Advertisement.

EnchantedLearning.com is a user-supported site.
As a bonus, site members have access to a banner-ad-free version of the site, with print-friendly pages.
Click here to learn more.

ad
(Already a member? Click here.)

Our subscribers' grade-level estimate for this page: 4th

Apes
Enchanted Learning
ALL ABOUT MAMMALS!

Geologic Time Chart
Introduction to Mammals Groups of Mammals Ice Age Mammals Mammalian Evolution Classification Mammal Index Activities and Worksheets

The Evolution of Mammals

The Evolution of Mammals

Mammals evolved during the Triassic period, about the same time that the first dinosaurs appeared. The first placental mammals appeared at the beginning of the Cretaceous period. The earliest mammals were tiny, shrew-like mammals.

The long line of animals leading to the mammals includes:


Mammals are advanced synapsids, animals distinguished by having extra openings in the skull behind the eyes; this opening gave the synapsids stronger jaw muscles and jaws (the jaw muscles were anchored to the skull opening) than previous animals. Synapsids include the mammals, and their ancestors, the pelycosaurs, therapsids, and cynodonts. Pelycosaurs (like Dimetrodon and Edaphosaurus) were early synapsids, they were mammal-like reptiles. Later synapsids include the therapsids and the cynodonts (with multicusped post-canine teeth; they lived from the late Permian through the Triassic period). The cynodonts led to the true mammals. Over time, the synapsid gait became more upright and tail length decreased.

Earliest-known Mammals:
Some of the earliest-known mammals (the Mesozoic Era) include Eozostrodon, Deltatheridium, Jeholodens, Megazostrodon, Triconodon, and Zalambdalestes.


EOZOSTRONDON

Eozostrodon was one of the first true mammals; it lived during the late Triassic period and early Jurassic period, about 210 million years ago. This small, primitive, egg-laying mammal fed the young with mother's milk. Eozostrodon was a quadruped with short legs, a long, pointed snout, five-toed feet with claws, and a long, hairy tail (it looked like a modern-day shrew). Eozostrodon was about 42 inches (107 cm) long. It was a triconodont that belonged to the family Morganucodontidae, which had true mammalian teeth (the cheek teeth were differentiated into simple premolars plus more complex molars, and the teeth were replaced only once, and the molars had triangular cusps).


DELTATHERIDIUM

(pronounced DEL-tah-ther-ID-ee-um) Deltatheridium was an early mammal from the late Cretaceous period, about 80 million years ago. This opossum-like quadruped was about 6 inches (15 cm) long and had a long tail. Its had sharp canine teeth and its cheek teeth were wide with triangular crowns. Deltatheridium was an insectivore that may have also eaten small reptiles and perhaps scavenged. Deltatheridium had characteristics of a very early marsupial (pouched mammal), and was a possible kangaroo ancestor. Fossils have been found in Mongolia.

JEHOLODENS

Jeholodens was a triconodont, an early mammal. Jeholodens jenkinsi, named by Luo Zhexi and Ji Qiang, dates from the middle Cretaceous period, about 125 million years ago. Its almost complete fossil was recently found in Lianong, China. This long-tailed, quadrupedal mammal was about 5 inches ( cm) long. An insectivore, it had relatively advanced, grasping hands, but a primitive pelvis and hind limbs. It may have had large eyes, and may have been nocturnal (most active at night).
MEGAZOSTRODON
Megazostrodon was one of the earliest mammals. It was a tiny quadruped with a long tail, a long body, and a long snout. It lived from the late Triassic period through the early Jurassic period. This primitive mammal was about 4 inches (10 cm) long and weighed only a few ounces. It may have eaten insects. A complete fossilized skeleton was found in Lesotho, South Africa.


ZALAMBDALESTES

Zalambdalestes was an early mammal that lived during the late Cretaceous period. Fossils of this shrew-like quadruped have been found in Mongolia. This tiny mammal was about 8 inches (20 cm) long and it's skull was only 2 inches long and. This insectivore (insect eater) had very sharp, interlocking teeth, a long, upturned snout, long foot bones, large eyes, a small brain, and a long tail. Classification: Subclass Pantotheria, Infraclass Eutheria (placental mammals), Genus Zalambdalestes

During the last Ice Age, there were many large, interesting mammals, like the saber-toothed cats, giant ground sloths, mastodons, and mammoths. These animals have long since gone extinct and are known mostly from fossils, from frozen, mummified carcasses, and even from ancient cave drawings.



Enchanted Learning®
Over 35,000 Web Pages
Sample Pages for Prospective Subscribers, or click below

Overview of Site
What's New
Enchanted Learning Home
Monthly Activity Calendar
Books to Print
Site Index

K-3
Crafts
K-3 Themes
Little Explorers
Picture dictionary
PreK/K Activities
Rebus Rhymes
Stories
Writing
Cloze Activities
Essay Topics
Newspaper
Writing Activities
Parts of Speech

Fiction
The Test of Time
iPhone app
TapQuiz Maps - free iPhone Geography Game

Biology
Animal Printouts
Biology Label Printouts
Biomes
Birds
Butterflies
Dinosaurs
Food Chain
Human Anatomy
Mammals
Plants
Rainforests
Sharks
Whales
Physical Sciences: K-12
Astronomy
The Earth
Geology
Hurricanes
Landforms
Oceans
Tsunami
Volcano
Languages
Dutch
French
German
Italian
Japanese (Romaji)
Portuguese
Spanish
Swedish
Geography/History
Explorers
Flags
Geography
Inventors
US History

Other Topics
Art and Artists
Calendars
College Finder
Crafts
Graphic Organizers
Label Me! Printouts
Math
Music
Word Wheels

Click to read our Privacy Policy

E-mail


Enchanted Learning Search

Search the Enchanted Learning website for:



Advertisement.



Advertisement.



Advertisement.


Copyright ©1998 EnchantedLearning.com ------ How to cite a web page