The anatomy of a dorsal head shield of an agnathan fossil Kalanaspis delectabilis, belonging to class Osteostraci from the Aeronian (early Silurian) Kalana Lagerstätte of Estonia was studied using the X-ray computer-tomography. Scanning exposed shallow superficial relief on the dorsal surface of the head shield, which unmasked the internal anatomy of the specimen. The Kalanaspis fossil displays a mosaic of morphological characters: the overall shape of the cephalothoracic head shield and the main structures, like lateral fields, are typical for the group, but instead of the open pineal foramen, common to most osteostracans, the organs in the pineal region are concealed by the shield. A combination of characters that are common to osteostracans, along with unique features known among other early vertebrates, refer to the position of K. delectabilis within the stem group of Osteostraci. The carbonaceous mode of preservation of the head shield of the Kalanaspis fossil, previously undocumented among vertebrate fossils, contradicts the conventional fossilization pattern and refers to an unusual taphonomic history. This anomalous type of preservation appears to be the key to unlock its fossilization history and the complex taphonomic conditions of the whole Kalana Lagerstätte. The main processes for the atypical fossilization of the specimen of K. delectabilis are most likely related to the microbially controlled dissolution of apatitic bone tissues and replacement with microbial biofilms, possibly by the invasion of collagenolytic bacteria.

中文翻译：

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来自爱沙尼亚志留纪 Kalana Lagerstätte 的脱矿质骨化石：揭示其内部解剖结构并揭示一种独特的化石类型

使用 X 射线计算机断层扫描研究了来自爱沙尼亚Aeronian（早期志留纪）Kalana Lagerstätte的无颌类化石Kalanaspis delectabilis的背头盾的解剖结构。扫描暴露了头罩背面的浅浅浮雕，暴露了标本的内部解剖结构。卡拉纳斯皮斯化石显示出一系列形态特征：头胸盾的整体形状和主要结构，如侧视野，是该群体的典型特征，但松果体孔不是大多数骨鳍鱼常见的开放式松果体孔，而是松果体区域的器官被盾牌遮住了。Osteostracans 常见的特征组合，以及其他早期脊椎动物中已知的独特特征，指的是K. delectabilis 在 Osteostraci 的茎组中的位置。Kalanaspis头盾的碳质保存模式化石，以前在脊椎动物化石中没有记载，与传统的化石模式相矛盾，指的是不寻常的埋葬历史。这种异常的保存方式似乎是解开其石化历史和整个 Kalana Lagerstätte复杂的埋葬条件的关键。K. delectabilis标本非典型石化的主要过程很可能与磷灰石骨组织的微生物控制溶解和微生物生物膜的替代有关，可能是由于胶原分解细菌的入侵。