Tag Archives: Fossils

When did the flowering plants evolve? Two new studies come to different conclusions

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The angiosperms (flowering plants) are far and away the most diverse group of plants ever to have evolved.  There are an estimated 350,000 to 370,000 species, more than all other groups of plants (ferns, conifers, cycads, mosses, etc.) combined, living and extinct.  The origin of the flowering plants was termed an “abominable mystery” by Charles Darwin – or perhaps it wasn’t: see this essay by Prof. Richard Buggs for an alternative view of what Darwin was describing, and this paper by Prof. William Friedman giving a different interpretation.

These disagreements about what Darwin meant are as nothing compared to disagreements about when the flowering plants actually evolved and how we interpret fossils and evidence from molecular phylogenies.  Two new studies illustrate this point: they use some of the same information to come to completely different conclusions.  I’ve copied the details and abstracts below, with links to the originals, and emphasised the areas of disagreement in bold text.  And I’m going to leave it at that; I don’t have a horse in this race and I have no idea which (if either) is correct.

There are, however, profound implications for understanding when and how relationships between flowering plants and their pollinators evolved, as I noted in my recent review of pollinator diversity.  If the much earlier, Triassic origin of the angiosperms is correct then perhaps the earliest flowering plants did not co-opt pollinators that were already servicing gymnosperms.  Perhaps the relationships between plants and pollinators originated with the (Triassic) angiosperms and the gymnosperms subsequently evolved to exploit this.  My feeling is that only more, better fossils will provide definitive answers.

Here’s the details of the studies:

Coiro et al. (2019) How deep is the conflict between molecular and fossil evidence on the age of angiosperms? New Phytologist

Abstract: The timing of the origin of angiosperms is a hotly debated topic in plant evolution. Molecular dating analyses that consistently retrieve pre‐Cretaceous ages for crown‐group angiosperms have eroded confidence in the fossil record, which indicates a radiation and possibly also origin in the Early Cretaceous. Here, we evaluate paleobotanical evidence on the age of the angiosperms, showing how fossils provide crucial data for clarifying the situation. Pollen floras document a Northern Gondwanan appearance of monosulcate angiosperms in the Valanginian and subsequent poleward spread of monosulcates and tricolpate eudicots, accelerating in the Albian. The sequence of pollen types agrees with molecular phylogenetic inferences on the course of pollen evolution, but it conflicts strongly with Triassic and early Jurassic molecular ages, and the discrepancy is difficult to explain by geographic or taphonomic biases. Critical scrutiny shows that supposed pre‐Cretaceous angiosperms either represent other plant groups or lack features that might confidently assign them to the angiosperms. However, the record may allow the Late Jurassic existence of ecologically restricted angiosperms, like those seen in the basal ANITA grade. Finally, we examine recently recognized biases in molecular dating and argue that a thoughtful integration of fossil and molecular evidence could help resolve these conflicts.

 

Li et al. (2019) Origin of angiosperms and the puzzle of the Jurassic gap. Nature Plants

Abstract: Angiosperms are by far the most species-rich clade of land plants, but their origin and early evolutionary history remain poorly understood. We reconstructed angiosperm phylogeny based on 80 genes from 2,881 plastid genomes representing 85% of extant families and all orders. With a well-resolved plastid tree and 62 fossil calibrations, we dated the origin of the crown angiosperms to the Upper Triassic, with major angiosperm radiations occurring in the Jurassic and Lower Cretaceous. This estimated crown age is substantially earlier than that of unequivocal angiosperm fossils, and the difference is here termed the ‘Jurassic angiosperm gap’. Our time-calibrated plastid phylogenomic tree provides a highly relevant framework for future comparative studies of flowering plant evolution.

 

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A fossilised flower in amber – with its pollinator!

