A group of extinct mammals from the Mesozoic Era that had unique teeth and skeletal features distinct from other mammalian groups.
From Greek 'allo-' (other) + 'theria' (beasts/mammals). The term was coined by paleontologists to classify these 'different beasts' whose characteristics didn't fit standard mammalian categories. It reflects 19th-century fossil classification when scientists discovered creatures that challenged existing taxonomic frameworks.
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