Dinosaur life histories are plicomcated

February 18, 2016

Zuniceratops ontogeny - Hone et al 2016 fig 2

Various methods that may be used to determine the age/ontogenetic status of a given dinosaur specimen. Central image is a reconstruction of the skeleton of an adult ceratopsian Zuniceratops, with surrounding indications of maturity (taken from multiple sources and do not necessarily relate to this taxon). (a) Development of sociosexual signals (adult left, juvenile right—modified from [9]), (b) surface bone texture (traced from [17]), (c) large size, represented here by an ilium of the same taxon that is considerably larger than that of a known adult specimen, (d) reproductive maturity, here based on the presence of medullary bone here shown below the black arrow (traced from [18]), (e) fusion of the neurocentral arch—location of the obliterated synchondrosis indicated by black arrow (traced from [19]), (f) asymptote of growth based on multiple species indicated by black arrow (based on [20]). Central image by Julius Csotonyi, used with permission. Hone et al. (2016: fig. 2).

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.

OMNH baby Apatosaurus

Apatosaurs large and small at the Sam Noble Oklahoma Museum of Natural History

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’.)

LACM Tyrannosaurus trio - Hone et al 2016 fig 1

Three growth stages of Tyrannosaurus on display at the Natural History Museum of Los Angeles County. Hone et al. (2016: fig. 1).

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

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39 Responses to “Dinosaur life histories are plicomcated”


  1. […] Edit: here’s a bonus – Mat Wedel’s sauropod-centered take on the paper […]

  2. Mike Taylor Says:

    Side-issue here, but: the caption to the first figure shows one more reason why number references are a plague. When you lift a single figure like this, its caption becomes hard to follow. “Development of sociosexual signals, modified from [9]” is not helpful; “Development of sociosexual signals, modified from Scannella et al. 2014” is.

  3. LeeB Says:

    If selection is acting on the sub-adult phenotype and there is selection for enormous size then there is the potential for the theoretical adult maximum size to beyond the maximum size sustainable in a land vertebrate.
    The outcome would be that the rare surviving near adult specimen would reach the maximum possible size while still growing and then die of size related consequences.

    If this occurred then the largest sauropods of different species should approach a similar size asymptote but they might be so rare that we haven’t realised this.

    LeeB.

  4. Anonymous Says:

    T. Lehman had a similar suggestion back in 2007 in his paper on Agujaceratops (then Chasmosaurus) mariscalensis in the Horns and Beaks book (yeah, yeah, I know, stinkin’ ornithischians, but bear with me). If I recall right, he suggested that because of the extremely high abundance of subadult individuals at Big Bend, he suggested that because of indeterminate growth the subadult individuals were essentially the “real” adults, with the super-sized, rare, and almost always lone adult individuals were old individuals that happened to survive to adulthood whereas the other members of their cohort died out. If this and the ideas you suggest are true, then skeletal maturity might be about as useful of an indication of adult status in dinosaurs as a widjet.

    Of course, the alternate is that the sutures simply never fused despite the animal being fully grown, or at least only fused in extremely senescent individuals that do not represent the general population. In marsupials, for example, the sutures of the skull remain separate throughout the animal’s life except in very rare cases. Histology might be a little more helpful in that case.

    Indeed, aren’t there some extant reptiles and/or amphibians that mate before being somatically mature? I seem to remember reading somewhere of some species of lizard or frog that have males that could mate as early as its first year of life, but generally doesn’t bother because it still needs one or two more years to reach a size where it can compete with the dominant males.


  5. Mike Taylor asked (https://twitter.com/MikeTaylor/status/700267690435465216) for my thoughts on T. rex as a fast runner *as a youngun* but not as an adult, so I’ve heeded that summons. My past (old) work on the biomechanics of running supported the idea (Currie, plenty of others) that juvenile tyrannosaurs, like smaller more “basal” tyrannosaurs, were leggier and better suited to being relatively fast runners.

