On the problem of power first tetrapods noticed Philip Anderson, an evolutionary paleobiologist from the University of Massachusetts (Amherst, USA). Together with his colleagues, Matthew Friedman of the University of Oxford and Marcello Ruta of the University of Lincoln (UK), he analyzed the jaw representatives of 89 genera of fossil vertebrates that lived on the Devonian (416 million years ago) to the Early Permian (295 million years ago). It was in this period were formed joints and limbs of tetrapods belt and separated from amphibians amniotes (reptiles, mammals and birds). Among the fossils studied the so-called four-legged fish Acanthostega, Ichthyostega and Tiktaalik, ancient amphibians, and more recent species, resembles the reptiles and mammals. The researchers analyzed ten biomechanical parameters of the jaws, took into account the known relation between the shape of the jaw and its function, as represented by the bones were attached muscles (they are unfortunately not preserved), have tried to calculate the bite force. Unfortunately, the fossil record can not tell how animals actually ate, is to rely on the calculations.
Jaws Acanthostega and Ichthyostega were like fish. In the first amphibians, they look different, but functionally changed little: the first tetrapods a very long time, about 80 million years could not reveal any significant fall in the corner, or close it with the necessary strength, and most importantly — the structure of the skull did not allow them to chew. What kind of herbivores!
Aquatic vertebrates usually suck in their prey with a stream of water, rapidly expanding the mouth and then swallow. To accomplish such air, which is about 800 times less dense tetrapod would expand the jaws 28 times faster than the water. Furthermore, most of the fish’s mouth is designed so that it is convenient to grab prey, located directly in front of «nose» and not lying on the ground.
Early tetrapods were forced to either hunt in the water and on land to make only brief forays, or to develop alternative strategies for hunting. About these strategies we can form some idea thanks to the work of specialists of the University of Ghent (Belgium) Sam Van Weissenberg («Nature»), which explores modern animals, the leading water-terrestrial life. One of its objects — eel catfish Channallabesapus, inhabitant of wetlands in tropical Africa, so named because of the long body and a tiny head. Eel catfish — a typical «aspirator» production, but it successfully catches terrestrial insects crawling on the beach, or leaning out of the water. Van Weissenberg filmed on camera hunting fish: catfish bends the front of the body so that his mouth is directly over the victim and falls on top of her, opening and expanding jaws. Whether catfish under water, sucked to the insect, but on the ground it just is «under the hood», perhaps a little crushed. After that, the fish quickly crawls back into the water and only there to swallow prey.
Unlike eel catfish, jumpers — 15-centimeter fish mangrove swamps — travel by land with the help of large paired fins. They eat too, on land and water is needed for this to carry a mouth. Seeing the prey, diver tilts his head, squeezes the side walls of the oral cavity, pushes the water directly to the extraction and then draws back together with the insects. The head of the fish is then in a position such that the victim could not escape.
Weissenberg Wang believes that the first tetrapods, adapting to the ground conditions could eat about the same. In order to hunt on the land, the animals do not necessarily have limbs that have maintained their body weight. Much more important is to bend the spine in the right direction. Apparently the fish and tetrapods, and early tetrapods such as Ichthyostega could bend back, and in this way to hunt on the beach.
The deplorable discrepancy strong ground limbs and weak jaws dragged on for 80 million years, all the while building mouthparts amphibians almost unchanged. Jaws tetrapods became more powerful and moving only 320 million years ago, when separated from amphibians and amniotes passed to the herbivores. At the rate of evolution of herbivores amniote mouthparts soared. Here are two interesting moment. First of all, surprisingly, that the formation of a land power so long. In addition it is not clear why the transition to herbivores accomplished only in amniotes. The four-legged fishes and the first tetrapods-anamniotes were predators, and among living amphibians known only example of herbivores — Indian frog Euphlyctis hexadactylus. Meanwhile, various groups amniote herbivores during evolution occurred repeatedly. Distribution of herbivores in amniotes in the background of her lack of other tetrapods demands an explanation. You would think that being in a certain status is an insuperable barrier of evolution.
It is possible that this is true. Discussing the power of early tetrapods Philip Andersen refers to the hypothesis that put forward in 2001 a professor of Brownian University (USA), Christine Janis («Acta Palaeontologica Polonica»). The researcher believes that the main difference between amphibians and amniotes is not the appearance of the embryonic amniotic membrane, which supplies the embryo fluid and allows the animals to breed on land and not in the presence of water-impermeable skin. The most important event in the evolution of tetrapods, which gave them a genuine independence from the water — the transition to the thoracic breathing.
First tetrapods breathing lungs, but the air is injected into these complex movements of the mouth, nostrils and larynx muscles. Everyone has seen how the frog breathes. Its «neck» constantly goes up and down, shaking the air, like a pump. To do this, as we expand the chest, amphibians simply can not, because the edges have short, straight and motionless. To the mouth muscles to cope with the role of the pump head must be flat, lower jaw wide, and neck — short. With this structure of the skull is limited jaw mobility and amphibians can not grind hard food.
According to Christine Janis, tetrapods, to feed on land, it was necessary first of all to free his head from the need to inject air into the lungs. This was made possible when the terrestrial tetrapods took shape chest: long rounded edges, some of which are connected to the sternum, intercostals and abdominal musculature. When the chest ventilation skull shape finally changed. He became more elongated, narrow and high; shifted jaw joints; part of the bone has disappeared and took the vacated seat jaw muscles. The lower jaw has gained the necessary mobility and began to move back and forth. The path to the pasture was open. These changes, according to the researchers, there were already at the earliest amniotes.
A new type of ventilation had another important consequence. Pomp breathing through the lungs removes carbon dioxide, most of it comes out through the skin. Breast ventilation is much more efficient, so that the entire gas exchange occurs in the lungs, and the skin is released from the respiratory function. Now it can be made impermeable to water and to live permanently on the land.
When we read: «Vertebrates landfall», we mean the emergence of a whole set of animals to adapt to life outside the water. After all, on the banks have to do everything differently: to move, to multiply, to breathe and eat. The first tetrapods feel uncomfortable in a new environment, but eventually adapted. However, the evolution of the set of symptoms does not mean their parallel transformation. Works Andersen and Janis clearly show that the formation of tetrapod occurred in stages and has not yet formed a functional unit, the other could not be reconstructed. Please change the destination, then the ventilation system, and only then — the structure of jaws and feeding mechanics. Of course, there were other changes: increased and lengthened the digestive tract, there intestinal microflora, necessary for the effective destruction of herbivorous fiber. Alas, the soft tissue of the first tetrapods did not reach us, so these stages of evolution, we do not know.
Philip Andersen notes that the development of new habitat or the acquisition of the key features of itself does not lead to a rapid change in other functions. Settle in new territory has allowed the emergence of vertebrate herbivores, not colonization of land. Yes, a process that took 80 million years, but it was worth it.