British researchers have identified the first-known fossils of baby tyrannosaurs, offering a rare glimpse into the earliest stages of the giant reptile’s life. The groundbreaking study was published in the journal Biology.
Paleontologist Nick Longrich of the University of Bath in the UK announced the discovery, stating that his team found the “vanishingly rare” remains while searching museum collections for smaller fossils.
The team initially mistook one of the samples for the foot bone of an adult theropod dinosaur. However, further examination revealed it belonged to a baby T. rex. Scientists determined this by noting the bone’s porousness, a characteristic of immature dinosaurs with a dense network of blood vessels supporting growth.
Close examination of other bone and foot fossils led researchers to conclude that juvenile tyrannosaurs were considerably smaller than previously thought. They measured approximately 30 inches long and weighed around 5.5 pounds, comparable in size to a Yorkshire terrier. Newly hatched T. rex likely weighed about 4 pounds, smaller than earlier estimates that suggested a length of three feet from nose to tail.
While a complete T. rex egg has never been found, the newly identified remains indicate that the eggs were small and likely laid in large clutches of 20 to 30, similar to some turtle species. This suggests a reproductive strategy involving producing numerous offspring with relatively little parental care.
Longrich theorized that this strategy marked an evolutionary transition between reptile and mammal reproductive styles. "Avian [and] mammal intensive parental investment and care seems to evolve gradually in the Mesozoic," he stated. "At the same time [that] tyrannosaurs are evolving to care for larger and fewer young (relative to reptiles), we see mammals and plesiosaurs—even insects—making similar shifts. Parental investment strategies change a lot in the Jurassic and Cretaceous periods."