i've mentioned before that merfolk hunt whales. it's what they evolved to do, picking up their highly social behavior to hit far above their own weight and spiraling out into even more benefits across the entirety of their lives, and they still do hunt whales to this day, albeit as more of a systematized structure than as something they're all doing, all the time.
this is why they retained hands and feet as mobile and specialized as they are, even as they moved to more marine environments. their specific method, literally hanging onto the sides of whales and other large animals, being much harder to dislodge without the use of hands or a ground to crush them against, biting and slicing further with their claws until the prey is exhausted and slowly dies of blood loss, is also still present in the most traditional methods, which is what most great nomadic families utilize regardless, as a cultural touchstone and a point of leverage. however, they aren't just limited to their own natural weapons anymore, and merfolk make great use of utilizing the full extent of what's available to them to make this easier.
hence, the hand crampons. these are much like the crampons for shoes, but specifically fitted to be worn on merfolk hands, usually with most spikes placed over the palms, and usually worn with a thick glove underneath to protect the hand. additional texturing is possible too, either on the glove part or through adding smaller spikes and hooks. these are designed and intended to be used by the portion of the hunting party dedicated solely to holding onto the whale, not in further worrying it, and if they are using any additional weapons they will be held within the mouth. these crampons help make it easier for a merfolk to hold onto the whale without risk of being dislodged, but also to open even larger wounds from which more blood is let. there are usually additional, matching sets for the feet, and sometimes they are even attached to the tail, effectively all to help turn any given merfolk into living velcro.
your common descent is tails and your fossil is anatosuchus: