临夏盆地晚中新世鸵鸟化石的特异保存

doi:10.19615/j.cnki.1000-3118.210309

摘要/Abstract

摘要：

报道了来自西北地区中新世晚期临夏盆地的一件鸵鸟化石,该标本包括鸵鸟的部分颈椎以及气管,由于缺乏物种级别的鉴定特征,被暂定为鸵鸟(Struthio sp.)。新标本还保留了平齿三趾马(Hipparion platyodus)的部分头骨。利用多种分析测试方法,对该鸵鸟骨骼的微观特征进行了详细研究,以探讨临夏鸵鸟的埋藏条件和古气候背景。在鸵鸟化石的一个脱矿化的骨碎片中发现了软组织(内源性血管和红细胞的化石残留)。同时光学显微镜和扫描电镜成像显示,化石组织切片中存在显著的细菌改变(骨侵蚀现象)。这是中新世临夏盆地脊椎动物遗体中软组织的首次报道。通过相关的地质和沉积学证据与新的古生物数据相结合,认为季风气候可能是造成鸟类化石早期埋葬期间微生物侵蚀的原因,接下来延续了8 Ma左右的盆地剧烈的干旱化作用,导致了微生物活动的停止,并进一步导致了成岩作用后期孔隙方解石的沉淀。这项工作显示出跨学科(包括形态学、沉积学、地球化学和软组织分析)研究可以更好地揭示中国西北临夏盆地柳树组的中新世晚期的动物群更替、气候和分子保存。

关键词: 临夏盆地, 中新世, 三趾马, 鸵鸟, 软组织, 血管, 细菌, 微观聚焦破坏

Abstract:

Here we report a new avian fossil from the Late Miocene Linxia Basin, Northwest China, with exceptional soft-tissue preservation. This specimen preserves parts of cervical vertebrae and tracheal rings that are typically ostrich-like, but cannot be diagnosed at the species level. Therefore, the fossil is referred to Struthio sp. The new specimen was preserved in association with a partial skull of Hipparion platyodus. To explore the soft tissue preservation in a fossil deposited in a terrestrial setting, we applied a combination of analytic methods to investigate the microscopic features of the fossilized avian bone. Bacterial alterations (bone bioerosion) were revealed by light microscopy and petrographic sections under SEM imaging. Soft-tissues (fossilized remnants of endogenous blood vessels and red blood cells) were preserved in one demineralized bone fragment and also observed in the in-situ ground-section. These are the first records of soft-tissue preservation in vertebrate remains from the Late Miocene Linxia Basin. Associated geological and sedimentological evidence combined with our new data provide insights into the postmortem taphonomic conditions of this ostrich specimen. A seasonal monsoon might have facilitated the microbial erosion penecontemporaneous with the burial of the specimen. This study encourages interdisciplinary research involving morphology, sedimentology, geochemistry, and histological soft-tissue analyses to better understand the Late Miocene faunal turnovers, climates, and fossil preservation in the Liushu Formation in northwestern China.

Key words: Linxia Basin, Miocene, Hipparion, Struthio, soft-tissues, blood vessels, bacteria, microscopic focal destructions

中图分类号:

Q915.865

李志恒, 艾莉达, Thomas A. STIDHAM, 王敏, 邓涛. 临夏盆地晚中新世鸵鸟化石的特异保存. 古脊椎动物学报, 2021, 59(3): 229-244.

LI Zhi-Heng, Alida M. BAILLEUL, Thomas A. STIDHAM, WANG Min, DENG Tao. Exceptional preservation of an extinct ostrich from the Late Miocene Linxia Basin of China. Vertebrata Palasiatica, 2021, 59(3): 229-244.

图/表 5

Fig. 1 Photographs of the fossil block (IVPP V 25336) from the Liushu Formation in Linxia Basin A. the block contains an assemblage of two species, with Struthio sp. (ostrich) cervical bones on the left side and a partial right maxilla of Hipparion platyodus on the right; B. close-up on the ostrich cervical vertebrae with a red arrow indicating the location of the fragment extracted; C. close-up of the ostrich tracheal rings, with a blue arrow showing where the fragment was extracted Abbreviations: Cerv. cervical vertebra; Ma. maxilla; To. tooth; Trach. tracheal rings

