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Original function altered or absent (=non-functional)
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Vestiges: Cormorant wing-drying & dog circling behaviors; mammalian dive reflex; dewclaw; cave animal eyes; coccyx; vermiform appendix; piloerection
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Circulatory shunts including fossa ovalis & ligamentum arteriosum; male nipples; inguinal canal & looped vas deferens from descent of testes
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Function, if any, continues, yet reveal past states of evolution (via phyletic analysis) or development
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Primate fingernails & stereoscopic vision as synapomorphies (shared, derived characters); cephalization with terminal rather than central head
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Cranial sutures & epiphyseal joints; umbilicus (navel) & other scars
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Function lost or shifted, yet compound clues reveal joint evo-devo history within lineage
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Atavisms: Sudden reversion to retention of tail in humans or of limbs in snakes or cetaceans; bird teeth; Exaptations: fish lungs before swim bladders; feathers for insulation before flight
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Homologies: Pharyngeal glands (e.g., tonsils) & chordate gill arches; male & female genitalia
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May be altered or non-functional; development in divergent lineages reveals clues to history/homology (in common origins & developmental processes)
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Phylogenetic (not merely phyletic) processes: mammalian ear ossicles as derived reptilian jaw bones; mammary glands as derivative sweat glands; perhaps sexual dimorphism in size and structures as conserved features
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