Ancestors of cetaceans or How did whales come to the ocean? Origin of whales Image reproduction of extinct mammals of the distant ancestors of whales

Both modern groups of whales, toothed whales (Odontoceti) and baleen whales (Mysticeti), appear suddenly in the fossil record. Speaking about the structure of the skull in both groups, Stahl notes:

"...a strange modification, absent, even in rudimentary form, at Basilosaurus and its relatives: due to the fact that the nostrils have moved back to the dorsal surface of the head, the nasal bones are reduced and moved upward, and the premaxillary and maxillary bones are expanded in the back and cover the roof of the skull."

Basilosaurus had very small hind limbs (obviously not adapted for movement on land), and "The Doctrine of Evolution..." reports that "they were most likely non-functional." But according to other evolutionists, they were used to hold a partner during mating. Thus, whale evolution specialist Philip Gingerich wrote: “It seems to me that these limbs could only be involved in sexual behavior and reproduction.”

Pakicetus machus, from the point of view of a number of evolutionists, it is another contender for the place of an intermediate form between whales and land mammals. According to evolutionary "dating" methods, its age is 52 million years. Because some teaching aids also announced Pakicetus"transitional form" (see figure below), we should talk about it in more detail, although it is not mentioned in "Teachings about evolution...". Apparently, the authors of this textbook do not consider Pabcetus a successful example of an intermediate form. Perhaps the point is that Pakicetus is known only from molars and fragments of the skull and lower jaw, which means we cannot judge how he moved. The figure compares the speculative reconstruction, who appeared on the cover of the magazine "Science" and in manuals for school teachers, with real remains. notice, that real fossil evidence are only areas of the skull, indicated by shading; everything else is "reconstructed". But we know for sure that the hearing aid Pakicetus was the same as that of land mammals - it was found in river sediments with the remains of other land animals. Thus, the facts indicate that Pakicetus was most likely a terrestrial creature, and not an “intermediate form.”

Russian whale specialist G. A. Mchedlidze expresses serious doubts that such creatures, as Pabcetus, Ambulocetus and others - even if it is proven that they lived in water - can be considered the ancestors of modern whales. He views them as a completely isolated group.

Evolution of whales?

What was actually found

RUDIMENTAL LEGS?

Many scientists support the hypothesis about the evolution of cetaceans, since it is believed that whales have vestigial hind limbs hidden in the thickness of the body. However, these "remnants" are not at all useless - they support the genitals (the bones of males and females differ greatly). Therefore, the existence of these bones is best explained from the point of view of creationism, rather than evolutionary teaching. As in the case of supposedly useless limbs Basilosaurus, we should not think that our ignorance of the function of an organ is tantamount to its lack of any function.

There are evolutionists who spread the myth that some whales have been found with hind limbs, even with thigh and knee muscles. Probably, this legend is based on a real case of finding a sperm whale with a swelling on the body 14 cm long, inside which a piece of bone 12 cm long was found. But the body length of a sperm whale is usually 19 meters, so this anomalous piece of bone is negligible compared to the body whale and does not look like a “leg” at all!

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 brain of basilosaurids was relatively small size, from which it can be assumed that they led a solitary lifestyle 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

Skeletal development

Write a review of the article "Evolution of Cetaceans"

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

An excerpt characterizing the evolution of cetaceans

August 12th, 2014

The evolution and origin of cetaceans has long remained a mystery to paleontologists. Due to the paucity of the fossil record, the question of the origin of whales has been the cause of fierce disputes between creationists and scientists who defend the doctrine of evolution. Fossil remains that shed light on the development and formation of this amazing group of animals were very rare until very recently. Undoubtedly, modern whales are secondary aquatic mammals - in the process of evolution, their ancestors first came out of the water, giving rise to amphibians and reptiles, and then returned to the water as mammals. This happened approximately 50-55 million years ago, in the late Paleocene-Eocene.

