High-impact research

There are few more-iconic Earth-history events than the tale of the dinosaurs being wiped out after an asteroid collided with Earth approximately 66 million years ago close to what today is the Yucatán Peninsula in Mexico. And far from being a local extinction episode (an extirpation), there is evidence that ‘dinosaurs and many of their contemporaries went extinct rapidly and simultaneously all across the globe’ from a study by Zoltán Csiki-Sava et al. Indeed, the collision and its aftermath are implicated as the cause of the Cretaceous–Paleogene (K–Pg) extinction event [formerly known as the Cretaceous–Tertiary (K–T) extinction], which saw a mass extinction of around three-quarters of the planet’s plant and animal species. The firestorm that is also inferred to have resulted from the impact has been considered widespread enough to have caused the attendant plant extinctions, or ‘a global firestorm that would have burned every twig, bush and tree on Earth’ in the more eye-catching prose associated with the science reporting of this research. Well, as sensational as that sounds, the latest view on that event suggests that there was no such global firestorm. In research that attempted to recreate the conditions of the impact in the laboratory(!!), Claire Belcher et al. found that the intense but short-lived heat near the impact site could not have ignited live plants, challenging the idea that the impact led to global firestorms. And, ‘because plants and ecosystems are generally resistant to single localized fire events, we conclude that any fires ignited by impact-induced thermal radiation cannot be directly responsible for plant extinctions, implying that heat stress is only part of the end-Cretaceous story’. Which could be viewed as proof of the saying that – and with Mr Cuttings’ profound apologies – the penstemon is mightier than the saurid. But there’s another twist to this asteroid’s fiery tale. For as much as ‘shock and awe’ may precede ‘regime-change’ in the shocking and awful affairs of the human world, so too in the natural world. The even greater degree of shock and awe that would no doubt have accompanied the Chicxulub bolide impact appears to have precipitated a major ‘regime change’ in the world of plants. Using fossil leaf measurements of minor-vein density and mass per area (as proxies for carbon assimilation rate and carbon investment, respectively), Benjamin Blonder et al. infer that plant species that survived the K–Pg extinction event had fast-growth ecological strategies corresponding to high assimilation rates and low carbon investment. Which is consistent with the loss of slow-growing evergreen species, and the ascendancy of deciduous angiosperms. ‘Potentially this also tells us why we find that modern forests are generally deciduous and not evergreen’, Boulder explains. And, as if to underline how dramatic (Earth-shattering almost…) an event this was, it spawned not just the one but two regime changes as the planet also witnessed the ascendancy of seed-bearing plants over the previously dominant, sporophyllous taxa. Although a truly extra-terrestrial origin of life on Earth – panspermia – is questionable, it does look like celestial bodies do have direct influence over – impact upon even(!) – the lives of some inhabitants on Earth. Phyto-astrology anyone? And the botanical relevance of this impact reverberates to this day in the form of asteroid P/2010 A2, a possible remnant cohort of the K–Pg impactor, and which is a member of the Flora family of asteroids.

[In the interests of balance, it should be mentioned that doubts exist amongst certain groups regarding the interpretation of the ‘Chicxulub incident’. Slightly less controversially, it is recognised that fire is a powerful life-giving component of natural ecosystems and has led to the development of so-called pyrophytes (‘plants which have adapted to tolerate fire’). For insights into some Mediterranean pyrophytes, Helen Roberts’ account on the University of Bristol‘s Botanic Garden blog is recommended. And putting the Yucatán bolide’s collision into the bigger context of the Earth’s chronological record, the BBC has produced an extremely… err… timely publication: ‘The 25 biggest turning points in Earth’s history’ – Ed.]