Mammals and birds are ubiquitous parts of modern ecosystems, but these familiar animal groups trace back hundreds of millions of years in evolutionary history. So when did the lineages leading to today’s diversity of mammalian and avian species first emerge?
If you’re short on time, here’s a quick answer to your question: Mammals and birds began evolving into their modern forms around 200-150 million years ago during the Jurassic period, following the extinction of non-avian dinosaurs.
Defining Modern Mammals and Birds
When discussing the development of modern mammals and birds, it is important to understand what is meant by the term “modern.” In this context, “modern” refers to the subclasses of mammals known as placentals and birds known as neognaths.
These are the groups that encompass the vast majority of mammalian and avian species we see today.
‘Modern’ refers to subclasses like placentals and neognaths
Placentals are mammals that give birth to live young instead of laying eggs. They have a placenta, which nourishes the developing fetus during pregnancy. Examples of placentals include humans, dogs, cats, and elephants.
On the other hand, neognath birds are a subclass of birds that have a specialized structure in their jaw, allowing for a wider range of beak shapes and functions. This includes the vast majority of bird species, such as sparrows, eagles, and penguins.
Excludes more primitive forerunners and extinct branches
When discussing the development of modern mammals and birds, it is important to note that we are excluding more primitive forerunners and extinct branches of these groups. For example, the subclass of mammals known as monotremes, which includes platypuses and echidnas, are considered more primitive and are not included in the “modern” mammal category.
Similarly, there are extinct branches of birds, such as the non-neognath group known as “archaeornithes,” that are not considered part of the modern bird lineage.
Key anatomical traits identifying each group
Several key anatomical traits help identify modern mammals and birds. In mammals, one defining characteristic is the presence of mammary glands, which produce milk for feeding their young. Mammals also have hair or fur, a unique feature that distinguishes them from other animals.
Birds, on the other hand, are characterized by their feathers, beaks, and the ability to lay hard-shelled eggs. These traits, along with others, help differentiate modern mammals and birds from other groups in the animal kingdom.
The Rise of Mammals in the Mesozoic Era
The Mesozoic Era, also known as the “Age of Reptiles,” was a pivotal period in the evolution of life on Earth. While dinosaurs dominated the land, a group of mammal-like reptiles known as cynodont therapsids paved the way for the emergence of modern mammals.
These early ancestors of mammals appeared in the late Permian period, around 260 million years ago.
Originated with mammal-like cynodont therapsids
The cynodont therapsids were a diverse group of reptiles that shared several characteristics with mammals. They had specialized teeth, a more efficient respiratory system, and a higher metabolic rate compared to their reptilian counterparts.
These adaptations allowed them to thrive in various habitats and gave them an evolutionary advantage.
One of the most well-known cynodont therapsids is the Dimetrodon, which lived around 280 million years ago. Although not a direct ancestor of mammals, Dimetrodon represents an important transitional stage in the evolution of mammalian traits.
Diversified alongside dinosaurs in the Triassic/Jurassic
During the Triassic and Jurassic periods, mammals continued to evolve and diversify alongside dinosaurs. While dinosaurs were dominating the terrestrial ecosystems, mammals occupied various niches, including burrowing, climbing, and insectivory.
One of the most notable mammal groups that emerged during this time was the multituberculates. These small, rodent-like mammals had specialized teeth with multiple cusps, allowing them to efficiently process tough plant material.
They were highly successful and remained a dominant group until the early Cenozoic era.
Coexisted with primitive placentals and marsupials
During the late Cretaceous period, around 90 million years ago, mammals underwent a significant diversification. Primitive placentals and marsupials emerged and coexisted with other mammal groups. Placentals, which include mammals like rodents, bats, and primates, have a more complex reproductive system compared to marsupials.
Marsupials, on the other hand, give birth to relatively undeveloped young, which then complete their development in a specialized pouch. This reproductive strategy allowed marsupials to successfully adapt to various environments, and they are still thriving today, particularly in Australia and South America.
For more information about the rise of mammals in the Mesozoic Era, you can visit the National Geographic website.
The Evolution of Neornithine Birds
Grew out of maniraptoran coelurosaur dinosaurs
The evolution of neornithine birds, which include all modern bird species, can be traced back to a group of dinosaurs known as maniraptoran coelurosaur dinosaurs. These dinosaurs lived during the Mesozoic Era, specifically the Cretaceous Period, around 145 to 66 million years ago.
Fossils of these dinosaurs show characteristics that are similar to both birds and other non-avian dinosaurs, suggesting a close evolutionary relationship between the two.
According to recent studies, it is believed that birds evolved from a subgroup of maniraptoran coelurosaur dinosaurs called theropods. This subgroup includes famous dinosaurs like Tyrannosaurus rex and Velociraptor.
The theropods that eventually gave rise to birds were smaller in size and had adaptations that allowed them to climb trees and glide. Over time, these adaptations evolved into the ability to fly, leading to the development of modern birds.
Jurassic Archaeopteryx had bird-like and ancestral traits
One of the key pieces of evidence for the evolution of birds is the fossil of Archaeopteryx, a dinosaur that lived during the Late Jurassic period, around 150 million years ago. Archaeopteryx had a unique combination of bird-like and ancestral dinosaur traits, making it a crucial transitional fossil.
