Serhiy Yastrebov. III. Type Vetulicolia: trace of the “Great Explosion of Life”

The 'Batrachos' Drawing Room. The final of three articles by Serhiy Oleksandrovych Yastrebov dedicated to the origin of the phylum Chordata. The first is here, the second is here. S. A. Yastrebov. Phylum Vetulicolia: a trace of the 'Big Bang of Life'.

One of the most important mysteries in animal evolution is the so-called Cambrian explosion—the sudden appearance of most modern phyla at the boundary of the Cryptic and Phanerozoic eras.

Living room “Batrachos”. The final of three articles by Serhiy Oleksandrovych Yastrubov devoted to the origin of the chordate type. The first is here, the second – here. S. A. Yastrubov Type Vetulicolia: the trace of the “Great Explosion of Life” One of the biggest mysteries in animal evolution is the so‑called Cambrian explosion – the sudden appearance of most modern phyla at the boundary of the Cryogenian and the Paleozoic. Let us recall some facts. The history of life on Earth lasts about 4 billion years (the age of the oldest discovered bacterial remnants is 3.8 billion). This huge interval is divided into two unequal parts. The first part, lasting more than three billion years, is called the Cryogenian or pre‑Cambrian. “Cryogenian” literally means “era of hidden life”. The second part, covering the last 540 million years, is called the Phanerozoic – literally “era of visible life”. Eras such as the Paleozoic, Mesozoic and Cenozoic are subdivisions of the Phanerozoic. They, in turn, are divided into periods. For example, the Paleozoic era is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods. Thus the modern geochronological scale looks like this: [IMG_1] Fig. 1. Geochronological scale (from https://www.sciencelearn.org.nz, with modifications). Cryogenian and Phanerozoic are the highest units of the scale; they are called eons. Archaea, Bacteria and Proterozoic make up the Cryogenian. Paleozoic, Mesozoic and Cenozoic are the Phanerozoic eras, shown in different colours and divided into periods. Note how much shorter the Phanerozoic is than the Cryogenian: its much more detailed subdivision had to be placed separately so as not to disrupt the scale’s proportion. All modern animals can be placed into roughly 30 phyla. Information on their phylogenetic relationships is found in any introductory zoology textbook. But here a problem arises. Many modern phyla appear in the fossil record almost simultaneously – at the beginning of the Cambrian – already in a fairly well‑formed state. This concerns such disparate, complex groups as arthropods, molluscs, echinoderms and chordates. Roughly speaking, it remains unclear when the different phyla diverged from one another. In a very interesting book by a team of American authors led by Robert Barnes, “Invertebrates: A New Integrated Approach” (its Russian translation was published exactly 20 years ago), the following calculations are presented. From paleontological data it is known that the average time required for one species to turn into another, for various invertebrate groups, is on the order of 10 million years². Suppose that genera differ from each other on average ten times more than species do, families ten times more than genera, and so on up to classes and phyla. How much time would be needed for two newly separated species to become different phyla? Answer: at a constant evolutionary rate – 1 000 billion years. To gauge this figure, note that the Big Bang, according to the modern hypothesis, occurred less than 14 billion years ago³. The first multicellular animals most likely appeared about one billion years ago. If the model above were correct, their descendants today would differ from each other as, for example, different families of beetles. Yet we know that already 400 million years ago many modern phyla existed, differing from each other roughly as they do now. Thus the rate of evolution is not constant. We record this conclusion and move on. It is noteworthy that modern animal phyla differ enormously in size. Almost half of them are the so‑called “small phyla”, each comprising no more than 500 species. At the same time there are giant phyla with hundreds of thousands of species (in the arthropod phylum – over a million). The extreme unevenness of species distribution among phyla is an important feature of the “diversity structure” of modern animals, which would be worthwhile to explain. One such explanation – controversial but interesting – was proposed by Robert Barnes and colleagues. The point is that many “small phyla” indeed have Cambrian ages⁴. But how did they survive to the present with such a low species count? The fewer the species, the higher the extinction risk. Barnes and colleagues concluded that this is statistically probable only if many more such phyla existed initially; most of them later went extinct. We are dealing with remnants of former diversity, the “tip of the Cambrian iceberg”. The conclusion about the non‑constant evolutionary rate implies that sometimes evolution proceeded slowly, and sometimes it accelerated explosively. Barnes and colleagues suggested that at the end of the pre‑Cambrian – the beginning of the Cambrian – an “explosion” occurred in animal evolution, possibly producing several hundred equivalent phylum‑level branches. Most of them quickly went extinct, and the survivors formed the modern fauna. We know of no animal phylum that certainly originated after the Cambrian; perhaps none exist. But where are the mysterious phyla that appeared in the Cambrian and then vanished? Have any traces of them survived? Modern paleontology advances rapidly, so the curtain over the Cambrian explosion mystery is gradually being lifted. Here is just one example. In 1987, a new animal genus was discovered in early Cambrian deposits of China and named Vetulicola. It was a rather odd creature, segmented and with a light external shell, like an arthropod. Yet Vetulicola had no legs (which a normal arthropod would be expected to have), and its shell was bivalved: such a feature is rare in arthropods, occurring only in some crustaceans. By other characters Vetulicola bears little resemblance to crustaceans. It resembles nothing else. Nevertheless, it was placed in the arthropod phylum, assigned to a separate class with unclear affinities. [IMG_2] Fig. 2. Vetulicola (from https://dic.nicovideo.jp). The Japanese artist, author of the illustration, brilliantly conveyed the slightly alien appearance of this organism In the meantime several more genera close to Vetulicola were uncovered, and in 2001 several paleontologists (among them the renowned Cambrian specialists – Chinese Degen Shu and English Simon Conway Morris) proposed a completely different interpretation of these animals. They declared them a distinct phylum, not related to arthropods but to chordates⁵. This phylum was named Vetulicolia. The main argument for the independence of the vetulicolids was the discovery of gill slits. This feature never occurs in arthropods, but is very characteristic of chordates. Yet vetulicolids lack any notochord. They cannot be assigned to any modern phylum. Renowned evolutionary biologist Professor Mykola Vorontsov once wrote that the discovery of a new animal phylum is equivalent in significance to the discovery of a new planet in the Solar System. That is exactly what happened here. [IMG_3] Fig. 3. Vetulicolia from early Cambrian China (from en.wikipedia.org). Top to bottom – representatives of the genera Didazoon, Pomatrum and Xidazoon What are vetulicolia? They are marine animals that swam in the water column, not very large – most often up to 10 cm in length (20‑cm individuals are already considered gigantic). It is not easy to say what they resembled. The most important characteristics of vetulicolia are: 1. Segmentation. Well expressed, externally similar to arthropod segmentation. The body is completely segmented, although the few anterior segments are often covered dorsally by a single solid shield⁶. Unlike chordates, segmentation involves the outer covering and is visible externally. 2. Gill slits. They have five pairs. By comparison, recently discovered early Cambrian chordates have six or seven pairs of gill slits. Vetulicolia may represent a more primitive state of this character. 3. An endostyle is found on the floor of the pharynx – a groove through which cilia push mucus together with food particles. This organ is very characteristic of lower chordates. 4. The mouth is terminal, i.e., located strictly at the anterior end of the body. In some genera it is round, and the surrounding marginal teeth are arranged concentrically, giving the oral apparatus radial symmetry. By this trait vetulicolia, surprisingly, vaguely resemble some roundworms. 5. The posterior part of the body forms a tail fin, externally very reminiscent of a chordate tail. The difference is that in chordates the tail lies behind the anal opening and contains no body cavity, whereas in vetulicolia the entire “tail” houses the intestine, with the anal opening at the very end. These body sections are analogous in function but differ in internal structure. 6. Appendages on the trunk are completely absent. A typical vetulicolian could be considered a worm if not for its characteristic “tadpole‑like” body shape with an expanded anterior region. What kind of animals are these? By characters (2) and (3) they resemble chordates, by character (1) – more like arthropods. The “portrait” of the animal is completed by characters (4), (5) and (6), whose significance is altogether enigmatic. Vetulicolia are a purely Cambrian group. They are unknown from any other time period. Apparently they represent one of the bursts of the “Cambrian explosion”: a unique branch worthy of the rank of a separate phylum, but low‑diversity and quickly extinct. In 1902 American paleontologist Henry Osborn introduced the concept of “adaptive radiation”: a relatively rapid splitting of an evolving group into many branches that specialize in different ways. This is one of the main phenomena studied by evolutionary theory. The Cambrian adaptive radiation is among the most powerful in the entire history of life on Earth. Therefore vetulicolian researcher Degen Shu called it the “Great Explosion of Life”. Brief bibliography Barnes R., Kellow P., Olive P., Holding D. Invertebrates: A New Integrated Approach. Moscow, “Mir”, 1992. Shu D. On the Phylum Vetulicolia. // Chinese Science Bulletin 2005 Vol. 50 No. 20 2342—2354. — Online. Aldridge R., Hou X., Siveter D., Siveter D., Gabbott S. The systematics and phylogenetic relationships of Vetulicolia. // Palaeontology, Vol. 50, Part 1, 2007, pp. 131–168.

