Why do species go from land back to sea?
For a long time, the answer to these questions was not clarified by science, and between the world of aquatic mammals and the land world of their ancestors, something like a missing link was seen. However, recent paleontological findings have brought some clarity to the topic. So which of the mammals lives in the ocean? Let's start with the most exotic - sirens. In 1741, during the second Kamchatka expedition, sad for the Danish-Russian navigator Vitus Bering, a very large marine animal was discovered near the Commander Islands. Possessing a spindle-shaped body (which ended with a bifurcated tail similar to a whale), it reached a weight of 5 tons and was up to 8 m in length. The animal was described by the participant in the expedition, the German naturalist Georg Steller, and an unprecedented creature was called the Steller cow. But why a cow? Not just because of the size.
Elephants and their underwater cousins
The giant animal was herbivore. Like a real cow, it grazed and plucked grass, or rather, sea kale in shallow water. Such a large and harmless animal, after being discovered by people, of course, could no longer count on a long life. By 1768, the “cabbage” had been knocked out, and now you can see the Steller’s cow only in the form of a skeleton or in the picture. But the unfortunate inhabitant of the Bering Sea has a close relative in the world. According to the zoological classification, the Steller’s cow belongs to the dugong family, which includes dugongs still living on the planet, and further to the detachment of sirens, which also includes manatees.
Seven attempts to go to sea
Leaving from land to sea - under pressure from natural enemies or in search of food - is not uncommon in the history of life on Earth. Reptiles and birds switched to aquatic and semi-aquatic lifestyles. Among mammals, seven such episodes are known. In addition to the sirens, pinnipeds, and cetaceans mentioned in the article, one can recall the polar bear, which, being a close relative of the brown bear, perfectly mastered in the marine environment, although its body did not undergo major morphological changes. The same can be said of the sea otter, or sea otter, representing the family of marten. It is hard to imagine floating sloths, but there were some. The genus of thalassocnus lived in the South America in the Miocene - these animals were herbivores and fed on shallow water vegetation. Finally, about 30-8 million years ago, demostilles lived on the shores of the Pacific Ocean - another detachment of marine mammals. Their limbs allowed them to walk on land, but in the water they seemed to move more confidently. Demostilles are relatives of sirens and proboscis.
All sirens are herbivorous (unlike whales or seals), however, they live exclusively in shallow water and cannot, like whales, go to the depths of the ocean or, like seals, get out to land. With whales sirens in common lack of hind limbs. But once upon a time these limbs were.
In 1990, in Jamaica, the American paleontologist Daryl Domning discovered a large location in the coastal deposits with petrified remains of marine vertebrates, as well as land animals like a primitive rhino. An almost complete skeleton of a creature living in the Eocene (about 50 million years ago) and previously unknown to science was found there. The find was called Pezosiren portelli. This same “pesosiren” had a heavy skeleton, very similar to the skeletons of current sirens. Sirens need powerful heavy ribs to give the body negative buoyancy, and, apparently, the ancient animal faced the same task, which indicates a semi-aquatic way of life. On the other hand, the pesosiren was clearly able to move on land, it had all four limbs and no tails and fins. In short, this animal, apparently, was similar in lifestyle to the hippo, as also pointed upward nostrils. But which of the living creatures is considered the closest relative of sirens? It turns out that they are not hippos.
Sirens are included in the superorder of placental mammals "afroteria", that is, "African animals". This branch, emerging from Africa, consists of several groups, and the closest relatives of the sirens are damans - rodent-like herbivorous animals the size of a domestic cat. Another detachment closely related to sirens and damans is proboscis, which today is represented exclusively by elephants.
Sirens are the only large taxon of marine mammals that have herbivorous ancestors. Pinnipeds - walruses, eared seals, real seals - came from predators, also originally land animals. However, many researchers are inclined to consider the concept of “pinnipeds” obsolete, since, according to the widespread opinion in science, pinnipeds are not a mono-but a polyphyletic group, that is, they come not from one, but from different branches of land animals. Nevertheless, pinnipeds undoubtedly belong to the order Carnivora - predatory placental mammals. This detachment is divided into two suborders - dog-shaped and cat-shaped. Psiforms are bears, martens, raccoons, of course, wolves and dogs, and cats, wyverres, mongooses, and hyenas are classified as catlike. Without going into the subtleties of classification, we can say that pinnipeds are part of the canine. But which ones? Supporters of the polyphyletic origin of pinnipeds believe that two lines led from land to sea. Walruses and eared seals (the Otarioidea superfamily) are closely related to the bear ones, while real seals (Phocoidea) originate from the marten. The similarity in the structure of pinnipeds in this case is explained by convergent evolution.