    But it all hinges on what “relative” means; it doesn’t mean faster in mph (absolute speed). Few species that have been thoroughly studied scientifically show a peak in maximal speed (in mph, meters per second, whatever) at juvenile ages and then a decline afterwards, although crocs etc. do. But tyrannosaurs are not “few species”, they are weird in being bipedal (simpler to study than pesky quadrupeds) and growing from ~5kg to >5000kg body masses in <20 years, as we discussed in our 2011 PLOS one paper. Yet the "error bars" on estimates of speed, whatever method one uses, mean that distinguishing relative speed vs. absolute speed is hard, or to be overly frank/cynical, delusional. I know palaeontologists like to go on and on about "cursorial" limb proportions in tyrannosaurs etc but I don't get excited by those- the singal vs. noise in limb proportions vs. absolute speed data are cause for much caution IMO.

    TLDR: we don't really know, scientifically, but younguns probably weren't worse off once they survived to ~10yrs old, speed-wise, based on the meager data available.

  6. Thomas Holtz Says:

    In response to what John said: keep in mind (as others have said in the past) that “cursorial” might mean a lot more than “maximum achieved speed” or “maximum acceleration”. There are modern ‘cursorial’ animals which might have wider home ranges than less ‘cursorial’ ones, and so on. Maximum speed is not the only performance metric involved.

  7. Thomas Holtz Says:

    In response to the OP, though: it isn’t just sauropods. Unless “Saurophaganax” is a skeletally-mature adult, none of us have seen a fully-grown and operational Allosaurus. The histological work on Allosaurus has, as far as I have seen, never shown an individual with an EFS.

  8. Matt Wedel Says:

    Thanks, everyone, for the comments.

    Mike: oh man, don’t get me or Andy or Dave started on the numbered references. They were a plague of near-Biblical proportions.

    LeeB: that is a plausible scenario that had not occurred to me. I know the validity of hypotheses is unrelated to their aesthetics, but that one has the feel of something that nature might actually do – intellectually stimulating, but pretty damn grim for the organisms that have to live through it.

    The most obvious counter I can think of is that surely there would be selection for growth to stop early, before the giant adults overshot the biological limits on large size. A sauropod that stopped growing just a little early – when it was big enough to be immense but not quite over the threshold of viability – would be effectively predator-immune. And since annual egg output scales up with increasing size, one of those threshold-of-viability giants would be able to spam the whole ecosystem with offspring, at least some of which would carry the same trait.

    But still, a most interesting idea, and one that could have happened if sauropods had some kind of “grow or die” genetic/developmental/physiological program.

    Anonymous: VERY interesting that Lehman proposed that back when. I don’t own that book – I’ll have to track down the paper and check it out.

    the alternate is that the sutures simply never fused despite the animal being fully grown

    I think we can rule that out for sauropods – for the big individuals with unfused joints, there is almost always something equally big or bigger with the same joint fused. For example, the three largest cervical vertebrae (of Supersaurus, Sauroposeidon, and Puertasaurus – see here) all have fused cervical ribs, so I’m pretty sure that the unfused cervical rib in the giant Oklahoma apatosaurine is actually telling us something useful about that individual’s ontogenetic stage.

    or at least only fused in extremely senescent individuals that do not represent the general population

    I admit that is much harder to falsify. And the regularities I’ve observed in sauropods may not – probably do not – apply to other clades of dinosaurs.

    John: thanks for the insights, much appreciated. Because you’ve done so much work on tyrannosaur locomotion, I’d be curious to hear your thoughts about how their changing proportions might (or might not) be related to changes in their ecology as they grew up.

    Tom: re: Allosaurus, you just blew my mind. I did not know that.

    What is the thinking on Saurophaganax these days? I know it’s always been controversial, but I thought that Chure made a better case than many that criticized his work later. I haven’t kept up with this, though, and I’d be grateful for any word from the theropod side.