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There have been only a handful of occasions in my professional life when I’ve been sent a manuscript to review that has caused my jaw to hit the floor with amazement.  The last time it occurred was July 2016 when I received a request to review a study that claimed to have found a fossil flower in amber, with an associated pollinator.  Not only that, but the flower appeared to belong to a species of asclepiad (Apocynaceae subfamily Asclepiadoideae) – the plant group on which I have focused a good deal of my attention over the years.  Even better, the study was by George Poinar, the originator of the idea to extract DNA from amber-encased fossils, thus inspiring Jurassic Park.  George is also the author of two books (Life in Amber and Quest for Life in Amber) that made a big impression on me when I was a PhD student and young post-doc*.

An asclepiad in amber?  From George Poinar?  How could I possibly refuse?!

One of the almost unique features of the asclepiads is that they disperse their pollen as discrete packages – pollinia – that attach en masse to their pollinators.  Only the unrelated orchids do anything similar, which means that identifying the pollinators of asclepiads is much more straightforward than for most plants, making them an ideal model group for studying plant-pollinator interactions.  I’ve had a deep interest in the asclepiads, and particularly their pollination ecology, for over 30 years. Over that time I’ve occasionally daydreamed that perhaps a fossil asclepiad flower in amber might be discovered in my lifetime, or an insect with a pollinarium attached, but I was amazed to see that this study had discovered both in the same piece of amber.  In the image at the top of this post, L and C point to some significant features of the flower, whilst T marks the pollinator.  And T stands for….

….a termite!  Termites are rare overall as pollinators and unknown in that role in relation to the asclepiads; P in this image points to the pollinia attached to the front of the head of the termite:

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You can get a better sense of the relationship between the flower and the pollinator from the photograph at the end of the this post.

After reading and re-reading and re-re-reading the manuscript I came to the conclusion that assigning the flower to a living group of asclepiads was problematical.  So (with the Editor’s permission) I solicited the views of a few colleagues with more experience than I of some of the less well-known groups of asclepiads, and related Apocynaceae s.s., the morphology of which may shed light on this intriguing fossil.  Without going into the technical details (which I’m happy to discuss with anyone who is interested) we concluded that the flower may well represent a transitional taxon as it has features of a number of different extant asclepiad lineages.  This amber, from the Dominican Republic, is estimated to be between 15 and 45 million years old, so it’s perhaps not unexpected that the set of floral features in this flower appear rather odd from a modern perspective: plants, and their flowers, evolve, just like all other organisms.

George has assigned the flower to a new genus and called it Discoflorus neotropicus  – the “different flower from the Neotropics”.  Millions of years ago this flower bloomed in the tropical forest that covered that part of the world, and attracted a worker termite to feed on its sweet nectar, pollinating the flower in the process.  Before that could take place both flower and pollinator found themselves entombed in the sticky sap being exuded by a leguminous tree called Hymenaea protera.  Many asclepiads are climbers so it’s quite possible that Discoflorus neotropicus was climbing through the branches of that tree when it got stuck.  A fateful day for flower and insect that has come down to the present day as an all-too-rare insight into ancient plant-pollinator interactions: the first fossil asclepiad flower and the first fossil asclepiad pollinator.

The study is published as follows, with a link to the journal:

Poinar Jr, G.O. (2017) Ancient termite pollinator of milkweed flowers in Dominican amber.  American Entomologist 63: 52-56

My sincere thanks to George for allowing me to highlight his research, and use his images, on my blog.  All images are (c) George O. Poinar Jr.

 

*I have a life-long interest in palaeontology that goes back to my youth, collecting fossils on the shale heaps produced from coal mining in the north of England (heaps, incidentally, that my father, grandfather, and paternal uncles had helped to create – all were coal miners in the area).  I very nearly became a professional palaeontologist but the lure of living ecosystems overcame my interest in those that are dead, though my fascination with fossils continues, particularly those in amber and (as I’ve related previously) human ancestors.

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Spiral Sunday #18 – fossils from Glasgow

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Spiral Sunday this week is brought to you by Fergus Chadwick, and is (in his words) a “spiral of spirals”: a spiral display of spiral ammonite fossils at the Kelvin Grove Museum in Glasgow.  Thanks for the photo Fergus!

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