OMG GONZO!!!! Omg you poor poor baby. You should definitely report this. I keep forgetting where you live, but that's definitely a breach of dog park rules. Furthermore, his dog can be labeled dog aggressive, and I really think you should take his license number and take pictures of all the wounds. Take it to the animal control place and tell them what happened. I hope you can label the dog dangerous and vicious. If I remember correctly, if the dog is labeled dangerous and vicious, they're required to muzzle the dog at all times in public. I just saw the episode of SF Animal Cops and they had a similar situation. Two rotties (poor breed-they and pits are getting a really bad rep) escaped through a fence and killed two guinea pigs and a chihuahua. Oh, I'm so sorry. I can't tell you how sorry I am.

BTW, you might want to call a dog behaviorist and ask about that Alpha roll thing. I know a lot of people are against for some reason or another. You could bring up the Alpha roll when talking to the animal control officers and possibly hint out that the Rottie's owner is incompetant and should be banned from dog ownership. Good Luck. I'm so sorry once again.