The image is a reproduction of extinct mammals, the distant ancestors of whales. Whales: galloping to the sea

Evolution is not always a movement towards something new. There are also retreats. But this is not always degradation: everything depends on the quality of changes in the body during development. This principle also applies to whales returning from land back into the water. They not only did not regress, but also managed to adapt to the aquatic environment in such a way that they left far behind them all other types of living beings that descended into the water at the same time or after them.

Who is the closest relative of the hippopotamus? No, not an elephant or a rhinoceros. The correct answer is whale. Of course, at first glance this may seem strange. The whale lives in the water and looks like a fish: it has both fins and a tail... But the physiology of cetaceans clearly indicates that many millions of years ago their ancestors walked the earth: whales are warm-blooded, breathe with lungs, and feed the young carried in the womb milk, like all mammals. But what exactly does the hippopotamus have to do with it? To understand this, let us turn to very distant evolutionary events.

Water immigrants

The return of land living creatures back to water during the process of evolution has happened more than once. Scientists name three possible reasons for such transitions: unfavorable climatic conditions, difficulties in finding food and the dominance of predators. Often these factors acted simultaneously.

Whales are classified as secondary aquatic animals, that is, those that have returned to the ocean elements, but they occupy a special position among them. Apart from Mesozoic sea lizards, this is the only group of vertebrates that “completely forgot” about their once land-based way of life, unlike crocodiles, walruses or penguins - also secondary aquatic, but not having lost contact with the shore. Whales have fully adapted to the aquatic environment, and while their ancestors walked on four legs and were covered with hair, modern cetaceans have a constitution ideal for moving in the aquatic environment.

One of the reasons that allowed the ancestors of cetaceans to master the aquatic environment could be favorable external conditions. If living conditions on land deteriorated, then water could well become a suitable refuge. The history of cetaceans begins in the Eocene era, 55 million years ago. The warm bays of the ancient Tethys Ocean were abundant in food, and the niche occupied by large marine predators remained relatively empty. Although sharks and crocodiles were doing very well in those days, large predatory marine reptiles - plesiosaurs and mosasaurs - went extinct along with the dinosaurs. Nature gave the whales' ancestors a chance, and they took it. It is also possible that an important role in the progressive evolution of whales was played by their brain, which, for still unknown reasons, is better developed in cetaceans than in all other modern secondary aquatic animals.

Who are cetacean-toed ungulates?

So, as we understand, nothing connects whales with land except... that's right - family ties with the hippopotamus. This was established in 1985 by comparing proteins of the mammalian immune system by Vincent Sarich, a professor at the University of California at Berkeley. However, for a long time this fact could not be directly confirmed by paleontological material. Those bone remains that were at the disposal of scientists gave reason to assert only that the relatives of whales were mesonychians - very distant predatory predecessors of artiodactyls, similar to massive dogs and who lived in the Paleogene period (63-33 million years ago). This was evidenced by the teeth of fossil whales: like those of mesonychians, they had a special shape - three-vertex. To this were added some similar features in the structure of the skull. New paleontological data appeared only in the last quarter of the twentieth century. In those years, in Pakistan, on the site of the coastline of the ancient Tethys Ocean, which separated Eurasia and Africa in the Eocene (55-37 million years ago), the famous American paleontologist Philip Gingerich conducted excavations. In 1979, he came across a piece of the skull of an unknown, apparently land animal the size of a large dog that lived about 52 million years ago. However, the structure of the auditory system of the find strangely resembled that of a whale. We were talking about the so-called auditory bulla, or rather, its medial thickening - a massive bone formation found only in modern sea giants. Soon, teeth and a jaw were discovered, confirming the connection of its owner with cetaceans. The find was named pakicet, that is, “whale from Pakistan.” At first, Pakicetus was represented as an amphibious predator similar to a seal, a transitional link from mesonychians to fossil whales.

It was only in 2001 that scientists received the entire skeleton of this animal. He also came from Pakistan, but he was discovered not by Gingerich, but by Hans Thewissen, a professor of anatomy at Northeastern University Medical College in Ohio. It turned out that the pakicet's appearance resembled a large-headed, long-muzzled dog the size of a wolf, which nevertheless had hooves and a long tail. He led a semi-aquatic lifestyle, as evidenced by two facts: on the one hand, the remains of Pakicetus are found in coastal marine or river sediments, on the other hand, his hearing aid was not adapted to function in an aquatic environment. These whale ancestors likely moved to shallow waters rich in fish and various invertebrates when climate change reduced land-based food resources and increased competition between predators. The fact is that in the Eocene there was a cooling: the temperature dropped on average from +28 to +16 ° C, which led to a reduction in the area of ​​tropical forests and the appearance of vast open spaces in their place. According to scientists, this was followed by an increase in the diversity and number of canids, which led to increased competition between predators.

But the most interesting thing is that the pakicetus was artiodactyl! This is evidenced by its talus (supraheel) bone, which forms the lower part of the ankle joint and transfers the body weight to the foot. Like all artiodactyls (and only them), in Pakicetus it consists of two blocks, which ensures the flexibility of the foot when running. In the opinion of paleontologists, this serves as direct evidence that the immediate ancestor of the whale (as evidenced by its ear bulla) belonged to the same order. So the supposed relationship of Pakicetus with the hippopotamus, also an artiodactyl animal, was proven. It has now become obvious that cetaceans separated from the ancient artiodactyls after the latter diverged from the mesonychians, so some scientists even combine the artiodactyls and cetaceans into one order, the so-called cetaceans (Cetartiodactyla).

I change my paws for a tail

After the discovery of Pakicetus, another fossil creature “took its place” in the evolutionary chain, the remains of which Thewissen discovered in 1992 on the Pakistani shores of the Tethys, in geological layers about 48 million years old. Now it turns out that it is ideally suited to serve as a transitional link between modern whales and their land-based ancestors. The unknown animal, which had three-vertex teeth, an auricular bulla and an talus bone, was called ambulocetus - “the walking whale.” The appearance of the ambulocetus resembled a large-headed crocodile up to three meters long. Large oar-like feet, ending with some kind of hooves, indicate that the animal was a good swimmer. Moreover, when moving in water, his body moved in a vertical plane, just like modern whales, seals or sea cows, and not in a horizontal plane, like fish. In turn, strong leg bones, mobile elbow and wrist joints indicate that the ambulocetus continued to feel good on land.

