History
Within the Saxifragales is a suprafamilial group known as the Saxifragaceae alliance. It comprises four families: Pterostemonaceae, Iteaceae, Grossulariaceae, and Saxifragaceae. These have long been known to be related to each other, but the circumscription of Saxifragaceae has changed dramatically. It is now a much smaller family than it had been. Crassulaceae and Tetracarpaeaceae have long been associated with Saxifragaceae. Penthorum has usually been associated with Crassulaceae, but sometimes with Saxifragaceae.
Two members of the core Saxifragales had sometimes been placed near Saxifragaceae, but usually elsewhere. Aphanopetalum was often placed in Cunoniaceae, a family in Oxalidales, even though there were good reasons to put it in Saxifragales. Aphanopetalum is now excluded from Cunoniaceae. Haloragaceae was often thought to be a family in Myrtales, but it is no longer included in that order.
Cercidiphyllaceae had for a long time been associated with Hamamelidaceae and Trochodendraceae and was often thought to be closer to the latter. Cercidiphyllaceae is now known to be a member of the woody clade of Saxifragales, along with Hamamelidaceae, Altingiaceae, and Daphniphyllaceae, but Trochodendraceae is in the basal eudicot order Trochodendrales. Altingiaceae was usually not separated from Hamamelidaceae until phylogenetic studies showed that its inclusion might make Hamamelidaceae paraphyletic. The recognition of Altingiaceae as a separate family received strong statistical support in 2008.
Daphniphyllum was always thought to have an anomalous combination of characters and it was placed in several different orders before molecular phylogenetic analysis showed it to belong to Saxifragales.
Paeoniaceae possesses many unique features and its taxonomic position was for a long time controversial. The idea has long persisted that Paeonia belongs in Ranunculales, close to Glaucidium. Paeoniaceae has been shown unequivocally to belong in Saxifragales, while Glaucidium is in the family Ranunculaceae.
The family Peridiscaceae underwent radical shifting and recircumscription from 2003 to 2009. Originally, it consisted of two closely related genera, Peridiscus and Whittonia. The APG II system placed the family in Malpighiales, based on a DNA sequence for the rbcL gene from Whittonia. This sequence turned out to be not from Whittonia, but from other plants whose DNA had contaminated the sample. After Peridiscaceae was finally placed in Saxifragales, it was expanded to include Soyauxia in 2007, and expanded again to include Medusandra in 2009.
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