Keratinous plates used in filtering food in living mysticetes. Its presence in a fossil may be inferred from associated anatomy but must not be confused with direct preservation of the plates. [ref]
Morphological and molecular evidence for a stepwise evolutionary transition from teeth to baleen in mysticete whales by Thomas A. Deméré and others. Systematic Biology 57, pp. 15–37; palatal foramina, tooth characters and enamel-gene pseudogenes
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