Producer: Popular Science China
Producer: Qin Zichuan (University of Bristol, UK)
Producer: Computer Network Information Center, Chinese Academy of Sciences
Manga author: Akiko
(Note: Latin names in the text should be italics)
In the Mesozoic Era, the famous "Age of Behemoths" in Earth's history, whether on land, sea or sky, it was ruled by huge dinosaurs, pterosaurs and marine reptiles. However, under the shadow of these behemoths, there is a different scene in all corners of the ecological environment. It is generally believed that on the night of the Mesozoic Era, those hidden corners often belonged to our ancestors: some small, hairy mammals. It is difficult for them to compete with the huge and fierce dinosaurs in the warm day, and they can only break out of the world at night with their advantages of constant temperature and developed five senses.
△ Ecological restoration map of the dinosaur era (Source: http://www.gxmuou.com/jiankang/19218.html)
In recent decades, the large number of feathered dinosaur fossils in northern China has shown another diverse and flourishing group of dinosaurs, the hand robbers, which are the ancestors and distant relatives of the birds we are familiar with. The hand robbers eventually evolved into a branch of birds, and instead of growing larger, they became lighter and lighter, even with hollow bones and flying wings, eventually conquering the sky. Other distant relatives of the bird branch, such as Oviraptorosaurs, Sickleosaurs, and Ornithisosaurs, were still inevitably caught up in the fierce 'arms race' of the Cretaceous Period, growing larger in the selection and selection, and the last few types that appeared were even comparable in size to members of the Tyrannosaurus rex, such as the "Mongol Dinosaur Triumvirate", Megalopods, Sickles and Dinosaurs.
△ Two famous giant theropod dinosaurs living in the late Cretaceous, the warrior Tyrannosaurus bataar and the turtle-shaped sickle dragon Theizinosaurus cheloniformis (Image: Raul Martin)
However, not all non-avian dinosaurs embarked on the road to large-scale, a mysterious small dinosaur, Alvarez dragons, but embarked on the path of miniaturization like distant relatives, and even became the smallest non-avian dinosaurs. Compared with the more famous "relatives", Alvarez dragons are not widely known, and their lives are far away from the "hustle and bustle of the world".
In fact, Alvarez Dragon can be said to be from a famous name. The earliest reports of Alvarezosaurs came from José Fernando Bonaparte, Argentina's most famous dinosaur hunter and paleontologist of the last century. The scientist, who shares the same surname as Napoleon, is one of the most reported scientists in the world in terms of achievements, and is revered by his peers as the "Master of Mesozoic".
Although Alvarez dragons have not received much media and public attention, they have always been one of the hot spots in the study of theropod dinosaurs. Their body structure is strange, their body shape has evolved into a mystery, their food and ecological niche are also controversial, and even the study of the basic question of which type of dinosaur they belong to is a twist and turn. They have slender hind limbs, but the forelimbs are short and thick, and the late members have only a huge claw, but no one knows their function. There is some speculation that they are burrowing animals; there are also some opinions that they are close to today's anteaters, digging nests with their forelimbs; and some scholars believe that they are real egg-stealing dinosaurs, but there is no definite statement. As a result, Alvarezosaurus has also been called "enigmatic dinosaurs" by paleontologists and enthusiasts alike.
△ Hawk-billed single-clawed dragons and their extremely specialized short and thick "small hands" (image from Wikipedia, the author has modified)
Since 2010, an international team of researchers led by Xu Xing, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, has reported on a series of important Alvarezosaurus fossil materials. Scientists from China, the United Kingdom, the United States and South Africa have studied these precious materials for many years, and finally this year, two new research results on the Alvarez dragons were published in the journals Science and Current Biology, gradually unveiling the mystery of these dinosaurs.
<h1 class= "pgc-h-arrow-right" > listens to six ways with small ears and looks in all directions with big eyes</h1>
What kind of dinosaurs were the Alvarez dragons? Paleontologists have always been curious about this strange little creature, trying to understand how this light and agile little animal survived the extremely dangerous Cretaceous period surrounded by giant beasts.
