The Many-Finned Brochoadmones

Dear Constant Readers,

There are a few fish out there with an unusual number of dorsal fins; bichirs and reedfish are basal Actinopterygians with a profusion of dorsal fins; some members of the family Scombridae (derived Actinopterygians) such as tuna and mackerel possess dorsal and anal "finlets". Nauen and Lauder 2001 note that during a stroke, finlets (with very similar morphology) showed a considerable difference in bending and appear to direct water flow and increase thrust. This abstract of a recent paper I couldn't access notes that finlets do not significantly reduce drag and enhance thrust in normal swimming. It is possible (the authors note) what difference they did cause either was meaningful over long periods of time or more useful at high speed. I haven't seen any discussion on what the mechanics of the many dorsal fins on a bichir are; since their lifestyle is pretty different from the super-performing scombrids I'm guessing they're not homologues. I've noticed that sturgeon have pre-dorsal scutes, but they may not play a role in swimming.

The butterfly kingfish (Gasterochisma melampus) showing dorsal and anal finlets. A public domain image from Fishbase by Robbie N. Cada.

A nice and old (therefore public domain) image of the bichir, Polypterurs bichir

These fish are, of course, not the focus of this particular blog post. They do illustrate that fish mechanics, particularly dealing with unconventional fin setups, are not as well understood as I anticipated. What then of a weird and obscure species from a weird and extinct group? Acanthodians, or "spiny sharks" are a radiation of jawed fishes distinct from the placoderms, chondrichthyans (sharks, skates/rays, chimeras), and bony fish. Acanthodians don't seem to have very much written on them at all; they're more closely related to bony fish than other groups (they're both Teleostomians), but their scales are more similar to those of chondrichthyans; and same species have ventral projections ("spines") appearing like multiple fins. All other fish have 2 paired fins, but some Acanthodians had many more (up to 6 spine pairs). While belonging to a unique group within Acanthodii, the recently re-described Brochoadmones sheds a lot of light on their bizarre anatomy.

Brochoadmones was a Devonian (430-435 mya) fish described all the way back in 1977. It had been demoted to a known Acanthodian family in '79 was put in its own sub-order in 1996 based on more evidence. As illustrated in Hanke & Wilson 2006, the reconstruction changed from a generalized fish with multiple finlets in 1977 to a humped salmon-like fish with shark-like gills. With two beautifully preserved fossils in the "Wonder Block" this fish is now almost completely know, and boy is it strange looking:

There was a photograph also used by Fish Feet, but at some point of time I'm going to make my own illustration based on the fossil. Note the shark-like gill slits and a pronounced pectoral fin. That last one is important...

The overall body shape of Brochoadmones is somewhat reminiscent of knifefish, the panther grouper, and a couple cichlids. These fish are stealthy ambush predators approach prey head-on facing downwards and capture them with a short lunge. Fish were visible inside some specimens, leaving no doubt that these were also piscivores. The authors speculate that if the pre-pelvic fins could move, then perhaps they aided subtly aided in stealth. Although not mentioned directly by the authors, I suppose it is implied that the multiple fins are analogues to the elongated anal fin of the knifefish and perhaps grouper. However, the authors note that other acanthodians were built for continuous swimming and some of them also have the pre-pelvic fins as well. Did pre-pelvic fins offer any advantage over the setup of modern fish groups? Over continuous fins in some instances? Why these fins were only present in this one group is apparently not clear and will likely remain so since the peculiar (non-analogous) multiple fins of scombrids and bichirs also have no clear function.

Brochoadmones had many features aside from the number of pelvic projections that made it unique and quite interesting. The projections themselves consist of six pairs of pre-pelvic spines and a web of skin with scales covering both sides. According to the authors, that feature has not been seen in any other vertebrate and look like expectations of what a fin evolving from a lateral fin-fold would look like. The largest spine is larger than that of the actual pelvic fin spine, although the attached fin of the latter is larger in area. Also noteworthy is that the pre-pelvic spines continue quite a ways closer to the head than the pectoral fin. There have been suggestions that both types of fins evolved from an ancestral lateral fin-fold, but the authors suggest that the radically different placement and development of the fins suggest different origins (both predating jaws). So there you have it, a peculiar and obscure fish that somehow has implications for the origins of limbs. The fusion of the anal and caudal fin (along with paddlefish development) suggest that the median fins actually do develop from an ancestral median dorsal/caudal/anal fin.

Other strange features hold implications for Acanthodian phylogenetics, but I'd say that is out of the focus of this current blog post. I want to be able to blog and still do homework, dangit. Recently it seems that I have been focusing quite a bit on aquatic creatures so I think next post it is time for me to branch out a little. Well, I really don't plan these out in advance, so I suppose the next thing that strikes my fancy will come up.


-Cameron

References:

Nauen, Jennifer C. & Lauder, George V. 2001. Three Dimensional Analysis of Finlet Kinematics in the Chub Mackerel (Scomber japonicus). Biol. Bull. 200, 9-19. Available: Here

Hanke, Gavin F. & Wilson, Mark V. H.. 2006. Anatomy of the early Devonian Acanthodian Brochoadmones milesi based on nearly complete body fossils, with comments on the evolution and development of paired fins. Journal of Vertebrate Paleontology 26 (3) 526-537. Available: Here




Tangent:


Should I even mention this thing? Brochoadmones also had pre-pelvic appendages extending anterior to the distinct pectoral fins as well as two dorsal fins and a somewhat humped profile. I'm not suggesting that there are giant whale-like acanthodians out there; but perhaps this bizarre Renaissance drawing was done with an Acanthodian in mind (like griffins from Protoceratops). The modern reports may have been influenced by this myth and illustration, but the sightings primarily seem to be of cetacean pods with a dash of polychaete thrown in.