Resizing Dunkleosteus: Russell Engelman’s view

 New Dunk article

 

Resizing the Dunk: Russell Engelman’s view

Photo: me and Dunk specimen CMNH 5936, important in Engelman's paper, at Denver Museum of Nature and Science.

Friends,

Russell Engelman’s paper revising the body proportions of Dunkleosteus terrelli, and thus its relatives in the gang called the arthrodires, made major waves in palaeoichthyology.  We’re used to seeing Dunkleosteus length estimates anywhere from 6 to 10 meters, 6-8 being most common. For comparison, the largest proven Great White shark (GWS) is slightly under 7 m. (Since someone will ask, I’m citing Ellis and McCosker.)

The paper involved is "A Devonian Fish Tale: A New Method of Body Length Estimation Suggests Much Smaller Sizes for Dunkleosteus terrelli" in the journal Diversity. 

It built on Engelman’s earlier paper, Giant, swimming mouths: oral dimensions of extant sharks do not accurately predict body size in Dunkleosteus terrelli (Placodermi: Arthrodira) [PeerJ] ) That one, critiquing a method of estimating Dunk size, was much less of a bombshell and sailed through peer review without major objections, See the “Review” section on the paper link just mentioned.   

Now to swipe a bit from the abstract of “A Devonian Fish Tale,” (footnotes removed, see original). For those of us outside scientific academia, it’s a long abstract with a lot of technical terminology. Some key sentences:

Dunkleosteus terrelli is a large arthrodire [placoderm] best known from the latest Devonian (late Famennian) Cleveland Shale of Ohio, USA…Arthrodires combine an ossified head and thoracic armor with a mostly cartilaginous post-thoracic skeleton (which includes the caudal region and major fins). Thus, the head and thoracic armor of arthrodires are frequently preserved in the fossil record but the rest of the body is typically lost during fossilization.

Post-thoracic remains are only known for a handful of arthrodire taxa. The best known of these is Coccosteus cuspidatus,… but estimating the size of Dunkleosteus using smaller arthrodires requires a significant degree of extrapolation. [Coccsosteus is barely 40 cm long.] The most recent attempt to estimate the size of Dunkleosteus [used] a proxy known to reliably predict size in large sharks (upper jaw perimeter) [but] these estimates …fail to control for anatomical differences between arthrodires and elasmobranchs (specifically, arthrodires having much larger mouths relative to body size).

Thus, it is clear a new method is needed to estimate size in Dunkleosteus and other arthrodires. Specifically, this method must accurately estimate length across fishes in general (e.g., lampreys, chondrichthyans, and bony fishes), be measurable in arthrodire fossils, and provide accurate length estimates for arthrodires known from complete remains. One such potential proxy is orbit-opercular length (hereafter OOL). This is the length from the anterior margin of the orbit to the posterior margin of the head. There are several biological reasons to believe OOL and total length would be highly correlated in fishes. …Here, I estimate body size (total length) and body mass (weight) in Dunkleosteus using a broad sample of extant fishes as well as arthrodires for which complete remains are known…

There are currently no individuals in the hypodigm of D. terrelli that could potentially pertain to an individual 4.7 m or greater in length, as would be necessary to produce the monstrous sizes reported for this species in previous studies. Indeed, larger (=likely adult) individuals of Dunkleosteus tend to be closer in size to CMNH 5768 than CMNH 5936. Therefore, although the maximal length of D. terrelli is likely ~4.1 m, 3.4 m is a more typical adult size for this species. (Those are specimen numbers for skull/armor fossils in Cleveland Museum of Natural History, which is basically Dunk Central.)

Naturally, this created quite a maelstrom. I contacted several experts, including Engelman. Russell was very generous with his time and answered all my questions in a series of emails in July 2023.  

This article covers that Q&A, condensed for space and to avoid repetition. This isn’t a point-by-point of clashing viewpoints. I haven’t read enough responses from other experts yet, and some are in journals I can’t access.  So this is based on an avocational Dunk lover’s questions.   

I began this very skeptical of the assertion that a specimen estimated to be 8.9 meters long was more likely 4.1 m, even if the weight didn’t shrink as much.  Engelman’s Dunk, rather resembling a tuna on steroids, didn’t look “right,” but more specifically it didn’t strike me as well-balanced or controllable enough.  It still doesn’t look right, but it’s not like the oarfish and the mola-mola look right, so I was willing to listen and ask questions.

