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This is a submission for the Anatomically Correct Series

Residing in Ireland, the Dobhar Chú is a creature of Irish folklore being depicted as:

  • being carnivorous
  • resembling an Otter
  • being 7 ft long
  • residing in lakes (optional)
  • hunting in pairs (optional)
  • being mostly aquatic but capable of walking on land

Given these characteristics, what species could the Dobhar Chú have evolved from, and what evolutionary pressures would lead to such a creature?

NOTE: magic does not exist in my story

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    $\begingroup$ What reasons do you have to suspect such a creature couldn't exist? You've just described a really big otter. $\endgroup$ – Zxyrra Feb 14 at 0:55
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    $\begingroup$ Just for curiosity, what exactly is the astonishing caracter in the description of this animal? $\endgroup$ – AlexP Feb 14 at 1:02
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    $\begingroup$ Pteronura brasiliensis is a real-life 6 ft (1.7 m) long otter, plus it has a 2 ft (0.7 m) long tail. Very large males reach a length of 8 ft (2.4 m). $\endgroup$ – AlexP Feb 14 at 1:09
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    $\begingroup$ There are also a bunch of extinct giant island otters. Megalenhydris from Sardinia was much bigger than any living otter (including Pteroneura), and a few other island otters seem to be pretty big. There are also some big mainland otters like Enhydriodon and Siamogale. Otters are one of the few groups of carnivores that easily disperse over water and Ireland has a lot of open niches for large predators only filled by wolves and brown bears. Honestly it's more surprising that something like the dobhar-chu didn't evolve in real life. $\endgroup$ – user2352714 Feb 14 at 3:23
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Otters already get this big

The Giant River Otter of the Amazon reaches up to seven feet in length and is a powerful predator. It’s highly social and lives throughout the Amazon basin. There is nothing astonishing or far fetched about a different species of otter getting this big in a cold climate like ireland, it just needs the right pressure.

Perhaps your Dobar Chú got big because they started to grab small land animals when the fish runs were low and have therefore gotten bigger and bigger to catch animals like small deer and unsuspecting humans.

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Competition favors large bodies, so the easiest solution to your problem is just to make otters bigger, provided the ecosystem can support them. The "evolutionary pressure" making otters large would be the lack of pressure. The abundance of food and lack of competition would allow them to keep getting bigger. While this is the most obvious solution, I don't think a lake ecosystem would be a great place to evolve large mammals; food is relatively finite. I'd like to prose that -

Dobhar Chú are extant cetaceans.

Larger mammals than otters have evolved to live in marine environments. I propose that the Dobhar Chú are an offshoot of the lineage that produced whales and dolphins that didn't fully evolve to live in water. Perhaps they inherited cetaceans' size, but not their fins.

If you really want lake otters, check out the seals of Lake Baikal. Marine mammals have gotten trapped in freshwater ecosystems before; it wouldn't be infeasible for it to happen again.

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