Life between extinctions: cracking open the Cretaceous period
One hundred million years ago, Earth experienced its first great peak in biodiversity. Flowers emerged and with them pollinators, dinosaurs towered over newly evolved mammals and marsupials, the steaming jungles were teeming with newly arrived ants and termites, and the oceans were filled with gigantic, air-breathing reptiles. This was life during the Cretaceous period, Earth between two great extinctions.
But before the abundant Cretaceous period, the Earth was covered in lava. The great landmass Pangaea broke apart about 200 million years ago, creating the basin that formed the Atlantic Ocean and causing numerous fissure vents. Lava flowed from these vents continuously for about 700,000 years, covering a portion of the planet’s surface roughly one-third the size of the Moon. As a result, the atmosphere filled with carbon dioxide and aerosols, creating a huge jump in greenhouse gas levels. The resulting global warming wiped out 50% of tetrapods (four-limbed animals) at the time, 50% of terrestrial plants and 20% of marine life.
Geologist Jessica Whiteside from Brown University and colleagues analyzed plant wax and wood remains from sedimentary rock embedded in these ancient lava flows. As they explain in a study published online March 22 in the Proceedings of the National Academy of Sciences, these fossilized remains indicate the depletion of heavy isotopes compared to light isotopes. This is evidence of significant disturbances in the carbon cycle which probably led to the Triassic-Jurassic extinction. As Whiteside says in a BBC article, “We are showing that these events are synchronous with the extinction and that the events all occur within a few tens of thousands of years of the eruption of these huge lava flows.”
The planet had changed: temperatures were warmer, the skies contained ash and the land masses began to resemble the present-day continents. Whiteside argues that this event created conditions which led to the extinction of the larger crurotarsans, ancestors of alligators and crocodiles and the dinosaurs’ main competitors, leaving room for landbound saurians to grow to immense sizes.
In the oceans, many marine reptiles survived this major extinction. Mosasaurs, plesiosaurs and pliosaurs dominated the oceans while dinosaurs started to take over the terrestrial landscape. The marine reptiles differed from the egg-laying dinosaurs because they were fully aquatic and bore live young tail-first to prevent suffocation, just as whales and other present-day sea creatures do.
Even today, all reptiles, with the exception of sea snakes, return to land to lay eggs. This is due to poor gas exchange through eggshells when they are submerged in water. Once the eggs are laid on land, the embryo’s sex is determined, not by chromosomes like in many other species, but by the surrounding temperature. Scientists have speculated whether there is a connection between the vitality of extinct marine reptiles and their ability to bear live young.
Chris Organ from Harvard University and colleagues analyzed reproductive processes in 94 living species of reptiles, birds and mammals and discovered that the selection of live birth is likely caused by the ability of a species to genetically control an offspring’s sex. That is, mosasaurs and other marine reptiles likely evolved to bear live young as a means of genetically controlling the sex of their offspring, instead of relying on temperature to do the job for them. As described in a recent New York Times article,
Once the genetic control of sex evolved, so could live birth. And with live birth, these animals were freed of the obligation to return to land to nest. That freedom allowed them to evolve large body size, and the fins and fluked tails that made them efficient swimmers. They dominated the open seas.
On land, however, reptiles were also flourishing in the developing climate 100 million years ago. Notorious dinosaurs like Tyrannosaurs Rex, and raptors such as the newly discovered Linheraptor, an 8-foot-tall, two-legged predator that lived in what is now northeastern China, grew to immense heights.
But closer to the ground, an explosion of biodiversity was underway during the Cretaceous period. Ants and termites made their first appearance. Flowering plants were arriving for the first time and bringing with them pollinators, fungi and herbivores, including the first marsupials. As speciation led to rare plants, other individual species began to develop to match the needs of seed and pollen dispersal. Parasites preferred specific hosts, plants arose in various microclimates and soils, and complex biotic interactions were on the rise. It is at this point that biodiversity on land overcame the diversity of marine organisms that had once ruled the planet.
This event is described in the March 12 edition of Science magazine, in which paleontologist and evolutionary biologist Geerat Vermeij is featured with colleague Richard Grosberg from the University of California, Davis. The scientists are researching how biodiversity on land evolved to be much more complex and dynamic than marine life—to date, roughly nine out of ten species inhabit the 30% of Earth that is not covered by water. (Although, particularly in marine environments, scientists say there are many as yet undiscovered species). The researchers attribute a large portion of this disparity between aquatic and land species diversity to the development of dispersed communities. As explained in the article, “High-density populations are at an increased risk of being eaten or wiped out by disease, while dispersed communities face reduced competition and predation.”
To explain why this division in biodiversity happened during the Cretaceous period, the researchers referenced a paper by Kevin Boyce from the University of Chicago, Illinois that traced the evolution of leaf-vein density in flowering plants. The earlier fern and conifer plants of the Triassic period had relatively low vein densities compared to the flowering plants that emerged during the Cretaceous period, which, according to fossils, had somewhere between three and ten times as many veins per millimeter. According to Boyce, since more veins lead to greater photosynthesis, the excess energy found in these flowering plants allowed for complex characteristics.
While these adaptations and mechanisms are similar to what we see in plant, animal and insect speciation today, they are occurring in relatively new species. That is, some 65.5 million years ago, many species ended with the Cretaceous period in the last great extinction: the Cretaceous-Tertiary extinction. Earlier this month, a panel of scientists confirmed in a Science study that the most likely explanation for the extinction of the dinosaurs was a massive asteroid.
Atmospheric particles would have prevented photosynthsis, and as a result, herbivores would have died with the plants. Ammonites, fresh water snails and mussels died as well, spelling death for the mosasaurs and other marine life that depended on them for sustenance. Those that survived were omnivores, scavengers, carrion-eaters and insectivores. Following the mass extinction, the recovery of biodiversity was spotty for several million years as plant-insect coevolution tried to regain footing. And eventually it did. Roughly 10,000 years ago, Earth experienced its most abundant peak in biodiversity.
Whiteside, J., Olsen, P., Eglinton, T., Brookfield, M., & Sambrotto, R. (2010). Compound-specific carbon isotopes from Earth’s largest flood basalt eruptions directly linked to the end-Triassic mass extinction Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1001706107
Organ, C., Janes, D., Meade, A., & Pagel, M. (2009). Genotypic sex determination enabled adaptive radiations of extinct marine reptiles Nature, 461 (7262), 389-392 DOI: 10.1038/nature08350
Pennisi, E. (2010). On Rarity and Richness Science, 327 (5971), 1318-1319 DOI: 10.1126/science.327.5971.1318
Boyce, C., Brodribb, T., Feild, T., & Zwieniecki, M. (2009). Angiosperm leaf vein evolution was physiologically and environmentally transformative Proceedings of the Royal Society B: Biological Sciences, 276 (1663), 1771-1776 DOI: 10.1098/rspb.2008.1919
Schulte, P., et al. (2010). The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary Science, 327 (5970), 1214-1218 DOI: 10.1126/science.1177265