Eighth Edition
by Harold L. Levin

Chapter 12 - page 10

Life of the Paleozoic

Phylum Chordata

The Amniotic Egg

The next major evolutionary development in vertebrate history was the ability to reproduce without returning to water. This was accomplished with the amniotic egg.

The amniotic egg is the key feature in the development of the reptiles.

Characteristics of the amniotic egg:

• Durable outer shell protects embryo from drying
• Egg can be laid on land
• Yolky part of egg provides nutrition; sac contains embryo and another sac collects waste products
• Eliminated need to lay eggs in water, allowing vertebrates to live and reproduce on dry land for the first time
• Amniotic egg probably evolved in Carboniferous
• First fossil eggs are Early Permian

The Reptiles

CLASS REPTILIA

Name: From "reptilis" meaning "creeping".

Chief characteristics: Two characteristics of the skull which can be used to distinguish reptiles from amphibians are:

1. The reptile skull is high and narrow, compared with the low, broad amphibian skull.
2. In reptiles, the roof of the mouth is arched, with small openings. In amphibians, it is flat with large openings.

Mode of life: Complete colonization of land was achieved by the reptiles, which can lay their eggs on dry land.

Geologic range: Pennsylvanian to Recent.

The oldest reptile fossils, genus Hylonomus, (300 m.y. old) are found in Nova Scotia inside fossilized hollow trees filled with sediment. These reptiles were about 24 cm (about 1 ft) long. They resemble modern insect-eating lizards.

Diadectes sp., from the Early Permian (280-250 m.y.) was a land-dwelling plant eater.
The skeletal anatomy is reptilian, but the skull resembles that of Seymouria, an amphibian.
Photo courtesy of Pamela Gore.

Dicynodon, a Late Permian (250-230 m.y.), plant-eating reptile. From Cape Province, South Africa.
Photo courtesy of Pamela Gore.

Various groups of vertebrates can be distinguished on the basis of the position and number of openings behind the eye on the side of their skulls.

1. Anapsida (no holes) - amphibians, the earliest known reptile (Hylonomus), and turtles
2. Diapsida (two holes) - dinosaurs, flying reptiles, birds, and all groups of living reptiles except turtles
3. Euryapsida (upper hole only) - extinct marine reptiles
4. Synapsida (lower hole only) - pelycosaurs, therapsids, and mammals

Diagram showing four vertebrate skull types.	Diagram showing the evolution of reptiles and synapsids.

The Synapsids

CLASS SYNAPSIDA

The synapsids had diverged from the reptiles by the Late Carboniferous. The synapsids were long considered to be a subclass of reptile, but more recent cladistic analysis shows that they diverged from ancestors completely different than Hylonomus and other true reptiles.

The synapsids were the dominant terrestrial vertebrate in the Permian.

This group was formerly called the "mammal-like reptiles", however the name has been abandoned because they are not really reptiles.

Synapsids include the pelycosaurs and the therapsids.

Pelycosaurs

Several species of pelycosaurs had fins or "sails" on their backs, supported by rod-like extensions of their vertebrae. These sails may have been used as temperature regulating mechanisms.

Two well known pelycosaurs, which evolved their sails independently were the carnivorous Dimetrodon, and the plant-eating Edaphosaurus.

The Permian pelycosaur, Edaphosaurus.
Photo courtesy of Pamela Gore.

Dimetrodon has a larger skull and teeth than does Edaphosaurus, suggesting that Dimetrodon was a meat eater.

Pelycosaurs lived in the Carboniferous and Permian. The sail-backed forms are characteristic of the Permian.

Therapsids

Therapsids were small to moderate-sized animals with several mammalian skeletal characteristics, such as:

• Fewer bones in the skull than the other reptiles
• Mammal-like structure of the jaw
• Differentiated teeth (incisors, canines, and cheek teeth)
• Limbs in more direct alignment beneath the body
• Reduction of ribs in the neck and lumbar regions, allowing greater flexibility
• Double ball-and-socket joint between the skull and neck
• Bony palate which permitted breathing while chewing (an important characteristic for animals evolving toward mammalian warm-bloodedness.) Efficient breathing provides oxygen needed to derive heat energy from food
• Whisker pits on the snout

Mammal-like features are well developed in the therapsid, Cynognathus. (From "kynos" meaning "dog" and "gnathos" meaning "jaw" or "tooth.")
Examination of the bone on the snout portion of the skull reveals probable "whisker pits", suggesting that they had hair, which may have functioned to insulate the animal and slow the rate of heat loss.

Cynognathus crateronotus, a therapsid from the Early Triassic (230-225 m.y.), Cape Province, South Africa.
Note the differentiated teeth. This animal was obviously a predator.
Photo courtesy of Pamela Gore.

Therapsids are found in Permian and Triassic rocks.