How the head was made to us: what is a skull and how did it form

After Precambria, with its strange multicellular organisms, an evolutionary leap occurs, as a result of which modern types of living creatures appear, in particular chordates. From them vertebrates also lead their kind. The primitive chordates, such as the lancelet, who had safely survived to our times, had no bone or cartilage spine, of course. It was preceded by a dorsal string of dense connective tissue. This flexible cord provided speed of movement due to the wave-like bends of the body, creating an advantage over creatures that erected an external carapace around them. More frisky, but seemingly almost defenseless, the chordates nonetheless embodied the most “advanced” construction and, as a result, stood at the origins of the type of animals, whose representatives will in the future become true masters of the planet.

The head is a descendant of the spine

The dorsal string was subsequently replaced by the simplest cartilaginous spine, whose function was to protect the spinal cord running along the back. It consisted of vertebrae in the form of cartilaginous rings. And by the way, why such fragmentation? The answer to this question can be easily obtained if we recall that the basis of the structure of vertebrates (as well as, for example, worms) is segmentation, that is, the repetition of some standard blocks and elements in the body structure. This design principle is very important in nature, as it allows you to lengthen the body, only adding standard elements. This phenomenon is called the term polymerization, known from chemistry. The reverse process, called oligomerization, also happens. It consists in merging elements to create a more compact design of a living creature. Thanks to oligomerization in vertebrates, in fact, a head with a skull box appeared.

Having developed from a chord - an elastic tourniquet made of connective tissue - the spine got a rather complicated structure over time. Several front vertebrae gave rise to the cerebral box, while the rest acquired specialization, forming different parts of the spinal column.

At first, in primitive vertebrates, the dorsal nerve cord had uniform segmentation. A pair of nerves departed from each vertebra containing a segment of the brain. The pair, departing from the frontmost vertebra, became olfactory, the second pair received visual function, the third - auditory. The need to process a large number of signals emanating from the senses led to a thickening of the three anterior segments of the spinal cord and their fusion into the brain. The cartilaginous vertebrae surrounding this important part of the nervous system also merged, as a result of which the so-called brain capsule arose. It was she who became the prototype of the skull. Thus, the brain and skull descended from a part of the spinal cord of the ancient chordate and the cartilaginous vertebrae surrounding it. And even the human head in embryonic development shows the division of the brain and skull into segments from which they once evolved.

Jaw, teeth, scales ...

In primitive vertebrates (such as lampreys), the cartilage box protects the senses and the brain. Lamprey belongs to the cyclostomes - with the mouth with a ring muscle, the animal draws in nutrient organic matter from the bottom sludge. It is not able to grab or bite anyone with its “mouth”, and it is understandable why - the early vertebrates do not have and did not have a jaw. Oddly enough, its appearance is associated with the development of respiratory function. In early fish, cartilaginous arches appear that allow more active control of the opening and closing of gill slits. They also have cartilaginous jaws. Moreover, as scientists were able to prove, the movable lower jaw is nothing but the anterior branchial arch, to which nature has found new application. The imaginary designer (as it almost always happens in nature) turned out to be extremely economical and not only created the jaw from the branchial arch in the same shark, but also pulled skin on it, the sharp scales of which, once in the oral cavity, increased in size and turned into teeth. Thanks to technological innovations, vertebrates from harmless fish-like creatures such as the lancelet turned into the animals that dominate the water, standing on top of the food pyramid.

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The giant brachiosaurus clearly did not belong to rapidly moving animals, but its long thin neck allowed it to actively manipulate its light little head.

Running a little ahead, it is worth noting that the bone skeleton that appeared in the future in fish, amphibians, reptiles, birds and mammals was formed during evolution from two different sources, the first of which were cartilage, and the second - skin. The history of the development of the jaw of cartilaginous fish just shows how exactly this happened.

Out of the water - to the exit!

The skull of even bone fish consists of many separate bones, interconnected very loosely. At the other end of the evolutionary chain - the continuous skull of homo sapiens - even sutures between the bones overgrow with old age. However, in the hundreds of millions of years that have passed since the first amphibians climbed onto land, before the domination of mammals, nature actively experimented with vertebrate skulls, offering a variety of designs. The transition to life on land could not do without serious changes in the structure of animals, and the head played an important role here. Relatively voluminous brain, cranial box - all this now had to be kept on weight, almost without the help of Archimedean force, which was a good help in the aquatic environment.

The figure shows the sequence of reconstruction of the head of a tyrannosaurus from the petrified remains of his skull. As we already wrote in the article “How the Lizards' Bones come to life” (“PM” No. 12'2009), such reconstructions, although they have a scientific basis, always contain a fair amount of hypothetics.