Ambulocetes probably hunted, lying in wait for prey in shallow water. Their powerful jaws were capable of grabbing a fairly large prey, the size of an average deer, and thanks to the special structure of the nose, which was no longer located at the end of the muzzle, like in dogs, but higher, like in a crocodile, these predators had the opportunity to eat their lunch without leaving water. The eyes of the ambulocetus already provided only lateral vision, and the auricles were absent. But unlike Pakicetus, he heard well in the aquatic environment: in his jaw a channel appeared, characteristic of all late whales, that conducted sound to the ear. The ambulocetus tracked the movement of its prey on land by pressing its head to the ground and picking up vibrations from its steps. And based on the results of a chemical analysis of the teeth, scientists came to the conclusion that the predator could hunt in both salt and fresh water bodies.

The next stage in the evolution of cetaceans were the so-called protocetids, which include protocete, rhodocete, eocete and some other species that lived 47-45 million years ago. Their skeletons are not so “scarce” and have been known to science since the first half of the twentieth century. Protocetids were the first cetaceans to move from shallow to deep waters. Evolution has awarded them a horizontal tail fin, which will be inherited by all generations of whales until the present day. But if today whales use this fin as the main organ of their movement, scientists are in no hurry to unequivocally say the same about protocetids.

Protocetids are known to have retained quite prominent hind limbs. But whether they could go onto land is unknown. Most likely, their lifestyle can be compared with modern walruses. Makaracet is also a protocetide. Its remains were discovered in 2004 in East Balochistan (Pakistan). This animal got its name for its resemblance to Makara, a character from Indian mythology who was something like our capricorn, only with the head of an elephant. Yes, yes, makaratset had a trunk! True, not very big. It is possible that he used it to collect mollusks or other small bottom inhabitants.

Protocetids are also considered to be the first cetaceans that managed to spread beyond the Indo-Pakistan region - their remains have been found in Africa and North America. Some scientists believe that all later cetaceans directly descended from protocetids. At least the protocetids set a general pattern for the further evolution of whales: a reduction in the number of sacral vertebrae rigidly attached to each other (due to this, the wave-like movements of the body were simplified), the disappearance of the already unnecessary sacropelvic joint, to which the hind limbs were attached, a reduction in the length of the cervical vertebrae, improving hydrodynamics, and moving the nostrils on the muzzle higher and higher.

The protocetids were inherited by the basilosaurs, which appeared on Earth 45 million years ago. Their fossils come primarily from the southern United States and Egypt, but they most likely had a worldwide distribution. Basilosaurs were giants: their serpentine body with a large tail fin reached 25 meters in length and weighed up to 6 tons. Like other ancient whales, they had conical premolars and serrated molars. The first skeleton of this sea predator was found back in 1840. The find was made in Louisiana, in the southern United States. But its first description was erroneous: Basilosaurus was mistaken for a huge sea lizard (hence its name - “king lizard”). Other species were later found in Egypt and Pakistan. Gingerich, already known to us, was most fortunate. He came across the most complete skeleton of a basilosaurus known to science, and even 18 meters long! This happened in 2005 during excavations in the so-called Valley of the Whale, off the southwestern outskirts of Cairo.

The structure of the spine of Basilosaurus suggests that when swimming, it could already significantly bend its body in a vertical plane (imagine how a whale or dolphin swims). However, it is unknown: was Basilosaurus capable of long swimming and deep diving? He did not go to land and, most likely, hunted large fish near the coast.

Basilosaurs still had hind limbs with several fingers and a movable knee joint, however, they were very small and not suitable for movement. Perhaps the males used them for mating embraces.

Evolution record holders

It must be said that the evolution of cetaceans proceeded at a fairly high pace: already 40 million years ago their modern suborders appeared: toothed and baleen whales. It is possible that climate change contributed to this: the level of the World Ocean dropped, new cold currents emerged, and an ice shell began to form in the Southern Hemisphere. At this time, whales began to explore the open ocean, learn to dive deep and stay under water for a long time.

Of course, the presented evolutionary scheme for the development of cetaceans is far from complete, just as paleontology as a whole as a science is far from it, and this is its peculiarity. There is still more discovery to be made that can bring us the next millimeter closer to the objective truth. But, probably, the general vector of whale development has already been determined and will not change. This is confirmed by a discovery made in 2006 by scientists from Northeastern University College of Medicine in Ohio. They were able to establish that dolphins have a gene responsible for the appearance of hind limbs in embryos in the first two months of gestation. Then the “countergene” is activated and unnecessary “paws” disappear. Such an argument should convince skeptics who do not want to believe in the wild twists of evolution.

Illustrations by Eldar Zakirov

It was also discovered by Thewissen that a similar ear arrangement was observed in the fossils of the small deer-like animal Indochyus. Indohyus lived about 48 million years ago in Kashmir. This small herbivore, the size of a domestic cat, had some features that brought it closer to whales and indicated adaptation to the aquatic environment. These include a thick and heavy bony shell, reminiscent of the bony shell of some modern semi-aquatic animals such as hippopotamuses, which helps reduce buoyancy and, as a result, allows you to remain underwater. This suggests that Indochyus, like the modern water fawn, dived underwater to hide from a predator.

Ambulocetides and remingtonocetids

The most notable of the ancient whales is Ambulocetus, known from the Eocene of Pakistan. Outwardly, this mammal resembled a three-meter crocodile. Ambulocetus was a semi-aquatic animal: its hind legs were better suited for swimming than for walking on land. It probably swam by bending its body in a vertical plane, like modern otters, seals and whales. It is assumed that ambulocetids hunted like modern crocodiles, lying in wait for fish and animals that came to drink.

Basilosaurids and dorudontids: fully marine cetaceans

Basilosaurus (discovered in 1840 and originally thought to be a reptile, hence the "reptilian" name) and Dorudon lived approximately 38 million years ago and were purely marine animals. Basilosaurus was as large as large modern whales, sometimes reaching 18 meters in length. Dorudontids were somewhat smaller, up to 5 meters.

Despite all the similarities with modern whales, basilosaurids and dorudontids lacked the frontal fatty protrusion, the so-called melon, which allows existing cetaceans to effectively use echolocation. The brains of basilosaurids were relatively small, suggesting that they were solitary and did not have such a complex social structure as some modern cetaceans. In connection with the transition to a purely aquatic lifestyle, basilosaurids experience degradation of the hind limbs - although they are well formed, they are small and can no longer be used for movement. However, perhaps they played an auxiliary role during mating. The pelvic bones of basilosaurids are no longer connected to the spine, as was the case in protocetids.

