Bone histology reveals the first record of titanosaur (Dinosauria: Sauropoda) from the Late Cretaceous of Bulgaria

The fossil record of Mesozoic tetrapods in Bulgaria is sparse and currently limited to the Maastrichtian limestones of the Kajlâka Formation.Herein we report on two bone fragments from the Upper Cretaceous, lower Santonian to/or lower Campanian, coal-bearing sedimentary succession of the Western Srednogorie, Western Bulgaria.Due to being very fragmentary in nature, it is not possible to assess their taxonomy based solely on osteological characters and a paleohistological analysis is used as an alternative method for taxonomic identification.Our analysis reveals an informative combination of histological characteristics, most notably: absence of free medullar cavity, thick cortex affected by extreme Haversian remodeling with up to five generations of secondary osteons, and laminar bone in the mid-cortex characterized by moderately to highly organized bone matrix.These results do allow us to tentatively assign the studied fossils to a titanosaurian sauropod.The interpretation of the new Bulgarian material as belonging to Titanosauria is intriguing, because it comes from a time interval when sauropods are rare or completely absent in the fossil record of Europe.The histologically assessed ontogenetic stage for one of the fragments suggests that it may come from a sexually mature animal.