Abrophyllaceae Nakai, 1943 accepted as Rousseaceae de Candolle, 1839 (extant flag source)
Acanthocollidae Schewiakoff, 1926 (extant flag source)
Acanthodrilidae Claus, 1990 (extant flag source)
Acanthoplegmidae Schewiakoff, 1926 (extant flag source)
Acanthothirididae † (extant flag source)
Acerentomidae Silvestri, 1907 (extant flag source)
Achromadoridae Gerlach & Riemann, 1973 (extant flag source)
Achromobacteriaceae accepted as Alcaligenaceae De Ley et al., 1986 (extant flag source)
Acidithiobacillaceae Garrity et al., 2005 (extant flag source)
Acidobacteriaceae Thrash & Coates, 2012 (extant flag source)
Acoelodidae Ameghino, 1901 † accepted as Notoungulata † (extant flag source)
Acrolepidae Aldinger, 1937 † accepted as Acrolepididae Aldinger, 1937 † (extant flag source)
Acropisthiidae Foissner & Foissner, 1988 (extant flag source)
Actinichonidae Jankowksi, 1973 (extant flag source)
Adapisoriculidae Van Valen, 1967 † (extant flag source)
Adiheterothripidae Shumsher, 1946 accepted as Stenurothripidae Bagnall, 1923 (extant flag source)
Aeginellidae Leach, 1814 accepted as Caprellidae Leach, 1814 (extant flag source)
Aegiridae P. Fischer, 1883 (extant flag source)
Aerococcaceae Ludwig et al., 2010 (extant flag source)
Aetosauridae Baur, 1887 † (extant flag source)
Afrotarsiidae Ginsburg & Mein, 1987 † (extant flag source)
Agaricostilbaceae (extant flag source)
Agdestidaceae Nakai, 1942 accepted as Sarcobataceae Behnke, 1997 (extant flag source)
Agnopteridae Lambrecht, 1933 † (extant flag source)
Agriidae (extant flag source)
Agrionidae (extant flag source)
Alagomyidae Dashzeveg, 1990 † (extant flag source)
Alatosporidae Schulman, Kovaleva & Dubina, 1979 accepted as Alatasporidae Shulman, Kovaleva & Dubina, 1979 (extant flag source)
Albionbaataridae † (extant flag source)
Alectoriaceae accepted as Parmeliaceae Zenker, 1827 (extant flag source)
Alicyclobacillaceae da Costa & Rainey, 2010 emend. Klenk et al., 2011 (extant flag source)
Alliaceae Borkhausen, 1797 accepted as Amaryllidaceae J. Saint-Hilaire, 1805 (extant flag source)
Allomyidae Marsh, 1877 † (extant flag source)
Aloaceae Batsch, 1802 accepted as Asphodelaceae de Jussieu, 1789 (extant flag source)
Alstroemeriaceae Dumortier, 1829 (extant flag source)
Alvinellidae Desbruyères & Laubier, 1986 (extant flag source)
Alycaeidae Blanford, 1864 (extant flag source)
Alzateaceae S.A. Graham, 1985 (extant flag source)
Amaryllidaceae J. Saint-Hilaire, 1805 (extant flag source)
Amesopodidae Stebbing, 1905 accepted as Arcturidae Dana, 1849 (extant flag source)
Ammatoideaceae accepted as Homoeotrichaceae Elenkin (basis of record)
Ammatoideaceae accepted as Homoeotrichaceae Elenkin (extant flag source)
Amoebidiidae Lichtenstein, 1917 ex Kirk et al., 2001 (extant flag source)
Amorphoscelididae accepted as Amorphoscelidae Stål, 1877 (extant flag source)
Amphilithidae Haeckel, 1887 (extant flag source)
Amphimonadidae (extant flag source)
Amphiplatysporidae (extant flag source)
Amphiporidae Rukhin, 1938 † (extant flag source)
Amphotrombiidae Zhang, 1998 (extant flag source)
Amylovoracidae (extant flag source)
Amynodontidae Scott & Osborn, 1883 † (extant flag source)