Although it is difficult to believe, looking at the modern blue whale, but all cetaceans, including whales, dolphins and porpoises, are descendants of land mammals of the artiodactyl order (of course, not modern, but ancient ungulates).

Previously, traditional views on the evolution of cetaceans were that their closest relatives and probably ancestors were mesonychians - an extinct order of predatory ungulates that resembled wolves with hooves instead of claws and were the sister group of artiodactyls. These animals had teeth of an unusual conical shape, similar to the teeth of cetaceans. In particular, because of this, scientists have long believed that cetaceans descended from some kind of ancestral mesonychy. However, new molecular genetic data indicate that cetaceans are close relatives of artiodactyls, in particular living hippopotamuses. Based on these data, it is now proposed to even include cetaceans in the order Artiodactyla and the name “Cetartiodactyla” is proposed for the monophyletic taxon that includes these two groups. However, the oldest known fossil remains of Anthracotherium, the ancestor of hippopotamuses, are still several million years younger than the age of Pakicetus, the oldest known ancestor of whales.

Basic scheme of whale evolution

The ear will tell you everything

During the expedition of American paleontologist Philip Gingerish (P. Gingerish) in Pakistan, scientists received very interesting 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, the 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.

Pakicetus

Let us remind you that sound waves propagate differently in water and air. 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. The structure of the skeleton of Pakicetus once again confirmed that whales are not direct descendants of mesonychids. On the contrary, the ancestors of whales separated from artiodactyls and switched to an aquatic lifestyle after the artiodactyls themselves separated from their common ancestors with mesonychids. Thus, proto-cetaceans were early forms of artiodactyls, which retained some characteristics characteristic of mesonychids (the conical shape of the teeth), lost by modern artiodactyls. Interestingly, the earliest ancestors of all ungulate mammals were probably part carnivores or "scavenger" omnivores.

Pakicetus were ungulates and are sometimes classified as early whales. They lived in what is now Pakistan (hence the name "whale of Pakistan") in the early Eocene, about 50 million years ago. It was an animal that resembled a dog in appearance, but with hooves on its toes and a long thin tail. It was initially assumed that the Pakicetus ear was well adapted for life under water, however, as further research showed, the Pakicetus ears are only suitable for the air environment, and if the Pakicetus ear is indeed the ancestor of whales, the ability to hear under water was a later adaptation of an already existing hearing aid. According to the American paleontologist Hans Thewissen, the teeth of Pakicetus already resemble the teeth of fossil whales.

Another reconstruction of Pakicetus - “with hair”

Thewissen also discovered that a similar ear structure was observed in the fossils of another unusual creature - the small deer-like animal Indohyus. Indohyus (literally “pig of India”) is a small (cat-sized) creature of fragile build, the remains of which were found in Kashmir (India). It is most often compared to modern African water deer; The similarity is broken only by the long tail - common feature various groups of primitive mammals of the early Cenozoic. The age of this creature is estimated at 48 million years. Indochyus is classified as a member of the family Raoellidae - primitive artiodactyls. It is considered a member of the sister group to early cetaceans based on the structural features of the same ear region. The auditory bulla of Indonychus, formed from the tympanic bone, is also very unusual in shape and demonstrates structural features characteristic of the most ancient whales discovered shortly before, and, in particular, the same Pakicetus. 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 Indohyus, like the modern water deer, dived underwater to hide from a predator. Thus, in its remains there is an increased content of the oxygen isotope 18O, which indicates an aquatic lifestyle. However, the 13C carbon isotope content suggests that it rarely fed in water. However, it is just as likely that its food could consist of higher aquatic plants (flowering plants). In any case, judging by the isotopic composition of tooth enamel, Indohyus was probably not part of food chains based on freshwater phytoplankton formed by algae rather than higher plants.

Indohyus

"Crocodile Among Mammals"

The most notable of the ancient whales is the well-known Ambulocetes, known from the Eocene of Pakistan. Outwardly, this mammal resembled a three-meter crocodile.

