The focus of study for nearly two centuries 1 , fossils of early gnathostomes-or jawed vertebrates -yield key clues about the evolutionary assembly of the bodyplan common to the group, as well the divergence of the two living gnathostome lineages: the cartilaginous and bony fishes 2,3 . A series of remarkable new palaeontological discoveries 4-10 , analytical advances and innovative reinterpretations of old fossils [11][12][13][14] have fundamentally altered a decades-old consensus on the relationships of extinct gnathostomes 15,16 , delivering a new evolutionary framework 3,6,10-14 for exploring major questions which remain unanswered, including the origin of jaws [17][18][19] .Jawed vertebrates (gnathostomes) comprise more than 99% of living vertebrate species, including humans. This diversity is built upon features including jaws, teeth, paired appendages, and specialised embryonic and skeletal tissues (Box 1); centuries of research have attempted to explain their origins 17,18,[20][21][22][23][24] . In particular, jaws and paired appendages have become flagship systems in the study of evolutionary novelty 23,24 -a key research programme in evolutionary biology 25 .The deepest split in the modern gnathostome tree is that between the chondrichthyans (sharks, rays, and chimaeras) and the osteichthyans (bony fishes and tetrapods). This divergence occurred in the Palaeozoic Era, at least 423 million years ago 8 , leaving a vast temporal and evolutionary gulf between modern lineages, with ample time for new innovations to overwrite primitive conditions. These complexities compel researchers to turn to the Palaeozoic fossil record to elucidate the origin of jawed vertebrates. A few wellpreserved fossil taxa from a handful of Silurian-Permian sites in Europe and North America 1 shaped late 19 th and early 20 th century hypotheses of gnathostome evolution 17,26,27 (Fig. 1). Many of these narratives persist to this day, either implicitly or explicitly. However, fossils once hailed as avatars for scenarios of jaw 27,28 or fin 17,29 origins often turn out to be specialized rather than primitive upon phylogenetic investigation 30,31 .Until they are placed in a phylogenetic tree, Palaeozoic fossils are mute on the question of gnathostome origins. In this review, we examine the progress made in the past two decades on the study of early gnathostome interrelationships, focusing on key fossil discoveries that have prompted a renewed intensity of phylogenetic investigation.
Phylogeny of extant gnathostomesFrom the perspective of modern lineages alone, deep vertebrate phylogeny is well resolved and there is little disagreement about the branching patterns surrounding the gnathostome crown node (Box 1). Morphological 3 and molecular 2 data unambiguously indicate that chondrichthyans and osteichthyans are each monophyletic sister taxa. Together, they form a clade to the exclusion of the cyclostomes: hagfishes and lamprey (Box 1). Molecular evidence strongly supports the monophyly of living agnathans with respect to jawe...