The Recapture Creek sauropod: the reveal
March 3, 2013
If you’re just joining us, this post is a follow-up to this one, in which I considered the possible size and identity of the Recapture Creek femur fragment, which “Dinosaur Jim” Jensen (1987: page 604) said was “the largest bone I have ever seen”.
True to his word, Brooks Britt at BYU got back to me with measurements of the Recapture Creek femur fragment in practically no time at all:
Length 1035 mm, width 665 mm. However, you cannot trust the measurements because Jensen put a lot of plaster on the proximal half of the bone.
Now, taking plaster off a bone is not going to make it any larger. So the plastered-up specimen is the best case scenario for the RC femur to represent a gigapod. And I know the stated width of 665 mm is the max width of the proximal end, because I sent Brooks a diagram showing the measurements I was requesting. The length is a little less than anticipated, and doesn’t quite jibe with the max proximal width–I suspect a little might have broken off from the distal end where the preservation looks not-so-hot.
Based on those measurements, it looks like Jensen got the scale bar in Figure 8 in his 1987 paper approximately right–if anything, the scale bar is a little undersized, but only by 5% or so, which is actually pretty good as these things go (scale bars without measurements are still dag-nasty evil, though). By overlapping Jensen’s photo with the femur of the Brachiosaurus altithorax holotype (FMNH P25107) to estimate the size of the element when complete, I get a total length of 2.2 meters–exactly the same size as about 8% bigger than the Brachiosaurus holotype (actual length 2.03 m). If the Recapture Creek femur is from a Camarasaurus, which I don’t think we can rule out, it was 2 meters long when complete, or 11% longer and 37% more massive than the big C. supremus AMNH 5761–about 35 tonnes or maybe 40 on the outside. So it’s a big bone to be sure, but it doesn’t extend only slightly extends the known size range of Morrison sauropods. (Updated 2014-05-19–as I related in the first post, I somehow got it fixed in my head that the holotype B.a. femur was 2.19 m when it is actually 2.03 m.)
So, as before, caveat estimator when working from scaled illustrations of single partial bones of possibly immense sauropods.
Now, here’s a weird thing. Let’s assume for the sake of this discussion that the Recapture Creek femur is from a brachiosaur. That gives us three individual Late Jurassic brachiosaurids–the Recapture Creek animal, the Brachiosaurus altithorax holotype, and the mounted Giraffatitan brancai–that are almost exactly the same size in limb bone dimensions (although B.a. had a longer torso). But we know that brachiosaurids got bigger, as evidenced by the XV2 specimen of Giraffatitan, and based on the lack of scapulocoracoid fusion in both FMNH P25107 and the mounted Giraffatitan. So why do we keep finding these (and smaller) subadults, and so few that were XV2-sized? I know that there gets to be a preservation bias against immense animals (it’s hard to bury a 50-tonne animal all in one go), but I would not think the 13% linear difference between these subadults and XV2-class adults would be enough to matter. Your thoughts?
Reference
Sauropod Vertebra Picture of the Week 
March 3, 2013 at 5:21 pm
What we need to do is find out just how good we are… Snatch out a femur/vertebra from a reasonably well known specimen, and see how closely a panel of paleontologists can estimate the beastie’s weight! Then check them against the published mass estimates. This could also be tested on living animal’s skeletons.
March 3, 2013 at 7:09 pm
Dean, I agree completely. And I have at least two things I’d like to try.
EXPERIMENT 1
Mike pointed out that he has often seen a bone in collections and thought, “Damn, that’s huge”, but then has done the math and realized that it’s no bigger than the same element in one of the famous mounted skeletons. I have had the same experience more than once. So maybe there is a perceptual effect where big bones look bigger when they’re isolated and not just one comparatively small part of an immense mounted skeleton. That could explain why Jensen mistook the Recapture Creek femur as being bigger than the Brachiosaurus altithorax holotype.
So, at the Field Museum they already have the B.a. humerus on display, inside, nowhere near the mounted skeleton, which is outside. That’s perfect for my purpose. I’d like to have people in one treatment group guess the length of the isolated humerus inside, and people in another guess the length of the cast humerus in the mounted skeleton, and see if there are any systematic differences.
