Osteichthyans, comprising sarcopterygians and actinopterygians, dominate modern vertebrate biodiversity1-3, yet their pre-Devonian fossil record remains scarce and fragmentary4,5. The oldest articulated sarcopterygian6 and stem osteichthyan7 date to the late Silurian, whereas undisputed actinopterygian fossils in articulation appear only in the Middle Devonian8. Here we report an articulated, near-complete osteichthyan from the early Silurian Chongqing Lagerstätte (approximately 436 million years ago)9-11, representing the oldest osteichthyan occurrence including microfossils. This tiny fish exhibits a fusiform, generalized osteichthyan body outline, with plesiomorphic osteichthyan characters, including the lack of lepidotrichia and the presence of serial median dorsal plates, pectoral and dorsal fin spines and an anal fin spine reported previously exclusively in stem chondrichthyans12 and one placoderm13. It also displays features, such as a single dorsal fin and caudal fulcra, seen commonly in actinopterygians. Bayesian inference and the 50% majority rule consensus of the maximum-parsimony analysis place the new fish on the osteichthyan stem, whereas the strict consensus leaves its position unresolved within osteichthyans. This discovery increases Silurian osteichthyan diversity and further populates the osteichthyan stem group. The morphological disparity among early osteichthyans implies a more extensive Silurian to Early Devonian radiation of bony fishes than previous lines of evidence suggested.

Osteichthyans (bony fishes and tetrapods) encompass 98% of modern vertebrate species. However, our understanding of the sequence of character evolution among stem osteichthyans has been substantially limited by the fragmentary nature of known stem osteichthyan fossils1-4. Here we investigate newly discovered articulated head and trunk material of Megamastax amblyodus5, which yields previously unseen morphological details of a Silurian stem osteichthyan. Megamastax-previously interpreted as a lobe-finned fish5-exhibits distinct osteichthyan traits in the dermatocranium, such as resorptive tooth shedding and the presence of extrascapular bones. However, the arrangement of its dorsal aortae is reminiscent of crown-group chondrichthyans. The premaxilla with extensive palatal lamina and the elongated post-hypophyseal region of the braincase recall the condition in maxillate placoderms6-8. Crucially, the discovery of an inner dental arcade of discrete tooth cushions on individual attachment bases aligns Megamastax with the fragmentary genera Lophosteus and Andreolepis2-4, corroborating the previous interpretation of isolated tooth cushions as part of the jaw dentition3,9 and verifying their identity as stem osteichthyans. Phylogenetic analysis places Megamastax within the osteichthyan stem, near the osteichthyan crown-group node, and provides a framework for exploring the sequence of character acquisition along the osteichthyan stem. Together, these new findings help to bridge the morphological gap between stem gnathostomes and modern osteichthyans, offering insights into the sequence of early evolutionary steps that shaped the osteichthyan lineage.