This is what the modern geochronological scale looks like: Fig. 1. Geochronological scale (from the website https://www.sciencelearn.org.nz, with modifications). The Cryptic and Phanerozoic are the highest units of the scale; they are called eons. The Hadean, Archean, and Proterozoic make up the Cryptic. The Paleozoic, Mesozoic, and Cenozoic are the eras of the Phanerozoic; they are shown in different colors and are divided into periods. Note how much shorter the Phanerozoic is than the Cryptic: its division (much more detailed) had to be moved to avoid disrupting the scale of the diagram.

All modern animals can be placed into approximately 30 phyla. Information about the phylogenetic relationships between them can be found in any zoology textbook 1. But here lies the problem. Many modern animal phyla appear in the fossil record almost simultaneously—at the beginning of the Cambrian, and already in a fully formed state. This applies to such diverse and dissimilar organisms as arthropods, mollusks, echinoderms, and chordates. Roughly speaking, it remains unclear how different phyla managed to arise from one another.

In the very interesting book 'Invertebrates: A New Synthesis' (its Russian translation was published exactly 20 years ago), written by a team of American authors led by Robert Barnes, the following calculations are presented. From paleontological data, it is known that the average time required for one species to transform into another is on the order of 10 million years for a wide variety of invertebrate groups 2. Assume that animal genera differ from each other on average 10 times more than species, families 10 times more than genera, and so on, up to classes and phyla. How much time would be needed in this case for two species that have just diverged to become different phyla? Answer: at a constant rate of evolution—1000 billion years. To appreciate this figure, it is enough to say that the Big Bang, according to the modern hypothesis, occurred less than 14 billion years ago 3.

