You enter a corridor with subdued light, where you meet destitute souls, tormented by pain and suffering. But there will be no peace for them here, because behind each of the doors more torment and fear awaits them, filling all the cells of the body and filling all thoughts. You come to one of the doors, behind which you can hear a hellish grinding and buzzing, making your way to the bones. Gathering the last of your courage into a fist, you stretch out a hand cold with horror to the door handle, when suddenly someone touches your shoulder from behind, and you turn around, shuddering in surprise. βThe doctor will be free in a few minutes. Sit down for now, we will call you, βthe gentle voice of the nurse tells you. Apparently, this is how some people imagine going to the dentist, who have an extremely negative attitude towards these βsadistsβ in white coats. But today we will not talk about dental phobia, we will talk about crocodiles. Yes, yes, it is about them, or rather about their teeth, which do not need dental treatment.
Scientists from the University of Missouri (USA) conducted a study of the teeth of crocodiles, which showed the amusing features of the enamel of these impeccable hunters, relying just on their jaws. What did scientists find out, how do the teeth of modern crocodiles differ from their prehistoric relatives, and what is the use of this study? We learn about this from the report of the research group.
Research basis
For most vertebrates, teeth are an essential attribute of receiving and eating food (anteaters do not count). Some of the predators rely on speed during the hunt (cheetahs), some on the collective (lions), and for some the strength of their bite plays a huge role. This also applies to crocodiles, which sneak up on their victims in the water and grab them with their powerful jaws. To prevent the victim from escaping, the grip must be powerful, and this results in heavy loads on the bone structure. To neutralize the negative effect of their powerful bites, crocodiles have a secondary bony palate, which is fixedly connected to the skull.
A visual demonstration of the closing and opening of the jaw of a crocodile.
One of the main features of crocodile teeth is their constant replacement with new ones when the old ones wear out. The fact is that the teeth of crocodiles resemble a nesting doll, inside which new teeth develop. Approximately every 2 years, each of the teeth in the jaw is replaced by new ones.
Pay attention to how tightly this "tooth trap" closes.
Crocodile teeth are divided into several categories according to their shape and corresponding functionality. At the beginning of the jaw there are 4 large fangs that are needed for effective capture of prey. In the middle are thicker teeth that increase along the jaw. This part is needed to cut prey. At the base, the teeth expand and become flatter, which allows crocodiles to crack open mollusk shells and turtle shells like seeds.
How strong is a crocodile's jaw? Naturally, this depends on its size and type. For example, in 2003, they found out that a 272-kilogram Mississippi alligator bites with a force of ο½9500 N (N - Newton, 1 N = 1 kg m / s2). But the 1308-kilogram combed crocodile showed a mind-blowing ο½34500 N. By the way, the absolute bite force in humans is approximately 1498 N.
The strength of the bite depends not so much on the teeth as on the jaw muscles. In crocodiles, these muscles are very dense and there are many of them. However, there is a big difference between the very developed muscles responsible for closing the mouth (which gives such bite force) and the weak muscles responsible for opening the mouth. This explains why a crocodile's closed mouth can be held in place with simple tape.
Come on, show me who called you a little one.
But crocodiles need a jaw not only for ruthless killings for food, but also for caring for their offspring. Female crocodiles often carry their cubs precisely in the jaws (it is difficult to find a safer place for them, because who would dare to climb there). The mouth of crocodiles is equipped with very sensitive receptors, thanks to which they can regulate the force of their bite, which allows them to better hold prey or carefully carry babies.
Human teeth, unfortunately, do not grow back after the old ones fall out, but there is something in common with crocodiles - enamel.
Image #1: Caudal tooth of a Mississippian alligator (Alligator mississippiensis).
Enamel is the outer shell of the crown of the tooth. This is the most durable part of the human body, like many other vertebrates. However, as we know, our teeth do not change for new ones, so our enamel should be thicker. But in crocodiles, worn teeth are replaced with new ones, so there is no need for thick enamel. Sounds quite logical, but is it really so?
