New paper shrinks Dunkelosteus! My impressions.

 

Thoughts on “A Devonian Fish Tale:  A New Method of Body Length Estimation Suggests Much Smaller Sizes for Dunkleosteus terrelli (Placodermi: Arthrodira)," a new paper by Russell K. Engelman, Department of Biology, Case Western Reserve University, Cleveland, OH 44106, USA, in Diversity

UPDATED 27 February 2023 after reading some of the scientific responses and having some discussions online.  Text corrections added 4 July 2023.

As Sam Gamgee once said, “This is an eye-opener, and no mistake.” There’s a lot here that strikes me as logical and innovative, although my conclusion as an interested amateur is that I interpret the variability in the figures presented a little differently than he does.   

Estimates of length for our best and largest adult skull specimen, CMNH 5768, range between 7 and 9 meters: up to 10m has been recently (2016)  claimed based on the inferognathal of one specimen, CMNH 5936, and that's not the only paper with that estimate. Engelman offers “…3.4 m for typical adults (CMNH 5768) with the largest known individuals (CMNH 5936) reaching 4.1 m.” The resulting animal looks very strange, but Engelman believed arthodires indeed have odd proportions compared to other fish. 

Me with a cast of CMNH 5768 at the Denver Museum of Nature and Science

I’m in complete agreement with the author’s first point: that inferring Dunk size and shape from the small Coccosteus cuspidatus (40 cm is a big one) is unreliable. I’ve said this many times: indeed, I hate the poor innocent animal more every time I see it dragged into the debate. (If you blow up the submersible Alvin 10 times, the result will be easily recognizable as a submersible, but it won’t be a workable one.) The point about the uncertainty in the use of shark jaws to calculate length also seems logical.

Here I have to stop again and emphasize my status as a well-read but necessarily limited amateur.  I'm a longtime student of Dunkleosteus, have read (I think) all the recent papers, and curate the FaceBook page Dunkleosteus terrelli. So I’m not qualified to judge the suitability of the orbit-opercular length (OOL) as a key component, but there’s a substantial margin of error. From my reading, we don't have a lot of high-quality skulls from large or even medium-sized placoderms to measure against, and the author has already noted arthrodires are proportionately weird.  The argument about relative sizes of body parts makes some sense, with the same caveat. Engelman notes that, in adapting the body mass formula used to estimate tarpon, he had to tweak it to allow for variations in the caudal fin reconstructions. That is logical, since the caudal fin has been argued over since the species was discovered. I’ve always been an advocate of a sharklike tail due to the surface area and muscle strength needed to move the tail fast while keeping precise angular control of a large animal whose weight is rather front-loaded, and I was very gratified when recent analyses (notably Ferron, et. al., 2017)  bore this out.

Engelman argues the apparent distance between the anus and tail, in the current picture of the animal, is unusually, and seemingly illogically, elongated. It looks like that could be so to me: it contributes (along with the  force/control point just made) to the fact I never liked the eel-like body plan as shown in Charles R. Knight’s illustrations (although I'm not exactly sure where the anus was supposed to be). Still, I don't know how this compares to similar-sized fishes. Engelman plunks for a short, squat body: while one known arthrodire (the oddly proportioned A. trinajsticae) is an exception, he argues the others line up reasonably well. Arthrodires, he notes, also have proportionately shorter snouts (although not greatly so) than other fish.

Everyone’s first question about Dunkleosteus is how long it was, and the second is what it weighed. Engelman looks at several ways to estimate body mass and concludes typical adult individuals of D. terrelli (i.e., the size of CMNH 5768) could reach weights of 950-1200 kg, although “More precise estimates might be obtainable via volumetric modeling.” (Interestingly, 950 is what a scientist in my yet-unpublished novel about this species comes up with, although she’s considering a conventionally proportioned Dunk about 7 m long.)  He notes that giant body size develops in animals because of pressures to become too big to be eaten or big enough to get high-quality prey and thinks the Dunk didn't need to be as large as we projected.  He suggests all the largest Devonian fishes, whether arthrodire, shark, or bony fish, topped out under 5 m, although he notes possible exceptions.  Still, it's not the general rule that matters most: it's what "made sense" to evolution at that time, with this species, and he needs more evidence.  

You’ve probably guessed by now that I think Engelman makes an interesting but not definitive case that our current estimates of length are too high, but I can’t picture a large adult measuring only 4 m. The massive head and armor needs, to me, a longer body for optimum balance and an adequate (but not overly long) moment arm going back to the caudal fin. A thunniform plan like a tuna’s makes for a very fast fish, but a tuna doesn’t have a couple hundred kilograms of extra weight at one end, a factor I think Engelman should have gone into a little more. I don't think his proposed Dunk in its shortest form would be fast enough or stable enough to catch prey more elusive than kelp.  The thoracic plates are short in proportion to length compared to smaller relatives, which might support Engelman's interpretation. Still, I went up to a museum in Denver specifically to look at this, and the plates appear to be going almost  straight back when they end, so the body's not starting to narrow significantly, vertically or horizontally,  at this point. I’m more inclined to shrink our friend CMNH 5768 to 7 m, although possibly a bit smaller. Basically, that means I’m guessing differently on how the uncertainty ranges in Engelman’s a paper come out in the flesh and how the overall body plan would work for a ocean predator. This is an important piece of work, but it's being disputed, and the argument over Dunk proportions is not over yet.

 Matt Bille is a writer, historian, and naturalist living in Colorado Springs. He can be reached at mattsciwriter@protonmail.com. Website: www.mattbilleauthor.com.

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