@D-a-n_L Looks like mud turtles need an enclosure 50/50 land and water, so he may be OK for a while but I would limit his ambient temperature to 22c to 27c or 78f-82f. If he is a Russian that temperature range won't harm him, but I don't know enough about turtles to know if you will roast him if he is a turtle otherwise if you have hot basking for a Russian. No hot basking till you work it out won't harm either breed for 24hrs so maybe do without.@D-a-n_L I'm nearly 99% sure Rex is a mid turtle, I'd guess from his shape or colour either a yellow or Eastern mud turtle. Yellow are endemic to the US so maybe more likely.
I hope others will come on line to confirm, but if you can give him a long soak it's a start.
Maybe you too can have a Google and have a look at possible care options.