Dinosaur diaries: Arctic duck-billed dinosaurs, dental work and dimorphism
In our Dinosaur diaries series, Museum experts share some of the latest science news and views from across the world of palaeontology.
Join them as they report on a re-examination of Edmontosaurus, explore sexual dimorphism in crocodilians and explain some embryonic dinosaur dental work.
Herbivorous dinosaurs from the deep north of Alaska
During the Mesozoic Era, dinosaurs lived all over the globe, including the Arctic. A new study has shed light on dinosaur material from North Slope, the northernmost borough of Alaska.
The authors took another look at skull bones belonging to small hadrosaurs, or duck-billed dinosaurs, that had been collected from the Prince Creek Formation near the Colville River. Hadrosaurs were a globally widespread group of plant-eating dinosaurs that lived during the Late Cretaceous. They are recognisable for their broad bills and complex teeth.
It was thought the Prince Creek bones were from a species called Ugrunaaluk kuukpikensis. However, following detailed examination of these skull bones, experts revealed that the remains probably represent different-sized juveniles of the much better known hadrosaur called Edmontosaurus.
It is not completely clear to which species of Edmontosaurus they should be assigned. Two species are currently recognised as valid, Edmontosaurus annectens and the slightly older Edmontosaurus regalis. The Prince Creek hadrosaur shares similarities with both of these species. Adult material needs to be recovered from Prince Creek to settle this question.
If these fossils do belong to Edmontosaurus, then this hadrosaur would have had one of the largest geographical ranges of any non-bird dinosaur. The Prince Creek bones are the northernmost examples, with the southernmost from Colorado.
The potentially enormous ranges of hadrosaurs like Edmontosaurus have interesting implications for the ecology of these animals and their environments.
The similarities of the Alaskan hadrosaurs to their more southern relatives may have been due to less dramatic temperature differences across North America during the Late Cretaceous than those we see today. Studies like this help to demonstrate that, like modern-day animals, dinosaur evolution was driven by the environments they lived in.
- Read the paper in the journal PLOS ONE: Re-examination of the cranial osteology of the Arctic Alaskan hadrosaurine with implications for its taxonomic status.
Can you tell male and female dinosaurs apart?
Male and female animals often look different, a phenomenon known as sexual dimorphism. Differences in size, shape, colour and behaviour can be used by individuals to attract a mate.
But can we tell if extinct animals like dinosaurs also had these differences, when all we have are fossilised bones? So far, no universally agreed feature has been found that could help us decide whether dinosaurs were sexually dimorphic.
People disagree on what we would need to look for. As a result, it has been hard for us to identify whether a dinosaur was female or male. To date, a handful of females have been spotted in museum collections, which we could identify because their bodies contain unhatched, complete fossil eggs.
Crocodiles, dinosaurs and birds all share a common ancestor, so we should expect them to share some traits in the way they grow, evolve and behave. In a study published last week, David Hone from Queen Mary University of London and colleagues looked at gharials, an endangered crocodilian, to see if differences in their nasal bones make them sexually dimorphic.
The team looked at 106 gharial skulls and found that as well as males being larger and heavier than the females, they have a bulbous structure made of soft tissue on the end of their snouts. This is called a ghara and it is probably used for sexual display.
The ghara is supported by a hollow cavity in the nose bones below, called the narial fossa, so this bony feature can be considered a male characteristic. Other than these structures there are few, if any, skeletal differences between the sexes.
It is unlikely the ghara would be preserved as part of a fossil because it is made of soft tissue, but the bony narial fossa would survive. However, it's only possible to know that the narial fossa is a male feature because we can see its associated soft tissue in living animals.
Identifying dimorphism within dinosaurs might be impossible without knowledge of their soft tissues and how they related to their bones. Even then we would need a large sample size to be sure, which is sadly rare in the fossil record.
- Read the research paper in the journal PeerJ: Ontogeny of a sexually selected structure in an extant archosaur Gavialis gangeticus (Pseudosuchia: Crocodylia) with implications for sexual dimorphism in dinosaurs.
Embryonic dinosaur dental work
A new study describing tiny embryonic skull bones from the early dinosaur Lufengosaurus might offer clues on how the gigantic plant-eating sauropods acquired their peculiar teeth and jaws. Lufengosaurus is a common dinosaur that lived in China in the Early Jurassic and is known from numerous skeletons, including babies and hatchlings.
It is a prosauropod dinosaur, an early representative of the group that goes on to include the more familiar sauropods, like Diplodocus. Robert Reisz from the University of Toronto and colleagues used CT-scans and high-powered microscopes to study the developing teeth of these 200 million-year-old dinosaurs and compared the features that they found to those in living alligators and extinct sauropods.
Reisz and colleagues found that the first teeth to form in embryonic Lufengosaurus were unlike those normally found in adults. Adults have teeth that are leaf shaped with large serrations, adapted for slicing through plants (and maybe occasional animals). By contrast, the first-formed teeth in the Lufengosaurus embryo are simple cylindrical pegs, only later-developing teeth acquire the typical adult-like shape.
In addition, in most dinosaurs, new teeth develop underneath the old teeth, pushing the old teeth out of the jaw as they are worn down. However, in a Lufengosaurus embryo these new teeth but lay alongside the old ones. Although these features aren't seen in adult Lufengosaurus they are, intriguingly, seen in some advanced sauropods, including Diplodocus, although it is not known if the first sauropods also had these features.
Using these results, the team suggested that an evolutionary process called paedomorphosis might have been involved in the origin of sauropod dentitions. Paedomorphosis (which means literally 'become child-shaped') occurs when features seen in embryos or babies that are usually lost as the animal grows up are kept in adults.
Reisz and colleagues suggest that sauropods with pencil-like stacks of teeth might have been retaining a Lufengosaurus embryo-like condition when adults. It's possible that their study of these tiny, ancient teeth might illuminate the evolution of the largest animals to walk the Earth.
- Read the research paper in the journal Nature Communications: Early Jurassic dinosaur fetal dental development and its significance for the evolution of sauropod dentition.