The discovery of a remarkable¹ 15-million-year-old Australian fossil limestone² cave（石灰岩洞） packed with even older animal bones has revealed almost the entire life cycle of a large prehistoric³（史前的，陈旧的） marsupial⁴（有袋类动物） , from suckling（尚未断奶的，哺乳的） young in the pouch⁵（育儿袋） still cutting their milk teeth to elderly adults. In an unprecedented⁶ find, a team of University of New South Wales [Sydney Australia] researchers in has unearthed⁸ from the cave floor hundreds of beautifully preserved fossils of the extinct browsing⁹ wombat-like marsupial Nimbadon lavarackorum, along with the remains¹⁰ of galloping¹¹（飞驰的） kangaroos, primitive¹² bandicoots（袋狸） , a fox-sized thylacine and forest bats.

By comparing the skulls¹⁴ of 26 different Nimbadon individuals that died in the cave at varying stages of life the team has been able to show that its babies developed in much the same way as marsupials today, probably being born after only a month's gestation¹⁵ and crawling to the mother's pouch to complete their early development.

Details of the find at a site known as AL90 in the famous Riversleigh World Heritage fossil field in Queensland are published in the Journal of Vertebrate Paleontology, by a team led by Dr Karen Black, of the UNSW School of Biological, Earth and Environmental Sciences. The research was supported by the Xstrata Community Partnership¹⁶ Program North Queensland and the Australian Research Council.

"This is a fantastic and incredibly rare site," says Dr Black. "The exceptional preservation¹⁷ of the fossils has allowed us to piece together the growth and development of Nimbadon from baby to adult. So far 26 skulls - ranging in age from suckling pouch young and juveniles¹⁸ right through to elderly adults - have been recovered, as well as associated skeletons.

"The animals appear to have plunged¹⁹ to their deaths through a vertical²⁰ cave entrance that may have been obscured by vegetation and acted as a natural pit-fall trap. These animals – including mothers with pouch young - either unwittingly（不知不觉地，不知情地） fell to their deaths or survived the fall only to be entombed（埋葬） and unable to escape.

"The ceiling and walls of the cave were eroded²¹ away millions of years ago, but the floor of the cave remains at ground level. We have literally²² only scratched its surface, with thousands more bones evident at deeper levels in the deposit.'

The site is also scientifically important because it documents a critical time in the evolution of Australia's flora²³ and fauna²⁴（动植物群） when lush greenhouse conditions were giving way to a long, slow drying out that fundamentally reshaped the continent's cargo²⁵ of life as rainforests retreated.

Dr Black notes that the Nimbadon skulls also reveal that early in life, the emphasis of its growth was on the development of bones at the front of the face, to help the baby to suckle from its mother. As it grew older and its diet changed to eating leaves, the rest of the skull¹³ developed and grew quite massive by way of a series of bony chambers²⁶ surrounding the brain.

Team member Professor Mike Archer⁷ says: "Yet we found that its brain was quite small and stopped growing relatively²⁷ early in its life. We think it needed a large surface area of skull to provide attachments²⁸ for all the muscle power it required to chew large quantities of leaves, so its skull features empty areas, or sinus（静脉窦） cavities. Roughly translated, this may be the first demonstration²⁹ of how a growing mammal 'pays' for the need to eat more greens - by becoming an 'airhead'.

"The abundance of Nimbadon fossils also suggests that they travelled in family groups or perhaps even larger gatherings³⁰: it's possible that this also reflects the beginning of mob behaviour in herbivorous marsupials, such as we see today in grey kangaroos."