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The Early Cretaceous Jehol Biota is a terrestrial lagerstätte that contains exceptionally well-preserved fossils indicating the origin and early evolution of Mesozoic life, such as birds, dinosaurs, pterosaurs, mammals, insects, and flowering plants. New geochronologic studies have further constrained the ages of the fossil-bearing beds, and recent investigations on Early Cretaceous tectonic settings have provided much new information for understanding the spatiotemporal distribution of the biota and dispersal pattern of its members. Notably, the occurrence of the Jehol Biota coincides with the initial and peak stages of the North China craton destruction in the Early Cretaceous, and thus the biotic evolution is related to the North China craton destruction. However, it remains largely unknown how the tectonic activities impacted the development of the Jehol Biota in northeast China and other contemporaneous biotas in neighboring areas in East and Central Asia. It is proposed that the Early Cretaceous rift basins migrated eastward in the northern margin of the North China craton and the Great Xing’an Range, and the migration is regarded to have resulted from eastward retreat of the subducting paleo-Pacific plate. The diachronous development of the rift basins led to the lateral variations of stratigraphic sequences and depositional environments, which in turn influenced the spatiotemporal evolution of the Jehol Biota. This study represents an effort to explore the linkage between terrestrial biota evolution and regional tectonics and how plate tectonics constrained the evolution of a terrestrial biota through various surface geological processes.

The Jehol Biota is an Early Cretaceous terrestrial fossil assemblage of paramount significance, and its core distribution areas are western Liaoning, northern Hebei, and southeastern Inner Mongolia. Despite with a research history of more than 150 years, it started yielding important fossils until early 1990s, which include feathered dinosaurs, early birds, early mammals, flower-visiting insects, and early angiosperms. These discoveries have implications for understanding the origins and early evolution of several major organismal groups, as well as the origin and initial formation of modern terrestrial ecosystem. This review presents a brief introduction of the major discoveries, research history, and current understanding of this biota, and also provides future prospects for studying the Jehol Biota.

The Lower Cretaceous Huajiying Formation of the Sichakou Basin in northern Hebei Province, northern China contains key vertebrate taxa of the early Jehol Biota, e.g., Protopteryx fengningensis, Archaeornithura meemannae, Peipiaosteus fengningensis, and Eoconfuciusornis zhengi. This formation arguably documents the second-oldest bird-bearing horizon, producing the oldest fossil records of the two major Mesozoic avian groups Enantiornithes and Ornithuromorpha. Hence, precisely determining the depositional ages of the Huajiying Formation would advance our understanding of the evolutionary history of the Jehol Biota. Here we present secondary ion mass spectrometry (SIMS) U-Pb zircon analysis results of eight interbedded tuff/tuffaceous sandstone samples from the Huajiying Formation. Our findings, combined with previous radiometric dates, suggest that the oldest enantiornithine and ornithuromorph birds in the Jehol Biota are ∼129–131 Ma, and that the Jehol Biota most likely first appeared at ∼135 Ma. This expands the biota’s temporal distribution from late Valanginian to middle Aptian with a time span of about 15 My.

The Early Cretaceous diversification of birds was a major event in the history of terrestrial ecosystems, occurring during the earliest phase of the Cretaceous Terrestrial Revolution, long before the origin of the bird crown-group. Frugivorous birds play an important role in seed dispersal today. However, evidence of fruit consumption in early birds from outside the crown-group has been lacking. Jeholornis is one of the earliest-diverging birds, only slightly more crownward than Archaeopteryx , but its cranial anatomy has been poorly understood, limiting trophic information which may be gleaned from the skull. Originally hypothesised to be granivorous based on seeds preserved as gut contents, this interpretation has become controversial. We conducted high-resolution synchrotron tomography on an exquisitely preserved new skull of Jeholornis , revealing remarkable cranial plesiomorphies combined with a specialised rostrum. We use this to provide a near-complete cranial reconstruction of Jeholornis , and exclude the possibility that Jeholornis was granivorous, based on morphometric analyses of the mandible (3D) and cranium (2D), and comparisons with the 3D alimentary contents of extant birds. We show that Jeholornis provides the earliest evidence for fruit consumption in birds, and indicates that birds may have been recruited for seed dispersal during the earliest stages of the avian radiation. As mobile seed dispersers, early frugivorous birds could have expanded the scope for biotic dispersal in plants, and might therefore explain, at least in part, the subsequent evolutionary expansion of fruits, indicating a potential role of bird–plant interactions in the Cretaceous Terrestrial Revolution. Birds and plants have a close relationship that has developed over millions of years. Birds became diverse and abundant around 135 million years ago. Shortly after, plants started developing new and different kinds of fruits. Today, fruit-eating birds help plants to reproduce by spreading seeds in their droppings. This suggests that birds and plants have coevolved, changing together over time. But it is not clear exactly how their relationship started. One species that might hold the answers is an early bird species known as Jeholornis . It lived in China in the Early Cretaceous, around 120 million years ago. Palaeontologists have discovered preserved seeds inside its fossilised remains. The question is, how did they get there? Some birds eat seeds directly, cracking them open or grinding them up in the stomach to extract the nutrients inside. Other birds swallow seeds when they are eating fruit. If Jeholornis belonged to this second group, it could represent one of the early steps in plant-bird coevolution. Hu et al. scanned and reconstructed a preserved Jeholornis skull and compared it to the skulls, especially the mandibles, of modern birds, including species that grind seeds, species that crack seeds and species that eat fruits, leaving the seeds whole. The analyses ruled out seed cracking. But it could not distinguish between seed grinding and fruit eating. Hu et al. therefore compared the seed remains found inside Jeholornis fossils to seeds eaten by modern birds. The fossilised seeds were intact and showed no evidence of grinding. This suggests that Jeholornis ate whole fruits for at least part of the year. At around the time Jeholornis was alive, the world was entering a phase called the Cretaceous Terrestrial Revolution, which was characterized by an explosion of new species and an expansion of both flowering plants and birds. This finding opens new avenues for scientists to explore how plant and birds might have evolved together. Similar analyses could unlock new information about how other species interacted with their environments.

