Unit 4: Cenozoic Life - The Age of Mammals

Aftermath of End Cretaceous Mass Extinction

The Cenozoic Era (66 Ma - Present) began immediately after the K/T mass extinction. The world was warm, quiet, and empty. The dominant land animals (dinosaurs) and marine predators (plesiosaurs) were gone.

A World of Opportunity:

This event created a massive ecological vacuum. All the jobs once held by dinosaurs—large herbivore, large carnivore, medium herbivore, etc.—were now open.

The survivors were small, generalist animals that could hide and eat anything (like insects, seeds, and carcasses). The most important of these survivors were the mammals, birds, and crocodilians.

The Cenozoic Era is defined by the explosive adaptive radiation of these surviving groups as they evolved to fill the empty niches. This is why it is known as the "Age of Mammals."

Radiation of Mammals in the Paleocene and Eocene

The Paleocene (66 - 56 Ma) and Eocene (56 - 34 Ma) were the first epochs of the Cenozoic. They were characterized by a very warm, "hothouse" climate, with forests stretching to the poles.

The Great Mammalian Experiment:

In the 10 million years after the extinction (the Paleocene), mammals rapidly evolved from small, shrew-like creatures into a huge variety of forms. This included:

• Early "Archaic" Mammals: Many were "experimental" forms that are now extinct (e.g., Creodonts, an early group of carnivores).
• The First Large Mammals: By the Eocene, large herbivorous mammals had evolved (e.g., Uintatherium, a rhino-sized animal with horn-like knobs).
• Appearance of Modern Orders: Critically, the ancestors of most modern mammal groups appeared during this time:
  • The first Primates (our ancestors).
  • The first Perissodactyls (odd-toed hoofed mammals, like early horses and rhinos).
  • The first Artiodactyls (even-toed hoofed mammals, like early camels and deer).
  • The first Cetaceans (whales, which evolved from land-dwelling artiodactyls).
  • The first Proboscideans (elephants).

Case Study: The Evolution of Whales

One of the best-documented evolutionary transitions occurred in the Eocene. Fossils show the step-by-step transition of a small, wolf-like land mammal into a fully aquatic whale.

1. Pakicetus: A wolf-like land animal that hunted fish in fresh water.
2. Ambulocetus: ("The Walking Whale") Had large hind legs but could swim with an undulating (up-and-down) spine, like an otter.
3. Rodhocetus: Had smaller, weaker hind legs (no longer used for walking) and a powerful tail. Lived in the sea.
4. Basilosaurus: A massive, 60-foot-long, fully aquatic whale with tiny, useless hind legs that were no longer connected to the spine.

Expansion of Modern Groups in the Oligocene

The Oligocene (34 - 23 Ma) marks a major turning point. The warm, "hothouse" Eocene ended with a significant cooling event. Antarctica became fully glaciated for the first time, and the climate became cooler and drier.

A Changing Landscape:

The main effect of this climate change was a shift in vegetation. The dense forests of the Eocene began to shrink, and were replaced by more open woodlands and grasslands.

Mammalian Response (Expansion of Modern Groups):

This new landscape drove the evolution of modern mammals. The "archaic" Eocene mammals died out, and were replaced by groups better adapted to running and eating tougher foods.

• Hoofed Animals (Ungulates):
  • The expansion of grasslands was a huge driver for horses, deer, and camels.
  • This led to evolutionary adaptations for running (longer legs, reduced toes) and for eating abrasive grass (taller, more complex teeth).
• Carnivores:
  • The archaic Creodonts were replaced by the two modern groups of Order Carnivora:
  • "Cat-like" feliforms (ancestors of cats, hyenas).
  • "Dog-like" caniforms (ancestors of dogs, bears, weasels).
• Primates:
  • Early prosimian-like primates (like lemurs) were common.
  • In the Oligocene, the first true anthropoids (the ancestors of monkeys, apes, and humans) appeared.

Rise of Modern Plants and Vegetation

The Cenozoic is also the "Age of Flowering Plants." Having evolved in the Cretaceous, they diversified into all available niches. The most important evolutionary event for Cenozoic vegetation was the rise of one specific family:

The Rise of Grasses (Poaceae)

Grasses are flowering plants, but they are highly specialized. They evolved in the Oligocene but became ecologically dominant in the Miocene.

Adaptations to Grazing:

Grasses are adapted to being eaten (grazed):

1. They grow from the base, not the tip. This means a mammal can eat the top of the leaf, and it will just keep growing from the bottom.
2. They are abrasive. Grasses incorporate silica (phytoliths) into their leaves, which rapidly wears down the teeth of herbivores.

This led to a powerful co-evolutionary "arms race" between grasses and grazing mammals:

• Grasses became tougher → Mammals evolved hypsodont (high-crowned) teeth that were very tall and would wear down slowly.
• Grasses spread across open plains → Mammals (like horses) evolved longer legs and reduced toes (from 3 toes to 1 hoof) to run faster and escape predators.

This rise of modern vegetation (grasslands) is directly responsible for the evolution of the modern mammal communities we see today, like the herds of the African savanna or the American plains.