The problem of the “missing link” existed here until the fossilized remains of an animal called Pujila were discovered by expedition of paleontologist Natalia Rybczynski in Polar Canada on Devon Island in 2007. Puyila lived in the Miocene, about 24 million years ago, probably in the area of the lake that existed at that time, surrounded by forest. The find was made by chance - the all-terrain vehicle broke down, and paleontologists came across a fossil, wandering around the neighborhood. Puyila was the owner of an elongated body 110 mm long and was able to move perfectly on land on four legs. She looked like a representative of the marten in her appearance, but the structure of the skull was already similar to the construction of the head of real seals. In addition, it was assumed that there were membranes between the fingers of the puila paws, indicating a semi-aquatic lifestyle of the beast associated with frequent movements through the water.
Before the discovery of the puila, the most ancient of the known pinnipeds was also the enaliarch - the "sea bear" who lived in the Miocene. This animal was already very well adapted for a long stay in the water, although it could hunt on land. Enaliarchus swam using all four limbs and had a special inner ear to perceive sound vibrations in the underwater environment. Some features of the structure bring enalyarct closer to sea lions, that is, to the subfamily of eared seals. Thus, the “sea bear” could be a link in the evolutionary chain leading from common with the bear ancestor to walruses and eared seals.
So, pinnipeds descended from predatory placental mammals and are obviously close relatives of bears and martens. The third large taxon of marine mammals - Cetacea - cetaceans, probably also descended from predators. But ... ungulates.
Yes, it’s quite true, there are no such people these days, but millions of years ago, very terrifying specimens ran on their hooves. The largest known terrestrial carnivorous mammal that has ever lived on Earth is considered to be the Andrews. Only his skull was found (in 1923), but the size of the fossil is amazing - 83 cm long and 56 cm wide. Most likely, the Andrewsarchus resembled a giant wolf, and not a real forest inhabitant, but such as the wolves are portrayed in cartoons. The giant was identified in the mezzanine squad, whose representatives lived 45–35 million years ago, and then became extinct. The mesonychiae were primitive ungulates with five- or four-fingered limbs, and each finger ended in a small hoof. The huge elongated skull of the Andrewsarch and the structure of the teeth led paleontologists to the idea of a close relationship with whales, and as early as the 1960s it was suggested that mesonychia are the immediate ancestors of cetaceans, and the latter, therefore, can be considered close relatives of artiodactyls.
However, molecular genetic studies of a later time led many researchers to the conclusion that cetaceans are not relatives of artiodactyls, but in fact they are, developed from their environment. So the term cetaceans appeared, denoting a monophyletic group - going back to a single ancestor - a group that includes both cetaceans and artiodactyls. Inside this group, the closest relatives of the whales were hippos. However, this does not at all follow that the ancestors of the whales were similar to hippos (although such a theory existed).
Due to the scarcity of the fossil record, the problem of the “missing link” between ungulates and cetaceans has not found a final solution and continues to cause debate, however, a number of finds of the last decades give quite convincing clues. If the genesis of pinnipeds took place somewhere in the Arctic regions of the planet, then the cetaceans owe their origin to the ancient Tethys Ocean - the water space constantly changing its configuration between the northern continent of Laurasia (future North America and Eurasia) and Gondwana (South America, Africa, Hindustan, Antarctica and Australia). In the Eocene epoch (56–34 million years ago), vast territories in the Near and Middle East were located under water, on the site of which there is now mountainous land. In the conditions of warm coastal shallow water, in which fish was abundant, some group of ancient ungulates reoriented themselves to finding food in the sea.
In 1981, in Pakistan, the skull of a creature was found, which it was called - Pakitset, "Pakistani whale" (Pakicetus). Outwardly, he had little in common with modern whales, the size of a dog, and he looked like a representative of canids. However, this predator was ungulate. Initially, it was recorded in a mezzanine, but later, at the beginning of the new millennium, when the paleontologists finally came across a complete skeleton of a pacicet, the animal was identified in artiodactyls, which separated from the mezzanine much earlier. Pakitset had an auditory bulla - a bone formation on the skull characteristic of cetaceans, which helps to perceive sounds underwater. And although the “Pakistani whale” obviously felt great on land, it had to be in the water often and the corresponding evolutionary adaptations had already begun. The hearing bull was also present in another fossil of the land animal, Indochius, a tiny artiodactyl whose remains were discovered in India. Indochius could not even be a predator at all, but a harmless herbivore that climbed into the water, escaping from natural enemies, for example, from birds of prey. And in 1992 in Pakistan they found the petrified bones of the ambulocet, ambulocetus natans - the “walking whale swimming”. With a great morphological similarity to cetaceans, the ambulocet could still move on land, led a semi-aquatic lifestyle, and was an ambush predator of the crocodile type. It took millions more years of evolution for the whales to switch to a completely aquatic way of life, and then to leave the coastal waters in the deep ocean. Pakitset, Indochius, ambulocet - all of them lived in the Eocene 50−48 million years ago. Due to the lack of genetic material in the fossils, it is impossible to say through which of these creatures there is a direct line to modern cetaceans, however, the general mechanism for converting artiodactyls into whales, dolphins and porpoises has become more understandable.The article “Enthusiastic about moisture” was published in the journal Popular Mechanics (No. 6, June 2015). Do you like the article?
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