来自中国西南贵州省晚三叠世海洋沉积层的一个保存完好的距今2.2亿年前的化石，为了解乌龟独特的身体结构形成中的中间步骤提供了线索。这一分支中的传统形式很罕见，使得该过渡阶段成为爬行类演化中的一个未解之谜。所发现的这种乌龟是已知最原始的乌龟。它有一个充分发育的胸甲（腹部皮肤上的盔甲），是在龟壳（龟的壳结构的背部上半部分）之前形成的。在这一化石中，龟壳仅由神经板（髓板）组成。这表明，龟壳是通过神经板的骨化和肋骨的增宽形成的——这个过程类似于今天小乌龟的发育模式。（生物谷Bioon.com）
生物谷推荐原始出处：
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.