Monotreme Sensory Systems
Monotremes are a unique group of mammals confined to Australia and New Guinea. The lineage leading to modern monotremes probably diverged from that leading to therians (modern placental and marsupial mammals) as much as 220 million years ago (Yu et al. 2012), so there has been abundant time for independent evolution of sensory systems and the brain. Modern monotremes cannot be considered primitive or relict, and they are certainly not models for the ancestors of placental mammals. Monotremes have undergone just as long a period of evolution as have therians. Furthermore, although they do exhibit some primitive characteristics, living monotremes are also highly specialized animals with some remarkably advanced neural adaptations (e.g., electroreception and isocortical expansion). Like all mammals (including humans), they are therefore the result of mosaic evolution, where primitive and derived...
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