New evidence of Zygophyllaceae evolution: insights from the Miocene fossil wood records and their associations with the orogeny of the Andes and arid climates

Abstract

The subfamily Larreoideae is the main representative of Zygophyllaceae in South America, although its fossil record remains extremely scarce. This study presents the first unequivocal fossil woods assigned to this family and subfamily from the early Miocene Mari~no Formation in Mendoza Province, Argentina. The specimens are described as a new fossil genus and species: Larreoxylon cuyensis gen. et sp. nov. and Larreoxylon sp. Wood anatomical features were examined through thin sectioning and scanning electron microscopy (SEM) imaging, and their phylogenetic affinities were evaluated using a matrix of 12 characters across 21 taxa. The fossil woods exhibit xeromorphic traits, including exclusively solitary vessels, vestured pits, and vasicentric tracheids, features associated with drought-adapted taxa in arid environments. These traits closely match those of extant members of the Larrea clade, suggesting a shared ecological strategy and providing robust evidence of early adaptation to aridity in the Miocene of southern South America. A cladistic analysis, based exclusively on secondary xylem characters, yielded a single most-parsimonious tree congruent with previous molecular phylogenies. Larreoxylon gen. nov. is placed within the Bulnesia-Larrea clade, defined by the presence of vestured pits, and specifically within a Larrea and Larreoxylon gen. nov. subclade characterized by exclusively solitary vessels. Both recognized fossil taxa occupy basal positions within this group, indicating an early stage in the anatomical diversification of the clade, preceding derived features such as crystals in axial parenchyma. Theresults show that the wood anatomy of Larreoideae retains informative traits for phylogenetic inference. Moreover, the fossils offer critical calibration points for understanding the evolutionary, ecological and biogeographical history of Larreoideae in the context of Andean uplift and Neogene aridification.