Question

Explain the origin and diversification of the amniotes, with special reference to amniote skull structure

Answer 1

Amniotes including all extant tetrapods such as amphibians, reptiles, birds, and mammals are first appear somewhere in the Devonian period of the Paleozoic era. Captorhinida, an earliest amniotes and the ancient reptilian group derived early in the Carboniferous (360-286 Million Years Ago; MYA) from the anthracosaurs, a group of Paleozoic amphibians (Carroll, 1988). In birds, it occurred during the Cretaceous (135-65 MYA) and the Tertiary period of the Cenozoic era (65-1.9 MYA). In mammals evolved from late Triassic / early Jurassic, about 200 million years ago. However, recent studies have been able to distinguish Paleozoic amniotes from their anamniotic tetrapod relatives to determine major patterns of interrelationships.

Amniotes skull have long been subdivided in following major parts:

(i) Anapsid called as temporal fenestrae, a region can either be solid or have openings, and is characteristic of turtles and most basal amniotes. Three basic patterns of fenestration are recognized.

(ii) Diapsid known as temporal fenestrae, one above the other on either side of the skull. These are the supratemporal (upper or dorsal) fenestra and the infratemporal (lower or ventral) fenestra. This is the condition present in most reptiles. A single fenestra occurs as two variants.

(iii) Euryapsid called supratemporal fenestra, characteristic of two reptile groups (ichthyosauromorphans and sauropterygians), whereas the synapsid skull bears an infratemporal fenestra and is characteristic of synapsids.

(iv) Synapsid formed mainly by the squamosal and postorbital bones in the dorsal border of the fenestra, and the ventral border is formed mainly by the squamosal and jugal bones.