Author: Cui Xueqin
Magazine cover Archaeovaranus lii restoration of Archaeovaranus lii (photo courtesy of Archaeovaranus lii, Chinese Academy of Sciences)
Archaeovaranus lii orthotype specimen of Archaeovaranus lii (photo courtesy of Archaeovaranus lii, Chinese Academy of Sciences)
On February 7, Dong Liping, Wang Yuanqing, Zhao Qi, Wang Yuan, and their British and Swiss counterparts from the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences published the research results on the evolution of monitor lizards in a cover article in the journal "Royal Society Journal of the Natural Science Society": The new genus of the Asian Eocene basal stem monitor lizard family supplemented the evidence chain of the Asian origin theory of the lizard family, indicating that there is a transition stage from the evolution of the monitor lizard type to the monitor lizard family in Asia.
Related thesis information: https://doi.org/10.1098/rstb.2021.0041
The family Varanidae is one of the most successful evolutionary clades of the scaly family , attracting much attention for its multi-body members. The only genus of living representatives is Varanus, a total of more than 80 species, distributed in Asia, Africa, and Oceania, including the largest living lizard, the Komodo dragon. The genus is similar in size, but has a very high morphological and ecological diversity. From the two- to three-centimeter-long Dampier monitor lizard to the three-metre-long Komodo dragon, Australia's extinct ancient monitor lizard can reach up to 5 metres in length. Some of them are good at climbing trees, some are good at swimming, and their diet is very different, insects, frogs, bird eggs, small and large mammals, and even crabs and berries can become food. Among the living lizards, the Borneo lizard, which is closest to the monitor lizard, has added a touch of mystery to the genus because of its only distribution on the island of Borneo and its hidden habits.
The monitor lizard family originated in the Late Cretaceous period distributed in Eurasia, such as the Upper Cretaceous in the Gobi Desert in the heart of Asia, which produced a variety of well-preserved fossils of monitor lizard type. The origin of the genus Monitor Lizard is still highly controversial, with several different hypotheses of Asian origin, African origin and Gondwana origin. The exact fossil record of the genus Monitor Lizard comes from the early Neogene period. Therefore, this period of paleogene is an important stage in the evolution of the monitor lizard type to the genus Monitor Lizard. However, the Paleogene monitor lizard fossil material is mostly preserved as scattered vertebrae, and the only well-preserved ones are several skeletons called the Saniwa lizard, but these materials come from North America, which is no longer distributed by monitor lizards, making the mystery of the origin of the genus even more puzzling.
In 2008, the mammal field team led by Wang Yuanqing found a lizard skeleton in the lower part of the Eocene in the Liguanqiao Basin in Hubei Province, and after careful repair, high-precision CT scans and systematic comparative studies, it was believed that it represented a new genus of monitor lizards, archaeopteryx lii, and the name of the genus Archaeopteryx originated from the systematic relationship between the species of this genus and the genus. The name of the species is taken from the surname of the late Professor Li Chuankuo, a famous ancient mammalian in the mainland who has done a lot of important work in the Liguanqiao Basin, as a memorial to him.
Archaeopteryx is about 1 meter long and is a medium-sized species among monitor lizards. Archaeopteryx has many typical features of the family Macrosqualis, such as the external nostrils are moved backwards, the snout end is lengthened, the dentition is located before the orbit, the plough bone grows like a rod, the lower jaw has a flexible intermaxillary joint, and the vertebral body has a pronounced anterior condylar contraction; but it is also significantly different from the genus Monitor Lizard, such as the subfrontal olfactory tube is not closed, the posterior edge of the orbit is closed by the yoke bone and the posterior orbital bone, there are two rows of small teeth on the palate surface, and the opening of the shoulder strap is reduced. The most distinctive feature of the Archaeopteryx is that its anterior and posterior limbs (near the middle segment, excluding the hands and feet) are similar in length. This is different from all monitor lizards, which have significantly longer hindlimbs than their forelimbs, especially in genera capable of bipedal standing. The near-equal length of the forehand and anterior limbs is also rare in the entire scales, suggesting that the Archaeopteryx may have adopted a special mode of movement, at least unlike its contemporaries, the Saniwa lizard. Bone histological analysis showed that the osteologic characteristics of archaeopteryx monitor lizards were similar to those of wedge-toothed lizards, and the individuals represented by the orthotype specimen reached sexual maturity at the age of 5 and died at the age of 16.
Based on morphological comparison and phylogenetic analysis, the team also explored the evolution of some traits related to feeding function in the family Monitor Lizard family. In the basal members of the family Monitor Lizard family, the posterior orbital margin is closed, the subfrontal olfactory tube is open, and the palatal teeth are retained, to the crown of the genus Monitor Lizard, the bony posterior orbital margin is open, and the subfrontal olfactory tube is closed into a tube, without palatal teeth. Previous functional studies of the Nile monitor lizard and the Taiga monitor lizard with similar body size and ecological habits have shown that the opening or absence of the posterior orbital margin and the openness of the olfactory tube may be two complementary features in terms of mechanical function. The opening of the posterior orbital margin during the evolution of the monitor lizard family increased the feeding efficiency of the skull, while the olfactory tube under the frontal bone was closed to counteract the increased stress caused by the opening of the posterior orbital margin. The disappearance of palatal teeth indicates that the tongue of the monitor lizard may have evolved functionally from the movement of complementary food in the mouth to the perception of chemical composition in the air, but fossil species such as archaeopteryx show that this evolutionary process may be complicated.