“The monster, lying motionless in the water among the mangroves, noticed its prey - an animal of suitable size that had come to drink. With a few energetic pushes of its hind legs, it approached the shore, sank its powerful teeth into the body of the victim and retreated back into the water. When the animal, tightly clamped in its jaws, unable to breathe, stopped beating, the predator crawled ashore to begin its 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 eyes protruding above the surface of the head. However, its body was covered not by shell plates, but by fur, its legs ended not with claws, but with something resembling hooves, and its teeth were the teeth of an animal, not a reptile...” - this is how ambulocetus, one of the first whales, looks like in the minds of paleontologists.

Earth in the Middle Eocene - 50 million years ago

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. In the jaw of the ambulocetus there was already the beginning of a canal characteristic of whales, conducting 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.

Close relatives of Ambulocetidae were Remingtonocetidae. Representatives of this family were smaller in size, had a more elongated snout and were better adapted to underwater life. It is assumed that their lifestyle resembled modern otters, hunting fish from ambush.

Representatives of both groups had nostrils located at the end of the snout, like those of land mammals.

The closest relatives of whales today are hippos

It is worth noting important changes skulls during the evolution of cetaceans - movement of the eye sockets from the upper (as in crocodiles) 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.

Ambulocet. The “crocodile-shaped” shape of the skeleton is clearly visible

"Protokits"

Protocetids form a large and diverse group known from finds in Asia, Europe, Africa and North America. This family includes a large number of genera, some of them are quite well studied (for example, Rhodocetus, known from the Tertiary deposits of Baluchistan). All known protocetids had well-developed forelimbs and hindlimbs that could support the body on the ground; They probably led an amphibiotic lifestyle, living both in aquatic environments and on land. It is not yet clear whether the protocetids had a caudal fin, like modern cetaceans, but it is obvious that they were well adapted to an aquatic lifestyle. For example, the sacrum - the part of the spine to which the pelvis is attached - in Rhodocetus consisted of five separate vertebrae, while the vertebrae in the sacrum of land mammals are fused. The nasal openings of protocetids moved even further up the snout - this is the first step towards the nostrils located on the top of the head of modern cetaceans. The version about the amphibious nature of protocetids is supported by the discovery of a pregnant female Maiaceti with a petrified fetus, with its head turned towards the exit hole. This suggests that Mayacet gave birth on land - otherwise the cub had a chance of choking.

Kuthicetus

The origin of early whales from ungulates is indicated by such features as, for example, the presence of hooves at the ends of the fingers of Rhodocet. In this cetacean, the bones of the lower 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. In general, during the Eocene, cetaceans made a sharp leap in morphological changes: from four-legged land animals, they turned into completely aquatic forms, completely different in appearance from their land-based ancestors and relatives. A possible reason for this phenomenon is the lack of competitors in the new habitat.

Rodocetus

Remingtonocet

Out to the ocean

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.

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.

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.

Georgiacet

Like modern whales, the shoulder of Dorudon and Basilosaurus remained mobile, and the elbow and wrist formed the front fin. 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.

Dorudon

Meanwhile, these whales were not yet “real whales.” 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. However, 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.

Basilosaurus

The emergence of “whalebone”

Baleen is unique to baleen whales, but toothed whales, while lacking it, are nevertheless also whales. Therefore, this feature cannot be considered fundamental: it is a particular adaptation of one group of cetaceans. 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.

Access to land and access to the ocean

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 little time later, 30 million years ago, real baleen and toothed whales were represented by several families.

Three years ago, in 2011, scientists found the fossilized remains of one of the oldest baleen whales, which turned out to be the “missing link” in the evolution of baleen. Researchers have discovered that the huge, elastic jaws of blue whales and their brethren evolved from more rigid structures.

On the upper jaws of baleen whales there are several hundred horny plates, which act as a filter that sifts out plankton from the water entering the animal’s oral cavity. Whales take a very large amount of water into their mouths, opening their mouths wide due to the fact that they do not have a rigid connection between the two halves of the lower jaw. In addition, almost all cetaceans have a very wide skull, which further increases the possible maximum volume of water entering the mouth. Thanks to their feeding method, whales were able to evolve to such impressive sizes.