Engelman’s silhouette Dunk image (copyright Russell Engelman)

 

First, some self-correction. In my writing to date, I’ve assumed the armor mass on a big Dunk added a hundred kilograms or more to the weight.  Engelman pointed me to a figure I’d never read – 30 kg. Even if that’s too conservative, it’s enough to make me revise my assumptions about what mass and length of post-armor body is needed to balance that body and how long a moment arm is needed for the tail to impart the necessary speed. I think his work also puts the final nail in the coffin of the old eel-like tail configuration. I doubted this for a long time, based on the tail surface area for the kind of push a Dunk would need. This was proven in a 2017 paper and is reinforced by Engelman’s animal, however much the latter may look like it chased a roadrunner fish into a painted tunnel on a reef. 

On to Mr. Engelman’s thinking.

MB: The paper says, “The model must accurately estimate body size in fishes regardless of phylogeny.” Why? Why should it not be specialized for something as strange as Dunkleosteus?

Russell: 

That is exactly the point. Because Dunkleosteus doesn't have any close living relatives, estimating its size requires a variable that near-universally correlates with body size among both living fishes (bony fishes and sharks) as well as extinct placoderms. Using only placoderms creates problems because all known species of complete arthrodires are only about 15-95 cm long, which is much smaller than even the smallest estimates for Dunkleosteus. Using placoderms only requires significant extrapolation of the data, and the further data is extrapolated beyond the range of values spanned by the dataset used to create it, the less accurate the result is.

Thus, in order to accurately estimate the length of Dunkleosteus, you need a single line that accurately predicts length in both living fishes as well as the few placoderms for which we have complete remains. If it worked on sharks + bony fish but didn't work on arthrodires, it was unlikely to work on Dunkleosteus. However, as mentioned in the paper, complete arthrodires like Millerosteus, Coccosteus, Watsonosteus, and Amazichthys all fall along the same regression line as other fishes.

The original point of the analysis was basically "statistically carpet bombing." It was very, very unlikely that Dunkleosteus would show head-body proportions outside the extreme diversity seen in extant fishes, especially when you have things like opah at one extreme and oarfish at the other. My original goal was to use this as a reality check to just rule out the most extreme estimates and give a more reasonable range of possible sizes. I had no idea beforehand head-body proportions correlated that strongly in fishes or that by themself they would be useful for approximating size.

This is also why I checked the method by scaling off of various armor plates or other measurements of Dunkleosteus. I got similar results no matter what I did. Indeed, if I run the same analysis using only arthrodires, I get lengths of only 3 m for CMNH 5768 (this is one reason why extrapolation can be an issue). Dunkleosteus just has an abnormally deep body compared to other arthrodires, and this is very evident if you scale their armors to the same size (Figure 11 in my Diversity paper).

MB: I’d always assumed the head looked like it was logically sized on a reconstruction like the full-size model at the Cleveland Museum of Natural History [the large model at CMNH] which appears a little shorter than the closest living cartilaginous comparison, the Great White shark. 

CMNH Dunk: see link for credit, license)

Russell: 

Because that model is only about 4 m (~13 feet) long. That model is relatively accurate for the time and place; it didn't take into account some details of arthrodire comparative anatomy. Specifically, the pelvic fins are located too far back, they need to be moved up slightly until they are near the end of the ventral armor. This, in turn, would result in a shorter caudal peduncle (i.e., distance to base of tail fin), because in basal fishes (including arthrodires) the pelvic fins are located about halfway along the total length of the body and are just behind the center of mass. These were things that weren't well known at the time that model was created… [The body anatomy in] my paper was helped a lot by the publications of Amazichthys by Jobbins et al. (2022) and Trinajstic et al. (2022)'s report of preserved arthrodire organs.

This is one of the big take home messages from the paper(s). Arthrodires are not sharks. They may show some broad-scale similarities to sharks, but arthrodires are a distinct group of animals and in fact seem to have evolved active swimming habits separately from either sharks or bony fishes. Thus, it probably should have been expected that arthrodires would differ in body shape and comparative anatomy from either sharks or bony fishes, because they are independently coming up with solutions to the same evolutionary problems. And this is kind of what we see: in some respects arthrodires are similar to sharks, in others they are more similar to bony fishes, and in some cases arthrodires seem to be doing something unlike almost any living fish. It's not possible to simply slap shark anatomy onto an arthrodire and call it a day, because arthrodire and shark body plans are very different.

