Simojoflorum mijangosii gen. et sp. nov. preserved in the Mexican amber unravels the polycarpellate condition in the tribe Mimoseae (Caesalpinioideae, Fabaceae)

An inflorescence preserved in amber from the middle-early Miocene from southern Mexico is identified as a new extinct member of Fabaceae. We described and identified the fossil material, comparing it with extant and fossil members of the Fabaceae using light microscopy and computed microtomography. The fossil inflorescence has morphological characters that allow a near comparison with Caesalpinioideae (Fabaceae), including flowers aggregated in a capitate inflorescence. Even more, bisexual and staminate flowers, valvate aestivation, many stamens, a superior, unilocular ovary with marginal placentation are characters that allow their comparison with the tribe Mimoseae, especially the polycarpellate gynoecium is comparable to the Acacieae and Ingeae tribes in the traditional classification. However, other floral characteristics, such as anther glands, partially united filaments, a basal nectariferous disk, and a stalk ovary, are comparable to Mariosousa Seigler & Ebinger, 2006, Senegalia Raf., 1838, and Parasenegalia Seigler & Ebinger, 2017. The phylogenetic relationship between them is unresolved, and information on the floral structure is limited for further comparison. Thus, we support the identification of Simojoflorum mijangosii gen. et sp. nov. as a new extinct member of the tribe Mimoseae that suggests the diversification of the lineage during the Miocene.