Why did birds lose one ovary on the way to evolution
During the Early Cretaceous, a series of characteristic changes occurred in the evolution of birds. Many key links in the middle are still a mystery. For example, why dinosaurs have ovaries and fallopian tubes on both sides, but birds have only left ovaries and fallopian tubes.
Our reporter Li He
What happened when birds evolved from dinosaurs? Following the publication of the paper "The Significance of the Preservation of Early Cretaceous Bird Follicles for the Study of Dinosaur Reproductive Behavior" in the journal Nature in 2013, Zhou Zhonghe's team from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences recently published an article in the journal Biological Communications. The latest related research results were published online: Bird soft tissue fossils can be preserved for a long time, and the loss of the right fallopian tube and ovary may occur during the transition from dinosaurs to birds.
About 120 million years ago in the Early Cretaceous, a series of evolutionary events from dinosaurs to birds occurred during this period. Compared with dinosaurs, a series of key characteristics of evolved birds have changed, and reproductive organs are one of them. Some specimens show that female dinosaurs are similar to current reptiles. They still preserve the ovaries and fallopian tubes on both sides, but the females of living birds only retain the left ovary and fallopian tubes. Since when did birds lose their ovaries and fallopian tubes? Is this related to the weight loss needed to adapt to flying?
The evolution of hollow skeletons and coccyx bones is still a mystery
The team of Zheng Xiaoting, director of Shandong Tianyu Natural Museum and professor of Linyi University, has been cooperating with Zhou Zhonghe's team on early bird evolution. Zheng Xiaoting told the reporter of Science and Technology Daily that the closest dinosaur to birds is like a crocodile, with two functional ovaries and fallopian tubes. However, in the early Cretaceous Rehe bird and anti-avian specimens, the researchers found that early birds only retained the left ovary, and speculated that the degradation of the right ovary of the birds may have occurred during the transition from dinosaurs to birds. It may be related to the weight loss needed to adapt to flight.
"For birds, in order to be more conducive to flying, they have always reduced their weight to the maximum in their evolution." Zheng Xiaoting said, "There are many aspects that reflect the weight loss of birds, such as simplified bone healing, and birds All of the bones are hollow, and these are all to minimize weight."
The unique hollow skeleton of birds is the most direct way to effectively reduce body weight. The skeleton of flying birds is thin and long, hollow and without bone marrow. The skeleton only accounts for 5% to 6% of body weight. But the weight of the skeleton of land animals accounts for nearly 40% of the total body weight.
The flying feathers and tail feathers in feathers are the flying feathers. The flying feathers grow on the wings and the tail feathers grow on the tail. Zheng Xiaoting said that the most primitive birds, including Archaeopteryx and Rehe bird, have a long bony tail like a reptile. Other birds have a shortened tail with a healing structure called the coccygeal bone. In living birds, the tail bone is usually connected to a muscle structure used to control the tail feathers-tail feather bulbs. The tail feather bulbs control the development of the tail feathers during flight. This control of the shape of the tail feathers greatly improves the flying function of the tail.
The Jehol Biota is an ancient biota that lived in East Asia about 120 to 135 million years ago. Its bird fossils preserve the earliest known birds with tail feathers, as well as various tail feather shapes. , But no direct fossil record of the transitional evolution of the tail has been found. The differences in the morphology of tail feathers and caudal healds among different branches of birds in the Early Cretaceous have led to the academic view that the tail feather bulbs originated with the plowhead-shaped caudal healds of living birds. Conclusive evidence preserved in the fossils of a newly discovered bird of the Penguinidae, the Chia piscina, shows that anti-birds also had fan-shaped tail feathers that helped them fly. The brachyura and fantail accompany, appear together in living birds and birds, and now appear in Pang bird family, which fully shows that these structures originated together, and together with the tail feather bulbs constitute a Compound tail. The simplest speculation is that the tail feather bulb is an ancestral sign of the caudal hemela, and then during the evolution process, some birds and anti-birds unfortunately lost this structure and become like today's ostriches.