Here we report on a new Early Cretaceous eutherian represented by a partial skeleton from the Jiufotang Formation at Sihedang site, Lingyuan City, Liaoning Province that fills a crucial gap between the earliest eutherians from the Yixian Formation and later Cretaceous eutherians. The newspecimen reveals, to our knowledge for the first time in eutherians, that the Meckelian cartilage was ossified but reduced in size, confirming a complete detachment of the middle ear from the lower jaw. Seven hyoid elements, including paired stylohyals, epihyals and thyrohyals and the single basihyal are preserved. For the inner ear the ossified primary lamina, base of the secondary lamina, ossified cochlear ganglion and secondary crus commune are present and the cochlear canal is coiled through 360°. In addition, plesiomorphic features of the dentition include weak conules, lack of pre- and post-cingula and less expanded protocones on the upper molars and height differential between the trigonid and talonid, a large protoconid and a small paraconid on the lower molars. The new taxon displays an alternating pattern of tooth replacement with P3 being the last upper premolar to erupt similar to the basal eutherian Juramaia. Parsimony analysis places the new taxon with Montanalestes, Sinodelphys and Ambolestes as a sister group to other eutherians. This article is part of the theme issue 'The impact of Chinese palaeontology on evolutionary research'.

The presented geological map at a scale of 1:75,000 shows a part of the Upper Mesozoic sedimentary belt exposed along the northern slopes of the Mongolian Altai in the Valley of Lakes (Western Mongolia). The Upper Jurassic to Lower Cretaceous formations of the Shiliin Nuruu Syncline, which have both lateral and vertical transitions, reach a total thickness of up to 1000 m. Poorly sorted conglomerates in the bottom of the sequence represent the proximal parts of the alluvial fans and grade up to the cycles of sandstones, mudstones, siltstones, marls, and coal seams of fluvial to lacustrine origin. Relics of terrestrial plants indicate a warm climate. The coalification corresponds to early brown coal. The rich Hauterivian–Aptian palaeontological association containing conchostracans, larvae of giant mayflies, gastropods, bivalves, and fish in the Anday Khudag and the Khulsan Gol formations could be compared with the Jehol Biota in northeast China.

In the last twenty years, the extraordinary discoveries of vertebrate fossils from the Jehol Biota not only have important implications for studying the evolution of major Mesozoic vertebrate groups, their paleobiostratigraphy and paleoenvironmentology, but also provide critical evidence for understanding the biodiversity changes of the Early Cretaceous ecosystem. Currently, the Jehol Biota in a narrow sense (i.e., distribution limited to western Liaoning, northern Hebei, and southeastern Inner Mongolia) comprises a vertebrate assemblage of at least 121 genera and 142 species. Among them are 13 genera and 15 species of mammals, 33 genera and 39 species of birds, 30 genera and 35 species of dinosaurs, 17 genera and species of pterosaurs, 5 genera and species of squamates, 5 genera and 7 species of choristoderes, 2 genera and species of turtles, 8 genera and species of amphibians, 7 genera and 13 species of fishes as well as 1 genus and species of agnathan. All these known 121 genera are extinct forms, and only a small percentage of them (e.g., agnathans, some fishes and amphibians) can be referred to extant families. The Jehol vertebrate diversity already exceeds that of the contemporaneous lagerstätten such as Santana Fauna from Brazil and the Las Hoyas Fauna from Spain, and is nearly as great as that of the Jurassic Solnhofen Fauna and the Eocene Messel Fauna from Germany. Therefore, The Jehol Biota undoubtedly represents a world class lagerstätte in terms of both fossil preservation and vertebrate diversity. The success of the Jehol vertebrate diversity had a complex biological, geological, and paleoenvironmental background. Analysis of the habitat and diet of various vertebrate groups also indicates that the habitat and dietary differentiation had played a key role in the success of the taxonomic diversity of vertebrates of various ranks. Furthermore, the interactions among vertebrates, plants, and invertebrates as well as the competitions among various vertebrate groups and some key morphological innovations also contributed to the success of the Jehol vertebrate diversity.