The emergence of echolocation

Toothed whales (Odontocetes) echolocate by producing a series of clicks at different frequencies. Sound pulses are emitted through the frontal fat pad (“frontal melon”), reflected from the object and recorded using the lower jaw. The study of Squalodon skulls suggests the primary occurrence of echolocation in this species. Squalodon lived from the early Middle Oligocene to the mid Miocene, about 33–14 million years ago, and had a number of features similar to modern toothed whales. For example, a strongly flattened skull and prominent jaw arches are most characteristic of modern Odontoceti. Despite this, the possibility of the origin of modern dolphins from Squalodon is considered unlikely.

Early dolphins

See also "Dolphins - development and anatomy (English)"

Skeletal development

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Notes

1. University of California, Berkeley (2005, February 7).. ScienceDaily. Retrieved February 1, 2010. .
2. Philip D. Gingerich, D. E. Russell (1981). " Pakicetus inachus, a new archaeocete (Mammalia, Cetacea) from the early-middle Eocene Kuldana Formation of Kohat (Pakistan)". Univ. Mich. Contr. Mus. Paleont 25 : 235–246.
3. Northeastern Ohio Universities Colleges of Medicine and Pharmacy (2007, December 21).. ScienceDaily. Retrieved December 21, 2007. .
4. J. G. M. Thewissen, E. M. Williams, L. J. Roe and S. T. Hussain (2001). "Skeletons of terrestrial cetaceans and the relationship of whales to artiodactyls". Nature 413 : 277–281. DOI:.
6. University Of Michigan (2001, September 20).. ScienceDaily. Retrieved December 21, 2007. .
7. Carl Zimmer.. ScienceBlogs (December 19, 2007). Retrieved December 21, 2007. .
8. Ian Sample.. Guardian Unlimited (December 19, 2007). Retrieved December 21, 2007. .
9. PZ Myers. . Pharyngula. ScienceBlogs (December 19, 2007). Retrieved December 21, 2007. .
10. Gingerich PD, ul-Haq M, von Koenigswald W, Sanders WJ, Smith BH, et al.. PLoS one. Retrieved February 4, 2009. .

Links

• Adapted from National Geographic, November 2001, Revised 2006 Dr. J.G.M. Thewissen

View this template Evolutionary processes Factors of evolution Population genetics Origin of life Historical concepts Modern theories Evolution of taxa

