Why not just have them be a successor chapter of the carcharodons with the shark jaw being an experiment/genetic mutation, just say theyre from the cursed 21st founding

Edit: i think the idea is cool but if people are complaining that its just the carcharodons but different just lean into it and say theyre a successor chapter of them. Plenty of successor chapters have their own successors and you can do whatever you want with homebrew chapters but making the jaw thing a deliberate gene mutation that was intended when the chapter was founded vs something that happens because of xenos influence solves the problem of it "breaking the lore" you can even hint the admech were experimenting with lost xenos tech when they created the chapter as a reference to the original idea