  9. Andrew Stuck Says:

    Might this settle the “Toroceratops” debate once and for all? I’m surprised you guys featured Zuniceratops instead.

  10. Mike Taylor Says:

    Leeb: very interesting proposition. I think that the scant evidence we have of super-giant sauropods — Bruhathkayosaurus, Amphicoelias fragillimus, the Broome Sandstone track-maker, that other track-maker in France — tends to corroborate the idea that even different lineages converged on a maximum size somewhere in the 150-tonne region.

    Anonymous, I seem to recall there there’s a frog which reproduces even in the tadpole stage.

    John, interesting stuff, thanks. I think there is a cottage industry growing up around the idea of fast juvenile tyrannosaurs; it should be more widely known that the person who’s most actually studied this doesn’t buy it. (I assume by relative speed you mean something like Froude number?)

    Tom, your comment on different meanings of “cursorial” reminds me that we really need to get our 2014 SVPCA talk written up!

  11. Thomas Holtz Says:

    Matt: currently agnostic on Saurophaganax: could be its own thing, could be adult Allosaurus. Work is planned to do the histological work on that specimen in the (hopefully near) future.

  12. Matt Wedel Says:

    Tom: very interesting! Hope the histo works out and gives useful data.

    Incidentally, folks from the OMNH and OSU have relocated some of Stovall’s old Morrison quarries in the Oklahoma panhandle, and they’re starting to produce. So it’s possible that we’ll get some more Saurophaganax at some point. I’m going out there next month to have a look around. Anne Weil, Kent Smith, and Rich Cifelli are mostly interested in the potential microverts and they need someone to offload the boring old sauropods onto. I’m happy to throw myself on that grenade.

    Anyway, if you need any ‘phaganax photos or measurements, let me know.

  13. David Hone Says:

    Andrew – we don’t ‘feature’ Zuniceratops in any way. We wanted a nice skeletal for the figure and a ceratopsian made sense and Julius had that and kindly allowed us to use it. If you read the fig caption, there’s nothing there that is actually about this animal, it’s all representative / conceptual.

  14. Mike Taylor Says:

    Andrew Stuck asks: “Might this settle the “Toroceratops” debate once and for all?”

    Hahahahahahahaha.

  15. Matt Wedel Says:

    Yeah, if anything, it shows why the “Toroceratops” debate is so tricky – except for a few obviously geriatric individuals, almost all dinosaurs ever found are not “fully adult” on one criterion or another.


  16. Mike re: cottage industry: yes don’t get me started/continuing… so tired of it all. In a way, it’s all not very interesting (self-deprecation fully intended here too), but in a less blase way, it is interesting because of the major changes in size across ontogeny, into elephantine-size realms, which have implications for general principles about how size influences athletic abilities (principles that we can’t fully test w/today’s animals).

    Matt: ecology of tyrannosaur growth: I agree w/what others have said, their ecology changed a ton, from vulnerable little ~chicken-sized kids to huge monsters that few other things could threaten. Much like crocodiles. Oh and sauropods, OK, but riding high in the saddle atop that food pyramid instead of grubbing closer to the base. ;-)

    Tom: yes, I tend to be more convinced by those that argue that cursorial morphology relates to economy of locomotion (e.g. Janis, Carrano), on average, but with substantial variation. No “one limb proportions theory to rule them all” maybe.

  17. Andrew Stuck Says:

    I see. I thought maybe since this implies that traditional notions of “fully adult” don’t seem to strictly apply to dinosaurs, that this would take all the wind out of the sails for that argument.

    (I see your point on Zuni. I guess it would’ve been more correct to say “figured” rather than “featured”.)

