Fossil Dental Exams Reveal How Tusks Evolved– Originated in “Very Weird Animals” Called Dicynodonts

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Dicynodont Illustration

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Life restoration of the dicynodontDicynodon Aside from the tusks in the upper jaw, many dicynodonts had a turtle-like beak that they utilized to chew their food. Image by Marlene HillDonnelly Credit: Marlene Hill Donnelly

Most individuals visualize an elephant when thinking of animals with tusks. But lots of other animals have tusks consisting of warthogs, hippopotamuses, Arctic- house walruses, and even a five-pound, guinea pig looking animal called hyraxes. Though the size of the animal and their tusks can differ they all have one distinct thing in typical because they are just discovered on mammals– there are no recognized fish, reptiles, or birds with tusks. Despite being a renowned function of modern-day and fossil mammals the secret stays of what evolutionary actions resulted in the advancement of this oral phenomenon and why are mammals the only animals today with tusks?

In a brand-new paper released October 27, 2021, in the Proceedings of the Royal Society B scientists trace the very first tusks back to ancient mammal family members that lived prior to the dinosaurs and clarified the development of mammalian tusks by very first specifying what makes a tusk a tusk.

“Tusks are this very famous anatomy, but until I started working on this study, I never really thought about how tusks are restricted to mammals,” stated lead author Megan Whitney, postdoctoral fellow in the Department of Organismic and Evolutionary Biology, Harvard University.

“We were able to show that the first tusks belonged to animals that came before modern mammals, called dicynodonts,” stated Kenneth Angielczyk, co-author and manager at Chicago’s FieldMuseum “They’re very weird animals.”

Dicynodont Fossil Skull

Left side of the skull of the dicynodont Dolichuranus (NMT RB554) fromTanzania The big tusk shows up at the lower left of the specimen. Credit: Ken Angielczyk

Dicynodonts, though not mammals, are far-off family members and are more carefully associated to mammals than dinosaurs and other reptiles. Dicynodonts lived in between 270–201 million years back and consisted of a varied variety of animals from small rat-like dicynodonts to big elephant-sized dicynodonts. They are understood for having a really strange plan of teeth. A specifying function of these animals, initially found 176 years back, is the extending tusks in their upper jaws. Most had 2 upper tusks that boiled down from the canine position, however they hardly ever had extra teeth. Instead, dicynodonts had a beak at the front of their mouths that was made from keratin and looked like a turtles beak.

The scientists were taking a lunch break throughout a paleontological dig when they understood for the research study. “We were sitting in the field in Zambia, and there were dicynodont teeth everywhere,” remembersWhitney “I remember Ken picking them up and asking how come they were called tusks, because they had features that tusks don’t have.”

Dicynodont Skull

A dicynodont skull still in the ground that is broken to expose the roots of their tusks/teeth (the white circular structures). Credit: Photo by K. Angielczyk

Not all extending teeth are technically tusks. “For this paper, we had to define a tusk, because it’s a surprisingly ambiguous term,” statedWhitney The scientists figured out that for a tooth to be a tusk it need to extend out from the mouth, be made completely of dentine– doing not have enamel discovered on many mammals’ teeth, and is ever-growing.

The scientists carried out paleohistology (the research study of fossil tissues) on paper-thin pieces of fossilized teeth from 19 dicynodont specimens, representing 10 various types. They utilized micro-CT to analyze how the teeth connected to the skull and to see if there was any proof of constant development.

Some of the dicynodont tusks that the group observed in Zambia didn’t appear to fit the meaning of a tusk either– they were covered in enamel rather of dentine. “There are many different kinds of dicynodonts and they appear to mostly all have tusks,” stated Whitney, “however, when you look at the micro structural details they’re very different in those groups.” Enamel teeth are harder than dentine however since of the geometry of how teeth grow in the jaw, if you desire teeth that keep growing throughout your life, you can’t have a total enamel covering. Animals like human beings developed long lasting however hard-to-fix teeth– there is no replacement for the loss of an adult tooth. Tusks are less long lasting than enamel-coated teeth, however they grow continually, even if they get harmed. “Enamel-coated teeth are a different evolutionary strategy than dentine-coated tusks, it’s a trade-off,” states Whitney.

