Hypsilophodontids
If you don't like 'em, stay away from south eastern Australia!



Hypsilophodontid The vast majority of dinosaur remains found in the south east of the Australian continent are from hypsilophodontid dinosaurs. These small ornithopod ("bird foot") herbivores were extremely successful, being one of the most long-lived and wide-spread ornithopod families. They lived from the Middle Jurassic to the Late Cretaceous (a span of about 110 million years) and have been found on every continent.

Hypsilophodontid dentary The name hypsilophodontid means "high crested tooth", refering to the leaf shaped teeth and the long ridges that ran along their length. The teeth of hypsilophodontid dinosaurs, like those of most dinosaurs, were constantly shed as they wore down, and were replaced by new teeth waiting below them. The image to the left shows a nearly complete dentary (lower jaw bone) from near Inverloch in southern Victoria, and is the type specimen of Qantassaurus. Part of the outer fossil bone has been removed (down to the obvious line) to reveal the unerupted teeth below the jaw line (the teeth are outlined in red). The four teeth seen in the lowest row would have originally been completely inside the jaw bone. As the teeth above them wore out, the new teeth below them pushed them out, eventually replacing them. The teeth were replaced one at a time as was required and not as a complete set. Although hypsilophodontids had cheek-teeth further back in their mouths, most species had a toothless beak at the front of the mouth for snipping off tough vegetation.

The femur (upper leg bone) is one of the most diagnostic bones of the hypsilophodontid skeleton, which is probably just as well since femora (the plural of femur) comprise much of the Victorian hypsilophodontid material. Hypsilophodontid femurThis image shows an isolated hypsilophodontid femur, from the Dinosaur Cove site in Victoria. The "hanging" fourth trochanter (red arrow) is a feature common to most hypsilophodontids, and one that they shared with more primitive relatives such as Lesothosaurus and Heterodontosaurus. The fourth trochanter acted as a site for the attachment of the leg muscles.

Other features that are characteristic of hypsilophodontids are elongated shins, four-toed feet and five-fingered hands. With shins longer than their thigh bones hypsilophodontids would have been among the fastest of dinosaurs. Their long tails, stiffened by tendons, would have helped them to swerve and change direction quickly while running. Sharp claws on their hind feet would have given them excellant traction. There would have been few predatory dinosaurs that could have run down a hypsilophodontid. In behaviour they have been compared to gazelles - fast running browsers built for speed rather than for fighting off attackers.

At least five (and may be more) species of hypsilophodontid are known from the Victorian sites. These include Leaellynasaura amicagraphica, Atlascopcosaurus loadsi, Fulgarotherium australe, Qantassaurus intrepidus, and some as-yet unnamed varieties. Nowhere else in the world is such a variety of hypsilophodontid species known from about the same time period (in this case the Aptian/Albian, 106-115 million years ago).

Diseased and healthy tibia One of the best preserved hypsilophodontid skeletons comes from Dinosaur Cove. It is an articulated partial skeleton consisting of most of the back half of the animal, including the hind legs, pelvis, and some of the tail. It was found at the Slippery Rock site in 1989, one of the three individual excavations at Dinosaur Cove, and was discovered only two metres away from the type specimen of Leaellynasaura. This may represent another specimen of the same species. The most striking feature of this animal is the difference between the tibiae (lower leg bones). One of them had become infected while the animal was alive, resulting in an advanced case of osteomyelitis. The right (healthy) tibia was 185 mm long and up to 30x10 mm wide. However, the infected bone was only 158 mm long, with a width of 40x30 mm. The amount of extra bone growth around the shaft, and the fact that the healthy bone in the other leg continued to grow another 27 mm, indicates that the animal lived for several years with the condition, most likely in constant agony. An abscess 41x16 mm wide and 8 mm deep had formed on the bone, and probably drained pus from an open wound in the skin.

Amazingly the infection itself may not have been what killed the animal, at least not directly. It probably died of complications brought on by malnutrition, since it would not have been able to walk quickly or far enough to find adequate food. The front half of the animal is missing, so there is the possibility that the animal was caught by a predator. However, most of the meat-bearing parts of the body would have been around the hind quarters, and there is no indication of predator-induced damage to the bones in this area (such as teeth marks). If this animal managed to live for several years without being able to run, and was not even scavenged when it finally died, then this may suggest how few predators were about in the polar forests, and perhaps a reason why the smaller and more defenceless hypsilophodontids prefered colder conditions. Certainly the number of juvenile hypsilophodontid bones from south eastern Australia indicates that the animals were capable of breeding there, so perhaps the lack of predators made this area such an attractive place to raise young - or to avoid being eaten.

During the Early Cretaceous southeastern Australia was within the Antarctic circle. The average annual temperature was probably somewhere between -6 and 3 celsius (21-37 fahrenheit). The winters were almost certainly frozen, with up to three months of darkness punctuated only by a moon that would have been visible for up to two weeks at a time. It has been suggested that hypsilophodontids may have been pre-adapted to living in colder climates. Studies of the way in which their bones grow have shown that they remained active all year round, whereas most other types of dinosaur show signs of seasonal stress that affected the way their bones grew (possibly due to being less active during winter months). One of the Dinosaur Cove hypsilophodonts (the eternally cute Leaellynasaura) has had a natural cast of the original brain preserved. As well as having large eye sockets, this individual also had enlarged optic lobes, the parts of the brain that process vision. Improved vision may indicate that these animals remained active during the long dark winters, whereas other species of dinosaur may have migrated or hibernated, or at least become less active than at warmer times of the year.

The number of juvenile and diseased hypsilophodontid specimens from the southern Victorian sites may indicate that these deposits were layed down by spring melt waters, washing any creatures that did not survive the winter (such as the very young or the sick) into river deposits and covering them quickly. There are few large dinosaurs represented in these fossil layers. These deposits were layed down by river and stream channels, which would have been moving fast enough to collect and redeposit small bones, but probably not of enough force to do the same for extremely large dinosaur bones. Large dinosaurs are only known from these deposits by the smallest bones in their bodies, and even these are rare when compared to the small herbivore remains.

References

Chinsamy, A., T.H.Rich & P.Vickers-Rich 1998 Polar dinosaur bone histology. Journal of Vertebrate Paleotology18:385

Gross, J.D., T.H.Rich and P.Vickers-Rich 1993 Dinosaur bone infection. National Geographic Research and Exploration 9:286-293.

Rich, T.H. and P.V.Rich 1988. A juvenile dinosaur brain from Australia. National Geographic Research. 4(2):148.

Rich, T.H. and P.V.Rich 1989 Polar dinosaurs and biotas of the Early Cretaceous of southeastern Australia. National Geographic Research 5:15-53.

Rich, P.V. and T.H.Rich 1999 Wildlife of Gondwana. The 500-million year history of vertebrate animals from the ancient southern supercontinent. Second Edition. Reed books, Sydney.

Rich, T.H. & P.Vickers-Rich 1999 The Hypsilophodontidae from Southeastern Australia In Y.Tomada, T.H.Rich and P.Vickers-Rich (Eds) Proceedings of the Second Gondwana Dinosaur Symposium. National Science Museum Monographs, No.15. Tokyo. pp.167-180

Rich,T.H., P.Vickers-Rich & R.A.Gangloff 2002 Polar Dinosaurs. Science 295:979-980

Vickers-Rich, P., T.H.Rich 2000 Dinosaurs of Darkness. Indiana University Press


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