The rosids are members of a large monophyletic clade of flowering plants, containing about 70,000 species, more than a quarter of all angiosperms. The clade is divided into 16 to 20 orders, depending upon circumscription and classification. These orders, in turn, together comprise about 140 families. The rosids and the asterids are by far the largest clades in the eudicots.
Fossil rosids are known from the Cretaceous period. Molecular clock estimates indicate that the rosids originated in the Aptian or Albian stages of the Cretaceous, between 125 and 99.6 million years ago.
The name "rosids" is based upon the name "Rosidae", which had usually been understood to be a subclass. In 1967, Armen Takhtajan showed that the correct basis for the name "Rosidae" is a description of a group of plants published in 1830 by Friedrich Gottlieb Bartling. This clade was later renamed "Rosidae" and has been variously delimited by different authors. The name "rosids" is informal, and not assumed to have any particular taxonomic rank like the names authorized by the ICBN. The rosids are monophyletic based upon evidence found by molecular phylogenetic analysis.
Three different definitions of the rosids are currently in use. Some authors include the orders Saxifragales and Vitales in the rosids. Others exclude both of these orders. The circumscription used in this article is that of the APG II classification, which includes Vitales, but excludes Saxifragales.
The rosids and Saxifragales form a clade. This is one of six groups that compose the Pentapetalae (core eudicots minus Gunnerales), the others being Berberidopsidales, Caryophyllales, Dilleniales, Santalales, and the asterids. Almost nothing is known about the relationships between these groups.
The rosids consist of two groups: the order Vitales and the eurosids (true rosids). The eurosids, in turn are divided into seven groups: Fabidae, Geraniales, Myrtales, Crossosomatales, Picramniales, Malvidae, and the unplaced family Apodanthaceae. The Fabidae are often called the fabids, or eurosids I. Likewise, the Malvidae are often called the malvids, or eurosids II.
The rosids consist of 17 orders and 2 families that are placed incertae sedis (not in any order). In addition to Vitales, Rosales, Geraniales, Myrtales, Crossosomatales, and Picramniales, there are 8 orders in Fabidae and 4 orders in Malvidae. In 2009, Hengchang Wang and co-authors proposed that Malvidae be expanded to include Geraniales, Myrtales, Crossosomatales, and Picramniales. This larger circumscription of Malvidae received strong statistical support (100% bootstrap percentage) in their analysis. Some of the orders have only recently been recognized. These are Vitales, Zygophyllales, Crossosomatales, Picramniales, and Huerteales.
Apodanthaceae is an enigmatic family of achlorophyllous parasites. They have been provisionally placed in Cucurbitales by some, but their affinities remain obscure. The chloroplast genes that have been used to infer plant phylogeny do not provide much phylogenetic information for plants that lack chlorophyll, because in this case, these genes are nonfunctional pseudogenes.
The family Huaceae is a member of the COM (Celastrales, Oxalidales, Malpighiales) clade of Fabidae. The question about Huaceae is whether it should be included in one of the COM orders or in an order by itself as a 4th member of the COM clade. Two studies have indicated that it should be placed in Oxalidales, while one has indicated that it should not.
The phylogeny shown below is adapted from Wang and co-authors (2009), with order names from the Angiosperm Phylogeny Website. Branches with less than 50% bootstrapping support are collapsed.[clarification needed] Other branches have 100% bootstrap support except where shown.[clarification needed]
The nitrogen-fixing clade contains a high number of actinorhizal plants (which have root nodules containing nitrogen fixing bacteria, helping the plant grow in poor soils). Not all plants in this clade are actinorhizal, however.
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