EXPERIMENT 2
We have basically no control on inter-observer error in mass estimates, because everyone rolls their own: Greg Paul uses his own models and drawings, Heinrich Mallison uses his own, Mike uses his own, and so on. What I’d like to do is get one multiview of a skeleton–probably using the dorsal and lateral photographs of the Apatosaurus model at BrantWorks–hand them out to a bunch of folks, including everyone named above, along with Scott Hartman, Zach Armstrong, Nima Sassani, and probably anyone else who wanted in. Everyone draws their own flesh envelope, calculates their own volumes, assigns their own densities, and comes up with their own mass estimate. Then everyone reports in, and we see not only how much the estimates vary, but where the variance creeps in–are we getting roughly similar volumes but assigning different densities, etc. And we publish the results as a big multi-authored paper with a title like, “The effects of inter-observer variation on volumetric mass estimates of dinosaurs.” (If anyone is interested in actually doing this, email me and I’ll be happy to coordinate it.)
March 18, 2013 at 2:05 am
>>”So why do we keep finding these (and smaller) subadults, and so few that were XV2-sized?”
Off-the-cuff entirely evidenceless answer: far more brachiosaurid individuals survived to HM SII size than survived to reach XV2-size; something led to high mortality around the age represented by HM SII etc.
March 18, 2013 at 7:04 am
Off-the-cuff entirely evidenceless answer: far more brachiosaurid individuals survived to HM SII size than survived to reach XV2-size; something led to high mortality around the age represented by HM SII etc.
Well, I wonder.
It’s hard to think about sauropod biology. All kinds of critters reach sexual maturity before they’re fully grown, but dinosaurs seem to have really decoupled onset of reproduction and cessation of growth. And that creates interesting possibilities. I wonder if there were sauropod populations that went centuries or maybe even millennia without producing any fully-grown individuals. Just subadults laying eggs for a few years and then getting whacked by predators or starving or succumbing to something else, and the babies–a few anyway–growing into the next generation of sexually-reproducing subadults, and on and on.
Maybe “fully grown” for many sauropods meant some kind of XV2 -> OMNH 1670 -> Amphicoelias fragillimus-class monster, but only a few individuals ever got that big, and only when conditions were just right–a good run of green years to feed them, and a few bad years for the predators to let them slip through until they were truly beyond challenge. You could have this really interesting situation where most of the time natural selection was acting almost entirely on juveniles and subadults, because they were all that were around for selection to act on. Since adults are practically by definition peramorphic juveniles, any allometric trends would be maxed out in adults. They might have really gnarly, possibly even maladaptive morphology, because selection would have crafted the developmental trajectory to produce the fittest subadults, not the fittest adults.
I’m not saying any of this is true. It’s speculation. One obvious counter is that reproductive scaling works out very advantageously for big egg-layers. Small, sexually mature but not fully-grown female snapping turtles (Chelydra serpentina) can lay as few as a dozen or two dozen eggs in a year, whereas big old matriarchs can lay about 80. So the giant, ancient females have lots more opportunities to get their genes into the next generation. If we imagine that the occasional female sauropod did survive to gigapod size, she should have been able to swamp the reproductive output of smaller females, and through her adult morphology would once more become visible to selection. So maybe my whole scenario is BS. But it’s interesting to ponder.
March 18, 2013 at 11:17 pm
Either scenario (evolutionary irrelevant ‘adults’ vs. super-productive but rare ‘adults’) would be quite interesting.
Is it possible that some of the skeletal fusions got pushed way “forward” in time or even stopped happening in some taxa – like neoteny?
March 18, 2013 at 11:23 pm
Is it possible that some of the skeletal fusions got pushed way “forward” in time or even stopped happening in some taxa – like neoteny?
