Evolution of Reptiles and Mammals

Study Notes

Evolution and Classification of Vertebrates

Dinosaurs and other reptiles dominated the world for nearly 130 million years, until the end of the Mesozoic era when most of them suddenly became extinct.

Comparison of Lower Vertebrata and Higher Vertebrata

Lower Vertebrata (Anamniota) include classes Cyclostomata, Chondrichthyes, Osteichthyes, and Amphibia.
Higher Vertebrata (Amniota) include classes Reptilia, Aves, and Mammalia.
Lower Vertebrata are predominantly aquatic, have a body consisting of three parts (head, trunk, and tail), and lack a neck.
Higher Vertebrata are predominantly terrestrial, have a neck, and a body typically divided into four parts.
Other key differences between Lower and Higher Vertebrata include:
- Appendages (fins or limbs)
- Exoskeleton (absent or dermal scales in Lower Vertebrata, and epidermal scales, feathers, hairs, claws, horns, etc. in Higher Vertebrata)
- Pharyngeal gill clefts (persistent in Lower Vertebrata, and disappearing in Higher Vertebrata)
- Notochord (persistent in Lower Vertebrata, and replaced by a vertebral column in Higher Vertebrata)
- Endoskeleton (mostly cartilaginous in Lower Vertebrata, and mostly bony in Higher Vertebrata)
- Heart (2-3 chambers in Lower Vertebrata, and 4 chambers in Higher Vertebrata)
- Thermoregulation (poikilothermous in Lower Vertebrata, and homoiothermous in Higher Vertebrata)
- Respiration (gills in Lower Vertebrata, and lungs in Higher Vertebrata)
- Kidneys (mesonephric in Lower Vertebrata, and metanephric in Higher Vertebrata)
- Cranial nerves (10 pairs in Lower Vertebrata, and 12 pairs in Higher Vertebrata)
- Copulatory organ (absent in Lower Vertebrata, and present in Higher Vertebrata)
- Fertilization (external in Lower Vertebrata, and internal in Higher Vertebrata)
- Development (metamorphosis in Lower Vertebrata, and indirect without metamorphosis in Higher Vertebrata)
- Amnion (absent in Lower Vertebrata, and present in Higher Vertebrata)

Early Vertebrate Evolution

The earliest vertebrates evolved around 480 million years ago, during the Cambrian period.
The first vertebrates were small, jawless, bony, fish-like forms related to modern cyclostomes.
The Ostracodermi became extinct, but some Cyclostomata (modern lampreys and hagfishes) are still present today.

Evolution of Fish and Tetrapods

During the Silurian and Devonian periods, fossil fish are found, with more species present in the Devonian period (known as the Age of Fishes).
Ostracoderms were jawless fish, but during the Devonian period, the first jawed fish, the placoderms, arose.
The placoderms became extinct without leaving living representatives, but are likely ancestors of cartilaginous and bony fishes.
Cartilaginous fishes (Chondrichthyes) did not give rise to bony fishes (Osteichthyes).
During the Carboniferous period, the lobe-finned bony fishes (Crossopterygii) gave rise to labyrinthodonts or primitive stem Amphibia, which were the first vertebrates to walk on land.

Mesozoic Era and Age of Reptiles

In the early Carboniferous, the very primitive amphibians also gave rise to the primitive reptiles.
Reptiles reached their peak and great abundance during the Mesozoic era, which is known as the Age of Reptiles.
They included, among others, the famed dinosaurs, ichthyosaurs, and pterodactyls or flying reptiles.