Mammals also appeared during Mesozoic era and were small in size, less than 33lbs until the dawn of Cenozoic which is termed as the “Age of mammals’. Cenozoic is termed as age of mammals because extinction of groups, gave a chance to mammals to flourish and diversify. First of the mammals got evolved from Therapsidsby the time when Triassic period was reaching its end.
Origin and Evolution Of mammals
First appearance of mammals was in the form of Synapsids in the late Carboniferous period. Synapsids started looking like mammals in the mid-Triassic period. Initial mammal –like – reptiles were really small with a few exceptions for e.g. Dicynodots (Lystrosaurus) were cow like in shape, Gorgonopsians were carnivorous and fanged in shape, Tritylodonts were tiny insectivorous mammals. By the middle of Triassic period one of the Synapsid group evolved into the very first mammals.
Mammalian forms are identified by some features which are shared by the mammals of today. This includes features that fossilize and the ones which don’t. Examples of features that don’t fossilize include: Warm blooded, covered with fur, sweat glands, and mammary glands. Features that do fossilizes to help us to identify the mammalian like creatures include: Only two sets of teeth, specialized teeth and development of crown like incisors, canines, molars and premolars, lower jaw made up of single bone and comprising of dentary and respiration using diaphragm(Encyclopedia Britannica, 2014).
Some major groups of mammals in Jurassic and Cretaceous period identified are:
Prototheria: Also known as Australosphenida. They were the earliest from of Monotromes from early Cretaceous period. Unique features include stumpy legs and tail, sprawling posture and reproduction by laying eggs. They were not a dominant group but moderately present in specific areas of the planet.
Eutriconodonta: Mode of reproduction is still unknown for this genera. Size ranged from tiny to opossum to badger sized. They were common in Jurassic and cretaceous period and were omnivores and carnivores. This genera got extinct after the cretaceous period.
Metatheria: One of the common genera of Mesozoic period, some mammals were really small in size but some were badger sized and represented the largest mammals of that time. Oldest fossil dates back to early cretaceous for Metatheria.