In 1942, the Cleveland Museum of Natural History
organized a fossil-hunting expedition through southeastern Montana. There, the crew prospected the Hell Creek
Formation, a world-famous deposit dating back to the very late Cretaceous Period - one that’s
known for fossils of Tyrannosaurus rex. And on that trip, they found a fossil that
sure looked like a T. rex. Clearly it belonged to a member of the same
family. Whatever this thing was, it lived at the same
time--and in the same place--as T. rex did. But at just 57 centimeters long, the skull
was less than half the size of the biggest known Tyrannosaurus heads. Today, some paleontologists call this fossil
the “Cleveland Skull.” And it wasn’t the last odd find from Montana. In the early 2000s, scientists found another
Tyrannosaurus-like fossil, which they nicknamed “Jane.” This one was a partial skeleton. Except, Jane didn’t look like your typical
T. rex, either. Its limbs were proportionately longer. And, like the Cleveland skull, it had narrower
teeth than a full-sized T. rex. And it was only about 6.5 meters long - much
smaller than a 12.3-meter adult T. rex. Which makes Jane a very unusual creature. See, in some parts of the dinosaur fossil
record, you’ve got your big carnivores and your small carnivores, without much in between
- call it the “carnivore gap.” The remains of medium-sized predatory dinosaurs
--animals weighing between 100 and 1000 kilograms-- are actually pretty rare in places where giant
predators like T. rex existed. Which is weird, because that’s just not
how ecosystems work today. For example, if South Africa’s Kruger National
Park had the same “carnivore gap” as some dinosaur communities did, then every meat-eating
mammal species at the park would either be lion-sized or no bigger than the 4-kilogram
bat-eared fox. So what’s going on here? How can we explain this dino size gap? Well, some experts think that predators like
T. rex went through some growing pains. And if that’s true, maybe the medium-sized
dinos we’ve been looking for were hiding in plain sight all along. Now at most dig sites around the world, small
to mid-sized dinosaurs in general aren’t very common. That goes for carnivores, herbivores, and
omnivores alike. One contributing factor might be the fossilization
process itself, because preservation issues can create biases in the fossil record. If you’re a small animal with little bones,
your skeleton is more likely to get broken up, eroded away, or picked apart by scavengers. That means big animals can be overrepresented
at fossil sites. Another problem is rock availability. We don’t have many fossil outcrops from
certain points in geologic time, so we know less about dinosaurs from those times. But it looks like there’s more to this dinosaur
size mystery. In 2012, two paleontologists analyzed the
published size data for over 2400 extinct animals, including 329 species of non-avian
dinosaurs. And they compared this to the available info
on living animals. Within all but one of the major animal groups
these authors looked at, they found that small species are way more diverse than large ones. Which makes sense - small animals tend to
need less food and less space, so it’s easier for big populations of them to coexist with
each other. But it looks like the non-avian dinosaurs
may have bucked this trend. The researchers found that small-bodied, non-avian
dinosaurs are vastly outnumbered by big ones in the fossil record. And if we blamed this all on preservation
bias… well, we’d be missing a whole lot of dinosaurs. For example, let’s assume the nonavian dinosaurs
had the same ratio of big species to small ones as the extinct land mammals of the Cenozoic
era did. If that were the case, then by the calculation
in that study, about 90% of all extinct dinosaurs weighing under 60 kilograms that
ever existed are completely absent from the global fossil record. Which is highly improbable, according to the
authors. It’s possible that nonavian dinos as a group
just naturally skewed larger than land mammals and other animals do. That said, we do still find the bones and
skeletons from some small dinosaurs. At the end of the Cretaceous Period, when
T. rex stalked North America, we know there were some little carnivores like Acheroraptor
and Saurornitholestes running around underfoot. Both probably weighed less than 25 kilograms. But again, medium-sized predatory dinosaur
bones are very uncommon in rocks where Tyrannosaurus fossils are found. And that’s where Jane and the Cleveland
skull come in. Many experts think that they were both juvenile
T. rex - Jane’s probably a juvenile something at least. One study from 2020 on the microstructure
of Jane’s femur and tibia found that it was only about 13 years old when it died and
other studies estimate that T. rex could potentially live into their late twenties. And Jane was a lot lighter than the oldest,
heaviest T. rex specimens on record. Weighing around 660 to 954 kilograms, it definitely
qualified as a “medium-sized” theropod -- and so did the smaller Cleveland skull’s
owner. Now those two animals--and a few similar fossils
that’ve turned up over the years--look very different from the huge T. rex specimens paleontologists
are used to finding. So much so that some authors think they represent
an entirely different species. Supporters of this idea say Jane and her midsized
