Faunal Turnover In The Morrison Formation

[DD1991]

While doing research on dino stratigraphy in the Morrison Formation, I noted that Hesperosaurus has been found at the bottom of the Morrison Formation and also the Salt Wash Member near Howe Quarry (where a specimen of Stegosaurus armatus has been found), roughly the same level as the plethora of Stegosaurus specimens found at Como Bluff Quarry 13. At the same time, I've noticed that the primitive apatosaurine Eobrontosaurus has been found to coexist with the advanced diplodocine Diplodocus in Garden Park, Colorado. Since the most primitive Morrison dinos (Eobrontosaurus, Hesperosaurus, "Allosaurus jimmadseni") are found in the lower part of the Salt Wash Member and coexist with the their more advanced counterparts at the Howe Quarry (which is situated near the boundary between the Salt Wash and Brushy Basin Members), would it be reasonable to assume that the rate of faunal turnover for dinosaurs in the Morrison Formation was a gradual one, with Hesperosaurus and Eobrontosaurus being eventually replaced by Diplodocus and Stegosaurus in the Brushy Basin Member?

[Auspex]

Confidence in any 'rate of succession' hypothesis will rise with sample-size. What is the relative abundance of each species, and how many specimens is that based on?

[Auspex]

I propose, as an idea to keep in mind during such discussions, that any concentration of dinosaur remains is an accident of taphonomy, not fecundity. It can also be a good idea to recognize that a period of several thousand years, within the mesozoic, is completely invisible in the geologic record.

[Regg Cato]

While doing research on dino stratigraphy in the Morrison Formation, I noted that Hesperosaurus has been found at the bottom of the Morrison Formation and also the Salt Wash Member near Howe Quarry (where a specimen of Stegosaurus armatus has been found), roughly the same level as the plethora of Stegosaurus specimens found at Como Bluff Quarry 13. At the same time, I've noticed that the primitive apatosaurine Eobrontosaurus has been found to coexist with the advanced diplodocine Diplodocus in Garden Park, Colorado. Since the most primitive Morrison dinos (Eobrontosaurus, Hesperosaurus, "Allosaurus jimmadseni") are found in the lower part of the Salt Wash Member and coexist with the their more advanced counterparts at the Howe Quarry (which is situated near the boundary between the Salt Wash and Brushy Basin Members), would it be reasonable to assume that the rate of faunal turnover for dinosaurs in the Morrison Formation was a gradual one, with Hesperosaurus and Eobrontosaurus being eventually replaced by Diplodocus and Stegosaurus in the Brushy Basin Member?

This is plausible; Auspex and Inyo made some very pertinent remarks as well.

It is worth noting however that the validity of Hesperosaurus and Eobrontosaurus at the generic level is not supported by all authors, and the amount of time and interval between deposition of the SW and BB members is worth considering as well.