CHARACTERS AND ANATOMY
Reptiles were the first fully
terrestrial vertebrates. The group includes lizards, snakes,
crocodiles, and turtles today. The Class 'Reptilia' is paraphyletic -
that means it is a group that had a single ancestor - some time in the
Carboniferous - but it excludes some ultimate descendants of that ancestor (namely
the birds and mammals).
Reptiles are amniotes - they lay eggs. This was one of the primary adaptations
that allowed them to colonise land. The egg shell
prevents the embryo from drying out before it is
ready to hatch.
All modern reptiles are cold-blooded (ectotherms).
This means that they cannot regulate their body temperature and
rely on the temperature of their environment to maintain body
heat. Some researchers have suggested that some, or all, of the dinosaurs
The skin of a reptile is covered in hard scales
made of a protein called keratin. This is the same substance that
forms bird feathers and mammal hairs.
Amniotes show four skull patterns:
- Anapsid: Probably
the primitive form of the amniote skill. It lacks any temporal fenestrae
and is thought to be common to
all early reptiles such as Hylonomus.
- Synapsid: This
form of skull has one temporal fenestra in a low position. It is seen
in the 'mammal-like' reptiles, such as the
cynodonts, and in mammals.
- Diapsid: The skull type
seen in the majority of reptiles both modern and fossil, including lizards,
crocodiles, birds, dinosaurs and pterosaurs. There are two
- Euryapsid: Probably derived
from the diapsid skull type, and perhaps more than once. It has one
temporal fenestra, but its position is higher
in the skull suggesting the loss on the lower fenestra of diapsids.
Seen in extinct
marine reptiles such as nothosaurs, plesiosaurs and ichthyosaurs.
Dinosaurs were the dominant group
of fossil reptiles. Many of them were huge and adapted to life on land. Their key
characters are in the hindlimb, and these reflect a major evolutionary change
through the Triassic, when the archosaurs evolved from a sprawling posture (the
primitive condition), through a semi-erect posture to the fully erect stance seen
in all dinosaurs:
- an erect (upright) posture,
- primitively bipedal (standing up on their hind legs),
- with a ball-and-socket hip joint,
- a straight hinge-like knee and ankle,
- roller-like main ankle bones (astragalus and
calcaneum) that were closely fixed to the bottom of the main shin bone (the tibia).
- most dinosaurs had more than the basic two sacral (hip) vertebrae seen
on other reptiles - sometimes three or four, or even as many as six or seven.
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