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Thursday 13 January 2022

The cannibal in the ocean

I’ve just learnt about a new shark – Orthacanthus – and maybe it’s Latin name will give you a clue as to why I hadn’t heard of it before: it’s extinct. But even when animals are long gone, the mysteries they leave in the ripples behind them continue to fascinate scientists. And all of us.
Orthacanthus. SaberrexStrongheart via

So what what was Orthacanthus? Imagine a long, eel-like body, and narrow, pointed teeth. These prehistoric sharks were xenacanthids – inhabitants of the Lower Carboniferous period 260 million years ago. They might have measured up to 5 metres long! Or 3 metres. It depends where you go for your information, because really, we don’t know for sure[1]. And they lived in freshwater. Or shallow oceans. Or brackish swamps. Actually, it’s highly likely they lived in all of them, which is where their story becomes interesting, because scientists think that they may have left their homes in the shallow oceans (why? We don’t know, and it seems like it was a bad idea in hindsight) to ascend rivers and have their babies somewhere “safer”.

And then they ate their babies.

Orthacanthus has been found all over Europe and North America, and inspection of their coprolites and gut contents tells us they were pretty indiscriminate eaters wherever. Carnivores and apex predators, they consumed mostly smaller, also eel-shaped sharks from the period, but they would also chomp down on other water-dwellers and, when push came to shove, their own babies.

Swampland © TWDK.
The theory goes that Orthacanthus started eating its own babies when the going got tough, and the going got tough when it went up the rivers and inhabited places where food was either scarce, or became scarce after getting ravaged by the Orthacanthus. Scientists think that this migration into freshwater could be a key evolutionary step in the story of fish colonisation of freshwaters, although its a mystery why they stayed there when it profoundly wasn’t working for them, and continued eating their own babies until they finally went extinct.

Scientists have also looked at the teeth of these animals, which can be useful in learning more about diet. Interestingly, large and small adult teeth have been found with different characteristics, suggesting two types of the shark. But scientists don’t think this signifies two species: instead they think it’s evidence of sexual dimorphism, with one of the male and female being bigger, and perhaps in other ways physiologically distinct, from the other. This also raises the question of whether the bigger Orthacanthus preyed on the smaller one, which, if one was male and the other female, could definitely have contributed to their final, sorry demise. 
why don't all references have links?

[1] Johnson, Gary D. Possible origin of the xenacanth sharks Orthacanthus texensis and Orthacanthus platypternus in the Lower Permian of Texas, USA. Historical Biology 24.4 (2012): 369-379. doi: 10.1080%2F08912963.2012.669128.

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