Janjucetus hunderi - one of the first baleen whales

Scientists do not know exactly what the sequence of appearance of these two characteristic features - a large skull and flexible jaw joints - was. Authors new job have described the bones of the ancient cetacean "Janjucetus hunderi" that lived in Earth's oceans about 25 million years ago, and new evidence supports the hypothesis that whales originally developed a broad skull.

“Early baleen whales lacked one of the characteristics of all living (and most fossil) baleen whales - a free joint of the lower jaw,” notes study author Erich Fitzgerald of Museum Victoria (Australia). “Without it, today’s baleen whales simply wouldn’t be able to eat the way they are used to.”

The scientist means the following: the whale opens its lower jaw at a very large angle; The elastic tissue attached to the jaw stretches, allowing the animal to take a huge amount of water into its mouth. Whalebone plates growing from the upper jaw act as a kind of sieve that filters out krill, the main source of food.

The new remains belonged to the species "Janjucetus hunderi", which lived about 25 million years ago off the coast of Australia and was probably about three meters in length, that is, the size of an average dolphin. It was adorned with large teeth for capturing and crushing prey, making it very different from today's baleen whales with their hair-like teeth. Moreover, as already mentioned, his lower jaw could not swing open so wide.

However, J. Hunderi" was a baleen whale, for it had a number of adaptations characteristic of this suborder. For example, this is a wide upper jaw, indicating that a large mouth appeared before the ability to filter. Two halves of the lower jaw "J. Hunderi" were firmly connected to each other and did not allow the ancient marine mammal to open its mouth very wide. At the same time, the upper jaws of the animal looked typical of modern cetaceans, and the skull itself was very wide. Scientists believe that "J. Hunderi" did not filter the incoming water, pushing it in the opposite direction from the mouth, but swallowed it along with the prey that was there.

Among modern large whales, the sperm whale provides, oddly enough, the opportunity to answer the question of how whales switched to a filter type of feeding. Usually in popular literature The sperm whale is depicted snacking on a giant squid. But this, although well-known, is far from being such a common prey. Naturally, at whaling stations, when cutting sperm whales, such mollusks were removed from their stomachs. But it is also known that large numbers of relatively small squids and fish were often found in the stomachs of sperm whales. Even if the fish sometimes reached a meter in length, it is still small compared to the multi-ton sperm whale. Modern toothed whales often feed on small fish and squid, simply sucking them into their mouths. Beaked whales have significantly reduced teeth - sometimes to only two large teeth, which are clearly not adapted for grasping prey such as fish and squid.

The blue whale has no teeth

In the earliest baleen whales ("baleen" due to their skeletal anatomy rather than the presence of baleen), teeth are quite rarely found on the jaws. It can be assumed that they gradually lost the function of capturing and directly holding prey, and served rather to “lock” the mouth. By the way, in the modern whale shark, small teeth perform exactly this role. The original prey of proto-baleen whales was most likely quite large, and small fish were able to slip between the teeth of the predator. Don't smile: modern herring also has a real chance of slipping out even from the much more advanced filtering apparatus of the humpback whale. As a matter of fact, this is how the modern crabeater seal (Lobodon) catches prey, small swimming crustaceans, whose teeth have acquired a specific shape, becoming multi-peaked and flat.

Canadian researcher Edward D. Mitchell was surprised by the unusual structure of the teeth of the whale Llanocetus: they sat in the jaws with large gaps, had shallow roots and deep grooves on the crown, dividing the tooth into blades. A popular article that talked about the discovery of this animal was called “Ancient whale smiled like a sieve.” This is clear evidence that the first baleen whales used their teeth to close the exit from their mouths to small food objects, which they grabbed several at a time. In the process of evolution, whales developed an adaptation that simultaneously allowed water to freely exit the animal's mouth, but more effectively retained small fish, crustaceans or squid that ended up in the whale's mouth. The key to understanding further changes in the hunting apparatus of cetaceans also comes from knowledge of the “spoiled” genes responsible for the development of teeth found in the genome of modern whales. With the further development of whalebone, the presence of teeth gradually became a neutral feature, and mutants with “damaged” genes responsible for the development of teeth did not differ in survival success from whales with intact genes, which grew anatomically complete teeth. Subsequently, the disappearance of teeth removed anatomical restrictions on the development of a more advanced filtering apparatus.

