Elephants are the largest land mammals, and one of their most striking features is their soft, flexible trunks. In the early stages of elephant evolution, including the Oligocene and Miocene epochs, all elephants had extremely elongated mandibles and lower incisors. Especially during the early to middle Miocene period, the morphology of the mandibles and lower incisors showed a high diversity.
How did elephants during this period use their mandibles and lower incisors of various shapes to feed? What role does the elephant trunk play in the process of feeding? However, in the latest Miocene and Pliocene, the elongated lower jaws of elephants were unexpectedly and invariably shortened. What are the internal mechanisms of this legendary evolutionary process?
△The position of the phylodontids, the phylodontids, and the phylodontidae in the phylogeny, as well as the comparison of the morphological characteristics of the mandibular and nasal regions of the three elephants
In response to these questions, researchers such as Li Chunxiao, a postdoctoral fellow at the University of Chinese Academy of Sciences, Wang Shiqi and Deng Tao, researchers at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, and Professor Zhang Ji from Huazhong University of Science and Technology, conducted in-depth research on the diversity and evolution of the proboscis mandible, as well as its co-evolution with the elephant trunk, and revealed the feeding methods of early elephants and the unique evolution of related organs. The relevant results were published in the international academic journal eLife on November 28.
△The relative abundance of three types of long-jawed elephants from the Early to Middle Miocene in northern China, the δ13C isotope value of tooth enamel, as well as the elephant fossil assemblages and corresponding biological ages in different locations
This study points out that as early as the middle Miocene climate optimum period of 17 to 15 million years ago, the star species among early elephants, Platybelodon with a shovel-shaped mandible, had developed a method of using its trunk to wrap up plants and cooperate with its mandibles to cut off plants for eating. This is the earliest evidence that the elephant trunk has realized the curling function. This study conducted a comparative study on the functional morphology of three main types of long-jawed mastodon: Shovelodon, Cogodon, and Bogodon. Their mandibular shapes are different: Shoveldon has wide and flat lower incisors like shovels, and the upper incisors are shorter and curved downward. Enamel band; the lower incisors of Cogodon are club-shaped, and the upper incisors are longer, also curved downwards, and have enamel bands; Bogodon has no lower incisors, but the mandibular symphysis is elongated and groove-shaped, and the upper incisors are thick and curved outward and upward, without enamel bands. In addition to this, their nasal area characteristics are also different. Compared with the other two types of early elephants, the shovel-toothed elephant has the smallest nasal bone and the most complex structure around the nostrils, even surpassing the degree of evolution of living elephants. These nose-related skeletal features all indicate that Shovedon elephants have a very developed nose (the smaller the nasal bone, the more developed the trunk is).
△Finite element mechanics analysis results and paleoecological environment reconstruction (painted by Guo Xiaocong)
In order to deeply reveal the feeding habits and feeding methods of the three elephants, the research team used a variety of research methods. Among them, tooth enamel carbon and oxygen stable isotope analysis and finite element mechanics analysis became the key means to reconstruct the feeding behavior of these early elephants: tooth enamel carbon and oxygen stable isotopes provided evidence of the different feeding preferences of the three elephants; finite element mechanics analysis became the key means to restore the feeding methods.
The research results show that the shovel-donted elephant, the ragweed and the codon had different feeding methods and living environments: the shovel-toothed elephant lived in a relatively open environment, and its mandibles were only suitable for cutting vertically growing plants. The shovel-toothed elephant used its flexible trunk to curl up on the plants, and then used its mandibles to cut horizontally. This feeding method is very efficient. The boar-odon lives in a relatively closed environment and is suitable for cutting branches that grow horizontally or obliquely. It uses its nose to assist in pressing the branches, and then cooperates with its mandibles to complete feeding. The habitat of the codon is between the two or overlaps with both. Its feeding methods are diverse and it can adapt well to both woodland and grassland. For early elephants, the elongated mandible and lower incisors were the main feeding organs, while the trunk was only used as an auxiliary tool. Early elephants with different mandibular shapes had different ecological adaptability. As the ecological environment gradually became dry and cold, shovel-toothed elephants were more able to adapt to the relatively open ecological environment and feed on herbaceous plants, which ultimately promoted the development of the elephant trunk's gripping function and flexibility.
△Three-dimensional restoration model of Spodoptera (produced by Wang Yu)
The expansion of spadedons into open habitats was terminated by the extinction event including spadedons caused by the Tortona extreme heat event at the beginning of the late Miocene. However, codons took over from spadedons and continued to spread into open habitats in the late Miocene. As the trunks continued to evolve towards higher flexibility and stronger gripping functions, the feeding function of elephants was completely transferred to the proboscis, which ultimately led to the shortening of the original feeding organs, the lower jaw and lower incisors. Therefore, the adaptability of feeding behavior in open environments is the "catalyst" for the evolution of the elephant's trunk grasping function. This study provides important evidence for our understanding of the evolutionary and ecological background of the trunk and mandible of this important group of elephants, and provides new insights into how proboscis adapt to their environment and how environmental changes shape the evolution of unique organs.
(CCTV reporter Shuai Junquan and Chu Erjia)