A big debate about small Tyrannosaurus rexes has been raging among the palaeontological community for decades. The debate rests on remains found in the US that are less than half the size of a full-grown T. rex: are these really juvenile T. rexes we’re looking at? Or are they a separate species? The opposing contender, some paleontologists claim, is Nanotyrannus lancensis, which roughly translates to “pygmy or dwarf tyrant”. Now, new research argues this is indeed what we’re looking at in the case of many “juvenile T. rex” specimens.
The first Nanotyrannus was discovered in 1942 in Montana based on a specimen now known as Chomper that was initially identified as
The scientific community has since gone
“As animals go from juveniles to adults, many things change,” explained palaeontologist at London’s Natural History Museum,
As Dr Maidment pointed out, Nanotyrannus does have features that are different from T. rex, but whether these are because it was not fully mature when it died, or because they’re different species is incredibly difficult to determine without a complete growth series of animals that we’re certain belong to T. rex.
“As juveniles grow rapidly (especially in dinosaurs, where they hatched out of relatively
“We can determine how fast it was growing by cutting up the bones and looking at them down a microscope, because bone tissue is organised differently in fast-growing animals vs. those in which growth had slowed or virtually ceased. The problem is that when a fossil is extremely significant, we might not want to cut it up, and sometimes the growth lines aren’t clearly preserved.”
New research from scientists at the University of Bath and University of Chicago states that Nanotyrannus is a distinct species as the fossils show it has unique traits not seen in T. rex. Were it the case that “Nanotyrannus” were actually just juveniles waiting to develop into adults, we would expect to see some crossover.
Furthermore, they state the growth rings in Nanotryannus also prove it wasn’t a juvenile T. rex. That’s because they are more closely packed towards the outside of the bone, indicating their growth rate was slowing down as it does in adulthood. Were it a fast-growing juvenile, we would expect the growth rings to be regularly spaced.
By modeling this data they were able to show that Nanotyrannus would’ve reached its maximum adult size at around 5 meters (16 feet) tall and weighing between 900 to 1,500 kilograms (1,984 to 3,307 pounds), making it just 15 percent the size of an adult T. rex, which as an adult weighed a hefty 8,000 kilograms (17,637 pounds).
“When I saw these results I was pretty blown away,” said Dr Nick Longrich in a
“We tried modelling the data in a lot of different ways and we kept getting low growth rates. This is looking like the end for the hypothesis that these animals are young T. rex,” he concluded.
So, if the proof was in the growth rings all along, why did it take so long to get here? As Dr Maidment explained, there are a lot of complexities when it comes to
“[D]ifferent bones appear to preserve growth records to different degrees,” Maidment continued. “Hind limb bones are generally considered to be the best for recording a growth record, but we might not want to cut up a very nicely preserved limb bone, and it’s not totally clear whether other bones, like ribs, for example, record an accurate record of growth.”
“Some dinosaur groups don’t appear to have deposited growth rings, growing continuously throughout their lives (this is especially true for some of the largest), while some record periods of slow growth, but it’s not clear that these are annual. Finally, we don’t really have a good grasp on whether or how much growth rate was affected by environment, and it’s likely that this varied in different dinosaur groups depending on their size, metabolism and life history strategies.”
As we’ve seen with the ongoing debate as to whether Spinosaurus was a “
So, for now, are you team juvenile T. rex? Or does tiny tyrant Nanotyrannus rule?
The study is published in