Llanocetus

Echolocation

True toothed whales (Odontocetes) echolocate by producing a series of clicks at different frequencies. Sound pulses are emitted through the frontal melon, reflected from the object and recorded through the mandible.

Echolocation- a system of orientation in space by delaying the return of a reflected sound wave - appeared in the ancestor of modern dolphins and toothed whales more than 28 million years ago. New York Institute of Technology Associate Professor Jonathan Geisler conducted a study of the fossil species Cotylocara macei, discovered near Charleston, South Carolina. “The most important findings of our study concern the evolution of echolocation and the complex anatomy that enables this ability. It arose around the same era when whales were diversifying - different body and brain sizes, different ways nutrition,” says Geisler.

Toothed whales, dolphins and porpoises produce high-frequency sounds through a closed area in the nasal canals, behind the blowhole - whereas all other mammals (including humans) produce sounds in the larynx. In toothed whales, the mechanism is very complex - it is a lot of muscles, air cavities and fat layers squeezed into a small area of ​​​​the face. Paleontologists believe that such a complex system developed gradually, step by step.

Cotylocara macei - one of the first whales with developed echolocation

According to Geisler, the whale "Cotylocara macei" was capable of echolocation. “The dense bones and air sinuses of its skull helped focus its sounds into a single stream of sound, which allowed the whale to search for food at night or in murky water,” the scientist says.

Through comparative analysis, Geisler and his colleagues determined that Cotylocara belonged to an extinct family of whales that diverged from other cetaceans at least 32 million years ago. A vestigial form of echolocation appears to have evolved in the common ancestor of Cotylocara and other toothed whales approximately 35-32 million years ago.

"Cotylocara" were distinguished by a number of unique anatomical features, including a deep cavity on the top of the head (hence the name of the species - "head with a sinus"), where the animals "stored" air when immersed in water - and the same cavity probably reflected sounds coming from the side of the face. Also noteworthy is the bone around the nasal openings, similar to a radar antenna, which could also reflect sound and improve the quality of echolocation. “The anatomy of the skull is very unusual. I have never seen anything like this in any whale, either living or fossil,” says Geisler.

The study of Squalodon skulls suggests the primary occurrence of echolocation in this species of whale. 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 modern dolphins descending from Squalodon is considered unlikely, although Squalodon does provide insight into early whale evolution.

Squalodon

In this post, I will not consider in detail the issue of the appearance and development of echolocation in toothed whales, otherwise I will have to write an entire scientific work here and consider many points of view on this issue. In general, we can say that the latest fossil remains of cetaceans once again brilliantly confirmed the correctness of Darwinism - the doctrine of evolution. Future discoveries will clearly prove the correctness of this thesis, both in the example of the evolution of whales and other groups of living organisms.

Gradual shift of the nasal sinuses to the back of the head in whales

It is interesting that in November 2006, off the coast of Japan, a bottlenose dolphin with underdeveloped hind limbs, but clearly visible from the outside, was caught alive. His photo probably made the rounds on all the news feeds. This atavism best indicates that the ancestors of whales lived on land.

A modern reconstruction of Leviathan, a giant toothed whale; its echolocation was also already developed:

For quite a long time, scientists have been confident that about 50 million years ago, the ancestors of whales and dolphins trampled the land with their hooves somewhere in what is now southern and southeast Asia. Later, for some reason - most likely in search of food - these animals went into the water and over the next several million years turned into modern cetaceans, now inhabiting all of the earth's oceans. The most ancient mammals, which gave rise to the modern sea giants known today, had webbed hind feet, could exist both on land and in the aquatic environment, and in their appearance were more reminiscent of crocodiles.