An excerpt characterizing the evolution of cetaceans

We came close... And I felt a wonderfully soft touch of a sparkling wave... It was something incredibly tender, surprisingly affectionate and calming, and at the same time, penetrating into the very “depths” of my surprised and slightly wary soul... Quiet “music” ran along my foot, vibrating in millions of different shades, and, rising upward, began to envelop me with something fabulously beautiful, something beyond any words... I felt that I was flying, although there was no flight it didn’t happen in reality. It was wonderful!.. Every cell dissolved and melted in the oncoming new wave, and the sparkling gold washed me through, taking away everything bad and sad and leaving only pure, pristine light in my soul...
I didn’t even feel how I entered and plunged into this sparkling miracle almost headlong. It was just incredibly good and I never wanted to leave there...
- Well, that’s enough already! A task awaits us! – Stella’s assertive voice burst into the shining beauty. - Did you like it?
- Oh, yes! – I exhaled. – I didn’t want to go out so much!..
- Exactly! So some “bathe” until their next incarnation... And then they never return here again...
-Where are they going? – I was surprised.
- Below... Grandma says that you also have to earn a place here... And whoever just waits and rests will “work off” in the next incarnation. I think it's true...
– What’s there below? – I asked interestedly.
“It’s not so nice there anymore, believe me.” – Stella smiled slyly.
- And this sea, is there only one or are there many of them here?
– You’ll see... It’s all different – ​​where there’s the sea, where there’s just a “view”, and where there’s just an energy field full of different flowers, streams and plants, and all this also “heals” souls and calms... just not like that- then just use it - you must first earn it.
– Who doesn’t deserve it? Don’t they live here? I didn’t understand.
“They live, but they don’t live so beautifully anymore...” the little girl shook her head. – Here it’s the same as on Earth – nothing is given for free, but the values ​​here are completely different. And whoever doesn’t want it, gets everything much simpler. All this beauty cannot be bought, it can only be earned...
“You speak now just like your grandmother, as if you had learned her words...” I smiled.
- The way it is! – Stella returned the smile. – I try to remember a lot of what she says. Even things that I don’t quite understand yet... But I’ll understand someday, right? And then, perhaps, there will be no one to teach... So that will help.
Here, we suddenly saw a very incomprehensible, but very attractive picture - on a shining, fluffy-transparent blue earth, like on a cloud, there was a cluster of entities that constantly replaced each other and took someone somewhere, and then returned again.
- And what's that? What are they doing there? – I asked, puzzled.
– Oh, they’re just helping the “newcomers” come, so they won’t be scared. This is where new entities come in. – Stella said calmly.
– Have you already seen all this? Can we have a look?
- Well, of course! – and we came closer...
And I saw an action absolutely breathtaking in its beauty... In complete emptiness, as if out of nothing, a transparent luminous ball suddenly appeared and, like a flower, immediately opened up, releasing a new entity, which looked around completely confused, still not seeing anything. understanding... And then, the waiting entities hugged the “newcomer” with a clot of warm sparkling energy, as if calming him down, and immediately took him somewhere.
“Do they come after death?..” I asked very quietly for some reason.
Stella nodded and answered sadly:
– When I arrived, we went to different “floors”, my family and I. It was very lonely and sad... But now everything is fine. I visited them here many times - they are happy now.
“They are right here, on this “floor”?.. – I couldn’t believe it.
Stella nodded her head sadly again, and I decided I wouldn’t ask anymore, so as not to disturb her bright, kind soul.
We walked along an unusual road that appeared and disappeared as we stepped on it. The road shimmered softly and seemed to lead, showing the way, as if knowing where we needed to go... There was a pleasant feeling of freedom and lightness, as if the whole world around had suddenly become completely weightless.
– Why does this road tell us where to go? – I couldn’t stand it.
– She doesn’t point, she helps. - The little girl answered. – Everything here consists of thoughts, have you forgotten? Even trees, the sea, roads, flowers - everyone hears what we are thinking about. This is a truly pure world... probably what people are used to calling Paradise... You cannot deceive here.
– Where is Hell then?.. Does it exist too?
– Oh, I’ll definitely show you! This is the bottom “floor” and there IS SUCH!!!... – Stella shrugged her shoulders, apparently remembering something not very pleasant.
We still walked further, and then I noticed that the surroundings began to change a little. Transparency began to disappear somewhere, giving way to a much more “dense” landscape, similar to the earth’s.
- What's going on, where are we? – I was wary.
- Everything is there. “The little girl answered completely calmly. - Only now we are already in the part that is simpler. Remember we just talked about this? The majority here are those who have just arrived. When they see a landscape that is similar to their usual one, it is easier for them to perceive their “transition” into this new world for them... Well, also, here live those who do not want to be better than they are, and do not willing to make the slightest effort to achieve something higher.
“So this “floor” consists of two parts?” I clarified.
– You can say so. - The girl answered thoughtfully, and suddenly switched to another topic - Somehow no one here pays any attention to us. Do you think they're not here?
After looking around, we stopped, not having the slightest idea what to do next.
– Shall we risk “lower”? – Stella asked.
I felt that the baby was tired. And I was also very far from my best form. But I was almost sure that she was not going to give up, so she nodded in response.
“Well, then we need to prepare a little...” the militant Stella said, biting her lip and seriously concentrating. – Do you know how to build strong protection for yourself?
- Seems Yes. But I don't know how strong it will be. – I answered embarrassedly. I really didn’t want to let her down right now.
“Show me,” the girl asked.
I realized that this was not a whim, and that she was just trying to help me. Then I tried to concentrate and made my green “cocoon”, which I always made for myself when I needed serious protection.
“Wow!..” Stella opened her eyes in surprise. - Well, then let's go.
This time our flight down was not nearly as pleasant as the previous one... For some reason, my chest felt very tight and it was hard to breathe. But little by little it all seemed to level out, and I stared in surprise at the eerie landscape that opened up to us...
The heavy, blood-red sun sparingly illuminated the dull, violet-brown silhouettes of distant mountains... Deep cracks crawled along the ground, like giant snakes, from which a dense, dark orange fog burst out and, merging with the surface, became like a bloody shroud . Strange, seemingly restless, essences of people wandered everywhere, looking very dense, almost physical... They appeared and disappeared, not paying any attention to each other, as if they saw no one but themselves and lived only in their own, closed from the rest of the world. In the distance, not yet approaching, sometimes dark figures of some monstrous animals appeared. I felt danger, it smelled terrible, I wanted to run away from here headlong, without turning back...
– Are we right in Hell or what? – I asked, horrified by what I saw.
“But you wanted to see what it looked like, so you looked.” – Stella answered, smiling tensely.
It was felt that she was expecting some kind of trouble. And, in my opinion, there was simply no way there could be anything else but trouble here...
“And you know, sometimes there are good beings here who just made big mistakes.” And to be honest, I feel very sorry for them... Can you imagine waiting here for your next incarnation?! Horrible!
No, I couldn’t imagine this, and I didn’t want to. And there was no smell of this same goodness here.
- But you’re wrong! – the little girl overheard my thoughts again. “Sometimes, it’s true, very good people end up here, and they pay very dearly for their mistakes... I really feel sorry for them...
– Do you really think that our missing boy ended up here too?! He certainly didn’t have time to do anything that bad. Do you hope to find him here?.. Do you think this is possible?
– Be careful!!! – Stella suddenly screamed wildly.
I was flattened on the ground like a big frog, and I just had time to feel as if a huge, terribly stinking thing was falling on me. mountain... Something was puffing, slurping and snorting, emitting a disgusting smell of rot and rotten meat. My stomach almost turned out - it’s good that we “walked” here only as entities, without physical bodies. Otherwise, I would probably get into the most unpleasant troubles.....
- Get out! Well, get out!!! - the frightened girl squealed.
But, unfortunately, this was easier said than done... The fetid carcass fell on me with all the terrible weight of its huge body and was already, apparently, ready to feast on my fresh vitality... But, as luck would have it, I couldn’t I couldn’t free myself from it, and panic was already starting to squeak treacherously in my soul, compressed by fear...
- Come on! – Stella shouted again. Then she suddenly hit the monster with some bright ray and screamed again: “Run!!!”
I felt that it became a little easier, and with all my might I energetically pushed the carcass hanging over me. Stella ran around and fearlessly hit the already weakening horror from all sides. I somehow got out, gasping for air out of habit, and was truly horrified by what I saw!.. Right in front of me lay a huge spiky carcass, all covered with some kind of sharply stinking mucus, with a huge, curved horn on a wide, warty head .
- Let's run! – Stella screamed again. – He’s still alive!..
It was as if the wind had blown me away... I didn’t remember at all where I was blown... But, I must say, it was carried very quickly.
“Well, you’re running...,” the little girl squeezed out, out of breath, barely pronouncing the words.
- Oh, please forgive me! – I exclaimed, ashamed. “You screamed so much that I ran away in fright, wherever my eyes were looking...
- Well, it’s okay, next time we’ll be more careful. – Stella calmed down.
This statement made my eyes pop out of my head!..
– Will there be a “next” time??? “I asked cautiously, hoping for a “no.”
- Well, of course! They live here! – the brave girl “reassured” me in a friendly manner.
– What are we doing here then?..
- We are saving someone, have you forgotten? – Stella was sincerely surprised.
And apparently, from all this horror, our “rescue expedition” completely slipped my mind. But I immediately tried to pull myself together as quickly as possible, so as not to show Stella that I was really, really scared.
“Don’t think so, after the first time my braids stood on end all day!” – the little girl said more cheerfully.
I just wanted to kiss her! Somehow, seeing that I was ashamed of my weakness, she managed to make me feel good again immediately.
“Do you really think that little Leah’s dad and brother could be here?..,” I asked her again, surprised from the bottom of my heart.
- Certainly! They could simply have been stolen. – Stella answered quite calmly.
- How to steal? And who?..
But the little girl didn’t have time to answer... Something worse than our first “acquaintance” jumped out from behind the dense trees. It was something incredibly nimble and strong, with a small but very powerful body, every second throwing out a strange sticky “net” from its hairy belly. We didn’t even have time to utter a word when we both fell into it... Frightened, Stella began to look like a small disheveled owlet - her big blue eyes looked like two huge saucers, with splashes of horror in the middle.
I had to urgently come up with something, but for some reason my head was completely empty, no matter how hard I tried to find something sensible there... And the “spider” (we will continue to call it that, for lack of a better one) in the meantime was quite apparently dragged us into his nest, preparing to “supper”...
-Where are the people? – I asked, almost out of breath.
- Oh, you saw - there are a lot of people here. More than anywhere... But they, for the most part, are worse than these animals... And they will not help us.

