These findings reveal that a key step in the evolution of back limbs happened in fish,
challenging previous theories that such appendages(附肢)evolved only after the move to land.
Scientists investigated fossils(化石)of a 375-million-year-old fish known as Tiktaalik,
discovered in 2004 in northern Canada's Ellesmere Island. Possessing a broad flat head and
sharp teeth, Tiktaalik resembled a cross between a fish and a crocodile, growing to a length of 9 feet (2.7 meters) as it hunted for its food in shallow freshwater.
This ancient creature was undoubtedly a fish. However, it also had features seen in modern tetrapods(四足动物).
This ancient fish had large forefins and shoulders, elbows and partial wrists, enabling it to support itself on ground. This makes it the best-known example of a link between finned(鳍)animals and limbed animals marking the evolutionary change from water to land for creatures with backbones.
Previous analyses of other fossils dating from the water-land change found their back
appendages were small and weak compared with their front appendages. This suggested the
earliest ancestors of tetrapods perhaps had a "front-wheel drive" form of movement that
depended more on their front limbs, and that a "four-wheel drive" form of movement with strong hips and back limbs only developed after tetrapods evolved.
【沈阳市2016届英语短文语法填空一轮练习(8)(含阅读理解)】相关文章:
★ 2017届高考英语二轮复习书面表达限时测验:1(含解析)
★ 贵州省贵阳市2014高考英语(新人教版)一轮阅读训练(33)附答案
★ 2017届高考英语二轮复习大题冲关秘籍完形填空之夹叙夹议文:创新押题(含解析)
★ 2017届高考英语二轮复习书面表达限时测验:5(含解析)
★ 湖南省汝城二中2014届高考英语一轮复习阅读训练 (73)
★ 湖南省汝城二中2014届高考英语一轮复习阅读训练 (1)
★ 湖南省岳阳市2014高考英语一轮单项选择训练题(1)及答案
★ 湖南省汝城二中2014届高考英语一轮复习阅读训练 (51)
最新
2017-04-24
2017-04-24
2017-04-24
2017-04-24
2017-04-21
2017-04-21