Whether soft tissue can be preserved for a long time still needs to be studied
After the 2013 paper "The Significance of the Preservation of Early Cretaceous Bird Follicles for Research on the Reproductive Behavior of Dinosaurs" was published, some scholars questioned whether the soft tissue could be stored for such a long time, and speculated that the follicle fossils were actually undigested in the stomach Plant seeds.
Zheng Xiaoting does not agree with this, he believes that the structure of plant seeds and follicle fossils are not the same. Zhonghe Zhou's research team used high-resolution CT, energy dispersive X-ray spectroscopy, and bone tissue slice staining to compare follicle fossils and living specimens. The research results also confirmed that this is an ovarian follicle fossil, showing high-resolution details of follicular tissue, including contractile muscles and vascularized tissue, which are similar to the surrounding tissues of egg yolks that are being formed in living birds. Follicles provide nutrients and help birds during ovulation to ovulate smoothly; the findings of smooth muscle fibers, collagen fibers, blood vessels and other soft tissues are consistent with the hypothesis put forward in earlier research papers, that is, similar to related tissues of living birds, and ingesting seeds The assumption does not match.
"Soft tissue is indeed not easy to preserve, but through rapid deoxygenation and other conditions, soft tissue can be better preserved." Zheng Xiaoting said, especially feather soft tissue, more preserved with fossils.
In recent years, in addition to the preservation of complete bones, delicate feathers and other skin derivatives, the fossils of the Jehol Biota, including alveoli and other soft tissue visceral tissues have also been continuously discovered in fossil studies, proving that soft tissues have undergone complex and long-term scales. After being buried, it may be preserved.
Zheng Xiaoting said that a study of a specimen of Confucius protothecoides that preserved follicular tissue and soft tissues of the wings found that the protocomitans had an anterior wing membrane and a posterior wing membrane. The internal net-like support structure preserved by the anterior wing membrane is similar to that of living birds. Very similar, the soft tissue structure shows that the anterior wing membrane can form an arc-shaped section and can generate lifting force.
"However, what's more interesting is that there are individual specimens, when feathers are well preserved, bone tissue is poorly preserved. There are also individual sturgeon specimens, when its digestive system and skin structure are preserved, bone tissue is preserved. Some specimens with better bone preservation tend to have poor preservation of soft tissues.” Zheng Xiaoting said, this reason needs further study.
Digestive system and other differences may be related to food structure
Zheng Xiaoting said that through the study of early birds, many interesting places will be discovered. For example, different birds have different digestive systems, which may be related to the different food structures of early birds.
For example, the study of several swallow bird specimens with fish remains in the crop found that when birds in the Early Cretaceous period were full, they would store a portion of their food in the esophagus for later digestion. The teeth of swallows are not used to chew food, but to capture food; different parts of the digestive tract of swallows have advanced muscle systems, which have different peristaltic functions in the process of digesting food, and advanced digestive tract systems have emerged In Cretaceous birds with teeth.
Zheng Xiaoting said that a study of an ancient beaked bird specimen from the Jiufotang Formation of the Early Cretaceous found that the preserved soft tissue morphology showed that the lungs of ancient beaked birds were very similar to those of living birds. This shows that specialized lungs that support birds to fly and provide effective oxygen have evolved in birds 120 million years ago.
Among the vertebrates that breathe air, birds have the most complex structure and the most effective respiratory system, which can meet the high energy consumption requirements of birds in flying in areas with low oxygen. The ancient-beaked bird is considered to be one of the most primitive living birds known. It may be in a relatively primitive stage of the respiratory system from the perspective of bone structure, but the microstructure of its lungs is very modern. This finding may indicate that the physiological evolution of many soft tissue structures that are vital to living birds, such as the digestive system and respiratory system, occurred before the adaptation and evolution of bone morphology.