Today I came upon a ginkgo tree dropping ripe “fruits.” (I know that is not the right term, because ginkgo is a gymnosperm; but visually, they look like drupes to me.) Since it has been a long time since I had ginkgo nuts, I took the opportunity to gather some. The foul odor of the “fruits” clung to my hands and the sack I was using.
That got me wondering: when a plant produces a fleshy covering for its seeds, it is because that plant relies on an animal to eat the fleshy part and disperse the seeds. What animal would eat a “fruit” as nasty as the ginkgo’s? I have a mental picture of a Stegosaurus munching on ginkgo “fruits” in a Jurassic forest, and I had the thought that those ancient reptiles must have had very different tastes than Holocene mammals like me.
But then it occurred to me that the ginkgo could not possibly have made it all the way to human times unless it adapted to other dispersers since. Who took over after the great reptiles died out? Then I found this article: Ginkgo: an icon of “ghost” mutualistic interactions. Except for the multituberculates (by analogy with Torreya today), it was rather vague, merely saying that “large and medium sized mammals” would have been the dispersers. Which still leaves me wondering: what mammals would eat something so nasty?
Avocados are the classic example. Thought to have been dispersed by the giant ground sloths, now only by people via domestication.
Many things like that shrink down to relictual ranges and survive because they happen to be in areas with relatively stable climates (because if they weren’t, they wouldn’t be able to survive very long).
This Wikipedia article has a long list of potential anachronistic species, which I find really interesting to read though. Ginkgo is there, is suggests maybe extinct squirrel-like mammals or small scavenging dinosaurs (since the fruit start to smell like rotten flesh).
What about birds? Some birds have a sense of smell and many seeds are dispersed by birds (some in Australia rely exclusively on the Casuary). Many raptors feed on roadkill too. Given, dinosaurs were already mentioned and birds are really just their surviving modern lineage.
It’s worth noting that a fruiting plant doesn’t need an animal to desperse its seeds, the fruit can fall to the ground and have the seeds sprout then. This is how Ginkgos reproduced, as well as plants like Osage-Orange. Ginkgos are very rare in the wild today (at least in their native range, Ginkgos are actually an established exotic in some parts of North America), but Osage-Oranges are still going strong, showing that this can be a reliable strategy for reproducing.
There’s evidence that Joshua trees were dispersed long-distance by giant sloths. Today the seeds are still cached by ground squirrels but they don’t travel as far as sloths. This means the Joshua trees can’t colonize over longer distances to new suitable habitats as the climate warms. Humans might need to start carrying the seeds around and burying them in fresh cow pies.
Buffalo Gourd (Cucurbita foetidissima) is listed in the Wikipedia article as one of the possible evolutionary anachronisms. I’ve seen the melons of this plant in the US Southwest that were broken open and chewed up, presumably by Rock Squirrels which I’ve seen in the same area. In one location, the burrow of a Rock Squirrel was right in the middle of a patch of these plants with a number of melons opened. Never photo-documented this, but I guess I should.
Osage oranges had reduced to a very restricted range until recent transport by people.
Squirrels do eat the seeds, but they serve little if any dispersal role, acting only as a seed predator. Historically mammoths would have eaten and passed the seeds in their dung.