Sponges are widely viewed as the most primitive metazoans. Sponge gemmule-like structures have been recovered from the lower part of the Doushantuo Formation in the Yangtze Gorges area (South China), that was deposited between 635 to 580 Ma. Gemmoscleres-like structures are embedded in the outer coat of the sponge structures. Electron probe microanalyses EPMA) of the gemmoscleres-indicate that they have siliceous composition. The naked subspheroidal sponge structures are composed of three layers, consistent with the preservation of a tri-layered theca embedded with the gemmosclere-like structures. The potential fossils of the Ediacaran sponge gemmules in the Doushantuo Formation may provide one of the earliest records of sponges.
1. Hongbo Zheng (a)
2. Peter D. Clift (b)
3. Ping Wang (a)
4. Ryuji Tada (c)
5. Juntao Jia (d)
6. Mengying He (e)
7. Fred Jourdan (f)
Affiliations:
a. School of Geography Science, Nanjing Normal University, Nanjing 210023, China;
b. Department of Geology and Geophysics, Louisiana State University, Baton Rouge, LA 70803;
c. Department of Earth and Planetary Science, University of Tokyo, Tokyo 113-0033, Japan;
d. School of Geosciences, China Petroleum University, Qingdao 266580, China;
e. School of Earth Science and Engineering, Nanjing University, Nanjing 210093, China; and
f. Western Australian Argon Isotope Facility, Department of Applied Geology and John de Laeter Centre, Curtin University, Perth, WA 6845, Australia
Abstract:
The development of fluvial systems in East Asia is closely linked to the evolving topography following India–Eurasia collision. Despite this, the age of the Yangtze River system has been strongly debated, with estimates ranging from 40 to 45 Ma, to a more recent initiation around 2 Ma. Here, we present 40Ar/39Ar ages from basalts interbedded with fluvial sediments from the lower reaches of the Yangtze together with detrital zircon U–Pb ages from sand grains within these sediments. We show that a river containing sediments indistinguishable from the modern river was established before ∼23 Ma. We argue that the connection through the Three Gorges must postdate 36.5 Ma because of evaporite and lacustrine sedimentation in the Jianghan Basin before that time. We propose that the present Yangtze River system formed in response to regional extension throughout eastern China, synchronous with the start of strike–slip tectonism and surface uplift in eastern Tibet and fed by strengthened rains caused by the newly intensified summer monsoon.
