Seems odd, that there are fossil remains in large numbers of these huge animals in close proximity to each other, seemingly buried rather rapidly, their skeletal remains largely intact, not gnawed or scattered by scavenging carnivores.
Fossilization is rare, and I’m of the view that death by rapid burial (as with the Burgess Shale, or death assemblages such as have been found in volcanic strata) or burial not long after death are generally the best explanations. OTOH, in Pompeii there are plaster of Paris casts made inside cavities left when buried victims’ remains decomposed — no bones. So, even that doesn’t happen every time.
Their descendants live on today. And they're still grouping up and jumping off cliffs en masse.