For a long time it was not possible to determine who was the last more or less land-based ancestor of these ancestors of whales and dolphins. On this score spoke out a wide variety of hypotheses. Among the ancient ancestors at different times were hippopotamuses, camels, “hoofed wolves” mesonychia and even canine-like predators, for example, bears and mustelids, whose relatives are other inhabitants of the seas - walruses and seals.

A later analysis of fossilized teeth of the apparent ancestors of whales showed their relationship with ungulates. The close relationship of the families is also indicated by the similarity of the proteins of the two modern orders of mammals, in particular, the proteins of their immune system. However, who exactly was the ancestor of whales and dolphins, and most importantly, why they needed to return to the sea, remained unknown.

It seems that scientists have found the “missing link” in the evolution of cetaceans. And apparently, it was not hunger, but fear that drove the whales into the water.

Renowned whale evolution specialist Hans Thewissen from Northeastern Medical College of Ohio University and his colleagues from the USA and India published in Nature, the results of a study of the almost complete skeleton of a creature found several decades ago in Kashmir by Indian geologist Rango Rao.

An ancient mammal called Indohyus (now there are two species in this genus), was not the direct ancestor of modern whales, but was the closest relative of all cetaceans, in comparison with which all other artiodactyls are cousins. Indohyus belongs to the prehistoric family Raoellidae of the artiodactyl order, and it is this family that is directly related to the ancestors of whales, dolphins and porpoises, which now make up the entire order of cetaceans.

Indohyus, which Thewissen and his colleagues studied, lived in what is now Kashmir 48 million years ago. At that time, here were the shores of the ancient shallow Tethys Sea, which by that time had already ceased to be the ocean of the same name. The fossilized remains of Indohyus were discovered in sediments from the bottom of an ancient river, now high in the mountains.

Having studied the structure of the teeth of this ancient mammal, scientists came to the conclusion that it was more likely a herbivore than a carnivore, and obtained its food on the shore.

In reservoirs, the Indohyus found refuge from predatory creatures that threatened its life.

Externally resembling a cross between a raccoon and a deer, the ancient brother of cetaceans was about 80 centimeters long and, apparently, spent most of its time in the water. This is indicated, in particular, by the heavy bones of the limbs, which helped the animal stay at the bottom. Apparently, Indohyus, like hippopotamuses, preferred to splash around in shallow water.

Skeleton of Indohys

The shaded parts of the skeleton are reconstructed based on data on the body structure of artiodactyls from the family raoellids.

The close relationship between Indochyus and whales is indicated by outwardly inconspicuous, but very convincing details for paleontologists. In particular, we are talking about the structure of the thickened umbrella bone (involucrum), which belongs to the area of ​​the middle ear. According to Thewissen himself, as soon as he saw her, it became clear to him that this was the closest relative of whales known to science. In addition, the structure of the premolar teeth of Indochyus and the ancestors of modern whales turned out to be similar. Previously, Jonathan Geisler from the University of South Georgia had noticed such similarities, but those results were not enough to make a firm statement about the relationship between whales and raoellids.

To establish the diet of Indohyus, paleontologists studied the structure of its teeth. Until now, scientists have argued a lot about the adaptation of cetaceans to eating fish and meat - whether it occurred before the arrival of mammals in the ocean or after. According to many evolutionists, the very return of these artiodactyls to the water served only one purpose - hunting (more precisely, fishing).

However, analysis of the relative abundance of stable isotopes of carbon and oxygen contained in the enamel of mammalian teeth showed the opposite.

Based on these facts, Thewissen put forward the hypothesis that the ancient ancestors of whales and dolphins did not think at all about conquering water spaces, but simply fled there, driven by predatory pursuers. The mechanism of feeding on fish and plankton was developed much later.

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 were asked general scheme 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 the movement of 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

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