[rrussell1962]

30 or so years since my comparative anatomy classes and I had to think about anything like this, but at risk of making a fool of myself here goes, photo 4 looks to me like a very degraded vertebra seen from the posterior side i.e. as if you were looking down at the animals back. I think the bits sticking out are the remains of the transverse and articular processes. Given the size you might be looking at something at least the size of a dugong.

As to photo 1, which I think is what you referred to as the tusk, I remember even less about teeth in Marine Vertebrates. I would say it's either an incisor or canine tooth though and not a molar, and from the upper jaw. It does look a bit like the end of a Dugong upper jaw, but that's a pure shot in the dark on my part. Google Dugong skull and you may see what I mean. Any dentists on here to comment on the tooth?