70 million years before the appearance of the greedy naked ape, whales began to populate the oceans. Cetaceans (lat. Cetacea) are a group of mammals fully adapted to life in water. In everyday life, all cetaceans are called whales, except dolphins and porpoises. Cetaceans are the largest known animals that have ever lived on Earth. The word cetus (Greek) means sea monster. The Russian word kit comes from it. Under the skin there is a layer of adipose tissue (blubum) with a thickness of 2.5 to 30 cm. This subcutaneous fat protects the body from hypothermia, helps retain water in the body that would otherwise diffuse into the environment, and also serves as an energy reserve during hunger strikes. Due to good insulation by the fat layer, body temperature ranges from 35 to 40 °C. In terms of immune system proteins, whales are closest to artiodactyls (deer, bulls, giraffes and hippos). Together with artiodactyls, cetaceans are sometimes classified as members of the superorder cetaceans. The key feature of the artiodactyl order is the talus of a characteristic shape with a notch-pulley at both ends. The key to solving the problem was the discovery of the so-called talus, which belonged to Rhodocetus - one of the ancient (still on land) ancestors of whales - connecting the bones of the knee with the bones of the ankle.

Phylogeny of whales

Skeletons of modern whales and their ancestors

Astragalus of mesonychids, modern artiodactyl and Rhodocetus

Oceans and continents 50 million years ago

Ancient whale Basilosaurus

Dorudon

It was assumed that whales descended from a group of land mammals called mesonychids, similar to wolves, but with hooves like cows and deer. They lived approximately 60 million years ago around the ancient Tethys Sea, in what is now the Mediterranean Sea and part of the Asian subcontinent. Mesonychids probably hunted fish and other aquatic animals in coastal swamps and estuaries. Comparison of the composition of various sections of the DNA of modern mammals - representatives of different orders - allowed us to talk about the relationship between whales and artiodactyls, possibly from the hippopotamus family. After the departure of dinosaurs 65 million years ago, the niches of large plant- and carnivorous tetrapods were freed up for mammals. It so happened that the ancestors of modern ungulates and proboscideans, the condylarthra, took shape first. At first they were not pure vegetarians; they even managed to be the first, before the appearance of the ancestors of true carnivores, to produce large scavengers and hunters. These ungulates, or rather artiodactyls, predators were mesonychids. They gave birth to the largest predatory beast that ever lived on Earth - Andrewsarchus. Unlike some of its smaller relatives, Andrewsar had more blunt teeth and was therefore more omnivorous - like bears. It weighed over one and a half tons, exceeding modern bears by one and a half to two times. The head of Andrewsarchus was “crocodile” - disproportionately elongated (up to 90 cm long and up to 60 cm wide at the back). The toothy mouth itself was relatively narrow.

It is unknown which artiodactyls gave rise to cetaceans. The origin of artiodactyls also remains unclear. Hippos, as an independent group, arose much later than the ancient cetaceans. Among their ancestors, as well as the ancestors of other artiodactyls, no known fossil species is better suited to the role of the ancestor of whales than mesonychids. Perhaps mesonychids are an independent branch of the artiodactyl order, which for some reason lost the characteristic shape of the talus? New paleontological finds are needed to understand the appearance of the very first whales.

Some paleontologists are inclined to believe that ancient cetaceans were related to the most ancient insectivores, and probably arose in the Late Cretaceous even before the ungulate and carnivorous orders branched off from them. 70 million years ago, the land-based ancestors of cetaceans moved into the water.
Perhaps Pakicetus was one of the first evolutionary steps along the transition from land animals to cetaceans. He was equally at home on land and in water and had a normal, functional eardrum that allowed him to perceive sounds traveling through the air. Being an ambush predator waiting for prey in the water, he could hear the footsteps of possible prey approaching the watering hole. Sound travels differently in water and air. Modern whales do not have an external ear, and the auditory canal leading to the middle ear is either extremely narrowed or absent altogether. The eardrum is thickened, immobile and does not perform the functions that are characteristic of land animals. In whales, they are taken over by the so-called auditory bulla - a special bone formation isolated by the sinuses.
Basilosaurs and dorudonts - the first completely aquatic whales - had an auditory system quite close to the auditory system of modern whales. Dolphin-like Dorudon are the possible ancestors of basilosaurs and modern whales, which gradually settled throughout all the seas of the globe. In the late Eocene, modern whales evolved from Basilosaurus.
The oldest family of baleen whales (Cetoteriidae), which included at least 20 species, lived in the Oligocene. Three modern families branched off from it - gray whales (Eschrichtiidae), right whales (Balaenidae) and minke whales (Balaenopteridae). Of the toothed whales, the most ancient group is the squalodontidae. From the squalodonts in the Oligocene and Miocene, 4 living families separated: sperm whales, beaked whales, river dolphins and sea dolphins with 3 subfamilies (dolphins, beluga whales, porpoises).
The ancestors of whales split into two lineages - baleen and toothed whales - about 30 million years ago. They form two suborders of modern cetaceans. The evolution of the two groups - toothed and baleen whales - followed different paths. The former developed the features necessary to produce and perceive high-frequency sounds used in echolocation, while the latter “specialized” in low-frequency acoustic signals that made it possible to communicate with each other over long distances. Around 28 million years ago, early toothed whales already had some of the bone structures necessary to perceive high-frequency sounds and, accordingly, a certain ability to echolocation.
Having discovered significant differences between suborders, some theorists considered it possible to assert that baleen and toothed whales originate from completely different ancestors and the external similarity between them should be explained by convergence. At the same time, other researchers, based on numerous signs of similarity in the structure of representatives of these groups, defend the view of the monophylithic origin (and therefore of the parallel, rather than convergent nature of development) of baleen and toothed whales.

