Take you to catch the "ghost" in the fossil

Produced by: Science Popularization China

Author: Cai Jiachen

Producer: China Science Expo

Although there are no "ghosts" in reality, there are some biological groups called "ghosts" in the field of paleontology.

These "ghosts" do not appear alone, they are called "ghost lineages". Seeing this, are you thinking, "Oh my god, you can also make a biological evolutionary tree of 'ghosts'? This is not scientific!"

Ghost evolution tree (Image source: self-made by the author)

No, no, but they do come from the analysis of biological evolution. And to tell this story, we have to start from 1970.

Before this point in time, paleontologists believed that the age of fossils was consistent with the age of the rocks around them. If we follow the logic of this view, it is not difficult to find that the order in which these fossilized organisms appeared on the earth's stage can be determined by the age of the rocks containing them. In other words, the older the age of the rocks in which the fossils are located, the earlier the organism appeared on the earth's stage.

Does it sound reasonable? If you think this sounds logical, you need to first understand the relationship between organisms, fossils, and rocks.

1. From jumping around to being slammed into a stone slab

If the remains of ancient organisms or the relics left behind during their lives are to become fossils, they first need to be buried. Then, as the covering material gradually increases, the pressure on the remains or relics also gradually increases, and then they are slowly compacted and formed into rock.

This process sounds simple, but in fact, every fossil has to go through many challenges when it is formed. The first problem comes from its body. Living things usually have both hard and soft parts. When they decay after death, the soft parts are almost all destroyed, and only the hard parts are preserved.

Most of the fossils we see are hard parts of organisms such as bones, and their soft parts have long disappeared over time. Image source: Wikipedia

Next comes the physical and chemical conditions when the body is exposed to the elements after death. If the energy of water or wind and sand is too strong, the body or remains will be worn away and destroyed, and there will be no chance of being buried. In addition, if the pH value and redox environment of the environment are not suitable, it will also affect the preservation of the organism.

Then there is the burial phase, where the composition of the covering material will also affect the integrity of the fossils. For example, the integrity of the fossils preserved in different environments such as mud and sand, glacial permafrost, and semi-cooled magma will vary greatly.

The next step is the formation of rocks, which is when fossils are actually formed. At this stage, physical compression from the outside world and the exchange of chemicals such as underground water solutions will also cause varying degrees of damage to the fossils.

Even after fossils are formed (that is, after they have completely become part of the rock), they will be destroyed during their long time underground due to continued compression, underground heat, or other factors, and will never be discovered by humans.

Two dinosaurs from death, burial, lithification to excavation. Image source: Wikipedia (https://zh.wikipedia.org/wiki/%E5%9F%8B%E8%97%8F%E5%AD%A6#/media/File:Fossilization_process.jpg)

Each step in these processes is a big selection. The fossils we can find now are only the remains and relics of a very small number of ancient creatures that have been fortunate enough to be preserved. In addition to them, the traces of countless creatures have long been erased by time.

Precisely because the remains and relics of most organisms cannot be preserved, the fossil record is very incomplete, both in terms of the types of ancient organisms and the time when they lived.

Image source: Weibo

Since 1970, paleontologists have gradually noticed this problem and started to seek a more logically rigorous method. Since rocks are unreliable, let's look at the fossils themselves. Paleontologists have extracted the information preserved in fossils and summarized the "features" that can describe fossils. These features are the key to helping restore the survival time of ancient organisms.

These characteristics can be divided into "ancestral characteristics" that reflect the characteristics of ancestors and "derived characteristics" that reflect the characteristics of descendants, depending on whether they can reflect the order in which organisms appeared. Ancestral characteristics and derived characteristics can be said to be a complementary definition.

The characteristics that are shared by all organisms are ancestral characteristics, such as all vertebrates have a spine; the corresponding derived characteristics are characteristics that only some organisms have but not others, just like birds and some dinosaurs have feathers, but many other dinosaurs do not have feathers. In this way, we can roughly determine the order in which different groups of organisms evolved in the history of the Earth, and then we can infer the time point when a certain group evolved its own distinct characteristics. This time point is the time when this group of organisms theoretically evolved.

