They only digested the pulp and then "sowed" the intact seeds: Paleontologists have uncovered the oldest known evidence of fruit consumption in a bird-like creature called Jeholornis. Skull features and fossil stomach contents indicate this diet. About 120 million years ago, Jeholornis could thus have been at the beginning of the cooperative coevolution between fruit-bearing plants and birds, the scientists say.
Instead of being naked, they form their seeds with fleshy coverings: a large group of plants produces structures that are called fruits. In many cases, the partly colored, sweet and nutrient-rich structures are intended to attract animals that eat them together with the seeds they contain. The hard kernels often survive the passage through the digestive system and are excreted as viable. This is how the fruit-bearing plants ensure their spread. They especially like to use flying messengers. In the course of evolution, many plant and bird species have adjusted to the mutually beneficial relationship. But how and when did it start?
“Birds are now major consumers of fruit and play an important role in seed dispersal, but so far there has been no direct evidence of early bird consumption of fruit. This complicates our understanding of the origins of this important plant-animal interaction," says Han Hu of Oxford University. To gain new insights into the subject, Hu and her colleagues looked at a creature that lived in what is now China in the early Cretaceous Period and is believed to be one of the most primitive members of the avian group: a raven-sized creature with feathered wings named Jeholornis.
Were seeds or pulp used for food?
"In the first Jeholornis fossil discovered, the stomach contents were superficially identified as seeds, suggesting the animal fed on them," says co-author Jingmai O'Connor of the Field Museum of Natural History in Chicago. But then the suspicion arose that Jeholornis might have eaten not just the seeds but whole fruit, leaving only the seeds as they were harder. "In this study, we therefore specifically investigated the question of whether Jeholornis only fed on seeds or rather on fruits," says the scientist.
The team examined the numerous other fossils that have since been discovered. The researchers selected the specimen with the best preserved skull and scanned it to create a detailed model of the structures. They report that the analyzes revealed that Jeholornis has many features that are more reminiscent of a dinosaur than a modern bird. However, the skull showed some features that could indicate a possible diet of fruit. Compared to other beings of time, Jeholornis possessed rather poorly developed tooth structures. Comparisons of cranial features with those of contemporary bird species that crack open seeds with their beaks also suggested that Jeholornis was probably incapable of seed cracking.
"However, one cannot yet clearly infer different diets from the characteristics," emphasizes O'Connor. Because it is known that some birds that can digest seeds break down the hard structures only in the stomach. "They swallow stones that help them break down their food," says O'Connor. The team reports that gastric stones have been found on some Jeholornis fossils. But they may have served to crush other tough food elements of the animals. Because in no case were they discovered together with seeds. The clearest finding, however, is that all seeds found in the stomach cavities of Jeholornis were always intact, the researchers emphasize. Accordingly, they were apparently not used for nutrition.
Beginning of a partnership coevolution
The bottom line is that the findings indicate that Jeholonis ate different foods at different times of the year, the paleontologists explain: if fruit was available, it ate it together with the seeds and later passed it on intact in the faeces. When there was no fruit, he ate other and harder food, which he crushed with the help of the stomach stones. The Jeholornis specimens with the intact seeds in their stomachs must have died during the season when certain plants were feared. This seasonal diet corresponds to a trait also found in many modern birds, the researchers explain.
"Our results are the earliest known evidence of fruit consumption in an animal," O'Connor said. The study sheds special light on the shared evolutionary history of fruit-bearing plants and birds. "They may have been recruited as highly mobile seed dispersers in the earliest stages of evolution," says Hu. This could have been an important factor in the intensive spread of fruit-bearing plants in the Cretaceous period. Conversely, the system may also have contributed to advances in animal flight technology: "A diet containing fruit may have increased evolutionary pressures on Jeholornis to be better flyers," says O'Connor. The team now hopes that their study will inspire other scientists to take a closer look at the factors behind the so-called terrestrial revolution in plants and birds in the Cretaceous period.
Source: Field Museum, Article: eLife, doi: 10.7554/eLife.74751