A small and light head could be held on a thin long neck. It’s easier to track small prey and grab it with a swift movement. Another option is a heavy massive head on a powerful short neck. An animal with such parameters was more suited to the role of a large predator awaiting a prey in an ambush.

The early amphibians had a structure close to the second type - the labyrinthodont wore a solid flat skull on a short neck. The reptiles that came after the amphibians were distinguished by a much greater variety of design options.

Flat monolithic skull of a primitive amphibian-labyrinthodont (Late Devonian - Triassic)

The skull of the thecodont - the ancestor of modern crocodiles. However, the crocodile's skull is much closer in structure to the head of the labyrinthodont.

Muscle freedom!

In primitive reptiles, such as pareiasaurs, the skull is also solid, and this had certain disadvantages. The fact is that the jaw muscles of these animals were inside the skull and therefore were short, and they did not have enough space to expand. As a result, the compressive force on the jaws was very weak. Over time, reptiles begin to gradually increase the height of the skull. Now the jaw muscles are lengthening, and their power is growing. However, there is still nowhere to expand - after all, they still remain inside a closed cavity. To clear space for muscles, the next stage of evolution takes place. The so-called temporal windows appear in reptile turtles. True, in the wild animals (which eventually became the ancestors of mammals), only the lower temporal windows developed in the skull, but in the diapsid reptiles (from which the snakes, lizards, as well as archosaurs, including dinosaurs, tecodonts and crocodiles) arose windows - upper and lower. The jaw muscles were attached from above to the edges of the windows and to the side wall of the brain box, and the external holes (primarily the lower one) were used so that when the jaws were compressed, the muscles could contract and expand. The appearance of temporal windows (back in the Carboniferous period) in the first animal-like and diapside reptiles was an innovation that allowed the development of higher groups of vertebrates.

Trumpeter with a comb. The skull of a duckbill dinosaur is one of the most interesting metamorphoses that occurred in the history of the development of diapsid reptiles. What is one crest that helped the animal make loud noises. There is no preorbital orifice, but a round neckline has appeared in the area of ​​the jaw joint.

Openwork and easy.

Thanks to the increased power, such jaws became much more functional both during the initial collapse and during the final compression of the prey.

True, the “perforation” of the skull of the archosaurs did not end there. Along with the temporal windows, they developed the so-called preorbital foramen located between the nostrils and the orbit. According to one hypothesis, there was a special gland responsible for the removal of excess salts from the body. Other researchers associate holes with the work of the maxillary pterygoid muscle. But there is another explanation leading us into the field of mechanics. Perhaps the skull of dinosaurs and tecodonts simply evolved towards facilitating the entire structure, which now resembled a lace farm with stiffeners along the vectors of application of the main forces and with voids in lightly loaded areas. The relatively light head did not require a powerful neck and massive, heavy forelimbs from the dinosaurs, which became the main prerequisite for the transition to upright posture and bipedism.

Head on springs

Another amazing property, especially developed in diapsid reptiles and the bird family tree that leads from them, is associated with the mobility of the skull elements - kinetism. Everyone knows the hyperkinetism of snakes, which are quite capable of pulling their skull, like a glove, for example, on a large egg. This option, however, is good for animals with a small brain and is completely unsuitable for more developed creatures such as birds. In birds, all strong mobility is carried to the periphery of the skull (square bone, jaw, beak), and that part of it that protects the brain remains motionless. The mobility of certain parts of the skull was distinguished by tecodonts - the ancient ancestors of crocodiles, but the current crocodiles themselves have lost both the kinetism of the head bones and temporal windows, returning to the monolithic skull of ancient amphibians. And of course, kinetism was more or less characteristic of dinosaurs.

Long-faced from China. In the skull of probactrosaurus (Probactrosaurus), a plant poultry-dinosaur dinosaur, considered to be a possible ancestor of duckbill dinosaurs, there is also no preorbital opening and the skull is quite massive. Moreover, he had an elongated lower jaw and elongated nostrils.

Moving skull gave its owners a number of advantages. For example, grasping a large prey often involves a strong blow, which takes the head. If the blow is not self-deprecating, it can simply break the bones of the skull. With kinetism, such blows are suppressed in moving areas. For the bones of these zones, there are corresponding muscles that either move or hold, spring the bone, creating an active tension.

Or another example. If a small animal such as a lizard grabs a prey comparable in size to the mouth of a predator, then when you try to squeeze the jaws to squeeze the victim, one of the forces that arise will work to push it out. To keep the prey, you will need either very long teeth, or ... movable bones of the lower jaw and skull. In this case, the lizard will be able to “hug” the victim with its jaws, preventing it from slipping out. Due to the mobility of the skull, birds manipulate the food found, as if pushing it into the esophagus. In general, kinetism turned out to be a rather "advanced" technology, but only mammals and homo sapiens themselves did not inherit such an interesting feature of the head structure. Why? Slightly pumped up ancestors.

