When the APG II system of plant classification was published in April 2003, fifteen genera and three families were placed incertae sedis in the angiosperms, and were listed in a section of the appendix entitled "Taxa of uncertain position".
By the end of 2009, molecular phylogenetic analysis of DNA sequences had revealed the relationships of most of these taxa, and all but three of them had been placed in some group within the angiosperms. In October 2009, APG II was superseded by the APG III system. In APG III, 11 of the genera listed above were placed in families, or else became families whose position within their orders was approximately or exactly known. The family Rafflesiaceae was placed in the order Malpighiales, close to Euphorbiaceae and possibly within it. Mitrastema became a monotypic family, Mitrastemonaceae. This family and Balanophoraceae were placed incertae sedis into orders, that is, their positions within these orders remained completely unknown. Metteniusa was found to belong to a supraordinal group known as the lamiids, which has not been satisfactorily divided into orders. Cynomorium was raised to familial status as Cynomoriaceae, and along with Apodanthaceae and Gumillea, remained unplaced in APG III. Five taxa were unplaced among the angiosperms in APG III because Nicobariodendron and Petenaea were added to the list.
Leptaulus There is no apparent reason for the inclusion of Leptaulus in the list of unplaced taxa, other than the time lag between submission and publication. In 2001, in a phylogenetic study based on morphological and DNA data, Leptaulus was found to belong to a group of six genera that most authors now consider to be the family Cardiopteridaceae. This was confirmed in a study of wood anatomy in 2008. The genus is placed in the Cardiopteridaceae in the APG III system of 2009. Before 2001, Leptaulus and the rest of Cardiopteridaceae had usually been placed in a broadly circumscribed Icacinaceae, which turned out to be polyphyletic. Some botanists do not recognize Cardiopteridaceae as a family of six genera. Instead, they segregate Cardiopteris into a monogeneric Cardiopteridaceae sensu stricto and place the other five genera in the family Leptaulaceae. The monophyly of Leptaulaceae has never been tested with molecular data.
Pottingeria
It had long been thought, at least by some, that the small Southeast Asian tree Pottingeria might belong in the order Celastrales. In a phylogenetic study of that order in 2006, Pottingeria was found to be a member of the order, but not of any of its families. It was in an unresolved pentatomy consisting of Parnassiaceae, Pottingeria, Mortonia, the pair (Quetzalia + Zinowiewia), and the other genera of Celastraceae. When the APG III system was published in October 2009, the Angiosperm Phylogeny Group expanded Celastraceae to include all members of the pentatomy mentioned above.
Dipentodon Dipentodon has one species Dipentodon sinicus. It is native to southern China, Burma, and northern India. In 2009, in a molecular phylogenetic study of the order Huerteales, it was shown that Dipentodon and Perrottetia belong together as the two genera of the family Dipentodontaceae.
Medusandra and Soyauxia In 2009, in a molecular phylogenetic study of Malpighiales, Kenneth Wurdack and Charles Davis sampled five genera and one family that had been unplaced in APG II. They placed some of these for the first time and confirmed the previous placement of others with strong statistical support. In their outgroup, they included four genera from Saxifragales. These were Daphniphyllum, Medusandra, Soyauxia, and Peridiscus. In their phylogeny, Medusandra and Soyauxia formed a strongly supported clade with Peridiscus, a member of the family Peridiscaceae, the most basal clade in Saxifragales. Wurdack and Davis recommended that Medusandra and Soyauxia both be transferred to Peridiscaceae. Thus the monogeneric family Medusandraceae is subsumed into Peridiscaceae. Soyauxia had been found to be close to Peridiscus in another study two years before. Wurdack and Davis also found that the family Rafflesiaceae and the genera Aneulophus, Centroplacus, and Trichostephanus belong in the order Malpighiales.
Aneulophus Aneulophus consists of two species of woody plants from tropical West Africa. Wurdack and Davis found the traditional placement of Aneulophus in Erythroxylaceae to be correct. Its position within the family remains uncertain. Erythroxylaceae is a family of four genera. Erythroxylum has about 230 species. Nectaropetalum has eight species and Pinacopodium has two. No one has yet produced a molecular phylogeny of the family.
Centroplacus Centroplacus has a single species, Centroplacus glaucinus, a tree from West Africa. It was found to be close to Bhesa, a genus that had only recently been removed from Celastrales. Bhesa was grouped with Centroplacus to become the second genus in Centroplacaceae. Bhesa consists of five species of trees from India and Malesia.
Trichostephanus Trichostephanus has two species, both in tropical West Africa. It had usually been assigned to Achariaceae, but it was found to be deeply embedded in Samydaceae. Many taxonomists do not recognize Samydaceae as a separate family from Salicaceae.
Rafflesiaceae
Several genera have been removed from Rafflesiaceae, so that it now consists of only three genera: Sapria, Rhizanthes, and Rafflesia. All of these are holoparasites and, as discussed below, finding their relationships by molecular phylogenetics has presented special challenges. Rafflesia and its relatives were the subject of several papers from 2004 to 2009, and as the world's largest flower, Rafflesia has attracted special interest. In 2009, Wurdack and Davis confirmed earlier work in which it was found that Rafflesiaceae is nested within Euphorbiaceae sensu stricto, a circumscription of Euphorbiaceae that excludes Phyllanthaceae, Picrodendraceae, Putranjivaceae, Pandaceae, and a few other very small groups that had been included in it until the 1990s. In order to preserve Rafflesiaceae, Wurdack and Davis split Euphorbiaceae sensu stricto into Euphorbiaceae sensu strictissimo and Peraceae, a new family comprising Pera and four other genera.
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