Skull of a Juvenile Heterodontosaurus Provides Information on Dinosaur Diets
A tiny skull, of an early ornithischian dinosaur is helping scientists learn more about dinosaur growth and their eating habits. The skull, one of the smallest dinosaur skulls known, has been identified and described by a group of researchers from London, Chicago and Cambridge and their findings published in the scientific journal “The Journal of Vertebrate Palaeontology”.
The skull belonged to a juvenile Heterodontosaurus, an early member of the ornithischian group, it is not complete but in life the skull of this animal would have measured 45 millimetres (less than the width of a credit card) and the juvenile would have weighed about 200 grammes.
Heterodontosaurus (the name means different-toothed lizard), was a small dinosaur of the Early Jurassic. Fossil finds have been dated at around 202 – 190 million years ago (Hettangian to Sinemurian faunal stages). It is regarded as one of the most primitive members of the dinosaur Order Ornithischia (bird-hipped dinosaurs) and regarded as an ancestral form of this huge and diverse Order. Ornithischian dinosaurs are believed to be entirely plant-eaters and are represented by types of dinosaurs as varied as the horned dinosaurs, the duck-billed dinosaurs and the armoured dinosaurs. The classification of the ornithischians was first outlined in a seminal paper written by the British palaeontologist Harry Govier Seeley, 120 years ago (first published in 1888). Seeley remarked on the similar types of pelvis certain dinosaurs had, he commented upon their “bird-like” features, as with modern birds the pubis bone at the front of the pelvis projected backwards. With birds, this position of the pubis is believed to be related to efficient breathing enabling muscles to be oxygenated for flight. However, with the ornithischians, it is believed the backward pointing pubis made room for a large gut – just what you need to digest tough vegetation.
Little is known about ornithischian origins, the oldest member of this Order is thought to be a tiny dinosaur called Pisanosaurus, dating from approximately 225 million years ago, but the classification of this dinosaur as a member of this group is controversial. It did possess skull and teeth characteristics of a “bird-hipped” dinosaur but other aspects of this animal’s anatomy are more saurischian (lizard-hipped).
It is believed that all dinosaurs evolved from meat-eaters, this new research into the juvenile skull of a Heterodontosaurus provides an insight into the transition from a meat-eating animal to a plant-eater (herbivore).
“It’s likely that all dinosaurs evolved from carnivorous ancestors,” pointed out study co-author Laura Porro, a post-doctoral student at the University of Chicago. “Since Heterodontosaurs are among the earliest dinosaurs adapted to eating plants, they may represent a transition phase between meat-eating ancestors and more sophisticated, fully-herbivorous descendants.”
The tiny skull of the juvenile, the first found for this species indicates that these dinosaurs were still in the middle of that transition.
Heterodontosaurus was named and described in the early 1960s after a single skull was found, but in 1976 an almost complete adult skeleton was discovered, one of the most complete of any dinosaur fossil found to date. Heterodontosaurus fossils have only been found in South Africa, thanks to the beautifully preserved fossil, scientists have been able to work out what this animal looked like. It was small, just over a metre long, fleet-footed, capable of bipedal running but the forelimbs were long and strong. The hand had five digits and it is possible that the thumb was opposable, so it may have been able to grasp vegetation, very efficiently, a feature seen in later ornithopods such as the iguanodonts. Opposable thumbs, the ability to grasp and manipulate objects using the thumb is also a feature of primates. Our own thumbs are capable of facing and touching the other fingers on our hands.
Laura Porro had heard rumours of a juvenile Heterodontosaurus skull in the collection of the South African Museum but no scientific paper or description had ever been produced about it.
As part of her research, Porro visited the Iziko South African Museum which is situated in Cape Town, to examine the adult fossils. When she was there, she got permission to explore the Museum’s collections and to examine the extensive archive of fossils. While going through drawers of material found during the original Heterodontosaurus excavations in the 1960s, she found two more heterodontosaur fossils, including the partial juvenile skull.
“I didn’t recognise it as a dinosaur at first,” Ms Porro said, “but when I turned it over and saw the eye looking straight at me, I knew exactly what it was.”
The study’s lead author, Richard Butler of the London Natural History Museum explained the importance of the juvenile skull.
“This discovery is important because for the first time we can examine how Heterodontosaurus changed as it grew. The juvenile Heterodontosaurus had relatively large eyes and a short snout when compared to an adult”.
The skull displays typical features of juveniles, big eyes in proportion to the rest of the skull are seen in other animals, many baby mammals possess this characteristic, a trait taken to extremes by artists and illustrators in many Disney films.
A specialist on the mechanics of feeding, Porro was particularly interested in the new fossil’s teeth. heterodontosaurs, have an unusual combination of teeth, with three pairs of stabbing jaws just behind the beak, the rearmost and largest looking like big canine tusks. The teeth towards the back of the jaws are closely packed and blunt, designed for crushing and grinding food. This little dinosaur may have possessed cheeks, another adaptation to help it chew its food. In contrast, most reptiles, including other dinosaurs have teeth which change little in shape along the length of the jaw.
This bizarre suite of teeth has led to debate over what these dinosaurs ate. Some scientists think heterodontosaurs were omnivores who used their differently-shaped teeth to eat both plants and small animals, such as lizards and insects. Others contend that heterodontosaurs were herbivores who ate only plants and that the canines were sexually dimorphic – present only in males, as in living warthogs. In that scenario, the canines could have been used as weapons by rival males in disputes over mates and territories.
Porro and colleagues found that the juvenile already had a fully-developed set of canines.
“The fact that canines are present at such an early stage of growth strongly suggests that this is not a sexually dimorphic character because such characters tend to appear later in life,” said Butler.
Instead, the researchers suspect that the canines were used as defensive weapons against predators, or for adding occasional small animals such as insects, small mammals and reptiles to a diet composed mainly of plants, the authors of this scientific paper refer to this as occasional omnivore behaviour, perhaps reflecting the fact that these little dinosaurs were opportunistic carnivores. However, the presence of well developed fang-like front teeth may be explained by the fact that like most reptiles, the diets of youngsters was very similar to the adults, although the could not tackle large prey or such a copious amount of vegetation. It has been suggested that with their strong forelimbs and sharp canine-like teeth these little dinosaurs may have also fed on roots. They could have dug them up with their five-fingered hands and then used the strong front teeth to break them open.
The study of this tiny skull may have shed light on heterodontosaur ontogeny (growth and development), but a new mystery has emerged. With the aid of X-rays and CT scans, Porro found a complete lack of replacement teeth in the adult and juvenile skulls.
Most reptiles, including living crocodiles and lizards, replace their teeth constantly throughout their lives, so that sharp, unworn teeth are always available. The same was true for dinosaurs, this is why when you look at the picture of a T. rex fossil skull, all the teeth are of different sizes as they are different ages. Most mammals, such as ourselves on the other hand, replace their teeth only once during their lives, allowing the upper and lower teeth to develop a tight, precise and efficient fit.
Heterodontosaurus was more similar to mammals, not only in the specialised, variable shape of its teeth but also in replacing its teeth slowly, if at all, and developing tight tooth-to-tooth contact. “Tooth replacement must have occurred during growth,” the authors conclude, “however, evidence of continuous tooth replacement appears to be absent, in both adult and juvenile specimens.”
This article has been reproduced from an article written by the University of Chicago Medical Centre entitled “Tiny Juvenile Dinosaur Fossil Sheds Light On Evolution Of Plant Eaters”.