In particular, almost all of the emeids and one Dinornis specimen had growth rings similar to those found on a tree. Many living animals, such as polar bears, have these markings today. The rings result from the different growth rates over the course of the seasons, Turvey said.
"In the spring and summer there's more food being eaten and faster bone growth. If [the weather] becomes harsher, then the growth slows down," he said. The changes in growth are reflected by changes in bone density. Growth can even temporarily stop altogether, leaving a distinct line running through the bone.
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In addition to the growth rings, the Dinornis specimens all contained numerous blood vessels characteristic of fast growth. Bone analysis suggests that, though larger than emeids, the Dinornis species were fully grown in three years. Some of the emeids grew for almost ten years.
Bird species today lack growth rings, because in most cases their growth phases are limited to one year and many birds' bodies "remodel" their bones throughout life, the researchers report.
Moas were ratites, flightless birds considered the sister group of all other birds. Living ratites include ostriches, emus, cassowaries, and kiwis.
The biggest moa was the largest bird that ever lived. It could have seen eye-to-eye with a professional basketball playerabout six feet (two meters) tall. It weighed as much as 500 pounds (225 kilograms). Smaller species were comparable to turkeys. All species were flightless and lived in New Zealand's forests, grasslands, and mountains, Turvey said.
A DNA analysis published in the June 7 issue of Proceedings of the National Academy of Sciences suggests the first moa evolved about 18.5 million years ago and diversified into at least ten species.
The exact number of moa species and relationships among them is controversial, Cracraft said. Recent DNA research suggests there were at least ten species, nearly all of which Turvey and colleagues studied.
Turvey and his colleagues' study, according to Cracraft, builds on this DNA work to examine the ecology of moas, providing insight to their lifestyle.
"The DNA results serve as an impetus for all this collaborative work," he said. "There's much more to be done, but piecing together biology like this on an extinct group is pretty remarkable."