Tyrannosaurus rex Baby Tooth and Sub-adult Tooth

Our highlighted fossil this month is not one, but two of our personally collected Tyrannosaurus rex teeth from South Dakota--one of which comes from a baby! 

Twice a year we go on a trip to Harding, South Dakota to dig and collect fossils from the Hell Creek Formation. The area we dig at is presumably a prehistoric river that once flowed into the Western Interior Seaway (a large Cretaceous-aged sea that divided North America into three sections) because we find numerous fossils from countless different freshwater animals and plants like fish, birds, pinecones, freshwater crocodiles, leaves, turtles, and amphibians. 

The species or bones to find are anyone’s guess as it appears the river pushed the fossils into random deposits, so you could find a massive, fragmented tibia from an Edmontosaur laying next to a small, extremely delicate skull that somehow emerged unbroken. However, roughly 95% of the fossils we find comes from both adult and juvenile Edmontosaur, or “Duck-billed dinosaurs.” This may be because a herd of these reptiles drowned while crossing the river or got caught in a flood. 

One lucky day on our last trip in October 2020, I was clearing the area to work on an Edmontosaur pubis bone when I saw a tiny tooth stuck in the matrix with it. I found out that this miniscule, half-inch long tooth probably belonged to a newly hatched or embryonic Tyrannosaurus rex--an almost unheard of fossil. It most likely was a tooth shed as T-rex was known to shed its teeth throughout its lifetime as it constantly grew newer and stronger teeth to replace the old and broken ones. 

I compared this newly-hatched T-rex tooth with a 12 ⅞ -inch sub-adult T-rex tooth I had uncovered years prior in the same site and in a similar way. I was working on getting a Duck-billed dinosaur tibia from out of the ground when I found a partially cracked T-rex tooth laying on the bone. I removed this tooth and flipped the tibia over when I saw another Tyrannosaur tooth (this one) laying undisturbed underneath it. Like the baby tooth, this adult tooth was also probably a shed. It has not been repaired or prepared at all (as visible by the matrix still in the base of it) other than a bit of glue at the base so it does not break. 

Tyrannosaurus rex was one of the largest and most fearsome theropods to walk western North America during the Cretaceous. Believed to grow over 40 feet in length and 12 feet high, it is thought this carnivore could weigh up to 9 tons. The sharp, knife-like teeth of T-rex indicate it did not chew its meat, but swallowed it whole. Its powerful jaw held 60 teeth that were an average of 9 inches each with a bite force that could easily go through bone. Although undoubtedly feared as a predator, T-rex was foremost a scavenger that followed its nose to lead it to dead animals. 

This predator sometimes lived in family groups, but it is still unknown whether parents stayed and cared for the estimated 20 elongated eggs and/or young as no fossil eggs or nests have been discovered. The newly-hatched Tyrannosaurus was probably the size of a very small turkey, yet grew at an exponential rate until it reached maturity at about 20 years. However, the mortality rate was considerably high due to disease and starvation, so if a T-rex hatchling made it past its first year then it had a great chance of reaching adulthood. 

Comparing the sizes of these two teeth helps with the understanding of the growth and development of the great tyrant lizard king from birth to adulthood. Both fossils have their serrated edges and glossy enamel intact and neither have been repaired, serving as excellent specimens of one of the most feared animals in history. 

 

Newborn Tyrannosaurus rex Tooth
Late Cretaceous
Hell Creek Formation
Harding County, South Dakota
Personally collected



Sub-adult Tyrannosaurus rex Tooth
Late Cretaceous
Hell Creek Formation
Harding County, South Dakota
Personally collected