Five questions from Tom Redd

Three years ago, Tom Redd made a very generous commitment to the SV-POW! Patreon, and he remains our most generous donor in total. When I wrote to thank him his reply included “I have thousands of questions about apatosaurus that I would like to ask you some day.”

It seemed only fair to invite him to ask some of those questions, so we asked him to give us five and said we’d try to answer them. When the questions came through, some of them were hard — not really in our area of expertise. But I promised we’d take a crack, and that we’d invite commenters to chip in where we get something wrong or leave something out.

Then a bit more than three years slipped past. Now, finally: here we go!

Question #1:

Do you think sauropods could have evolved long necks as a defensive strategy? (Better to see your enemies from a long way off). The sigmoid curve and the neutral position is another issue!

I recently read a couple of research papers where the researcher used “radiological imaging” to determine the neutral position of the cervical vertebrae. The result was a gently downward curving neck beginning at the pectoral girdle, with the skull only 2 meters off the ground. (I agree with Matt that this would be an ambush predator’s delight!)

Also, could compressive or tensional forces of cartilage and ligaments affect the neutral position of the cervical vertebra? (I believe Matt also alluded to this condition.)

This is the longest question, but maybe the one I’m best positioned to comment on.

First of all, the perennial question of what sauropods’ long necks were for. There are a few candidate explanations out there. The obvious one is that they enabled high browsing, and on the whole we feel that’s the strongest single explanation. Another candidate factor is sexual selection — that sauropods were particularly attracted to long-necked individuals of the opposite sex — but we don’t feel that is a strong explanation for reasons explained in our 2011 paper (Taylor et al. 2011). Predator avoidance would certainly have been aided by the long visual distances allowed by elevated heads, and we are confident that at least some sauropods used their necks in combat — primarily in intraspecific combat (Taylor 2015) but no doubt also against predators when the occasion arose. And we may well have missed other good uses for long necks.

In reality of course all these factors likely played a role: structures do not always, or even often, evolve for a single reason. When people who know much more about ceratopsians tell me “The horns of Triceratops were for intraspecific display and combat”, I don’t doubt them. But I also don’t doubt that, whatever the primary purpose of the horns, a Triceratops confronted by a Tyrannosaurus would do its damnedest to stick its horns into it. In the same way, while high feeding seems like the strongest driver of sauropod neck elongation, the other factors will surely have played in, too.

I’m not sure what radiological imaging papers you have come across, but the one I know about is Berman and Rothschild (2005) in the Thunder Lizards edited volume. This paper rather questionably partitions all sauropod cervicals into two bins, “robust” and “gracile”, and concludes, based on functional stress analysis, that “the robust-type centrum supported a neck held in a vertical, or near-vertical, pose, whereas the gracile-type centrum supported a neck held in a horizontal, or near-horizontal, pose”. If this is right, and their categorization holds, then Camarasaurus and an unidentified titanosaur had vertical necks; and Diplodocus, Apatosaurus, Haplocanthosaurus, Barosaurus and Brachiosaurus all had horizontal necks. We find every part of this unconvincing. At some point we should explain why in detail; but it is not this day.

Finally, yes, compressive and tensile forces in cartilage and ligaments definitely did affect neutral posture. My 2014 paper (Taylor 2014) shows this rather dramatically.

