Derek Turner writes . . .
Living Fossils
Cycads, along with coelacanths, horseshoe crabs, and chambered nautiluses, are important examples of “living fossils.” (I wrote a bit about living fossils in my last post as well.) Most people give credit to Charles Darwin for coining that term in the Origin of Species, though it also has quite a strange prehistory that goes back well before Darwin, to stories that miners used to tell about discovering living (presumably antediluvian) toads and other creatures hibernating in solid rock.
Today, however, many scientists are somewhat ambivalent about the notion of a living fossil. (And some are downright hostile.) Part of the problem is that the status of some of the classic cases of living fossils—including cycads—is murky. (For an interesting defense of the notion of a "living fossil," as well as a helpful discussion of cycad biology, see this blog essay by Jennifer Frazer.)
I took this picture of a cycad in Port Elizabeth, South Africa, while traveling there with colleagues in 2013.
Cycads do seem to present us with an interesting case of morphological stasis. Not long ago, a team of Chinese paleontologists reported the discovery of a remarkable cycad megafossil.[1] If you look at an image of the cycad megafossil, it is very hard to distinguish it from a run-of-the mill sago palm (also a cycad) that many people like to keep as a houseplant. (You can read the paper and check out the image here.) Pretty much anyone can keep a houseplant that is a lot like the plants that the dinosaurs liked to munch on.
Indeed, one possibility is that the extinction of the dinosaurs that they coevolved with helps explain the post-Mesozoic decline of cycad biodiversity. Competition from flowering plants may also part of the story.
Fossil cycads from N. America, courtesy of Wikimedia commons.
A Recent Radiation
But it’s not totally obvious that cycads deserve to be considered living fossils, either. One difference between cycads and other living fossils is that there are still quite a lot of cycad species hanging around—about 300 species belonging to 6 genera, even though their diversity was much greater during the Mesozoic. Contrast that with, say, the coelacanth (2 species), or the wollemi pine (just 1 species), New Zealand’s tuataras (1 species), or nautiluses (6 species).
Not only that, but when scientists recently did a molecular clock study on a large sample of extant cycad species, they found that most of those species were not actually very old! [2]
Molecular clock studies use comparisons of DNA and/or protein structure across species, together with assumptions about the rate at which changes in those chemical structures accumulate, to draw conclusions about how far back in the past the species shared a common ancestor. In the case of the cycads, the scientists found that there was a big burst of speciation—a major radiation—only about 12 million years ago, during the late Miocene. Most of the living species of cycads have common ancestors that lived around that time. This recent burst of evolutionary activity doesn’t fit too well with the idea of living fossils as the last remnants of a group whose evolutionary glory days lie far back in the mists of deep time. Instead, the cycads have been making a more recent evolutionary comeback.
It turns out that cycads are not the only putative living fossils whose status has been challenged. Other scientists have raised doubts about coelacanths, too, though on different grounds. [3] One might wonder whether paleontology and evolutionary biology even need the concept of a living fossil. What sort of work does that concept do?
Distinct Evolutionary Phenomena
Part of the confusion, perhaps, is that “living fossil” is often used to cover a variety of different evolutionary phenomena, but those phenomena do not always co-occur.
(1) Species longevity. Sometimes a particular species can hang on for a really long time. (See our earlier discussion of expiration dates and species lifespans.)
(2) Long-term morphological stability. Sometimes particular traits or structures persist for a very long time in the fossil record without much change. Of course, whether you “see” stability depends on which trait you’re looking at, on the grain at which you describe the traits, and so on.
(3) Persistence in the wake of severe of biodiversity reduction and/or loss of abundance. In some cases, there are groups that were abundant and diverse in the deep past. Some members of the group could hang on for a long time with very low abundance, even after most of their biodiversity has been lost.
When we talk about “living fossils,” we should be precise about which of these phenomena we have in mind. Cycads seem to afford a great example of (2) long-term morphological stability. But they are not such a good example of (3), given their relatively recent radiation. On the other hand, coelacanths (for example) are better examples of (3) persistence in the wake of biodiversity loss, though their degree of morphological stability is up for discussion.
Habitat Tracking?
Importantly, these three distinct phenomena might call for different sorts of evolutionary explanations. To make this a little more concrete, habitat tracking is one mechanism that scientists sometimes invoke to explain morphological stasis. When environmental conditions change, some populations that can do so simply follow their preferred habitat, rather than adapting to the new conditions. Habitat tracking could be an important part of the story about morphological stasis in some living fossil cases, and it could even help explain why some particular species persist for a long time, but it’s a little harder to to see how that could explain stasis in the cycad case, given what we know about their recent radiation and large geographic range. Habitat tracking might work best as an explanation where (2) stasis co-occurs with either (1) or (3).
Cycads exhibit a lot of morphological stability (though their recent radiation surely did not occur without some evolutionary change). But one of the classic explanations of stability (habitat tracking) is off the table in this case, because it doesn’t fit well with what we now know about the cycads’ recent evolutionary radiation.
A Macroevolutionary Puzzle
The story of the cycads seems to be one of decline (both in abundance and in diversity), persistence, and resurgence around 12 million years ago. This sort of pattern poses a peculiar explanatory challenge. Whatever factors initially caused the cycads to go into decline (the loss of their dinosaurian evolutionary partners, increased competition from flowering plants, etc.) were probably still in place 12 million years ago. So what exactly happened during the late Miocene to give the cycads their second wind, so to speak?
Nagalingum and colleagues point to some late Miocene paleogeographic and climate trends as possible triggers. Seaways between North and South America, and between Africa and Eurasia, were closed as the continents assumed their current positions. And the planet was getting somewhat cooler. Could these events have set the stage for a cycad comeback? (How, exactly?) Or might their comeback have had something to do with the weevils that pollinate many cycad species today?
Even if we could explain the cycad radiation, we might also want to know why it was not accompanied by more significant morphological change. Niles Eldredge and Stephen Jay Gould’s punctuated equilibria model suggests that morphological change happens rapidly during speciation episodes. But the cycads seem to have experienced a lot of speciation without much drastic morphological change. So what was going on, exactly?
I hope that this post leaves you scratching your head a little bit. If you have ideas, or if you know something about cycads that I don't, please do share below.
The fact that cycads are so puzzling, from an evolutionary perspective, makes it even sadder that Fossil Cycad National Monument no longer exists. Hopefully we can do a better job protecting the living cycad species than we did protecting the fossils!
[1] Wang, X., Nan, L., Wang, Y., and Zheng, S. (2009), “The discovery of whole-plant fossil cycad from the upper Triassic in western Liaoning and its significance,” Chinese Science Bulletin S4: 3116-3119.
[2] Nagalingum, et al. (2011), “Recent Synhronous Radiation of a Living Fossil,” Science 334: 796-799.
[3] Casane, D., and P. Laurenti (2013), “Why coelacanths are not ‘living fossils’,” Bioessays 35: 332-338