peers were a kind of “dwarf” tyrannosaur that’s been called “Nanotyrannus lancensis.” This would be really significant, because
if “Nanotyrannus” existed, it would help plug the carnivore gap we see in Late Cretaceous
North America. The smaller animal would’ve likely filled
a different ecological niche than T. rex, its contemporary cousin. However, many recent studies don’t think
“Nanotyrannus” is a separate genus, or even a valid dinosaur. Experts on this side of the debate tend to
identify Jane and the Cleveland skull as young Tyrannosaurus rex. And if that’s the case, then T. rex dramatically
changed shape with age. Proportionately, Jane and Jane-like skeletons
have longer legs than full-sized T. rex. And anatomical research published in 2020
found that Jane would have been a better sprinter--and more agile overall-- than a grown Tyrannosaurus. But the authors of that same paper also found
mature T. rex were better “marathon” walkers - they could travel more efficiently over
great distances. Also, with its narrower jaws and blade-shaped
teeth, Jane probably couldn’t bite as hard as the big guys did. And mathematical models back this up. Paleontologists have calculated that while
Jane’s maximum bite force was around 2,400 to 3,850 Newtons, a full-sized T. rex could
exert a bone-splintering 64,000-plus Newtons when it closed its jaws. Yikes! So, if Jane and the Cleveland skull were juvenile
T. rex, then the teenagers in this species were better sprinters and more agile than
the grownups, but their bites were weaker. The adults may have been slower and less agile,
but at least they had bone-crushing jaws to make up for it. Take all this information together, and it
starts to look like young and old T. rex were built for two very different lifestyles. It’s possible that the speedier juveniles
hunted small herbivores, like the roughly 2-meter Leptoceratops or the 4.5-meter Pachycephalosaurus--while
bite marks on fossils tell us the adults ate big game, such as Triceratops. So it looks like we’ve got a scenario where
T. rex adults filled the role of “giant predator” in Late Cretaceous North America,
and their teenage offspring acted as mid-sized predators. And maybe that helps explain the “carnivore
gap” in that environment. Maybe the reason why we don’t really find
adult, meat-eating dinosaurs of medium size in Tyrannosaurus’ ancient habitat is because
those animals would’ve faced stiff competition from juvenile T. rex. So, ecologically, one species did double duty. Basically, a single population of T. rex would’ve
occupied two different niches at the exact same time, all thanks to physical differences
between the age groups. Ecologists call this ontogenetic niche shifting. Komodo dragons do the same thing today. Young Komodos weighing under 20 kilograms
prefer smaller prey than adults do and they actively wander around foraging for food. Whereas grown dragons are “sit and wait”
carnivores who prefer to hang out and let food come to them. Their niche changes with age. And just like dinosaurs, komodos lay eggs. Egg-laying explains the huge size differences
between the most colossal dinosaurs and their hatchlings. Physically, eggs can only get so large. Bigger eggs need thicker shells for protection,
but if the shell gets too thick, the embryo inside won’t get enough oxygen. The biggest nonavian dinosaur eggs we’ve
ever found are around 61 centimeters long. But the vast majority are a lot smaller, with
most dino eggs probably weighing under 10 kilograms in life. And here’s the thing. Remember how I mentioned the mammals of Kruger
National Park way back at the beginning? If we compare giant dinos hatching out of
very small eggs to a mammal like an African elephant, we see much greater size changes
in dinosaurs from newborns to adults than we see in most mammals. Because of that, hatchlings and juveniles
of some extra-large dinosaurs couldn’t have eaten the same things their parents did. And these dinos would’ve changed in size,
shape, and diet a lot more throughout their lives than mammals do today - passing through
more different niches along the way. Now this didn’t just affect Tyrannosaurus
rex. The hadrosaur Edmontosaurus got dramatically
larger with age and so did Camarasaurus, an herbivore that lived in the Jurassic Period. Juvenile Camarasaurus also had proportionately
shorter necks than grownups did, which could point to differences in lifestyle and behavior
between the age groups. Bottom line: growing up is hard. Physics put huge constraints on baby dinosaurs,
from T. rex to Camarasaurus to Edmontosaurus. So as some of these little newborns matured,
and new dietary options became available, they changed proportions and took on different
niches. And by recognizing this - by thinking about
these dinosaurs as living animals that actually grew up - we may have finally solved the mystery
of the missing, medium-sized dinosaurs. We'd like to thank Dr. Thomas R. Holtz Jr.,
paleontologist at the University of Maryland, for his help with today’s episode. And thanks to this month’s dino-mite Eontologists:
Sean Dennis, Jake Hart, Annie & Eric Higgins, John Davison Ng, and Patrick Seifert! By becoming an Eonite at patreon.com/eons,
you can get fun perks like submitting a joke for us to read, like this one from Jules What did the T. Rex say to the Pachycephalosaurus? “Don’t be such a bone head!” And as always thanks for joining me in the
Konstantin Haase studio. Subscribe at youtube.com/eons for more evolutionary
escapades.
