Evolution is one of the most important processes in the life of any organism on earth. Of the modern species on planet Earth, only a small percentage abandoned evolution many millions of years ago, most have changed all this time and continue to change. In matters of the development and history of our species, much attention was paid to the intellect, the structure of the hand, which allows us to hold the tools of labor in our hands, upright posture, etc. However, rarely the question arose, where did we get such a jaw? Scientists from Uppsala University (Uppsala, Sweden) and ESRF (particle accelerator in Grenoble, France) decided to conduct a fossil study to find out the evolutionary origin of the vertebrate jaw. What were the difficulties of the study, what methods were used and what new did the scientists learn? Their report will tell us about this. Go.
Research basis
Remember how we pulled out baby teeth in childhood. Sometimes it was necessary to visit a terrible dentist, sometimes a thread and a door were enough, and sometimes an annoying wobbly tooth fell out on its own. Regardless of the method of getting rid of milk teeth, its essence is the same - the "old" tooth makes room for the "new" one. This is inherent in many species of animals, but the mechanism of this process is different.
Dunkleosteus is one of the most famous representatives of Arthrodiriformes.
Let's go back to the past, 300-400 million back. In those days there were no sharks yet, but the World Ocean was inhabited by other predators - Arthrodiriformes . Arthrodiras are a group of extinct vertebrates from the placoderm class.
One of the most famous and well-studied types of arthrodir is dunkleosteas (Dunkleosteus ), who lived on the planet 415-360 million years ago. At one time, this species was the largest predator (4.5-6 m body length), although this statement is very shaky, since most of the studied fossils are fragments, according to which it is very difficult to determine the exact size.
The documentary "Prehistoric Predators", which also tells about Dunkleosteus.
But it is not the dimensions that distinguish these creatures from their contemporaries, but the method of hunting, or rather the tool - the jaw. Dunkleosteus were not the first to have jaws, but their variant was well developed and adapted to the lifestyle and gastronomic preferences of Dunkleosteus. Instead of teeth, in the modern sense of the term, these predators had large bony plates. The force of the jaw was approximately 5 MPa (like that of the Mississippi alligator), and the opening speed was 1/50 second, which allowed the predator to literally suck in prey.
Representatives of arthrodir, including dunkleostea, were more often than others the object for the study of ancient fish, as well as their anatomy. However, in order to understand the entire evolutionary path of the jaw, it is necessary to consider an even earlier version of it. Therefore, scientists decided to study the fossils of Acanthothoraci - another early group of fish, which is considered more primitive than arthrodiras and is closely related to the very first jaw vertebrates. But studying the fossils of Acanthothoraci has the same problem as studying the Arthrodiriformes - the lack of fossils and their fragmented nature.
The best sample to study was found about 100 years ago in the Czech Republic. Previously, it was extremely difficult to study it, since the bones were almost completely sealed in the rock, and its destruction would lead to damage to the sample.
Exterior of the ESRF research complex.
But in the hundred years since the discovery of this sample, much has changed. For example, in 1994, the ESRF research complex was built with a synchrotron 844.4 m long. This miracle of modern science produces powerful X-rays that can be used for non-destructive scanning of matter, including for the study of fossil fossils.
As the authors of the study note, the stem group Osteichthyans (bony fish) and most Chondrichthyans(cartilaginous fishes) have transversely arranged spiral teeth, while the crown group Osteichthyans have longitudinal dentition (diagram below).
Image # 1: Distribution of types of dentition among the maxillary teeth.
In arthrodir, new teeth are added in diverging rows from the predecessor tooth or the predecessor region. The number of rows varies greatly, and the arrangement of additional rows can be both labial (protruding) and lingual twelve (inward) or longitudinal.
Recent discoveries of the marginal corneous jaws and arthrodiral body anatomy in Entelognathus (423–416 Mya) and Qilinyu (419 Mya) led to the theory of jaw evolution, in which these two genera form a bridge between arthrodiral jaw shapes and bony fish jaws.
* Mya - million years ago.However, the dentition of Entelognathus and Qilinyu is still poorly understood.
In this work, scientists presented the results of a study of the dentition of Radotina , Kosoraspis , Tlamaspis (Czech Republic) and the only sample of the dentition Acanthothoraci (CPW.9 / Canadian Arctic Archipelago).
Research results
The mechanism for adding teeth to the CPW.9 row, which includes a pair of supragnatal plates on the snout, has previously been described as concentric. As a result, a detached dermal tessera ( scale- like) with concentrically positioned odontodes * from the same formation led to a misinterpretation of the specimen as the Acanthothoraci maxilla .
