Stop Saying That There Are Too Many Sauropod Dinosaurs, Part 5

If you’ve been visiting TetZoo over recent weeks, you’ll know why we’re here. Yes, we’re here to continue with the Too Many Damn Dinosaurs (TMDD) series, in which I argue that it’s wrong to argue – that is, on principle, rather on detailed evaluation of the evidence – that the world famous Late Jurassic Morrison Formation contains too many sauropods. In the previous four parts of this series we introduced the DMDD contention, looked at the fact that Paleogene mammals are not especially relevant to the TMDD contention, and then at the fact that modern giraffes are not especially relevant to the TMDD contention either.

Caption: a montage of images relevant to previous articles in this series. Image: Prothero (2019), Darren Naish, Bakker (1993).

Caption: a montage of images relevant to previous articles in this series. Image: Prothero (2019), Darren Naish, Bakker (1993).

And – most recently – we looked at the fact that the TMDD contention over-states the case for Morrison sauropod sympatry. They weren’t all packed into the same one environment at the same one point in time, but actually lived over a vast distance, in different environments, and over some considerable span of time. Now, in the fifth part of the series, it’s time to consider something else that shows why there aren’t TMDD.

Assumption 1: we have a handle on megadinosaur populations. An inherent assumption behind the TMDD contention is that there wasn’t the ecological ‘space’ for large populations of the respective multiple giant species, with ‘space’ referring to literal, physical space but mostly to food plant biomass (e.g., Prothero 2019, p. 111). We know – it’s been demonstrated many times, most spectacularly and sadly in African elephants – that big herbivores can and will eat themselves into (local) extinction when trapped in an area where there’s insufficient plant material. A Late Jurassic North American continental western interior occupied by around 30 sauropod species, for example, surely just couldn’t be home to all those tens of thousands, or hundreds of thousands, or millions of plant-smashing, ever-hungry megaherbivores. No way!! Right?

Caption: droughts and shortages of edible browse can cause elephant die-offs. Surely packed masses of Mesozoic megaherbivores were afflicted by this sort of thing too? This photo was taken in Tsavo Park in 1971. The dying animal is the group matriar…

Caption: droughts and shortages of edible browse can cause elephant die-offs. Surely packed masses of Mesozoic megaherbivores were afflicted by this sort of thing too? This photo was taken in Tsavo Park in 1971. The dying animal is the group matriarch. Image: Spinage (1994).

But here’s the thing. We have no real idea what the populations of any of these species really were. Ok, we can make some guesses based on our knowledge of how many individuals of a giant reptile species are required to prevent inbreeding depression: the ballpark minimum viable population (MVP) for big terrestrial vertebrates is supposedly between 500 and 1000, with one recent study supporting a higher figure of around 4000 (Traill et al. 2007). Furthermore, we can make guesses based on their inferred geographic ranges and ecological preferences: in wild elephants, home ranges for individuals are between 100 km2 and over 10,000 km2 (Ngene et al. 2017).

Caption: Morrison Formation environments included open spaces and ecotones like the sort shown here. But there were also dense, thickly vegetated stretches of forest, at least at times. Image: Liam Elward (original here), used with permission.

Caption: Morrison Formation environments included open spaces and ecotones like the sort shown here. But there were also dense, thickly vegetated stretches of forest, at least at times. Image: Liam Elward (original here), used with permission.

But any assumption about sauropod populations being high – if not problematically high – is not just an assumption, it may well be a very wrong assumption. I actually think it’s likely that these species existed – as adults (that’s an important caveat, keep it in mind) – at very low population densities, as in… in the low thousands or even the hundreds. Why do I think this?

Firstly, sauropod fossils are rare, surprisingly so given how gigantic and potentially resistant to destruction many of their bones were. Of the approximately 30 currently recognised Morrison sauropods, around 20 are known from less than five specimens. Only Camarasaurus lentus and Diplodocus carnegii can be considered abundant, both being known from “hundreds” of isolated elements (Upchurch et al. 2004). Camarasaurus as a whole (that is, including all four recognised species*) is known from over 530 specimens belonging to a minimum of 200 individuals (Woodruff & Foster 2017; they cite Foster 2003 for some of the relevant data). These figures are good as dinosaurs go, but are they good enough to demonstrate adult populations in the high thousands, tens of thousands or more?

* This assumes inclusion of C. lewisi within the genus (it arguably warrants recognition as the distinct genus Cathetosaurus). Arguments that other specimens included within Camarasaurus might not belong there have been made too. Looking at you, Uintasaurus douglassi.

