a few taxa within Flagellicaudata because Amphicoelias altus is distinguished by Mannion et al. (2021) from all other diplodocoids in having the apex of the posterior dorsal neural spine with rounded, non-tapered lateral projections resulting from the expansion of spinodiapophyseal laminae and little material is preserved in AMNH 5764.
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| Stratigraphic chart of dinosaur localities in the Morrison Formation (from Turner and Peterson 1999). Despite the opinion of some that the diversity of diplodocoids in the Morrison Formation has been inflated, the type localities of Haplocanthosaurus delfsi (CO-5) and Brontosaurus yahnanpin (WY-44) are stratigraphically low in the Morrison Formation, and type locality of Amphicoelias altus (CO-71) is situated near the top of the Brushy Basin Member of the Morrison Formation, being stratigraphically higher than the type localities of Apatosaurus ajax, A. louisae, Brontosaurus excelsus, and B. parvus. Moreover, three different groups of the diplodocoids (haplocanthosaurids, diplodocids, and dicraeosaurids) have been found at the Felch Quarry 1 (CO-3) in Garden Park, Colorado. |
When addressing the question of whether or not some Morrison diplodocoid species are growth stages of well-known taxa as hinted by Woodruff (2019), Mannion et al. stress that the basal position of the genus Haplocanthosaurus within Diplodocoidea is not attributable to ontogeny given that known specimens of H. priscus and H. delfsi are of the adult/near-adult stage. When taking into account the cladistic diversity and stratigraphic distribution of sauropods within the Morrison Formation, it should be noted that Brontosaurus (=Eobrontosaurus) yahnahpin and Haplocanthosaurus delfsi hail from the lower half of the upper part of the Salt Wash Member of the Morrison Formation whereas Amphicoelias altus was found near the top of the Brushy Basin Member (Turner and Peterson 1999, fig. 7) and that no members of Turiasauria or Mamenchisauridae have yet been reported from the Morrison Formation, although the Lourinhã and Tendaguru Formations have yielded members of Diplodocoidea, Macronaria and Turiasauria. Additionally, given that Whitlock and Wilson Mantilla (2020) note that the juvenile diplodocine specimens CM 3452 and CM 11255 (the latter probably Barosaurus; Melstrom et al. 2016) differ from Kaatedocus, Smitanosaurus, Suuwassea, and MOR 592 in lacking a postparietal foramen despite being juveniles, although the adult apatosaurine specimen BYU 17096 has this feature, it is not hard to imagine four dicraeosaurid taxa existing in the Morrison Formation because known specimens of Kaatedocus, Smitanosaurus, and MOR 592 were found in the upper part of the Salt Wash Member and lowermost part of the Brushy Basin Member, whereas Suuwassea probably was found high in the Morrison Formation (Harris and Dodson 2004; Turner and Peterson 1999). While I agree with Mannion et al. (2021) that ontogeny is an important factor to take into account when determining whether small or primitive sauropod specimens from the Morrison Formation are juveniles of existing species or more basal than well-known diplodocids, the assignment of Suuwassea and MOR 592 to Dicraeosauridae by Whitlock (2011) took into account the possibility that the sub-adult status of the Suuwassea holotype was why Suuwassea defied precise classification within Diplodocoidea when first described by Harris and Dodson (2004), while bearing in mind the fact that some characteristics used to refer MOR 592 to Amphicoelias by Wilson and Smith (1996) were likely to be found in other diplodocoid taxa. Moreover, since Brontosaurus yahnanpin was found lower in the Morrison Formation than Amphicoelias or other species of Brontosaurus, it is possible that it is actually more basal than either B. excelsus, B. parvus, or Apatosaurus because the holotype of Amphicoelias altus contains a few skeletal elements and was found in the uppermost layer of the Brushy Basin Member.
References:
Melstrom, K.M., D’Emic, M.D., Chure, D.J., and Wilson, J.A., 2016. A juvenile sauropod dinosaur from the Late Jurassic of Utah, USA, presents further evidence of an avian style air-sac system. Journal of Vertebrate Paleontology e1111898.
2015. A specimen-level phylogenetic analysis and taxonomic revision of Diplodocidae (Dinosauria, Sauropoda). PeerJ 3:e857 https://doi.org/10.7717/peerj.857
2017. Osteology of Galeamopus pabsti sp. nov. (Sauropoda: Diplodocidae), with implications for neurocentral closure timing, and the cervico-dorsal transition in diplodocids. PeerJ 5:e3179 https://doi.org/10.7717/peerj.3179
Turner, C.E. and Peterson, F., 1999. Biostratigraphy of dinosaurs in the Upper Jurassic Morrison Formation of the Western Interior, U.S.A. pp. 77–114. In: Gillette, D.D. (ed.), Vertebrate Paleontology in Utah. Utah Geological Survey Miscellaneous Publication 99-1.
Wilhite, D.R. 2005. Variation in the appendicular skeleton of North American sauropod dinosaurs: taxonomic implications. pp. 268-301. In: Tidwell, V., and Carpenter, K. (eds.), Thunder-lizards: the Sauropodomorph dinosaurs. Indiana University Press, Bloomington.
Wilson, J.A., and Smith, M., 1996. New remains of Amphicoelias Cope (Dinosauria: Sauropoda) from the Upper Jurassic of Montana and diplodocoid phylogeny. Journal of Vertebrate Paleontology 16 (supp. to volume 3): 73A.
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