Do I have a predator in the tank?

[coralsandfishes]

I just found this in the Red Sea 750XL this morning. I dont know if it is part of a skeleton or molting but dont have anything in the tank that look like this. I started about a year ago with mostly dead rock and live sand. about 8 months back I got some rock (30lb) from someone breaking down a tank and I thought I did a pretty thorough inspection before put it in the tank. can someone help me ID this?

 

[Alexraptor]

Looks like part of the rear end of a horseshoe crab to me.

 

[coralsandfishes]

Looks like part of the rear end of a horseshoe crab to me.

Thanks.... That make sense... I did lost one a while back while on vacation..