MB: Asked follow-up questions about Dunk anatomy and the CMNH model, as it’s one of the most widespread popular images of the Dunk and inspired many others.

Russell: 

That model is about the longest Dunkleosteus could plausibly be based on the head length data; the head/trunk armor is about the size of CMNH 5768 but the model has a head-body proportion more similar to Amazichthys (so ~10-20% longer than predicted by head dimensions). However, in arthrodires, the pelvic girdle is always closely associated with the end of the ventral armor. In that model there is a significant gap of 1 to 2 feet, Amazichthys gets away with this because it has a very long ventral armor which allows the position of the pelvic fins to remain consistent despite the longer body. Dunkleosteus does not, and in fact has a very deep torso relative to the dimensions of its armor.

With the description of preserved organs from the Gogo arthrodires, it has become very apparent that the ventral armor of placoderms is sort of like the ribcage in tetrapods in that it protects the organ cavity. The ventral armor then ends very close to the pelvic girdle and anus. Specifically, think of something like a crocodile, where the rib cage/gastralia extend all the way to the vent of the animal. Both the anus and the pelvic fins have a very consistent location in arthrodires, sharks, and basal bony fishes in terms of their relative length from the tip of the snout in the animal measured from the snout (~45-55% total length). This would mean in order to keep the proportions more consistent with what we see in other arthrodires, the body in that model needs to be shortened a bit such that the pelvic fins are closer to the ventral armor. I would estimate the model is somewhere between½ to ¾ of a head too long.

The other thing is that the model …was more or less deliberately designed to look like a shark. The back half has proportions very similar to a carcharhinid or a lamnid, but because the torso of Dunkleosteus is so deep in order to maintain a shark-like shape the posterior body has to be scaled up to be a bit longer in order to maintain the same length-width ratio, rather than allowing the body to be deeper similar to a lamnid or a thunnin. Thus, because the reconstruction has a stereotypically shark-y shape, it looks intuitively “right” and visually appealing to us, even though once you get familiar with arthrodire body plans the shape looks a bit off…. keep in mind this "shark-like" depiction was not super popular at the time and well into the early 2000s most people still depicted Dunkleosteus as a copy-paste of Coccosteus.

MB: I’ve always thought people relied too much on Coccosteus.  I don’t know hydrodynamics. I do know something about aerodynamics, and you can’t blow up a small business jet and get a working airliner.

Russell: 

[A] big take home point from the study. We can no longer simply assume that all arthrodires had the same body shape where every taxon is a copy-paste of Coccosteus. Arthrodires spanned nearly sixty million years of geologic time and it stands to reason they would be at least as diverse in body shape as modern sharks. Even just comparing one to the other Dunkleosteus clearly has a much deeper trunk armor than Coccosteus, and it seems to be deeper in height rather than reduced in length.

Even Coccosteus doesn't really look like Coccosteus. More complete specimens have turned up since the publication of Miles and Westoll (1968) and they suggest that the reconstruction in Miles and Westoll might be a little too long in the post-thoracic and tail regions than what is seen in actual complete specimens.

MB: Is measuring the OOL, the head to the back edge of the operculum, always an incorrect practice if it does not line up with the gill chamber?

Russell: 

OOL is literally just "head length minus snout length." Head length has a very consistent definition in ichthyology, measured from the snout to the end of the braincase or gill chamber, whichever is more posterior. The posterior regions of the head (braincase and gill chamber) have some pretty severe restrictions on how big they can be relative to the body, which means they should have a relatively good correlation with body size. By contrast, the snout can vary a bit more based on ecological habits (think the long snout of a trumpetfish). The other issue is placoderms have systematically shorter snouts than other fishes. Thus, using total head length results in systematic underestimates of placoderm body length, whereas OOL produces values closer to the actual specimens.

Additionally, I hadn't been aware of this at the time of my publication but Carr et al. (2009) did some work that suggested the heads of arthrodires aren't constructed that differently from other fishes. Specifically, the anteroposterior locations of the vagus and glossopharyngeal nerves are the same between placoderms and other fishes, but in placoderms the cranial skeleton has expanded backwards such that the gill chamber is covered by the cranium and cheek plates from the top and sides. Thus, all the major organs are located in the same place between arthrodires and other fishes, it's just that the external covering has been rearranged, and placoderms show pretty much the same pattern as other fishes.

