What the heck is this?

So, because I’ve hit a slow patch and I’m battling my current Tarbosaur painting, I thought I’d do another sneak peek but with added mystery!
I’ve added a feature to my big unpleasant meat eater which I haven’t encountered in other dinosaur reconstructions, but which I did see on ostriches at that great bastion of zoological parks, Zoodoo, here in Tasmania.
So all you paleo buffs out there, write your guesses on a stamped, self addressed envelope… or just in the comments section below to attain the glory of being first to identify the amorphous lump depicted in the above picture!(and possibly a prize!)