Enlarged Caniniforms of Diictodon Have Enamel

The bigger caniniforms of Diictodon have enamel making them more like teeth than tusks. The cross-section exposes a ring of enamel around the beyond the tusk that is lit up under polarized light. Credit: Image by M. Whitney

Analyzing the histological thin areas of dicynodont specimens from South Africa, Antarctica, Zambia, and Tanzania the scientists discovered that, similar to human teeth, these animals appeared to lower the quantity of replacement teeth at the canine position and had a soft tissue accessory to the jaw. Interestingly, this is a mix of functions that is distinct to mammals. Mammals, like human beings, change primary teeth with adult teeth just as soon as unlike many other vertebrates– for example sharks have constant teeth production. Mammal teeth are connected to the jaw by gomphosis which is a soft-tissue, or ligament, accessory. Most vertebrate teeth, nevertheless, are connected to the jaw by ankylosis, which is a hard-tissue combination of bone to tooth.

“If you have these two things, a reduced amount of tooth replacement and a soft-tissue attachment, an ever-growing tooth allows the animal to get around the fact that it cannot replace the tooth. Instead it evolves to continuously deposit the same tooth tissues,” statedWhitney “And as the animal continues to deposit the tissue, the tooth begins to move outside of the mouth to become functional.”

Dicynodont Lystrosaurus

An example of an ever-growing, real tusk in the dicynodontLystrosaurus When scientists cut into the face of Lystrosaurus, the root of the tusks is made up of a broad open pulp cavity that recommends dentine was continually being transferred. Credit: Image by M. Whitney

The scientists discovered that real tusk development just took place at a later phase of development in this group– early members of this group had a huge tooth instead of a real tusk. Late in their evolutionary history dicynodonts developed a real tusk that was ever growing, and remarkably did so convergently in several various sort of dicynodonts. “I kind of expected there to be one point in the family tree where all the dicynodonts started having tusks, so I thought it was pretty shocking that we actually see tusks evolve convergently,” statedWhitney “This is a similar story to what we see in elephant evolution in that it mirrors a lot of the patterns that have been studied on how elephants got their tusks.”

“Dicynodonts were the most abundant and diverse vertebrates on land just before dinosaur times, and they’re famous for their ‘tusks.’ The fact that in reality only a few have true tusks, and the rest have big teeth, is a beautiful example of evolution we can document. We can see how to build a tusk!” stated co-author Brandon Peecook, manager at the Idaho Museum of Natural History.

Dicynodont Canine Cross Section

Cross- area through a dicynodont dog under cross-polarized light. The dentine core of the tooth remains in grey and the topping enamel is displayed in blue/purple. This enamel covering recommends that this caniniform tooth is more like a routine tooth than a real ever-growing tusk. Credit: Photo by M. Whitney

The scientists state that the research study, which reveals the earliest recognized circumstances of real tusks, might assist researchers much better comprehend how development works.

“Tusks have evolved a number of times, which makes you wonder how—and why? We now have good data on the anatomical changes that needed to happen for dicynodonts to evolve tusks.  For other groups, like warthogs or walruses, the jury is still out,” stated co-author Christian Sidor, manager at the University of Washington Burke Museum.

The numerous sort of teeth animals have actually developed can inform researchers about the pressures those animals dealt with that might have produced those teeth. For circumstances tusks can work in a range of methods consisting of defense, competitors, burrowing, sexual choice, and even help with mobility– as in the walrus which utilizes its tusks to raise itself upon to the ice from the water. A continually growing tusk might have permitted these dicynodonts to get rid of the difficulties of just having one set of replacement teeth throughout their lives.

“We don’t really know what functions the dicynodonts tusks may have had because we can’t observe them and see what they were doing with them,” statedWhitney “That’s a lingering question about dicynodonts, even more so now.”

“Dicynodont tusks can tell us a lot about mammalian tusk evolution in general,” statesAngielczyk “For instance, this study shows that reduced rates of tooth replacement and a flexible ligament attaching the tooth to the jaw are needed for true tusks to evolve. It all ladders up to giving us a better understanding of the tusks we see in mammals today.”

Reference: “The evolution of the synapsid tusk: insights from dicynodont therapsid tusk histology” 27 October 2021, Proceedings of the Royal Society B
DOI: 10.1098/ rspb.20211670