Dinosaur life histories are plicomcated February 18, 2016

New paper out in Biology Letters:

Hone, D.W.E., Farke, A.A., and Wedel, M.J. 2016. Ontogeny and the fossil record: what, if anything, is an adult dinosaur? Biology Letters 2016 12 20150947; DOI: 10.1098/rsbl.2015.0947.

The idea that dinosaurs had unusual life histories is not new. The short, short version is that it is usually pretty straightforward to tell which mammals and birds are adults, because the major developmental milestones that mark adulthood – reproductive maturity, cessation of growth, macro-level skeletal fusions, histological markers of maturity – typically occur fairly close together in time. This is radically untrue for most dinosaurs, which started reproducing early, often well before they were fully grown, and for which the other signals of adulthood can be wildly inconsistent.

Puny ‘pod

We don’t talk about this much in the paper, but one aspect of dinosaur life history should be of particular interest to sauropodophiles: most of the mounted sauropod skeletons in the world’s great museums belong to animals that are demonstrably not mature. They’re not the biggest individuals – witness the XV2 specimen of Giraffatitan, the giant Oklahoma Apatosaurus, and Diplodocus hallorum (formerly “Seismosaurus”).* They’re not skeletally mature – see the unfused scapulocoracoids of FMNH P25107, the holotype of Brachiosaurus mounted in Chicago, and MB.R.2181, the lectotype of Giraffatitan mounted in Berlin. And histological sampling suggests that most recovered sauropods were still growing (Klein and Sander 2008).

* The Oklahoma Museum of Natural History does have a mounted (reconstructed) skeleton of the giant Apatosaurus, and the New Mexico Museum of Natural History has a mounted reconstructed skeleton of Diplodocus hallorum. But as nice as those museums are, in historical terms those mounts are brand new, and they have not shaped the public – and professional – conception of Apatosaurus and Diplodocus to anywhere near the same degree as the much smaller specimens mounted at Yale, AMNH, the Field Museum, and so on.

Basically, very little of what we think we know about sauropods is based on animals that were fully grown – and the same problem extends to many other groups of dinosaurs.

This is kind of a methodological nightmare – a colleague on Facebook commented that he had pulled his hair out over this problem – and in the paper we suggest some ways to hopefully alleviate it. I mean, the biology is what it is, but we can minimize confusion by being really explicit about which criteria we’re using when we assign a specimen to a bin like “juvenile”, “subadult”, and so on.

Supposed Former Evolution Junkie

Personally, I’m more excited about the possibilities that dinosaur life history weirdness open up for dinosaur population dynamics and ecology.

Confession time: I am a recovering and relatively high-functioning evolutionary theory junkie. In grad school I was on the heavy stuff – I read tons of Gould and Dawkins and admired them both without being smitten by either. I took seminars on Darwin and evolutionary morphology, and lots of courses in ecology – ever mindful of Leigh Van Valen’s definition of evolution as “the control of development by ecology”. I read a fair amount of Van Valen, too, until “Energy and evolution” (Van Valen 1976) burned out most of my higher cognitive centers.

I say “recovering” evolutionary theory junkie because after grad school I mostly went clean. The problem is that dinosaurs are good for a lot of things, but exploring the inner workings of evolution is usually not one of those things. As products of evolution, and demonstrations of what is biomechanically possible, dinosaurs are awesome, and we can look at macroevolutionary patterns in, say, body size evolution or morphospace occupation, but we almost never find dinos in sufficient numbers to be able to test hypotheses about the tempo and mode of their evolution on the fine scale. I suppose I could have switched systems and worked on critters in which the machinations of selection are more visible, but for me even the charms of evolutionary theory pale next to the virulent allure of sauropods and pneumaticity.

Anyway, keeping in mind that Van Valenian dictum that evolution stands with one foot in the organism-internal realm of genes, cells, tissue interactions, and other developmental phenomena, and the other in the organism-external world of competition, predation, resource partitioning, demographics, and other ecological interactions, then it stands to reason that if dinosaurs had weird ontogenies – and they did – then they might have had weird ecologies, and weird evolution full stop. (Where by ‘weird’ I mean ‘not what we’d expect based on modern ecosystems and our own profoundly mammal-centric point of view’.)

Actually, we can be pretty sure that the weird ontogenies and weird ecologies of dinosaurs were intimately linked (see, for example, Varricchio 2010). Like the tyrannosaurs shown here – they didn’t all fill the same ecological niche. This casts some old arguments in a new light. Was T. rex adapted for fast running? Prrrrobably – just not as a full-size adult. The skeleton of an adult tyrannosaur is that of a 500 kg cursor pressed into service hauling around 10 tons of murder. And all of this has some pretty exciting implications for thinking about dinosaurian ecosystems. Whereas mammals tend to fill up ecospace with species, dinosaurs filled up their world with ecologically distinct growth stages.

Does all of this add up to weird evolutionary dynamics for dinosaurs? Possibly. As we say in the paper,

Correct identification of life stage also is relevant to fundamentals of evolution—if the onset of sexual reproduction substantially preceded cessation of growth in dinosaurs then the ‘adult’ phenotype may not have been the primary target of selection. In fact, once juveniles or subadults are capable of reproducing, it is conceivable a population could exist with potentially no individuals making it through the survivorship gauntlet into ‘adulthood’ and close to maximum body size. The occasional hints from the fossil record of anomalously large sauropods like Bruhathkayosaurus [51], and the Broome trackmaker [52] might be explained if many sauropods were primarily ‘subadult’ reproducers, and thus extremely large adults were actually vanishingly rare.

Did that actually happen? Beats me. But it’s consistent with what we know about sauropod life history, and with the observed scarcity of skeletally mature sauropods. And it might explain some other oddities as well, such as the high diversity of sauropods in seasonally arid environments like the Morrison Formation (see Engelmann et al. 2004), and the fact that sauropods – and large dinosaurs generally – are much larger than predicted based on the land areas available to them (see Burness et al. 2001). Because the age structure of sauropod populations was so skewed toward juveniles, the average body size of most sauropod populations was probably fairly modest, even though the maximum size was immense. So maybe a continuously reproducing population didn’t require as much food or space as we’ve previously assumed.

If we can falsify that, cool, we’ll have learned something. And if we can falsify the alternatives, that will be even cooler.

I’ll stop waving my arms now, lest I achieve powered flight and really inspire controversy. Many thanks to Dave and Andy for bringing me on board for this. It was a fun project, and we hope the paper is useful. You can read Dave’s thoughts on all of this here.

References