Anarthriaceae D.F. Cutler & Airy Shaw, 1965 accepted as Restionaceae R. Brown, 1810 (extant flag source)
Anchariidae Glaw & Vences, 1994 (extant flag source)
Anchilophidae McKenna & Bell, 1997 † accepted as Palaeotheriidae Bonaparte, 1850 † (extant flag source)
Ancylostomidae accepted as Ancylostomatidae Looss, 1905 (extant flag source)
Andrassyidae Tchesunov & Gagarin, 1999 (extant flag source)
Aneuridae (extant flag source)
Angeiocystidae Léger, 1911 (extant flag source)
Anhangueridae Campos & Kellner, 1985 † (extant flag source)
Anisolabididae (extant flag source)
Anomalocaridae † accepted as Anomalocarididae Raymond, 1935 † (extant flag source)
Anoplodactylidae † accepted as Phoxichilidiidae Sars, 1891 (extant flag source)
Anoplogasteridae accepted as Anoplogastridae Gill, 1893 (extant flag source)
Anopsiidae Pruvot-Fol, 1922 accepted as Hydromylidae Pruvot-Fol, 1942 (extant flag source)
Anthericaceae J.G. Agardh, 1858 accepted as Asparagaceae de Jussieu, 1789 (extant flag source)
Antoniaceae Hutchinson, 1959 accepted as Loganiaceae R. Brown ex Martius, 1827 (extant flag source)
Anziaceae accepted as Parmeliaceae Zenker, 1827 (extant flag source)
Aphloiaceae Takhtajan, 1985 (extant flag source)
Aphyllanthaceae Burnett, 1835 accepted as Asparagaceae de Jussieu, 1789 (extant flag source)
Aplocheilichthyidae Myers, 1928 accepted as Poeciliidae Bonaparte, 1831 (extant flag source)
Aplysinellidae Bergquist, 1980 (extant flag source)
Apodanthaceae van Tieghem ex Takhtajan, 1987 (extant flag source)
Apsiktratidae (extant flag source)
Aptandraceae Miers, 1853 accepted as Olacaceae de Jussieu ex R. Brown, 1818 (extant flag source)
Apusomonadidae Karpov & Mylnikov, 1989 (extant flag source)
Arachnactidae McMurrich, 1910 (extant flag source)
Aralidiaceae Philipson & B.C. Stone, 1980 accepted as Torricelliaceae Hu, 1934 (extant flag source)
Archaeolemuridae Grandidier, 1905 † (extant flag source)
Archaeonycteridae † (extant flag source)
Archaeopterygidae Huxley, 1872 † (extant flag source)
Archinomidae accepted as Amphinomidae Savigny in Lamarck, 1818 (extant flag source)
Archirhizidae (extant flag source)
Argophyllaceae Takhtajan, 1987 (extant flag source)
Arguimuridae † (extant flag source)
Arguitheriidae † (extant flag source)
Ariciidae accepted as Orbiniidae Hartman, 1942 (extant flag source)
Arkhangelskiellaceae Bukry, 1969 † (basis of record)
Armilliferidae (extant flag source)
Arthessidae C.R. Boettger, 1963 accepted as Volvatellidae Pilsbry, 1895 (extant flag source)
Ascomyzontidae Thorell, 1859 accepted as Asterocheridae Giesbrecht, 1899 (extant flag source)
Ascoporiaceae Kutorga & D. Hawksworth, 1997 (extant flag source)
Asiatheriidae † (extant flag source)
Asparagaceae de Jussieu, 1789 (extant flag source)
Asperococcaceae accepted as Chordariaceae Greville, 1830 (basis of record)
Asperococcaceae accepted as Chordariaceae Greville, 1830 (extant flag source)
Asperspinidae Rankin, 1979 (extant flag source)
Asphodelaceae de Jussieu, 1789 (extant flag source)
Asteliaceae Dumortier, 1829 (extant flag source)
Asteranthaceae R. Knuth, 1939 accepted as Lecythidaceae A. Richard, 1825 (extant flag source)
Asteropeiaceae Takhtajan ex Reveal & Hoogland, 1990 (extant flag source)