Abstract
The Upper Jurassic Morrison Formation is both geographically extensive and well-sampled, making it an ideal candidate for biogeographic analysis at both coarse and ‑ner scales. Historically, however, this has not translated into a consensus on patterns of ecological structure and connectivity, particularly with regard to the characteristic dinosaur faunas. Here, we use both traditional (genus richness, alpha and beta diversity) and bipartite network-based (biogeographic connectivity, local endemism, and average occurrence) measures to examine patterns of structure on a per-locality basis. Given the broad geographic range of the formation, we subdivide the Morrison Formation into four discrete regions based roughly on latitude and lithology—north (Montana, South Dakota, and northern Wyoming), west (Utah and western Colorado), east (central and eastern Colorado and southern Wyoming), and south (Arizona, New Mexico and Oklahoma). Further investigation revealed many coeval sites (ca. 152 Ma) in the east and west regions. Presence-absence data were also compared using network analysis to determine the presence and content of discrete subassemblages within the larger region-level assemblages. Based on our results, we favor reconstructions of the Morrison Formation as a ‘mosaic’ type environment over most of its depositional history, with patches of open environments interspersed with more closed, forested regions. is is suggested by relatively low rates of local endemism (patches are consistent in plant and animal structure) and connectivity across the majority of the formation, as well as the recovery of three non-overlapping assemblages dominated by dierent guilds of herbivorous dinosaurs.
Highlights
Similarity is lower between the north and west regions (65%), and is lowest between the south region and all others (56-58%), with the north and south being most di erent. is trend is generally conserved when the samples are restricted by taxonomic group, with the exception of sauropods, where similarity is highest between the south region and the west and east regions (83%), again the north and south are least similar (63%)
Shared taxonomic scope is reasonably high in the Morrison Formation. is is exempli ed by the large-bodied sauropods, where 5 of 12 taxa occur in all four regions and 8 of 12 occur in at least two. e regionally endemic taxa, Dyslocosaurus, Dystrophaeus, Kaatedocus, and Suuwassea, are known from only a single locality each, which suggests that their rarity is either a function of their rarity in the fauna as a whole or the rate at which they are misidenti ed by collectors, or both. eropod shared taxonomic scope is less robust, with only two of the largest bodied taxa (Allosaurus and Ceratosaurus) occurring in all four regions, and two others occurring in three
Half (4/10) of theropod taxa are known from a single region only, and there is the possibility of a great many more singleton taxa that would increase this total. ese regionally endemic theropods are invariably small-bodied
Summary
Because the Morrison Formation incorporates both a large geographic and temporal range, the 291 localities sampled were divided into four regions: north (65 localities), inclusive of localities in Montana, South Dakota, and northern Wyoming (above 43°N); west (72 localities), inclusive of localities in Utah and Colorado just east of the city of Gunnison, Colorado (Cabin Creek [see Turner and Petersen, 1999] and westward); east (128 localities), inclusive of Colorado east of Cabin Creek and Wyoming below 43°N and the lithologically similar Snowmass locality (Foster and Wedel, 2014); and south (26 localities), inclusive of localities in Arizona, New Mexico and Oklahoma ( gure 1). Ese localities are examined independent of the rest of the data set to provide further insight into potential regional di erences independent of time dilation e ects. One caveat with using data of this sort is that we are, at best, two steps removed from the original material (collector->compiler->present authors), each step having the potential to introduce errors ranging from incorrect taxonomic assignment to typographic issues. Dinosaur workers in general and those familiar with the Morrison Formation speci cally will no doubt already be aware of the di culty in correctly assigning many fossil specimens, as well as the potential for “hidden” taxonomic diversity (e.g., Galeamopus, “Elaphrosaurus;” Chure, 2001; Carrano and Sampson, 2008; Tschopp and others, 2015; Tschopp and Mateus, 2017). We note the potential for taphonomic bias towards larger bodied taxa, which we attempt to mitigate again by limiting our scope to presence-absence rather than abundance
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