Abstract:
The evolutionary history of testate amoebae can be traced back to the early Neoproterozoic. The earliest testate amoebae are represented by vase-shaped microfossils found in shallow marine carbonate rocks and shales in North America, North China, Norway, Australia, and other areas. In the Phanerozoic, the earliest fossil record of testate amoebae occurs in the Lower Devonian, whereas there has been no unambiguous fossil evidence of such protists known in the early Paleozoic rocks for a long time. By means of standard palynological acid maceration techniques, we obtained fossil specimens preserved in situ by grinding thin slices of rocks. We found several organic microfossils bearing strange ornamentations from the Balang Formation (Stage 4, Series 2, Cambrian) at the Jiaobang section in Jianhe County, eastern Guizhou. Based on their micromorphological characteristics and the results of laser Raman spectroscopy, these organic-walled microfossils are proposed as remains of the early Cambrian testate amoebae, and they are morphologically very similar to those of Euglyphida. In particular, they can be morphologically compared with extant testate amoebae Assulina Greef, 1888. Thus, our specimens, previously described as acritrach Plagasphaera balangensis and Plagasphaera sp. A, are now re-classified here as Palaeoassulina balangensis gen. et sp. nov., and ?Palaeoassulina sp. A, respectively, based on their morphological similarity to extant testate amoeba Assulina Greef, 1888. This discovery not only extends the fossil record of testate amoebae from the previously known late Paleozoic to the early Cambrian, but also provides key fossil evidence for understanding the evolution of testate amoebae.
