Baby Tyrannosaurs Born Ready to Hunt

A new scientific paper published this week suggests that tyrannosaurs were able to hunt and to look after themselves soon after they hatched.  In addition, tyrannosaur hatchlings were surprisingly large, perhaps more than a metre long when they broke out of their eggs and if this the case, then tyrannosaur eggs would have been colossal, perhaps larger than any other dinosaur egg known to science.

A Life Reconstruction of a Baby Tyrannosaur

A life reconstruction of a juvenile tyrannosaur.  This illustration by the talented palaeoartist Julius Csotonyi, depicts a baby tyrannosaur covered in a coat of insulating protofeathers.

Picture credit: Julius Csotonyi

As Big as a Collie Dog

Writing in the latest edition of the Canadian Journal of Earth Sciences, the scientists which include such eminent figures as Phil Currie, “Jack” Horner and Stephen Brusatte, have written up an on-line presentation from last October which took place at the virtual Society of Vertebrate Palaeontology Conference and they indicate that young tyrannosaurs were big babies.  With a length of in excess of 1 metre, that’s about the size of a border collie dog.

A Model of a Young Tyrannosaurus rex

A young T. rex.  Research suggests that Late Cretaceous tyrannosaurs may have been around a metre in length when very young.  Rare fossil bones from perinatal tyrannosaurs from North America also suggest that these predators were highly developed and capable of hunting for themselves – precocial development – mobile and relatively fully developed when first hatched.

The model (above) is from the CollectA Prehistoric Life Age of Dinosaurs range.

To view this range: CollectA Prehistoric Life Figures.

Perinatal Tyrannosaurid Bones and Teeth

Perinatal tyrannosaurid bones and teeth from the Campanian–Maastrichtian of western North America provide the first window into this critical period of the life of a tyrannosaurid.  An embryonic dentary (Daspletosaurus horneri) from the Two Medicine Formation of Montana, measuring just 3 cm in length, already exhibits distinctive tyrannosaurine characters like a “chin” and a deep Meckelian groove, and reveals the earliest stages of tooth development.  When considered together with a remarkably large embryonic claw bone (ungual) from the Horseshoe Canyon Formation of Alberta and believed to have come from an Albertosaurus sarcophagus, a minimum hatchling size for tyrannosaurids could be estimated by the research team.

Corresponding author for the paper, Gregory Funston (University of Edinburgh), stated:

“It appears that tyrannosaurs were born ready to hunt, already possessing some of the key adaptations that gave tyrannosaurs their powerful bites.  So, it’s likely that they were capable of hunting fairly quickly after birth, but we need more fossils to tell exactly how fast that was.”

Tyrannosaur Babies Bigger than Other Dinosaur Babies

The dentary and the claw bone indicate that Late Cretaceous tyrannosaurs were bigger than any other known dinosaur babies.  The researchers conclude that they must have hatched from enormous eggs, perhaps exceeding the 43 cm length of largest dinosaur eggs described to date.

The Embryonic Tyrannosaur Dentary

The fossilised lower jawbone (dentary) of a Daspletosaurus horneri, one of the first baby tyrannosaurs ever discovered.

Picture credit: Gregory F. Funston (University of Edinburgh)

Studying Embryonic Fossil Material

Co-author of the paper, Mark Powers a PhD student at the University of Alberta (Canada), commented:

“Tyrannosaurs are represented by dozens of skeletons and thousands of isolated bones or partial skeletons, but despite this wealth of data for tyrannosaur biology, the smallest identifiable individuals are aged three to four years old, much larger than when they would have hatched.  No tyrannosaur eggs or embryos have been found even after 150 years of searching—until now.”

The study, focused on the two fossils representing perinatal development of tyrannosaurids.  The ungual was found near Morrin in the province of Alberta, whilst the dentary came from Montana. The ungual is approximately 71.5 million years old, and the jawbone a little older at around 75 million years old.

