A new angiosperm fruit, Dilcherifructus mexicana gen. et sp. nov, from the Middle Jurassic of Mexico. This is the currently earliest record of angiosperms in the North America, and its geographical position indicates that angiosperms were already widespread in the North Hemisphere during the Jurassic. This new information prompts a rethinking on the history of angiosperms and related hypotheses.
Recently, Prof. WANG Xin from the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGPAS), published this result in the international academic journal Biosis: Biological Systems. This is also the first time that Chinese scientists have independently studied early angiosperm fruit fossils, which from North America.
The fossil was collected from an outcrop of the Simon Formation near Tezoatlán, nine miles south of the city Tezoatlan, Oaxaca, Mexico. The fossils are preserved as coalified compressions embedded in yellowish siltstones (Fig. 1a). By using stereomicroscope equipped with a digital camera, the researcher observed and photographed the fruits are round-shaped, with an apical and a basal depressions, 10-13.6 mm long, 10-13 mm wide, including a seed and an enclosing pericarp (Figs. 1a-b,). The pericarp has a smooth surface, the seeds are inside the fruits, round in shape, approximately 8 mm long and 6-9 mm wide (Fig. 1a-b).
A persistent style 76 μm long and 30 μm wide is in the apical depression (Fig. 1b-c). Epidermal cells of the exocarp are polygonal or rectangular in shape, 20-60 μm long, 13-38 μm wide (Figs. 2d-e). Anomocytic stoma is surrounded by approximately five epidermal cells, 36 μm long, 36 μm wide, with a slit 25 μm long, 5.4 μm wide, level with neighboring epidermal cells (Figs. 2d-e).
If Dilcherifructus were taken as a seed, less hard seed content enclosed by a hard seed coat should not be visible for an observer. The situation in Dilcherifructus (Figs. 1b) is on the contrary: the inside content is obvious. This observation suggests that the internal body in Dilcherifructus is harder than the enclosing layer, a case frequently seen in angiosperm fruits: seeds inside ovary usually are harder than the enclosing fleshy ovarian wall and thus visible when squashed. The distal projection in Dilcherifructus is interpreted as a persistent style on the tip of a fruit, as frequently seen in angiosperms.
Its Jurassic age suggests that origin of angiosperms is much earlier than widely accepted, while its occurrence in the North America indicates that angiosperms were already widespread in the Jurassic, although they were still far away from their ecological radiation, which started in the Early Cretaceous.
This research was supported by the Strategic Priority Research Program (B) of Chinese Academy of Sciences and the National Natural Science Foundation of China.
Reference: Wang Xin, The currently earliest angiosperm fruit from the Jurassic of North America, Biosys: Biological Systems, 2021, 2(4): 416-422, doi:10.37819/biosis.001.04.0160. https://eaapublishing.org/journals/index.php/biosis/article/view/160/229.
Fig. 1. General morphology of Dilcherifructus mexicana gen. et sp. nov and its details
Fig. 2. Stomata of fossil fruits under microscope
Contact:
LIU Yun, Propagandist
Email: yunliu@nigpas.ac.cn
Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences
Nanjing, Jiangsu 210008, China
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