Archaeopteryx had feathers, wings, and a wishbone, which are all characteristics of birds. However, it also had teeth, a long tail, and claws on its wings, which are more reminiscent of its dinosaur ancestors.
This mix of features suggests that Archaeopteryx was an early bird that had not fully evolved into the modern avian form.
Diversification after extinction of non-avian dinosaurs
The extinction event that wiped out non-avian dinosaurs, known as the Cretaceous-Paleogene (K-Pg) extinction event, occurred approximately 66 million years ago. This event paved the way for the diversification of neornithine birds and their dominance in the avian world.
After the extinction of non-avian dinosaurs, the ecological niches that were previously occupied by these reptiles became available for other organisms to exploit. Birds, with their ability to fly and adapt to various environments, were able to fill these niches and undergo rapid diversification.
Today, there are over 10,000 species of neornithine birds, ranging from tiny hummingbirds to majestic eagles. They have evolved a wide range of adaptations to suit their specific habitats and lifestyles, making them one of the most diverse groups of animals on Earth.
Flourishing After the Cretaceous-Paleogene Extinction
The Cretaceous-Paleogene extinction event, often referred to as the K-Pg extinction, marked a major turning point in the history of life on Earth. This cataclysmic event, which occurred approximately 66 million years ago, led to the disappearance of non-avian dinosaurs and opened up new opportunities for mammals and birds to thrive in the aftermath.
Non-avian dinosaurs disappeared 66 million years ago
The extinction of non-avian dinosaurs, including iconic species like Tyrannosaurus rex and Triceratops, occurred abruptly at the end of the Cretaceous period. The exact cause of their extinction is still a subject of scientific debate, but the most widely accepted hypothesis is that it was triggered by a massive asteroid impact in what is now the Yucatan Peninsula in Mexico.
This catastrophic event caused widespread devastation, leading to the extinction of around 75% of all species on Earth.
Allowed mammals and birds to expand roles
With the disappearance of the dominant non-avian dinosaurs, mammals and birds were able to fill ecological niches that were previously occupied by these reptilian giants. Mammals, which had existed alongside dinosaurs for millions of years but remained relatively small and inconspicuous, suddenly had the opportunity to diversify and expand their roles in ecosystems.
Birds, which had evolved from theropod dinosaurs, also experienced a surge in their evolutionary trajectory.
This expansion of mammals and birds was facilitated by the availability of new resources and habitats that were left vacant by the extinction of dinosaurs. Mammals began to occupy a wide range of ecological niches, with some species evolving into large-bodied herbivores, predators, and even marine mammals.
Birds, on the other hand, diversified into various forms and sizes, occupying different habitats and developing adaptations for flight and specialized feeding strategies.
Rapid speciation led to modern orders and families
The post-extinction period witnessed a rapid burst of speciation among mammals and birds, leading to the development of modern orders and families that we see today. This period of intense evolutionary activity resulted in the emergence of diverse mammalian groups, such as primates, carnivores, and ungulates, as well as various bird lineages, including passerines, raptors, and waterfowl.
Through adaptive radiation, these newly formed orders and families quickly diversified and spread across different continents and habitats. This diversification continues to shape the biodiversity of mammals and birds, with new species being discovered and described on a regular basis.
The Success of Mammals and Birds Today
Mammals and birds have achieved remarkable success in the modern world. There are currently over 20,000 species of birds and 6,000 species of mammals, making them two of the most diverse groups of vertebrates on the planet.
These numbers alone speak to their adaptability and ability to thrive in various environments.
Over 20,000 bird species and 6,000 mammal species now
The sheer number of bird and mammal species today is truly astounding. From tiny hummingbirds to majestic eagles, birds have evolved to occupy a wide range of habitats and fulfill numerous ecological roles.
Similarly, mammals have diversified into various forms, from the smallest shrews to the largest whales. This incredible diversity is a testament to their evolutionary success.
Have adapted to fill diverse ecological niches globally
Birds and mammals have successfully adapted to fill an incredibly diverse array of ecological niches around the world. Birds, for example, have evolved different beak shapes and sizes to suit their specific feeding habits.
Some have long, slender beaks for probing flowers for nectar, while others have strong, hooked beaks for tearing into flesh. Mammals, on the other hand, have adapted to various diets, including herbivory, carnivory, and omnivory.
They have also evolved specialized adaptations for locomotion, such as the ability to fly, swim, or burrow, depending on their environment.
Make up a significant portion of modern vertebrate biomass
In addition to their impressive diversity, birds and mammals also contribute significantly to the overall vertebrate biomass on Earth. Their population sizes and collective mass have a substantial impact on ecosystems worldwide.
Birds, for instance, play crucial roles in pollination, seed dispersal, and insect control. Mammals, on the other hand, contribute to seed dispersal, soil aeration, and nutrient cycling. Without their presence and ecological contributions, the balance of many ecosystems would be greatly disrupted.
In summary, while early predecessors existed hundreds of millions of years ago, modern mammals and birds really came into their own around 150-200 million years ago. Their rise was fueled by the extinction of dinosaurs, opening up new evolutionary opportunities.
Understanding their origins provides insight into the composition of modern ecosystems.