Angiosperms, the flowering plants, are the most diverse group of land plants. As they’re so successful, you might think their origins are well known. However, over the past year research has found that the earliest ancestor of the flowering plants is difficult to place in time, and there’s a good reason for this.
One way to find the earliest flowering plant would be to find the earliest fossil of one. So far this has been from around 125 million years ago, in the Cretaceous period. Once flowering plants start appearing in the fossil record, they diversify rapidly. If you follow the fossils, it seems they cooperate with pollinating insects to colonise the plant some time in the early Cretaceous.
Another way to estimate the age would be to look at the molecules of modern plants. Organisms mutate from a common ancestor, and the rate of this mutation is reasonably constant. So if you know how much two organisms have diverged from each other, then you have a molecular clock. If you do this, you can get a date as early as 350 million years ago. Ofir Katz noted this had quite a clash with the fossil record, saying: “[T]here is no pre-Cretaceous fossil that can be confidently assigned to the angiosperms. However, the lack of fossil evidence thus far does not negate the new molecular chronology. If anything, it shows that more attention should be put into this possible 200Myr gap.”
Working independently, a team led by Jose Barba-Montoya also tried to reconcile the difference between the molecular clock and the fossil record. They said: “The discordance between molecular clock estimates and unequivocal fossil evidence of crown‐angiosperms implies a cryptic interval to their early evolutionary history, in which angiosperms existed but are unrepresented in the fossil record, which could be as much as 121 Myr, but as little as 23 Myr.”
Cryptic, in this case, means that the flowering plants are there but hidden in the fossil record. Katz has proposed they have already been found. “[A]ngiosperm ancestors probably did not differ morphologically and anatomically from gymnosperms, and their fossils that did not differ from those of other land plant clades were classified as pteridosperms or gymnosperms.”
Another problem is that the earliest fossil flower found is, just, the earliest fossil flower. It’s not necessarily the earliest common ancestor of the angiosperms. Brown and Smith have said that there’s a statistical problem in dating angiosperms based on fossils, and their analysis of the data suggests that rather than 120 or 140 million years ago, 200 million years would be a better estimate from the fossil evidence for the earliest common ancestor.
Katz recently returned to the problem of how the two methods can vary in dating. He argues that for fossils to appear, there need to be three steps. First, you need the genetic and physiological divergence from the ancestral group. Then you need a trait to emerge, along with the clades of plants bearing that trait. After that you need it to be ecologically successful.
Of course, if an earlier flowering plant fossil could be found, then the discrepancy between the two dating methods could be reduced.
Fu and colleagues may have found such a fossil. They published “An unexpected noncarpellate epigynous flower from the Jurassic of China“. The study is part of an ongoing research programme by Xin Wang of the Chinese Academy of Sciences. In their commentary on the paper Winship Taylor and Li note that it’s not the first time Wang has published a Jurassic flowering plant. Herendeen and colleagues have examined some of these plants and concluded they’re not flowering plants, with many features being similar to gymnosperms. This led to Wang publishing a sharp rebuke.
Some of this argument may be due to the increasing need for precision is defining exactly what an angiosperm is. Winship Taylor and Li say that this new fossil flower, Nanjinganthus dendrostyla “[D]oes exhibit strong evidence that the seeds are within an ovary, which falls within a rather narrow definition of an angiosperm…”
If Katz’s most recent paper is right, then this is an argument that is likely to continue for a while yet. He’s drawn a diagram showing how fossil and molecular evidence can appear to give such different results.
If the earliest angiosperms are cryptic, like Katz and Barba-Montoya et al. believe, then the details of how you define what an angiosperm is will matter. Winship Taylor and Li conclude: “Nanjinganthus is clearly an important fossil, but additional characteristics need to be documented, the similarities to angiosperms need more careful justification, and its relationships to other species should be analyzed phylogenetically.”
The molecular evidence strongly suggests that there are Jurassic angiosperms somewhere in fossil beds waiting to be found. After publication, it is now for other researchers to decide if Fu and colleagues have found one.
Additional reading: In Defense of Plants