  18. Andy Farke Says:

    Great discussion so far. As an added note, I think part of why I was so keen to write this paper is to counter a bit of the nihilism that seems to be floating around regarding the issue of “adult” dinosaurs. I think to some extent it is more prominent in the pop-culture/dino-fan sector, but it also has circulated to some extent among professionals too. By nihilism I mean the “oh, it’s all juvenile so we can’t do any [real phylogeny/biomechanics/taxonomic diagnosis/etc.]”. As discussed in the paper and above blog post and comments, and as noted by others in the literature, the biology of dinosaurs is so odd that I think we get into hazardous territory when trying to overstate the relevance of considering only “adult” animals. And it’s complicated even more by the fact that so much of the necessary groundwork in extant taxa simply hasn’t been done yet. It’s better than it used to be, certainly, but how many extant sauropsids really have solid life history data, or histological data, or sutural fusion data, or whatever? And almost none of them have all of these together! So I suppose I’m nihilistic too, just in a different way.

  19. Matt Wedel Says:

    Nihilists! Farke – I mean, say what you want about the tenets of the “Toroceratops” hypothesis, dude, at least it’s an ethos.

  20. Mike Taylor Says:

    John: “principles that we can’t fully test w/today’s animals” — that is exactly the problem we run into all the time with sauropods. Stupid extant fauna *grumble* *moan*

  21. David Hone Says:

    Following on from Andy’s point, in my case I was at least partly motivated though my interest in the issues of definitions themselves and how they are used. I’ve been very frustrated by lots of palaeontologists (and yes, I too have been guilty of this in the past I’m sure) throwing around actually quite fundamental terms like ‘adult’ or ‘social’ or ‘predator’ without considering what that might really mean or providing any definition or support for that use and it’s really critical when trying to compare across specimens / taxa / ideas and we need to get better at it.

  22. Matt Wedel Says:

    Yes, well said. This is an interesting problem on two levels – both the underlying biology, and the methodological question of how we interpret that.


  23. I’m going to ask about a factor that many may predict, and that’s how much of a biomechanical constraint on properties of so-called “adulthood” are being assessed?

    When it comes to things like display features, proportion of bone to soft-tissue, size of cranial crests and the presence of fenestrae in them (sunken areas expanding into apertures), and features such as sutural closure and synovial fusion — note that there is no such thing as a postcranial suture: sutures are cranial only. These features all seem to have actual, relevant, and perhaps unaccounted-for biomechanical properties. John Hutchinson and Tom Holtz both touched on cursoriality in animals grading through size, and that’s another issue involved here: In birds, preccociality is primarily indicated by early fusion of postcranial synovial contacts, such as the fibula to the tibia, the proximal tarsals to the tibia, scapula to coracoid, etc. Fusion of bone contacts are primarily as a response to strain, and higher activity and higher strain would result in higher degrees of fusion.

    Anecdotally, we might assume then that at some point in their evolution, preccociality was selected for as young animals incurred higher strain at younger ages, and survived longer. What, if anything, tells us that at certain sizes fusion of bones is even relevant to the question of “is this an adult”? This is actually begging the question: Why are we assuming being an adult is relevant to sexual reproduction?

  24. brian engh Says:

    Great discussion!! Ahhh, how I love it when the hard science kicks down the door to the realm of wild speculation!

    So, as a paleoartist / the significant-other to a wildlife biologist I am duty bound to cast both complexity and doubt into the discussion about “cursorial” predator ecology/niche partitioning!

    For starters, here’s a video of a grizzly bear running down a young elk (note the relative ‘legginess’ of each animal): https://www.youtube.com/watch?v=aBBmdays-c4

    i think i heard somewhere the Athabascan word “grizzly” translates to “cursorial”…

    Some thoughts & questions on T-rex legs:
    >Do we have any real reason to believe that young t-rex legginess had anything to do with running faster? How do we know they weren’t using their long legs and impressively clawed feet as modern theropods do – to kick the snot out of prey before putting their face in harms way? Here’s a highly “cursiorial” looking secretary bird doing that to a snake: https://www.youtube.com/watch?v=QJKBPyavWlI
    Also, young t-rex leg proportions don’t strike me as being markedly different from more basal coeleurosaurs (some of which show adaptations for using their feet as weapons, btw). Isn’t it possible that leggier/relatively longer armed youngins are just the more basal state? On some “Ontogeny recapitulates phylogeny” or somethin.

