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Our Jurassic Past
Did dinosaurs in ancient times walk upon our mountains green? Yes, they did, but we know relatively little about them.
by Vicki Hyde
Dinosaurs may be running rampant across the cinema screens of New Zealand, but our country has had relatively little experience of dinosaurs in the flesh. Just a handful of bones exist to tell palaeontologists that New Zealand once hosted a few examples of the "terrible lizards".
Geological processes played a large part in determining New Zealand's unique flora and fauna, and kept the country relatively dinosaur-free. When much of the early dinosaur development was under way in the huge ancient landmass of Gondwana, the area that was later to become New Zealand was under the ocean.
"There wasn't much land, just great masses of sediment spread over the sea floor," says Bradshaw.
Earth movements, sediment accumulation and tectonic plate activity saw the rise of an ancestral landmass. It didn't look anything like the country today, stretching as far north as New Caledonia and south to Campbell Island. This landmass took part in a geological gavotte, rotating and spinning away from the other continents over the course of millions and millions of years, rising and falling until the now-familiar outlines emerged only 3-4 million years ago.
"New Zealand began as an arc of volcanic islands off the cost of Gondwana," says Dr Roger Cooper of the Institute of Geological and Nuclear Sciences. " New Zealand as we know it now is an amalgam of 13 or more separate fragments that came together between 250 million and 130 million years ago."
The oldest of these fragments, or terranes, is found in north-west Nelson, with a piece of the same belt in Fiordland. At 530 million years old, it sports the oldest rocks -- and the oldest fossil remains -- in the country. Sandwiched in the rocks of Cobb Valley are the broken, moulted skeletal fragments of trilobites, ancient marine creatures which looked something like giant wood lice (slaters).
Almost 450 million years separate these oldest inhabitants of New Zealand from the half-dozen small bones that are all that remain of this country's dinosaurs. During the Jurassic Period, 205-135 million years ago, New Zealand became home to the ancestors of the weta, moa, kiwi and tuatara, and saw the growth of its first major forests. Relatively little is known about land conditions at that time, as much of the country's fossil-bearing Jurassic rock is marine in origin, and is buried under later deposits.
"The rocks of the Dinosaur Age found in New Zealand are very nearly all marine, and nearly all the fossils found are marine," says Bradshaw.
Geology and circumstance have combined to limit the amount of dinosaurs remains preserved in rock. Dinosaurs are likely to have reached the New Zealand area in the Jurassic Period, before the Tasman Sea had formed and cut the New Zealand landmass from Australia. However, New Zealand's turbulent geological history and the chancy nature of the preservation process means that there is very little evidence about land life at that time.
Marine creatures have a relatively good chance of ending up as fossils. The sand and muds which sweep over the ocean floor act to preserve bodies lying on the bottom, particularly hard skeletons or shells. The action of minerals, pressure and time result in fossils embedded in the rock. On land, such processes are not so common Lakes and swamps may act as suitable preservation sites, but most land animals die and disappear without trace.
"The chances of a dinosaur being washed out to sea are not that great," remarks Bradshaw.
Bones and Bits
Sometime between 87 and 65 million years ago, in what geologist term the Late Cretaceous Period, a shallow sea rested where inland Hawkes Bay now lies. Rocks deposited at that time have yielded bone fragments of two types of small dinosaurs, the therapod and the ornithopod.
Therapods are part of a large group of dinosaurs, called Saurischians, which were characterised by a reptilian hip structure supporting large leg muscles. While there were plant-eating Saurischians, it has been the carnivorous therapod members -- such as the infamous Tyrannosaurus rex -- which are best known. One backbone and a couple of toe bones shows that New Zealand was once home to a small therapod, weighing some 400 kilograms and standing as high as a person.
The ornithopods had a birdlike hip structure which helped support the large digestive system these herbivorous animals required. It was a single bone from this distinctive hip arrangement which revealed that some type of small ornithopod once grazed in what is now inland Hawkes Bay. The bones were found by amateur palaeontologist Joan Wiffen, and caught New Zealand experts by surprise.
"Nobody had realised there might be dinosaurs around," says geologist Margaret Bradshaw. "People were used to finding fish bones, marine reptiles and turtles. Nobody had their eye in for dinosaurs."
Further discoveries in the Hawkes Bay area have seen Wiffen dubbed "the Dinosaur Lady". The 70-year-old enthusiast has worked in conjunction with overseas experts over the last 20 years to identify the various bones she has found. Her most recent discovery, still under investigation, looks like being a leg bone from the huge plant-eating diplodocus -- the first such discovery in New Zealand.
