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The Irano-Turanian floristic region spans a topographically complex and climatically continental territory, which has served as a source of xerophytic taxa for neighboring regions and is represented by a high percent of endemics. Yet, a comprehensive picture of the abiotic and biotic factors that have driven diversification within this biota remains to be established due to the scarcity of phylogenetic studies. Acantholimon is an important component of the subalpine steppe flora of the Irano-Turanian region, containing c. 200 cushion-forming subshrubby pungent-leaved species. Our recent molecular phylogenetic study has led to enlarging the circumscription of this genus to include eight mono- or oligospecific genera lacking the characteristic life-form and leaves. Using the same molecular phylogeny, here we investigate the tempo and mode of diversification as well as the biogeographic patterns in this genus, to test the hypothesis that a combination of key morphological innovations and abiotic factors is behind Acantholimon high species diversity. Molecular dating analysis indicates that Acantholimon s.l. started to diversify between the Late Miocene and the Pliocene and the biogeographic analysis points to an Eastern Iran–Afghanistan origin. Macroevolutionary models support the hypothesis that the high diversity of the genus is explained by accelerated diversification rates in two clades associated with the appearance of morphological key innovations such as a cushion life-form and pungent leaves; this would have favored the colonization of water-stressed, substrate-poor mountainous habitats along the newly uplifted IT mountains during the Mio-Pliocene. Given the apparent similarity of mountain habitats for most species of Acantholimon, we hypothesize that its current high species diversity responds to a scenario of nonadaptive radiation fueled by allopatric speciation rather than evolutionary radiation driven by ecological opportunity. Similar scenarios might under

Acantholimon riyatguelii is described as a new species. Diagnostic morphological characters, a full description and detailed illustrations are provided on the basis of the type specimen and wild exemplars. This outstanding new species is a strict gypsophyte, which is characterized by the dwarf caespitose perennial habit, not forming dense thorny cushions; leaves are homomorphic, hairy, short and quite soft, not pungent but mucronate, swollen-fleshy and calcareous-punctate; spikes are unbranched, very compact, distichous, longer than leaves; spikelets are numerous and quite regularly imbricate; and the calyx is 5-lobed, with whitish limb. These combination of morphological characters in not known in any other Turkish Acantholimon species, and allows easy identification. The main morphological traits of A. riyatguelii suggest inclusion in A. sect. Staticopsis, though in an isolated position for which a new subsection, A. subsect. Exacantha, is described. Morphological affinities and divergences to other members of that section are dicussed. Furthermore, data are reported on the conservation status of A. riyatguelii, which is suggested to be labelled as CR according to the IUCN categories.

Acantholimon riyatguelii is described as a new species. Diagnostic morphological characters, a full description and detailed illustrations are provided on the basis of the type specimen and wild exemplars. This outstanding new species is a strict gypsophyte, which is characterized by the dwarf caespitose perennial habit, not forming dense thorny cushions; leaves are homomorphic, hairy, short and quite soft, not pungent but mucronate, swollen-fleshy and calcareous-punctate; spikes are unbranched, very compact, distichous, longer than leaves; spikelets are numerous and quite regularly imbricate; and the calyx is 5-lobed, with whitish limb. These combination of morphological characters in not known in any other Turkish Acantholimon species, and allows easy identification. The main morphological traits of A. riyatguelii suggest inclusion in A. sect. Staticopsis, though in an isolated position for which a new subsection, A. subsect. Exacantha, is described. Morphological affinities and divergences to other members of that section are dicussed. Furthermore, data are reported on the conservation status of A. riyatguelii, which is suggested to be labelled as CR according to the IUCN categories.