Scientists say that understanding the changes in enamel within a single taxon will allow us to better remember how the structure of enamel changes depending on the biomechanics and diet of the animal.
Crocodiles, that is alligator mississippiensisare well suited for this study for a number of reasons. Firstly, their teeth, bite force and enamel structure change with age and size of the individual, which is also due to a change in diet. Secondly, crocodile teeth have different morphologies depending on their position in the jaw.
Image #2: a and b show the difference in teeth between large and small individuals, c-e show the teeth of the fossil ancestors of modern crocodiles.
The rostral teeth are thin and used for prey capture, while the caudal teeth are blunt and used for crushing with higher bite forces. In other words, the load on the tooth depends on its position in the jaw and on the size of the owner of this jaw.
This study presents the results of analysis and measurements of absolute enamel thickness (AET) and size-standardized (relative) enamel thickness (RET) of crocodile teeth.
AET is an estimate of the average distance from the enamel-dentin junction to the outer surface of the enamel and is a linear measurement. And RET is a dimensionless value that allows you to compare the relative thickness of enamel on different scales.
Scientists assessed the AET and RET of the rostral (at the βnoseβ of the jaw), intermediate (in the middle of the row) and caudal (at the base of the jaw) teeth in seven individuals of the species alligator mississippiensis.
It is also important to note that the structure of the enamel may depend on the diet of the individual and the species as a whole. Crocodiles have a very extensive diet (whatever they catch will be dinner), but it differs from that of their relatives, who have long since died out. To test this in terms of enamel, the scientists analyzed AET and RET fossils. Protosuchidae (UCMP 97638), Iharkutosuchus (MTM VER 2018.837) and Allognathosuchus (YPM-PU 16989). Protosuchidae is representative of the Jurassic period, Iharkutosuchus - the Cretaceous period, and Allognathosuchus from the Eocene.
Before starting the actual measurements, the researchers brainstormed and proposed several theoretical hypotheses:
- Hypothesis 1a - since AET is a linear measure and should depend on size, it is assumed that the variance in AET will be best explained by skull size;
- Hypothesis 1b β since RET is standardized for size, it is assumed that the variance in RET will be best explained by tooth position;
- Hypothesis 2a - since AET and skull length are linear measures of size, they should be scaled with an isometric slope;
- hypothesis 2b - since the caudal teeth experience the greatest bite forces in the dentition, therefore the RET will be higher in the caudal teeth.
The tables below present sample data (skulls of crocodiles of the species alligator mississippiensistaken from the Rockefeller Preserve in Grand Chenier, Louisiana, and fossils).
Table #1: Crocodile tooth scan data (rostral, intermediate and caudal).
Table 2: teeth data (LSkull - skull length, hCrown - crown height, VE - enamel volume, VD - dentin volume, SAEDJ - enamel-dentin interface area, AET - absolute enamel thickness, RET - relative enamel thickness).
Results of the study
According to the dental data presented in Table 2, the scientists concluded that the thickness of the enamel scales isometrically with the length of the skull, regardless of the position of the tooth.
Table 3: AET and RET values ββdepending on the variables.
Image #3: AET/RET scaling in relation to skull length.
At the same time, the thickness of the enamel on the caudal teeth is significantly greater than on the others, but this also does not depend on the length of the skull.
Table No. 4: average values ββof enamel thickness in higher vertebrates (Crocodyliform - non-taxon group of crocodiles, Dinosaur - dinosaurs, Artiodactyl - artiodactyls, Odontocete - suborder of cetaceans, Perissodactyl - equids, Primate - primates, Rodent - rodents).
Image #4: The caudal teeth are thicker than the other teeth.