Caption: how sauropod-busy were Morrison environments? Were there MILLIONS of giant adult sauropods, of multiple species, roaming the land? Mostly there were not, read on. Image: Darren Naish.

Caption: how sauropod-busy were Morrison environments? Were there MILLIONS of giant adult sauropods, of multiple species, roaming the land? Mostly there were not, read on. Image: Darren Naish.

I should note at this point that Farlow et al. (2010) modelled the possible abundance of giant herbivorous dinosaurs in the Morrison Formation, using estimates of dinosaur energetic demands and the productivity of the landscape. They also looked at the possibility of high-metabolism, elephant-like sauropods in addition to lower metabolism, varanid-like ones. Their conclusions? That Morrison sauropods existed at “a few tens of individuals of large size, per square kilometre” if they were varanid-like, and at “a few tens of individuals of all sizes/ages, and a few adult and subadult individuals of large size, per square kilometre” if they were of mammal-like metabolism (p. 423). The Morrison Formation has an areal extent of 1.2 million square kilometres (Maidment & Muxworthy 2019), suggesting a conservative possible total adult sauropod population (across all species) in the low millions, assuming mammal-like metabolisms (this estimate also assumes that all Morrison sauropods lived at the same time, and that the extent of the Morrison ‘habitat’ was consistent across its duration, and neither of those things are true). This is easily enough to allow for minimum viable populations (and then some) of 30 species (since 30 species existing at the high end theoretical MVP of 4000 individuals would only need 120,000 km2).

Caption: Farlow et al. (2010) includes various diagrams which function to model sauropod population density in Late Jurassic North American environments. Some of their assumptions about biomass and so on might be off but it’s a valuable study.

Caption: Farlow et al. (2010) includes various diagrams which function to model sauropod population density in Late Jurassic North American environments. Some of their assumptions about biomass and so on might be off but it’s a valuable study.

And it has to be noted that Farlow et al.’s (2010) take on Morrison environmental productivity is almost certainly substantially too conservative: they assumed that the habitats in question were savannah-like, and with relatively low plant biomass. A lot has happened in our understanding of the Morrison Formation within the last ten years, and new data – soon to be published – demonstrates that we’ve vastly under-estimated the productivity of Morrison environments. The habitats concerned weren’t savannah-like all the time, but supported vast, complex, densely vegetated forests (with gargantuan trees) during some or much of their history. We’ll come back to the subject of plants and productivity in a later article.

Caption: Carpenter (2006) produced this reconstructed cross-section of a Morrison woodland to show how giant trees forming the canopies of gallery forests were large enough to provide shade and cover for giants like Maraapunisaurus. Image: Carpenter…

Caption: Carpenter (2006) produced this reconstructed cross-section of a Morrison woodland to show how giant trees forming the canopies of gallery forests were large enough to provide shade and cover for giants like Maraapunisaurus. Image: Carpenter (2006).

Secondly, there are good reasons for thinking that sauropod population structure was unusual relative to what some people seem to expect. The sauropod fossil record shows us that these animals produced large clutches of eggs (as in, 20-40), en masse, and that breeding events were near explosive. Enormous numbers – millions, perhaps – of hatchlings were produced during these events. Each breeding female could have produced between 500 and 4000 eggs during her reproductively active years (Paul 1994). This strategy means that sauropods were good at replacing themselves and quick to recover from times of high mortality (including – but not limited to – extinction events) (Janis & Carrano 1992, Paul 1994, Sander et al. 2011), and also that reproductively active animals could have been few in number – perhaps close to, or at, MVP – and yet still able to maintain a large total population over time.

Caption: there are good reasons for thinking that gigantic sauropods really were rare objects on the landscape. This fantastic painting shows how the very biggest specimens - like this immense dark Barosaurus - would have dwarfed more ‘normal sized’…

Caption: there are good reasons for thinking that gigantic sauropods really were rare objects on the landscape. This fantastic painting shows how the very biggest specimens - like this immense dark Barosaurus - would have dwarfed more ‘normal sized’ sauropods. Image: John Conway (original here).

What this means is that sauropod populations were skewed towards juveniles; that the population of a given species was formed – mostly – of small animals, and that adults, especially giant adults, were rare. Taphonomic biases mean that the bones and footprints of juveniles are less preservable than those of adults. But even so, juveniles of Morrison sauropods are relatively well represented, in cases being present at nearly 90% of relevant sites (Foster 2005). And trackways indicate that about 70% of sauropods in some areas (meaning worldwide, not in the Morrison biota specifically) were small juveniles (Lockley 1994).

