A lost lake, a different world
In the early Miocene epoch, between 19 and 16 million years ago, a huge freshwater lake covered 5,600 square kilometers of Central Otago. This was Lake Manuherikia, the center of a subtropical ecosystem vastly different from modern New Zealand. The climate was warm, supporting vegetation like casuarinas, eucalypts, and palms. The sediments of this ancient lake, preserved today in the Bannockburn Formation, contain many fossils known as the St. Bathan's Fauna. These remains record a "ghost ecosystem" that vanished as the climate cooled, long before the arrival of humans.
The fossils reveal a New Zealand full of unexpected life. The discovery of a terrestrial crocodilian, a member of the extinct mekosuchine group, changed the belief that tuatara were the only ancient reptiles to survive in New Zealand. Though known only from jaw fragments, estimates suggest the crocodile had a skull about 20 centimeters long. It shared this warm, wet world with turtles, relatives of the tuatara, geckos, and skinks.
Mammals, giant bats, and monster parrots
Perhaps the most surprising discoveries from St. Bathan's are the mammals. Before these finds, only two native bat species were thought to constitute New Zealand's entire land mammal history. The fossils of Mystacina miocenalis, a giant burrowing bat, showed this group has been in New Zealand for at least 16 million years. Weighing around 40 grams, it was three times the size of its modern relatives and likely spent much of its time walking on the forest floor, foraging for fruit and ground-dwelling prey.
Even more enigmatic is the "St. Bathans mammal." Known from just a few bone fragments, this mouse-sized creature was not a bat, a placental, or a marsupial. It represents a completely extinct lineage of archaic mammal, a ghost whose ancestors likely walked Zealandia when it split from Gondwana over 80 million years ago.
The bird life was equally strange. Alongside ancestors of the moa and kiwi, at least 45 other bird species have been found. The most spectacular is Heracles inexpectatus, the largest parrot ever known. Standing up to a meter tall and weighing about 7 kilograms, this flightless giant, nicknamed "Squawkzilla", had a massive beak that could cracking tough nuts and possibly even preying on other animals.