I change my teeth every 76 days, just to eat

The importance of teeth to us humans is self-evident. If your teeth are not in good condition, you can’t eat hot, cold, sour or sweet food as you like, and it may breed bacteria and affect your health. Moreover, we humans only change our teeth once in our lifetime. Since we change our permanent teeth in childhood, once there is a problem with our teeth, we can only repair and replace them on the existing basis, which is painful and expensive.

However, some animals do not have this worry, especially some dinosaurs, because they may replace a mouth full of teeth every few months .

A recent study published in the international journal PeerJ found that titanosaurs that lived in Ruyang, Henan 100 million years ago replaced their teeth on average every 76 days.

The picture shows the giant Ruyangosaurus, a type of titanosaur dinosaur. Reference [3]

The teeth are replaced every 76 days.

Not for eating.

Although the titanosaurs in Ruyang are vegetarians, they are not "vegetarians": in terms of size, they are one of the largest dinosaurs in the world. It is not easy to maintain such a large size. They have to eat a lot of ferns or gymnosperms every day - these hard foods will cause great wear and tear on their teeth . In order to eat and eat to maintain their physique and avoid the impact of worn teeth on their eating, this type of titanosaur dinosaurs has evolved the ability to "change teeth every 76 days."

Researchers from the Henan Museum of Natural History, the Institute of Vertebrate Paleontology and Paleoanthropology of the Chinese Academy of Sciences, and the University of Birmingham in the UK have discovered the secret of how this dinosaur replaced its teeth by studying the fossil of the front left lower jaw of a titanosaur dinosaur. This partial left lower jaw fossil of a titanosaur dinosaur was discovered in Ruyang, Henan in 2008. | References [1]

The researchers speculate that for titanosaurs, part of the purpose of "growing big" was to be able to fight against natural enemies .

Carcharodontosaurus is one of the terrifying natural enemies that the titanosaurs had to face. At the place where the fossil specimens were unearthed, researchers also found a large number of sharp carcharodontosaurus tooth fossils. Carcharodontosaurus is a carnivorous dinosaur that is as ferocious as Tyrannosaurus . It preyed on titanosaurs or other animals and occupied the top of the food chain at that time. Under such circumstances, the titanosaurs in the Ruyang Basin of Henan Province chose to eat non-stop, eat until they became big and had a good physique, and live in groups with the idea of ​​"more dragons, more power" , so that they could better survive the dangerous Cretaceous period.

This specimen is a carcharodontosaurian tooth found in the Ruyang Basin in Henan Province. The crown height is 7 cm, which is about the same size as the largest maxillary tooth of Carcharodontosaurus. The tooth has sharp serrations on the edge. | Reference [2]

Wait, here’s a question: if teeth are replaced so frequently, won’t there be a few days when we can’t eat properly?

Don't worry, titanosaurs have a solution. In order to avoid frequent tooth replacement affecting eating, they evolved a particularly clever interval tooth replacement mode : one tooth is replaced every other tooth socket - this ancient secret of tooth replacement is hidden in fossil specimens.

Billions of years later, when human researchers studied these tooth fossils, they found that the first five tooth sockets contained replacement teeth, that is, the dinosaurs' replacement teeth. This shows that Titanosaurs had the ability to replace teeth multiple times, and the tooth replacement pattern was a typical interval replacement, so that replacing new teeth would not affect eating .

3D rendering of the specimen, showing the replacement teeth at four different stages of growth and development, for example, the shortest teeth are at stage 1 and the longest teeth are at stage 4. Reference [1]

From the 3D perspective of the specimen, we can see that the teeth grow rapidly. According to the length of these replacement teeth, they are obviously in four different growth and development stages, with an average increase of about 20 mm in length from stage 1 to stage 4. However, the width of the teeth only increases slightly, so it can be said that this is a set of dinosaur teeth that "only grow taller but not fatter."

76 days is not fast, just average

Titanosaurs can replace their teeth every 76 days, which is much faster than humans, but in their dinosaur family, this speed is only average .

According to previous studies, it only takes 62 days for Camarasaurus to replace teeth, while the later branch of Titanosaurs, the Rock Helmosaurs, can replace new teeth every 20 days. Of course, there are some dinosaurs that are slower than Titanosaurs, such as Brachiosaurus, which takes 83 days to replace teeth, and Mamenchisaurus, which takes 98 days.

The rate of tooth replacement in dinosaurs varied among evolutionary branches , with the abelisauridae of the theropods, the hadrosauridae of the ornithischians, and the late-diversifying titanosaurs showing increasingly rapid rates of tooth replacement.

