Alienacanthus, the placoderm with a chin for days

[Halichoeres]

Absolutely wild arthrodire placoderm published today in Royal Society Open Science. The inferognathal (lower jaw) of Alienacanthus had a long point sticking out in front of the upper jaw, and it bore backward-pointing teeth.

The skull:


Artwork by Beat Scheffold and Christian Klug:


Paper: https://royalsocietypublishing.org/doi/10.1098/rsos.231747
And some news coverage: https://www.nytimes.com/2024/01/30/science/fish-fossil-underbite.html

[Concavenator]

That chin looks so fragile. I'm assuming the fish was highly specialised for its chin to have meant an advantage.

[Newt]

He's going full ballyhoo! The teeth on the extended beak are interesting. They're not oriented laterally, so no good for sawfish-style slashing; I wonder what their function was.

[Paleo Flo]

What's that?! Really an alienish fish!

[BlueKrono]

What's that?! Really an alienish fish!

I don't think it's that bizarre since I've seen halfbeaks, like @Newt mentioned above.

[Faelrin]

Pretty bizarre for a placoderm alright. Also thanks @Newt since I knew I had seen an extant fish like this before but couldn't remember what it was called (unless another is out there like this).

[Splonkadumpocus]

It's like a reverse swordfish.

[Halichoeres]

Half-beaks have a bunch of lateral line sensors along their lower jaw that they use to track prey. It's possible Alienacanthus was using its jaw the same way, to sense either pressure waves or electric fields generated by smaller animals.

[Dusty Wren]

Oh, man, I love this weird little guy. What a cool find.

Question: In the abstract, the authors put the word "fish" in quotes:

This armoured 'fish' expands the morphological and ecological diversity during one of the first radiations of jawed vertebrates with a combination of features so far unrecorded for arthrodires.

Are placoderms not considered fish? I admit I don't know much about them.

[Halichoeres]

'Fish' is a little like 'crustacean' in that it tells you something about ecology, but not about phylogeny. Some biologists have started to restrict the usage of 'fish' to actinopterygians, which would exclude sharks, lampreys, coelacanths, etc., as well as tetrapods, which are phylogenetically included if those other things are. So they're just indicating that they are using the word in its looser ecological sense: a vertebrate that lives in water.

[Concavenator]

Some biologists have started to restrict the usage of 'fish' to actinopterygians, which would exclude sharks, lampreys, coelacanths, etc., as well as tetrapods, which are phylogenetically included if those other things are.

That was interesting to read! I'm a biologist too, but I'm more into the molecular stuff, and I'm not totally up-to-date with phylogenetic analyses. That said, I find them interesting even if sometimes they get to seemingly weird results.

Similar to what you said, I remember from our zoology lessons that the concept of toads is artificial, and that their differences with frogs are purely morphological and cursorial. 

[Halichoeres]

Some biologists have started to restrict the usage of 'fish' to actinopterygians, which would exclude sharks, lampreys, coelacanths, etc., as well as tetrapods, which are phylogenetically included if those other things are.

That was interesting to read! I'm a biologist too, but I'm more into the molecular stuff, and I'm not totally up-to-date with phylogenetic analyses. That said, I find them interesting even if sometimes they get to seemingly weird results.

Similar to what you said, I remember from our zoology lessons that the concept of toads is artificial, and that their differences with frogs are purely morphological and cursorial. 

And I'm definitely not up to date with molecular biology and biochemistry!

Yep, all toads are frogs, and all tetrapods are fish, and all insects are crustaceans. But colloquially, all those terms still have their uses!

[Newt]

I have a whole spiel about the frog/toad issue, phylogenetic versus vernacular classification, whether scientists really ought to claim their use of non-scientific words is the correct one, the effect of colonialism on nomenclature, and the impact of England's depauperate herpetofauna on Anglophone concepts of herpetofaunal classification. You all get the gist, I'll spare you the rant.

Back on topic, I'm intrigued by the body form in the reconstruction. The keeled caudal peduncle and deeply forked tail are reminiscent of a fast pelagic swimmer such as a billfish or a lamniform shark. Is there any evidence for this body plan existing in placoderms generally or Alienacanthus in particular?  

[Halichoeres]

I have a whole spiel about the frog/toad issue, phylogenetic versus vernacular classification, whether scientists really ought to claim their use of non-scientific words is the correct one, the effect of colonialism on nomenclature, and the impact of England's depauperate herpetofauna on Anglophone concepts of herpetofaunal classification. You all get the gist, I'll spare you the rant.

Back on topic, I'm intrigued by the body form in the reconstruction. The keeled caudal peduncle and deeply forked tail are reminiscent of a fast pelagic swimmer such as a billfish or a lamniform shark. Is there any evidence for this body plan existing in placoderms generally or Alienacanthus in particular? 

It's funny how the lingua franca of science (i.e. English) results in some odd outcomes, like "all snakes are lizards." I see your point that if English had evolved in a different place, we might have more words for scaly quadrupeds.

As for speedy placoderm body plans, it was long predicted, but I'd say Amazichthys is the most direct evidence that they could pull it off.



Is that the kind of thing you're thinking of?

[Newt]

Yes indeed, thank you.