I was hoping there would be an article to provide further explanation of the video.  Bottom line is that this find is unique and so well preserved  from a likely random selection of events.  I am thinking the animal must have been caught in a massive flood incident, drowned, floated downriver and drifted far out to deep waters where scavenging sea animals were rare or the turbulence from the flood waters made feeding waters inedible, or materials in the river/sea water kept the body hidden where it eventually sank.  Perhaps the weight of the armour kept most of the body submerged and out of sight.

What an incredible find and extraordinary preservation.