Author: Ren Fangyan
New research suggests that many species flapped their wings when birds first evolved to fly. For example, some small theropod dinosaurs, their movements are very close to the real sense of flight, although different from the evolutionary path of birds, some of them may still be able to fly with their feet off the ground and fly to the sky. (Image source: The University of Hong Kong)
Pei Rui, an associate researcher at the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences, is observing a theropod dinosaur fossil at the Museum of Natural History in New York, a warehouse for specimens.
He observed very carefully, using a variety of measurement tools, and even recorded the order in which each toe of a certain dinosaur was placed.
These exhaustive records help to understand how the birds' close ancestors evolved the ability to fly.
At the beginning of August, the relevant results were published in Contemporary Biology, and Pei Rui was the co-first author of the article.
"Previous studies have suggested that only a few birds' close ancestors had similar flight movements, but in fact, this range is larger than we thought," corresponding author Michael Pittman, a research assistant professor at the University of Hong Kong, told China Science Daily.
Reconstruct the evolution tree
Prior to joining the Institute of Paleovertebrates, Pei was engaged in postdoctoral research at the University of Hong Kong. By that time he had collected a certain number of theropod dinosaur specimens. Most of these mammalian dinosaurs were small in size, among them, chiropods and plesiosaurs, as dinosaurs, are considered to be close relatives of birds, and the more well-known small pirates belong to the chiropod family.
In reconstructing the evolutionary tree of the little raptors, Pei Rui noticed that as evolution continued, its size tended to gradually become smaller.
"So we added some data related to morphological function in the analysis, including the proportion of the front and back limbs, weight, etc., which seemed to hint at another trend: the flight ability of the small raptors was gradually increasing." Pei Rui realized that this might be a separate flight origin event.
In order to clarify the development of paleontology, researchers often need to use the genealogical evolution tree. But in the evolution of theropod dinosaurs into real birds, many parallel evolutionary events occurred. Due to differences in fossil calibration and feature selection, there are still ambiguities in some evolutionary trees.
For example, near birds are one of the evolutionary branches of theropod dinosaurs, which are divided into three branches: dinosaurs, chiloosaurs, and "wounded toothed dinosaurs", which have different orders in different evolutionary trees.
In order to avoid uncertainty as much as possible, the evolutionary tree reconstructed by Pei Rui et al. contained data on all small theropod dinosaurs as much as possible.
The researchers reconstructed the evolutionary tree of small theropod dinosaurs. (Image source: Contemporary Biology)
In collaboration with researchers from Argentina's National Council for Scientific and Technological Research (CONICET), they optimized the workflow of evolutionary genealogy analysis, assigning different weights to different evolutionary features to avoid ambiguous analysis results in parallel evolution.
Ultimately, the team identified two key indicators of whether near-bird birds have flight potential: wing load, which determines whether individual animals can fly continuously, and takeoff dynamics, which is whether the force generated by flapping wings can keep the animal off the ground.
Three breakthroughs
For paleontologists, a major obstacle to exploring animal evolution is the inability to see real-world scenes in the past. "We can't see how an animal can make an action or achieve what effect." Pei Rui said.
A roundabout approach is to compare paleontological specimen data with live animals. The research team also collected data on the wingspan length and weight of living birds, including flying birds and non-flying birds, and based on the data of flying birds, they reversed the wing load and take-off power, and brought them into the evolutionary tree for comparison.
In the article, taking the living birds as a reference standard, if the wing load of the fossil species exceeds 2.5 grams per square centimeter and the take-off power exceeds 9.8 Newtons per kilogram, it can be counted as passing the flight threshold set by the researcher, that is, it has the potential to actively fly.
The results showed that in addition to the broad bird: bird wing, two other species passed the threshold: the small robber dragon belonging to the chidrosaur class and the "bird dragon".
But in other species, researchers have still observed signs of active flight potential, such as long-feathered tarposaurs and Xiaoting dragons. "They've broken through at least one indicator, and probably have an active flight-like action, and they're adapting in their own way." Pei Rui said.
The analysis found that a significant proportion of near-bird dinosaurs may have had flight potential. (Image source: Contemporary Biology)
"Most people believe that dinosaurs' ability to fly is limited, but the potential of animals is beyond human imagination." Pei Rui said.
If you observe the movement behavior of a parrot, you will find that the order of placement of its front and back toes changes when climbing branches. "Sometimes one toe is in front, three toes are behind, sometimes two in front and two back, sometimes three before and one back, and the structure of the body is flexibly used according to the different scenes," Pei Rui said.
But in paleontological studies, it is generally only based on the fossil morphology of a certain animal that is most likely to speculate about the single movement of its toes.
In addition, in living birds, researchers have also observed an action: birds flapping their wings while running up the slope, and this near-flight move is also likely to exist in small theropod dinosaurs.
"Next, we'll delve into the interesting phenomenon of these non-birds flapping their wings and find out more reasons why dinosaurs didn't really fly." Pittman concluded.
Related thesis information: https://doi.org/10.1016/j.cub.2020.06.105