Egg

Because of their hard shell, eggs do not need water to hatch.

Musée du Séminaire de Sherbrooke

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What great evolutionary innovation allowed vertebrates to emerge from the water 340 million years ago? The egg shell! This shell enclosed the membranes, liquids and food reserves that allowed an embryo to develop completely before hatching. The first creatures hatched from an egg shell (or an amniotic egg) are called amniotes. They spawned several lines of descendants, which are classified according to their anatomical structure, particularly the openings on each side of the skull. Birds, reptiles and mammals are amniotic cousins. The term reptile meaning the crawling animals of today is of little significance to a palaeontologist. In terms of its evolution, the chicken is more closely related to the crocodile than the lizard is.
What great evolutionary innovation allowed vertebrates to emerge from the water 340 million years ago? The egg shell! This shell enclosed the membranes, liquids and food reserves that allowed an embryo to develop completely before hatching. The first creatures hatched from an egg shell (or an amniotic egg) are called amniotes. They spawned several lines of descendants, which are classified according to their anatomical structure, particularly the openings on each side of the skull. Birds, reptiles and mammals are amniotic cousins. The term reptile meaning the crawling animals of today is of little significance to a palaeontologist. In terms of its evolution, the chicken is more closely related to the crocodile than the lizard is.

© 2001, CHIN. All Rights Reserved.

The warm, dry climate of 220 million years ago was more favourable to reptiles than mammals. So reptiles evolved and diversified more quickly. Dinosaurs didn’t drag their bellies like modern reptiles. Instead, they had a more upright posture, which allowed them to expand their territory and dominate the planet.

The Cretaceous period saw further evolution of dinosaurs. The huge Jurassic dinosaurs had died out to be replaced by smaller, more sophisticated ones. The dinosaur fauna from the Upper Cretaceous of North America was among the richest in the world. Most dinosaurs of the time were herbivores. Ornithopods (consisting chiefly of hadrosaurs or the duck-billed dinosaurs) were the most abundant and diverse group. They had strange head crests and the most highly evolved teeth among the dinosaurs. Other dinosaurs of the period include pachycephalosaurs, ceratopsians and ankylosaurs. The carnivorous dinosaurs of the time were quick and intelligent (like Troodon and Ornithomimidae) or gigantic and terrifying (like Tyrannosauridae).
The warm, dry climate of 220 million years ago was more favourable to reptiles than mammals. So reptiles evolved and diversified more quickly. Dinosaurs didn’t drag their bellies like modern reptiles. Instead, they had a more upright posture, which allowed them to expand their territory and dominate the planet.

The Cretaceous period saw further evolution of dinosaurs. The huge Jurassic dinosaurs had died out to be replaced by smaller, more sophisticated ones. The dinosaur fauna from the Upper Cretaceous of North America was among the richest in the world. Most dinosaurs of the time were herbivores. Ornithopods (consisting chiefly of hadrosaurs or the duck-billed dinosaurs) were the most abundant and diverse group. They had strange head crests and the most highly evolved teeth among the dinosaurs. Other dinosaurs of the period include pachycephalosaurs, ceratopsians and ankylosaurs. The carnivorous dinosaurs of the time were quick and intelligent (like Troodon and Ornithomimidae) or gigantic and terrifying (like Tyrannosauridae).

© 2001, CHIN. All Rights Reserved.

As the Cretaceous period drew to a close, all the dinosaurs suddenly died out along with several other species. Much evidence now supports the theory that this mass extinction (called the K/T extinction) was partly caused by a large asteroid hitting the Earth.
As the Cretaceous period drew to a close, all the dinosaurs suddenly died out along with several other species. Much evidence now supports the theory that this mass extinction (called the K/T extinction) was partly caused by a large asteroid hitting the Earth.

© 2001, CHIN. All Rights Reserved.

Dinosaurs

The tyrannosaurids, ceratopsians, and hadrosaurs were some of the last living dinosaurs in North America.

Musée du Séminaire de Sherbrooke

© Musée du Séminaire de Sherbrooke. All Rights Reserved.


Mammals appeared at nearly the same time as dinosaurs. At first, they were small, warm-blooded vertebrates that fed on insects, worms, small reptiles and dinosaur eggs. They even laid eggs themselves. Some had fur covering their bodies. Mammals evolved throughout the Cretaceous period. In North America, they mainly consisted of two categories: Allotheria (often called multituberculates) and Theria.

Multituberculates were numerous. They lived in trees, ranged between the size of a mouse and squirrel, ate almost anything, and were probably active at night. Their mouths had incisors for gnawing and molars for grinding. These creatures were eventually replaced by the rodent ancestors of beavers and rats.

In the middle Cretaceous period, therian mammals divided into two groups: marsupials and placentals. Marsupials gave birth to poorly developed young that completed their growth in their mother’s pouch. Though today’s marsupials live mostly in Australia, the oldest marsupials were found in deposits of the Upper Cretaceous of North America. They were many diverse species at the time. The only surviving marsupial in North America is the Virginia opossum.
Mammals appeared at nearly the same time as dinosaurs. At first, they were small, warm-blooded vertebrates that fed on insects, worms, small reptiles and dinosaur eggs. They even laid eggs themselves. Some had fur covering their bodies. Mammals evolved throughout the Cretaceous period. In North America, they mainly consisted of two categories: Allotheria (often called multituberculates) and Theria.

Multituberculates were numerous. They lived in trees, ranged between the size of a mouse and squirrel, ate almost anything, and were probably active at night. Their mouths had incisors for gnawing and molars for grinding. These creatures were eventually replaced by the rodent ancestors of beavers and rats.

In the middle Cretaceous period, therian mammals divided into two groups: marsupials and placentals. Marsupials gave birth to poorly developed young that completed their growth in their mother’s pouch. Though today’s marsupials live mostly in Australia, the oldest marsupials were found in deposits of the Upper Cretaceous of North America. They were many diverse species at the time. The only surviving marsupial in North America is the Virginia opossum.

© 2001, CHIN. All Rights Reserved.

Opossum

The opossum is the only marsupial found in North America.

Musée du Séminaire de Sherbrooke

© Musée du Séminaire de Sherbrooke. All Rights Reserved.


Placentals developed entirely in the mother’s abdomen. In the late Cretaceous period, they were less abundant and diverse than multituberculates and marsupials. They ranged between the size of a shrew and a groundhog and ate a varied diet. Little is known about most placentals of the Cretaceous period. Yet studying them is important since most mammals living today, including human beings, evolved from these creatures.
Placentals developed entirely in the mother’s abdomen. In the late Cretaceous period, they were less abundant and diverse than multituberculates and marsupials. They ranged between the size of a shrew and a groundhog and ate a varied diet. Little is known about most placentals of the Cretaceous period. Yet studying them is important since most mammals living today, including human beings, evolved from these creatures.

© 2001, CHIN. All Rights Reserved.

Shrew

The placental mammals of the Cretaceous may have looked like this shrew.

Musée du Séminaire de Sherbrooke

© Musée du Séminaire de Sherbrooke. All Rights Reserved.


Learning Objectives

The learner will:
  • Develop enthusiasm and continuing interest in the study of science
  • Describe some of the reptiles and mammals of the Cretaceous and their ecology
  • Describe how evolution shaped the vertebrate communities of the Cretaceous, with examples
  • Describe the rise and extinction of the dinosaurs

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