That's a very interesting point. It was animals that weighed over 200-250 pounds that died off mostly. Since very large critters require more food, some sudden decline in food sources makes sense to me as the cause for the big animal die-off. I've never bought into the idea that human hunters caused the megafauna extinctions. They may have picnicked on already-dead mammoths and even killed one now and then but they were much more likely to go after smaller and less dangerous prey given their primitive weapons. In general this new theory makes sense to me.
Ask yourself if “bullfighting” is an easy occupation.
Then imagine that the bull was twenty times bigger!
;-)
Also, with short summers at the Northern limits of the temperate zone most of the area wold be one big refrigerator for most of the year. Humans with the capability of entering into the near-tundra could dine well on the animals who’d simply dropped dead.
If the most significant cause was an extraterrestrial object, then the smaller animals would have been more likely to be able to hide and be protected from the effects. I think that this is why the reptiles which survived the great dinosaur extinction event were snakes, turtles, crocodiles and others which lived in the water or underground; also, of course, our small mammalian ancestors.
Regarding a comment from SC, I was referring to underwater structures, not underwater wrecked shipping.