Living room “Batrachos”. The final of three articles by Serhiy Oleksandrovych Yastrubov devoted to the origin of the chordate type. The first is here, the second – here. S. A. Yastrubov Type Vetulicolia: the trace of the “Great Explosion of Life” One of the biggest mysteries in animal evolution is the so‑called Cambrian explosion – the sudden appearance of most modern phyla at the boundary of the Cryogenian and the Paleozoic. Let us recall some facts. The history of life on Earth lasts about 4 billion years (the age of the oldest discovered bacterial remnants is 3.8 billion). This huge interval is divided into two unequal parts. The first part, lasting more than three billion years, is called the Cryogenian or pre‑Cambrian. “Cryogenian” literally means “era of hidden life”. The second part, covering the last 540 million years, is called the Phanerozoic – literally “era of visible life”. Eras such as the Paleozoic, Mesozoic and Cenozoic are subdivisions of the Phanerozoic. They, in turn, are divided into periods. For example, the Paleozoic era is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods. Thus the modern geochronological scale looks like this: [IMG_1] Fig. 1. Geochronological scale (from https://www.sciencelearn.org.nz, with modifications). Cryogenian and Phanerozoic are the highest units of the scale; they are called eons. Archaea, Bacteria and Proterozoic make up the Cryogenian. Paleozoic, Mesozoic and Cenozoic are the Phanerozoic eras, shown in different colours and divided into periods. Note how much shorter the Phanerozoic is than the Cryogenian: its much more detailed subdivision had to be placed separately so as not to disrupt the scale’s proportion. All modern animals can be placed into roughly 30 phyla. Information on their phylogenetic relationships is found in any introductory zoology textbook. But here a problem arises. Many modern phyla appear in the fossil record almost simultaneously – at the beginning of the Cambrian – already in a fairly well‑formed state. This concerns such disparate, complex groups as arthropods, molluscs, echinoderms and chordates. Roughly speaking, it remains unclear when the different phyla diverged from one another. In a very interesting book by a team of American authors led by Robert Barnes, “Invertebrates: A New Integrated Approach” (its Russian translation was published exactly 20 years ago), the following calculations are presented. From paleontological data it is known that the average time required for one species to turn into another, for various invertebrate groups, is on the order of 10 million years². Suppose that genera differ from each other on average ten times more than species do, families ten times more than genera, and so on up to classes and phyla. How much time would be needed for two newly separated species to become different phyla? Answer: at a constant evolutionary rate – 1 000 billion years. To gauge this figure, note that the Big Bang, according to the modern hypothesis, occurred less than 14 billion years ago³. The first multicellular animals most likely appeared about one billion years ago. If the model above were correct, their descendants today would differ from each other as, for example, different families of beetles. Yet we know that already 400 million years ago many modern phyla existed, differing from each other roughly as they do now. Thus the rate of evolution is not constant. We record this conclusion and move on. It is noteworthy that modern animal phyla differ enormously in size. Almost half of them are the so‑called “small phyla”, each comprising no more than 500 species. At the same time there are giant phyla with hundreds of thousands of species (in the arthropod phylum – over a million). The extreme unevenness of species distribution among phyla is an important feature of the “diversity structure” of modern animals, which would be worthwhile to explain. One such explanation – controversial but interesting – was proposed by Robert Barnes and colleagues. The point is that many “small phyla” indeed have Cambrian ages⁴. But how did they survive to the present with such a low species count? The fewer the species, the higher the extinction risk. Barnes and colleagues concluded that this is statistically probable only if many more such phyla existed initially; most of them later went extinct. We are dealing with remnants of former diversity, the “tip of the Cambrian iceberg”. The conclusion about the non‑constant evolutionary rate implies that sometimes evolution proceeded slowly, and sometimes it accelerated explosively. Barnes and colleagues suggested that at the end of the pre‑Cambrian – the beginning of the Cambrian – an “explosion” occurred in animal evolution, possibly producing several hundred equivalent phylum‑level branches. Most of them quickly went extinct, and the survivors formed the modern fauna. We know of no animal phylum that certainly originated after the Cambrian; perhaps none exist. But where are the mysterious phyla that appeared in the Cambrian and then vanished? Have any traces of them survived? Modern paleontology advances rapidly, so the curtain over the Cambrian explosion mystery is gradually being lifted. Here is just one example. In 1987, a new animal genus was discovered in early Cambrian deposits of China and named Vetulicola. It was a rather odd creature, segmented and with a light external shell, like an arthropod. Yet Vetulicola had no legs (which a normal arthropod would be expected to have), and its shell was bivalved: such a feature is rare in arthropods, occurring only in some crustaceans. By other characters Vetulicola bears little resemblance to crustaceans. It resembles nothing else. Nevertheless, it was placed in the arthropod phylum, assigned to a separate class with unclear affinities. [IMG_2] Fig. 2. Vetulicola (from https://dic.nicovideo.jp). The Japanese artist, author of the illustration, brilliantly conveyed the slightly alien appearance of this organism In the meantime several more genera close to Vetulicola were uncovered, and in 2001 several paleontologists (among them the renowned Cambrian specialists – Chinese Degen Shu and English Simon Conway Morris) proposed a completely different interpretation of these animals. They declared them a distinct phylum, not related to arthropods but to chordates⁵. This phylum was named Vetulicolia. The main argument for the independence of the vetulicolids was the discovery of gill slits. This feature never occurs in arthropods, but is very characteristic of chordates. Yet vetulicolids lack any notochord. They cannot be assigned to any modern phylum. Renowned evolutionary biologist Professor Mykola Vorontsov once wrote that the discovery of a new animal phylum is equivalent in significance to the discovery of a new planet in the Solar System. That is exactly what happened here. [IMG_3] Fig. 3. Vetulicolia from early Cambrian China (from en.wikipedia.org). Top to bottom – representatives of the genera Didazoon, Pomatrum and Xidazoon What are vetulicolia? They are marine animals that swam in the water column, not very large – most often up to 10 cm in length (20‑cm individuals are already considered gigantic). It is not easy to say what they resembled. The most important characteristics of vetulicolia are: 1. Segmentation. Well expressed, externally similar to arthropod segmentation. The body is completely segmented, although the few anterior segments are often covered dorsally by a single solid shield⁶. Unlike chordates, segmentation involves the outer covering and is visible externally. 2. Gill slits. They have five pairs. By comparison, recently discovered early Cambrian chordates have six or seven pairs of gill slits. Vetulicolia may represent a more primitive state of this character. 3. An endostyle is found on the floor of the pharynx – a groove through which cilia push mucus together with food particles. This organ is very characteristic of lower chordates. 4. The mouth is terminal, i.e., located strictly at the anterior end of the body. In some genera it is round, and the surrounding marginal teeth are arranged concentrically, giving the oral apparatus radial symmetry. By this trait vetulicolia, surprisingly, vaguely resemble some roundworms. 5. The posterior part of the body forms a tail fin, externally very reminiscent of a chordate tail. The difference is that in chordates the tail lies behind the anal opening and contains no body cavity, whereas in vetulicolia the entire “tail” houses the intestine, with the anal opening at the very end. These body sections are analogous in function but differ in internal structure. 6. Appendages on the trunk are completely absent. A typical vetulicolian could be considered a worm if not for its characteristic “tadpole‑like” body shape with an expanded anterior region. What kind of animals are these? By characters (2) and (3) they resemble chordates, by character (1) – more like arthropods. The “portrait” of the animal is completed by characters (4), (5) and (6), whose significance is altogether enigmatic. Vetulicolia are a purely Cambrian group. They are unknown from any other time period. Apparently they represent one of the bursts of the “Cambrian explosion”: a unique branch worthy of the rank of a separate phylum, but low‑diversity and quickly extinct. In 1902 American paleontologist Henry Osborn introduced the concept of “adaptive radiation”: a relatively rapid splitting of an evolving group into many branches that specialize in different ways. This is one of the main phenomena studied by evolutionary theory. The Cambrian adaptive radiation is among the most powerful in the entire history of life on Earth. Therefore vetulicolian researcher Degen Shu called it the “Great Explosion of Life”. Brief bibliography Barnes R., Kellow P., Olive P., Holding D. Invertebrates: A New Integrated Approach. Moscow, “Mir”, 1992. Shu D. On the Phylum Vetulicolia. // Chinese Science Bulletin 2005 Vol. 50 No. 20 2342—2354. — Online. Aldridge R., Hou X., Siveter D., Siveter D., Gabbott S. The systematics and phylogenetic relationships of Vetulicolia. // Palaeontology, Vol. 50, Part 1, 2007, pp. 131–168.

Therefore, the rate of evolution is not constant.

Let's note this conclusion and move on.

It is noteworthy that modern animal phyla vary greatly in size. Almost half of them are so-called 'small phyla', comprising no more than 500 species each. At the same time, there are 'giant phyla' with hundreds of thousands of species (the phylum Arthropoda has over a million). The enormous unevenness in the distribution of species among phyla is an important feature of the 'structure of diversity' of modern animals, which is worth explaining somehow. One such explanation—controversial but interesting—has been proposed by Robert Barnes and his colleagues.

The fact is that many 'small phyla' are precisely Cambrian in age 4. But how did they survive to our days with such a small number of species? After all, the fewer species, the higher the risk of extinction. Barnes and colleagues concluded that this is statistically probable only if there were many more such phyla initially. Simply put, most of them did go extinct. We are dealing with the remnants of past diversity, with the 'tip of the Cambrian iceberg'.

The conclusion about the non-constant rate of evolution means that sometimes it proceeded slowly, and sometimes it explosively accelerated. Barnes and colleagues suggested that at the end of the Precambrian—beginning of the Cambrian, such an 'explosion' occurred in animal evolution, which gave rise to perhaps several hundred equivalent phylum branches. Most of them quickly went extinct, and the survivors formed the modern fauna. We know of no animal phylum that demonstrably originated after the Cambrian; perhaps there are none.

But where are the mysterious phyla that appeared in the Cambrian and went extinct at the same time? Are there any traces left of them?

Modern paleontology is developing very rapidly, so the veil over the mystery of the Cambrian explosion is gradually lifting. Here is just one example.