Taylor (2014:figure 3). Effect of adding cartilage to the neutral pose of the neck of Apatosaurus louisae CM 3018. Images of vertebra from Gilmore (1936:plate XXIV). At the bottom, the vertebrae are composed in a horizontal posture. Superimposed, the same vertebrae are shown inclined by the additional extension angles indicated in Table 1. If the slightly sub-horizontal osteological neutral pose of Stevens and Parrish (1999) is correct, then the cartilaginous neutral pose would be correspondingly slightly lower than depicted here, but still much closer to the elevated posture than to horizontal. (Note that the posture shown here would not have been the habitual posture in life: see discussion.) ” data-medium-file=”https://svpow.com/wp-content/uploads/2014/11/figure3-cm3018-juxtaposition.jpeg?w=300″ data-large-file=”https://svpow.com/wp-content/uploads/2014/11/figure3-cm3018-juxtaposition.jpeg?w=480″ tabindex=”0″ role=”button” class=”wp-image-11005″ src=”https://svpow.com/wp-content/uploads/2014/11/figure3-cm3018-juxtaposition.jpeg?w=480″ alt=”" srcset=”https://svpow.com/wp-content/uploads/2014/11/figure3-cm3018-juxtaposition.jpeg?w=480 480w, https://svpow.com/wp-content/uploads/2014/11/figure3-cm3018-juxtaposition.jpeg?w=960 960w, https://svpow.com/wp-content/uploads/2014/11/figure3-cm3018-juxtaposition.jpeg?w=150 150w, https://svpow.com/wp-content/uploads/2014/11/figure3-cm3018-juxtaposition.jpeg?w=300 300w, https://svpow.com/wp-content/uploads/2014/11/figure3-cm3018-juxtaposition.jpeg?w=768 768w” sizes=”(max-width: 480px) 100vw, 480px” />

What we’re seeing here is what the neutral posture would be if cartilage is added to a neck that is otherwise articulated in horizonzal pose. The importance of intervertebral cartilage has often been overlooked, but can make a dramatic difference to neck posture.

Question #2

I recently read a report that indicated Apatosaurus survived at the species level for a period of approximately 8 million years ! So is this a success, average, or a short run?

I’m not sure where you read that, but the problem here is that no-one really knows what Apatosaurus means. We have the type species Apatosaurus ajax, sure, and the referred species Apatosaurus louisae, and the genus Brontosaurus based on the species Brontosaurus excelsus which is sometimes but not always synonymised with the genus Apatosaurus yielding the combination Apatosaurus excelsus, and don’t even get me started on Apatosaurus parvus, Apatosaurus laticollis, Atlantosaurus and whatever the heck AMNH 460 is.

So if we say that Apatosaurus survived for 8 million years, what exactly are we saying? That Apatosaurus ajax is known from sediments that differ in age by 8 million years? That would be interesting if true, but it’s very hard to establish because the referral of any given individual to a particular sauropod species tends to be very uncertain — largely because most specimen are so fragmentary and distorted. And if all we mean is that 8 million years separate the oldest and youngest specimens that have been referred to the genus Apatosaurus — well, that statement is all but meaningless, given the huge uncertainty about what is and is not part of that genus, if indeed genera even really mean anything.

Putting it all together, I’m not confident that there is any reason to think that Apatosaurus was particularly longer lived than other sauropods. Probably Camarasaurus outlasted it if you include all the taxa that have been referred to Camarasaurus. But then I’m far from convinced that that’s the right thing to do, too.

Question #3

Why so many heavy predators during the age of apatosaurs? Predator to prey ratios were in the 6 to 8% range as compared to modern ratios of 2 to 3%!

I’m not sure I can say much about this without knowing the source of the figures, but I assume that what’s being counted here is the number of individuals represented in the fossil record, and the ratio of a predator species to prey species. The problem is that there is a huge amount of vagueness in these numbers, but it’s not obvious that the apatosaur-age figures are comparable to the modern ones.

Consider first the modern ratio. Which animals are counted in each category? In the Serengeti, lions prey on zebras. So far, so simple, but there are also dwarf moongooses, which are predators — but they don’t hunt zebras. So do we count them in the numbers? If so, do we count their prey animals, too? Including invertebrates? And if not, then where do we draw the line between predators that we consider do and do not hunt the prey animals that we’ve decided we’re interested in?

Then there’s the matter of which animals get counted. If you do your Serengeti counts on dead animals, you might find disproportionately many predators because prey animals tend to be consumed. Or you might find disproportionately many prey animals because they tend to die in areas where the corpses are more easily found and counted. You might be able to do better by counting live animals, but then you might easily undercount secretive predators, or perhaps overcount predators because they stand fearlessly around to be counted.

