專題：Nature報導
來自中國西南貴州省晚三疊世海洋沉積層的一個保存完好的距今2.2億年前的化石，為了解烏龜獨特的身體結構形成中的中間步驟提供了線索。這一分支中的傳統形式很罕見，使得該過渡階段成為爬行類演化中的一個未解之謎。所發現的這種烏龜是已知最原始的烏龜。它有一個充分發育的胸甲（腹部皮膚上的盔甲），是在龜殼（龜的殼結構的背部上半部分）之前形成的。在這一化石中，龜殼僅由神經板（髓板）組成。這表明，龜殼是通過神經板的骨化和肋骨的增寬形成的——這個過程類似於今天小烏龜的發育模式。（生物谷Bioon.com）
生物谷推薦原始出處：
Nature 456, 497-501 (27 November 2008) | doi:10.1038/nature07533
An ancestral turtle from the Late Triassic of southwestern China
Chun Li1, Xiao-Chun Wu2, Olivier Rieppel3, Li-Ting Wang4 & Li-Jun Zhao5
1 Laboratory of Evolutionary Systematics of Vertebrates, Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of SciencesPO Box 643, Beijing 100044, China
2 Canadian Museum of Nature, PO Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada
3 Department of Geology, The Field Museum, 1400 South Lake Shore Drive, Chicago, Illinois 60605-2496, USA
4 Geological Survey of Guizhou Province, Guiyang 550005, China
5 Zhejiang Museum of Nature History, Hangzhou 310012, China
The origin of the turtle body plan remains one of the great mysteries of reptile evolution. The anatomy of turtles is highly derived, which renders it difficult to establish the relationships of turtles with other groups of reptiles. The oldest known turtle, Proganochelys from the Late Triassic period of Germany1, has a fully formed shell and offers no clue as to its origin. Here we describe a new 220-million-year-old turtle from China, somewhat older than Proganochelys, that documents an intermediate step in the evolution of the shell and associated structures. A ventral plastron is fully developed, but the dorsal carapace consists of neural plates only. The dorsal ribs are expanded, and osteoderms are absent. The new species shows that the plastron evolved before the carapace and that the first step of carapace formation is the ossification of the neural plates coupled with a broadening of the ribs. This corresponds to early embryonic stages of carapace formation in extant turtles, and shows that the turtle shell is not derived from a fusion of osteoderms. Phylogenetic analysis places the new species basal to all known turtles, fossil and extant. The marine deposits that yielded the fossils indicate that this primitive turtle inhabited marginal areas of the sea or river deltas.