Saxifragales is a relatively small
angiosperm order, having only 15
families, about 100
genera and about 2,470
species.
History 's description of Saxifragales 1853|alt=First page of Lindley's Saxifragales from 1853 Saxifragales was first described in 1820 by
Berchtold and Presl as a group of plants, Saxifrageae, with five genera, including
Saxifraga, lending their names as the
botanical authority (Bercht. & J.Presl). At times, that authority has also been given to
Dumortier, due to a later publication (1829). Dumortier first used the word Saxifragaceae. By the time of
John Lindley's
The Vegetable Kingdom (1853), the term Saxifragales was in use, which Lindley called an Alliance, containing five families. Later, the Saxifragales were placed in the
angiosperm class Dicotyledons, also called
Magnoliopsida.
Phylogeny The
order Saxifragales has undergone considerable revision in both placement and composition, since the use of
molecular phylogenetics, and the use of the modern
Angiosperm Phylogeny Group (APG) classification. They are identified as a strongly
monophyletic group. In the initial APG publication (1998), the Saxifragales were identified within the
core eudicots clade but its relationship to other clades was uncertain. The core eudicots consist of the order
Gunnerales and a large clade of
Pentapetalae (so named for having a
synapomorphy of
pentamerous (5 part) perianths), the latter representing about 70% of all angiosperms, with eight major lineages. Later (2003), the order was described as "one of the major surprises of molecular phylogenetic analyses of the angiosperms", having elements previously placed in three or four separate subclasses based on morphology. This was eventually resolved in the third APG system (2009), placing Saxifragales as a
sister group to the
rosids (Rosidae), within the Pentapetalae clade. This large combination has subsequently been given the name
superrosids (Superrosidae), representing part of an early diversification of the
angiosperms. Among the rosids, they share a number of similarities with the
Rosales, particularly
Rosaceae, including a hypanthium, five part flowers and free floral parts. As circumscribed, Saxifragales account for 1.3% of eudicot diversity. }}
Biogeography and evolution Diversification among Saxifragales was rapid, with the extensive
fossil record indicating that the order was more
diverse and more
widespread than an examination of the
extant members suggests, with considerable
phenotypic diversity occurring early. The earliest fossil evidence is found in the
Turonian-
Campanian (late Cretaceous), suggesting a minimum age of 89.5
Myr. However, molecular
divergence time estimation suggest an earlier time of 102–108 Myr, into the early Cretaceous, for the
crown and
stem groups respectively. Within the order Saxifragales, the molecular data imply a very rapid initial diversification time of about 6–8 Myr, between 112 and 120 Myr, with major lineages appearing within 3–6 Myr. The ancestral state appears to be woody, as in Peridiscaceae and the woody clade, but is also ancestral to Grossulariaceae. A number of independent transitions to a herbaceous habit occurred in the ancestors of Crassulaceae, Saxifragaceae and the base of the Haloragaceae-Penthoraceae clade (the other two families in Haloragaceae
s.l. remaining woody), while other taxa reverted to a woody habit, especially Crassulaceae. Most of Saxifragales have a superior
ovary, but some families show frequent transition with inferior or subinferior position, particularly Saxifragaceae and to a lesser extent Hamamelidaceae. Almost all Grossulariaceae have an inferior ovary. The ancestral
carpel number is two, with transition to higher numbers, such as four in Haloragaceae
s.l. and Peridiscaceae with five in Penthoraceae. The ancestral carpel number for Crassulaceae is five, decreasing to four in
Kalanchoe, where it is synapomorphic for the genus, though the most frequent transition in this family is 6–10, but only where
stamen number is increased above five. Some
Macaronesian taxa (Aeonieae) have 8–12, with up to 32 carpels for
Aeonium. The ancestral petal number is five, with three major transitions; 5 to 0, 5 to 4, 5 to 6–10. Increased petal number is seen in Paeoniaceae and Crassulaceae, particularly where stamen number is also increased. Cercidiphyllum + Daphniphyllum, Chrysosplenium and
Altingia are examples of the complete loss of petals. The ancestral stamen:petal ratio is 1, with transitions characterising several clades, e.g. Paeonicaceae+woody clade >2, Crassulaceae 2 (but
Crassula 1). Overall there has been a decrease over evolution, but independent of a decrease in petal number, so that it is the stamen number that has decreased. The ancestral habitat appears to be forests, followed by early diversification into desert and aquatic habitats, with shrubland the most recent colonization. Species diversification was rapid following a transition from a warmer, wetter Earth in the
Eocene (56–40 Myr) to early
Miocene (23–16 Myr), to the cooler drier conditions of the mid-Miocene (16–12 Myr). However, this appears to not have coincided with ecological and phenotypic evolution, which are themselves correlated. There is a clear lag, whereby increase in species diversification was followed later by increases in niche and phenotypic lability.
