Chapter 5
The Reptiles
Reptiles are considered ph yletically to represent th e first truly terrestrial vert ebrates . They originated in the early Mesozoic period from an amphibian-like ancestor. In those times the y became the predominant tetrapod vertebrates, living
not only on the dry land, but also in the fr esh water of lakes and rivers, and in
the sea. Four principal groups of reptiles have persisted to the present day. The
Chelonia (turtles and tortoises) have changed little since their origin in early Triassic times and today are represented by about fift y genera. These reptiles have a
world wide distribution; they are usuall y aquatic (o r more strictly amphibious)
in their habits. Five species live in the sea although the y must return to dry land
in order to lay their eggs. A number of Chelonians have adopted a life in arid desert
regions and these include the North American desert tortoise, Gopherus agassiz ii,
and the Mediterranean tortoise, Testudo graeca . The Crocodilia (9 genera) have
existed in a relatively unchanged form since the y first appeared in the late Triassic
period. They are mostly aquatic, living in the vicinity of fresh water, but at least
one species , Crocodilus porosus, ventures into the sea for periods of uncertain duration. The Squamata, numerically the principal contemporary reptiles, consist of
two main groups; the Lacertilia (lizards) and Ophidia (snakes), which originated
in the Jurassic and Cretaceous periods respectively. Today the y are each represented by about 300 genera. The lizards have the widest geographic distribution
of the reptiles and are even found on man y oceanic islands. The marine iguana of
the Galapagos islands, Amblyrbyncbus cristatus, spends much of its time feeding ·
on the algae in the sea. The snakes also have a wide distribution and one famil y,
the H ydrophiidae (15 genera) lives principally in the sea. Terrestrial snakes and
lizards live in habitats ranging from tropical rain forests to dry desert regions. The
remaining major group is the Rh ynchocephalia, a relict branch of the reptiles with
one surviving species , Sphenodon (the tuatara), which is now confined to a wet
temperate environment on a few small islands situated off the coast of New Zealand.
Compared with th eir amphibian ancestors, reptiles are considered to be relativel y independent of th e proximity of fresh water. Many species, however, continue to live in the vicinity of Jakes and rivers, though, unlike almost all amphibians
they produce an egg that , by virtue of its shell and amnionic membrane, can be
laid on land. Some of the sea snakes, Pelamis , however, do not even need to return
to land for this purpose, as the y give birth to live yo ung in the sea. As compared
to the amphibians the reptiles have a relatively impermeable skin which limits exchanges of their water and salts with the environment. Many reptiles, especially
those living in arid areas , can form uric acid (instead of urea or ammonia) as the
principal end-product of nitrogen metabolism, so that the y require little water for
the renal excretion of catabolic nitrogen. The three aforementioned factors , amniotic egg, relatively impermeable inte gument and uricotelism, distinguish the reptiles osmotically from their ancestral amphibian form s and must cont ribute substantially to the cosmopolitan radiation of this group into regions of potential
osmotic hostility.
135
The Reptiles
Reptiles are considered ph yletically to represent th e first truly terrestrial vert ebrates . They originated in the early Mesozoic period from an amphibian-like ancestor. In those times the y became the predominant tetrapod vertebrates, living
not only on the dry land, but also in the fr esh water of lakes and rivers, and in
the sea. Four principal groups of reptiles have persisted to the present day. The
Chelonia (turtles and tortoises) have changed little since their origin in early Triassic times and today are represented by about fift y genera. These reptiles have a
world wide distribution; they are usuall y aquatic (o r more strictly amphibious)
in their habits. Five species live in the sea although the y must return to dry land
in order to lay their eggs. A number of Chelonians have adopted a life in arid desert
regions and these include the North American desert tortoise, Gopherus agassiz ii,
and the Mediterranean tortoise, Testudo graeca . The Crocodilia (9 genera) have
existed in a relatively unchanged form since the y first appeared in the late Triassic
period. They are mostly aquatic, living in the vicinity of fresh water, but at least
one species , Crocodilus porosus, ventures into the sea for periods of uncertain duration. The Squamata, numerically the principal contemporary reptiles, consist of
two main groups; the Lacertilia (lizards) and Ophidia (snakes), which originated
in the Jurassic and Cretaceous periods respectively. Today the y are each represented by about 300 genera. The lizards have the widest geographic distribution
of the reptiles and are even found on man y oceanic islands. The marine iguana of
the Galapagos islands, Amblyrbyncbus cristatus, spends much of its time feeding ·
on the algae in the sea. The snakes also have a wide distribution and one famil y,
the H ydrophiidae (15 genera) lives principally in the sea. Terrestrial snakes and
lizards live in habitats ranging from tropical rain forests to dry desert regions. The
remaining major group is the Rh ynchocephalia, a relict branch of the reptiles with
one surviving species , Sphenodon (the tuatara), which is now confined to a wet
temperate environment on a few small islands situated off the coast of New Zealand.
Compared with th eir amphibian ancestors, reptiles are considered to be relativel y independent of th e proximity of fresh water. Many species, however, continue to live in the vicinity of Jakes and rivers, though, unlike almost all amphibians
they produce an egg that , by virtue of its shell and amnionic membrane, can be
laid on land. Some of the sea snakes, Pelamis , however, do not even need to return
to land for this purpose, as the y give birth to live yo ung in the sea. As compared
to the amphibians the reptiles have a relatively impermeable skin which limits exchanges of their water and salts with the environment. Many reptiles, especially
those living in arid areas , can form uric acid (instead of urea or ammonia) as the
principal end-product of nitrogen metabolism, so that the y require little water for
the renal excretion of catabolic nitrogen. The three aforementioned factors , amniotic egg, relatively impermeable inte gument and uricotelism, distinguish the reptiles osmotically from their ancestral amphibian form s and must cont ribute substantially to the cosmopolitan radiation of this group into regions of potential
osmotic hostility.
135