|A 2018 study showed that the number of recognized mammal species has increased over time from 4,631 species in 1993 to 5,416 in 2005 / Photo by: Cinemanikor via Shutterstock|
A 2018 study showed that the number of recognized mammal species has increased over time from 4,631 species in 1993 to 5,416 in 2005. Last year, the figure rose to 6,495 species, representing nearly a 20% increase in overall mammal diversity.
One of the myths surrounding mammals is that they only existed after the dinosaurs went extinct 65 million years ago and that the last dinosaurs evolved into the first mammals. However, this is far from the truth. Previous studies have revealed that the first mammals evolved from a population of vertebrates called therapsids at the end of the Triassic period, which began 252 million years ago and ended 201 million years ago.
According to ThoughtCo., one of the largest and most comprehensive learning, information, and education sites online, these early mammals even coexisted with dinosaurs throughout the Mesozoic Era, which lasted almost 180 million years from approximately 250 to 65 million years ago. Another myth is that mammals have always been small. While most of these species were indeed small, there came a period when their size increased.
These false narratives have been popularized in books, lectures, and even scientific papers. “This is a very old idea, which makes it very hard to defeat. But this view of mammaliaforms simply doesn’t stand up to what we and others have found recently in the fossil record,” David Grossnickle, a postdoctoral researcher in the biology department at the University of Washington, said.
The Evolution of the First Mammals
Mammals came from members of the reptilian order “Therapsida.” While some of these species had larger sizes, only a small population of early mammals were larger than mice. The first mammals lived in spaces high up in trees or underground, hunted at night (when predatory dinosaurs were less active), and fed on plants, insects, and small lizards.
According to Futurity, a nonprofit website that aggregates news articles about scientific research conducted at prominent universities in the US, mammals and their relatives have gone through significant “ecological radiations” in their history. This only occurs when a particular lineage invades and adapts to new ecological niches. A review article published in Trends in Ecology and Evolution found that two of the three ecological radiations happened during the Jurassic and Cretaceous periods when dinosaurs were thriving.
The first true mammals and their closest relatives experienced the oldest mammaliaform ecological radiation that ran from 190 to 163 million years ago. The second one occurred 90 million years ago in the Late Cretaceous Period, while the last once began 66 million years ago around the time of the K-Pg event, ending about 34 million years ago. These ecological radiations can explain how mammals have evolved throughout the years. “The presence of this diversity of mammaliaforms in the Jurassic and Cretaceous overturns a classical interpretation of how mammals evolved,” Greg Wilson, an associate professor of biology and curator of vertebrate paleontology at the Burke Museum of Natural History & Culture, said.
Contrary to popular belief, mammals began to flourish well before the end of the dinosaur age. A 2016 study conducted by researchers from the Universities of Southampton and Chicago revealed that mammals began to adapt 10 to 20 million years before the dinosaurs died out, contradicting the traditional view that the extinction of dinosaurs around 66 million years ago allowed mammals to evolve and thrive.
|Mammals came from members of the reptilian order “Therapsida.” While some of these species had larger sizes, only a small population of early mammals were larger than mice / Photo by: Lvova via Wikimedia Commons|
How Mammals Thrived After the Dinosaurs Died Out
About 66 million years ago, a sudden mass extinction killed almost three-quarters of the animal and plant species on Earth. The Cretaceous-Paleogene (K-Pg) extinction marked the end of all dinosaurs during that time. It was caused by an unusually high number of extremely large asteroid impacts. While this resulted in the extinction of the large, dominant land dinosaurs, it had allowed smaller animals to survive and thrive.
Recently, Denver Museum of Nature and Science paleontologist Tyler Lyson and colleagues released a study about the discovery of a place called Corral Bluffs. According to the Smithsonian, the official journal published by the Smithsonian Institution in Washington, D.C., the graveyard is filled with animals that lived 66 million years ago, including the remains of turtles, crocodiles, and, most of all, mammals.
100,000 years after the extinction, mammals that were once small grew larger. By the 300,000-year mark, the biggest mammals were about the size of large beavers. Mammal species that lived 700,000 years after weighed over a hundred pounds. The researchers found out that extinction is not the only factor that influenced their growth. A warming global climate changed the makeup of forests and allowed for the evolution of new plants.
|About 66 million years ago, a sudden mass extinction killed almost three-quarters of the animal and plant species on Earth. The Cretaceous-Paleogene (K-Pg) extinction marked the end of all dinosaurs during that time / Photo by: Herschel Hoffmeyer via Shutterstock|
Different plant species evolved for the first time, including legumes, the ancestors of beans, and energy-rich plants. “For the first time, we are able to link changes in plants and animals together, and more importantly, we are able to place all of these changes in a high-resolution temporal framework,” Lyson said. While older species of mammals failed to survive, they paved the way for more modern mammals to take up the ecological roles previously filled by other species.
A 2017 study showed that the behavior of mammals that lived after the dinosaur extinction changed rapidly. According to Science Mag, the peer-reviewed academic journal of the American Association for the Advancement of Science and one of the world's top academic journals, the species started venturing out in the daylight after living a primarily nocturnal existence. The change may have even sparked the eventual evolution of humankind.
There’s still a lot to discover and learn about the evolution of mammals. Nonetheless, these studies provide a glimpse into how these species managed to thrive over the years.