    > IF leggy t-rex legs = fast t-rexes how do we know that had anything to do with running down prey? In modern crocs and monitor lizards smaller individuals are in danger of being cannibalized by larger ones, despite the fact that all sizes will feed together semi cooperatively on a big kill. Seems to me that leggy young rexes might just be needing to outrun/maneuver bigger t-rexes whenever the big ones haven’t made a big kill.

    > Looking at modern predators we see 8 bajillion surprising and innovative behavioral strategies for overcoming various prey species’ defenses. Cheetahs are the exception in that they just blast on full speed to try and run down the fastest prey in an open sprint. But even cheetahs are only successful in catching their prey (which generally have better endurance than cheetahs) about half of the time, and are often too tired to resist other predators taking their prey after a prolonged sprint. Crocs, monitor lizards, large constricting snakes, birds of prey, cats (including cheetahs) all use an element of stealth and ambush in order to overtake prey, and many of these groups frequently take prey that can out-sprint them in an open chase or outpace them over long distances. Even among crocs there are varying degrees of social or cooporative hunting, and a number of other factors such as terrain, condition of the prey animal, weather, season, time of day/night, etc., can all affect when and how a predator makes a kill.

    >Looking at apex predator niche partitioning we see that many of them kill and eat a wide variety of prey items, and their prey focus shifts throughout their life. Many predators “graduate” through various food sources, at times losing the capacity to efficiently hunt one prey menu as they gain the ability to hunt other species. Young leopards for example hone their stalking and ambushing skills by catching wary rodents and birds (many of which leopards could easily out-run in an open chase, but which evade predators by hiding or taking flight). As leopards gain in size and experience they then graduate to monkeys (which can out-climb them) and gazelle (which can out run them over long distances). They only attain the skill and physical strength neccessary to tackle things like larger antelope species, wildebeests and small zebra when they reach adult size, and even at adult size usually only large male leopards will tackle the largest prey species, while being a bit too big and heavy to effectively hunt some of the smaller prey they grew up on. Crocs and monitors similarly work their through different species as they grow, and there appears to be an element of learned hunting strategy and individual preference with them as well.

    So yeah. Uh. I have no idea how to conclude this longass comment. Got totally carried away. Sorry about that!

    Great work on the paper!!! Really fascinating stuff!!

    Here’s a couple rad documentaries about big cats growing up, learning to hunt different prey items, sometimes hunting cooperatively etc:
    Leopards > https://www.youtube.com/watch?v=nHMLg6zgeUI
    Puma > https://www.youtube.com/watch?v=z3bhkrmjfhY

    and here’s a doc about salt water crocs showing complex heirarchical predation/scavenging/prey focus stuff:

    And here’s a video of a pack of the most murderously destructive and bizarre apex predators of all time killing a kudu (which can run faster than them) by using bipedal endurance running as a weapon to exhaust it to the point of collapse!:

  25. Matt Wedel Says:

    Yep, those are good points. And thanks for the videos. Here’s one that was suggested as a follow-on from the grizzly chasing the elk:

    There is something primally terrifying about seeing a big predator coming right at you, even if it’s in a photo. I have a very healthy respect for bears anyway, but those photos of the charging grizzly made my blood run cold.

  26. Mike Taylor Says:

    Bears are incredible animals. It beggars belief that anyone thinks they’re cute. They are gigantic super-atheletic murderous clumps of awesome.

