Up until the end of the last ice age, megafauna roamed North America down to Central America. These included the 9 ton Columbian Mammoth, 6 ton mastidons, 600 pound armadillos, and ground sloths weighing up to 3 tons. The last of these disappeared from the fossil record just 13,000 years ago, an instant in the life of the earth.
Their extinction has been attributed to over hunting, climate change and loss of food. The American Forest article points out that they had a bad habit of disappearing around the time of human arrival, be it West Indies about 6,000 years ago or Australia 50,000 years ago. We know that humans hunted them. The first evidence of this was finding a Clovis point in mastodon bones at Mastodon State Park south of St. Louis.
Thorn trees such as honey locust and hawthorn presumably had a good reason to expend the energy to produce these protective spikes. Likewise Osage Orange grow large fruit which is shunned by most animals. It would be of no value in spreading the species unless something large ate it and spread the seeds in fecal deposits. The current theory is that megafauna were the answer. If this is true, why haven't the thorns and hedge apples disappeared, since they would no longer be of value. The answer in the words of Whit Bronaugh:
It’s true that such adaptations are now anachronistic; they have lost their relevance. But the trees have been slow to catch on; a natural consequence of the pace of evolution. For a tree that lives, say, 250 years, 13,000 years represents only 52 generations. In an evolutionary sense, the trees don’t yet realize that the megafauna are gone.
The first Americans could not have known they were causing extinctions, and they could not have understood the implications. But we no longer have such an excuse. As Aldo Leopold has advised, “The first rule of intelligent tinkering is to save all the pieces.” We have tinkered, lost some of the most important pieces, and tried to put many where they don’t belong. That we will continue to tinker there is no doubt. Everything will depend on how intelligently we do it. And that will depend, in part, on our ability to see the ghosts that haunt our trees.All this and much more can be found at this The Trees that Miss the Mammoths link. For more on megafauna of Missouri try this MDC site.