Comparing the Tyrannosaurid Fossil Material with Dr Funston and an Adult Albertosaurus

This diagram compares the size of a full-grown Albertosaurus with that of palaeontologist Greg Funston and the two dinosaur embryos whose toe claw and jawbone were identified in a newly published study.

Picture credit: Gregory F. Funston (University of Edinburgh)

Mark Powers, who completed the research as a master’s student supervised by Phil Currie added:

“The discovery of embryonic material is a huge find in our efforts to understand how some of the most popular and charismatic dinosaurs began their life and grew to immense sizes.  It provides a much-needed—and until now, missing—data point depicting the starting point for tyrannosaur growth.”

Surprising Results

The researchers were surprised to find that the small tyrannosaur teeth in the lower jaw were distinct from the teeth of older tyrannosaurids.  They had not developed true serrations running along the cutting edges.  In addition, the toe claw (specimen number UALVP 59599), came from an animal estimated to be about 1.1 metres long, whilst the tiny jawbone (MOR 268), came from a tyrannosaur around 71 cm in length.

The size estimates for perinatal tyrannosaurs based on this study reinforce the work of the late American-Canadian palaeontologist Dale Russell, who back in 1970 provided some of the first insights into tyrannosaur development and ontogeny.  This study was published in a special issue of the Canadian Journal of Earth Sciences which honours the contribution made to vertebrate palaeontology by Professor Russell.

Everything Dinosaur acknowledges the assistance of a media release from the University of Alberta in the compilation of this article.

The scientific paper: “Baby tyrannosaurid bones and teeth from the Late Cretaceous of western North America” by Gregory F. Funston, Mark J. Powers, S. Amber Whitebone, Stephen L. Brusatte, John B. Scannella, John R. Horner and Philip J. Currie published in the Canadian Journal of Earth Sciences.

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Fossil of Dinosaur Sitting on Eggs (Late Stage Incubation)

Fossils of dinosaurs are rare, fossils of articulated dinosaur skeletons rarer still and any fossils that preserve evidence of behaviour, these are amongst the rarest of all, truly scientific treasures.  A newly published paper describes the fossilised remains of a Late Cretaceous oviraptorid from east China’s south-eastern Jiangxi Province, a specimen that ticks all these boxes.  The fossils represent the partial, articulated remains of oviraptorosaur preserved sitting on a clutch of eggs in a brooding position.  Such fossils have been found before, but uniquely these 70-million-year-old remains include fossils of baby dinosaurs preserved inside the eggs, the first time this has been recorded in the non-avian dinosaur fossil record.

The Partially Preserved Adult Oviraptorid On the Nest

The fossilised remains of an oviraptorid with preserved eggs that contain the remains of baby dinosaurs.  Note scale bar = 10 cm.

Picture credit: Shundong Bi et al (Science Bulletin)

Sitting Atop a Dinosaur Nest

The multinational team of researchers includes Dr Shundong Bi (Indiana University of Pennsylvania) and Xing Xu (Institute of Vertebrate Palaeontology – Beijing) along with Dr Matt Lamanna (Carnegie Museum of Natural History) and scientific illustrator Andrew McAfee.

Commenting on the significance of this discovery, Dr Bi stated:

“Dinosaurs preserved on their nests are rare, and so are fossil embryos.  This is the first time a non-avian dinosaur has been found, sitting on a nest of eggs that preserve embryos, in a single spectacular specimen.”

Fossils of brooding dinosaurs have been found before, perhaps the most famous is “Big Mamma” the fossilised remains of an adult Citipati osmolskae (also an oviraptorid) on display at the American Museum of Natural History (New York).

Citipati osmolskae Fossil Sitting Atop a Nest of Eggs

The Citipati fossil sitting on a nest “Big Mamma”.

