#sideneck turtle

Yellow-spotted River Turtle (Podocnemis unifilis), family Podocnemididae, with Julia butterfly (Dryas iulia), family Nymphalidae, Tambopata National Reserve, Peru

Huh, I’m not sure I’ve heard of this (though admittedly I’ve never kept musk turtles). My first thoughts are that either there’s something stuck on his shell (perhaps an area that is shedding?), it’s some kind of itch, or possibly he isn’t getting enough of some nutrient (which can sometimes cause animals to try to eat weird things)? I doubt it’s to trim his beak as generally turtles are able to do this through regular wear and tear while eating, foraging, etc.

Biting the basking area is another odd one. Some turtles just want to try eating everything in their tank (which is why gravel that is too big to swallow is so important) so it could just be that. Or, again, it could be something lacking in the diet and he is looking for it in other places. If the basking dock is made of something he can take a bite out of (like the foam some floating docks are made out of) you might want to change it as I have known people who had their turtle rip chunks out of the dock and swallow them.

I would double check that your water quality and diet are up to snuff, but otherwise as long as your turtles seem healthy and are behaving normally I’d chalk it up to turtles being weird. You could also try asking around The Turtle Forum (there’s a musk specific forum as well as a sideneck specific forum) and see if any of those keepers have encountered similar behavior.

Plates from Zoology of New Holland ([1793]-1794) by George Shaw, illustrations by James Sowerby

"Zoology of New Holland" describes and illustrates birds, reptiles, mammals and a fish and crustacean from Australia, where England had established a penal colony at the end of the eighteenth century. This colony, and previous expeditions, made Australian flora and fauna more accessible to English naturalists who subsequently published about it in natural history books. 

Red-belly Sideneck Turtle (Emydura subglobosa), family Chelidae, found in QLD, Australia and New Guinea