Living room “Batrachos”. The final of three articles by Serhiy Oleksandrovych Yastrubov devoted to the origin of the chordate type. The first is here, the second – here. S. A. Yastrubov Type Vetulicolia: the trace of the “Great Explosion of Life” One of the biggest mysteries in animal evolution is the so‑called Cambrian explosion – the sudden appearance of most modern phyla at the boundary of the Cryogenian and the Paleozoic. Let us recall some facts. The history of life on Earth lasts about 4 billion years (the age of the oldest discovered bacterial remnants is 3.8 billion). This huge interval is divided into two unequal parts. The first part, lasting more than three billion years, is called the Cryogenian or pre‑Cambrian. “Cryogenian” literally means “era of hidden life”. The second part, covering the last 540 million years, is called the Phanerozoic – literally “era of visible life”. Eras such as the Paleozoic, Mesozoic and Cenozoic are subdivisions of the Phanerozoic. They, in turn, are divided into periods. For example, the Paleozoic era is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods. Thus the modern geochronological scale looks like this: [IMG_1] Fig. 1. Geochronological scale (from https://www.sciencelearn.org.nz, with modifications). Cryogenian and Phanerozoic are the highest units of the scale; they are called eons. Archaea, Bacteria and Proterozoic make up the Cryogenian. Paleozoic, Mesozoic and Cenozoic are the Phanerozoic eras, shown in different colours and divided into periods. Note how much shorter the Phanerozoic is than the Cryogenian: its much more detailed subdivision had to be placed separately so as not to disrupt the scale’s proportion. All modern animals can be placed into roughly 30 phyla. Information on their phylogenetic relationships is found in any introductory zoology textbook. But here a problem arises. Many modern phyla appear in the fossil record almost simultaneously – at the beginning of the Cambrian – already in a fairly well‑formed state. This concerns such disparate, complex groups as arthropods, molluscs, echinoderms and chordates. Roughly speaking, it remains unclear when the different phyla diverged from one another. In a very interesting book by a team of American authors led by Robert Barnes, “Invertebrates: A New Integrated Approach” (its Russian translation was published exactly 20 years ago), the following calculations are presented. From paleontological data it is known that the average time required for one species to turn into another, for various invertebrate groups, is on the order of 10 million years². Suppose that genera differ from each other on average ten times more than species do, families ten times more than genera, and so on up to classes and phyla. How much time would be needed for two newly separated species to become different phyla? Answer: at a constant evolutionary rate – 1 000 billion years. To gauge this figure, note that the Big Bang, according to the modern hypothesis, occurred less than 14 billion years ago³. The first multicellular animals most likely appeared about one billion years ago. If the model above were correct, their descendants today would differ from each other as, for example, different families of beetles. Yet we know that already 400 million years ago many modern phyla existed, differing from each other roughly as they do now. Thus the rate of evolution is not constant. We record this conclusion and move on. It is noteworthy that modern animal phyla differ enormously in size. Almost half of them are the so‑called “small phyla”, each comprising no more than 500 species. At the same time there are giant phyla with hundreds of thousands of species (in the arthropod phylum – over a million). The extreme unevenness of species distribution among phyla is an important feature of the “diversity structure” of modern animals, which would be worthwhile to explain. One such explanation – controversial but interesting – was proposed by Robert Barnes and colleagues. The point is that many “small phyla” indeed have Cambrian ages⁴. But how did they survive to the present with such a low species count? The fewer the species, the higher the extinction risk. Barnes and colleagues concluded that this is statistically probable only if many more such phyla existed initially; most of them later went extinct. We are dealing with remnants of former diversity, the “tip of the Cambrian iceberg”. The conclusion about the non‑constant evolutionary rate implies that sometimes evolution proceeded slowly, and sometimes it accelerated explosively. Barnes and colleagues suggested that at the end of the pre‑Cambrian – the beginning of the Cambrian – an “explosion” occurred in animal evolution, possibly producing several hundred equivalent phylum‑level branches. Most of them quickly went extinct, and the survivors formed the modern fauna. We know of no animal phylum that certainly originated after the Cambrian; perhaps none exist. But where are the mysterious phyla that appeared in the Cambrian and then vanished? Have any traces of them survived? Modern paleontology advances rapidly, so the curtain over the Cambrian explosion mystery is gradually being lifted. Here is just one example. In 1987, a new animal genus was discovered in early Cambrian deposits of China and named Vetulicola. It was a rather odd creature, segmented and with a light external shell, like an arthropod. Yet Vetulicola had no legs (which a normal arthropod would be expected to have), and its shell was bivalved: such a feature is rare in arthropods, occurring only in some crustaceans. By other characters Vetulicola bears little resemblance to crustaceans. It resembles nothing else. Nevertheless, it was placed in the arthropod phylum, assigned to a separate class with unclear affinities. [IMG_2] Fig. 2. Vetulicola (from https://dic.nicovideo.jp). The Japanese artist, author of the illustration, brilliantly conveyed the slightly alien appearance of this organism In the meantime several more genera close to Vetulicola were uncovered, and in 2001 several paleontologists (among them the renowned Cambrian specialists – Chinese Degen Shu and English Simon Conway Morris) proposed a completely different interpretation of these animals. They declared them a distinct phylum, not related to arthropods but to chordates⁵. This phylum was named Vetulicolia. The main argument for the independence of the vetulicolids was the discovery of gill slits. This feature never occurs in arthropods, but is very characteristic of chordates. Yet vetulicolids lack any notochord. They cannot be assigned to any modern phylum. Renowned evolutionary biologist Professor Mykola Vorontsov once wrote that the discovery of a new animal phylum is equivalent in significance to the discovery of a new planet in the Solar System. That is exactly what happened here. [IMG_3] Fig. 3. Vetulicolia from early Cambrian China (from en.wikipedia.org). Top to bottom – representatives of the genera Didazoon, Pomatrum and Xidazoon What are vetulicolia? They are marine animals that swam in the water column, not very large – most often up to 10 cm in length (20‑cm individuals are already considered gigantic). It is not easy to say what they resembled. The most important characteristics of vetulicolia are: 1. Segmentation. Well expressed, externally similar to arthropod segmentation. The body is completely segmented, although the few anterior segments are often covered dorsally by a single solid shield⁶. Unlike chordates, segmentation involves the outer covering and is visible externally. 2. Gill slits. They have five pairs. By comparison, recently discovered early Cambrian chordates have six or seven pairs of gill slits. Vetulicolia may represent a more primitive state of this character. 3. An endostyle is found on the floor of the pharynx – a groove through which cilia push mucus together with food particles. This organ is very characteristic of lower chordates. 4. The mouth is terminal, i.e., located strictly at the anterior end of the body. In some genera it is round, and the surrounding marginal teeth are arranged concentrically, giving the oral apparatus radial symmetry. By this trait vetulicolia, surprisingly, vaguely resemble some roundworms. 5. The posterior part of the body forms a tail fin, externally very reminiscent of a chordate tail. The difference is that in chordates the tail lies behind the anal opening and contains no body cavity, whereas in vetulicolia the entire “tail” houses the intestine, with the anal opening at the very end. These body sections are analogous in function but differ in internal structure. 6. Appendages on the trunk are completely absent. A typical vetulicolian could be considered a worm if not for its characteristic “tadpole‑like” body shape with an expanded anterior region. What kind of animals are these? By characters (2) and (3) they resemble chordates, by character (1) – more like arthropods. The “portrait” of the animal is completed by characters (4), (5) and (6), whose significance is altogether enigmatic. Vetulicolia are a purely Cambrian group. They are unknown from any other time period. Apparently they represent one of the bursts of the “Cambrian explosion”: a unique branch worthy of the rank of a separate phylum, but low‑diversity and quickly extinct. In 1902 American paleontologist Henry Osborn introduced the concept of “adaptive radiation”: a relatively rapid splitting of an evolving group into many branches that specialize in different ways. This is one of the main phenomena studied by evolutionary theory. The Cambrian adaptive radiation is among the most powerful in the entire history of life on Earth. Therefore vetulicolian researcher Degen Shu called it the “Great Explosion of Life”. Brief bibliography Barnes R., Kellow P., Olive P., Holding D. Invertebrates: A New Integrated Approach. Moscow, “Mir”, 1992. Shu D. On the Phylum Vetulicolia. // Chinese Science Bulletin 2005 Vol. 50 No. 20 2342—2354. — Online. Aldridge R., Hou X., Siveter D., Siveter D., Gabbott S. The systematics and phylogenetic relationships of Vetulicolia. // Palaeontology, Vol. 50, Part 1, 2007, pp. 131–168.

