Tuesday, December 19, 2006
Fossil discovery turns scientific theory on its head
An international team led by University of Adelaide palaeontologist Trevor Worthy ("Mr Moa") has discovered a unique, primitive type of land mammal that lived at least 16 million years ago on New Zealand.
The discovery of tiny fossilised bones of a mouse-like creature in the Central Otago region is the first hard evidence that New Zealand once had its own indigenous land mammals. The finding could prompt a major rewrite of prehistory textbooks, say scientists.
Mr Worthy, a University of Adelaide PhD student in the School of Earth & Environmental Sciences, and fellow team members from New Zealand and the University of NSW, discovered fossilised parts of a jaw and a leg from the mammal, unearthed in sediment from the St Bathans lake bed in the South Island. It represents an evolutionary stage that pre-dates the split between pouched marsupials and placental mammals.
The find adds a whole new insight into the evolution of mammals in New Zealand, putting paid to the theory that the country's diverse prehistoric groundbird fauna evolved there because they had no competition from land mammals.
"Scientists have long held the view that New Zealand has this weird and wonderful avian biota that lived on the ground because there were no mammals to impede or compete with birds. It appears that this little mouselike animal was part of the fauna on the ancient Gondwana* supercontinent and it got stuck on New Zealand when the latter separated more than 80 million years ago," Mr Worthy says.
The discovery also challenges geological claims that New Zealand was entirely submerged beneath the sea from 25 to 30 million years ago and re-colonised by plant and animal species from nearby land masses like Australia once it re-emerged.
"While a lot of the land disappeared temporarily, there is evidence that some of it was emergent because there are floral pollen records that indicate there were living plants throughout the period. The Tuatara reptiles, also found in New Zealand, have no living relatives in the fossil record until 65 million years ago - and there is no evidence of them in Australia, so we can only assume they have been in New Zealand all the time."
Mr Worthy and his team expect to unearth more mammal fossils from the St Bathans site, perhaps even other species that pre-date the split between pouched marsupials and placental mammals, surviving for millions of years in isolation in New Zealand.
"The deposition site was a big lake - 5600 km2 - and the three mammal bones from the mouse were discovered in a 36 m2 area, so it's reasonable to believe we will find more. We have already found bats of three families at this site, of which two are new in the world. We have also unearthed 24 kinds of extinct birds from this sediment that were previously unknown."
The Australian Research Council recently awarded the team a $513,902 grant over three years to further explore the St Bathans site. Mr Worthy is organising the next field trip in early January.
Mr Worthy, a world bird expert, joined the University of Adelaide from New Zealand in August 2005.
[Source: Adelaide University, Australia]
See the earlier post "Fossils reveal New Zealand's indigenous 'mouse'" which gives a link to a related open access/free Proceedings of the National Academy of Sciences (PNAS) paper (see below) and Trevor Worthy's involvement in discovering New Zealand's first snake fossil:
"Miocene mammal reveals a Mesozoic ghost lineage on insular New Zealand, southwest Pacific"
Abstract (Full Text available at same link)
New Zealand (NZ) has long been upheld as the archetypical example of a land where the biota evolved without nonvolant terrestrial mammals. Their absence before human arrival is mysterious, because NZ was still attached to East Antarctica in the Early Cretaceous when a variety of terrestrial mammals occupied the adjacent Australian portion of Gondwana. Here we report discovery of a nonvolant mammal from Miocene (19-16 Ma) sediments of the Manuherikia Group near St Bathans (SB) in Central Otago, South Island, NZ. A partial relatively plesiomorphic femur and two autapomorphically specialized partial mandibles represent at least one mouse-sized mammal of unknown relationships. The material implies the existence of one or more ghost lineages, at least one of which (based on the relatively plesiomorphic partial femur) spanned the Middle Miocene to at least the Early Cretaceous, probably before the time of divergence of marsupials and placentals >125 Ma. Its presence in NZ in the Middle Miocene and apparent absence from Australia and other adjacent landmasses at this time appear to reflect a Gondwanan vicariant event and imply persistence of emergent land during the Oligocene marine transgression of NZ. Nonvolant terrestrial mammals disappeared from NZ some time since the Middle Miocene, possibly because of late Neogene climatic cooling.
*Info on Gondwana:
The southern supercontinent Gondwana (originally Gondwanaland) included most of the landmasses in today's southern hemisphere, including Antarctica, South America, Africa, Madagascar, Australia-New Guinea, and New Zealand, as well as Arabia and the Indian subcontinent, which are in the Northern Hemisphere. The name is derived from the Gondwana region of India. (More)