Darwin called it a "pesky mystery" in the early 19th century, puzzled by the discovery of a sudden abundance of angiosperms in the Strata of the Cretaceous period (145m to 66m years ago), which seemed to contradict the evolutionary view that evolution is gradual.
Four years ago, a sino-American team led by the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences (NANJING Institute of Palaeontology) discovered new fossils in Inner Mongolia, proving that angiosperms did not appear in "sudden abundance" and that their ancestral groups date back 126 million years.
The mystery of the origin of angiosperms
Angiosperms are considered to be large groups with well-evolved and rich species (about 300,000 species) and complex ecological habits in the plant kingdom. They are also an important part of human survival and closely related to human food, clothing, housing and transportation. For example, the vast majority of food, linen clothing, and building wood are derived directly or indirectly from angiosperms.
The origin and early evolution of angiosperms have always been important scientific questions in evolutionary biology because of their vital influence on humans and other life on earth.
"Flowers are the most important feature that distinguishes angiosperms from other large groups of plants, and are an important advantage for angiosperms to become the dominant group in terrestrial ecosystems." The origin and evolution of flowers are key to understanding the origin of angiosperms and their relationship to the lineage development of large groups of other seed plants, said shi Gongle, lead author of the paper and a researcher at the Nanjing Institute of Paleontology. The mystery of how flowers evolve is one of 125 world class scientific puzzles listed by Science.
According to Shi gongle, seed plants include cycads, Ginkgo biloba, conifers, Ma Ma and angiosperms. The ovules of Cycads, Ginkgo biloba, conifers and Ma Ma are called gymnosperms when pollen directly enters the micropore tube at fertilization.
Previously, phylogenetic reconstruction of seed plants by molecular genealogical studies had confirmed that the living gymnosperms were a monophyletic group that formed sister groups with angiosperms, but which separated before the Permian period, which began 299 million years ago and ended 250 million years ago.
"Living gymnosperms are evolutionally distant from angiosperms and have reproductive organs very different from flowers, making it difficult for us to understand the origin and evolution of flowers." "We therefore need to look for close relatives and ancestral groups of angiosperms in extinct fossil groups of seed plants, which hold clues about the origin and early evolution of flowers."
Look for close relatives and ancestors
In the summer of 2017, Shi gongle and a TEAM from the United States discovered a particularly buried early Cretaceous silicified flora (dating back 126 million years) in the Huolinhe Basin in eastern Inner Mongolia, China.
"The silicified fossil is an ideal material for palaeobotany research because it not only preserves the three-dimensional morphology of plant organs, but also details of plant tissues and cells," he said.
In cretaceous silicified fossils, the team found a large number of well-preserved seed-bearing organs of heliosetia, an extinct Mesozoic seed plant. The team then used traditional acetate membrane tearing techniques and advanced microscopic tomography (CT) techniques to reconstruct the three-dimensional appearance and internal anatomy of the fossil plants.
Comparative morphology and paleo-plant anatomy show that the curved shell enclosing the seeds of Heliosetia nei is very similar to the outer integument of anatrophic ovules in the more primitive groups of angiosperms.
Previous studies have shown that gymnosperms have only one layer of nucellus to cover and protect the seeds, while angiosperms have two layers of nucellus. The outer integuage, carpel (pericarp) and stamen of angiosperms are considered to be the three most important features that distinguish angiosperms from gymnosperms.
The monolayer of gymnosperms' seeds is thought to correspond to the endotubule of angiosperms, Shi notes. The outer integument of angiosperms is a unique structure, its development is related to the inward-bending of the ovule, and the genes controlling the development of the outer integument and the inner integument are different.
It is found that the outer and inner integument of angiosperms seeds have different origins. Evolutionary biologists have long realized that the seeds of Mesozoic sedum ferns were encased in a curved shell that may be related to the origin of angiosperms.
"But the vast majority of Mesozoic fossils with curved shells are poorly preserved and lack anatomical details, hindering our understanding of the relationship between the curved shells of fossil plants and the outer integument of angiosperms." "Said Skunk.
"Curved shell" found homology
In addition to the new fossil material from the Early Cretaceous in Inner Mongolia, the joint research team used microtomography to re-examine the Opening plant from the Jurassic in Yorkshire, England, and the Petriellaea fossil from the Triassic in Antarctica.
Comparative morphological studies show that the Mesozoic extinct seed plants with curved shells include helmetheophytes, open plants, Petriellaea, etc. The curved shells enclosing their seeds are essentially the same type of structure, and these curved shells are also homologous with the outer integume of angiosperms ovule.
It is concluded that the Mesozoic extinct seed plants with curved shells are close relatives or ancestors of angiosperms. The inverted ovules and epicycles of angiosperms are a continuation of the early evolutionary features of the ancestral groups of angiosperms. The emergence of the inverted ovules and epicycles is much earlier than the appearance of carpel and stamen of angiosperms, which are two key characteristics of angiosperms that distinguish angiosperms from gymnosperms.
"Based on the published seed plant morphological data matrix and our new understanding of the homology of seed organs in seed plants, the research team reconstructed a morphological data matrix consisting of 31 taxa and 83 traits." The team used maximum minimalism, maximum likelihood and Bayesian methods to analyze the phylogenetic development of large groups of seed plants based on the latest genomic data as a molecular framework constraint, Shi said.
The results of the above three analysis methods all indicate that the Mesozoic extinct seed plants with curved shells, including helmetheophytes, open plants and Petriellaea, are close relatives of angiosperms, and it is likely that the ancestral groups of angiosperms are hidden in these groups.
"We refer to the large group of plants that includes angiosperms and their relative fossil groups as Angiophyte. The Mesozoic extinct seed plants with curved shells are the basal stem group of the angiosperms, while the modern angiosperms are the living angiosperms." According to Shi gongle, the identification of the basal stem groups of subclassed plants and their key features partly answers the question of the origin of angiosperms and flowers.
It is reported that the basal stem group of subclass plants with curved shells had rich diversity and wide distribution in Mesozoic era. Among them, the earliest fossil record of helmet seed plants can be traced back to the Late Permian, indicating that the ancestral group of angiosperms had been differentiated 260 ~ 250 million years ago. The team also found that glossophytes could also have been subclassed in the Permian.
"The morphology and homology of the seed-bearing organs of the basal stem group of the subclass of angiosperms are very important for our understanding of the origin and evolution of the carpel, a key feature of the angiosperms," says Shi.
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