Living room “Batrachos”. The final of three articles by Serhiy Oleksandrovych Yastrubov devoted to the origin of the chordate type. The first is here, the second – here. S. A. Yastrubov Type Vetulicolia: the trace of the “Great Explosion of Life” One of the biggest mysteries in animal evolution is the so‑called Cambrian explosion – the sudden appearance of most modern phyla at the boundary of the Cryogenian and the Paleozoic. Let us recall some facts. The history of life on Earth lasts about 4 billion years (the age of the oldest discovered bacterial remnants is 3.8 billion). This huge interval is divided into two unequal parts. The first part, lasting more than three billion years, is called the Cryogenian or pre‑Cambrian. “Cryogenian” literally means “era of hidden life”. The second part, covering the last 540 million years, is called the Phanerozoic – literally “era of visible life”. Eras such as the Paleozoic, Mesozoic and Cenozoic are subdivisions of the Phanerozoic. They, in turn, are divided into periods. For example, the Paleozoic era is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods. Thus the modern geochronological scale looks like this: [IMG_1] Fig. 1. Geochronological scale (from https://www.sciencelearn.org.nz, with modifications). Cryogenian and Phanerozoic are the highest units of the scale; they are called eons. Archaea, Bacteria and Proterozoic make up the Cryogenian. Paleozoic, Mesozoic and Cenozoic are the Phanerozoic eras, shown in different colours and divided into periods. Note how much shorter the Phanerozoic is than the Cryogenian: its much more detailed subdivision had to be placed separately so as not to disrupt the scale’s proportion. All modern animals can be placed into roughly 30 phyla. Information on their phylogenetic relationships is found in any introductory zoology textbook. But here a problem arises. Many modern phyla appear in the fossil record almost simultaneously – at the beginning of the Cambrian – already in a fairly well‑formed state. This concerns such disparate, complex groups as arthropods, molluscs, echinoderms and chordates. Roughly speaking, it remains unclear when the different phyla diverged from one another. In a very interesting book by a team of American authors led by Robert Barnes, “Invertebrates: A New Integrated Approach” (its Russian translation was published exactly 20 years ago), the following calculations are presented. From paleontological data it is known that the average time required for one species to turn into another, for various invertebrate groups, is on the order of 10 million years². Suppose that genera differ from each other on average ten times more than species do, families ten times more than genera, and so on up to classes and phyla. How much time would be needed for two newly separated species to become different phyla? Answer: at a constant evolutionary rate – 1 000 billion years. To gauge this figure, note that the Big Bang, according to the modern hypothesis, occurred less than 14 billion years ago³. The first multicellular animals most likely appeared about one billion years ago. If the model above were correct, their descendants today would differ from each other as, for example, different families of beetles. Yet we know that already 400 million years ago many modern phyla existed, differing from each other roughly as they do now. Thus the rate of evolution is not constant. We record this conclusion and move on. It is noteworthy that modern animal phyla differ enormously in size. Almost half of them are the so‑called “small phyla”, each comprising no more than 500 species. At the same time there are giant phyla with hundreds of thousands of species (in the arthropod phylum – over a million). The extreme unevenness of species distribution among phyla is an important feature of the “diversity structure” of modern animals, which would be worthwhile to explain. One such explanation – controversial but interesting – was proposed by Robert Barnes and colleagues. The point is that many “small phyla” indeed have Cambrian ages⁴. But how did they survive to the present with such a low species count? The fewer the species, the higher the extinction risk. Barnes and colleagues concluded that this is statistically probable only if many more such phyla existed initially; most of them later went extinct. We are dealing with remnants of former diversity, the “tip of the Cambrian iceberg”. The conclusion about the non‑constant evolutionary rate implies that sometimes evolution proceeded slowly, and sometimes it accelerated explosively. Barnes and colleagues suggested that at the end of the pre‑Cambrian – the beginning of the Cambrian – an “explosion” occurred in animal evolution, possibly producing several hundred equivalent phylum‑level branches. Most of them quickly went extinct, and the survivors formed the modern fauna. We know of no animal phylum that certainly originated after the Cambrian; perhaps none exist. But where are the mysterious phyla that appeared in the Cambrian and then vanished? Have any traces of them survived? Modern paleontology advances rapidly, so the curtain over the Cambrian explosion mystery is gradually being lifted. Here is just one example. In 1987, a new animal genus was discovered in early Cambrian deposits of China and named Vetulicola. It was a rather odd creature, segmented and with a light external shell, like an arthropod. Yet Vetulicola had no legs (which a normal arthropod would be expected to have), and its shell was bivalved: such a feature is rare in arthropods, occurring only in some crustaceans. By other characters Vetulicola bears little resemblance to crustaceans. It resembles nothing else. Nevertheless, it was placed in the arthropod phylum, assigned to a separate class with unclear affinities. [IMG_2] Fig. 2. Vetulicola (from https://dic.nicovideo.jp). The Japanese artist, author of the illustration, brilliantly conveyed the slightly alien appearance of this organism In the meantime several more genera close to Vetulicola were uncovered, and in 2001 several paleontologists (among them the renowned Cambrian specialists – Chinese Degen Shu and English Simon Conway Morris) proposed a completely different interpretation of these animals. They declared them a distinct phylum, not related to arthropods but to chordates⁵. This phylum was named Vetulicolia. The main argument for the independence of the vetulicolids was the discovery of gill slits. This feature never occurs in arthropods, but is very characteristic of chordates. Yet vetulicolids lack any notochord. They cannot be assigned to any modern phylum. Renowned evolutionary biologist Professor Mykola Vorontsov once wrote that the discovery of a new animal phylum is equivalent in significance to the discovery of a new planet in the Solar System. That is exactly what happened here. [IMG_3] Fig. 3. Vetulicolia from early Cambrian China (from en.wikipedia.org). Top to bottom – representatives of the genera Didazoon, Pomatrum and Xidazoon What are vetulicolia? They are marine animals that swam in the water column, not very large – most often up to 10 cm in length (20‑cm individuals are already considered gigantic). It is not easy to say what they resembled. The most important characteristics of vetulicolia are: 1. Segmentation. Well expressed, externally similar to arthropod segmentation. The body is completely segmented, although the few anterior segments are often covered dorsally by a single solid shield⁶. Unlike chordates, segmentation involves the outer covering and is visible externally. 2. Gill slits. They have five pairs. By comparison, recently discovered early Cambrian chordates have six or seven pairs of gill slits. Vetulicolia may represent a more primitive state of this character. 3. An endostyle is found on the floor of the pharynx – a groove through which cilia push mucus together with food particles. This organ is very characteristic of lower chordates. 4. The mouth is terminal, i.e., located strictly at the anterior end of the body. In some genera it is round, and the surrounding marginal teeth are arranged concentrically, giving the oral apparatus radial symmetry. By this trait vetulicolia, surprisingly, vaguely resemble some roundworms. 5. The posterior part of the body forms a tail fin, externally very reminiscent of a chordate tail. The difference is that in chordates the tail lies behind the anal opening and contains no body cavity, whereas in vetulicolia the entire “tail” houses the intestine, with the anal opening at the very end. These body sections are analogous in function but differ in internal structure. 6. Appendages on the trunk are completely absent. A typical vetulicolian could be considered a worm if not for its characteristic “tadpole‑like” body shape with an expanded anterior region. What kind of animals are these? By characters (2) and (3) they resemble chordates, by character (1) – more like arthropods. The “portrait” of the animal is completed by characters (4), (5) and (6), whose significance is altogether enigmatic. Vetulicolia are a purely Cambrian group. They are unknown from any other time period. Apparently they represent one of the bursts of the “Cambrian explosion”: a unique branch worthy of the rank of a separate phylum, but low‑diversity and quickly extinct. In 1902 American paleontologist Henry Osborn introduced the concept of “adaptive radiation”: a relatively rapid splitting of an evolving group into many branches that specialize in different ways. This is one of the main phenomena studied by evolutionary theory. The Cambrian adaptive radiation is among the most powerful in the entire history of life on Earth. Therefore vetulicolian researcher Degen Shu called it the “Great Explosion of Life”. Brief bibliography Barnes R., Kellow P., Olive P., Holding D. Invertebrates: A New Integrated Approach. Moscow, “Mir”, 1992. Shu D. On the Phylum Vetulicolia. // Chinese Science Bulletin 2005 Vol. 50 No. 20 2342—2354. — Online. Aldridge R., Hou X., Siveter D., Siveter D., Gabbott S. The systematics and phylogenetic relationships of Vetulicolia. // Palaeontology, Vol. 50, Part 1, 2007, pp. 131–168.