Subdivision The first APG classification (1998) placed 13 families within the order Saxifragales: •
Altingiaceae •
Cercidiphyllaceae •
Crassulaceae •
Daphniphyllaceae •
Grossulariaceae •
Haloragaceae •
Hamamelidaceae •
Iteaceae •
Paeoniaceae •
Penthoraceae •
Pterostemonaceae •
Saxifragaceae •
Tetracarpaeaceae This was subsequently revised to 15, in the fourth version (2016). The Saxifragales families have been grouped into a number of informally named suprafamilial subclades, with the exception of the
basal split of Peridiscaceae, which thus forms a
sister group with the rest of Saxifragales. The two major ones are (Paeoniaceae + the woody clade of primarily woody families) and the "core" Saxifragales (i.e. the primarily herbaceous families), with the latter subdivided into two further subclades, (Haloragaceae
sensu lato + Crassulaceae) and the Saxifragaceae alliance. In the clade Haloragaceae
sensu lato (s.l.) + Crassulaceae the genera constituting Haloragaceae
s.l. are all small, and APG II (2003) proposed merging them into a single larger Haloragaceae
s.l., but transferred
Aphanopetalum from
Cunoniaceae to this group. The Saxifragaceae alliance represents Saxifragaceae together with a number of woody members of the traditional Saxifragaceae
sensu Engler (1930). Within this, APG II (2003) proposed placing the two species of
Pterostemon that constitute Pterostemonaceae within
Iteaceae, and all subsequent versions have maintained this practice. Thus Saxifragales
sensu APG II consisted of only 10 families. The third version (2009) added
Peridiscaceae (from
Malpighiales), as sister to all other families, but re-expanded Haloragaceae to provide for a narrower circumscription, Haloragaceae
sensu stricto (
s.s.), to give a total of 14 families. APG IV (2016) added the parasitic family Cynomoriaceae to provide a total of 15 families, although its placement within the order remained unclear. Of the 15 families included in APG IV, the basal divergence 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 placement in Saxifragales, it was expanded to include
Soyauxia in 2007, and
Medusandra in 2009. In the first of the subclades of the remaining Saxifragales, Paeoniaceae possesses many
unique features and its taxonomic position was controversial for a long time, and
Paeonia was placed in
Ranunculales, close to
Glaucidium, prior to transfer to Saxifragales as sister to the woody clade. In the woody clade, the genus
Liquidambar was included in Hamamelidaceae until
molecular phylogenetic studies showed that its inclusion might make Hamamelidaceae
paraphyletic, and was segregated as a separate monotypic family, Altingiaceae in 2008. Cercidiphyllaceae was for a long time associated with Hamamelidaceae and
Trochodendraceae and was often thought to be closer to the latter, which is now in the basal
eudicot order
Trochodendrales.
Daphniphyllum was always thought to have an anomalous combination of characters and was placed in several different orders before molecular phylogenetic analysis showed it to belong to Saxifragales. In the core Saxifragales, Crassulaceae and Tetracarpaeaceae have been associated with Saxifragaceae, while
Penthorum has been associated both with Crassulaceae and Saxifragaceae, before being placed here.
Aphanopetalum was often placed in
Cunoniaceae, a family in
Oxalidales, even though there were good reasons to put it in Saxifragales, and it was subsequently transferred. Haloragaceae was included in
Myrtales, before being placed in Saxifragales. The other "core" group, the Saxifragaceae alliance comprises four families: Pterostemonaceae, Iteaceae, Grossulariaceae, and Saxifragaceae, which have long been known to be related to each other, but the
circumscription of Saxifragaceae has been much reduced and Pterostemonaceae submerged as
Pterostemon in Iteaceae. Most of the families are
monogeneric.
Choristylis is now considered a synonym of
Itea, but the addition of
Pterostemon, gives Iteaceae two genera.
Liquidambar and
Semiliquidambar are also submerged into
Altingia, making Altingiaceae monogeneric. About 95% of the species are in five families:
Crassulaceae (1400),
Saxifragaceae (500),
Grossulariaceae (150–200),
Haloragaceae (150), and
Hamamelidaceae (100). The relationships of the Saxifragales families to each other is shown in the following
cladogram. The
phylogeny in this cladogram still has some uncertainty as to the exact relationships, and the phylogenetic tree is subject to further revision.
Cynomoriaceae, previously placed in
Santales or
Rosales is included in Saxifragales, but unplaced within it. Li et al. (2019) have slightly different relationships, and also place Cynomoriaceae as the first branch in the Crassulaceae+Haloragaceae
s.l. tree, i.e. as sister to those two families. The number of genera in each family is shown in parentheses: }}
Families == Distribution and habitat ==