A demineralized osteostracan fossil from the Silurian Kalana Lagerstätte of Estonia: revealing its internal anatomy and uncovering a unique type of fossilization

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.