Endochondral bone is the main internal skeletal tissue of nearly all osteichthyans—the group comprising more than 60,000 living species of bony fishes and tetrapods. Chondrichthyans (sharks and their kin) are the living sister group of osteichthyans and have primarily cartilaginous endoskeletons, long considered the ancestral condition for all jawed vertebrates (gnathostomes). The absence of bone in modern jawless fishes and the absence of endochondral ossification in early fossil gnathostomes appear to lend support to this conclusion. Here we report the discovery of extensive endochondral bone in Minjinia turgenensis, a new genus and species of ‘placoderm’-like fish from the Early Devonian (Pragian) of western Mongolia described using X-ray computed microtomography. The fossil consists of a partial skull roof and braincase with anatomical details providing strong evidence of placement in the gnathostome stem group. However, its endochondral space is filled with an extensive network of fine trabeculae resembling the endochondral bone of osteichthyans. Phylogenetic analyses place this new taxon as a proximate sister group of the gnathostome crown. These results provide direct support for theories of generalized bone loss in chondrichthyans. Furthermore, they revive theories of a phylogenetically deeper origin of endochondral bone and its absence in chondrichthyans as a secondary condition.

软骨几乎是所有骨锁骨的主要内部骨骼组织，该组由六万多种活骨鱼和四足动物组成。软骨鱼类（鲨鱼及其亲属）是骨软骨鱼类的活体姐妹群，主要有软骨内骨骼，长期以来一直被认为是所有颌骨脊椎动物（棘齿动物）的祖先条件。现代无颌鱼类中没有骨头，早期化石破骨动物的软骨内骨化也似乎为这一结论提供了支持。在这里我们报告在Minjinia turgenensis中发现广泛的软骨内骨的发现，是使用X射线计算机断层扫描技术描述的来自蒙古西部泥盆纪（古希腊）的一种新的“编轴类”鱼类的物种。该化石由部分颅骨顶和脑壳组成，并具有解剖学细节，提供了在gnathostome茎组中放置的有力证据。但是，它的软骨内空间充满了细小梁的广泛网络，类似于小骨鱼类的软骨内骨。系统发育分析将这种新的分类单元定位为gnathostome冠的最接近的姐妹群。这些结果为软骨干软骨普遍骨质流失的理论提供了直接的支持。此外，他们振兴了软骨内骨骼的系统发育较深起源的理论，并认为软骨下骨骼的缺如是继发性疾病。