The hadrosauroid remains from Kajlâka Formation (Upper Maastrichtian) limestones of the Labirinta Cave, NW Bulgaria, are most interesting for their small size but without paleohistological data it was not possible to affirm whether they belong to young individual/s or to small-sized mature animal/s. To elucidate their histology and ontogenetic state, 6 associated cortical fragments and one partial diaphysis were sectioned and studied. Thin-sections reveal cortex build of highly vascularized tissues of the woven-parallel complex. Vascularization patterns are somewhat intermediate between those of derived hadrosauroids and non-hadrosauroid ornithopods, yet characterized by thick sequences of laminar bone. Bone tissue matrix consists largely of parallel-fibered or lamellar bone tissue. Cortical tissues are affected by processes of secondary remodeling that locally results in dense Haversian tissue reaching the subperiosteal cortex. No growth marks or external fundamental system are observed in any of the specimens. The transition from bone tissues with predominantly longitudinally oriented osteons to true laminar bone, the thickness of the latter, the extensive secondary bone remodeling and Haversian tissue with at least 3 generations of secondary osteons, as well as the presence of endosteal bone in one of studied specimens all suggest that the material pertains to animal/s at a late sub-adult ontogenetic stage. The high presence of parallel-fibered and lamellar tissues in the cortex indicates significantly slower growth rates in comparison with similarly sized but ontogenetically younger derived hadrosauroids. The osteohistology of the Bulgarian hadrosauroid reveals yet another example of dinosaurian insular dwarfism in latest Cretaceous European archipelago.
Brief conference report on a new Late Cretaceous tetrapod fossil site near the town of Tran (Western Srednogorie), Western Bulgaria, with notes on its dating and palaeoecology. Collected fossil material indicates presence of at least two groups of non-avian dinosaurs, crocodylomorph, and testudines.
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