Jurassic insects (a, fossil photograph; b, Restoration image) and comparison with extant Echinocephalus (c). The scale bars are 5.0 mm (a, b) and 0.0 mm (c). Courtesy of Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences

南古所新發現的來自1.6億年前侏羅紀的棘頭蟲化石。 金陵晚報/紫金山新聞記者 張安琪 攝

□Reporter of Jinling Evening News/Purple Mountain News

Angel Zhang

Echinocephalus is a class of parasites that seriously harm freshwater fish, and can also infect humans, pigs, dogs, cats, etc. Its body configuration is highly specialized, and the question of its origin has not been effectively answered.

　　近日，記者從中國科學院南京地質古生物研究所（以下簡稱“南古所”）獲悉，該院發現了一枚產自約1.6億年前侏羅紀的棘頭蟲化石，通過國際合作系統研究，確認其演化位置位於棘頭蟲的最根部，換言之是現代棘頭蟲的“鼻祖”。該研究填補了棘頭蟲的演化空白，為解決棘頭動物門的起源之謎提供了實證。成果於2025年4月9日發表於英國《自然》（Nature）雜誌。

　　長期以來，棘頭蟲一直被認為是一個獨立的門，即棘頭動物門。分子系統學分析表明，棘頭蟲最可能是輪蟲動物門中一個特化的類群。但現代棘頭蟲的“長相”與輪蟲相比實在相距甚遠，同時，由於棘頭蟲是體內寄生蟲，很難保存為化石，此前唯一的化石記錄是來自晚白堊世（距今1億—660010,000 年前）鱷形動物糞便中四枚疑似的棘頭蟲卵。因此，人們對棘頭蟲的起源和早期演化知之甚少。

　　南古所新發現的棘頭蟲化石來自1.6億年前的侏羅紀。記者在現場看到，化石長約2釐米，整體約手指關節大小，一端有狼牙棒狀的突起。

"That's the hooked snout process, which is also similar to that of modern echinocephalus, and at the same time it is important evidence that it has evolved endoparasitic habits." Wang Bo, a researcher at the Southern Institute of Ancient Studies, said that this fossil was first discovered in 2018 years, and its unique shape immediately attracted the attention of the scientific research team. According to the place of discovery, researchers named it "Daohugou Jurassica spinocephalus" (referred to as "Jurassia"), "We speculate that Jurassica excavated the host's body before the death of the host, which was preserved in fossil form." Otherwise, the host's internal organs will accelerate its corrosion. ”

Subsequently, the research team conducted a detailed anatomical study with the help of scanning electron microscopy and energy spectroscopy analysis and found that the body of the Jurassic worm was spindle-shaped and divided into three distinct parts, namely the snout process, neck and trunk. Jurassic worms have hardened, slightly downward curved spines and about 32 pairs of longitudinal hairs extending only to a small part of the body, similar structures commonly found in living echinocephates. The digestive tract is preserved in the center of the snout, but no obvious digestive tract is found on the entire trunk, and there is a structure at the end of its body that resembles a copulative umbrella of a living Echinocephalus male.

It is worth mentioning that Jurassica still retains its teeth and anus, but modern echinocephalus no longer has these two organs. "We suspect that these two organs were primitive features of modern echinocephalus and later disappeared during the long evolution." Through the PPT photos shown by Wang Bo, the reporter saw that the teeth of the Jurassic worm are located in the front of the torso, and the teeth in the front are smaller, gradually larger backwards, and the number of teeth is also more. The teeth in the jaw are oriented towards the front of the body, and all the teeth do not exceed the body boundary of the Echinocephalus. It is known that similar apparatus is widely present in the possible ancestors of Echinocephalus, the class of exozoa that contains rotifers (including the phylum Orozoa, the phylum Micromoria, and the phylum Rotichridae, of which the phylum Rotichrida contains the phylum Rotifers and the phylum Echinocephalica).

To further determine the evolutionary location of Jurassic worms, the research team also constructed an up-to-date morphological data matrix of various living and fossil helminths, and conducted phylogenetic analysis. The results showed that the evolutionary position of Jurassica was located at the most root of Echinocephalus and was the basal group of Echinocephalus. Jurassians are a transitional group of rotifers to echinocephals. This is similar to the results of molecular phylogenetic analysis in recent years, which said that "Echinocephalus is most likely a specialized group in rotiformes", which successfully resolved the gap between morphological studies and phylogenetic genomics studies.

So who is the host of the Jurassic worm? Wang Bo said that the team speculated that it was an amphibian salamander, "because in Daohugou, where it was found, almost no fish fossils were found, and there were more salamander fossils." ”

It is reported that Wang Bo and Luo Cihang, a doctoral student at the Nangu Institute, and related researchers will continue to carry out fossil research, "We have also found a lot of fossils with special morphologies, and we will continue to dig deeper." Wang Bo said that although molecular biology can solve some phylogenetic relationships that are difficult to solve in traditional morphological studies, practice shows that transitional fossils still play a very important role in exploring the revolutionary evolution of animal body configuration.

This research was supported by the National Natural Science Foundation of China, the International Science Program of Deep Time Digital Earth, and the Jiangsu Excellence Deep Time Digital Earth Research Center. The stratigraphic paleontology big data center GBDB of Nanjing Institute of Paleontology provided data support. Yang Dinghua, a painter at the Nanjing Institute of Paleontology, drew a restoration of the Jurassic insect.