During the Eocene, the ancestors of whales gradually adapted to life at sea, filling the ecological niche that was vacated after the extinction of mosasaurs and plesiosaurs. Over millions of years of evolution, cetaceans have adapted well to life in the seas and oceans. They have acquired a streamlined torpedo-shaped body, which allows them to overcome water resistance, which is 800 times greater than in air. They lost their skin hair, ears and hind limbs, which turned into a caudal fin.
The forelimbs have turned into pectoral, flattened hard fins - the elevators, turns, braking and organs that ensure the movement of the body backwards, the rear ones have atrophied. although some species show rudiments of pelvic bones. The tail ends in a large horizontal fin. They can swim at a speed of 50 km/hour. The pectoral, dorsal and especially caudal fins of cetaceans have variable elasticity, which changes depending on the speed of swimming and is regulated by specific blood vessels.
A thick subcutaneous layer of fat protects the body from hypothermia and is used as an energy reserve during seasonal hunger strikes of cetaceans, so it fluctuates greatly according to the seasons.
The skeleton of whales is spongy, soaked in fat. The spine has from 41 to 98 vertebrae, forming four sections: cervical (very short, but always consisting of seven free or fused vertebrae), thoracic, lumbar and caudal. The thoracic region of cetaceans bears 10-17 pairs of ribs, of which only the first 2-8 pairs articulate with the sternum.
The maxillary, premaxillary and mandibular bones are elongated due to the development of the fining apparatus (whalebone) or numerous single-vertex teeth. The nasal bones are reduced, the parietal bones are shifted to the sides so that the superoccipital bone is in contact with the frontal bones. The blowhole - one or two external nasal openings - is located at the top of the head and opens only at the moment of a short respiratory act of exhalation - an inhalation performed immediately after surfacing. In cool weather, when exhaling, condensed steam flies up, forming a so-called fountain, by which the type of whale can be distinguished.
Here are the most important changes that ultimately allowed whales to spread throughout the world's oceans:
1. A highly developed brain and a powerful cerebral cortex, which has become a subtle tool for adaptive activity and complex behavioral reactions in a new environment. The brain mass of cetaceans is the largest among mammals in absolute numbers, reaching 7.8-9.2 kg in the sperm whale, but relative to body weight it is small. Thus, in the blue whale it makes up only 0.007% of the total mass. The brain is highly differentiated, the sensory organs are developed, although not equally. The hearing organs are greatly modified. The auricle is missing. The auditory canal is narrowed and opens behind the eye with a small opening; Apparently, it serves as a separate sensory organ that perceives changes in pressure. The eardrum is curved outward (baleen whales) or inward (toothed whales). Cetaceans perceive sound through the bones of the skull and lower jaw, the posterior end of which is close to the area of ​​the inner ear and innervated by a branch of the trigeminal nerve. The structure of the inner ear in cetaceans is very complex, with an enlarged cochlea. Sound for aquatic organisms is the most important source of information, since sound vibrations travel 5 times faster in water than in air. Cetaceans are able to detect sound waves in the range from 150 Hz to ultrasonic vibrations at 120-140 kHz. The hearing of toothed whales is most acute; in baleen whales it is worse compared to land mammals.

2. Perfect echolocation as the main method of spatial orientation in the water column. Some toothed whales, like some bats, are capable of directional echolocation. They have developed a special echolocation apparatus, consisting of a fat pad and a concave front surface of the skull, which act as a sound lens and reflector, concentrating the emitted ultrasonic signals and directing them to an object in the form of a sound beam.

3. A complex of morphophysiological adaptations that ensured the creation of oxygen reserves necessary for long-term and deep diving. Cetaceans are able to stay under water from 2-10 to 30-40 minutes (sperm whale - up to 1.5 hours). The duration of the dive is ensured by a large lung capacity and an increased content of myoglobin in the muscles. The oxygen capacity of the blood is increased due to the high content of hemoglobin and an increase in its concentration in red blood cells.
4. Complex of transformations in the respiratory organs. The nostrils are paired, slit-like, and open at the crown. Water does not enter the respiratory tract either when swallowing food or breathing. The lungs are monolobed with highly developed smooth muscles, which allow one exhalation-inhalation to renew air by 80-90% (in humans only 15%). When ascending to the surface, the forcefully exhaled air of the whale, in contact with the colder outside air, forms a column of condensed steam (fountain).

5. Progressive development of lactation and increasing the energy value of milk. It is thick, usually cream-colored, with a fat content of up to 54%, with a surface tension 30 times greater than that of water, so the stream of milk does not spread in the water. Female whales produce from 200-1200 g (dolphins) to 90-150 l (fin whale) and 200 l (blue whale) of milk per day.

6. Restructuring of the nutritional organs. Cetaceans swallow prey whole without chewing. The stomach is complex, multi-chambered; capable of stretching and holding up to 1.5 tons of food (in the blue whale). The length of the intestine exceeds the length of the body: from 4-5 times (in the Gangetic dolphin and bottlenose dolphins) to 15-16 times (sperm whale) and even up to 32 times (La Plata dolphin).
7. Transformation in the organs of movement.

How did whales come to the ocean?

"The monster, lying motionless in the water among the mangroves, noticed its prey - an animal of suitable size that had come to drink. With several energetic pushes of its hind legs, it approached the shore, plunged its powerful teeth into the body of the victim and retreated back into the water. When tightly gripped in the jaws of the animal, unable to breathe, stopped beating, the predator crawled ashore to begin a meal on solid ground. At first glance, the monster looked like a crocodile - with short legs, a massive tail, a long elongated muzzle and high-set, protruding heads eyes. However, his body was covered not by plates of armor, but by fur, his legs ended not with claws, but with something resembling hooves, and his teeth were the teeth of an animal, not a reptile..."

This is what paleontologists think of as Ambulocetus, one of the first whales. Or rather, a creature that is a “link in the evolutionary chain”, connecting land mammals of the beginning of the Cenozoic era and modern cetaceans: blue, gray and humpback whales, sperm whales, narwhals, killer whales and dolphins - in total about 40 genera and 80 species of animals roaming the expanses of the World ocean.

The evolutionary history of whales, as one of the most specialized orders of mammals, remained a mystery for biologists even in the 20th century. When, in 1945, J. Simpson compiled a system of evolutionary relationships between mammals, based on the paleontological data available at that time, he noted that there was very little data about cetaceans and placed this order independently of the others, connecting its history only with a common hypothetical ancestor placental.

After two decades, paleontologists found a large number of remains of ancient whales and their ancestors, which belonged to the Eocene period (55-34 million years ago). This is the time when ancient whales (Archaeoceti) just began to “enter” the water. Cetacean bones were found in later Oligocene (34–24 million years ago) layers. Then the formation of two modern suborders of this group took place - toothed and baleen whales. A reassessment of the family relationships between living orders of mammals, based on the similarity of immune system proteins, showed that whales are closest to artiodactyls (deer and bulls, giraffes and hippos).

