A short survey of the archaic mammals will be informative to gain a clear idea regarding the ancestry of mammals. Mammals became well flourished at the beginning of Cenozoic era but have started their true career in the Mesozoic era. So the description of some Mesozoic and Cenozoic mammals will serve the purpose.
The mammal-like reptiles were quite abundant in Permian period. But at the early Triassic time, those forms became gradually extinct and ultimately they became almost entirely absent by the end of Triassic. But again in the Jurassic and subsequent periods fossil mammals became more abundant.
possessed some basic characteristics
- small size,
- sharp-pointed teeth,
- teeth were generally heterodont
- habits were varied.
The fossil forms were discovered in upper Triassicperiod. Dentition was more like that of reptile than mammalian. But the simple jaw consisting of a single bone established their mammalian affinity. The fossil records of this group arc rather incomplete.
They were very important group amongst the extinct mammals and their fossils were recorded in the upper Triassic rocks of Germany. The most important mammalian characteristic was the nature of the molar teeth which had two to three longitudinal rows of tubercles. The presence of these tubercles gave the name Aluitituberculata to this group.
These fossils were recorded in Jurassic period. The molars had three cusps arranged in a single longitudinal row.
This group was confined to the upper Jurassic period of England and North America. The molars had three main cusps arranged in a triangular fashion.
The symmetrodonta was distantly related to this group. The molars had four or more cusps. The three principal cusps assumed the form of a trigon. Their fossils were recorded throughout the Jurassic period.
Besides the archaic mammals, a few important Cenozoic mammals need mentioning in connection with the ancestry of mammals. They belong to three groups, Creodonta, Condylarthra and Amblypoda Creodonta had small reptile-like brain, carnivore-like teeth and terminal phalanges with claws.
Condylarthra represented the primitive ungulates and structurally formed a sort of connecting link between the hoofed and clawed mammals. The cranial portion was located behind the orbit. Teeth were suited for herbivorous habit. Molars were low crowned. Amblypoda was short footed and hoofed forms of almost elephantine size.
Fate of Archaic Mammals
With the close of reptilian age, the Mesozoic mammals became well flourished in the Mesozoic era and exhibited wide adaptive radiation. But very quickly they became extinct and were replaced by modernised mammals which were fully equipped to face all eventualities of nature.
The cause of extinction of archaic mammals is primarily due to their incapabilities to respond to new environmental conditions. The archaic mammals had a brief evolutionary career and they were subjected to racial extinction.
The challenges of life were received by modernised mammals who flourished well in time and space. Some of the archaic forms transmutated into higher types while some became emigrant.
Marsupials passed beyond the limits of South America and crossed the Antarctic land-bridge to come to Australia where they still persist.