This is part of the reason I tested with several genera of coccosteomorphs and Amazichthys. If the method worked on all of these taxa, then the chances it would produce inaccurate lengths in Dunkleosteus are low. And that's pretty much what happened; the method based on extant fishes predicted length in these other taxa relatively accurately, and the error suggested arthrodires showed similar patterns of variation to living fishes.

The operculum not lining up with the gill chamber is an issue restricted to groupers and a few other fish, and that's mostly because they have a huge skin flap that extends its margins. Counting that would be like measuring the skull of a basset hound including the ears. It's not really an issue with placoderms.

MB: asked about some of the more interesting models on the market.

1.       The dual model on Etsy from HellbenderMuseum with a Dunk in both body plans. (No review by me yet: )

Russell:

I think what the seller is calling the "conservative" model is probably more accurate. The smaller model seems to have a head-body ratio of 1:4.5, whereas the conservative one has a head-body ratio of 1:5.5, which are the proportions I had gotten with my model.  

2.       The PNSO model, which I praised highly except for dorsal fin placement, and the Paleozoo model, my all-time favorite. Matt's Sci/Tech Blog: Dunkleosteus Model: Gorgeous New One from PNSO (mattbille.blogspot.com)   Matt's Sci/Tech Blog: Review: A wonderful Dunkleosteus model from Paleozoo (mattbille.blogspot.com)

Russell:

I think what's going on is that people are trying to fit the known dimensions of the Dunk specimens with previously cited lengths of 5-6 or more meters. The problem is if you try to fit the known dimensions of Dunkleosteus specimens to previously cited lengths you get this extremely elongated, eel-like animal, which looks very strange and doesn't have anatomical data to support the unusual body shape.

I actually noticed several laypeople had noticed something was odd with Dunkleosteus' proportions when I was reviewing previous size estimates of Dunkleosteus because they also produced these extremely eel-y reconstructions. However, all of them just kind of assumed their estimation had to be wrong because 5-6 m was the typically cited length for Dunkleosteus, or that the larger length estimates must have been based on some giant, unpublished specimens in the bowels of the CMNH that served as the basis for those suggested lengths of 8-10 m.

MB: asked about two interesting models, from PNSO and Paleozoo. The latter is my favorite for accuracy, but I praised PNSO highly except for placing the dorsal fin too far back.(PNSO credit Matt Bille, Paelozoo by the manufacturer)

 

Russell:

The PNSO [model] likely used the length estimates from Ferron et al. for CMNH 5768 and used that to determine proportions. PNSO is usually pretty rigorous about anatomical inferences when they make models and Ferron et al. 2017 has been the only paper directly concerning Dunkleosteus' appearance since 1932. [Edit: he added, "There's one more, Carr et al., 2010."]

[MB: The Ferron paper is here: https://doi.org/10.7717/peerj.4081: I have not seen these authors’ response to Engelman’s critique of their methods in his “Swimming Mouths” paper [https://doi.org/10.7717/peerj.15131] but following the branches of critiqued and cited papers knowledgably requires a very long article by a true expert; as for me, “Man’s got to know his limitations.” ]

MB: I wondered about placement of the dorsal in your graphic: I understand unpublished Moroccan fossils show the dorsal fin starts right behind the dorsal armor.

Russell:

Define “right behind.” The dorsal fin in my reconstruction is as close to the trunk armor as physically possible, it just looks like it is further posterior than it actually is due to the shorter body. The dorsal fin in Dunkleosteus is limited in how far forward it can be placed by the huge carinal process sticking out of the back of the mediodorsal. If the fin is too far forward the basal (which are also a midline structure) conflict for space with the big, bony spike and associated submedian dorsal plate extending out of the trunk.

Also note that the dorsal fin position in my reconstruction is considerably further forward than it is in specimens of Coccosteus, Incisoscutum, etc. In most of the coccosteomorph arthrodires the dorsal fin begins behind the pelvis, whereas in this reconstruction of Dunkleosteus it begins before the pelvis, more similar to the aspinothoracidans we have dorsal fins for. So in a sense it is further forward than in other arthrodires.

MB: I sent him my article “Dunkleosteus: First King of the Oceans,” Prehistoric Times #126, Summer 2018.

Russell:

There are a few things you might be interested in given the material in the Prehistoric Times article.

