Beware! What would a parasite from 100 million years ago look like if it were preserved today?

When we were young, if we didn't wash our hands frequently before eating, adults would scare us with "parasites in our stomachs." This word always reminds us of some disgusting, long, worm-like creatures, such as roundworms (a type of nematode) and tapeworms (a type of flatworm). In particular, people infected with parasites will develop various diseases, which can even be life-threatening in severe cases.

In fact, parasites are very common in nature. In addition to parasitizing the human body, parasites also parasitize various animals, plants, and even fungi. It can be imagined that not only in modern times, but also in ancient times, various creatures also suffered from parasites. However, due to the very hidden lifestyle of parasites, our current understanding of their evolutionary history and lifestyle in ancient times is extremely limited.

Fortunately, scientists recently discovered a large number of parasites in the Cretaceous Kachin amber from Myanmar 100 million years ago, which opened a window for us to observe parasites in the Cretaceous.

Part.1 The deadly killer that empties your stomach - the nematode

Among the major categories of parasites found in amber, nematodes, especially nematodes, occupy an important position. Nematodes are widely distributed in almost all surface environments and are an important part of the food web. Nematodes are a special type of nematodes that parasitize various invertebrates such as insects, millipedes, spiders, snails and earthworms. They can change the morphology and physiological characteristics of the host and even manipulate the host's behavior.

Compared with other nematodes, nematodes are large in size and can occupy almost the entire body cavity of the host. They often leave the host early when disturbed (such as when the host is wrapped in amber), so there are many fossils of nematodes and their hosts preserved together in amber. However, such fossils were mainly reported in Cenozoic amber before, and fossil records before the Cenozoic are very rare. Therefore, we know very little about the early evolution of parasitic behavior of nematodes.

The abdomen of ants infected with nematodes turns red, much like berries, thus attracting birds that like to eat berries to prey on them. The eggs will spread to farther places with the bird feces and further parasitize other ants. (Image source: Wikipedia)

Recently, a research team led by the Nanjing Institute of Geology and Paleontology, Chinese Academy of Sciences, conducted research on the fossilized nematodes in Kachin amber in Myanmar. Scientists have discovered a total of 16 specimens of nematodes preserved with their hosts. These nematodes widely parasitize different groups of insects, including stone lice (a type of primitive wingless insects), dragonflies, earwigs, crickets, cockroaches, wax cicadas, gnawing insects (commonly known as book lice) and midges. The discovery of these specimens shows that the phenomenon of nematodes parasitizing insects was already very common during the Cretaceous period, and it is likely that they played a role in controlling the number of insect populations. For insects at that time, being infected by nematodes was almost a dead end, so nematodes are well-deserved "nightmare-like deadly killers."

Insects parasitized by nematodes in Burmese amber (Image source: Luo et al., 2023)

However, compared to modern times, the hosts of nematodes in the Cretaceous period were somewhat different. According to the current fossil analysis results, scientists have found that the nematodes at that time were more inclined to parasitize incomplete metamorphosis insects. Insects can be divided into two categories according to their development process: complete metamorphosis insects and incomplete metamorphosis insects. Complete metamorphosis insects refer to insects that go through four stages in their life: eggs, larvae, pupae, and adults. Most modern insects are complete metamorphosis insects. Incomplete metamorphosis insects refer to insects that only have three stages: eggs, nymphs, and adults, but no pupae stage.

Holometabolous insects appeared later than incomplete metamorphosis. Although the Cretaceous period was already the mainstream, the statistical results of amber fossils show that in the Kachin amber of Myanmar in the middle Cretaceous period, only one of the nine insect orders parasitized by nematodes belongs to the holometabolous insects, while in the Baltic amber, four of the six insect orders are holometabolous insects, and in the Dominican amber, all three insect orders are holometabolous insects.

Fossil records of parasitic insects of nematodes. (Image source: Luo et al., 2023)

Scientists further counted all previous nematode fossil records and found that the same situation not only appeared in the nematodes, but also in the entire nematode group. In the Kachin amber from the middle Cretaceous, only about 40% of the hosts were holometabolous insects, but in the Baltic and Dominican amber from the Cenozoic era, 80% of the hosts were holometabolous insects. Therefore, at least before the middle Cretaceous, nematodes were more inclined to parasitize non-holometabolous insects. It may not be until the mass extinction event at the end of the Cretaceous that the widespread parasitic relationship between nematodes and holometabolous insects appeared.

Part 2 : Tapeworms: They have no mouths, but they can eat with their skin

Theoretically, only terrestrial organisms can be preserved in amber, because amber comes from the oil secreted by terrestrial trees. Therefore, parasites can only be preserved on terrestrial organisms. However, nothing is absolute. Among the many pieces of amber, scientists have also found a piece of amber that preserves marine parasitic tapeworms.

