The Elements of Evolution (22) Yucatán Big Bang

Yucatán Big Bang

There is nothing to suggest that dinosaurs were doomed to extinction. Without that asteroid, the dinosaurs would probably still be here, and we very probably would not. ~ English vertebrate paleontologist Richard Butler

66 MYA abruptly brought the final phase of the Cretaceous–Paleogene (K–Pg) extinction event, courtesy of an 9-km diameter meteorite whacking into Mexico’s Yucatán peninsula at Chicxulub. The collision produced the most powerful earthquake of all time, releasing energy equivalent to a billion atomic bombs, heating the atmosphere into an incandescent furnace. Impact shock waves coursed through the Earth, catalyzing massive floods of basalt and volcanic eruptions on the other side of the planet.

The impact vaporized sulfur-rich rock, creating a vast cloud of sulfur trioxide (SO3). This mixed with water vapor within days of the collision, to besiege the oceans and land with lethal acid rain.

The meteorite strike shot so much dust and soot into the atmosphere that surface seawater temperature near the impact site dropped 2 °C for lack of sunlight. The soot darkened the skies for nearly 2 years.

At first it would have been about as dark as a moonlit night. ~ American atmospheric and oceanic scientist Owen Toon

Earth’s atmosphere chilled 26 °C in 5 years. The soot came from raging wildfires. Underneath the bolide strike point was a rich reservoir of volatile crude oil, with extensive forests nearby.

The K–Pg event included a major marine regression. Sea levels dropped enough to reduce the continental shelf area, which is the most species-rich part of the sea.

Inland (epeiric) seas were also affected, greatly altering surrounding terrestrial ecosystems. Freshwater environments grew, as continental runoff had a longer run to the ocean.

The Chicxulub bolide was the 3rd act in the grand finale to the Mesozoic era. The 1st act was climate change.

Continental drift was altering the face of the planet and the flow of its waters. The food chain was weakened by a dearth of diversity among the large plant-eating dinosaurs which were the meal ticket for major predators. The mass extinction this invoked was modest, especially compared with what was to come.

Dinosaur evolutionary success had started to stall 140 MYA. Several families of large dinosaurs went extinct in the early Cretaceous, 135 MYA. By 90 MYA, dinosaurs were in decline, as extant species were going extinct faster than new ones emerged.

Then, a million years before the bolide struck, massive volcanic eruptions on the Indian subcontinent filled the atmosphere with aerosols. (These eruptions created the geophysical formation known as the Deccan Traps.) It was a magma fountain gushing for 750,000 years, flooding an area the size of Mongolia with lava. The resultant climate change was intense: altering seasons and rapidly heating the seas 8 ºC or more.

Volcanism kicked off the extinction of much marine life. But this would not have led to wholesale dinosaur extinction.

The Yucatán impact was legendary bad luck in happening at the worst possible time and place. Besides setting an ocean of oil ablaze, the impact occurred at a precipitous time, when ecosystems were particularly vulnerable. If the bolide had struck a few million years earlier, when species were more diverse and food chains more robust, or a few million years later, after adjusting to other environmental calamities, dinosaurs may have pulled through.

As with other mass extinction events, the deck of life was severely shuffled in the K–Pg extinction event. No land animal heavier than 25 kilos survived, which is about the heft of a bulldog or dalmatian.

Dinosaurs were not the only land animals wiped out: so too 83% of existing lizards and snakes. The climatic calamity was simply too much to bear for all sorts of creatures, even arthropods. All told, 75% of terrestrial animal species, and about as many plants, were lost in the K–Pg extinction event. Pollinating bees suffered terribly.

Half of marine species were decimated, but only 10–22% of those in freshwater. 90% of bony fish species survived. Ray-finned fishes dominated the seas in the aftermath of K–Pg extinction. The demise of the large reptilian sea monsters of the Cretaceous opened the way for cetaceans.

K–Pg was a reenactment of extinction patterns from previous events. Large animals are inherently less well-adapted to withstand dramatic changes than smaller ones, as some niches remain habitable, whereas larger landscapes are drastically diminished during such events. That withstanding, rapid climate change can doom specialist species, as K–Pg showed with the dramatic decline of reptiles.

The toll on small animals was still high. Over 90% of mammal species worldwide were wiped out. Mammal comeback after the extinction event was surprisingly swift, abetted by the loss of predators and habitat competitors.

Amphibians, reliant upon freshwater, were largely unaffected. Insects, tortoises, and crocodiles fared relatively well.

Many insects are detrivores. Furthermore, most have at least 1 durable resting stage that allows extended dormancy.

Crocodiles and tortoises can survive for long durations without food, and both can be scavengers. All 6 groups of turtles alive at the time made it through.

One trick to making it past the extended winter that followed the Yucatán big bang was being small and mobile, and to be able to survive on what food was available. Fussy eaters did not fare well.

Able to flee devastated areas, tiny dinosaur descendants winged their way past the K–Pg boundary – or at least some of them did. Being toothless, but with strong beaks, meant that birds could survive on seeds or fish the seas. Still, that as many species endured as did is surprising.

Their hardiness served them well. Birds proliferated to become the most speciose tetrapod.

Zoologists have tended to look upon the current geological era as being dominated by mammals, but it as much belongs to birds.