More than 100 million years have passed, and it is still a baby

Among amphibians, in addition to the "number one family" Anura (frogs and toads), the most prosperous category is the Caudata, which includes various "salamanders" in a broad sense. From the common Oriental salamander to the giant salamander that is more than 1.5 meters long, the Caudata is distributed throughout the middle and high latitudes of Eurasia and North and South America. These smooth and moist, lizard-like animals can be found from the surface of humid forests, mountain streams to dark mines, and even in trees.

There are many species of Caudata in China, many of which are endemic to China. Do you know what is the oldest known Caudata in China? It is the protagonist of today: Jeholotriton.

Weird Jehol Salamander fossil specimen | Yuan Wang / Journal of Vertebrate Paleontology (2009)

Ancient "baby"

The fossils of Jehol Salamander were found in the Daohugou Formation in Chifeng City, Ningcheng County, Inner Mongolia, in the strata of the Middle and Late Jurassic period 146 million years ago. In 2000, Wang Yuan, an amphibian expert at the Institute of Vertebrate Paleontology and Paleoanthropology, first described Jehol Salamander and published his research in the Journal of Vertebrate Paleontology in 2005.

There is only one species in the genus Jeholotriton: Jeholotriton paradoxus, which looks similar to existing salamanders: a wide, flat, porous skull, short limbs and a long tail. But it also has many unique features: many existing amphibians have teeth on the vomer of the upper jaw in addition to the upper and lower jaws, which is convenient for them to hold the smooth prey tightly when hunting and assist in swallowing. The vomer teeth of Jeholotriton grow in the middle of the jaw surface, forming a tooth plate, and have a rear process full of teeth extending backwards. The wing bones of the upper jaw of Jeholotriton do not point to the back end of the maxilla like those of existing salamanders, but to the middle of the jaw surface. It is these characteristics of the skull and teeth that gave it the species name "strange".

Skull of Jehol Salamander | Yuan Wang / Journal of Vertebrate Paleontology (2009)

The tail of Jehol Salamander is flattened laterally, indicating that it is a type of salamander that mainly lives in water. There are dark strips on both sides of the head of the fossil, which are traces of the existence of external gills: like frogs, most salamanders also lay eggs and reproduce in water. The larvae (tadpoles) living in water have feather-like external gills on both sides of their heads to absorb dissolved oxygen in the water. Just as the tadpoles of the Anura will metamorphose into frogs and toads, most of the larvae of the Caudata will gradually grow limbs and lungs, and the external gills will shrink and disappear.

However, from the fossils of Jehol salamanders, it is not a juvenile, but a fully mature adult. Why does the adult Jehol salamander have the external gills of the juvenile? The answer can be found in the existing Caudata: when the environment is suitable, some salamanders do not undergo metamorphosis, but directly maintain the juvenile form and develop into sexually mature adults.

Mexican axolotl, which still retains feathery gills as an adult | Pixabay

For example, the Mexican axolotl (commonly known as the "six-horned dinosaur"), which is very common in the pet market, will only shed its external gills and metamorphose into a newborn under very specific and harsh conditions. This phenomenon is called neoteny. The discovery of the Jehol axolotl proves that this peculiar developmental mode already existed in the Caudata amphibians as early as the Jurassic period.

The life world of Jurassic

Jehol Salamander lived from the Callovian to the Oxfordian stages of the Jurassic period. It probably spent most of its life in forest rivers and ponds, crawling and swimming slowly on the bottom of the water in search of food. Judging from the fossilized stomach contents of the scutellar arthropods (a class of arthropods living in freshwater, belonging to the order Conchostraca, which was very prosperous at the time), Jehol Salamander was also a carnivore, just like the existing salamanders.

They may have poor eyesight like the existing aquatic salamanders, relying on their keen sense of smell to find food in the turbid water, or lying motionless near rocks, sunken wood and other hidden objects, waiting for careless prey to fall into their trap. When small fish (such as the common wolf-fin fish Lycoptera at the time), aquatic insects and other small animals entered the attack range, the Jehol salamander may also suddenly open its mouth like the existing Mexican amphibian and spotted mud salamander, using the suction force to suck the prey into its mouth and swallow it whole.

Wolffish | James St. John / Wikimedia Commons

The Jehol salamander is not the only Caudata in the Daohugou Formation. It also coexists with the Chunerpeton tianyiensis. Like the Jehol salamander, the Chunerpeton tianyiensis also has the characteristics of neoteny, which may be the adaptive evolution of the two salamanders to the same living environment. Both the Jehol salamander and the Chunerpeton tianyiensis are at the base of the Caudata evolutionary tree and are the "old predecessors" of various existing salamanders.

Tianyi Chusaur, from the Beijing Museum of Natural History | Bjoertvedt / Wikimedia Commons

There was also a strange animal living in the freshwater at that time: Castorocauda lutrasimilis, which belonged to the extinct mammaliaformes and was an ancient relative of modern mammals rather than a direct ancestor. Castorocauda lutrasimilis had webbed limbs, a flat tail similar to that of a beaver, and sharp teeth similar to those of a seal. It mainly fed on fish and other small vertebrates living in freshwater. It was perhaps the most formidable natural enemy of Jehol salamanders and protosaurids.

Reconstruction of the otter-shaped raccoon | Nobu Tamura / Wikimedia Commons

More than 100 million years have passed since the Jurassic period. There are still at least 85 species of caudate amphibians living in China today. These small animals have sensitive and fragile skin, weak migration ability, and are highly dependent on the microhabitats they depend on for survival. They are particularly vulnerable to adverse factors such as the abuse of pesticides and fertilizers, water pollution, habitat destruction, invasion of alien species, and climate change. Many caudate species unique to China, such as the Zhenhai Acanthus and the South China Giant Salamander, are now in a precarious situation and on the verge of extinction, and the once numerous Dianchi salamander has disappeared from the earth forever. We can only know this interesting creature through specimens soaked in alcohol. The future of these magical, unique, and fragile small animals is still in the hands of humans.

This article comes from the Species Calendar, welcome to forward