Picture credit: The American Museum of Natural History

An Oviraptorid – A Very Bird-like Dinosaur

The Chinese fossil material has been assigned to the Oviraptoridae, although Everything Dinosaur are not aware of any new taxon being announced.  Oviraptorid dinosaurs are mainly associated with Asia, but the closely related Caenagnathidae are also known from North America.  The bones of these dinosaurs are very bird-like and they do belong to a great linage of theropods that are related to modern birds (the Maniraptora).

The specimen represents an incomplete, articulated skeleton of a large, (presumably adult) oviraptorid crouched in a bird-like brooding posture over a clutch of at least twenty-four eggs.  The adult appears to have perished whilst brooding the clutch.  The researchers identified the preserved remains of seven unhatched dinosaurs entombed inside the eggs.  The late stage of development of the embryos suggests that, just like modern birds, oviraptorids brooded their eggs, rather than simply guarding them as observed in extant crocodilians.

A Caring Parent

Dr Lamanna explained:

“This kind of discovery, in essence, fossilised behaviour, is the rarest of the rare in dinosaurs.  Though a few adult oviraptorids have been found on nests of their eggs before, no embryos have ever been found inside those eggs.  In the new specimen, the babies were almost ready to hatch, which tells us beyond a doubt that this oviraptorid had tended its nest for quite a long time.  This dinosaur was a caring parent that ultimately gave its life while nurturing its young.”

As part of their research, the scientists conducted an oxygen isotope study that demonstrated that the eggs were incubated at high bird-like temperatures, further evidence to support the idea that the adult died whilst brooding its clutch of eggs.  Analysis of the tiny baby dinosaur bones preserved inside their eggs indicate that some babies were more fully developed than others, this suggests that the eggs might have hatched at different intervals, a hatching strategy known as asynchronous hatching.  This strategy is found in many types of birds today such as Shoebill storks and numerous species of birds of prey such as raptors and owls.

Oviraptorids – Evidence that they were Caring Parents

A pair of oviraptorosaurs.  Scientists suggest that these dinosaurs incubated their eggs and that they were caring parents.

Picture credit: Mike Skrepnick

Asynchronous hatching appears to have evolved independently in oviraptorids and modern avians.

The Evolutionary Benefits of Asynchronous Hatching

If the eggs of oviraptorids did hatch at different intervals, then this too can provide an insight into the behaviour of these Late Cretaceous dinosaurs.  Biologists have identified a number of reasons why some kinds of bird alive today have evolved asynchronous hatching.

The evolutionary benefits of asynchronous hatching:

• To reduce the losses from predators due to the whole brood not being present in the nest at the same time.
• The younger animals are a back-up plan in case earlier hatched animals die.
• When food resources are scarce the adults can dedicate the bulk of these scarce resources to the older babies and let the younger siblings perish.
• Reduces the demands on the parents as they do not have to care for all the babies at the same time.

Gastroliths Identified

The researchers also noted the presence of gastroliths (stomach stones).  A cluster of tiny pebbles had accumulated in the body cavity of the adult dinosaur.  Gastroliths are associated with many different types of dinosaur, but this is the first time that undoubted gastroliths have been found in an oviraptorid.  As such, these stones may provide new insights into the diets of these very bird-like dinosaurs.

Dr Xu concluded:

“It’s extraordinary to think how much biological information is captured in just this single fossil.  We’re going to be learning from this specimen for many years to come.”

To read a related article on dinosaur parenting skills: Doting Fathers – A Parenting Strategy Amongst the Dinosauria.

Everything Dinosaur acknowledges the assistance of a press release from the Carnegie Museum of Natural History (Pennsylvania).

The scientific paper: “An oviraptorid preserved atop an embryo-bearing egg clutch sheds light on the reproductive biology of non-avialan theropod dinosaurs” by Shundong Bi, Romain Amiot, Claire Peyre de Fabrègues, Michael Pittman, Matthew C. Lamanna, Yilun Yu, Congyu Yu, Tzuruei Yang, Shukang Zhang, Qi Zhao and Xing Xu published in Science Bulletin.

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