Now consider trying to measure the predator/prey ratio in the Morrison formation. You have all the problems I already mentioned, plus a bunch of others. If you only count complete-ish articulated skeletons then your sample size is too small to be meaningful. If you count isolated elements, you’re at risk of registering multiple instances of the same individual. Counting individuals represented in bonebeds is difficult because of these problems. Assigning an element to a taxon is error-prone (though should generally be OK at the high level of sauropod vs. theropod — or are you?). Bones of different taxa may survive taphonomy better or worse than others. Life history differences will mean that the fossils of long-lived taxa under-represent their live populations. And so on, and on, and on.

Putting it all together, I would tend to be very sceptical that a difference in ratios of 6–8% to 2–3% is necessarily telling us anything.

With all that said, it’s perfectly possible that the average predator:prey body-size ratio was closer in the Morrison than in modern ecosystems. But we’d do better trying to measure that directly from body-fossils than to infer it from population densities.

Question #4

Are all apatosaur tracks on emergent surfaces? (Some depth of water over the prints)

This I don’t know. But then I wouldn’t know how to pick out apatosaur tracks from those of other diplodocids, and I bet no-one else does, either. Tracks are notoriously variable in shape, and can very wildly from the that of the feet that made them. Given that diplodocid feet were mostly pretty similar anyway, I would not be easily persuaded that any track can be confidently identified down to the genus level.

One other thing to be aware of is that there is often not agreement on the conditions under which a given track is made. One palaeontologist may think a given a track is a direct print, another will think it’s an underprint. I don’t mean to say that it’s hopeless and all we can do is throw our hands up in despair — good work is being done on interpreting tracks, but we have a long way to go. And this is not an area that I’m at all expert in.

Question #5

I read recently that in order for a skin impression to be made the Dermal tissue must undergo a type of chemical alteration! Do you think this is what allows the impression to be made?

That doesn’t sound right to me. The first thing that has to happen for an animal to be fossilized is that it needs to be buried in sediment really fast after it dies — before it’s eaten by scavengers. For something as fine as skin impressions to be preserved, that sediment needs to be very fine — which sadly tends to conflict with the first condition, since course sediments can be deposited more quickly than fine ones. It’s really hard for enough fine sediment to be laid down quickly enough to cover an animal of the size of a sauropod, which is why we don’t have sauropod specimens like those gloriously preserved theropods from the Yixian Formation in China(*). So sauropod skin impressions are pretty rare.

(*) Alternatively: there are spectacularly preserved partial sauropod specimens in the Yixian, but Chinese researchers can’t be bothered to write them up because they’d rather spend their time getting a slam-dunk Nature paper out of yet another little feathered theropod. Unduly cynical? Maybe. But I continue to live in hope.

Well, that about wraps up the five questions — to the best of my ability at least. But I’d love to hear from people who know more than I about these topics: leave a comment, and fame and glory could be yours!

And finally … if you, too, would like to have us answer five questions on the sauropod-related topic of your choice, quite possibly in the less than three years, you should consider getting yourself across the The SV-POW! Patreon and making an unreasonably extravagant financial commitment.

References

• Berman David S., and Bruce M. Rothschild. 2005. Neck posture of sauropods determined using radiological imaging to reveal three-dimensional structure of cervical vertebrae. pp. 233–247 in: Virgina Tidwell and Ken Carpenter, Thunder-lizards: the sauropodomorph dinosaurs. Bloomington: Indiana University Press.
• Taylor, Michael P. 2014. Quantifying the effect of intervertebral cartilage on neutral posture in the necks of sauropod dinosaurs. PeerJ 2:e712. doi: 10.7717/peerj.712
• Taylor, Michael P., David W. E. Hone, Mathew J. Wedel and Darren Naish. 2011. The long necks of sauropods did not evolve primarily through sexual selection. Journal of Zoology 285:150-161. doi: 10.1111/j.1469-7998.2011.00824.x
• Taylor, Michael P., Mathew J. Wedel, Darren Naish and Brian Engh. 2015. Were the necks of Apatosaurus and Brontosaurus adapted for combat?. p. 71 in Mark Young (ed.), Abstracts, 63rd Symposium for Vertebrate Palaeontology and Comparative Anatomy, Southampton. 115 pp.

Source: https://svpow.com/2024/10/24/five-questions-from-tom-redd/