Plant-eating dinosaurs probably arrived in the northern half of the world millions of years after meat-eating dinosaurs, a recent study found. Their late arrival was likely a result of climate changes that took place hundreds of millions of years ago.

The study used a new way to ascertain the age of dinosaur remains found in Greenland. Researchers found that the plant-eating dinosaurs were about 215 million years old. The mineralized（石化的）remains of bones are called fossils. Earlier, those fossils were thought to be as old as 228 million years. The findings could change how scientists think about dinosaur movements, or migrations.

The earliest dinosaurs all seemed to first develop about 230 million years ago or longer. They appeared in what is now South America. Then, the creatures moved north and spread all over the world. The new study suggests not all dinosaurs moved at the same time.

So far, scientists have not found any evidence of plant-eating dinosaurs living in the Northern Hemisphere before 215 million years ago. One of the best examples of these is the Plateosaurus, a two-legged, 7-meter-long plant-eater that weighed around 4,000 kilograms. However, meat. eaters lived worldwide by at least 220 million years ago.

During the Triassic（三叠纪）period, 230 million years ago, CO₂ levels were 10 times higher than now. The Earth was hotter with no ice sheets at the poles and two areas of extreme deserts north and south of the equator（赤道）. It was so dry in those areas that there were not enough plants for the plant-eaters to survive. But there were enough insects so that meat-eaters could.

About 215 million years ago, CO₂ levels dropped. The drop resulted in more plant life in the deserts. The plant-eaters were then able to make the trip. Triassic changes in CO₂ levels were from volcanoes and other natural forces. The changes were different from those in our times, when the burning of coal, oil and natural gas are considered the main causes of climate change.

Black rhinos(犀牛) have terrible eyesight. Even so, these African giants easily fend off hungry predators such as lions and hyenas. Their basic body plan has proved to be good enough to survive on African grasslands full of large predators, being very large, thick-skinned, big sword-like horns.

But those protective traits(特征) are no match for humans with guns. Today the species is critically endangered largely due to poaching. But the rhinos may have an unlikely ally against poachers: the red-billed oxpecker. The chatty, sociable birds often hang out on the backs of rhinos, feasting on parasitic ticks(寄生虫).And actually, research has shown that the tick is the favorite diet of an oxpecker.

The birds also get nutrients by picking at sores on the rhinos’ bodies. Victoria University of Melbourne behavioral ecologist Roan Plotz suspected that the rhinos put up with this indignity because the oxpeckers make loud alarm calls whenever they see humans approach, giving the rhinos an early warning to flee.

To test whether the oxpeckers do, in fact, act as noisy lookouts, Plotz and his team implanted(植入) radio transmitters in the rhinos’ horns. This didn’t hurt the rhinos and allowed the researchers to track the animals.

“And approach them undetected, when the oxpeckers were on the back and when the oxpeckers were not on their backs, to get an idea of if the birds do alarm them and if the rhinos do listen.”

The researchers found that rhinos without oxpeckers detected an approaching human only 23 percent of the time, at an average distance of 27 meters. But when oxpeckers were present, the birds alarmed the rhinos 100 percent of the time. And they detected the human 61 meters away on average. And the more oxpeckers on the scene , the greater the detection distance.

“It makes sense because the more eyes you have on your back, looking out for you, the more chance that they can pick up anything coming. There can be no doubt that the rhino is listening on the oxpecker alarm call.”

How to Grow Plants in the Classroom

Growing plants in the classroom is an excellent way to get students excited about science!

Start with seeds（种子）in the classroom. 【小题1】 They can do this by putting seeds or beans in wet paper towels and then in a clear plastic bag. The seeds should begin coming out in a few days, which can give the students an idea of what is happening underground when plants begin to grow.

Continue with seeds in the classroom. At the same time as the seeds are beginning to come out, plant the same type of seeds in soil. 【小题2】 They can also predict（预测）how long it will be before the seeds come out and are seen above the soil.

【小题3】 A quick Internet search should let you know which plants are native to your area. This can connect the plant-growing in the classroom with natural history and other social studies lessons. Also, after growing the plants in your   classroom, you can take the students on field trips to see where the plants grow naturally.

Explore the needs of plants. 【小题4】 You can know what they actually need by growing plants in the classroom and making observations along the way.

In all, growing plants in the classroom is sure to be an exciting science lesson. 【小题5】 They can train their creativity with their own gardens!