Other have already made some good points, the difficulty or resource poverty to repair damaged buildings, and cultural changes that meant some buildings suddenly become undesirable.

​

At which point they have often been converted into a five fingered discount quarry.

​

They have been very, very vulnerable to resource looting. Sometimes for ordinary buildings, sometimes for other monuments and other religious buildings.

Often new churches were built on the same site as the former temples, renovating or reusing older parts of the structures.

​

Some were also used as churches without much adaptation, like the Pantheon, which is still to this day an active church. Though a pope did mug it during the 17th century and absconded with a lot of bronze and marble to build the Barberini Palace.

​

In the eastern part of the roman empire (byzantine empire) many temples and shrines were demolished by decree and reused to build new churches.

A church in Hagios Kosmas used stones and columns from a shrine to Aphrodite. As far as researchers can tell just about every bit of that shrine is gone and more or less recycled into various structures, some which are also long gone by now.

​

Quite a bit of the Colosseum that had been damaged by earthquakes and then some was carted off to build medieval and renaissance Rome, until a pope in 1744 put his slipper down and banned the practice as well as declaring it a protected site that could not be demolished. Parts of it is in Barberini Palace, Piazza Venezia and St. Peter's.

​

You can find stones from a building dedicated to Ramses II (Died 1213 BC) used miles and miles away from it's original place to build the gateway for Shoshenq III (Died 798 BC).

For when you still wants that glorious builder prestige but haven't quite got the same budget. In that case Ramses was from a very different family and time, but Shoseng III wasn't averse to taking from his direct ancestors either.

​

Recycled sarcophaguses too. Some previous occupants might have been dumped who knows where, and some found themselves reinterred in less fancy environments with new roommates.

And some dispensed with relocating entirely and just cleared out the previous incumbent, relabeled stuff and moved in after death.

​

If people ever wonder why we don't know where more pharaohs, their relatives and other dignitaries mummies are and who's whom, this is also one of the reasons. You can't even trust these people to stay in their own graves.

Depends on place, time, presence of competition and how long since a major extinction event, and what else is around. Enough big prey animals to support sustainable groups of large predators, not enough major apex predators around to limit herbivore size etc.

Most extinction events hit the large species harder because they need more food to sustain their bodies, and a major hit to their diet goes very badly. In the time after though, after whatever went wrong is over or stabilized to a new normal, some of the smaller species start to grow to fill the gap left behind.

One reason for a marine animal to get bigger is insulation. Even warm water can draw heat from a body. It makes it easier to regulate your own body temperature and that means you can stay in the water for longer and travel further, and may reach harder to access food sources.

The time for giant penguins was between 60-23 million years ago. There were more than this one species. But they all died out.

Their fall coincides with the diversification of dolphins who might have competed for the same spot in terms of diet and size and they were much better at it, or they may have acted as predators to the penguins. Or both.

As for place several of these giant penguin species have been found in or near new Zealand. At their time there was also the Zealandia continent, which since around 23 million years ago is almost completely submerged. Besides new Zealand itself and some islands poking out. But for many millions of years there was more land, and as it sunk it brought nutrients that could support rich fishing grounds, but not low enough to make it very passable for major marine animals that couldn't go in shallow waters. Which could have made a very roomy and rich niche for something like a penguin. Until it sank even further down. And that is when dolphins and whales hit their stride in this region.