The early evolution of gnathostomes (jawed vertebrates) has recently become a major research topic in vertebrate biology, challenging the long-established views on interrelationships and character patterns. It was originally thought that, of the two extant gnathostome groups, the chondrichthyans (cartilaginous fishes) retained many more primitive characters than the osteichthyans (bony fishes and land vertebrates). Similarities between osteichthyans and the so-called ‘placoderms’ (jawed stem gnathostomes belonging to the common ancestral stock of osteichthyans and chondrichthyans), such as the presence of large dermal bones in the skin, were dismissed as convergent. Over the last decade this perception has flipped and osteichthyans are now believed to retain a range of primitive characters inherited from stem gnathostomes, whereas the lack of these characters in chondrichthyans is derived.

Writing in Nature Ecology & Evolution, Brazeau and colleagues1 add to this discussion by reporting an unexpected discovery of extensive endochondral bone — the main internal skeletal tissue of most osteichthyans — in a unique fossil specimen of a stem-group gnathostome, a member of the lineage that gave rise to both osteichthyans and chondrichthyans. This discovery represents a major advancement in our knowledge of the origins of this tissue, which developed an important function called haematopoiesis (red blood cell production)2 later in evolution.

In vertebrates, bones reinforce the body and support its axis, protect the internal organs and provide attachment surfaces for muscles. There are different modes of bone formation and not all vertebrates are capable of them all. The key distinguishing feature is whether cartilage is involved. In the skin, bone can form without direct connection to a cartilage in a process called dermal ossification3. This appears to have been the first kind of ossification to evolve among the earliest jawless vertebrates, and it persists in osteichthyans. Even some human skull bones are dermal in origin.

By contrast, the ossification of the internal skeleton (endoskeleton) is always connected to cartilage. Either the bone forms on the surface of an existing cartilaginous element — a process called perichondral ossification4 — or the cartilage is gradually replaced by bone from within by means of endochondral ossification. This process produces so-called ‘trabecular’ or ‘cancellous’ bone, which is porous and well-vascularized.

Among extant vertebrates, all of these modes of ossification are exclusive to osteichthyans. Living chondrichthyans do not develop any bone at all and their endoskeleton is cartilaginous, although this cartilage can be calcified. However, the extinct jawless and jawed vertebrates of the gnathostome stem group produced dermal bone. In addition, more developed stem gnathostomes (such as osteostracans and ‘placoderms’) covered their cartilaginous endoskeleton with a layer of perichondral bone, which implies that the ability to form dermal and perichondral bone was secondarily lost in chondrichthyans5,6,7. Endochondral bone, on the other hand, has been thought to be a unique invention of osteichthyans.

Brazeau and colleagues describe extensive endochondral, as well as perichondral and dermal bone in Mininjia, a newly established fossil fish genus discovered in the Early Devonian rocks of western Mongolia, which are around 400 million years old. The specimen consists of a partial braincase covered by the dermal bones of the skull roof, mostly preserved as a mould. It has been studied by means of X-ray computed microtomography, which allows the internal anatomy to be visualized without damaging the specimen. The phylogenetic analyses the researchers performed place Minjinia in the ‘placoderm’ part of the tree, as a close sister group to the modern jawed vertebrate lineages, suggesting that endochondral bone evolved in the gnathostome stem and was subsequently lost in chondrichthyans (Fig. 1). If this is correct, it further supports the contention that osteichthyans generally retain the ancestral character complex, whereas chondrichthyans are derived.

Fig. 1: Bone formation evolution.
figure 1

A simplified evolutionary tree of vertebrates showing the distribution of dermal (brown), perichondral (orange) and endochondral (blue) bone. The position of Minjinia and the distribution of endochondral bone formation ability is based on Brazeau and colleagues’ work1. The authors suggest that endochondral bone, in addition to dermal and perichondral bone5,6,7, may have been secondarily lost in chondrichthyans.

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Over the course of the past five years of early vertebrate research, a number of papers have boosted the debate on the deep origins of bony fish, providing new questions rather than definite answers. Unique stem-group gnathostome specimens, often discovered in under-sampled or previously marginalized parts of the world, display unexpected characters such as braincase features of both osteichthyans and chondrichthyans5, the presence of osteichthyan-like marginal jawbones (maxilla, premaxilla and dentary)6,8 or crown gnathostome-like teeth9. The discovery by Brazeau and colleagues is a valuable addition to this progressively evolving research field.