The Mesozoic is the age of dinosaurs. Lasting from 251 million to 65 million years ago, it includes the Triassic, Jurassic and Cretaceous periods. Most of the greywacke rocks dominant in the New Zealand mountains, including the Southern Alps and those in the central North Island, originally accumulated as sediment on the floor of the Pacific Ocean during this time.
These rocks were uplifted and incorporated into the eastern margin of continental Gondwana. Then, about 85 million years ago, the New Zealand continent, Zealandia, broke away, carrying with it a cargo of plants and animals, including dinosaurs.
Triassic (251–200 million years ago)
Worldwide, the Triassic period is noted for the emergence of marine molluscs and marine reptiles, while on land the earliest dinosaurs and mammals appeared. However, their fossils have not been found in New Zealand. The oldest terrestrial animal fossil is some 245 million years old. It is an amphibian like a large salamander (about 3 metres long), found in rocks near Mataura Island, Southland.
Fossil-bearing Triassic rocks are scattered around the country. Fossils include representatives of most marine invertebrate groups. Vertebrate fossils of conodonts, fish and marine reptiles are generally rare. Dinoflagellates (a type of marine plankton) first appear in younger Triassic rocks. Plant fossils (wood and leaves), fossil seeds, spores and pollen are also present.
Jurassic (200–145 million years ago)
During the Jurassic period, brachiopods (lamp shells) declined globally while molluscs such as ammonites and belemnites (relatives of squid) flourished. On land, dinosaurs and birds rose to prominence and flowering plants first appeared. Jurassic marine fossils include plankton, corals, many other marine invertebrates and rare sharks and bony fish.
Jurassic land fossils include well-preserved tree stumps (Curio Bay, Southland). Fossil wood, leaves, seeds, spores and pollen have been documented from numerous layers. The oldest New Zealand fossil insect (an ancestor of the wētā) is Jurassic. A single Jurassic dinosaur fossil has been found near the Waikato River mouth.
Cretaceous (145–65 million years ago)
The Cretaceous period is noted for the global rise of calcareous plankton (major contributors to chalk and limestone formation), marine reptiles (mosasaurs, elasmosaurs), and on land the spread of the flowering plants.
New Zealand’s oldest Cretaceous fossils are plankton. Younger Cretaceous marine fossils include plankton, invertebrates and vertebrates (fish and marine reptiles).
Terrestrial rocks include coal layers with wood, leaf, seed and pollen fossils. New Zealand’s oldest fossil flowers are of Cretaceous age – found near Pakawau, Golden Bay. Rare insect and vertebrate fossils are also known. A single fossil locality (inland Hawke’s Bay) has yielded dinosaur, turtle, pterosaur and perhaps bird fossils from sedimentary rocks which formed in a shallow sea about 75 million years ago. Late Cretaceous dinosaur fossils are known from one other locality, on the Chatham Islands. Fragmentary theropod bones have been found in marine sedimentary rocks that are not older than 65 million years.
A turkey-sized dinosaur
In 1995 Brendan Hayes, a fossil collector, was walking along the coast near the Waikato River mouth when he found a small bone encased in rock. The rock revealed its secrets when he extracted the bone and had it examined. He had found New Zealand’s first Jurassic dinosaur bone – a finger bone from a small theropod dinosaur that was about the size of a turkey.
The dinosaurs’ demise
The most common explanation of the dinosaurs’ extinction 65 million years ago is a catastrophic meteorite impact. Estimated to have been 10 kilometres in diameter, the meteorite completely vaporised on impact. It gouged a 200-kilometre-wide crater on the northern Yucatan Peninsula in Mexico. From investigations of fossilised vegetation, scientists think it hit in July. The entire surface of the earth was plunged into darkness for at least three months because sunlight could not penetrate the resulting dust cloud. Paleontologists think all animals weighing more than about 20 kilograms perished. Smaller animals survived by hibernating, living underground, or living off reduced food supplies. Among the survivors were ancestors of modern mammals.