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     The mammals have been so successful that the Cenozoic Era (the last 65 million years) is often referred to as the Age of Mammals.  Where did mammals come from?  Mammals are actually a very old group of vertebrates, having evolved only shortly after the dinosaurs appeared.   Early in the history of amniotes (the group which is composed of reptiles, birds, and mammals), two groups of higher reptiles evolved from the anapsid reptiles which lacked a hole in the cheek region of their skull.   These two groups possessed holes in the cheek region of the skull for jaw muscles but they differed in the number of holes they possessed: the diapsids possessed two holes and the synapsids possessed one hole.
MAMMAL GROUPS ARCHAEOTHYRIS
HUMAN SKULL WITH TEMPORAL OPENINGS

     The diapsids were very successful, evolving into a number of groups such as the dinosaurs, pterosaurs (flying reptiles), crocodilians, birds, snakes, and lizards.  Unfortunately, many who consider prehistoric life consider only the success of the diapsids and not the success of the synapsids.  The synapsid reptiles were very successful and were the dominant group of reptiles in many areas during the Permian and Early Triassic Periods.  Although two mass extinctions (one at the end of each of these periods) in addition to the evolution of dinosaurs ended their time of dominance on the earth, they did evolve into a diversity of groups, including one which has survived to the present day: the mammals.

     The evolution of early synapsid reptiles into mammals involved a slow progression through groups known as pelycosaurs, therapsids, and cynodonts.  Although pelycosaurs were the earliest “mammal-like reptiles”, there are only a few characteristics which have been modified from the ancestral condition of the anapsid reptiles.  The most prominent are the synapsid opening and the presence of canine teeth.

PELYCOSAUR
The majority of the notable features of the pelycosaurs were primitive reptilian traits.
THERAPSID
     Over millions of years, some synapsids became more and more mammal-like.  This is evident if one examines the transitions between the early therapsids,
advanced therapsids, LYCAENOPS
early cynodonts, THRINAXODON
MASETOGNATHUS
and late cynodonts.

WHAT DID ALL THESE SKELETAL CHANGES MEAN?

     As pelycosaurs evolved into therapsids which evolved into cynodonts from the Carboniferous Period through the Triassic Period, there were certainly a large number of modifications to the ancestral anapsid body plan.  What do all these changes mean?  Many of them are inter-related, which perhaps can be illustrated by emphasizing two points:

1)     Cynodonts were faster, smarter, and warmer than pelycosaurs.  Their limbs longer and were held under the body for an upright posture.  Their back did not undulate when they moved as in primitive reptiles and their tails were smaller.  Expanded bone regions (such as in the vertebrae and hip) allowed for larger limb muscles.  These changes would have made them faster.  Their brains were larger and there is evidence that from the time of the therapsids, the development of some degree of endothermy had been achieved. 

2)     All of the above adaptations required more energy.  Upright body stance, larger brains, and a higher metabolism which produces excess heat are energetically expensive.  Therefore, these ancestors of mammals had to evolve modifications to produce energy more efficiently.  They evolved the ability to process their food more efficiently—they increased their jaw musculature, evolved the ability to chew, and modified their reptilian teeth to produce both tearing and grinding teeth.  Obviously, greater oxygen supplies would be needed to convert these foodstuffs into energy and so they evolved a secondary palate which ensured that they could continue to breathe, even while eating.  The loss of lumbar ribs and the evolution of the mammalian diaphragm muscle allowed them to increase the volume of their lungs when inhaling by pushing down on the digestive organs.

   By the end of this process, synapsid reptiles had evolved into the advanced cynodonts, which were the ancestors of the first mammals.

 

MAMMALS

MORGANUCODON
     The first mammals were not very different from advanced cynodonts.  Unlike cynodonts, their teeth were replaced only once in their lives.
teeth

     Another key feature of the mammals is that two tiny bones in the jaws of cynodonts were incorporated into the middle ear to improve hearing. 

 

MAMMALS BELONGING TO THE THREE MODERN GROUPS

     All modern mammals share a number of characteristics in addition to the skeletal characteristics already mentioned: they possess at least some hair, they are endothermic, and they produce a fatty, protein rich sweat called milk which nourishes their young.  A mother cat nursing her kittens is pictured below.

cat nursing

1)  MONOTREMES

   The most primitive mammals alive today are three genera from Australia classified as monotremes: the duck-billed platypus and 2 genera of echidna.  Modern monotremes still possess a number of primitive features: they lay eggs, have a more reptilian structure of their reproductive tract, possess a cloaca (common opening for urinary, reproductive, and digestive tracts), and lack a scrotum in males.  The shoulder is similar to that of cynodonts and primitive Mesozoic mammals (in the retention of both coracoid bones and the interclavicle; there no supraspinous fossa of the shoulder blade).  The cervical ribs are not fused to vertebrae.  They have a somewhat sprawling posture and low metabolic rate.

     Monotremes do possess a number of derived mammalian characteristics: they have a mammalian jaw joint, 3 inner ear bones, hair, and mammary glands (although there is no nipple and milk is secreted onto females chest like sweat and is licked off of tufts of hair).

echidna
Advanced mammals are described in the following chapters.