This tapeworm belongs to the order Trianosaurus. In the story inferred by scientists, the experience of this tapeworm is legendary: in the middle of the Cretaceous period, the climate was much warmer than it is now, and the power of storms was more terrifying than it is now. A ray that was unlucky was washed ashore by a storm. Even more unlucky was that a dinosaur happened to pass by at this time, and naturally could not let go of this delicious meal delivered to its door. So this "lucky" ray became the dinosaur's meal.

However, while the dinosaur was feasting, something seemed to fall out of the ray's stomach and landed in the resin secreted by a nearby Araucaria (a tall tree that flourished during the Cretaceous period).

Time flies, and in the blink of an eye, the resin of that year turned into amber through geological action, and this amber just happened to attract the attention of scientists, because the things that fell out of the belly of the stingray that year were really strange!

Careful observation shows that the fossil is worm-like in shape, with a large number of hook-like structures on its surface. The hook-like structures gradually become smaller from front to back, and the distribution of the hooks is in a spiral pattern. At the same time, high-precision micro-CT scans revealed that the front of the fossil preserved layers of cone-like structures. Through these features, scientists determined that this strange creature is actually part of the body of a type of marine endoparasitic tapeworm, the Conodontidae tapeworm.

Tapeworms are common parasites belonging to the phylum Platyhelminthes. They are all parasitic and can infect almost all vertebrates, including humans. They are widely distributed in almost all terrestrial, freshwater and marine ecosystems. They absorb food through their epidermis and have no mouth or digestive tract.

The Conodontidae tapeworm is one of the most numerous tapeworm groups in the ocean, and mainly parasitizes the gastrointestinal tract of cartilaginous fishes of the order Elasmobranch (mainly sharks and rays). Molecular biology speculates that the Conodontidae tapeworm may have originated at the turn of the Triassic and Jurassic periods (about 200 million years ago), but no fossil evidence has been found so far. In short, due to the parasitic habits of tapeworms and the easy degradation of their soft bodies, their fossil records are extremely rare. Therefore, scientists know very little about the early evolution of tapeworms.

Before the Quaternary, the only confirmed tapeworm fossils came from tapeworm eggs in shark feces fossils in the Permian strata of Brazil. Although one of these tapeworm eggs may have preserved a developing tapeworm embryo, it was not widely accepted due to poor preservation. This tapeworm fossil found in amber is the only tapeworm body fossil we know of so far, indicating that tapeworms could already "compete for food" in the stomachs of cartilaginous fish in the Cretaceous period, providing direct evidence for understanding the early evolution of tapeworms.

In fact, the entire Flatworm phylum, to which tapeworms belong, has very few body fossils, with only a few highly controversial records. The oldest fossil is a hook-like structure found in fish from the Devonian period, which is consistent with the characteristics of monogenean trematodes, but no other structures are preserved. Two structures similar to trematode metacercariae have also been found in a lizard in Burmese amber, but there is a lack of clear morphological evidence. There are also studies that claim to have found monocoelenterates in Baltic amber, which were later proved to be just bubbles. Therefore, this fossil is also the most reliable record of the body fossil of the Flatworm phylum.

In addition, since the Conodontidae tapeworm is a purely marine parasite, the discovery further confirms the scientists' previous inference that the deposition environment of Myanmar's Kachin amber is close to the sea, so there is an opportunity to capture ancient marine animals.

Part 3 Conclusion

In today's global climate change, as the hosts of parasites decline, many parasites are endangered or heading for extinction. Although protecting parasites sounds ridiculous, in nature, there is no distinction between good and bad ways of survival, and parasites are also an important part of the ecosystem.

By studying parasite fossils in amber from 100 million years ago, scientists have restored the role of parasites in the ecosystem at that time, allowing us to further understand the evolution of parasites in the past and assess their future extinction risk. These studies highlight the importance of amber research in paleoparasitology. Amber, a time capsule that has traveled through ancient times and has been preserved to this day, will surely reveal more mysteries of ancient life to us.

References:

[1]Luo Cihang, Poinar GO, Xu Chunpeng, Zhuo De, Jarzembowski EA, Wang Bo, 2023. Widespread mermithid nematode parasitism of Cretaceous insects. eLife 12, e86283. https://doi.org/10.7554/eLife.86283

[2]Luo Cihang, Palm HW, Zhuang Yuhui, Jarzembowski EA, Nyunt TT, Wang Bo, 2024. Exceptional preservation of a marine tapeworm tentacle in Cretaceous amber. Geology. https://doi.org/10.1130/G52071.1

Source: Science Institute