Fig. 2 SEM photographs of the fragment of fossil ostrich trachea (IVPP V 25336) A. corresponding SEM image of tracheal sections in Fig. 4. It shows compact bone with very few vascular spaces and calcite infillings in the vascular spaces. B. SEM close-up of well-preserved bone, with an osteocyte lacuna. Some intralacunar material is present, and it is most likely the fossilized remnants of an original osteocyte. C. second histological cross section of the fragment. D. close-up of unaltered bone and osteocyte lacunae. E. high magnification on the osteocyte lacuna filling, most likely a fossilized remnant of the original ostrich osteocyte. F. close-up of porous area with exposed fibrous material, undoubtedly collagen fibers Abbreviations: CFs. fossilized collagen fibers; O. fossilized remnant of osteocyte; OL. osteocyte lacuna; Trach. tracheal ring; ub. unaltered bone matrix

Fig. 3 Photomicrographs of another demineralized fragment of Struthio cervical bone (IVPP V 25336) A. partially demineralized bone using EDTA in sterile conditions. The bone is translucent and bacterial colonies are clearly visible. B. fossilized remnants of original blood vessels (freed from the bone matrix after complete demineralization) which present the same morphological characteristics of extant ostrich blood vessels (Schweitzer et al., 2005, 2007). The vessel is hollow, translucent, and shows clear walls. C. same image as panel B shown in a slightly different focal depth to emphasize remnants of fossilized endothelial cell nuclei within the blood vessel walls. D. fragment of blood vessel with round brown microstructures that are consistent in location and morphology with fossilized blood content. E. fossilized intravascular content (potential blood breakdown products). F. fragment of blood vessel with internal content. G. close-up of endothelial cells in the thick blood vessel walls, and three distinct, circular to oval, brown structures that are most likely the fossilized remnants of original red blood cells This vessel is not complete, but a branching pattern is obvious Abbreviations: bc. bacterial colony; BV. fossilized remnant of original blood vessel; BVW. fossilized remnant of blood vessel walls; ec. fossilized remnants of endothelial cell nuclei; RBCs. fossilized remnants of original red blood cells

Fig. 4 Photographs of the fragment of fossil ostrich trachea (IVPP V 25336) A. photograph of the fragment (outlined in blue) prior to extraction; B. first histological cross section of the fragment seen under transmitted light; C. same image seen under polarized light, some brown material, potentially original soft-tissues with remnants of organic molecules is on top of the trachea; D. close-up of unaffected bone with no evidence of bacterial invasion; E. close-up of unaffected bone with dendritic structures (yellow arrows); F. partially demineralized bone with dendritic structures (yellow arrows) They emanate from the osteocyte lacunae, but are also present directly on the bone matrix so they may represent some type of diagenetic recrystallization Abbreviations: BOM. potential brown organic matter; OL. osteocyte lacuna; Trach. tracheal ring; ub. unaltered bone matrix

Fig. 5 Histological and SEM photographs of the fragment of Struthio sp. cervical vertebra (IVPP V 25336) A. photograph of the fragment prior to extraction, outlined in red; B. histological cross section of the fragment under transmitted light; C. close-up showing highly altered bone microstructure; D. higher magnification shows elongated, beige areas that correspond to bacterial colonies within a lighter and unaffected bone matrix; E. corresponding SEM image of (C); F. a close-up of the bacterial colonies under the SEM; G. Higher magnification shows that the colonies are characterized by thin channels and pores (bc); H. second histological cross section of the fragment, with high pneumaticity and little amounts of bone; I. close-up showing microbially attacked bone; J. close-up on another region showing unaltered bone (without any bacterial colonies) and a vascular canal in the center; K. close-up of the vascular canal shows it is filled with a brown material, most likely organic remnants, some round structures are also seen, and they present the right size and location to be the fossilized remnants of original red blood cells; L. close-up (slightly more dorsal) shows a hollow, tubular structure with walls. It presents all of the morphological characteristics of extant ostrich blood vessels and blood vessel walls (Schweitzer et al., 2005, 2007) All light microscopy photographs in this figure are shown under transmitted light Abbreviations: bc. bacterial colony; BOM. potential brown organic matter; BV. blood vessel (or fossilized remnants of original blood vessel); BVW. blood vessel wall (or fossilized remnants of original blood vessel wall); Cerv. cervical vertebra; OL. osteocyte lacuna; RBCs. red blood cells; ub. unaltered bone matrix; VC. vascular canal

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