The really sad thing about this is that clearly some people can only get excited about alien life if it's large and complex. If life existed on Mars it probably never had a chance to get going beyond microbial life but discovering that would be no less incredible.
Put it this way: if the bones of large animals can be found on Mars then the search for any trace of life there at all would have turned something up long ago. That large animal would require a large and varied ecosystem to support it, such a system would have taken many millions of years to develop, and even if it no longer existed then the soil would be rich with chemical signatures that screamed "life was here!" so loudly that it would have been easily detected by now.
Reasoning backwards from the comparative lack of evidence so far tells you that life on Mars didn't hold on for very long and stayed at a small scale, if it existed at all.