Therefore, the tooth replacement rate can reflect the evolutionary position and kinship of animals to a certain extent . Titanosaurs at different evolutionary positions lived in different times, and the flora at that time - that is, the food source of Titanosaurs - was also different, and their tooth structure also changed accordingly. According to the research results of Titanosaur teeth discovered around the world, the more primitive Titanosaurs were, the slower they replaced their teeth.

Take a CT scan of a tooth that was 100 million years old

How do modern people know how long it took for the extinct dinosaurs to replace their teeth? In fact, it is through CT scans (computed tomography). The 3D perspective images of teeth we saw earlier were also obtained in this way.

The researchers performed a CT scan on the partial left lower jaw fossil of this titanosaur dinosaur, obtaining three-dimensional (coronal, sagittal, and transverse) scan images of each layer, and then estimated the tooth formation time and replacement rate based on the length of the intact teeth.

According to the algorithm derived from the incremental line data of the Camarasaurus tooth slices, the researchers can calculate the time (days) of tooth formation for this specimen, that is, y=-0.0078x2 + 2.6771x + 109.44, where x is the length of the tooth and the calculated y is the time of tooth formation. With the time of tooth formation, the tooth replacement rate can be calculated, that is, the time required to replace a tooth in a certain alveolus. The number of days obtained by subtracting the ages of two adjacent teeth in the same alveolus is the tooth replacement rate of this alveolus . The average replacement rate of the three alveoli is 76 days.

The tooth replacement rate can be calculated by subtracting the age of the new tooth from the age of the old tooth. For example, the age of tooth 1 (days) minus the age of tooth 2 (days) in the figure. Reference [1]

Teeth are the hardest part of an animal's body and can withstand harsh preservation environments better than bones. They are one of the important materials for paleontological research. Tiny teeth contain tiny records about the tooth owner: whether it was a big dinosaur, what food it ate, how it ate, etc. Based on the various characteristics of teeth, researchers can further understand the living habits and even living environment of the tooth owner.

The teeth in the dinosaur mandibles studied this time can be used to determine the phylogenetic position of the tooth owner in the dinosaur family . By analyzing the shape, size, number of replacement teeth and other characteristics of the teeth, the tooth owner may belong to the early differentiated non-rock helmet dinosaur titanosaur.

From a local perspective, four types of titanosaur dinosaurs from the Aptian to Albian periods of the Early Cretaceous have been discovered in Ruyang, Henan, namely, Ruyang Huanghe Titanosaurus (2007), Giant Ruyangosaurus (2009), Shijiagou Xianshanosaurus (2009), and Ruyang Yunmengosaurus (2013). Which of these dinosaurs could this mandible fossil belong to?

At present, researchers are not sure what specific dinosaur species the tooth owner belongs to, but the tooth owner is likely to be most closely related to the Xianshanosaurus.

This is the specimen and 3D color perspective model exhibited in the Dinosaur Hall of Henan Natural History Museum｜Provided by the author

This is the first discovery of a skull with teeth from the Early Cretaceous Ruyang titanosaur fauna . The analysis of teeth can help us understand the secret of the replacement of teeth of titanosaur dinosaurs, and also enrich our understanding of the Ruyang titanosaur fauna. Through small teeth, modern people can find traces of ancient times, but compared with mammals, we have done much less research on the growth of reptile teeth. This time, the researchers have put together a small piece of the giant puzzle of studying the growth of reptile teeth.

[1] Chang H, You HL, Xu L, Ma W, Gao D, Jia S, Xia M, Zhang J, Li Y, Wang X, Liu D, Li J, Zhang J, Yang L, Wei X. 2021. Relatively low tooth replacement rate in a sauropod dinosaur from the Early Cretaceous Ruyang Basin of central China. PeerJ 9:e12361 http://doi.org/10.7717/peerj.12361

[2] Junchang Lü, Li Xu, Hanyong Pu, Songhai Jia, Yoichi Azuma, Huali Chang and Jiming Zhang, 2016. Paleogeographical significance of carcharodontosaurid teeth from the late Early Cretaceous of Ruyang, Henan Province central China. Historocal Biology, 28(1-2), 8–13. http://dx.doi.org/10.1080/08912963.2014.947287

[3] Pu Hanyong, Lü Junchang, Xu Li, Zhang Jiming, Jia Songhai, 2014. “Giant Dragon Appears: A Record of the Giant Ruyang Dragon”, Henan People’s Publishing House, pp. 136-137.

Author: Chang Huali

Editor: Towel, Malt Yang

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