Ecological environment of Li's source predators and reconstruction of Lujiatun fauna (Zhao Chuangyi)
Once upon a time, the auditory organs of mammals were connected to the chewing organs. Imagine how loud the "bar chirp" sound is transmitted directly into the ear through the bones during a big meal! Eating less pleasantly, and having difficulty being vigilant enough to be in the wind and grass outside the world when eating, nature cleverly wields the "hand of evolution" to separate the two organs, each working better, and more conducive to the survival of living things.
However, when exactly did the two organs of "eating" and "listening" separate? Paleontologists have been looking for conclusive fossil evidence. Recently, this puzzle has been solved by Chinese paleontologists.
The day before yesterday, the journal Science published online the research results of Mao Fangyuan, Wang Yuanqing and Meng Jin of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences on the early chalky basal stem mammal Li's source predator: In the evolution of the lower pore vertebrates, it was once an integrated auditory and chewing structure, regulated by their respective genetic mechanisms, and adapted to natural selection in mammals to improve the efficiency of hearing and chewing, showing a modular divergence evolution Lee's source skimming perfectly shows the phenotypic characteristics of the evolution and separation nodes of the two modules in the basal stem mammals; the isolated auditory and chewing modules enhance their variability or evolutionability, becoming one of the intrinsic drivers of the possible radiation evolution of mammals.
Schematic diagram of modular evolutionary separation of mammals (mammals) hearing and chewing morphology (Courtesy of Mao Fangyuan)
Modular evolution is a concept that combines evolutionary biology and developmental biology. Organisms can be broken down at different levels into small morphological, developmental, or functional units. The forelimbs of vertebrates are an example of homologous module evolution that can evolve into wings, fins, or human hands, but do not affect the morphology and function of other parts, such as the hindlimbs.
The researchers propose that the mammalian jaw and auditory bone are a particular example of modular evolution. Its uniqueness lies in the fact that this evolution has the characteristics of diversification and functional specificity of homologous modules, and gradually evolves from a complex structure of complex morphological and functional integration to two organ modules with completely independent morphology and function.
Based on six specimens preserved in three dimensions in the early Cretaceous Rehe biota (Lujiatun Layer), the researchers used microscopic tomography (CT) and three-dimensional reconstruction methods to establish a new genus of the rodent: Li's source predator, one of the authors of the memorial article, and one of the founders of early Chinese mammal research who died in October this year.
Origolestes lii mode CT-reconstruction
The buried forms of the specimens all show that these animals died in a resting state, and the fossils were basically not disturbed by the later stages, and the pairs of preserved specimens may reflect some social behavior of the basal mammals.
Evidence of the same genus, dentition, jawbone, and abrasion marks of the same genus show that in addition to opening and closing movements, the jaw of the animal has lateral movement and rotation along the long axis in addition to opening and closing movements. The multidirectional movement process of the jaw during jaw chewing is likely to be one of the selective pressures that lead to the detachment of the auditory bone of the middle ear of the mammal from the tooth bone and the myrnoid cartilage.
Origolestes lii teeth morphology, abrasive details and bite kinematic relationships
A large number of studies of morphological anatomy, developmental biology, and genetics have long proposed that the mammalian auditory bone (hammer bone, anvil bone, external drum bone) and the reptile posterior bone of the tooth bone (joint bone, anterior joint bone, square bone, corner bone) have a homologous relationship. Studies in recent years have further shown that these bones are regulated by the same genetic mechanisms during the early development of various taxa. These mechanisms can even be traced back to the development of the jaws of fish.
In the past two decades, the ossified Mai's cartilage of reptiles and other mesozoic mammals found in western Liaoning, China, and the hearing bones of Liao-tipped beasts provide evidence of the evolutionary transition pattern from the middle ear of the lower jaw to the middle ear of a typical mammal. However, in the transitional middle ear, the auditory bone, although detached from the dental bone, is still tightly twisted with the ossified Myrdier cartilage, which is connected to the dental bone. Therefore, the functions of hearing and chewing have not yet been completely separated, and there are still influences on each other.
For the first time, multiple specimens of Lee's genie exhibit a key feature, namely the absence of a boneless link between the auditory bone and myrnelloy cartilage, representing the key node in the separation of the auditory and chewing modules in mammalian evolution, bridging the gap between the transitional middle ear and the typical mammalian middle ear in the evolutionary process of phenotypic characteristics, representing a more advanced evolutionary stage in the phylogenetic and characteristic evolution of basal mammals.
Morphological and anatomical position of the middle ear, inner ear, and Mych cartilage in Lee's source. H, the arrow points to the point where the auditory bone is separated from the Myrnos cartilage. I, the arrow points to the bone junction of the transitional middle ear ossicles and the Myrdochondrium cartilage.
The establishment of this isolated phenotype provides a reference and validation of evolutionary time and phenotype for models and hypotheses on the evolution of the middle ear in mammals in developmental biology.
In terms of morphological function, the separated auditory and chewing modules eliminate the physical constraints that interfere with each other, increasing the possibility of evolution and multi-directional adaptation of the two modules; the auditory organs have the potential to develop to high-sensitivity and high-frequency hearing, and the chewing organs have also acquired the possibility of diversifying teeth and bite patterns to ingest different foods.
Thanks to the results of high-precision CT scans, Lee's genie also demonstrated near-complete morphological features of the ossic bones of basal mammals (even among all known Mesozoic mammals), providing a credible fossil basis for further in-depth study of the evolution of mammalian osseous bones. In particular, in addition to the stapes, hammer bones, external drum bones, and anvil bones that all mammals have, the hearing bones of the Lee's source skimmer retain the upper bone.
The structure of the inner ear of Lee's source skimmer and the morphology and relative relationship of the brain cavity (A-I), and the comparison with single-hole (J) and marsupial (K). Yellow is the inner mold of the inner ear structure, and blue is the vascular system associated with the inner ear.
Since the study of the hearing bone of the Liao-tipped and Ahocenes has proposed the possibility of the existence of the upper bone, there is growing evidence that the upper bone entered the middle ear in the early evolution of mammals. The Lee's Source Predator provides the most credible three-dimensional evidence of the Upper Bone to date.
The presence of the upper bone in the basal stem also poses a challenge to paleontological research and modern developmental biology: is this ossuary bone directly lost in the evolution of mammals or in what way is it fused into a certain part, resulting in the absence of the upper bone in the middle ear of the living species? The discovery of more relevant fossils and more detailed research into developmental biology may be able to answer this question.
The inner ear, which is closely related to the auditory bone of the middle ear, as an important part of the auditory organ, also embodies a unique evolutionary structure in the trinosaurus-mammal. Compared with the living mammals and single holes, this cochlear tube with secondary bone plate is elongated in a straight line to the middle of the skull base, and the reticular veins are enriched around the cochlea; the length and width of the cochlear tube of lee-source maragulers are even to the limit of the rock bone, and its volume is larger relative to the proportion of the brain and skull, almost to the limit, which can be regarded as an evolutionary "experiment" of different elongation methods of the mammalian cochlea.
Basal mammals (including humans) have enhanced their variability or evolutionability in various experiments, eventually succeeding as one of the protagonists of the dominant Earth ecosystem.
This research has been funded by the National Natural Science Foundation of China, the Strategic Pioneering Science and Technology Special Project of the Chinese Academy of Sciences (Category B), and the Youth Innovation Promotion Association of the Chinese Academy of Sciences, and special thanks to the Chaoyang Jizantang Museum for its strong support.
Author: Xu Qimin
Photo: Unless noted, all are provided by Mao Fangyuan Editor: Xu Qimin Responsible Editor: Ren Tsuen