Propelling Dunkleosteus

 

THOUGHTS ON DUNKLEOSTEUS PROPULSION AND STEERING (TAILS AND FINS)

By Matt Bille

This is one interested amateur’s view. I’m the creator of the FaceBook Dunkleosteus terrelli site and author of the article “Dunkleosteus: First King of the Ocean,” in the Summer 2018 Prehistoric Times (which, ironically, had an editing error by me that swapped mentions of upper and lower tail lobes).  I have fiction and nonfiction projects related to Dunk in progress, I’ve talked to a lot of the experts, and I've looked at fossils and at models from toys to life-size museum exhibits.

The three tail options:

Eel-like tail: 

Despite its presence in some illustrations and some scientific papers, I never liked it.  I’ve never studied hydrodynamics, but I took a course in aerodynamics and have worked a bit with plane and missile designers. The main difference is that water is 830 times denser, and any propulsive movement that has to displace water is more difficult. That also means drag is worse.

If you look at big eels like congers, the head isn’t much bigger than the body, and they’re not supporting any big, heavy structure at the front end.  Wolf eels aren’t much different.  I think the eel body plan works for a larger creature than known eels (I’ve suggested that an eel of 8 or 9 meters might cover some still-puzzling “sea serpent” reports), but not something like a Dunk. You’re pushing a lot of water to move that front end. This tail is common on some smaller placoderms, like Diandongpetalichthys and the cool armor-plated Acanthothoraci, but nothing with the layout or mass of a Dunk. I just don’t think that tail surface is big enough. I think there are also stability and leverage problems stretching the body out well past the center of mass: it might balance while static, but the front end is so much more dense than the rear end that it’s hard for the latter to move the former.

Heterocercal tail with a large upper lobe and a small or nonextant lower one with a fin (I call this the scimitar tail): 

The most common in scientific and popular works, and it may be correct. Still, you’re sweeping with a fin well aft of the body, and there’s a lot of water to push: for a predator that has to move fast (at least in spurts) and has maybe a ton of weight at the head, the surface and the musculature are both questionable. This one is so common because, with no impression fossils of Dunk to tell us what the tail looked like, the logical supposition is that it looks like a smaller placoderm of similar layout (Coccosteus cuspidatus) enlarged.  

It’s a sensible approach: if you take a foot-long dogfish shark (Squalus acanthias) and blow it up the size of a whale shark, it’s still the same body plan and instantly recognizable as a shark. However: I don’t think our giant dogfish would move well, with the long, slender tail stock (peduncle) of the regular dogfish.  That’s where I wonder about projections for the Dunk. It looks, generally, like Coccosteus, but Coccosteus is a foot long: blow it up to match a 20-foot Dunk and you have 20x20x20=800 times the mass.   The body will not be identical. 

It may be a reach, but what comes to mind is an anaology from aircraft and aerodynamics. You can look at a small jet, like a two-place Air Force T-37 (29 feet long, 3,800 pounds), and the C-17 Globemaster (174 feet, 500,000 lbs with moderate load).  The latter is about 6 times longer and 132 times heavier. You can see they are both airplanes and both are designed to the same principles, but they’re very different in layout, proportions, materials, etc.  An enlarged T-37 wouldn’t work: the drag's too much, the propulsion's too weak, the structure won't hold up.

There are a lot of variations in the scimitar tail, of course: I think the ones with the tail lobe rising at a more acute angle are more likely to work.

It goes without saying that this stuff has been worked on and modeled by icthyologists and  paleoicthyologists with a zillion times my knowledge and sophisticated tools, and I’m just spitballing, but if I had to bet I’d say that tail is wrong.

The tail problem is also related to the reason I don’t like the fin “wrist” joints outside the body: I don’t think the leverage is good.  This configuration appears in some scientific papers, and again I could be wrong, but the analogous modern fish in length and bulk are big sharks like the great white, and the fins look quite different: the joint is closer in to the pelvic girdle to reduce the “stroke length.” The pelvic fins are moving a lot of water .

Sharklike Tail: 

So this is what I’ve thought for a long time, and I was delighted to see real experts with real tools come out with this in 2017:

https://peerj.com/articles/4081/

“Ecomorphological inferences in early vertebrates: reconstructing Dunkleosteus terrelli (Arthrodira, Placodermi) caudal fin from palaeoecological data”

Humberto G. Ferrón​, Carlos Martínez-Pérez, Héctor Botella

Their modeling follows the shark analogy and concludes the tail is sharklike.  I think they’re right.  It takes major muscle mass to move a fin quickly through water in a two-ton animal: I think convergent evolution is going to work its magic here.  “…body design of fishes is determined, to a large extent, by their swimming mode and feeding niche, making it possible to recognize different morphological traits that have evolved several times in non-closely related groups with similar lifestyles.” They produced logic similar to what I’d always guessed about: “Our proposal suggests a caudal fin with a well-developed ventral lobe, narrow peduncle and wide span, in contrast to classical reconstructions founded on the phylogenetic proximity with much smaller placoderms known from complete specimens.”

NOTE: I've looked for any response questioning this article and have not found one yet.

That’s why I think the Paleozoo model hits it on the head.

https://www.paleozoo.com.au/Dunkleosteus.php

There is, by the way, considerable debate on the skin covering of the Dunk and its close relatives. The Paleozoo model used denticles. The CollectA Dunk, one of my favorites, used more prominent denticles borrowed from the modern wolf fish.  Predator fish today use denticles (sharks) or scales of varying sizes (easily visible on barracuda and tarpon, very small on tuna). The popular but problematic Scheich Dunk adds a row of huge scutes based on.. well, I don't know what. We many never be certain.