#romerodus

For about a year, now, I've been wracking my mind on the subject of the latest reconstruction of the extinct holocephalan[1] Helicoprion[2]. By all other accounts the latest reconstruction, which can be viewed below, is the best it's ever been. The iconic Eugeneodontid[3] tooth whorl fits into the jaw in a logical way and lends credence to our present understanding of its diet.

But, as you gaze further posterior, you may notice something queer: no pelvic or anal fins. Now, a not-insignificant number of living chondrichthyes[4] lack anal fins. This is not out of the realm of imagination. But the same cannot be said of the pelvic fins, which aid in control and stabilization and also serve as the girdle for which claspers are attached to in males. All living chondrichthyes possess pelvic fins; from chimaeras like Hydrolagus colliei[5], to sharks like Hexanchus griseus[6], to skates[7] like Raja binoculata[8].

(Hydrolagus colliei)

(Hexanchus griseus)

(Raja binoculata)

So, why are they missing in our latest reconstruction? I made attempts last year to reach out to the researcher responsible for our modern understanding of Helicoprion, Jesse Pruitt (whose lecture on his research set me off on this journey), through LinkedIn, but I never got a response. Looking through my message history, though, the conversation appears to have disappeared, making me wonder if I'd actually sent it at all.

This morning, I tried again. I drafted up my question and reasoning and sent it to Pruitt's LinkedIn and ResearchGate profiles, and I sent it to the Idaho Visualization Laboratory (IVL) which made the Helicoprion model. I had expected considerable delay, but to my mirth, I got a response this afternoon.

On behalf of IVL, Leif Tapanila responded to my email with answers. As it were, other Eugeneodontids from the Carboniferous and Permian periods were preserved better than our Helicoprion specimens. Specimens of Fadenia[9] and Romerodus[10] were preserved very well with full-body imprints in consistent shale rock.

(Fadenia)

(Romerodus)

These imprints lack pelvic fins. Considering their close relation to Helicoprion, this absence was assumed for the visualization. I've inquired after Edestus[11], another Eugeneodontid of the same era, since there are many specimens and imprints, however fragmentary. I have yet to receive an answer, but when I do, I will update this post.

(Edestus)

I was determined to find some oversight, since holocephalan specimens from the Devonian period include pelvic fins in their reconstruction, but it seems the facts have dissolved my suspicions. I wanted to share this with you all, since I cannot edit Wikipedia articles (much to my dismay, as I have learned much and more in my studies of H. griseus that I want to add to its page). This knowledge deserves to be shared, since I find it ponderous.

Until next time,

[1] Subclass of chondrichthyes, containing contemporary chimaeras.

[2] Extinct eugeneodontid. Etymology translates to "Spiral saw"

[3] Order of holocephalan characterized by tooth "whorls" and inability to shed teeth.

[4] Class of chordate (vertebrate) characterized by a cartilaginous skeleton

[5] Chimaera known as the "spotted ratfish"

[6] Hexanchiform known as "bluntnose sixgill shark"

[7] Order of elasmobranch "Rajiformes," adjacent to rays.

[8] Rajiform known as "big skate" (no, really. haha)

[9] Eugeneodontid from the Carboniferous, Permian, and Triassic period

[10] Eugeneodontid from the Carboniferous period