In conclusion, the TMDD contention makes assumptions about the population size of adults that are almost certainly very wrong. Even if taxonomic diversity in the sauropods of a given region was seemingly high (and as we saw in Part 4, it wasn’t as high as implied by the TMDD contention, and…. wait for a later article in this series), it does not follow that there was some problematic number (thousands, tens of thousands, hundreds of thousands or millions) of big, adult individuals in the area concerned. Put another way, Morrison Formation palaeoenvironments were easily large enough, and easily productive enough, to host the relatively low numbers of adults and relatively large numbers of juveniles of those sauropod species identified so far, even when several species were sympatric.

And thus we end this part here. But there’s still more to come, stay tuned.

For the previous article in this series, see…

For previous TetZoo articles on sauropods, brontotheres, giraffes and related issues (linking where possible to wayback machine versions), see…

And here's your reminder that the persistence and success of Tetrapod Zoology now depends entirely on funding via patreon. Thanks to those who support me! The more secure my funding, the more time I can and will spend generating content for Tet Zoo.

Refs - -

Carpenter, K. 2006. Biggest of the big: a critical re-evaluation of the mega-sauropod Amphicoelias fragillimus Cope, 1878. New Mexico Museum of Natural History and Science, Bulletin 36, 131-137.

Farlow, J. O, Coroian, I. D. & Foster, J. R. 2010. Giants on the landscape: modelling the abundance of megaherbivorous dinosaurs of the Morrison Formation (Late Jurassic, western USA). Historical Biology 22, 403-429.

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.

Foster, J. R. 2005. New juvenile sauropod material from western Colorado, and the record of juvenile sauropods from the Upper Jurassic Morrison Formation. In Tidwell, V. & Carpenter, K. (eds) Thunder-Lizards: The Sauropodomorph Dinosaurs. Indiana University Press (Bloomington & Indianapolis), pp. 141-153.

Janis, C. M. & Carrano, M. 1992. Scaling of reproductive turnover in archosaurs and mammals: why are large terrestrial mammals so rare? Annales Zoologici Fennici 28, 201-216.

Lockley, M. G. 1994. Dinosaur ontogeny and population structure: interpretations and speculations based on fossil footprints. In Carpenter, K., Hirsch, K. F. & Horner, J. R. (eds) Dinosaur Eggs and Babies. Cambridge University Press, pp. 347-365.

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.

Ngene, S., Okello, M. M., Mukeka, J., Muya, S., Njumbi, S. & Isiche, J. 2017. Home range sizes and space use of African elephants (Loxodonta africana) in the Southern Kenya and Northern Tanzania borderland landscape. International Journal of Biodiversity and Conservation 9, 9-26.

Paul, G. S. 1994. Dinosaur reproduction in the fast lane: implications for size, success, and extinction. In Carpenter, K., Hirsch, K. F. & Horner, J. R. (eds) Dinosaur Eggs and Babies. Cambridge University Press, pp. 244-255.

Prothero, D. R. 2019. The Story of the Dinosaurs in 25 Discoveries. Columbia University Press, New York.

Sander, P. M., Christian, A., Clauss, M., Fechner, R., Gee, C. T., Griebeler, E.-M., Gunga, H.-C., Hummel, J., Mallison, H., Perry, S. F., Preuschoft, H., Rauhut, O. W. M., Remes, K., Tütken, T., Wings, O. & Witzel, U. 2011. Biology of the sauropod dinosaurs: the evolution of gigantism. Biology Reviews 86, 117-155.

Spinage, C. A. 1994. Elephants. T & A D Poyser, London.

Traill, L. W., Bradshaw, C. J. A. & Brook, B. W. 2007. Minimum viable population size: a meta-analysis of 30 years of published estimates. Biological Conservation 139, 159-166.

Upchuch, P., Barrett, P. M. & Dodson, P. 2004. Sauropoda. In Weishampel, D. B., Dodson, P. & Osmólska, H. (eds) The Dinosauria, Second Edition. University of California Press (Berkeley), pp. 259-322.

Woodruff, D. C. & Foster, J. R. 2017. The first specimen of Camarasaurus (Dinosauria: Sauropoda) from Montana: the northernmost occurrence of the genus. PLoS ONE 12 (5): e0177423.