The origins and development of the arid and highly seasonal steppe-desert biome in Central Asia, the largest of its kind in the world, remain largely unconstrained by existing records. It is unclear how Cenozoic climatic, geological, and biological forces, acting at diverse spatial and temporal scales, shaped Central Asian ecosystems through time. Our synthesis shows that the Central Asian steppe-desert has existed since at least Eocene times but experienced no less than two regime shifts, one at the Eocene–Oligocene Transition and one in the mid-Miocene. These shifts separated three successive “stable states,” each characterized by unique floral and faunal structures. Past responses to disturbance in the Asian steppe-desert imply that modern ecosystems are unlikely to recover their present structures and diversity if forced into a new regime. This is of concern for Asian steppes today, which are being modified for human use and lost to desertification at unprecedented rates.
The Ephedra lineage can be traced at least to the Early Cretaceous. Its characteristically polyplicate pollen is well-represented in the fossil record and is frequently used as an indicator of paleoclimate. However, despite previous efforts, knowledge about variation and evolution of ephedroid pollen traits is poor. Here, we document pollen morphology of nearly all extant species of Ephedra, using a combination of scanning electron microscopy (SEM) and light microscopy (LM), and reconstruct ancestral states of key pollen traits. Our results indicate that the ancestral Ephedra pollen type has numerous plicae interspaced by unbranched pseudosulci, while the derived pollen type has branched pseudosulci and (generally) fewer plicae. The derived type is inferred to have evolved independently twice, once along the North American stem branch and once along the Asian stem branch. Pollen of the ancestral type is common in Mesozoic fossil records, especially from the Early Cretaceous, but it is less commonly reported from the Cenozoic. The earliest documentation of the derived pollen type is from the latest Cretaceous, after which it increases strongly in abundance during the Paleogene. The results of the present study have implications for the age of crown group Ephedra as well as for understanding evolution of pollination syndromes in the genus.
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