At the paleontological level, such a connection could presumably be confirmed based on the study of the remains of various representatives of the ancient (and completely extinct) order of Condylarthra - the supposed common ancestors of ungulates, proboscis and cetaceans. A clear similarity was discovered between the three-peaked teeth of newly discovered fossil whales and a group of predatory condylarthra - Mesonychia. Similarities were also discovered between the dental characteristics of artiodactyls and another group of condylarthra, Arctocyonids, very close to mesonychians. The researchers who carried out this work came to the conclusion that cetaceans descended from carnivores that resembled mesonychian wolves in their appearance, and they, in turn, had a common ancestor with artiodactyls (diagram A)

Ten years later, paleontologists, during the expedition of the American paleontologist Philip Gingerish (P. Gingerish) in Pakistan, received additional material. They looked for remains of Eocene land mammals in places where they were already found, but they only came across marine organisms. About 50 million years ago, this area contained the changing coastline of the ancient Tethys Sea, which separated Eurasia and Africa for most of the Eocene period. Among the remains of fish and shellfish, paleontologists found two fragments of pelvic bones that clearly belonged to relatively large “walking” animals. At the same time, in another part of Pakistan, the jaw of a primitive artiodactyl was discovered.

Two years later, another strange find was discovered by the Gingerish expedition in Northern Pakistan. It was a piece of the skull of a strange creature the size of a wolf. Nearby, remains of other mammals were discovered, this time terrestrial, living about 50 million years ago. However, the skull of the unknown animal found had features that resembled some details of the structure of the auditory system of modern cetaceans.

In water and air, sound waves travel differently. Whales that live today do not have an external ear, and the auditory canal leading to the middle ear is either extremely narrowed or absent altogether. The eardrum is thickened, immobile and does not perform the functions that are characteristic of land animals. In whales, they are taken over by the so-called auditory bulla - a special bone formation isolated by the sinuses. The bulla in the skull of an unknown animal discovered by Gingerish, although it was not truly “whale-like” and clearly could not provide good underwater hearing, was nevertheless distinguished by quite characteristic changes. It turned out that this creature - it was named Pakicetus after the place where it was found - could be one of the first evolutionary steps along the path of transition from land animals to cetaceans. At the same time, it could be assumed that the mysterious beast also had a normal functional eardrum, allowing it to perceive sounds traveling through the air - it so far spent no less time on land than in water.

Around the same time, another group of paleontologists also discovered the remains of this animal, namely a fragment of the lower jaw and several individual teeth. Their structure fully confirmed the connection between pakicetus and mesonychids, whose remains, by the way, were known from approximately the same area and the same time layers. So everything seemed to be falling into place: mesonychids or their close relatives really “could lay claim” to the role of the ancestors of the first cetaceans.

In the Eocene, the sea was very favorable for the settlement of mammals. The warm bays of Tethys were teeming with fish, promising a satisfying life, and the niche of large sea predators turned out to be relatively free. Although sharks and crocodiles continued to do very well, numerous aquatic reptiles in the Mesozoic - plesiosaurs, mosasaurs and the like - have now disappeared from the sea.

Relationship between land and sea 50 million years ago

But soon it became impossible to conduct further research in Pakistan - in 1983, the war began in Afghanistan and the entire region turned into a “hot spot”. Carried away by the search for whales, Gingerish moves his work to Egypt, to a desert valley 95 miles southwest of Cairo - the Zeuglodon Valley, named after the work done here at the beginning of the 20th century. finds of archaic Zeuglodon whales. In the Eocene, most of Egypt, like Pakistan, was the bottom of the Tethys Sea - now the remains of animals that plowed its waters lie embedded in sandstone. After several field seasons, Gingerish and his team discovered thin hind limbs that belonged to an 18-meter “sea snake” - the ancient whale Basilosaurus.

Finds of basilosaurs - animals that undoubtedly led an exclusively aquatic lifestyle and never went onto land - were known earlier. Basilosaurs swam in ancient seas approximately 40–37 million years ago. But no one knew anything about the presence of hind legs because they found only separate parts of the femurs, which paleontologists considered vestigial. Gingerish showed that basilosaurs had legs! True, they are very small - less than half a meter. Whether they were used in any way or were simply rudiments is unclear. It is unlikely that they could be of much help during swimming and certainly could not be used to reach land. It has been suggested that the huge serpentine basilosaurs may have used them for mating embraces, holding each other in the rippling waves. Importantly, 10 million years after Pakicetus, some of the ancient whales still had limbs. It turned out that it was possible to establish a connection between these creatures, although only speculatively. “Whales with legs” turned out to be a reality.

A very important discovery was made in 1992. The team of American Hans Thweissen discovered in Northern Pakistan, in layers about 48 million years old, an almost complete skeleton of a “transitional link” between modern whales and their land-based ancestors. The animal’s large paddle-shaped feet and powerful tail indicated that it was a good swimmer, and strong leg bones and movable elbow and wrist joints indicated that it moved well on land. The “transitional link” was called Ambulocetus natans, “a walking and swimming whale.”

And even later, Thewissen, Jinderish and other paleontologists managed to find fossils that made it possible to reconstruct the successive stages of the transition of whales from a semi-aquatic to an oceanic way of life. The remains of long-nosed Remingtonocetids (for example, Kutchicetus) and Protocetids (for example, Rodhocetus), also descended from some “Pakicetus-like” ancestors on the shores of Tethys, were found. From the protocetids came the completely “dolphin-like” Dorudon - the possible ancestors of basilosaurs and modern whales, which gradually settled throughout all the seas of the globe.

During the Oligocene period following the Eocene, sea levels dropped. “Proto-India” connected with Asia (the result of this “collision” was the emergence of the Himalayas), and Australia and Antarctica moved away from each other, resulting in the formation of a wide, free ring of seas in the Southern Hemisphere. A southern circumpolar current emerged and an ice shell began to form. This created new conditions for mammals living in the seas, which, according to some experts, led to the emergence of modern suborders - baleen and toothed whales. The oldest known transitional form between them and the ancient archaeocetes is Llanocetus, a primordial baleen whale found in Antarctic sediments about 34 million years old. Apparently, he could easily feed on krill. Toothed whales, according to experts, arose around the same time, developing the ability to echolocation, which made it possible to actively hunt in the depths.

Unfortunately, finds of remains of the first representatives of the two modern orders are extremely rare. Lower sea levels in the Oligocene dried up coastal areas that may have contained these remains, and they were destroyed. But excavations in later layers show that a short time later, 30 million years ago, real baleen and toothed whales were represented by several families.