Odontodes * (horny teeth) are horny skin formations in some vertebrate species.Reanalysis of CPW.9 scans by microcomputer tomography (micro-CT) shows that the upper dental plates are located labially relative to the plate of the premedial cornea. Consequently, the addition of teeth is radial from the base of the lip area and not on the lip itself. Instead, the oldest small teeth are overgrown with larger horny teeth, as observed in the Andreolepis stem group .
Phase contrast microscopy has allowed scientists to detect previously unexplored dentition in Radotina , Kosoraspis and Tlamaspis . They are all significantly different from CPW.9.
Image # 2: the structure of the Radotina tesselata jaw. Radotina
teeth structure(images above) consists of four rows located on the ventral surface of the large horny cheekbone attached to the outer surface of the tent square (dorsal component of the mandible).
This ventral edge forms a longitudinal groove with teeth on the labial side, and rows of vascular grooves are located on the lower part of the groove. Thus, the dentition is uniquely marginal, located inside the edge of the jaw, but outside the tent square ( 2C ), and is separated from the outer part of the cheek by a clear labial edge of the exposed bone. Rows of teeth are added lingually (relative to the tongue) and a pronounced gradient of mineralization is visible from the oldest to the youngest teeth. The teeth have an elongated, star-like blade-like shape ( 2E). The shape of these teeth is similar to the shape of the marginal odontodes on the cheek tessera. However, unlike these odontodes, the teeth are fused into rows using separate basal connective tissue.
No signs of tooth resorption or replacement were found. However, a broken tooth in the lingual row itself clearly went through the process of restoration with the formation of a new crown in the damaged area.
Kosoraspis and Tlamaspis have several short, horny jaw bones on which teeth are located. They vary in length and shape according to their position, but all are associated with a faceplate. At Kosoraspisthe obverse plate bears dentate odontodes, which merge into pointed teeth on the oral plate, forming obliquely transverse spiral rows. They differ in shape from the star-shaped odontodes on the cheek.
Image # 3: the structure of the Kosoraspis peckai jaw.
Both odontodes and teeth are composed of pallial dentin, while circumpulpal dentin, which fills the pulp cavity in mature teeth, is tubular (tube-shaped) dentin tissue with fewer cellular spaces.
And here is the faceplate of Tlamaspismostly labially uncovered, with the exception of two or three rows of tubercles, which abruptly merge into one row of conical teeth, lingually delimited by a toothless ridge. This resembles the marginal jaw bones of a crown group of bony fish, although the teeth are not shed or replaced.
Image # 4: Tlamaspis inopinatus jaw structure.
Taking into account the phylogenetic analysis, Acanthothoraci are separated from the jawgroup node by four intermediate nodes, i.e. four intermediate stages of evolution. However, there is a clear similarity between the dentition of Acanthothoraci and bony fish.
It is noteworthy that multiple short horny jaws similar to those of Kosoraspis and Tlamaspis, are present in Lophosteus (an extinct genus of prehistoric bony fish). The cheek structure of Kosoraspis also strongly resembles that of primitive bony fish.
For a more detailed acquaintance with the nuances of the study, I recommend that you look into the report of scientists .
Epilogue
The above jaws of different types have a lot in common, despite the apparent variety of external characteristics, which are due to functional aspects.
Functionally, the CPW.9 dentition appears to be adapted for crushing, the Radotina dentition is adapted for cutting, and Kosoraspis and Tlamaspis are adapted for different styles of holding and piercing prey.
From an architectural point of view, the most striking difference was found between Radotina, which has teeth that resemble ornamental odontodes and are attached to the buccal plate, and three other taxa, each of which have separate horny jaws and teeth that do not resemble their horny ornament.
If we compare the dentition of Acanthothoraci andArthrodira , there are significant differences. Like sharks, bony fish and land animals, Acanthothoraci added new teeth exclusively on the inside: the oldest teeth are located at the edge of the jaw.
The bones on which the teeth are located also bear small dentin elements of the skin on their outer surfaces, characteristic of primitive bony fish, but not of Arthrodira . This distinctive feature is extremely important as it shows that the bones of the Acanthothoraci jaw were located right at the edge of the mouth, while those of the Arthrodira jaw were deeper.
It is also surprising that one representative of Acanthothoraci , namely Kosoraspis, there is a gradual transition from these dentin elements to the adjacent true teeth. On the other hand , Radotina's real teeth are almost identical to its dentin skin elements.
Such findings may indicate that real teeth were formed only recently from elements of dentin on the skin.
Consequently, Acanthothoraci , although they are an earlier and primitive genus of jaw vertebrates, it is their teeth that in many ways resemble modern teeth, in contrast to the more developed Arthrodira .
Evolution is a very complicated process that can hardly be called linear or predictable. The development of the jaws and teeth is a clear confirmation of this.
Thanks for your attention, stay curious and have a great weekend, guys! :)
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