” We had the ability to reveal that the very first tusks belonged to animals that came before modern-day mammals, called dicynodonts,” said Kenneth Angielczyk, co-author and manager at Chicagos Field Museum. “Theyre very odd animals.”
Left side of the skull of the dicynodont Dolichuranus (NMT RB554) from Tanzania. The big tusk is visible at the lower left of the specimen. Credit: Ken Angielczyk
They are known for having an extremely peculiar arrangement of teeth. A defining function of these animals, initially discovered 176 years back, is the protruding tusks in their upper jaws. Most had two upper tusks that came down from the canine position, but they hardly ever had additional teeth.
The researchers were taking a lunch break during a paleontological dig when they understood for the study. “We were being in the field in Zambia, and there were dicynodont teeth all over,” recalls Whitney. “I keep in mind Ken selecting them up and asking how come they were called tusks, due to the fact that they had features that tusks dont have.”
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 needed to define a tusk, since its a surprisingly ambiguous term,” said Whitney The researchers identified that for a tooth to be a tusk it must extend out from the mouth, be made totally of dentine– doing not have enamel discovered on many mammals teeth, and is ever-growing.
The researchers carried out paleohistology (the research study of fossil tissues) on paper-thin pieces of fossilized teeth from 19 dicynodont specimens, representing 10 different types. They used micro-CT to take a look at how the teeth connected to the skull and to see if there was any proof of constant growth.
A few of the dicynodont tusks that the group observed in Zambia didnt appear to fit the meaning of a tusk either– they were covered in enamel rather of dentine. “There are lots of different sort of dicynodonts and they appear to mainly all have tusks,” stated Whitney, “nevertheless, when you take a look at the micro structural information theyre very various in those groups.” Enamel teeth are tougher 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 cant have a total enamel covering. Animals like humans progressed hard-to-fix but resilient teeth– there is no replacement for the loss of an adult tooth. Tusks are less resilient than enamel-coated teeth, but they grow continuously, even if they get harmed. “Enamel-coated teeth are a various evolutionary technique than dentine-coated tusks, its a trade-off,” states Whitney.
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 brightened under polarized light. Credit: Image by M. Whitney.
Examining the histological thin areas of dicynodont specimens from South Africa, Antarctica, Zambia, and Tanzania the scientists found that, much like human teeth, these animals appeared to decrease the amount of replacement teeth at the canine position and had a soft tissue accessory to the jaw. Interestingly, this is a mix of features that is unique to mammals. Mammals, like humans, replace infant teeth with adult teeth only once unlike many other vertebrates– for circumstances sharks have continuous teeth production. Mammal teeth are connected to the jaw by gomphosis which is a soft-tissue, or ligament, attachment. Most vertebrate teeth, however, are attached to the jaw by ankylosis, which is a hard-tissue combination of bone to tooth.
” If you have these two things, a reduced quantity of tooth replacement and a soft-tissue accessory, an ever-growing tooth enables the animal to navigate the fact that it can not change the tooth. Rather it develops to constantly deposit the very same tooth tissues,” said Whitney. “And as the animal continues to deposit the tissue, the tooth begins to move outside of the mouth to end up being practical.”
An example of an ever-growing, real tusk in the dicynodont Lystrosaurus. When researchers cut into the face of Lystrosaurus, the root of the tusks is made up of a broad open pulp cavity that suggests dentine was constantly being deposited. Credit: Image by M. Whitney
The researchers discovered that true tusk evolution just took place at a later phase of evolution in this group– early members of this group had a huge tooth rather than a real tusk. Late in their evolutionary history dicynodonts developed a real tusk that was ever growing, and remarkably did so convergently in numerous different sort of dicynodonts. “I sort of anticipated there to be one point in the family tree where all the dicynodonts began having tusks, so I thought it was quite stunning that we in fact see tusks evolve convergently,” said Whitney. “This is a comparable story to what we see in elephant development because it mirrors a lot of the patterns that have actually been studied on how elephants got their tusks.”
The fact that in reality only a few have real tusks, and the rest have big teeth, is a gorgeous example of evolution we can document.
The dentine core of the tooth is in grey and the capping enamel is revealed in blue/purple. This enamel covering suggests that this caniniform tooth is more like a routine tooth than a real ever-growing tusk.
The researchers say that the study, which reveals the earliest recognized circumstances of true tusks, might assist scientists better understand how evolution works.
” Tusks have evolved a number of times, that makes you question how– and why? We now have excellent data on the anatomical modifications that required to happen for dicynodonts to evolve tusks. For other groups, like walruses or warthogs, the jury is still out,” said co-author Christian Sidor, curator at the University of Washington Burke Museum.
The numerous type of teeth animals have evolved can inform scientists about the pressures those animals dealt with that might have produced those teeth. Tusks can function in a variety of methods including defense, competition, burrowing, sexual selection, and even assist with locomotion– as in the walrus which utilizes its tusks to raise itself upon to the ice from the water. A continuously growing tusk may have permitted these dicynodonts to get rid of the challenges of just having one set of replacement teeth throughout their lives.
” We do not actually know what functions the dicynodonts tusks may have had because we cant observe them and see what they were finishing with them,” stated Whitney. “Thats a lingering concern about dicynodonts, a lot more so now.”
” Dicynodont tusks can tell us a lot about mammalian tusk advancement in basic,” says Angielczyk. “For instance, this research study shows that minimized rates of tooth replacement and a versatile ligament attaching the tooth to the jaw are needed for real tusks to evolve. Everything ladders approximately giving us a better understanding of the tusks we see in mammals today.”
Referral: “The advancement of the synapsid tusk: insights from dicynodont therapsid tusk histology” 27 October 2021, Proceedings of the Royal Society B.DOI: 10.1098/ rspb.2021.1670.
The size of the animal and their tusks can vary they all have one distinct thing in common in that they are only discovered on mammals– there are no recognized fish, reptiles, or birds with tusks. Some of the dicynodont tusks that the group observed in Zambia didnt appear to fit the meaning of a tusk either– they were coated in enamel instead of dentine. The scientists discovered that real tusk development just took place at a later stage of advancement in this group– early members of this group had a huge tooth rather than a true tusk. “I kind of anticipated there to be one point in the family tree where all the dicynodonts started having tusks, so I believed it was pretty shocking that we really see tusks evolve convergently,” stated Whitney.” Dicynodont tusks can tell us a lot about mammalian tusk advancement in basic,” says Angielczyk.
Life reconstruction of the dicynodont Dicynodon. Aside from the tusks in the upper jaw, many dicynodonts had a turtle-like beak that they used to chew their food. Image by Marlene Hill Donnelly Credit: Marlene Hill Donnelly.
Many other animals have tusks consisting of warthogs, hippopotamuses, Arctic-dwelling walruses, and even a five-pound, guinea pig looking animal called hyraxes. The size of the animal and their tusks can differ they all have one unique thing in typical in that they are just found on mammals– there are no recognized fish, reptiles, or birds with tusks.
In a new paper released October 27, 2021, in the Proceedings of the Royal Society B scientists trace the very first tusks back to ancient mammal relatives that lived prior to the dinosaurs and clarified the evolution of mammalian tusks by first defining what makes a tusk a tusk.
” Tusks are this really popular anatomy, but until I began dealing with this study, I never really considered how tusks are limited to mammals,” said lead author Megan Whitney, postdoctoral fellow in the Department of Organismic and Evolutionary Biology, Harvard University.