On the skull of the oldest tecodont of the archosaur Archosaurus rossicus Tatarinov, who lived in the Perm period, all the main features inherent in the skulls of diapside reptiles are clearly visible. The preorbital foramen located between the nostrils and the orbit may have exited the secretions of a special gland that removed salt from the body. Other researchers have linked the openings to the pterygoid muscle, which made it easier for the tecodont to snap its mouth shut. The third explanation is lightweight construction. Joints of bones that provide mobility of the skull, in particular prokinetism and streptognathia, are also clearly visible.

How our ancestors tore meat

As already mentioned, kinetism is mainly characteristic of diapsid reptiles. In those days when terapsids dominated the planet - bestial reptiles, diapsids escaped them in a small class. These were brisk animals like the current lizards, hunting for relatively small prey, which could immediately be captured by the mouth. It was then that kinapsism began to develop in diapsids, which was the best suited to this method of hunting and nutrition. Probably, kinetism initially developed also in beast-like reptiles, but when they reached enormous sizes, large herbivorous dinosaurs became their prey. You cannot roll such an animal in your mouth, and therefore, in return for the light and moving head, the therapsids developed a heavy monolithic skull with massive jaws to easily tear pieces from large carcasses. The compressive movement of the jaws was more in demand than the grasping one. So the way of feeding directed this branch of reptiles in a completely different direction of development. By the irony of evolution, it is the beast-like, first decreasing, and then turning into mammals, that eventually became our ancestors. True, unlike the ancient therapsides, in mammals, including humans, all the muscles of the jaw apparatus are brought out of the skull, and the temporal region is strongly transformed, only the zygomatic arch is preserved.

Jaw ear

The imaginary designer who created the forms and mechanisms of the living for hundreds of millions of years was extremely inventive and economical. Once turning an extra branchial arch into a jaw, he later used the jaw itself to construct a perfect hearing aid. The lower jaw of a person consists of one bone - the tooth. For reptiles, the situation was different: in addition to the dental bone, the lower jaw also included a complex of bones located in the back, near the joint. When the beast-shaped reptiles changed the way of feeding and they needed more compressive force on the jaws, an interesting thing happened: the joint connecting the lower jaw with the skull “moved” forward, directly to the tooth bone. And those bones that remained behind gradually lost their muscles, became thinner and partly disappeared. But one of them - articulated together with the square bone of the skull, with which it initially formed the jaw joint in reptiles, became the basis for the construction of the middle ear. The hammer and the anvil - these small bones that transmit sound from the eardrum (outer ear) to the inner via the stapes, show a clear resemblance to the articular and square bones, which seemed to be unnecessary, and the joint between them is the original jaw joint of the reptiles that fell into the middle ear mammals.

Regressions are also characteristic of evolution, as are constructive innovations. Ceratops are a group of horned dinosaurs that lived in the late Cretaceous. Unlike most dinosaurs, whose skulls have an openwork structure with well-defined openings and mobile bones, the ceratops developed a massive practically monolithic skull, in which through holes were overgrown and turned into hollows. But nature rewarded these animals with a bone collar, the purpose of which remains the subject of debate, and a horn. The most famous representative of the group - Triceratops, got three horns.

If we take the relatives of diapsid reptiles - a lizard or a bird, we will see that their eardrums are located quite high on their heads. Obviously, animals of this type are historically used to receiving sound through the air. They lifted up a small head and listened. In the extinct massive animal hunters with a large head on a short neck, the eardrum, on the contrary, was very low, at the level of articulation with the lower jaw, and was directed to the ground. How can one not recall Russian fairy tales in which Ivan Tsarevich put his ear to the ground in order to hear the thunder of the Basurm horses or the tread of the Serpent Gorynych. So, according to one hypothesis, these beast-shaped reptiles did not raise their heads, but put their ear almost to the ground, listening to whether a predator or prey was walking somewhere there. The sound went not only through the eardrum and stapes, but also through the back bone of the lower jaw and the square bone of the skull articulated with it, which were adjacent to the soil. В итоге эта функция усиливалась, и кости оказались со временем включены в конструкцию слухового аппарата. Так что Иван-царевич из сказки не придумал ничего нового, а лишь использовал технологию далеких зверообразных предков человека.

Редакция благодарит администрацию и сотрудников палеонтологического института РАН им. but. А. Борисяка за предоставленные для съемки экспонаты коллекции и за помощь в подготовке материала

Андрей Герасимович Сенников — кандидат биологических наук, старший научный сотрудник Палеонтологического института РАН им. A.A. Борисяка

Статья «Как нам делали голову» опубликована в журнале «Популярная механика» (№6, Июнь 2010). Do you like the article?

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