Angiosperms, http://name.umdl.umich.edu/IC-HERB00IC-X-1515471%5DMICH-V-1515471, https://quod.lib.umich.edu/cgi/i/image/api/thumb/herb00ic/1515471/MICH-V-1515471/!250,250, The University of Michigan Library provides access to these materials for educational and research purposes. Some materials may be protected by copyright. If you decide to use any of these materials, you are responsible for making your own legal assessment and securing any necessary permission. If you have questions about the collection, please contact the Herbarium professional staff: herb-dlps-help@umich.edu. If you have concerns about the inclusion of an item in this collection, please contact Library Information Technology: libraryit-info@umich.edu., https://www.lib.umich.edu/about-us/policies/copyright-policy

Angiosperms, http://name.umdl.umich.edu/IC-HERB00IC-X-1515473%5DMICH-V-1515473, https://quod.lib.umich.edu/cgi/i/image/api/thumb/herb00ic/1515473/MICH-V-1515473/!250,250, The University of Michigan Library provides access to these materials for educational and research purposes. Some materials may be protected by copyright. If you decide to use any of these materials, you are responsible for making your own legal assessment and securing any necessary permission. If you have questions about the collection, please contact the Herbarium professional staff: herb-dlps-help@umich.edu. If you have concerns about the inclusion of an item in this collection, please contact Library Information Technology: libraryit-info@umich.edu., https://www.lib.umich.edu/about-us/policies/copyright-policy

Angiosperms, http://name.umdl.umich.edu/IC-HERB00IC-X-1515470%5DMICH-V-1515470, https://quod.lib.umich.edu/cgi/i/image/api/thumb/herb00ic/1515470/MICH-V-1515470/!250,250, The University of Michigan Library provides access to these materials for educational and research purposes. Some materials may be protected by copyright. If you decide to use any of these materials, you are responsible for making your own legal assessment and securing any necessary permission. If you have questions about the collection, please contact the Herbarium professional staff: herb-dlps-help@umich.edu. If you have concerns about the inclusion of an item in this collection, please contact Library Information Technology: libraryit-info@umich.edu., https://www.lib.umich.edu/about-us/policies/copyright-policy

Angiosperms, http://name.umdl.umich.edu/IC-HERB00IC-X-1515472%5DMICH-V-1515472, https://quod.lib.umich.edu/cgi/i/image/api/thumb/herb00ic/1515472/MICH-V-1515472/!250,250, The University of Michigan Library provides access to these materials for educational and research purposes. Some materials may be protected by copyright. If you decide to use any of these materials, you are responsible for making your own legal assessment and securing any necessary permission. If you have questions about the collection, please contact the Herbarium professional staff: herb-dlps-help@umich.edu. If you have concerns about the inclusion of an item in this collection, please contact Library Information Technology: libraryit-info@umich.edu., https://www.lib.umich.edu/about-us/policies/copyright-policy

Angiosperms, http://name.umdl.umich.edu/IC-HERB00IC-X-1515474%5DMICH-V-1515474, https://quod.lib.umich.edu/cgi/i/image/api/thumb/herb00ic/1515474/MICH-V-1515474/!250,250, The University of Michigan Library provides access to these materials for educational and research purposes. Some materials may be protected by copyright. If you decide to use any of these materials, you are responsible for making your own legal assessment and securing any necessary permission. If you have questions about the collection, please contact the Herbarium professional staff: herb-dlps-help@umich.edu. If you have concerns about the inclusion of an item in this collection, please contact Library Information Technology: libraryit-info@umich.edu., https://www.lib.umich.edu/about-us/policies/copyright-policy

Angiosperms, http://name.umdl.umich.edu/IC-HERB00IC-X-1497392%5DMICH-V-1497392, https://quod.lib.umich.edu/cgi/i/image/api/thumb/herb00ic/1497392/MICH-V-1497392/!250,250, The University of Michigan Library provides access to these materials for educational and research purposes. Some materials may be protected by copyright. If you decide to use any of these materials, you are responsible for making your own legal assessment and securing any necessary permission. If you have questions about the collection, please contact the Herbarium professional staff: herb-dlps-help@umich.edu. If you have concerns about the inclusion of an item in this collection, please contact Library Information Technology: libraryit-info@umich.edu., https://www.lib.umich.edu/about-us/policies/copyright-policy

