I recently read paleontologist Riley Black’s lovely book When the Earth Was Green: Plants, Animals, and Evolution’s Greatest Romance, which looks at the prehistory of life on Earth through the lens of paleobotany.
While I know the charismatic megafauna like dinosaurs and saber-toothed tigers and giant ground sloths garner most of the attention, I’ve always found ancient plants equally interesting. Part of that comes from my ongoing love of both gardening and wild plants, something I’ve experienced since I was about six and discovered F. Schuyler Mathews’s Field Book of American Trees and Shrubs, with its hundreds of pages of descriptions and range maps and wonderful illustrations. I can’t even begin to estimate the amount of time I spent poring over its pages (and I still own my copy of it).
Once I gained a passing knowledge of the trees and shrubs and wildflowers I saw every day, I was shocked to find out that if I were to go back a few million years, I’d find an entirely different assemblage of plant species. I know, it shouldn’t have been a surprise; if the animals had changed, there’s no reason the plants wouldn’t have as well. But I still found it astonishing when I found out that (for example) at the moment, there is exactly one extant species of ginkgo (the familiar, and beautiful, Ginkgo biloba), but in the past there had been hundreds, perhaps thousands, of species in the family:
A sampler of now-extinct Jurassic ginkgo species [Image licensed under the Creative Commons Peter R. Crane, Pollyanna von Knorring, Fossil Ginkgoales, CC BY 4.0]
Riley Black does a masterful job of tracing the evolutionary history of plants from their origins to recent times, and her signature lucid writing style makes the subject completely captivating. One of the chapters deals with an odd period of Earth’s history—the Cretaceous Resinous Interval, a span of about fifty million years during which there was intense diversification amongst gymnosperms, a group that includes not only ginkgos, but the superficially palm-like cycads and the much more familiar conifers.
Anyone who has ever leaned up against a pine or spruce tree knows about their impossibly sticky, golden-brown, aromatic sap. This glop, so unfortunate for skin and clothing, evolved as a way of sealing wounds and preventing insect damage. So in a relatively short time, we see the evolution of hundreds of species of plants that produced the stuff—and, when it met the right conditions, hardening into amber.
Most of the world’s amber, whether from Burma or the Baltic region or the highlands of Ecuador and Peru, formed during this time. Amber has been popular for jewelry-making since the time of the ancient Greeks, and probably before; in fact, an interesting linguistic side-note is that the Greek name for amber, ἤλεκτρον, is where our words electron and electricity come from (due to amber’s property of gaining a static charge when rubbed with a silk cloth). But amber really came into the popular consciousness because of Jurassic Park, wherein some scientists extract dinosaur blood from bloodsucking insects trapped in amber, and use it to clone dinosaurs, with predictable results.
[Nota bene: it’s thought that the upper bound for the survival of DNA in amber, even with optimal conditions, is around a million years, not the hundreds of millions required by Jurassic Park. And even that is likely to be an overestimate. In 2013, scientists tried—and failed—to extract intact DNA from a bee trapped in ten thousand year old copal, an amber precursor.]
That doesn’t mean it can’t have phenomenal paleontological significance, however, even if we’re unlikely to have velociraptors stalking us any time soon. The reason the topic comes up is a paper in Communications Earth about 112-million-year-old amber unearthed in an Ecuadorean quarry, which contained so many inclusions of insects, pollen, and seeds that it’s being called a “Cretaceous time capsule.”
A midge from the Ecuadorean amber. Check out how well preserved those compound eyes and antennae are! [Image credit: Mónica Solórzano-Kraemer]
The number of insect and arachnid taxa represented, as well as the pollen and other plant fossils discovered, paint a remarkably detailed picture of the ecosystem back then. The authors write:
The new palaeobotanical evidence suggests the presence of a diverse and humid, low-latitude forest in north-western Gondwana during the early Albian... The strata in this quarry reveal a vertical evolution of various palaeoenvironments, including proximal braided rivers, lacustrine systems, hyperpycnal [high-density, high-sediment] flows, and distal braided rivers during the Albian... Pollen and plant macrofossils show abundant ferns and fern-allies that likely grew in the understory and/or near water bodies, in a forest dominated by araucariacean resinous trees. The overall palynological and plant macrofossil association found in the Genoveva quarry, particularly the high diversity of pteridophytes and the presence of moderately thick coal seams in the stratigraphic sequence, indicates a humid environment, similar to previous reports in other but less studied north-western tropical South American sites.
The presence of relatively abundant chironomid flies and one trichopteran as bioinclusions—both insect groups with aquatic larval stages—further supports the interpretation of predominantly humid conditions during resin production and deposition.
Fascinating to think that if you went back there, in that thriving humid lowland forest, you wouldn’t see a single modern plant species. Not one. Groups, sure—we still have araucariacean trees around today (the most familiar being the Norfolk Island pine and the monkey-puzzle tree)—but our modern forests, even in habitats with similar climates, have no species in common with those that produced the 112-million-year-old Genoveva amber.
Change is always the way of things, but still, it strikes me as sad that all those many forms most beautiful and most wonderful (to swipe Darwin’s pithy phrase) are gone. Last spring at the Tompkins County Friends of the Library Used Book Sale—a twice-a-year, three week long, must-attend event for any bibliophiles within driving distance of Ithaca, New York, and which offers a quarter of a million used books each go round—I picked up a real prize in a lovely illustrated paleobotany text, with drawings and fossil photographs representing over a thousand different species of plants no longer to be found anywhere on Earth.
I think this morning I’ll spend some time flipping through its pages, and dream of wandering through the ghostly forests of prehistory.
Before plants there were, apparently, giant fungi! Imagine that.
This was so fascinating I just got Riley Black's book When the Earth Was Green from our local library. Thanks.