Acanthaceae de Jussieu, 1789 (extant flag source)
Acanthocephala (awaiting allocation) (extant flag source)
Acarnidae Dendy, 1922 (extant flag source)
Acetabulariaceae Nägeli, 1847 accepted as Polyphysaceae Kützing, 1843 (extant flag source)
Acidostomatidae Stoddart & Lowry, 2012 (extant flag source)
Acidostomatidae Stoddart & Lowry, 2012 (habitat flag source)
Acrolophidae Fracker, 1915 accepted as Tineidae Latreille, 1810 (extant flag source)
Acrostichaceae Mettenius ex A.B. Frank, 1874 † accepted as Pteridaceae E.D.M. Kirchner, 1831 (extant flag source)
Acteonidae d'Orbigny, 1835 (habitat flag source)
Actinastridae (extant flag source)
Actinellida (awaiting allocation) (extant flag source)
Actiniaria (awaiting allocation) (extant flag source)
Actiniopteridaceae Pichi Sermolli, 1962 accepted as Pteridaceae E.D.M. Kirchner, 1831 (extant flag source)
Actinophycaceae † (extant flag source)
Adamystidae Cunliffe, 1957 (extant flag source)
Adelphacaridae Grandjean, 1954 (extant flag source)
Adeorbidae Monterosato, 1884 accepted as Tornidae Sacco, 1896 (extant flag source)
Aextoxicaceae Engler & Gilg, 1920 (extant flag source)
Agamaceae † (extant flag source)
Agaricales incertae sedis (extant flag source)
Ailuropodidae Grevé, 1894 accepted as Ursidae Gray, 1825 (extant flag source)
Alcyonaria (awaiting allocation) (extant flag source)
Allocreadiidae Looss, 1902 (habitat flag source)
Altingiaceae Horaninow, 1841 (extant flag source)
Alucitidae Leach, 1815 (extant flag source)
Alysidiidae Levinsen, 1909 (extant flag source)
Amathinidae Ponder, 1987 (extant flag source)
Amblypteridae Romer, 1945 † (habitat flag source)
Amblystegiaceae (extant flag source)
Ammotheidae Dohrn, 1881 (extant flag source)
Amphibolidae Hubendick, 1945 (habitat flag source)
Amphicynodontidae Simpson, 1945 † (habitat flag source)
Amphinectidae Forster & Wilton, 1973 accepted as Desidae Pocock, 1895 (extant flag source)
Amphisoleniaceae (habitat flag source)
Amphitholaceae accepted as Amphilothaceae (habitat flag source)
Anabaenaceae accepted as Nostocaceae Eichler, 1886 (extant flag source)
Anachoropteridaceae † accepted as Psalixochlaenaceae † (extant flag source)
Analgidae Trouessart & Mégnin, 1884 (extant flag source)
Anaspididae accepted as Anaspidesidae Ahyong & Alonso-Zarazaga, 2017 (extant flag source)
Anatomidae McLean, 1989 (extant flag source)
Anaulaceae (Schütt) Lemmermann, 1899 (extant flag source)
Ancorabolidae G.O. Sars, 1909 (habitat flag source)
Angiopteridaceae Fée ex J. Bommer, 1867 (extant flag source)
Anguillavidae Hay, 1903 † (habitat flag source)
Animalia (awaiting allocation) (habitat flag source)
Annectocymidae Hayward & Ryland, 1985 (extant flag source)
Anniellidae (extant flag source)
Anoplocephalidae Cholodkowsky, 1902 (habitat flag source)
Anthemiphylliidae Vaughan, 1907 (extant flag source)
Anthocerotopsida (awaiting allocation) (extant flag source)
Anthomedusae (awaiting allocation) (extant flag source)
Anthozoa (awaiting allocation) (extant flag source)
Antipathidae Ehrenberg, 1834 (extant flag source)
Antipatidae accepted as Antipathidae Ehrenberg, 1834 (extant flag source)
Anura (awaiting allocation) (extant flag source)
Apatopygidae Kier, 1962 (extant flag source)
Apiales (awaiting allocation) (extant flag source)
Apioceridae Bigot, 1857 (extant flag source)
Apionacaridae Gaud & Atyeo, 1977 (extant flag source)
Aporcelaimidae Heyns, 1965 (extant flag source)
Apseudidae Leach, 1814 (extant flag source)
Arachnacthidae McMurrich, 1910 accepted as Arachnactidae McMurrich, 1910 (extant flag source)
Arachnoidiscaceae accepted as Stictodiscaceae (extant flag source)
Archaeosigillariaceae † (extant flag source)
Ariophantidae (extant flag source)
Arnoldiellaceae accepted as Cladophoraceae Wille, 1884 (extant flag source)
Artocarpaceae von Berchtold & J. Presl, 1820 † accepted as Moraceae Gaudichaud-Beaupré, 1835 (extant flag source)
Ascomycetes incertae sedis (extant flag source)
Ascomycota incertae sedis (extant flag source)
Asparagales (awaiting allocation) (extant flag source)
Asperifoliaceae † accepted as Boraginaceae de Jussieu, 1789 (extant flag source)
Aspidiaceae G.H. Mettenius ex A.B. Frank in Leunis, 1877 accepted as Dryopteridaceae Herter, 1949 (extant flag source)
Aspleniaceae Newman, 1840 (extant flag source)
Assimineidae H. Adams & A. Adams, 1856 (habitat flag source)
Asterales (awaiting allocation) (extant flag source)
Asterinaceae C.G. Hansford, 1946 (extant flag source)
Astrolithiidae accepted as Astrolithidae Haeckel, 1887 (extant flag source)
Astrophorida (awaiting allocation) (extant flag source)
Asymmetronidae accepted as Branchiostomatidae Bonaparte, 1846 (extant flag source)
Asymmetrontidae accepted as Branchiostomatidae Bonaparte, 1846 (extant flag source)
Atherinopsidae Fitzinger, 1873 (extant flag source)
Athyriaceae Alston, 1956 (extant flag source)
Atopocrinidae Messing in Hess & Messing, 2011 † (extant flag source)
Aturidae Thor, 1900 (extant flag source)
Audycoptidae Lavoipierre, 1964 (extant flag source)
Aulacodiscaceae (Schütt) Lemmermann, 1903 (extant flag source)
Aulacothyropsidae (extant flag source)
Aulocystidae Sollas, 1887 (extant flag source)
Aurantiaceae de Jussieu, 1789 † accepted as Rutaceae de Jussieu, 1789 (extant flag source)
Austrodecidae Stock, 1954 (extant flag source)
Autophyllitaceae † (extant flag source)
Azoricidae Sollas, 1888 (extant flag source)
Bacillariaceae Ehrenberg, 1831 (extant flag source)
Baicaliidae P. Fischer, 1885 accepted as Amnicolidae Tryon, 1863 (extant flag source)
Bairdiidae Sars, 1865 (extant flag source)
Balanidae Leach, 1806 (extant flag source)
Balanitaceae M. Roemer, 1846 accepted as Zygophyllaceae R. Brown, 1814 (extant flag source)
Balantiopsidaceae (extant flag source)
Bankeraceae (extant flag source)
Barrandeinaceae † accepted as Blasariaceae † (extant flag source)