  27. LeeB Says:

    Once sauropods get big enough you would think their risk of predation would drop down to near zero; for sauropods reaching that size their chances of continuing on to full adulthood would seem quite good.
    Yet very large ‘fully adult’ sauropods seem to be extremely rare.
    This suggests that either something is continuing to kill even very large sauropods at an appreciable rate (starvation, fighting among themselves, lightning strikes, disease etc.) or that they never reach an “adult phenotype” and continue to grow albiet at a reduced rate.
    If the latter is true then the very large specimens that appear adult may not be the same species after all but specimens of some other species that is either very rare or has wandered in from some other region.

    LeeB.


  28. Great to see this eye-opening article, especially about some really big sauropods on display not being skeletally-mature!
    I wonder how sexual maturity and sexual selection relate to the ability to reproduce before total ‘adulthood’ (whatever that may mean) and the supposed trait of continuing to grow life-long.
    For example, did a sauropod individual attaining a certain huge size rule out its finding a compatible/accepting mate? Although largest indiduals often may win in mating rivalries among mammals, I can picture the most massive sauropod male losing the agility to mount, or becoming too risky and hazardous as a partner for significantly-smaller females; and can picture the largest sauropod female finding herself lacking a local male large enough to mount her, in a local population.
    Similarly for carnosaurs, etc.
    Might there, then, have been significant sub-populations of non-breeding but healthy individuals of the largest sizes in the highest age range, in various dinosaur species?

    What then might have been their interactions with their smaller, younger con-specifics?
    Might they have maintained parental (or family, or communal) oversight of the younger ones long-term – defending their genetic investment?
    The largest sauropods might have been effective guardians of a family group – as with elephant matriarchs.
    I imagine the largest carnosaur could bring down prey with less risk to itself (and tear apart large carcasses), big enough to feed both itself and with left-overs, so helping its smaller relatives to survive.


  29. With regards to the relative paucity of mega huge sauropods in the fossil record: there is also a relative paucity of mega huge trees and tree trunks preserved in tact in the fossil record, yet we know that modern species that grow really goddamn big (redwoods, sequoias, araucaria etc) have survived relatively unchanged since the mesozoic. So where are all the bigass logs? That question might be best answered with another question: how do you bury a huge tree trunk so that it can be preserved? Or, rather than a tree trunk, howabout a giant, deliciously nutritious, airsac and bacteria filled sauropod?

    Ask Sharon McMullen in a few months! Science is happening!

  30. Mickey Mortimer Says:

    To answer about Saurophaganax, from the Database- “Though generally accepted as a species distinct from A. fragilis, Saurophaganax is represented by multiple individuals with uncertain referral of particular elements to each one, and reportedly distinctive characters are not stated to be present in multiple specimens except for distal chevron expansion and astragalar buttress strength. Indeed, nine of the characters can only be evaluated for one specimen. Thus the apparent distinctiveness of this taxon may be due to individuals each having a few unique characters, as is true of most well described Morrison allosaurs. Resolution will involve more detailed information on each specimen and more description of the variation in Morrison allosaurids. Smith (2008) found no morphometric differences between maximus and fragilis in at least the humerus and femur. “

  31. Mike Taylor Says:

    Brian, this very issue — the difficulty of burying and fossilising a giant sauropod — is part of the subject of a paper that I have in review right now. (It’s a bit stalled because the reviews were critical but totally fair … the worst combination, as it gives me the most extra work to do :-))

  32. Mike Taylor Says:

    Armadillozenith raises the interesting possibility of kin selection in sauropods. Just as worker bees do not directly pass on their own genes but enable their sister, the queen to pass on hers which are closely related, so the largest sauropods may have acted as predator deterrents for their herd even when they were past the age of being able to effectively mate. You could imagine (and that’s all that this is, of course) a situsation where each sauropod herd maintains one or two super-giant individuals for this purpose.


  33. […] life less ordinary. Dinosaurs. Everything about them was unusual. Great take, by Matt […]

  34. Duane Says:

    A more interesting question to me is not where are all the mature, giant sauropods but where are all the elk, buffalo, rhino, hippo, and elephant sized immature sauropods? These smaller but still large size classes must have been relatively more abundant than the few scattered giant individuals. Still large enough to have been easily preserved.