While actual dinosaurs may have been thin on the ground in New Zealand, the country did have a number of creatures often incorrectly tagged as dinosaurs -- plesiosaurs, mosasaurs and pterosaurs. Despite their names, these marine and aerial reptiles were cousins to the dinosaurs; all dinosaurs were land-based, being unable to swim or fly.
Another major difference was in the hip, leg and ankle structure of the dinosaurs and their reptilian cousins. Like mammals, dinosaurs had their legs positioned directly underneath their bodies, supporting them in well-balanced bipedal or four-footed stances. Lizards and crocodiles -- two forms of modern reptiles -- have legs that project sideways or which are only just able to lift their bodies off the ground.
One reptile that was able to get well off the ground was the pterosaur, or winged lizard. They were the first vertebrate to make it into the air, becoming widespread in Jurassic times and predating birds and bats by around 150 million years. An arm bone and a tooth are all that have been found to show that a pterosaur with a wingspan of around 3.75 metres once flew in New Zealand skies.
In the seas swam plesiosaurs, marine reptiles with the long, narrow necks and big flippered bodies. They're one of the better known examples of the ancient marine reptiles, having achieved a certain notoriety as the possible model for the Loch Ness Monster.
The remains of plesiosaurs have been collected in New Zealand since the middle of the last century, when bones from about 150 creatures were collected from Waipara and Haumuri Bluff in the South Island. Sadly, most of these disappeared with the loss at sea of the ship taking them to England for further study. One clearly identified species is Mauisaurus haasti, named after early geologist and bone collector Julius von Haast.
The study of plesiosaurs continues. Researchers at Canterbury Museum have been patiently chipping away at two siltstone boulders holding the partial skeleton discovered in Waipara ten years ago.
In 1983, three amateur geologists found a broken boulder and were excited to see a seam of bone within the rock. Bradshaw accompanied them to the site and was startled to find that the break was a fresh one, with little weathering. The team were able to spot where the boulder came from, the other half still embedded in the cliff. Bradshaw recalls the excitement as the group begged, borrowed and bagged sponsorship to get a front-end loader and large truck to drag out the nine-and six-tonne rocks.
Ironically, after the initial examination the find remained in storage in the basement of Canterbury Museum, as funds were lacking to take things further. It wasn't until 1990 that a $25,000 lottery grant came through and provided the tools needed to excavate the bones. It's been a difficult task, and the ancient marine reptile probably won't be on permanent display until 1997 or so when the museum gets a new geology gallery.
Bradshaw believes that this plesiosaur is an example of a new species, as comparisons with existing specimens has indicated some significant differences. This brings the number of different plesiosaurs species found in New Zealand to three. Last century, collectors believed they had identified 12 species of plesiosaur in New Zealand. However, the bulk of these were found to be merely different growth stages of M. haasti.
A more diverse grouping existed amongst the mosasaurs, which had seven species endemic to New Zealand. One genus name, Taniwhasaurus, seems particularly appropriate for this large, toothy marine lizard. Measuring up to nine metres in length, these fast-moving carnivores used a powerful tail and four stubby paddles to chase prey in New Zealand's early waters. One favoured food was the ammonite, squid-like creatures related to the Nautilus (see cover picture). Fossil ammonites have been found bearing imprints of mosasaur teethmarks.
One ammonite that might have been more than a match for a mosasaur is the one discovered by the NZ Geological Survey in 1978. It measured almost 1.5 metres in diameter and is one of the largest, most complete ammonites to have been discovered. This particular fossil has attracted international attention because it is the oldest of all the giant ammonites to be found, dating back to the Late Jurassic Period some 142 million years ago.
All these creatures -- ammonites, plesiosaurs, mosasaurs, pterosaurs, therapods, ornithopods and the rest -- died out at the end of the Cretaceous Period 65 million years ago. Arguments still rage concerning why this great die-off happened. The Late Cretaceous was a period of intense volcanic activity, which some scientists speculate was enough to deplete atmospheric ozone, leading to major changes in habitat availability and weather patterns. In recent years, there's been growing support for the suggestion that a massive meteorite strike caused the mass extinction, and some of the evidence for this has been sourced from New Zealand.
Large gaps in our fossil record remain to be filled. Bradshaw speculates as to what might be found in the Jurassic and Cretaceous rocks buried deep below the gravels and sediments across much of the country.
"Who knows what's down there?"
Vicki Hyde is the editor of New Zealand Science Monthly.
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