October 2, 2013
Mammal Diversity Faltered When Flowering Plants Arrived
Brett Smith for redOrbit.com - Your Universe Online
As mammals were trying to emerge from the shadows of dinosaurs 100 million years ago, there was a dramatic proliferation of flowering plant species.However, instead of early mammals benefiting from new food and shelter opportunities that would have been provided by the plants, they experienced a decline during the mid-Cretaceous.
Using a morphological analysis, two researchers were able to provide these new insights about mammalian evolution in their report that was recently published in the journal Proceedings of the Royal Society B.
"At the middle of the Cretaceous, a time when the early angiosperms are radiating, we find a surprising decrease in the diversity of mammals," said study author David Grossnickle, currently a doctoral student in geology at the University of Chicago. "It's not until the end of the Cretaceous, close to the time of the extinction of the dinosaurs, that we actually see a rebound in mammalian diversity and the first appearance of purely herbivorous mammals."
Previous research had indicated that the spread of angiosperms and the evolution of pollinating insects may have increased mammalian diversity. The theory was based on the notion of more food sources, such as seeds and fruits, being beneficial for the furry omnivores.
However, the Indiana researchers discovered that while the number of mammals may have increased, their overall biodiversity decreased, resulting in surviving mammals that were small, insect-eating animals.
"From the fossil record, the time of the angiosperm radiation doesn't look like a very good time for mammals," Grossnickle said. "There's not as much variation as there was before and after that time, and there's not as much as you would expect at a time when angiosperms were radiating."
The study team reached their conclusion through a thorough examination of the size and shape of mammalian jaws in the fossil record. They also used dental mechanical functionality and molar mass to determine evolution in mammal morphology.
One particular group of mammals, while mostly small and very similar, actually flourished during the mid-Cretaceous. These early mammals, called therians, would eventually give rise to most modern mammals, including humans.
"Without the ecological changes brought about by the Cretaceous radiation of flowering plants," Grossnickle said, "the world would be a very different place and may not have triggered crucial adaptations of our clever primate ancestors."
The Indiana study comes just as another study published in the journal Frontiers in Plant Science said that flowering plants actually developed 100 million years earlier than previously thought.
In the study, the scientists described how drilling cores have been found to contain well-preserved 240-million-year-old pollen grains, the oldest known fossils from flowering plants.
“An uninterrupted sequence of fossilized pollen from flowers begins in the Early Cretaceous, approximately 140 million years ago, and it is generally assumed that flowering plants first evolved around that time,” study researchers from the University of Zurich said in a statement.
“But the present study documents flowering plant-like pollen that is 100 million years older, implying that flowering plants may have originated in the Early Triassic (between 252 to 247 million years ago) or even earlier,” they added. “Many studies have tried to estimate the age of flowering plants from molecular data, but so far no consensus has been reached. Depending on dataset and method, these estimates range from the Triassic to the Cretaceous.”
Image Below: This figure shows a decline in mammal variety by dental type in the mid-Cretaceous. Note the increase, however, in mammals with tribosphenic molars, small insectivores that gave rise to modern-day mammals. Credit: Indiana University-Bloomington