The main argument in favor of the distinctness of vetulicolids was the discovery of gill slits in them. This feature is never found in arthropods, but is very characteristic of chordates. However, no notochord has been found in vetulicolids. They cannot be assigned to any of the modern phyla at all.

The renowned evolutionary biologist Professor Mykola Vorontsov once wrote that the discovery of a new animal phylum is equivalent in significance to the discovery of a new planet in the Solar System. This is precisely what happened in this case. Fig. 3. Vetulicolids from the Early Cambrian of China (from en.wikipedia.org). Top to bottom—representatives of the genera Didazoon, Pomatrum, and Xidazoon.

What are vetulicolids? They are marine animals that swam in the water column, not very large—most often up to 10 cm in size (20-cm vetulicolids are already considered giants). It is difficult to say what they resemble. The most important characteristics of vetulicolids are as follows:

1. Segmentation. Clearly expressed, externally similar to arthropod segmentation. The body is fully segmented, although the first few anterior segments are often covered by a single, rigid shield 6. Unlike chordates, segmentation affects the integument and is visible externally.

2. Gill slits. There are 5 pairs. For comparison, early Cambrian chordates recently discovered have 6 or 7 pairs of gill slits. It is possible that in vetulicolids, we are seeing a more ancient state of this characteristic.

3. At the bottom of the pharynx, an endostyle has been found—a groove along which cilia push mucus along with food particles. This organ is very characteristic of lower chordates.

4. The mouth is terminal, meaning it is located strictly at the anterior end of the body. In some genera, it is rounded, and the covering denticles are arranged concentrically around it, so that the mouth apparatus has radial symmetry. In this respect, vetulicolids, strangely enough, remotely resemble some annelids.

5. The posterior part of the body forms a tail fin, externally very similar to the tail of chordates. The difference is that in chordates, the tail is located behind the anus and does not contain a body cavity, whereas in vetulicolids, the intestine runs through the entire 'tail', with the anus at the very end. These body sections are analogous in function but have different internal structures.

6. Limbs on the trunk are completely absent. A typical vetulicolid could be considered a worm if not for the characteristic 'tadpole-like' body shape with a widened anterior part.

What kind of animals are these? By characteristics (2) and (3), they resemble chordates; by characteristic (1), they are more like arthropods. Characteristics (4), (5), and (6), whose significance is generally mysterious, complete the 'portrait' of the animal.

Vetulicolids are a purely Cambrian group. They are not known from any other time. This is likely one of the outpourings of the 'Cambrian explosion': a unique branch, worthy of the rank of a separate phylum, but small in number and quickly extinct.