Professor Jona Jonier, a paleontologist at the University of the Witwatersrand in South Africa, is also interested in the question of the discoverer of the earliest known Alvarez-like dinosaurs. After observing the skulls of many theropod dinosaurs, he suddenly recalled that the skull of the earliest type of Alvarez dragon he had reported, the dexterous Jane-hand dragon, seemed to be slightly different from the skulls of other theropods: the Simple-handed Dinosaur had an unusual pair of large eyes. Professor Jonah quickly observed and measured the size of the orbits of several other Alvarez dragons and found that this "big eye" situation was so common among such small dinosaurs!
Their study compared data on the relative orbit size of Alvarezosaurus with data from living birds, lizards, and other theropod dinosaurs, and was surprised to find that both early and late Alvarez dragons had disproportionately large eyes, a proportion that was even comparable to that of typical nocturnal birds and lizards, much larger than most large dinosaurs. Why do these little dinosaurs have big eyes? Quite simply, because the big eyes allow animals to see their prey and enemies clearly in the dimly lit wild, like owls that prey on mice at night.
The skulls of desert bird-faced dragons show that they have huge eye sockets, so they can have better vision in the low light at night. (Image: Mick Ellison/American Museum of Natural History)
To survive the brutal struggle for survival in the Cretaceous, the owl's vision alone may not be enough, but also sensitive hearing. Professor Jonah led a team of researchers to perform high-precision CT scans of the beautifully preserved fossil skulls of the Alvarez dragons and found that the Specialization of the Alvarez Dragons was not just vision. The ear area of the two complete skull fossils shows a clear cochlear structure, and the Alvarez dragons were found to have large diameter cochlear tubes like owls. This indicates a very interesting phenomenon that the Alvarez dragons not only had a vision system that was highly adapted to low-light conditions, but also a powerful hearing system that could keep the slightest movement in silence.
Ecological restoration of the desert bird-faced dragon, hunting insects in the night (Viktor Radermaker, https://creativecommons.org/licenses/by-nd/4.0/)
The work of Professor Jonah's team, published in the world-renowned academic journal Science, reveals that both early and late Alvarez dragons had highly developed, sensitive senses that existed only in nocturnal animals. The Alvarez dragon may have been a small animal that was perfectly adapted to nocturnal life, which could also avoid the large carnivorous dinosaurs that haunted during the day, and could also rely on sensitive senses to search for insects in the dark of the night. This nocturnal hypothesis also fits somewhat with the previous hypothesis about alvarezosaur burrowing or insect feeding, because many burrowing animals, or insectivores, are mainly active at night.
<h1 class= "pgc-h-arrow-right" > life is not easy, become smaller and avoid</h1>
Another important feature of The Alvarezosaurus is that they were particularly small, and some of their members were even the smallest non-avian dinosaurs. For example, it was found in Zhang Shixi, Henan Province, China
The gorge clawed dragon was only the size of a larger pigeon.
(Manga Author: Akiko)
Professor Jonah and Researcher Xu Xing collaborated on a species of Alvarezosaurus that lived in the late Jurassic, the dexterous Jane-handed Dragon. Although in the Mesozoic Era, this "dragon era" is not remarkable, but in terms of body size, the size of the simple hand dragon is relative to their descendants, can be regarded as a giant, two to three meters long, weighing nearly 50 kg. On the other side of the globe, the Patagonian Plateau in Argentina, some Late Cretaceous Alvarezosaurs have also been reported, which are the largest representatives of body size, nearly three meters long, and can weigh 50-70 kg. However, by the end of the Cretaceous period, almost all of the reported members of the Alvarez dragons around the world had become very "miniature". What happened to make these dinosaurs suddenly smaller?
To answer this question, the first question to be solved is, are these small dinosaurs real? There are some doubts that these small-bodied Alvarez dragons were actually juvenile individuals. To solve this question, it is necessary to use the research method of bone histology. Bone histology is to study the microstructure associated with ontology recorded on these sections by sectioning the femur and other long bones, which can be used as evidence to estimate the ontology of different specimens, and then exclude the negative impact of juvenile individuals on body size estimation. Over the past decade, paleontologists have discovered more than a dozen important fossil specimens of The Alvarez dragons from the Jurassic to the end of the Cretaceous period in China, enabling extensive osteotologic studies.
