2.2.1 skeletons  (Page 4/4)

Despite these skeletal developments in adapting to land, they are still dependent on water in that:

• They have a naked skin and are restricted to moist areas as they have no protection against desiccation.
• They have not developed an egg that is suited to terrestrial conditions and they thus have to lay their eggs in water in order for the tadpole to mature into a frog.

Reptiles

The two major advantages that reptiles have over amphibians with regard to adaptations to terrestrial life are

• The development of a tough dry scaly skin which offer protection against desiccation and physical injury.
• The development of a shelled egg containing food and protective membranes which allow embryonic development to take place on land.

With regard to the skeleton

• All reptiles except those that are limbless ( i.e. snakes) have better body support than the amphibians and more efficiently designed limbs for travel on land .
• Most reptiles including crocodiles, most lizards and tortoises do not have an efficient form of locomotion as the limbs are spread out at the sides of thebody and the walk is a slow and waddling one. This is exhausting as a great deal of muscular effort is needed to keep the body off the grounds. Suchreptiles cannot move rapidly and cannot support a large size.
• Some reptiles overcame this problem by becoming bipedal i.e only using the hind limbs for locomotion. Others gradually over time shifted their limbs to under the body. Many dinosaurs followed this route and were ableto support a greater weight which is why some dinosaurs became huge.
• The development of ribs with a sternum(breastbone) has allowed for larger lungs and a more efficient method of inhaling and exhaling air by the inward and outward movement of the ribs.

Birds

Birds differ from the other vertebrates in that as a group they are able to fly and are designed for flight. and many of their skeletal adaptations are linked to flight . The main feature which identify a bird are its feathers and they are the only modern animals to have them

• Feathers are modified reptilian scales and there are two types
  • Contour feathers which are attached to the wings in such a way that they overlap to produce a broad flat surface beneficial for flight..
  • Down feathers provide excellent insulation against body heat loss. This is important as birds, like mammals are endothermic i.e. they generate heat in order to maintain a constant body temperature .
• The paired forelimbs are usually adapted for flying ( an exception is in penguins where they are used as paddles for swimming).
• The bones are hollow and light due to the presence of air cavities .
• Jaws are covered by horny beaks which have varied shapes according to the nature of the food source
• The sternum/breastbone is enlarged and has a keel -like extension which provides attachment for the strong muscles used for flying.. Flightless birds such as ostriches do not have a keel.
• The feet are also adapted to the bird’s mode of life e.g. talons for birds of prey, webbed feet for waterfowl. Perching feet for songbirds.

Mammals

• Body is covered by hair but mammals differ in the amount, distribution and type of hair.
• Apart from two species, mammals do not lay eggs, instead they are retained in a uterus and are nourished by an organ called the placenta
• After birth the young are nourished by milk from mammary glands .
• Have 2 sets of teeth (milk teeth replaced by permanent teeth)
• Have 4 different types of teeth namely;
  • Incisors : mainly for biting, snipping or gnawing – prominent in rodents and grazing animals. Elephant tusks are incisors
  • Canines : have long cone shaped crowns (the part of the tooth that s above the gums) and are well developed in carnivores. They have sharp edges for tearing andpiercing.
  • Premolars : have compressed crowns and one or two cusps (ridges) used for shearing and slicing
  • Molars : with large bodies and variable cusp arrangement, are used for crushing and mastication.
• There are four limbs (reduced or absent in some) adapted for many forms of locomotion.
• In most four legged mammals the leg bones are held directly underneath the body . In this position they act as props or struts and it is the bones rather than the muscles that take most of the strain of the body’s weight. For thisreason the animal is able to support the body clear of the ground for long periods of time without tiring.
• Like birds they are endothermic and can maintain a constant body temperature.
• Have a muscular partition or diaphragm between the thorax (chest region) and abdomen to make breathing more effective.
• They have a highly developed cerebrum (the brain’s most complex region).