Article: There is an exceptional Dunk specimen in the Cleveland Museum of Natural History estimated to have been 8.9m long in life, making it the size of a large male orca.

Russell:

That's CMNH 5936. That's the same specimen I re-evaluated in the length(s) papers and came up with an estimate of about 4.1 m. Depending on the error bars, something as high as 4.5ish meters might be possible.

Article: There is some dispute about the similar-sized Titanichthys being a filter feeder.

Russell: 

From what I am aware of, there is no dispute over the dietary habits of Titanichthys. Dunkle and Bungart (1942) suggested it could have eaten benthic invertebrates, but then Dunkle and Bungart (1942) also suggested it had a prehensile lip like an elephant (which no one seriously considers anymore). More specifically, it has been suggested on and off that Titanichthys was a filter-feeder since its discovery, but no one had actually tested this hypothesis until Coatham et al. (2020) ran finite element analyses on the lower jaw and found it was more similar to megaplanktonivores like basking sharks than predatory species. Exactly how Titanichthys fed is still a mystery, since there are multiple types of feeding strategies for megaplanktonivores.

Article: Dunkleosteus was larger than any of the extant sharks. Russell agreed but added interesting stuff.

Russell:

There are some really frigging big sharks known from the later Paleozoic. Edestus probably got around 6-7 m in length (and this seems fairly reliable). Helicoprion and some Helicoprion-adjacent taxa have been suggested to reach lengths of 12 m, but then again these taxa were basically Paleozoic sperm whales. Some of the ctenacanths got fairly large, but exactly how large isn't clear. All of this kind of relates to a broader issue in that there are a lot of "Paleozoic fish stories," where fragmentary remains get extrapolated to colossal sizes and it's really hard to double-check them simply because there are so few complete specimens it is hard to tell what the actual proportions were. This is especially the case when claims of giant sizes are based on 50–100-year-old estimates on specimens (i.e., back when paleontology was less quantitative and people liked to exaggerate) that are now missing or lost and have never been revisited based on new data.

Article: Described Dunk as strictly marine.

Russell:

There are some passing reports of dunkleosteids (probably not Dunkleosteus) from estuarine/freshwater sediments in Pennsylvania and West Virginia. Some are clearly estuarine but I think at least one is in freshwater sediments. It's not entirely clear what the osmotic regulation of arthrodires was, but it's possible they cared a lot less about salinity than modern sharks do (more like bony fish) as we know Coccosteus was apparently able to colonize the Achanarras lakes from the ocean. The idea of "what is salt water in the Devonian, really" is actually very controversial for a number of reasons. Chondrichthyans are very picky with their salinity, they tend to stick with either freshwater (xenacanths, freshwater rays, freshwater hybodonts) or saltwater (everything else) and never leave due to their wacky biochemistry (i.e., changes in osmotic tolerance seem to be hard to evolve and difficult to reverse). Again, sharks are weird and it's not entirely clear if their physiology should be taken as standard for placoderms, especially given sharks do some wacky stuff with their biochemistry that circumstantial fossil evidence suggests arthrodires may not have. 

My comments:

Engelman has reopened, in a big way, the discussion of how large a very prominent species is and what it looked like. He got his entire community talking, (That’s a huge feat for any grad student: Colleen Canavaugh’s realization of the symbiosis in giant tube worms comes to mind.) I agree we should consider the Dunk as usually pictured is too long, but how much too long? I don’t think that’s certain yet.

The easiest way to explain my current thinking is to look once again at the models. The PNSO model, which I praised highly, is still the most lifelike model but now looks to me like it may have an extra meter of Dunk in there. I think the Paleozoo dunk, which places the dorsal fin further forward and doesn’t extend the body as much, is the most authentic Dunk model out there (even better than the CollectA model, which has the dorsal and tail off but is the best-looking, or the Mojo), and I still think so after talking to Engelman. I think there is still a bit of wiggle room, despite the way so many fish fit close to Engelman’s line, because we don’t have enough material from big arthodires, and other experts are having their say on the OOL measurement. But I could be wrong. Following further scientific debate here is going to be fascinating.

Interested in zoology? Read Of Books and Beasts: A Cryptozoologist's Library.! 

Don't forget to sign up for the blog.  

www.mattbilleauthor.com  //  Twitter: MattWriter  // mattsciwriter@protonmail.com.