In the previous articles in this series (see part 1 here, part 2 here and part 3 here) we looked at the ‘too many damn dinosaurs’ (TMDD) contention, this being the claim that the Upper Jurassic Morrison Formation simply has too many sauropod dinosaurs. You’ll need to check those previous articles out before reading this one. The previous parts of the series introduce the TMDD contention and then discuss whether arguments made about Paleogene fossil mammals and modern giraffes are relevant. Here, we move on to something else.
Caption: beautiful Morrison Formation scene by Johan Egerkrans, showing Ceratosaurus at left and some apatosaurines at right. Rhamphorhynchid pterosaurs and a docodont mammal appear as well. Image: (c) Johan Egerkrans, used with permission.
It’s true that sauropod diversity in the Morrison Formation as a whole is high. The TMDD contention would have it that it’s too high, and that the contemporaneity of these many, often similar giant animals is problematic. Prothero (2019) stated “Tschopp, Mateus, and Benson record 14 different species clustered in nine genera of diplodocines …. from a single formation that covers a limited geographic area and approximately 7-11 million years of time” (p. 109). There’s a technical error there, by the way: the taxa he lists are not ‘diplodocines’ but in fact include diplodocines (Diplodocus, Barosaurus, Supersaurus, Kaatedocus and Galeamopus), apatosaurines (Apatosaurus and Brontosaurus), dicraeosaurids (Suuwassea) and non-diplodocid diplodocoids (Amphicoelias). More importantly…
Those Morrison sauropods weren’t all contemporaneous, and the view of a packed ecosystem has been overstated. I’ve been saying throughout this series of articles that the Morrison is home to around 30 sauropod species. But, as just noted in that quoted bit of text from Prothero’s 2019 book, the Morrison spans a long time: 7 million years is a long time! If you were to travel, right now, 7 million years into the past you’d be in the Tortonian age of the Miocene, a time pre-dating the appearance of hominins. Furthermore, it simply isn’t the case that all Morrison sauropods were contemporaneous. Indeed, most of the species are specific to certain sections of the Morrison (known technically as members) and each seemingly lasted for about a million years: they weren’t all living alongside one another for several million years, and certainly not for the entire duration of Morrison deposition. In recent years, some considerable effort has been directed to pinning down the ages of the different sections of what is a hugely complex, multi-layered geological succession, and those efforts continue today (e.g., Trujilo et al. 2014, Trujilo & Kowallis 2015, Maidment & Muxworthy 2019). All of which means that we’re gradually getting a better handle on which Morrison dinosaurs really were contemporaneous, and which were not.
Caption: a reminder of the extent of the Morrison Formation, and of the distances between various of its outcrops. Not only does the Morrison span a substantial chunk of Mesozoic time, it was deposited over a vast geographical area. Image: Maidment & Muxworthy (2019).
Prothero (2019) noted that Morrison sauropod diversity was so high – this ecosystem was so unrealistically packed – that you might find 7 or 8 “species of huge sauropods from a single interval of time and a single place, all crowding together and sharing common resources” (p. 110). He specifically mentions Carnegie Quarry as one place where 7 or 8 species might be found alongside one another.
Caption: perhaps the most famous Carnegie Quarry sauropod find is this remarkably complete, articulated Camarasaurus lentus juvenile (CM 11338). Incidentally, it’s one of several sauropod specimens which confirms that these animals habitually held their necks in erect poses. The tail in this specimen was originally bent right over the back. Image: James St. John, CC BY 2.0 (original here).
There’s no getting away from the fact that a good number of Morrison sauropods were indeed sympatric, a number well above anything that seems normal for the Holocene. I think that there are reasons for this, and we’ll be coming back to them in later articles in this series.
However, it’s not accurate or honest to say (1) that there are 7 or 8 species at Carnegie Quarry, and (2) that such a high count is typical for the Morrison. On point (1), Carnegie Quarry has not yielded 7 or 8 sauropod species. According to Turner & Peterson’s (1999) list it has yielded 4 species (Camarasaurus lentus, Diplodocus longus, Barosaurus lentus and Apatosaurus louisae) (see also Carpenter 2013). And on point (2): yes, there are a lot of Morrison sauropods, but they’re never all found packed together. In fact, a check of the faunal lists for all dinosaur-bearing Morrison Formation quarries shows that virtually all sauropod-yielding localities yield just one or two species, not a long list of them. In addition to Carnegie Quarry, the exceptions are the Jensen-Jensen Quarry in Utah, Nail Quarry and Zane Quarry in Wyoming (all of which yield 3 sympatric species), Red Fork Powder River Quarry B in Wyoming (which yields 4 if the Haplocanthosaurus record there is valid) and Howe Quarry in Wyoming (which has yielded 5 if the records of Barosaurus and Diplodocus there are valid) (Turner & Peterson 1999, Foster 2003).
