What is the Evolutionary History of Cetaceans?

The submarine whale (janjucetus) is a prehistoric whale. A baleen whale has sharp teeth.

Submarine whale

Australian scientists have discovered a very ferocious looking ancient whale fossil with sharp teeth. These fossils were found near a small town called JanJuc in Victoria in the southeast, so they named it "Janjucetus". At the end of the last century, a teenager surfing on Victoria Beach discovered the fossil.

Question 1

Why is Janjucetus a baleen whale?

"It belongs to the baleen whale," Professor Fitzgerald said. "Its ear structure clearly shows the characteristics of the baleen whale."

In cetaceans, ear structure is regarded as an important judgment mark. Because sound travels differently in water and air. Humans and other terrestrial animals have thin, flat ear drums (also called tympanic membranes) that receive sound waves using air as a propagation medium; the ear structures of modern cetaceans are thick and long drum ligaments that cannot accept sound waves. It is a bone called "bulla". The modern cetacean's bubbling is very dense, so it can pass the sound transmitted by the dense medium to the inner ear.

Take an ancient whale discovered in Pakistan in 2000: Take the Pakicetus as an example. Its bubbling has evolved in a modern direction, but they still retain ear drums similar to terrestrial mammals. Down has no effect. Based on this feature, the discoverer Dr. Thewissen speculated that just as the turtle heard the ground vibrations transmitted through the shell of the turtle, the Bucky Whale might use bubbles to capture the sound transmitted from the ground. After considering the newly discovered body bones and ear structure, he imagined the Bucky Whale as a predator with ambush as the main strategy. Like crocodiles, they may lurk in the shallow water of the river bank, heading towards the shore, attacking animals who come to drink.

The evolution of whale and dolphin hearing was late, and the tooth whale and the baleen whale parted ways. Tooth whales evolved to receive and produce high-frequency sounds, so they can use echolocation to search for prey, while baleen whales receive and produce extremely low-frequency sounds, which can communicate with each other over long distances. The cetacean ear bone fossils show that about 28 million years ago (late Oligocene), early toothed whales already had some bone structure that received high-frequency sounds, so they have at least the ability to locate echoes. Although the origin of their low-frequency hearing is still unknown, as far as fossil records are concerned, baleen whales developed low-frequency hearing structures 34 million years ago (early Oligocene). It is precisely because the ear structure of the baleen whale was found in Janjucetus, so it is determined that it belongs to a baleen whale.

Question 2

Why baleen whales have teeth

As the name implies, the difference between a baleen whale and a toothed whale is that the former's epidermis is fully keratinized and extends into baleen, about 150 to 400 pieces, while the latter has conical shaped teeth that are not changed for life.

Fitzgerald found that not only did Janjucetus have no whale, but his teeth were very sharp, almost the length of his fingers, so he believed that Janjucetus was a ferocious predator. Fitzgeral estimates that it is only 3.5 meters in length and is considered to be a very small individual in the baleen whale family. But it is unique in that it can use its sharp teeth to catch larger fish, including other whales, and even sharks can become its prey.

In fact, one of the important values of Janjucetus fossils is the fact that people have re-examined the neglect of the simple classification of tooth and whale. Scientists have long discovered that baleen whales have teeth during the embryonic stage, and the teeth do not become baleen until after birth. This problem is related to the problems of gill fissures in reptile, bird, and mammalian embryos, as well as chordal cords in higher vertebrate embryos. It questions the early evolution theory. Darwin first pointed out in The Origin of Species that the only reasonable The explanation is that these strange forms are the heritage of their ancestors: "The same embryonic structure reveals that the ancestors are the same." Later, German zoologist Ernst Haeckel proposed the law of biological repetition, claiming that personal development is a repetition of phylogeny, reflecting the simplification And compressed evolutionary processes. By studying embryonic development, we can understand the evolutionary processes of animals. This is obviously an oversimplification and extreme proposition, because embryonic development repeats only a specific structure of the ancestor, and does not repeat the overall shape of the ancestor.

Question 3

What does a huge eye socket mean?

Other notable features on Janjucetus's skull include the location of the eye sockets. Earlier Bucky and Whale whales resemble crocodiles, with eye sockets at the top of their heads, and aquatic primitive whales and later whales and dolphins at their heads. Janjucetus's eyes were very large, Fitzgerald said, and it was the pair of huge eye sockets that were 24 cm in diameter. This shows that, in terms of the size ratio of Janjucetus (about 3.5 meters in length according to Fitzgerald's estimation), it has the largest eyes among all known cetaceans. Although the skull is similar in size to modern bottle-nose dolphins, the diameter of the eye sockets is double that of the latter. Its keen eyes will help it track its prey, before tearing it into pieces.

Perhaps the characteristics of the baleen whales and the way they use the baleen to filter the sea water and eat fish and shrimp are not the basic characteristics of the baleen whale since its differentiation. More likely, after breaking away from the tooth whale suborder, the baleen whale presents a complex evolutionary picture, and one of them has evolved into a gentle, slow-moving species such as the blue whale and fin whale that are now common, and the baleen whale Other members of the family, stubbornly retaining the characteristics of hunting large fish such as teeth, therefore embarked on a strange evolutionary direction and became the so-called "problem teenagers" within the baleen whale family. In the words of Fitzferald: "Janjucetus is a peculiar branch of whale development and evolution, like a cousin of a modern baleen whale, not a direct ancestor."

Evolutionary history

Whales are descendants of camels? --- The Mystery of Cetacean Land Ancestors

Scientists believe that modern cetaceans originated from carnivorous four-footed mammals, whose ancestors were a bit like large wolves, good at roaming and tracking prey. Until 57 million years ago, at the beginning of the Eocene, these carnivores faced harsh food sources, and because the dinosaurs were extinct, the food resources in the ocean were very abundant, so some mammals resolutely jumped into the sea to open up one. The new world, as a result, experienced a profound evolution in their bodies, eventually becoming the look of modern cetaceans.

However, questions about how cetaceans have occupied an important position in the evolutionary history of mammals, and who are the ancestors of cetaceans, have not been able to accurately answer and controversy because no definitive terrestrial fossil evidence has been found. Based on the morphological characteristics of teeth and ear regions, paleontologists argue that cetaceans are closest to middle-clawed beasts (an extinct, carnivorous ungulate in the early Tertiary); but molecular biologists believe that cetaceans It belongs to the artiodactyl group (such as sheep, cattle, pigs, camels, deer) and is a sister group of hippos (a family of artiodactyls).