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This is based on the notion of adaptive radiation, a process in which organisms diversify rapidly from an ancestral species into a multitude of new forms, particularly when a change in the environment makes new resources available, creates new challenges, or opens new environmental niches.

The most popular example is the mammal class exploding with diversity in the aftermath of the Mesozoic-Cenozoic extinction event.

In this alternate MC extinction event, 65 million years ago, whole ecosystems around the world collapsed not because of a comet slamming into Mexico's Yucatán Peninsula, but a series of flood basalt eruptions in Siberia, releasing enough greenhouse gases to plague the entire world with extreme heat and drought.

The floral casualty list is as follows:

  • Gnetophyta

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  • Ginkgophyta

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  • Cycadophyta

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  • Sciadopityaceae

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  • Araucariaceae

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  • Podocarpaceae

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  • Sequoioideae

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  • Bryophyta

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  • Marchantiophyta

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  • Anthotocerotophyta

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  • Pteridophyta

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  • Pteridospermophyta

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Such a list would no doubt result in an overwhelmingly explosive adaptive radiation of the angiosperms, or flowering plants. Fast-forward to the 21st century, and the question isn't which flowering plant would fill in which void, but what would the landscapes look like?

What would today's marshes, bogs and swamps look like without the ferns, mosses or other waterlogged plants that we associate those habitats with?

Who will be the dominant trees of the rainforests of western North America, Chile or New Zealand?

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Actually scientists are still divided over what caused the MC event and whether the mass extinction was caused by just meteorites or meteorites in conjunction with climate change, volcanic activity and changing continental orientations which posed multiple shocks to the system.

You have made the question a bit easier by not including all the conifers though they aren't flowering plants - these occupy massive stretches of North Eurasia and America and would indeed leave a massive vacancy. But I'm a bit confused, why would an extreme heat-and-drought event wipe out just the named divisions/orders/families? Indeed, mosses are the oldest land plants and a resistant bunch, ranging from the tropics to polar regions. The humble moss and its spores can survive under all weather conditions and repeat droughts. Moss has even been revived after as long as 1500 years. If really pushed, why wouldn't mosses build upon their drought tolerance capabilities and evolve to survive? A couple of more links: Moss myths, why moss is drought resistant, ancient moss revived and Arid bryophyta. Ferns adapt so well that they have turned a pest in many areas. Again like moss, they grow under a variety of conditions including arid regions. If it's hot enough to kill these, it would be hot enough to kill most else.

Mosses and worts - bryophyta - would be the hardest to replace. Ancient moss is said to have brought on the Ordovician ice age and I'm not sure what would replace this voracious carbon sink. They reduce erosion along streambanks, retain water in tropical forests, form a layer of soil/vegation over polar regions, fix CO2 and provide nutrition to the animal species around. Some of this niche could be taken over by sedge grasses, lichen and algae.

As to the other species, their passing would not mean much difference. Some of these plants such as cycads are not found in single stands. Though they do have some medicinal and food value (ephedrine, gingko), most are living fossils and curiously resistant and thriving relics of a bygone era. They do not actually occupy large niches that can't be replaced by some other species. The current landscape as it stands in most places would not look very different without these.

Refuge 'islands' such New Zealand, Chile or western NorAm would have the existing competing flowering plants/trees/conifers take over the vacancy. Take New Zealand, for example, home to most of the southern Gondwana paleo-flora. Take out the podocarps but there are still the southern beeches. Australia, the nearest neighbour, once had similar vegetation, which retreated to a few isolated stands in the east and south once the climate got drier and favoured the eucalypt family.

And umm, have you checked Pteridospermophyta? Seed ferns died out mostly by the Cretaceous and are no longer around. And it's Anthocerotophyta, not Anthotocerotophyta.

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  • $\begingroup$ 1) The ferns, worts and mosses are gone because they are waterlogged plants that grow via spores--really pathetic compared to the more vascular, seed-bearing plants. 2) Lycopods are pteridophytes, so they're gone, too. 3) Why make that remark about the seed ferns? 4) It's not species--it's divisions, orders, families and subfamilies. $\endgroup$ – JohnWDailey Apr 26 '16 at 13:09
  • $\begingroup$ 1) As they say 'Underestimate not the staying power of simplicity'. I wouldn't call moss pathetic by any means. 2) Acknowledged and edited to remove lycopods. 3) All plants belonging to Pteridospermatophyta division or seed ferns are extinct with most of them dying out by Cretaceous and the few that were left gone by Eocene (en.wikipedia.org/wiki/Pteridospermatophyta). $\endgroup$ – artemissunshine Apr 27 '16 at 1:37
  • $\begingroup$ 3) I realize that, but this is an alternate mass extinction so much more serious that NONE of the seed ferns made it to the Eocene. $\endgroup$ – JohnWDailey Apr 27 '16 at 2:18
  • $\begingroup$ Didn't think they made a difference by existing till the Eocene actually. Interesting really, that flowering plants should considered robust when several of these depend on the survival of pollinating insects and insect distributing birds and animals, setting up a domino effect were any component of the cycle were to be disturbed. $\endgroup$ – artemissunshine Apr 27 '16 at 4:25

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