Anatomy and systematics of the diplodocoid Amphicoelias altus supports high sauropod dinosaur diversity in the Upper Jurassic Morrison Formation of the USA.

Philip D Mannion,Emanuel Tschopp,John A Whitlock

doi:10.1098/rsos.210377

Abstract

Sauropod dinosaurs were an abundant and diverse component of the Upper Jurassic Morrison Formation of the USA, with 24 currently recognized species. However, some authors consider this high diversity to have been ecologically unviable and the validity of some species has been questioned, with suggestions that they represent growth series (ontogimorphs) of other species. Under this scenario, high sauropod diversity in the Late Jurassic of North America is greatly overestimated. One putative ontogimorph is the enigmatic diplodocoid Amphicoelias altus, which has been suggested to be synonymous with Diplodocus. Given that Amphicoelias was named first, it has priority and thus Diplodocus would become its junior synonym. Here, we provide a detailed re-description of A. altus in which we restrict it to the holotype individual and support its validity, based on three autapomorphies. Constraint analyses demonstrate that its phylogenetic position within Diplodocoidea is labile, but it seems unlikely that Amphicoelias is synonymous with Diplodocus. As such, our re-evaluation also leads us to retain Diplodocus as a distinct genus. There is no evidence to support the view that any of the currently recognized Morrison sauropod species are ontogimorphs. Available data indicate that sauropod anatomy did not dramatically alter once individuals approached maturity. Furthermore, subadult sauropod individuals are not prone to stemward slippage in phylogenetic analyses, casting doubt on the possibility that their taxonomic affinities are substantially misinterpreted. An anatomical feature can have both an ontogenetic and phylogenetic signature, but the former does not outweigh the latter when other characters overwhelmingly support the affinities of a taxon. Many Morrison Formation sauropods were spatio-temporally and/or ecologically separated from one another. Combined with the biases that cloud our reading of the fossil record, we contend that the number of sauropod dinosaur species in the Morrison Formation is currently likely to be underestimated, not overestimated.

Highlights

The Upper Jurassic Morrison Formation of the western USA has yielded a high diversity of sauropods, including some of the most iconic dinosaurs, such as Brachiosaurus, Brontosaurus and Diplodocus [1,2,3]
Two further species of Amphicoelias were named by Cope from nearby Morrison Formation localities—Amphicoelias latus [9] and Amphicoelias fragillimus [35]—but neither species is currently considered to belong to the genus
The numbers of most parsimonious trees (MPTs) and steps for all analyses are summarized in table 5

Summary

Introduction

The Upper Jurassic Morrison Formation of the western USA has yielded a high diversity of sauropods, including some of the most iconic dinosaurs, such as Brachiosaurus, Brontosaurus and Diplodocus [1,2,3]. Apatosaurus and Camarasaurus were abundant components of this fauna, whereas a number of other sauropod taxa are known from far fewer remains [4]. Some of these have only been recognized this century, namely Galeamopus [5], Kaatedocus [6], Smitanosaurus [7] and Suuwassea [8], whereas others have been known for much longer, consisting of Amphicoelias [9], Barosaurus [10], Haplocanthosaurus [11] and Supersaurus [12]. The type species, Amphicoelias altus, was erected by Cope in 1877 for two dorsal vertebrae, a partial pubis and a femur (figures 1 and 2). No further remains can currently be unambiguously referred to Amphicoelias, making it one of the rarest taxa in the Morrison Formation, despite being known for over 140 years

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Conclusion