It’s possible. BUT. For almost every possible ontogenetic fusion in the skeleton, we know of at least one individual who has it. And we’ve got geriatric sauropods like the holotype of Camarasaurus lewisi that are all fused up. AND we’ve got evidence from histology that a lot of what we normally consider to be big sauropods had not laid down external fundamental systems, so they hadn’t stopped growing. AND we’ve got giant individuals of known taxa, like XV2 for Giraffatitan and “Seismosaurus” for Diplodocus. So the hypothesis that SII, CM 84/94, etc. were subadults that had not reached max size looks pretty good to me. If we find an EFS in a sauropod that still has the scapulocoracoid joint unfused, or some of the cervical ribs unfused, then we’ll have evidence that some of these fusions stopped happening in some taxa.
March 19, 2013 at 12:08 am
Oh, I had forgotten the EFS thing, but now that you mention it, I remember that that was discussed on SV-POW before.
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I kind of wonder if death by predation was common even at HM SII size. I’m sure it happened sometimes – even modern elephants aren’t completely safe – but unless allosaurs were pack hunters, a 20-30 ton sauropod would pose a pretty big risk to even a Saurophaganax. And it must have taken years and years to reach that size, so there would be much more vulnerable, smaller individuals available year-round.
It seems sort of odd that so many could starve to death, though… but maybe it happens in modern herbivore populations, I dunno.
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Does the (tiny bit of) known morphology of A. fragillimus rule out it being an ultra-huge individual of, say, Supersaurus or Diplodocus hallorum?
March 19, 2013 at 3:11 pm
I kind of wonder if death by predation was common even at HM SII size. I’m sure it happened sometimes – even modern elephants aren’t completely safe – but unless allosaurs were pack hunters, a 20-30 ton sauropod would pose a pretty big risk to even a Saurophaganax. And it must have taken years and years to reach that size, so there would be much more vulnerable, smaller individuals available year-round.
It seems sort of odd that so many could starve to death, though… but maybe it happens in modern herbivore populations, I dunno.
Yeah, that’s one problem with the scenario I outlined above, which is that the HM SII/CM 3018-sized individuals should already be beyond the reach of all the but the biggest and most desperate predators.
Also, when times get lousy, it’s the little ones that starve first, not the big ones.
That’s another way that selection on adult morphology could stay “in play”: the thousand-year and ten-thousand-year droughts (i.e., ones that happen that often, not that last that long, by analogy with thousand-year, etc. floods) might wipe out all but the very largest. In the immediate aftermath of such an event, you’d have this odd situation where the entire population was made up of gigapods. And then a few months later, it would be only gigapods and hatchlings, with nothing in between.
Even if that scenario never came to pass (but, surely, somewhere, somewhen…), Morrison paleoecology must have been W-E-I-R-D.
Does the (tiny bit of) known morphology of A. fragillimus rule out it being an ultra-huge individual of, say, Supersaurus or Diplodocus hallorum?
Not to my knowledge. Heck, I don’t even think we can totally rule out the possibility that it’s Apatosaurus. Apatosaurus dorsal vertebrae are no more heavily constructed than those of other diplodocids, and the giant dorsals from Oklahoma have more and bigger pneumatic fossae than most “normal” Apatosaurus dorsals. So if degree of pneumaticity scales positively with size within an individual over its ontogeny–and there is some support for that, based on some of my as-yet-unpublished data–an A. fragillimus-sized Apatosaurus might be expected to have dorsals that big and delicate.
Obviously if A. fragillimus is a scaled-up Apatosaurus rather than a scaled-up Diplodocus, we’re going to have to revisit those mass estimates…
April 8, 2013 at 5:38 am
[…] big ilium. I’m not going to claim it’s the biggest bone I’ve ever seen–that rarely ends well–but sheesh, it’s gotta be pretty freakin’ big. And apparently a brachiosaurid, or […]
May 19, 2014 at 12:09 pm
[…] The first point I need to make here is that photos like these are attention-grabbing but they don’t really tell you much. Partly because they’re hard to interpret, and partly because they almost always look more impressive than they really are. For example, I am 6’2″ tall (1.88 meters). If I lie down next to a bone that is 7’2″ (219 cm) long, it is going to look ungodly huge–a full foot longer than I am tall. But that is the length of the femur of the Brachiosaurus holotype–we’ve known of sauropod femora that big for a century now. People get tripped up by this sort of thing all the time–even scientists. […]