Living room “Batrachos”. The final of three articles by Serhiy Oleksandrovych Yastrubov devoted to the origin of the chordate type. The first is here, the second – here. S. A. Yastrubov Type Vetulicolia: the trace of the “Great Explosion of Life” One of the biggest mysteries in animal evolution is the so‑called Cambrian explosion – the sudden appearance of most modern phyla at the boundary of the Cryogenian and the Paleozoic. Let us recall some facts. The history of life on Earth lasts about 4 billion years (the age of the oldest discovered bacterial remnants is 3.8 billion). This huge interval is divided into two unequal parts. The first part, lasting more than three billion years, is called the Cryogenian or pre‑Cambrian. “Cryogenian” literally means “era of hidden life”. The second part, covering the last 540 million years, is called the Phanerozoic – literally “era of visible life”. Eras such as the Paleozoic, Mesozoic and Cenozoic are subdivisions of the Phanerozoic. They, in turn, are divided into periods. For example, the Paleozoic era is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods. Thus the modern geochronological scale looks like this: [IMG_1] Fig. 1. Geochronological scale (from https://www.sciencelearn.org.nz, with modifications). Cryogenian and Phanerozoic are the highest units of the scale; they are called eons. Archaea, Bacteria and Proterozoic make up the Cryogenian. Paleozoic, Mesozoic and Cenozoic are the Phanerozoic eras, shown in different colours and divided into periods. Note how much shorter the Phanerozoic is than the Cryogenian: its much more detailed subdivision had to be placed separately so as not to disrupt the scale’s proportion. All modern animals can be placed into roughly 30 phyla. Information on their phylogenetic relationships is found in any introductory zoology textbook. But here a problem arises. Many modern phyla appear in the fossil record almost simultaneously – at the beginning of the Cambrian – already in a fairly well‑formed state. This concerns such disparate, complex groups as arthropods, molluscs, echinoderms and chordates. Roughly speaking, it remains unclear when the different phyla diverged from one another. In a very interesting book by a team of American authors led by Robert Barnes, “Invertebrates: A New Integrated Approach” (its Russian translation was published exactly 20 years ago), the following calculations are presented. From paleontological data it is known that the average time required for one species to turn into another, for various invertebrate groups, is on the order of 10 million years². Suppose that genera differ from each other on average ten times more than species do, families ten times more than genera, and so on up to classes and phyla. How much time would be needed for two newly separated species to become different phyla? Answer: at a constant evolutionary rate – 1 000 billion years. To gauge this figure, note that the Big Bang, according to the modern hypothesis, occurred less than 14 billion years ago³. The first multicellular animals most likely appeared about one billion years ago. If the model above were correct, their descendants today would differ from each other as, for example, different families of beetles. Yet we know that already 400 million years ago many modern phyla existed, differing from each other roughly as they do now. Thus the rate of evolution is not constant. We record this conclusion and move on. It is noteworthy that modern animal phyla differ enormously in size. Almost half of them are the so‑called “small phyla”, each comprising no more than 500 species. At the same time there are giant phyla with hundreds of thousands of species (in the arthropod phylum – over a million). The extreme unevenness of species distribution among phyla is an important feature of the “diversity structure” of modern animals, which would be worthwhile to explain. One such explanation – controversial but interesting – was proposed by Robert Barnes and colleagues. The point is that many “small phyla” indeed have Cambrian ages⁴. But how did they survive to the present with such a low species count? The fewer the species, the higher the extinction risk. Barnes and colleagues concluded that this is statistically probable only if many more such phyla existed initially; most of them later went extinct. We are dealing with remnants of former diversity, the “tip of the Cambrian iceberg”. The conclusion about the non‑constant evolutionary rate implies that sometimes evolution proceeded slowly, and sometimes it accelerated explosively. Barnes and colleagues suggested that at the end of the pre‑Cambrian – the beginning of the Cambrian – an “explosion” occurred in animal evolution, possibly producing several hundred equivalent phylum‑level branches. Most of them quickly went extinct, and the survivors formed the modern fauna. We know of no animal phylum that certainly originated after the Cambrian; perhaps none exist. But where are the mysterious phyla that appeared in the Cambrian and then vanished? Have any traces of them survived? Modern paleontology advances rapidly, so the curtain over the Cambrian explosion mystery is gradually being lifted. Here is just one example. In 1987, a new animal genus was discovered in early Cambrian deposits of China and named Vetulicola. It was a rather odd creature, segmented and with a light external shell, like an arthropod. Yet Vetulicola had no legs (which a normal arthropod would be expected to have), and its shell was bivalved: such a feature is rare in arthropods, occurring only in some crustaceans. By other characters Vetulicola bears little resemblance to crustaceans. It resembles nothing else. Nevertheless, it was placed in the arthropod phylum, assigned to a separate class with unclear affinities. [IMG_2] Fig. 2. Vetulicola (from https://dic.nicovideo.jp). The Japanese artist, author of the illustration, brilliantly conveyed the slightly alien appearance of this organism In the meantime several more genera close to Vetulicola were uncovered, and in 2001 several paleontologists (among them the renowned Cambrian specialists – Chinese Degen Shu and English Simon Conway Morris) proposed a completely different interpretation of these animals. They declared them a distinct phylum, not related to arthropods but to chordates⁵. This phylum was named Vetulicolia. The main argument for the independence of the vetulicolids was the discovery of gill slits. This feature never occurs in arthropods, but is very characteristic of chordates. Yet vetulicolids lack any notochord. They cannot be assigned to any modern phylum. Renowned evolutionary biologist Professor Mykola Vorontsov once wrote that the discovery of a new animal phylum is equivalent in significance to the discovery of a new planet in the Solar System. That is exactly what happened here. [IMG_3] Fig. 3. Vetulicolia from early Cambrian China (from en.wikipedia.org). Top to bottom – representatives of the genera Didazoon, Pomatrum and Xidazoon What are vetulicolia? They are marine animals that swam in the water column, not very large – most often up to 10 cm in length (20‑cm individuals are already considered gigantic). It is not easy to say what they resembled. The most important characteristics of vetulicolia are: 1. Segmentation. Well expressed, externally similar to arthropod segmentation. The body is completely segmented, although the few anterior segments are often covered dorsally by a single solid shield⁶. Unlike chordates, segmentation involves the outer covering and is visible externally. 2. Gill slits. They have five pairs. By comparison, recently discovered early Cambrian chordates have six or seven pairs of gill slits. Vetulicolia may represent a more primitive state of this character. 3. An endostyle is found on the floor of the pharynx – a groove through which cilia push mucus together with food particles. This organ is very characteristic of lower chordates. 4. The mouth is terminal, i.e., located strictly at the anterior end of the body. In some genera it is round, and the surrounding marginal teeth are arranged concentrically, giving the oral apparatus radial symmetry. By this trait vetulicolia, surprisingly, vaguely resemble some roundworms. 5. The posterior part of the body forms a tail fin, externally very reminiscent of a chordate tail. The difference is that in chordates the tail lies behind the anal opening and contains no body cavity, whereas in vetulicolia the entire “tail” houses the intestine, with the anal opening at the very end. These body sections are analogous in function but differ in internal structure. 6. Appendages on the trunk are completely absent. A typical vetulicolian could be considered a worm if not for its characteristic “tadpole‑like” body shape with an expanded anterior region. What kind of animals are these? By characters (2) and (3) they resemble chordates, by character (1) – more like arthropods. The “portrait” of the animal is completed by characters (4), (5) and (6), whose significance is altogether enigmatic. Vetulicolia are a purely Cambrian group. They are unknown from any other time period. Apparently they represent one of the bursts of the “Cambrian explosion”: a unique branch worthy of the rank of a separate phylum, but low‑diversity and quickly extinct. In 1902 American paleontologist Henry Osborn introduced the concept of “adaptive radiation”: a relatively rapid splitting of an evolving group into many branches that specialize in different ways. This is one of the main phenomena studied by evolutionary theory. The Cambrian adaptive radiation is among the most powerful in the entire history of life on Earth. Therefore vetulicolian researcher Degen Shu called it the “Great Explosion of Life”. Brief bibliography Barnes R., Kellow P., Olive P., Holding D. Invertebrates: A New Integrated Approach. Moscow, “Mir”, 1992. Shu D. On the Phylum Vetulicolia. // Chinese Science Bulletin 2005 Vol. 50 No. 20 2342—2354. — Online. Aldridge R., Hou X., Siveter D., Siveter D., Gabbott S. The systematics and phylogenetic relationships of Vetulicolia. // Palaeontology, Vol. 50, Part 1, 2007, pp. 131–168.

