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For all kinds of creatures on Earth, eating is not a simple thing. From the formal point of view, it can be described as strange, some are fast and elegant, such as reptile chameleons by catapults and adhesives to swallow food (Picture 1); some are clumsy and funny, such as the whale-headed stork of birds repeatedly swallowing and torturing food, and are ridiculed by netizens to make themselves endangered by strength (Figure 1, lower left). In terms of feeding patterns, mammals have a high sense of superiority over amphibians, reptiles, and birds that swallow dates because they tend to be able to chew food comfortably in their mouths (bottom right, Figure 1).
Figure 1. Chameleons, whale-headed storks and mammalian camels feed. The image comes from the Internet
So, is the way of eating the core competitiveness of mammals that have come after evolution in nature? Scholars such as Bhart-Anjan S. Bhullar of Yale University in the United States have tried to find answers from mammalian chewing studies.
In order to achieve the purpose of the study, the chewing characteristics of Therian, the dominant branch of mammals, must be analyzed from an evolutionary perspective. This clade contains major mammal types such as even-hoofed, rodents, primates, and even humans. And the best object of study is the common ancestor of the euphrasiaceae, at the node of the evolutionary branch, obviously, such paleontology is already fossilized today. But Bhart-Anjan S. Bhullar's team identified as a study object a mammal that was very similar to the ancestors of the euparathmic subclass, the "living ancestor" of the North American possum (Didelphis virginiana), which is also a "net red drama master" (in nature, it is often performed to fake death to avoid predators, Figure 2 left, there are even possums in German zoos that only fight cocks, right).
Figure 2. The North American possum brothers pretending to be dead in The Ice Age and the cock-eyed possum in a German zoo. The image comes from the Internet
The research technology means in this paper are also full of innovation. Previous mammalian chewing studies, whether it is anatomical studies that have lasted for two centuries or two-dimensional X-ray imaging technology, have not been able to fully capture information in real time, study dynamic processes, look at internal conditions, and reproduce 3D construction. And these are precisely the information urgently needed to study the chewing process. Therefore, this paper uses marker-based X-ray motion morphology reconstruction (XROMM) and microcomputer-based tomography enhanced contrast study (μCT) technology to solve these problems.
Previous studies have found that early mammals had a tribosphenic dentition and that there was a long-axis rotation along the long axis during complex chewing processes. There is an asymmetry between the working and balanced sides of the mandibular when chewing, the working and balanced sides are valgus when the mandible is open, the working side is turned inward when the mandible is closed, and the etropenion of the equilibrium side is turned (Figure 3). However, data reconstruction studies relied on idealized upper and lower molar pairs, not taking into account the diversity of the tooth row. Historical toothwear theory and physical mathematical models simply resulted in a simple inverted-valgus chewing process without grinding. Therefore, they used the North American possum as the ancestor model animal of the mammalian suborder, using the two advanced technologies mentioned above, and constructed 3D dynamic data on the possum chewing, and the real animation restoration results were more exquisite and interesting than previous studies.
Figure 3. Reconstructed possum 3D skull with muscular schematics, including up-down, lateral unilateral, anterior and posterior angles. Cyan: superficial masseter muscles, dark blue: deep masseter muscles, pink: deep temporal muscles, dark green: pterygium muscles, light green: inner wing muscles. Image credit: Nature
The middle half of the jaw can be moved independently during chewing. The movement of the lower jaw is rhythmic and symmetrical, with the lower jaw rapidly closing until it comes into contact with food (the rapid closing phase), followed by a slow-closing chewing phase followed by a slow opening. These dynamic processes involve the shortening of the adductors in the mandibular, especially the masseter and temporal muscles, during closure. The rotation along the long axis during chewing can pull the underlying teeth to a greater extent, and compared with the previous study, the relative degree of rotation of the semi-jaw-long axis can be analyzed. In contrast to previous studies reporting asymmetry of semi-mandibular rotation along the long axis, all experiments by Bhart-Anjan S. Bhullar et al. have shown that the rotation of the long axis is symmetrical. The half-jaw is slightly valgus or open at rest, which makes the middle and lower canine teeth in the dentition on the outside of the upper canine teeth. When the half-jaw is closed, the teeth are symmetrically turned inwards, the lower and upper teeth are closed into the occlusal position, at which time both the closure and the inversion are slowed down, and the teeth are involved in the process of food entering the mouth.
Possum chews the middle half of the jaw in lateral viewing angles. Attached muscles as shown in the image above. Image credit: Nature
At the maximum closure of the possum's mouth, the continuous process of inversion-valgus that they observed had never been reported before, and they called the rotational grinding stroke. This rapid and important movement is close to universal in chewing. Changes in the relative position of muscle attachment are reconstructed, showing shortening of the superficial masseter muscles during valgus and shortening of the medial pterygium during valgus. Therefore, they speculate that the roughly symmetrical opening of the lower jaw is accompanied by valgus, the closure of the lower jaw is accompanied by inversion, and the rotational grinding process may be characteristic of subropods.
The possum chews the middle half of the jaw at a top-down perspective, and changes in the surface masseter muscles (dark blue) and medial wing muscles (dark green) can be seen. Image credit: Nature
They looked along the entire dentition to a single tricuspid molar pair at the time of closure. It was found that the mouth slowly closed and opened during chewing, and the teeth were turned inward and valgus continued, while food was also processed in the mouth, which led to additional relative lateral movement of the teeth. This is different from the previous assumptions. In addition, the rotary grinding process forms an inverted "mortar" and "pestle" process, with the lower molar far central basin area being the "mortar" and the upper molar proto-cone being the "pestle", which represents a complement to the traditional molar interaction model. In this experiment, they examined the position of each tooth individually and found that their molar closure patterns were generally similar along the tooth sequence. The two anterior molars located at the rear are large enough to close, and their interaction is tightly approached in a simple edge-to-edge cutting method, highly anastomosing when chewing for maximum inversion.
Movement of a single molar pair. Image credit: Nature
From this conservative 3D dynamic analysis of mammals of theropod mammals, they concluded two core observations: (1) the half-jaw rolls outward along the long axis when chewing open, so that the dentition is symmetrically valgus, and when chewing and closing, the dentition is turned inward by rolling inward on the long axis. (2) At the deepest part of the bite of the tooth, the mandible is first valgus and then inward, and the food is further processed during the rotational grinding process.
From this, they speculate that at some critical time in early mammalian evolution, the lower jaws of the theropods (who are also the ancestors of this branch of our humanity) evolved to roll in harmony with precise occlusal fits, and evolved a rotational grinding process that coincided with the far central basin of the lower molars of the triangular teeth. It is these evolutions that enhance chewing function that allow mammals to mash food by shearing and grinding, making it small chunky to facilitate further slow, intact chemical breakdown of food. This plays a key role in the development of mammalian body and brain power. The giant reptiles, which can only swallow, are in decline due to their inability to adapt to changes in the natural world, and this "mammalian critter" that was originally inconspicuous in the biological world has gradually risen to become the "winner" on this planet (Figure 4).
Figure 4. The North American possum brothers who are good at chewing in The Ice Age. The image comes from the Internet
original
Rolling of the jaw is essential for mammalian chewing and tribosphenic molar function
Nature, 2019, 566, 528–532, DOI: 10.1038/s41586-019-0940-x
(This article is contributed by Mizumura Yamaku)