Evolution of Mammals - Triassic Takeover

Triassic Takeover

The catastrophic Permian-Triassic mass extinction slightly more than 250 million years ago killed off about 70 percent of terrestrial vertebrate species and the majority of land plants.

As a result, ecosystems and food chains collapsed, and the establishment of new stable ecosystems took about 30 million years. With the disappearance of the gorgonopsians, who were dominant predators in the late Permian, the way was open for the cynodonts to compete with another previously obscure group, the archosaurs, for dominance of the carnivorous niches. The archosaurs include the ancestors of crocodilians, dinosaurs and birds.

The archosaurs quickly became the dominant carnivores, a development often called the "Triassic takeover." Their success may have been due to the fact that the early Triassic was predominantly arid and therefore archosaurs' superior water conservation gave them a decisive advantage. All known archosaurs have glandless skins and eliminate nitrogenous waste in a uric acid paste containing little water, while the cynodonts probably excreted most such waste in a solution of urea, as mammals do today; considerable water is required to keep urea dissolved.

But the Triassic takeover may have been a vital factor in the evolution of the mammals. Two groups stemming from the early cynodonts were successful in niches with minimal competition from the archosaurs: the tritylodonts, who were herbivores, and the mammals, most of whom were small nocturnal insectivores although some, like Sinoconodon, were carnivores that fed on vertebrate prey, while still others were herbivores or omnivores. As a result:

• The therapsid trend towards differentiated teeth with precise occlusion accelerated, because of the need to hold captured arthropods and crush their exoskeletons.
• As the body length of the mammals' ancestors fell below 50 mm (2 inches), advances in thermal insulation and temperature regulation may have become necessary for nocturnal life.
• Acute senses of hearing and smell became vital.
  • This accelerated the development of the mammalian middle ear.
  • The increase in the size of the olfactory lobes of the brain increased brain weight as a percentage of total body weight. Brain tissue requires a disproportionate amount of energy. The need for more food to support the enlarged brains increased the pressures for improvements in insulation, temperature regulation and feeding.
• Probably as a side-effect of the nocturnal life, mammals lost two of the four cone opsins, photoreceptors in the retina, present in the eyes of the earliest amniotes. Paradoxically, this may have improved their ability to discriminate colors in dim light.