Living room “Batrachos”. The final of three articles by Serhiy Oleksandrovych Yastrubov devoted to the origin of the chordate type. The first is here, the second – here. S. A. Yastrubov Type Vetulicolia: the trace of the “Great Explosion of Life” One of the biggest mysteries in animal evolution is the so‑called Cambrian explosion – the sudden appearance of most modern phyla at the boundary of the Cryogenian and the Paleozoic. Let us recall some facts. The history of life on Earth lasts about 4 billion years (the age of the oldest discovered bacterial remnants is 3.8 billion). This huge interval is divided into two unequal parts. The first part, lasting more than three billion years, is called the Cryogenian or pre‑Cambrian. “Cryogenian” literally means “era of hidden life”. The second part, covering the last 540 million years, is called the Phanerozoic – literally “era of visible life”. Eras such as the Paleozoic, Mesozoic and Cenozoic are subdivisions of the Phanerozoic. They, in turn, are divided into periods. For example, the Paleozoic era is divided into the Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian periods. Thus the modern geochronological scale looks like this: [IMG_1] Fig. 1. Geochronological scale (from https://www.sciencelearn.org.nz, with modifications). Cryogenian and Phanerozoic are the highest units of the scale; they are called eons. Archaea, Bacteria and Proterozoic make up the Cryogenian. Paleozoic, Mesozoic and Cenozoic are the Phanerozoic eras, shown in different colours and divided into periods. Note how much shorter the Phanerozoic is than the Cryogenian: its much more detailed subdivision had to be placed separately so as not to disrupt the scale’s proportion. All modern animals can be placed into roughly 30 phyla. Information on their phylogenetic relationships is found in any introductory zoology textbook. But here a problem arises. Many modern phyla appear in the fossil record almost simultaneously – at the beginning of the Cambrian – already in a fairly well‑formed state. This concerns such disparate, complex groups as arthropods, molluscs, echinoderms and chordates. Roughly speaking, it remains unclear when the different phyla diverged from one another. In a very interesting book by a team of American authors led by Robert Barnes, “Invertebrates: A New Integrated Approach” (its Russian translation was published exactly 20 years ago), the following calculations are presented. From paleontological data it is known that the average time required for one species to turn into another, for various invertebrate groups, is on the order of 10 million years². Suppose that genera differ from each other on average ten times more than species do, families ten times more than genera, and so on up to classes and phyla. How much time would be needed for two newly separated species to become different phyla? Answer: at a constant evolutionary rate – 1 000 billion years. To gauge this figure, note that the Big Bang, according to the modern hypothesis, occurred less than 14 billion years ago³. The first multicellular animals most likely appeared about one billion years ago. If the model above were correct, their descendants today would differ from each other as, for example, different families of beetles. Yet we know that already 400 million years ago many modern phyla existed, differing from each other roughly as they do now. Thus the rate of evolution is not constant. We record this conclusion and move on. It is noteworthy that modern animal phyla differ enormously in size. Almost half of them are the so‑called “small phyla”, each comprising no more than 500 species. At the same time there are giant phyla with hundreds of thousands of species (in the arthropod phylum – over a million). The extreme unevenness of species distribution among phyla is an important feature of the “diversity structure” of modern animals, which would be worthwhile to explain. One such explanation – controversial but interesting – was proposed by Robert Barnes and colleagues. The point is that many “small phyla” indeed have Cambrian ages⁴. But how did they survive to the present with such a low species count? The fewer the species, the higher the extinction risk. Barnes and colleagues concluded that this is statistically probable only if many more such phyla existed initially; most of them later went extinct. We are dealing with remnants of former diversity, the “tip of the Cambrian iceberg”. The conclusion about the non‑constant evolutionary rate implies that sometimes evolution proceeded slowly, and sometimes it accelerated explosively. Barnes and colleagues suggested that at the end of the pre‑Cambrian – the beginning of the Cambrian – an “explosion” occurred in animal evolution, possibly producing several hundred equivalent phylum‑level branches. Most of them quickly went extinct, and the survivors formed the modern fauna. We know of no animal phylum that certainly originated after the Cambrian; perhaps none exist. But where are the mysterious phyla that appeared in the Cambrian and then vanished? Have any traces of them survived? Modern paleontology advances rapidly, so the curtain over the Cambrian explosion mystery is gradually being lifted. Here is just one example. In 1987, a new animal genus was discovered in early Cambrian deposits of China and named Vetulicola. It was a rather odd creature, segmented and with a light external shell, like an arthropod. Yet Vetulicola had no legs (which a normal arthropod would be expected to have), and its shell was bivalved: such a feature is rare in arthropods, occurring only in some crustaceans. By other characters Vetulicola bears little resemblance to crustaceans. It resembles nothing else. Nevertheless, it was placed in the arthropod phylum, assigned to a separate class with unclear affinities. [IMG_2] Fig. 2. Vetulicola (from https://dic.nicovideo.jp). The Japanese artist, author of the illustration, brilliantly conveyed the slightly alien appearance of this organism In the meantime several more genera close to Vetulicola were uncovered, and in 2001 several paleontologists (among them the renowned Cambrian specialists – Chinese Degen Shu and English Simon Conway Morris) proposed a completely different interpretation of these animals. They declared them a distinct phylum, not related to arthropods but to chordates⁵. This phylum was named Vetulicolia. The main argument for the independence of the vetulicolids was the discovery of gill slits. This feature never occurs in arthropods, but is very characteristic of chordates. Yet vetulicolids lack any notochord. They cannot be assigned to any modern phylum. Renowned evolutionary biologist Professor Mykola Vorontsov once wrote that the discovery of a new animal phylum is equivalent in significance to the discovery of a new planet in the Solar System. That is exactly what happened here. [IMG_3] Fig. 3. Vetulicolia from early Cambrian China (from en.wikipedia.org). Top to bottom – representatives of the genera Didazoon, Pomatrum and Xidazoon What are vetulicolia? They are marine animals that swam in the water column, not very large – most often up to 10 cm in length (20‑cm individuals are already considered gigantic). It is not easy to say what they resembled. The most important characteristics of vetulicolia are: 1. Segmentation. Well expressed, externally similar to arthropod segmentation. The body is completely segmented, although the few anterior segments are often covered dorsally by a single solid shield⁶. Unlike chordates, segmentation involves the outer covering and is visible externally. 2. Gill slits. They have five pairs. By comparison, recently discovered early Cambrian chordates have six or seven pairs of gill slits. Vetulicolia may represent a more primitive state of this character. 3. An endostyle is found on the floor of the pharynx – a groove through which cilia push mucus together with food particles. This organ is very characteristic of lower chordates. 4. The mouth is terminal, i.e., located strictly at the anterior end of the body. In some genera it is round, and the surrounding marginal teeth are arranged concentrically, giving the oral apparatus radial symmetry. By this trait vetulicolia, surprisingly, vaguely resemble some roundworms. 5. The posterior part of the body forms a tail fin, externally very reminiscent of a chordate tail. The difference is that in chordates the tail lies behind the anal opening and contains no body cavity, whereas in vetulicolia the entire “tail” houses the intestine, with the anal opening at the very end. These body sections are analogous in function but differ in internal structure. 6. Appendages on the trunk are completely absent. A typical vetulicolian could be considered a worm if not for its characteristic “tadpole‑like” body shape with an expanded anterior region. What kind of animals are these? By characters (2) and (3) they resemble chordates, by character (1) – more like arthropods. The “portrait” of the animal is completed by characters (4), (5) and (6), whose significance is altogether enigmatic. Vetulicolia are a purely Cambrian group. They are unknown from any other time period. Apparently they represent one of the bursts of the “Cambrian explosion”: a unique branch worthy of the rank of a separate phylum, but low‑diversity and quickly extinct. In 1902 American paleontologist Henry Osborn introduced the concept of “adaptive radiation”: a relatively rapid splitting of an evolving group into many branches that specialize in different ways. This is one of the main phenomena studied by evolutionary theory. The Cambrian adaptive radiation is among the most powerful in the entire history of life on Earth. Therefore vetulicolian researcher Degen Shu called it the “Great Explosion of Life”. Brief bibliography Barnes R., Kellow P., Olive P., Holding D. Invertebrates: A New Integrated Approach. Moscow, “Mir”, 1992. Shu D. On the Phylum Vetulicolia. // Chinese Science Bulletin 2005 Vol. 50 No. 20 2342—2354. — Online. Aldridge R., Hou X., Siveter D., Siveter D., Gabbott S. The systematics and phylogenetic relationships of Vetulicolia. // Palaeontology, Vol. 50, Part 1, 2007, pp. 131–168.

1 A brief account of the modern (as of 2008) version of the animal phylogenetic tree can be found here. To the article text. 2 For multicellular animals this estimate is slightly exaggerated, but for their predecessors – unicellular eukaryotes – it is even underestimated. According to American paleontologist Stephen Stanley, the average fossil species duration is a little over 2 million years for beetles, 5 million for ammonites, 10 million for marine gastropods, 11‑14 million for bivalves, over 20 million for foraminifera and 25 million for marine diatom algae. To the article text. 3 Source. To the article text. 4 List of modern “small phyla” known from the Cambrian: graptolites, brachiopods, hemichordates, priapulids, onychophorans, tardigrades, foraminifera, sipunculids. Remains of some other phyla may simply not have been preserved. For utmost precision one may note that Barnes’s statements made more than 20 years ago now require some corrections: for example, tardigrades now include over 500 species, and some systematists no longer consider sipunculids a separate phylum. Whether this will change the overall picture remains to be seen. To the article text. 5 British paleontologist Richard Jeffries a few years ago even suggested that vetulicolia are chordates, but his hypothesis received little support. To the article text. 6 What the solid external covering of vetulicolia was composed of is still unknown. Several possibilities have been proposed (bone‑like material, lime, tunicin, chitin …); researchers have not yet reached a consensus. To the article text.