The results of the determination of the age of zircon grains from tuffites of the Leskovo Unit of the Unda–Daya Basin, Eastern Transbaikalia, are presented for the first time. The age of the youngest population of zircon grains is 145.8 ± 3.8 Ma, approximately corresponding to the Jurassic–Cretaceous boundary and indicating the Early Cretaceous age of most of the Leskovo Unit. Given that the similar taxonomic composition of ostracods from the middle part of the Leskovo Unit and the Valanginian–Lower Hauterivian Dabeigou Formation of northeastern China, our U–Pb age from the lower part of the Leskovo Unit allows confident correlations of these lithostratons.

The Istiodactylidae is a group of pterodactyloids characterised by large nasoantorbital fenestrae and labiolingually compressed teeth, with several records reported from the Early Cretaceous of northeastern China and western Europe. Here we report a new istiodactylid, Lingyuanopterus camposi gen. et sp. nov. from the Jiufotang Formation of Lingyuan, Liaoning, northeastern China. The holotype is represented by a near-complete skull, mandible and atlas-axis complex. It is distinguished from other istiodactylids by several characters, including two autapomorphies: short triangular tooth crowns with sharp mesial and distal carinae limited to the distal teeth, mandibular symphysis occupying approximately a quarter the mandible length. We also report the presence of helical jaw joints in istiodactylids, and provide a revised diagnosis of the clade Istiodactylidae, which includes five genera: Istiodactylus , Liaoxipterus , Nurhachius , Luchibang and Lingyuanopterus . Four pellets containing fish fragments were observed and are tentatively interpreted as bromalites of Lingyuanopterus . Although members of this clade possess similar skull morphologies, istiodactylids vary in terms of their dentition, with at least three forms from the Jiufotang Formation alone. This may represent different feeding strategies, and also indicate a similarity between the pterosaur assemblages of northeastern China and Britain during the Early Cretaceous.

Background Birds are key indicator species in extant ecosystems, and thus we would expect extinct birds to provide insights into the nature of ancient ecosystems. However, many aspects of extinct bird ecology, particularly their diet, remain obscure. One group of particular interest is the bizarre toothed and long-snouted longipterygid birds. Longipterygidae is the most well-understood family of enantiornithine birds, the dominant birds of the Cretaceous period. However, as with most Mesozoic birds, their diet remains entirely speculative. Results To improve our understanding of longipterygids, we investigated four proxies in extant birds to determine diagnostic traits for birds with a given diet: body mass, claw morphometrics, jaw mechanical advantage, and jaw strength via finite element analysis. Body mass of birds tended to correspond to the size of their main food source, with both carnivores and herbivores splitting into two subsets by mass: invertivores or vertivores for carnivores, and granivores + nectarivores or folivores + frugivores for herbivores. Using claw morphometrics, we successfully distinguished ground birds, non-raptorial perching birds, and raptorial birds from one another. We were unable to replicate past results isolating subtypes of raptorial behaviour. Mechanical advantage was able to distinguish herbivorous diets with particularly high values of functional indices, and so is useful for identifying these specific diets in fossil taxa, but overall did a poor job of reflecting diet. Finite element analysis effectively separated birds with hard and/or tough diets from those eating foods which are neither, though could not distinguish hard and tough diets from one another. We reconstructed each of these proxies in longipterygids as well, and after synthesising the four lines of evidence, we find all members of the family but Shengjingornis (whose diet remains inconclusive) most likely to be invertivores or generalist feeders, with raptorial behaviour likely in Longipteryx and Rapaxavis. Conclusions This study provides a 20% increase in quantitatively supported fossil bird diets, triples the number of diets reconstructed in enantiornithine species, and serves as an important first step in quantitatively investigating the origins of the trophic diversity of living birds. These findings are consistent with past hypotheses that Mesozoic birds occupied low trophic levels.