Caption: Carnegie Quarry is at the core of the huge Dinosaur National Monument complex, and the quarry wall is famous for its numerous dinosaur bones. It’s just about impossible to get the whole wall in the same photo. Quite why and how so many giant dinosaurs became preserved in the same small area remains the issue of discussion (Carpenter 2013). Image: Wild About Utah (from here).
As mentioned above, the stratigraphy and sedimentology of the Morrison Formation is extremely complicated. Not only is the Morrison vast in terms of area and thickness and spans a considerable chunk of geologic time, it’s diverse in terms of lithology and in the sorts of environments and habitats its rocks preserve. There’s evidence for freshwater wetlands in the south-eastern part of the region covered by the Morrison, a desert and saline lake system in the south-west, highlands in the north, well-drained floodplains, evidence for cooler, wetter conditions at the top of the Morrison in Montana, and more (Maidment & Muxworthy 2019). Overall, the Morrison depositional environment was, during most of its history, a complex mosaic of different habitats (Whitlock et al. 2018).
Caption: sedimentological evidence shows that the environments present in Late Jurassic North America (and representing those areas whose sediments now form part of the Morrison Formation) were diverse, and changed over the relevant timespan. These two maps show just some of the changes that occurred across the Bighorn Basin of Wyoming during the time the Morrison Formation was deposited. Note the scale bars! Image: Maidment & Muxworthy (2019).
The sauropods we’re talking about here are not found across the whole of the Morrison (as we’ve seen, it spans at least 7 million years). They are, instead, specific to certain geological segments of the Morrison, and to specific areas: some do not appear to be especially endemic (though note that the studies that say this – e.g., Whitlock et al. 2018 – talk about genera and not species), but others are currently highly endemic. We’ll be coming back to this issue in another article in this series, though from a somewhat different angle.
Caption: the numerous Morrison sauropods were far from contemporaneous. This diagram - from Bakker (1993) - depicts the stratigraphic distribution of Morrison sauropod and theropod species through a 91.5 m succession in Wyoming. Image: Bakker (1993).
In short, there are reasons for thinking that some, if not most, of the Morrison Formation sauropod taxa come from different location, different habitats, and maybe even different ecosystems. Arguments that just too many species were packed alongside one another are almost certainly overstated. And there’s a lot more to this part of the argument, stay tuned…
Caption: the environments represented by the Morrison Formation would have included rolling hills, plateaus and other highland areas - we mustn’t imagine the entire area of deposition as a gigantic flat floodplain. In this fantastic illustration by Vladimir Nikolov, a Stokesosaurus consider hiding inside a diplodocid carcass already occupied by two sheltering dryosaurs. Vladimir’s artwork can be seen here and here. Image: Vladimir Nikolov, used with permission.
Do taxonomic decisions about genera really mean all that much when it comes to actual diversity? As most readers of TetZoo will recall, Tschopp et al.’s (2015) substantial study of diplodocid phylogeny got a lot of news coverage back when it was published. Is this because it was an especially grand, detailed study of diplodocids? No, it’s because it included the intriguing (but actually somewhat anticipated) proposal that Brontosaurus should warrant generic recognition, since the species included within this genus don’t – according to Tschopp et al. (2015) – belong within Apatosaurus after all. It’s the resurrection of Brontosaurus that caused Prothero to call TMDD in the first place, his main take being that the ‘addition’ of yet another genus to the Morrison Formation sauropod assemblage makes Morrison sauropod diversity too high.
It should go without saying, however, that the adoption of a particular generic name tells you more about taxonomic convention than how packed an ecosystem is. If you’re really using the resurrection of a given taxonomic name to make complaints about there being TMDD, you need to object to the existence of species, not genera, since the species included by Tschopp et al. (2015) within Brontosaurus would still exist – and still contribute to the supposed TMDD problem – whether Brontosaurus were recognised or not. In other words, complaining about the resurrection of Brontosaurus (within the context of the TMDD contention) totally misses the point.
Caption: yes, there are a lot of Morrison Formation sauropods - this montage nicely illustrates that. But, as we’ve established by now, the species concerned were not all contemporaneous, and did not all live in the same place, and did not all frequent the same environments. Image: Muhammad Rizky Maulana (using skeletals made by Gunnar Bivens, Scott Hartman, Henrique Paes, and Gregory S. Paul), used with permission.
