Question

VISUAL SKILLS Based on the phylogeny shown in Figure 34.25, identify the sister group for (a) reptiles; (b) squamates; and (c) the clade that includes crocodilians and birds. Concept Check 34.6 Contrast monotremes, marsupials, and eutherians in terms of how they bear young. Identify at least five derived traits of primates. MAKE CONNECTIONS Develop a hypothesis to explain why the diversity of mammals increased in the Cenozoic. Your explanation should consider mammalian adaptations as well as factors such as mass extinctions and continental drift (review Concept 25.4).

Answer 1

On the basis of the type by which they bear young the Monotremes are the oviparous or the egg laying mammals. they lay the eggs just like the birds and the reptiles and they protect the egg from the gee eaters or the predators and the egg hatches eventually and give rise to the young ones.

The marsupials are Ovoviviparous that is they lay egg and the development occurs through that egg but the fertilized egg is retained withing the maursupial sack or the marsupium of the female body and the baby is raised within that sack. This is seen in the case of the kangaroos.

In case of the Eutherians or the higher mammals they are vivparous that is the egg after fertilization is implanted within the speciallised chamber knows as the uterus and the development of the young one occurs within the placenta and the birth is given as the form of live birth through the vaginal canal of the female.

2. Some of the most important derived traits in the primates are-

a. opposable thumb and presence of big toes.

b. presence of nails instead of claws in the digits

c. prehensile hands and feet.

d. reduced sense of smell and increased reliance on vision

e. Ability of sit in an upright position for very long time without using the hands or the fore limbs for balance.