Shortly after posting my blog article yesterday, I realized that mastodons rather than giant tortoises were the more likely dispersal agent of torreya seeds during the Pleistocene. Connie Barlow came to the conclusion that giant tortoises were the most important dispersal agent of this now relic species of tree, but several flaws in her reasoning occurred to me.
1. She thinks mammal teeth would destroy the torreya seeds and accordingly, the seeds would be more likely to survive tortoise consumption because tortoises have no teeth. However, mastodon teeth have big ridges. A tiny conifer seed could easily escape the grinding molars of these teeth and pass through to the alimentary canal. Mastodon coprolites have been found with intact seeds such as acorns, hazelnuts, persimmon and wild squash. Mastodons were not thorough chewers.
Mastodon tooth. Note the large spaces between ridges. A tiny conifer seed could easily pass through the teeth into the alimentary canal without getting crushed.
Torreya seed cone. The seed inside this cone is small enough for a mastodon to swallow without chewing.
2. Ms. Barlow claims the turpene found in torreya cones is toxic to mammals but not to reptiles–further evidence that tortoises must have been their main disperser. Turpene is found in all conifer needles. It may be toxic to some mammals, but it obviously was not toxic to mastodons. Although mastodons ate a wide variety of plant foods, conifer needles were usually the most common item in their diet. Mastodon coprolites found in the midwest almost entirely consist of spruce needles. Cypress needles along with buttonbush twigs were the most common item in mastodon coprolites found in Florida.
3. Healthy torreya trees grew to 60 feet tall. A giant tortoise would not be tall enough to reach most of the cones. A mastodon could easily reach most of them and tear down the tree to reach the ones at the top. They probably injested some cones while feeding upon the needles.
4. The closest living relative of the extinct giant tortoise is the gopher tortoise, and it feeds upon succulent plants growing in open sunny savannahs and desert scrub habitats. This is precisely the type of habitat where torreyas can not grow. Giant tortoises were likely a denizen of these open habitats that were unsuitable for torreyas.
5. The last torreya trees are found along rivers in protected environments. Isotopic studies show that mastodons migrated seasonally up and down river system corridors. This may explain why the torreya’s last stand is located by rivers. It’s where they would have been most abundant, if they had depended upon mastodon transportation for dispersal.
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