The scaling data (Table 3) confirmed hypothesis 1a, explaining the dependence of the AET value on the length of the skull, and not on the position of the tooth. But the RET values, on the contrary, depend on the position of the tooth in the row, and not on the length of the skull, which confirms hypothesis 1b.
The remaining hypotheses (2a and 2b) were also confirmed, which follows from the analysis of the average thickness of tooth enamel with different positions in the row.
A comparison of the enamel thickness between the modern Mississippi alligator and its ancient ancestors showed much in common, but there were also differences. So, in Allognathosuchus, the thickness of the enamel is about 33% greater than in modern crocodiles (image below).
Image #5: Comparison of average enamel thickness in alligators and fossil crocodilians by crown height.
Summing up all the above data, scientists came to the conclusion that the thickness of the enamel directly depends on, so to speak, the role of the teeth. If these teeth are needed for crushing, then their enamel will be much thicker. Previously, it was found that the pressure (compression force) of the caudal teeth is higher than that of the rostral ones. This is due precisely to their role - to hold prey and crush bones. Thus, thicker enamel prevents damage to the teeth, which are subjected to maximum stress during nutrition. Indeed, the evidence suggests that crocodilian caudal teeth break much less frequently despite severe stress.
In addition, it was found that the teeth Allognathosuchus the enamel is considerably thicker than that of the other crocodilians studied. It is believed that this fossil preferred to eat turtles, and crushing their shells requires strong teeth and thick enamel.
The scientists also compared the thickness of the enamel of crocodilians and some dinosaurs, corresponding to the estimated weight and size. This analysis showed that crocodile had thicker enamel (diagram below).
Image #6: comparison of crocodile and dinosaur enamel thickness.
Curiously, tyrannosaurid enamel was almost as thick as that of the much smaller Allognathosuchus and even modern crocodiles. It is logical that the structure of the tooth of crocodiles is explained by their habits in terms of hunting and diet.
However, despite their records, the enamel of archosaurs (crocodiles, dinosaurs, pterosaurs, etc.) is thinner than that of mammals.
Image #7: Comparison of enamel thickness (AET) between crocodiles and some mammals.
Why is the enamel of hunters who rely so heavily on their jaws thinner than that of mammals? The answer to this question was at the beginning - the replacement of worn teeth with new ones. Although crocodiles have strong teeth, they do not need, so to speak, heavy-duty ones, in view of the fact that a new tooth will always come to replace a broken one. Mammals (for the most part) have no such talent.
Image #8: Enamel thickness (RET) comparison between crocodiles and some mammals.
To be more precise, the thickness of the enamel in archosaurs varies from 0.01 to 0.314 mm, and in mammals from 0.08 to 2.3 mm. The difference, as they say, on the face.
For more detailed information about the nuances of the study, I recommend looking at
Finale
Teeth, as strange as it may sound, are an extremely important tool in the extraction of food. Yes, a modern person can always correct any defect associated with teeth, but there are no dentists among representatives of the wild nature. Even a person did not always know what dental treatment is. Therefore, some species choose strong and durable teeth, while others prefer to change them like gloves. Crocodiles and their distant relatives can be placed in both groups. The enamel on the teeth, which are necessary for effective retention of prey and crushing bones, is quite thick in crocodiles, but given the serious loads, their teeth still wear out and sometimes break. In such a new tooth takes the place of the old tooth.
For a human, one of the hallmarks is the opposable thumb, which has helped us tremendously in many endeavors, ranging from "pick up a stick and slap the annoying neighbor on the branch" to "pick up a pen and write a sonnet." For crocodiles, such a tool is their jaws, in particular their teeth. It is this part of the body that makes crocodiles such dangerous and deadly hunters, which should be avoided.
Friday off-top:
A very curious and aesthetically beautiful short cartoon in which a crocodile is not quite a crocodile.
A cartoon about how you can not trust suspicious "logs" in the water, especially if you are a wildebeest.
Thanks for watching, stay curious and have a great weekend everyone! π
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Source: habr.com