A and B are two biological groups with a common ancestor; T1 is the time point when these two biological groups separated in evolution; T2 is the time point when group B was discovered; T2 is always equal to or later than T1.

Schematic diagram of the discovery time and evolutionary appearance time of fossil species

However, reality is always unsatisfactory, let alone restoring the "truth" that spans hundreds of millions of years. The preservation of fossils is incomplete, and the missing information will cause the time node of evolution to appear earlier than the earliest fossil record. Since there is no fossil evidence, during this period of time, this type of creature has become a theoretical existence but invisible in reality. Thus, the "ghost" was born. During this period of time without fossil evidence, the evolutionary tree that should theoretically exist is its "ghost lineage".

In addition, a term called "ghost taxa" was proposed at the same time as the ghost lineage, which means "ghost taxa". As the name suggests, it is a "ghost" itself, but the difference from the ghost lineage is that although it has not been discovered, it can be inferred theoretically that it has offspring. From an evolutionary perspective, its extinction evolved into its offspring. For example, a type of theropod dinosaur evolved into birds. This type of dinosaur is now extinct, but its descendants still exist. If this type of dinosaur has not been discovered so far, then this type of dinosaur is a ghost taxa. In other words, the ghost taxa is a type of group that only exists in evolutionary analysis and is a product of pure theoretical analysis. For example, E in the figure can be called a ghost taxa.

A, B, C, and D are four species. Figure I shows the evolutionary relationship between these four species; E and F in Figure II are the ancestors of A, B and C, D respectively; Figure III is the time range of the existence of these fossil species, where the ordinate 1-10 represents the time sequence, 10 is the oldest and 1 is the newest, the solid line is the time recorded in fossils, and the dotted line is the existence time inferred based on systematic evolution. The dotted lines of A and D are called ghost lineages, and E is called ghost group; what about F? Because C appeared in 10, the ancestor F must have appeared earlier than 10, so it is naturally not visible in this figure.

Explanation diagram of ghost lineages and ghost groups (modified from Norell, 1993)

The clever friends must be drawing inferences from this. Could there be a ghost lineage that is simply inferred by theory and for which no actual fossils have been found?

The answer is yes, as long as it lives long enough and its fossils have not been discovered! For example, Latimeria can still be seen in the ocean today.

2. Living Ghost

One of the most famous "ghost lineages" in living fish is the Latimeria, also known as the lancetail, which belongs to the genus Latimeria of the order Coelacanths. It is a type of deep-water fish with a dark blue or brown base, fleshy fins and white spots.

A Latimeria fish found off the coast of South Africa in 2019. Photo credit: Bruce Henderson

In addition to being of interest to biologists and paleontologists, it is also a favorite character of Japanese game companies that love to add fishing elements.

Latimer fish in FF14, screenshot

Latimer in Monster Hunter World: Iceborne, screenshot

Ancient Coelacanth in Pokémon, from wiki.52poke.com

In 1938, people discovered the West Indian Ocean Latimeria chalumnae alive in South Africa. This is the source of the most classic Latimeria image, which is the one with a dark blue background. In 1997, another brown-gray Indonesian Latimeria (Latimeria menadoensis) was discovered in Indonesia.

Latimer fish from Indonesia. Image source: Wikipedia

At present, the genus Latimeria consists of only these two species. From the evolutionary tree obtained by analysis, the genus Latimeria separated from its sister group in the early Carboniferous period. The results of molecular clock analysis show that these two species of Latimeria began to form their own differences from a common ancestor 40 million to 30 million years ago, and officially embarked on their own evolutionary paths about 13 million years ago. However, to this day, paleontologists have not found any fossils of the genus Latimeria except for the two existing Latimeria species. This gap in the fossil record makes Latimeria a "ghost branch" from the early Carboniferous period to the present.

