Fieldwork in the Early Cretaceous Jehol Group, northeastern China has revealed a plethora of extraordinarily well-preserved fossils that are shaping some of the most contentious debates in palaeontology and evolutionary biology. These discoveries include feathered theropod dinosaurs and early birds, which provide additional, indisputable support for the dinosaurian ancestry of birds, and much new evidence on the evolution of feathers and flight. Specimens of putative basal angiosperms and primitive mammals are clarifying details of the early radiations of these major clades. Detailed soft-tissue preservation of the organisms from the Jehol Biota is providing palaeobiological insights that would not normally be accessible from the fossil record.

Most of Mesozoic bird diversity comprises species that are part of one of two major lineages, namely Ornithurae, including living birds, and Enantiornithes, a major radiation traditionally referred to as ‘opposite birds’. Here we report the largest Early Cretaceous enantiornithine bird from north‐east China, which provides evidence that basal members of Enantiornithes share more morphologies with ornithurine birds than previously recognized. Morphological evolution in these two groups has been thought to be largely parallel, with derived members of Enantiornithes convergent on the ‘advanced’ flight capabilities of ornithurine birds. The presence of an array of morphologies previously thought to be derived within ornithurine and enantiornithine birds in a basal enantiornithine species provides evidence of the complex character evolution in these two major lineages. The cranial morphology of the new specimen is among the best preserved for Mesozoic avians. The new species extends the size range known for Early Cretaceous Enantiornithes significantly and provides evidence of forelimb to hind limb proportions distinct from all other known members of the clade. As such, it sheds new light on avian body size evolution and diversity, and allows a re‐evaluation of a previously proposed hypothesis of competitive exclusion among Early Cretaceous avian clades.

Ornithuromorpha is the most inclusive clade containing extant birds but not the Mesozoic Enantiornithes. The early evolutionary history of this avian clade has been advanced with recent discoveries from Cretaceous deposits, indicating that Ornithuromorpha and Enantiornithes are the two major avian groups in Mesozoic. Here we report on a new ornithuromorph bird,Archaeornithura meemannaegen. et sp. nov., from the second oldest avian-bearing deposits (130.7 Ma) in the world. The new taxon is referable to the Hongshanornithidae and constitutes the oldest record of the Ornithuromorpha. However,A. meemannaeshows few primitive features relative to younger hongshanornithids and is deeply nested within the Hongshanornithidae, suggesting that this clade is already well established. The new discovery extends the record of Ornithuromorpha by five to six million years, which in turn pushes back the divergence times of early avian lingeages into the Early Cretaceous.

The study of the Early Cretaceous terrestrial Jehol Biota, which provides a rare window for reconstruction of a Lower Cretaceous terrestrial ecosystem, is reviewed with a focus on some of the latest progress. A newly proposed definition of the biota based on paleoecology and taphonomy is accepted. Although the Jehol fossils are mainly preserved in two types of sedimentary rocks, there are various types of preservation with a complex mechanism that remains to be understood. New discoveries of significant taxa from the Jehol Biota, with an updated introduction of its diversity, confirm that the Jehol Biota represents one of the most diversified biotas of the Mesozoic. The evolutionary significance of major biological groups (e.g. dinosaurs, birds, mammals, pterosaurs, insects, and plants) is discussed mainly in the light of recent discoveries, and some of the most remarkable aspects of the biota are highlighted. The global and local geological, paleogeographic, and paleoenvironmental background of the Jehol Biota have contributed to the unique composition, evolution, and preservation of the biota, demonstrating widespread faunal exchanges between Asia and other continents caused by the presence of the Eurasia–North American continental mass and its link to South America, and confirming northeastern China as the origin and diversification center for a variety of Cretaceous biological groups. Although some progress has been made on the reconstruction of the paleotemperature at the time of the Jehol Biota, much more work is needed to confirm a possible link between the remarkable diversity of the biota and the cold intervals during the Early Cretaceous. Finally, future directions for the study of the Jehol Biota are proposed that highlight the great potential of more comprehensive and multidisciplinary studies to further our understanding of the biological and geological implications of the Jehol Lagerstätte.