Recently, scientists from China, britain, the United States and South Africa published their research on bone histology, body size estimation and body type evolution of Alvarez dragons in the world-renowned academic journal Current Biology. On the basis of bone histology research, the scientists constructed an Alvarezosaurus "adult weight dataset" that successfully excluded the influence of juvenile individuals. Based on this new weight dataset, they used the latest ancestral state restoration methods in the field of macroevolution to study the overall body size evolution trend of the entire Alvarez dragon family.
Alvarez plesiosaur bone histology, macroevolution of body weight and trends of diversity (courtesy of Qin Zichuan)
Macroevolution research is the study of evolution on large time scales, which do not focus mainly on the details and characteristics of each species, but are concerned with large-scale changing trends and trends. Through the restoration of ancestral states based on different mathematical methods, we try to explore the "past that has not been recorded by fossils". The work restored the weight status of the ancestor nodes on the Alvarez dragon evolutionary tree and estimated that the common ancestor weight of all Alvarez dragons was about 23 kg.
The miniaturization of the entire Alvarezosaurus was of a single source, occurring mainly between 110 million and 85 million years ago, changing extremely quickly, losing weight by more than two orders of magnitude in a short period of time. As their body size declines, species diversity increases significantly, more than three times more than it did before miniaturization. The scientists also introduced the method of evolutionary model testing, and by comparing different model fitting data, found that this miniaturization process is actually a body type "transition" process. Between 110 million and 85 million years ago, the weight of the Alvarez dragons suddenly shrank from the weight range of 5-50 kilograms, a typical small and medium-sized theropod dinosaur, to the very small weight range of 0.5-5 kilograms.
What is the reason for this when many distant relatives of the Alvarez dragons are getting bigger and smaller? One more likely guess is that there was a shift in feeding habits in the Alvarez dragons.
Interestingly, in the biological world, this "transition" is often associated with a specific evolutionary event, that is, there is one or more reasons that lead to the occurrence of the "transition". For Alvarez dragons, what is the reason for this?
One more likely guess is that during this time, there was a shift in feeding habits among the Alvarez dragons. Previous anatomical studies of Alvarezosaurus have shown that these dinosaurs had many morphological characteristics similar to those of today's anteaters, such as straight and large forelimb claws, short but very strong forelimb bones, and a snout with missing incisors (which provide space for sticking out a slender tongue to stain ant-eaters). In addition, according to the study of living animals, due to the limitation of energy transfer efficiency, the single insect feeding will limit the size of the animal to a very small range, which also coincides with the "jump" miniaturization of the Alvarez dinosaurs found in this study.
Based on the anatomical characteristics of Alvarezosaurus, it has been speculated that Alvarezosaurus may have been antevorous, but the earliest fossil record of Alvarezosaurus appeared in the Late Jurassic, earlier than the era of the widespread distribution of eusocial insects represented by termites, and could not be corresponded in time. This study demonstrates that the miniaturization process of Alvarez dragons may be a rapid miniaturization process that accompanies the transition of ecological niches from carnivorous to insect-eating, and also coincides with the adaptive radiation of eusocial insects in time, further supporting the idea that late small Alvarez dragons may be specialized anteivists. Of course, more evidence from functional and ecological studies is needed to concretely prove the Alvarez-lepiosaurus ant-eating hypothesis.
Ecological restoration of four representative Alvarez dragons, Haplocheirus sollers (Late Jurassic, left), Patagonykus puertai (Late Cretaceous, upper middle), Linhenykus monodactylus (Late Cretaceous, lower middle) and Bannykus wulatensis (Early Cretaceous, lower right), also showing differences in body size and feeding habits of Alvarezosaurus dinosaurs at different evolutionary stages. (Image source: Han Zhixin painting)
bibliography:
1 Choiniere, J. N. et al. Evolution of vision and hearing modalities in theropod dinosaurs. Science 372, 610-613 (2021).
2 Qin, Z. et al. Growth and miniaturization among alvarezsauroid dinosaurs. Current biology