The red line is the ghost lineage. The ghost lineage of Latimeria fish (drawn by the author)

3. Ghosts in Fossils

In 2008, the Early Vertebrate Research Group of the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences discovered a nearly complete bony fish in the late Silurian strata of Qujing, and published the research in the British journal Nature. This fish is Guiyu oneiros. Although it is a bony fish, it combines the characteristics of bony fish, cartilaginous fish, placoderms and spiny fish, bringing the relationship between bony fish and "relatively ancient" placoderms closer.

Specifically, its head armor can be divided into a top armor and a rear top armor in morphology, which is the same as that of early lobe-finned fish; its cheek bones are similar to those of early ray-finned fish, and the ridged patterns on the membranous bones are also closer to those of ray-finned fish; its shoulder girdle has a spine in front of the pectoral fin, which is similar to early lobe-finned fish (such as spotted fish), early cartilaginous fish, acanthids and placoderms; the large bone plate on the midline of its back may be equivalent to the bone plate in the same position in placoderms. Before it was rediscovered, researchers had already discovered that spotted fish among early lobe-finned fish had the same spines as placoderms, acanthids and cartilaginous fish, but because the spotted fish materials found at that time were all scattered bone plates, this feature that linked placoderms, acanthids, cartilaginous fish and bony fish in early jawed fishes has always made researchers doubtful. After the discovery of the ghost fish, its phylogenetic analysis showed that it was located at the basal position of lobe-finned fish. This confirms that the spotted fish has the same wonderful feature of spines as armored fish, acanthosaurs and cartilaginous fish, filling a gap in the evolution of early vertebrates. Since it fills a gap, it can be regarded as a ghost before it is discovered.

The evolutionary position of the ghost fish. Image source: Reference [8]

After discovering such a ghost, paleontologists will naturally not let it go easily. The naming of paleontology follows the binomial nomenclature of biological naming, which consists of two parts, the genus name and the species name. The species name of the dream ghost fish is the Latin "oneiros", which comes from Oneiros, the god of dreams, one of the sons of the goddess of the night in Greek mythology, which means the fish in the dream. Its genus name is "Guiyu", which is a simple and straightforward Chinese pinyin. On the one hand, it means "ghost talent", implying strange characteristics; on the other hand, it is called the "ghost branch" of the ghost branch. Not only does it have strange characteristics, but it is also a 400 million-year-old ghost that paleontologists on the evolutionary tree have been dreaming about and have always wanted to find.

Image source of the restored image of the fantasy ghost fish: Reference [6]

References

[1] Norell M A. Tree-based approaches to understanding history; comments on ranks, rules and the quality of the fossil record[J]. American Journal of Science, 1993, 293(A): 407.

[2] Encyclopedia Latimer

[3] Inoue JG, Miya M, Venkatesh B, et al. The mitochondrial genome of Indonesian coelacanth Latimeria menadoensis (Sarcopterygii: Coelacanthiformes) and divergence time estimation between the two coelacanths[J]. Gene, 2005, 349: 227-235.

[4] Sugeha HY, Pouyaud L, Hocdé R, et al. A thirteen-million-year divergence between two lineages of Indonesian coelacanths[J]. Scientific reports, 2020, 10(1): 1-9.

[5] Zhu M, Zhao W, Jia L, et al. The oldest articulated osteichthyan reveals mosaic gnathostome characters[J]. Nature, 2009, 458(7237): 469-474.

[6] Yu XB, Zhu M, Zhao W J. The Origin and Diversification of Osteichthyans and Sarcopterygians: Rare Chinese Fossil Findings Advance Research on Key Issues of Evolution[J]. Paleoichthyology, 2010, 24(2): 71-75.

[7] Qiao Tuo, Zhu Min. Brain morphology of the Silurian sarcopterygian Guiyu oneiros[J]. Science China: Earth Sciences, 2010 (9): 1191-1203.

[8] Lu Jing, Zhu Youan, Zhu Min. Our ancestors came from water: the origin and early evolution of bony fishes[J]. Nature, 2016, 38(6): 391-398.

[9] Zhao Wenjin. Fantasy ghost fish: the oldest bony fish[J]. Collection, 2009, 6.

(Note: The Latin part of the text is italicized)