Abstract:

Because ancient proteins have a higher preservation potential than ancient DNA, proteomic studies can help shed light on the biology of some extinct biological groups that are beyond the reach of the field of ancient DNA. The oldest peptide discovered so far is part of the protein struthiocalcin (SCA-1) involved in eggshell mineralization and found within an ostrich egg from the Late Miocene Linxia Basin of Northwest China. It was originally hypothesized that SCA-1 was evenly distributed within the eggshell and was able to enter the fossil record for so long, because it was bound to calcite crystals. We conducted histological, scanning electron microscopy and Raman spectroscopic analyses on this same fossil egg to test if any protein or organic matter could be observed within specific regions of the eggshell and indeed bound to calcite crystals. Our results show that the eggshell is made entirely of calcite except at the base layer, which is made of mammillary knobs at least partially made of apatite. These knobs were secondarily phosphatized during diagenesis. After decalcification of this material, the fossilized mammillary knobs showed fibrous residues consistent in location and morphology with remnants of original organic material forming a network. This network was similar to the organic matrix observed in an extant ostrich eggshell with this same method. The results here suggest that SCA-1 may have been concentrated at the mammillary knobs, rather than evenly throughout the eggshell. Phosphatization may be another taphonomic process that favors organic preservation in deep-time. The paleoclimate and taphonomic environment of the Linxia Basin may have provided favorable conditions for the molecular preservation of this egg. More in-depth histochemical and mineralogical analyses will certainly increase our understanding of organic and ancient protein preservation in this basin.

Key words: fossil organics, struthiocalcin, apatite, phosphatization, ostrich eggshell, ancient proteins

摘要：

古蛋白质比古DNA具有更高的保存潜力，因此蛋白质组学研究可以帮助阐明一些超出古DNA研究领域的灭绝生物群体的生物学特征。迄今为止最古老的多肽发现于中国西北地区晚中新世临夏盆地的鸵鸟蛋壳化石中，是与蛋壳矿化相关的蛋白质struthiocalcin (SCA-1) 的一部分。前人认为SCA-1在蛋壳中均匀分布，并因其与方解石晶体结合的特性而得以在地质历史中长时间保存。本次对同一鸵鸟蛋壳化石进行了组织学、扫描电子显微镜和拉曼光谱分析，发现蛋壳内侧锥体层的晶核含有部分磷灰石，其他部位则完全由方解石构成；这些晶核部分应当是在成岩作用过程中经历了磷酸盐化。在对该化石蛋壳样品脱钙处理后，其锥体层晶核部分存在残留物，呈现网络状纤维结构，其位置和形态与现生鸵鸟蛋壳中脱钙后残留的有机质相似。结果表明，该化石蛋壳中的古多肽可能集中保存在锥体层晶核处，而非在整个蛋壳中均匀分布。磷酸盐化可能是另一个有利于有机物长期保存的埋藏过程。临夏盆地的古气候和埋藏环境可能为该古蛋白分子的保存提供了有利的条件。建议在未来研究中进行更深入的组织化学和矿物学分析，以进一步了解该盆地有机质和古蛋白的保存机制。

关键词: 化石有机质, 生物矿化蛋白, 磷灰石, 磷酸盐化, 鸵鸟蛋壳, 古蛋白