It would be a subordinate cause, for it can't stand on its own. Think about it: Since early this morning. What happened since early morning? Since it doesn't provide that information, it wouldn't be able to be an independent cause.
Also, I don't believe that the sentence is grammatically correct :)
Answer:
The definition of tragic in coorelation to this sentence would be devastating or unexpected, but I would go with devastating because that's a synonym for tragic.
Explanation:
This is because the puppy's return to the animal shelter was devastating/unexpected. However devastating is a synonym for tragic.
The answer would be Golden Gate Bridge. I hope this helps you.