The discoveries of more ancient archaeocetes and their ancestors made it possible to reconstruct the path of transition of cetaceans to permanent life in the ocean.

As was said, Pakicetus already had certain transformations in the auditory system, characteristic of whales. Thewissen suggested that he could use them to perceive sounds propagating in the ground. Being an ambush predator waiting for prey in the water, he could hear the footsteps of possible prey approaching the watering hole. It is quite possible that Ambulocetus did the same, in the jaw of which there was already the beginning of a canal characteristic of whales that conducts sound to the ear. By placing its lower jaw on the ground - as crocodiles do - Ambulocetus “located” the movement of its potential victims along the shore.

Luo (Zhe-Xi Luo), a paleontologist and employee of the Museum of Natural History in Pittsburgh, showed that in basilosaurs and dorudonts - the first completely aquatic whales - the auditory system in structure was already quite close to the auditory system of modern whales. Later, the evolution of the two groups - toothed and baleen whales - took different paths. The former developed the features necessary to produce and perceive high-frequency sounds used in echolocation, while the latter “specialized” in low-frequency acoustic signals that made it possible to communicate with each other over long distances. Around 28 million years ago, early toothed whales already had some of the bone structures necessary to perceive high-frequency sounds and, accordingly, a certain ability to echolocation.

Other important changes to the skull during cetacean evolution included the movement of the eye sockets from a superior (crocodilian) position in Pakicetus and Ambulocetus to the sides of the head, as in protocetids and modern whales. The nostrils moved from the top of the snout in Pakicetus to the top of the head (the blowhole) in modern whales. The teeth became simple and monotonous - adapted only for holding, and not chewing, prey. In baleen whales they disappeared completely; their “whalebone” - horny plates, are in no way connected with teeth.

Analysis of the isotopic composition of oxygen atoms present in the teeth of fossil whales allows us to draw conclusions about whether they lived in fresh or sea water - the latter contains a large proportion of the 18O isotope. It turned out that the body of pakicetus received only fresh water, ambulocetus could live in both fresh and salt water, and protocetids were already real marine animals.

One of the most important adaptations found in whales is their ability to swing their tails vertically to allow them to swim quickly. The structure of the pelvic girdle of Ambulocetus still allowed the animal to maintain its weight when moving on land, although the disproportionately large hind limbs of this animal and paddle-shaped feet made such movement difficult. But such limbs were very well suited for rowing when moving in water. In Rhodocetus, a representative of the protocetids, the bones of the lower part of the forelimb were compressed and already resembled flippers, and the long, delicate feet may have been webbed. The ligaments between the vertebrae that form the sacrum were weakened in Rhodocetus, allowing the spine to bend to create undulating vertical movements of the tail. According to Gingerish, it swam “like a dog” on the surface, and moved under water by combining the pushes of its paddle-shaped hind legs and tail. Most likely, this animal had not yet completely broken with the terrestrial environment and periodically came to land, where it moved in jerks, like modern eared seals.

Basilosaurs and dorudontids were, as already said, completely aquatic creatures. Like modern whales, their shoulder remained mobile, and the elbow and wrist formed the front fin. The hind legs and pelvic bones were small and weak. However, the question of exactly when the whales finally lost their hind limbs remains open. For example, a real baleen whale, whose remains were recently discovered in layers 27 million years old, still had well-formed legs.

In the caudal region of Dorudon there was a rounded vertebra, similar to that found in modern whales at the base of the caudal fin. So, perhaps Dorudon and Basilosaurus already had a completely whale-like tail fin.

So, a series of paleontological discoveries in the 70s–90s. XX century made it possible to trace the main stages of the evolutionary formation of whales on their path from terrestrial to exclusively aquatic animals. However, the question of their more distant ancestors continued - and continues to - remain open. Comparison of the composition of various sections of the DNA of modern mammals - representatives of different orders - allowed researchers to once again talk about the high degree of relatedness between whales and artiodactyls. While mesonychids, with their unclear systematic position, seemed to have nothing to do with it... Other analyzes showed a great similarity of whales with a separate and very specific, it must be said, group of artiodactyls - hippopotamuses (schemes B, C).

Skeletons of modern whales and their ancestors

At first, paleontologists reacted very negatively to such an interpretation of the results of DNA analysis - after all, we were talking about a comparison of living animals; the DNA of Pakicetus and Ambulocetus, of course, was not preserved. Mesonychids were very suitable for the role of the ancestors of whales - they were distinguished by “suitable” morphological characteristics, and their remains were found precisely in those regions and in those layers where and when the formation of cetaceans should have begun. But there were no remains of ancient artiodactyls that would confirm the assumptions of molecular biologists.

The key to solving the problem could be the discovery of the so-called talus, which belonged to one of the ancient (still on land) ancestors of whales, connecting the bones of the knee to the bones of the ankle. In all artiodactyls, the talus has a very characteristic shape with a notch-pulley at both ends. Actually, this is one of the key characteristics of the detachment.

The talus of a representative of mesonychids (1), a modern artiodactyl (2) and a newly discovered representative of protocetids – Rodhocetus astralagus (3). The arrow shows the position of the talus on the limb of Rhodocetus

In the fall of 2001, Gingerish reported the discovery of another species of rodhocetus, Rodhocetus astralagus, and a new form of protocetid, artiocetus, in eastern Balochistan. And Thewissen and his colleagues at the same time dug up in Punjab the skeleton of Pakicetus (previously only the skull was known) and the skeleton of another animal close to Pakicetus (Ichtyolests). And all the newly found skeletons had an talus of a shape characteristic of artiodactyls. It turned out that mesonychids, in which this bone had a different shape, really had nothing to do with the emergence of whales? Did their remains just happen to be “at the right time and in the right place”? But what artiodactyls then gave rise to cetaceans? Hippos, as an independent group, arose much later. Among their ancestors, as well as the ancestors of other artiodactyls, no known fossil species is better suited to the role of the ancestor of whales than mesonychids.

However, it should be noted that the very origin of artiodactyls remains unclear. Perhaps mesonychids are an independent branch of this order (and not the order Condylarthra), which for some reason lost the characteristic shape of the talus? Then they may well have common ancestors with cetaceans. Now there is such a hypothesis (scheme D). The debate continues, and most scientists agree that the mystery of the very first ancestors of whales has not yet been solved. New paleontological finds are needed.

Based on an article by Kate Wong // Scientific American May 2002