Paleontologists have revealed two separate fossils of embryonic tyrannosaurids. Dr. Gregory Funston of the University of Edinburgh recently detailed these specimens at the annual meeting of the Society of Vertebrate Paleontology.
The two fossils – a foot claw and a lower jawbone – represent some of the youngest specimens of tyrannosaurs known to science. Both have been dated to 71-75 million years ago, placing these creatures' brief lifespans sometime at the tail end of the Cretaceous Period. At this time, large, efficient-hunting theropods like the tyrannosaurids roamed across Asia and North America.
Embryonic skeletons, even fragmental ones as in this case, are rare and coveted for the scientific insights they offer regarding the appearance and growth of prehistoric creatures.
Funston was excited about the discovery, particularly the jawbone, saying that studying the fossil would offer a substantial amount of new information about juvenile tyrannosaurs.
Funston originally saw the jaw as a graduate student and, at first glance, did not believe it belonged to a tyrannosaur at all.
Upon further examination with a 3D scanner and comparison with extant tyrannosaur specimens, he changed his mind. The fact that the jaw belonged to embryonic tyrannosaurids was all the more engrossing.
Observing the minuscule teeth on the jawbone, scientists have speculated on the diet of tyrannosaur hatchlings. They likely fed on animals like insects and tiny lizards.
The two fossils were taken from locales that have become known for their concentration of dinosaur remains. The jawbone was excavated in 1983 from the Two Medicine Formation in Montana, and the tiny claw was extracted in 2018 from the Horseshoe Canyon Formation of Alberta, Canada.
Horseshoe Canyon has garnered notoriety in other recent headlines after a twelve-year-old boy and his dad were hiking in one of the canyon's conservation areas and discovered the bones of a 3 or 4-year-old hadrosaur.
Because other dinosaur species (including examples of young specimens) have been found at both of these sites, Funston deduces that tyrannosaurs likely could have built nests and laid eggs in the same general vicinity as other dinosaurs.
Estimates as to the size of the two tyrannosaurs suggest the jaw belonged to a specimen that was two and a half feet long, while the claw came from a specimen approximated to be a little over three feet long.
Given these dimensions, and taking into account the fact that the embryos would have been curled up inside their eggs, scientists have determined the length of the eggs to have been a whopping 17 inches (43 centimeters).
These fossils contribute to what we know about the natural development of tyrannosaurs and their nesting procedures. Funston is optimistic that more advancements in our understanding of tyrannosaurid development will be made in the years to come.