Another thing: if you’re going to complain that the resurrection of Brontosaurus somehow contributes to a perceived TMDD problem (as Prothero (2019) did) – and are calling out Tschopp et al. (2015) for contributing to said perceived problem – what about acknowledging the generic synonymisations they proposed? You see, Tschopp et al. (2015) didn’t just raise and resurrect Morrison generic names, they advocated the abandonment of some too, namely Eobrontosaurus and Elosaurus. If you’re saying that the number of genera creates a TMDD problem (which – let’s note again – it doesn’t given that genera are formed of species), it’s wrong to state or imply that dinosaur workers like Tschopp et al. (2015) are on some sort of reckless genus-naming frenzy. They aren’t. Rather, the taxonomy they chose to adopt reflects the phylogenetic structure they recovered.
Ok, that’s where we’ll end things for now, but we’re not done yet. Far from it. In the next part of this series we’ll look at sauropod population biology.
For the previous article in this series, see…
Stop Saying That There Are Too Many Sauropod Dinosaurs, Part 1, April 2020
Stop Saying That There Are Too Many Sauropod Dinosaurs, Part 2, April 2020
Stop Saying That There Are Too Many Sauropod Dinosaurs, Part 3, April 2020
For previous TetZoo articles on sauropods, brontotheres, giraffes and related issues (linking where possible to wayback machine versions), see…
Giraffes: set for change, January 2006
Biggest…. sauropod…. ever (part…. I), January 2007
Biggest sauropod ever (part…. II), January 2007
The hands of sauropods: horseshoes, spiky columns, stumps and banana shapes, October 2008
Thunder beasts in pictures, March 2009
Thunder beasts of New York, March 2009
Sauropod dinosaurs held their necks in high, raised postures, May 2009
Inside Nature’s Giants part IV: the incredible anatomy of the giraffe, July 2009
Testing the flotation dynamics and swimming abilities of giraffes by way of computational analysis, June 2010
Paul Brinkman’s The Second Jurassic Dinosaur Rush, March 2011
The sauropod viviparity meme, May 2011
Necks for sex? No thank you, we’re sauropod dinosaurs, May 2011
The Second International Workshop on the Biology of Sauropod Dinosaurs (part I), December 2011
The Second International Workshop on the Biology of Sauropod Dinosaurs (part II), January 2012
Greg Paul’s Dinosaurs: A Field Guide, February 2012
Junk in the trunk: why sauropod dinosaurs did not possess trunks (redux, 2012), November 2012
That Brontosaurus Thing, April 2015
Unusual Giraffe Deaths, November 2015
Burning Question for World Giraffe Day: Can They Swim?, June 2016
10 Long, Happy Years of Xenoposeidon, November 2017
The Life Appearance of Sauropod Dinosaurs, January 2019
Refs - -
Bakker, R. T. 1993. The dinosaur renaissance. In Calhoun, D. (ed) 1993 Yearbook of Science and the Future. Encyclopaedia Brittanica (University of Chicago), pp. 28-40.
Carpenter, K. 2013. History, sedimentology, and taphonomy of the Carnegie Quarry, Dinosaur National Monument, Utah. Annals of the Carnegie Museum 81, 153-232.
Foster, J. R. 2003. Paleoecological analysis of the vertebrate fauna of the Morrison Formation (Upper Jurassic), Rocky Mountain Region, U.S.A. Bulletin of the New Mexico Museum of Natural History & Science 23, 1-72.
Maidment, S. C. R. & Muxworthy, A. 2019. A chronostratigraphic framework for the Upper Jurassic Morrison Formation, western U.S.A. Journal of Sedimentary Research 89, 1017-1038.
Trujilo, K. C. & Kowallis, B. J. 2015. Recalibrated legacy 40Ar/39Ar ages for the Upper Jurassic Morrison Formation, Western Interior, U.S.A. Geology of the Intermountain West 2, 118.
Trujilo, K. C., Foster, J. R., Hunt-Foster, R. K. & Chamberlain, K. R. 2014. A U/Pb age for the Mygatt–Moore quarry, Upper Jurassic Morrison Formation, Mesa County, Colorado. Volumina Jurassica 12, 107-114.
Turner, C. E. & Peterson, F. 1999. Biostratigraphy of dinosaurs in the Upper Jurassic Morrison Formation of the western Interior, U.S.A. Vertebrate Paleontology in Utah, Miscellaneous Publications of the Geological Survey 99-1, 77-114.
Whitlock, J. A., Trujillo, K. C. & Hanik, G. M. 2018. Assemblage-level structure in Morrison Formation dinosaurs, Western Interior, USA. Geology of the Intermountain West 5, 9-22.
