Fossils confirm early diversification of spiny plants in central Tibet

Spinescence (a general term for the phenomena of spines, prickles, and prickles on plants) is an important functional trait shared by many plant families worldwide and primarily provides physical protection against vertebrate herbivores. Even though spiny plants are distributed worldwide, our understanding of their evolutionary history remains incomplete, largely due to a lack of fossils.

In a study published in Nature Communications, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG), the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences (CAS), the University of Bristol and the The Open University in the United Kingdom have reported exceptionally rich assemblages of plant spine fossils collected from late Eocene (about 39 million years ago) sediments in central Tibet.

These fossils confirm an early diversification of thorny plants in the Tibetan region contemporary with the emergence of open, semi-forested habitats in the late Eocene and early transition from central Tibet to full plateau formation. .

The researchers documented a total of 44 spine-bearing fossil specimens collected from two fossil localities in the central Tibetan Bangong-Nujiang Suture Zone: the Dayu (32°20′ N, 89°46′ E) and the Xiede ( 31° 58′ N , 88° 25′ E) localities. They classified the fossil spines into a total of seven morphotypes comprising prickles and spines according to two criteria: the spines come from the epidermis of plant organs such as stems, leaves and petioles, while the spines are branches. modified and contain internal vascular bundles.

To study the emergence and early diversification of spiny plants, the researchers not only performed molecular phylogenetic analyzes but also used proxy and modeling data to reconstruct the vegetation, climate, and herbivory that favored the evolution of spiny plants in central Tibet during the Eocene. Additionally, they reconstructed the evolutionary history of spines across woody eudicot species in Eurasia within the plant mega-phylogeny.

According to ZHANG Xinwen of XTBG, modeling and proxy data indicate “a drying and cooling climate” in the central valley of Tibet during the mid-Eocene. This change was accompanied by an increase in altitude of about one kilometer about 47 million years ago.

In this context, plant fossils record the beginnings of vegetation opening under a drying/cooling climate as the modern Tibetan Plateau developed from the late Eocene. The observed spinescence marks the early development of physical defense mechanisms against the feeding pressure of large herbivores in the region.

“Our study shows that regional aridification and the expansion of herbivorous mammals may have driven the diversification of functional spinescence in the forests of central Tibet, about 24 million years earlier than similar transformations in Africa,” said said SU Tao of XTBG.

A typical thorny plant Gleditsia microphylla (Fabaceae) native to Asia. (Picture by ZHANG Xinwen)

Morphotypes of spiny fossils from the Eocene sediments of the central Tibetan plateau. (Picture by ZHANG Xinwen)

A spiny fossil from the Eocene sediments of the central Tibetan plateau. (Picture by ZHANG Xinwen)

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