  35. Matt Wedel Says:

    That’s a great question.

    First, we should acknowledge that sometimes we do get immature sauropods, in herds even. See, for example, Myers and Fiorillo (2009) on age-segregated herds of juvenile sauropods at the Mother’s Day Quarry in the Morrison and in the Late Cretaceous at Big Bend.

    Myers, T.S., and Fiorillo, A.R. 2009. Evidence for gregarious behavior and age segregation in sauropod dinosaurs. Palaeogeography, Palaeoclimatology, Palaeoecology 274:96-204.

    Second, my impression from lots of museum visits is that there actually are a fair number of juvenile-ish sauropods in museums, they just tend to be incomplete (in the same way that all sauropods tend to be incomplete) and not described in the literature. For example, I know of small individuals of Apatosaurus, Diplodocus, and Camarasaurus at the Carnegie Museum that have never received more than a cursory mention in the literature, and there are juvenile Brachiosaurus cervicals at BYU and the Smithsonian that have never been properly described.

    Third, my personal pet theory is that most mid-sized juvenile sauropods – say, large enough to be part of the reproducing population, but still a long way from full size – had one of two fates: they grew up, or they got processed into theropod poop. And before anyone argues that Morrison theropods were too wimpy to “disappear” an entire elephant-sized (or larger) sauropod, there is a camarasaur ilium from Cleveland Lloyd that is bitten through at a point where it must be 5 or 6 inches (125-150mm) thick. Anything that can bite through that much solid bone will have no problem transmuting an elephant-size carcass into feces and mayhem.

    Fourth, assuming I’m right in the previous point, where are all the big sauropod skeletons, then? One answer is that if sauropods spent a lot of time on long hikes between patchily distributed resources like forests and water sources, many of them probably died in places where they were unlikely to be rapidly buried. And those that did die in depositional environments were not guaranteed to get buried before they decayed. There is a very necessary taphonomy/sedimentology project to be done on the likely exponential fall-off in preservability for big terrestrial animals. In short, the flood necessary to bury and fossilize a 50-ton animal probably comes along a lot less frequently than the one necessary to bury a 5-ton animal – but how much less frequently?

    BUT with all of that said, relating the likely population structure of sauropods to what is actually present in museum collections is a great project that still needs doing.

  36. Mike Taylor Says:

    Duane, don’t forget that the age-distribution of a standing population is usually very different from that of its corpses. In most human societies, the majority of the population is aged 40 or below, but the great majority of corpses are aged over 40. The same was likely true of sauropods.

  37. briunghengh Says:

    With regards to elephant/rhino/hippo sized sauropods: doesn’t the trackway evidence also indicate what we’d expect? i.e.. super giants are very rare, properly big ones a bit less so, and elephant+ ish sizes quite abundant…? Might be an interesting grad student project just in running the numbers on that. Certainly at the Mill Canyon site there are a pile of sauropod tracks and all of them are in the elephant ish size range. In fact the largest sauropod tracks there are only about the same size, if not smaller than, the largest theropod, ornithopod and ankylosaur tracks at that site… Though that is thought to be a pretty arid/sparsely vegetated desert environment, so who knows how climate/vegetation type might’ve selected in favor of one dino group or another.

    Great discussion. I love the idea of a herd structure where there’s a huge matriarch or 2 and a bunch of smaller ones who crowd around it like huts around a castle keep.


  38. […] target for Saurophaganax and other large Morrison theropods: sauropods that were not fully-grown, which was almost all of them. I am hip to the fact that golden eagles kill deer, and some lions will attack elephants – as […]


  39. […] developmental programs were ‘messy’ compared to those of mammals, both in having weird timings for things like onset of reproduction, and in varying a lot among closely related taxa. […]


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