Please don't list this on a work's page as a trope. Examples can go on the work's YMMV tab.
Tear Jerker: Walking With Dinosaurs
The death of the Giant FlyerOrnithocheirus in Giant of the Skies and the eventual extinction of the dinosaurs. But "Giant of the Skies" especially. Not to mention the poor youngcynodonts from the first episode...
Just the music itself in Giant of the Skies is heartbreaking enough.
The Walking with Dinosaurs Special about Big Al : Many people felt sad when the allosaur dies still young from his injuries and starvation.
The poor old Liopleurodon getting beached in Walking with Dinosaurs. This series is not kind to it's older characters.
Walking With Beasts has also a sad scene when a young Indricotherium is violently driven off by its own mother as she got a new calf. The scene when the Gastornis chick is eaten alive by ants is an odd mixture of this and Nightmare Fuel, just like the aforementioned cynodonts. Still another example from Beasts is the opening with the australopithecines "mourning" their elder female in "Next of Kin", probably because of how human they look.
Also in Walking With Beasts, the mother bear dog whimpering while she digs her dead pups out of the collapsed den.
The entire Late Permian in Monsters is a slow journey to death for all creatures, since the infamous Permian/Triassic extinction is going to take place soon. And that was the worst mass extinction ever, the Cretaceous one coming only in second or third place : 90 % of marine and 70-80 % of terrestrial lifeforms went extinct.