Angiosperms, http://name.umdl.umich.edu/IC-HERB00IC-X-1515474%5DMICH-V-1515474, https://quod.lib.umich.edu/cgi/i/image/api/thumb/herb00ic/1515474/MICH-V-1515474/!250,250, The University of Michigan Library provides access to these materials for educational and research purposes. Some materials may be protected by copyright. If you decide to use any of these materials, you are responsible for making your own legal assessment and securing any necessary permission. If you have questions about the collection, please contact the Herbarium professional staff: herb-dlps-help@umich.edu. If you have concerns about the inclusion of an item in this collection, please contact Library Information Technology: libraryit-info@umich.edu., https://www.lib.umich.edu/about-us/policies/copyright-policy

Angiosperms, http://name.umdl.umich.edu/IC-HERB00IC-X-1515470%5DMICH-V-1515470, https://quod.lib.umich.edu/cgi/i/image/api/thumb/herb00ic/1515470/MICH-V-1515470/!250,250, The University of Michigan Library provides access to these materials for educational and research purposes. Some materials may be protected by copyright. If you decide to use any of these materials, you are responsible for making your own legal assessment and securing any necessary permission. If you have questions about the collection, please contact the Herbarium professional staff: herb-dlps-help@umich.edu. If you have concerns about the inclusion of an item in this collection, please contact Library Information Technology: libraryit-info@umich.edu., https://www.lib.umich.edu/about-us/policies/copyright-policy

Angiosperms, http://name.umdl.umich.edu/IC-HERB00IC-X-1515473%5DMICH-V-1515473, https://quod.lib.umich.edu/cgi/i/image/api/thumb/herb00ic/1515473/MICH-V-1515473/!250,250, The University of Michigan Library provides access to these materials for educational and research purposes. Some materials may be protected by copyright. If you decide to use any of these materials, you are responsible for making your own legal assessment and securing any necessary permission. If you have questions about the collection, please contact the Herbarium professional staff: herb-dlps-help@umich.edu. If you have concerns about the inclusion of an item in this collection, please contact Library Information Technology: libraryit-info@umich.edu., https://www.lib.umich.edu/about-us/policies/copyright-policy

Angiosperms, http://name.umdl.umich.edu/IC-HERB00IC-X-1515472%5DMICH-V-1515472, https://quod.lib.umich.edu/cgi/i/image/api/thumb/herb00ic/1515472/MICH-V-1515472/!250,250, The University of Michigan Library provides access to these materials for educational and research purposes. Some materials may be protected by copyright. If you decide to use any of these materials, you are responsible for making your own legal assessment and securing any necessary permission. If you have questions about the collection, please contact the Herbarium professional staff: herb-dlps-help@umich.edu. If you have concerns about the inclusion of an item in this collection, please contact Library Information Technology: libraryit-info@umich.edu., https://www.lib.umich.edu/about-us/policies/copyright-policy

Angiosperms, http://name.umdl.umich.edu/IC-HERB00IC-X-1515471%5DMICH-V-1515471, https://quod.lib.umich.edu/cgi/i/image/api/thumb/herb00ic/1515471/MICH-V-1515471/!250,250, The University of Michigan Library provides access to these materials for educational and research purposes. Some materials may be protected by copyright. If you decide to use any of these materials, you are responsible for making your own legal assessment and securing any necessary permission. If you have questions about the collection, please contact the Herbarium professional staff: herb-dlps-help@umich.edu. If you have concerns about the inclusion of an item in this collection, please contact Library Information Technology: libraryit-info@umich.edu., https://www.lib.umich.edu/about-us/policies/copyright-policy

Angiosperms, http://name.umdl.umich.edu/IC-HERB00IC-X-1497392%5DMICH-V-1497392, https://quod.lib.umich.edu/cgi/i/image/api/thumb/herb00ic/1497392/MICH-V-1497392/!250,250, The University of Michigan Library provides access to these materials for educational and research purposes. Some materials may be protected by copyright. If you decide to use any of these materials, you are responsible for making your own legal assessment and securing any necessary permission. If you have questions about the collection, please contact the Herbarium professional staff: herb-dlps-help@umich.edu. If you have concerns about the inclusion of an item in this collection, please contact Library Information Technology: libraryit-info@umich.edu., https://www.lib.umich.edu/about-us/policies/copyright-policy