Angiosperms, or Flowering Plants, are dominant members of almost all terrestrial (and some marine) ecosystems today. For a long time their origin was considered mysterious, but in the past two decades a wealth of new fossil material has gone a long way to unravelling this mystery. However, while these fossils can help us understand the phylogeny of the early Angiosperms, the way in which they came to dominate modern food webs through interactions with other organisms is harder to unravel. Modern Birds have a complicated relationship with Angiosperms, often being herbivores, but also pollinators and seed dispersers, but, as with Angiosperms, the fossils of early Birds tell us a limited amount about their ecological roles.
The Early Cretaceous Jehol Biota of northeastern China is an important resource for our understanding of the early evolution of both Angiosperms and Birds, preserving some of the earliest known Angiosperm fossils as well as a wide diversity of Birds. The Jehol Birds show a wide variation in skull shape, dentition, and general morphology, suggesting a range of ecological and dietary specializations, although it is believed that the majority of these Birds were arboreal (tree-dwelling), and had a diet not dissimilar to modern arboreal Birds, including Insects, small Vertebrates, and some plant matter. These dietry assumptions have been supported by some direct fossil evidence, from the preserved stomach contents of Jehol Birds, as well as the presence of what are believed to have been gastroliths in the gizzards of some specimens. Nevertheless, this falls a long way short of the dietary variation seen in modern Birds, and leaves questions about the extent to which the rise of the Angiosperms and that of the Birds might be connected.
In a paper published in the journal Nature Communications on 28 July 2023, Yan Wu of the Key Laboratory of Vertebrate Evolution and Human Origins at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, Yong Ge, also of the Key Laboratory of Vertebrate Evolution and Human Origins, and of the Department of Archaeology and Anthropology at the University of the Chinese Academy of Sciences, Han Hu of the Department of Earth Sciences at the University of Oxford, Thomas Stidham, again of the Key Laboratory of Vertebrate Evolution and Human Origins, and of the College of Earth and Planetary Sciences at the University of Chinese Academy of Sciences, Zhiheng Li and Alida Bailleul, again of the Key Laboratory of Vertebrate Evolution and Human Origins, and Zhonghe Zhou, once again of the ey Laboratory of Vertebrate Evolution and Human Origins and College of Earth and Planetary Sciences at the University of Chinese Academy of Sciences, present the results of a study in which they attempted to gain a deeper understanding of the relationship between early Birds and Angiosperms by examining the stomach contents of a Bird from the Jehhol Biota for phytoliths.
The specimen examined, IVPP V14978, is a subadult Jeholornis prima, one of the earliest branching Birds other than Archaeopteryx, and known from over a hundred well-preserved specimens. Jeholornis prima retains the long bony tail and akinetic skull of non-Avian Theropod Dinosaurs, but is starting to show the reduced dentition and other derived Bird-like features. A morphometric analysis of mandible and alimentary tract has suggested that it was at least partially frugivorous.
Phytoliths are opaline silica crystals which build up in the leaves of Plants as dissolved monosillicic acid taken up with groundwater is precipitated out as water is lost from the leaves via transpiration. Phytoloths have a long fossil record, are known to be resilient to dissolution, and, importantly, are taxonomically significant, with different groups of plants producing different shaped phytoliths. Previous studies have been able to reconstruct the diets of Dinosaurs by examination of phytoliths recovered from their teeth and faeces.
Specimen IVPP V14978 s one of the smallest known Birds assigned to the Jeholornithidae, and is considered to be a subadult Jeholornis prima, based on the presence of a long bony tail, a strut-like coracoid, and a largely edentulous jaw. About two grams of carbonized material, interpreted as potential food residue within the gastric region of the Bird, was scraped from this specimen with a clean razor blade for analysis. Processing of this sample produced 418 phytoliths, from the gastric region of the Bird, but none from the surrounding sediment, supporting the hypothesis that these were derived from the Bird's stomach contents, not the contamination from the surrounding environment.
The majority of these phytoliths (68%) have a blocky body with wavy ridge ornamentations and a size of ranging between about 45 and 90 μm. A blocky form and wavy ridgeline is consistent with several types of Plants, including Ferns, Conifers, Grasses, Broad-leaved Trees, and members of the Magnoliales. While Grasses, Ferns, and Conifers do produce blocky phytoliths with wavy ridgelines, these are rectangular, while those from the sample are polyhedral. Furthermore, the phytoliths of most Broad-leafed Trees have very weak ridgelines, while those of the sample are well developed, making it probable that the phytoliths from the sample came from a Magnoliid. Comparison of the phytoliths to a large selection (over 4000 samples) of modern phytoliths from known plants found that they were most similar in form to those of the Coconut Magnolia, Lirianthe coco, and Machilus nanmu, a type of Laurel found in south China, both members of the Magnoliales.
Also present in the sample are two phytoliths with a round shape and a protuberant centre, resembling the phytoliths of modern Eudicots, and 132 phytoliths which could not be identified. While these phytoliths cannot be identified, they do provide evidence that the living Bird was consuming foliage from more than one sort of Plant.
The Magnoliales and Laurales are currently thought to have diverged about 130 million years ago, based on molecular clock dating, with comprehensive adjustments derived from fossil calibrations. Other estimates for this divergence are 117 million years, based on a mega-phylogeny of plastid genomes, and o a reconstructed time frame derived from transcriptomes and nuclear genomes. None of these estimates is at odds with the age of the Jiufotang Formation, from which specimen IVPP V14978 was obtained, which has been dated to about 120 million years before the present, on the basis of radioisotope dating of a volcanic ash layer. The palynological record also suggests that the Jehol Biota included members Angiosperms such as Magnoliapollis, Liliacidites, Asteropollis, Knemapollis, and Chloranthus. Wu et al.'s data suggests that Magnoliids were present and that their leaves formed part of the diet of Jeholornis prima.
The fossils of the Jehol Biota were laid down in lake environments, within an ecosystem with a mixture of Gymnosperm and Angiosperm Plants, as well as a wide variety of Vertebrate and Invertebrate Animals. Many modern Birds are arboreal specialists, and Birds such as Jeholornis prima appear to have been early pioneers of this environment, showing adaptations to an arboreal environment, and having a diet which included fruits, seeds, and leaves. An ecological shift to living in the treetops combined with a dietary shift away from carnivory to include a range of Plant foods could have been a significant driver of Avian evolution away from the ecological niche occupied by their closest relatives, the Dromaeosaurs and Troodontids.
An ecological shift to living in trees combined with an ecological shift to a largely herbivorous diet in forests increasingly becoming dominated by Angiosperms could potentially have had a significant impact on the evolution of the early Birds, during their Cretaceous radiation. Such a transition appears to have been marked by the adoption of gastroliths, as well as a reduction in dentition, a thinning of the enamel on the remaining teeth, and changes in the shape of the skull. A prior morphometric analysis of the mandible of Jeholornis found that it was a generalist herbivore, possibly with a diet similar to that of modern Anserform or Galliform Birds, although it also showed a strong similarity to the Hoatzin, Opisthocomus hoazin, a modern specialist leaf-eater. Wu et al.'s study supports the idea that Jeholornis was a herbivore, with leaves forming at least part of its diet. Leaves are generally considered to be a poor diet for Birds, due to the low amounts of energy they provide, which is not sufficient to support the energy requirements of sustained flight. Jeholornis appears to have had a diet which included seasonally available fruits, and to have used gastroliths during an alternative season to help it process other foods. It is quite possible that it consumed some leaves as part of a wider herbivorous diet, as it the case with modern groups such as Mousebirds.
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