【轉載】東歐「演化孤島」衍生迷你物種

[誠惶誠恐]

【誠惶誠恐按】這段新聞《明報》的標題是「侏儒島恐龍矮如馬 東歐『進化孤島』衍生迷你物種」，其實矮如馬的恐龍不算很小，有些恐龍甚至小到不足一米（約 3 呎）長（Wannanosaurus）。這次發現的特別之處在於這裡的恐龍比起境外的同類或近親恐龍小很多。

＊　＊　＊

轉自：
http://hk.news.yahoo.com/article/100221/4/go28.html

侏儒島恐龍矮如馬 東歐「進化孤島」衍生迷你物種
(明報)2010年2月22日 星期一 05:10

【明報專訊】在一個世紀前，匈牙利人諾普喬（Baron Franz Nopcsa）曾表示在家中的莊園，發現了「侏儒恐龍」化石，但被不少人嗤之以鼻。但最近科學家不單證實了「侏儒恐龍」的存在，還發現了一個「迷你恐龍島」，揭開這些恐龍「縮小」的原因。

來自布里斯托大學的本頓（Michael Benton）講解這個古代侏儒生物島時說：「其他在島上和恐龍一同生活的動物——魚、青蛙、兩棲動物、烏龜、哺乳類動物……普遍來說都比正常小，但卻不如恐龍般，和牠們的大陸遠親在體積上有如此大分別」。

這個古代「侏儒島」名叫哈采格（Hateg），位於現今羅馬尼亞中部。該處現時位處內陸，但在650萬年前，它是一個有如現今不列顛島一半大小的島嶼，四周是汪洋。本頓說：「很多我們熟悉的恐龍都是住在白堊紀末期的遼闊大陸上……有趣的是歐洲當時很大部分都是被海洋覆蓋，東歐則是一堆群島」。

體型縮小適應狹小缺糧生態

本頓解釋，遠古時期，絕大部分巨型恐龍都在遼闊大陸上生活，在那裏牠們的食物充裕，牠們的巨型體積亦保護了牠們。但在侏儒島情况便不大一樣。本頓教授說，比起其他後白堊紀化石，島上很多恐龍化石在進化過程上仍然是處於非常原始的階段，顯示這些恐龍在島上被孤立起來。古生物學家相信，歐洲海平面上升，是導致這些恐龍與居於古大陸恐龍斷絕往來的主因，迫使牠們適應「相對狹小」的居住環境。

大恐龍被困在食物和空間有限的小島上，結果如何？本頓說﹕「有證據顯示，那是個熱帶島，有豐富的植被以及昆蟲，但要支撐數以百計動物，進化的壓力令牠們要麼縮小，要麼死亡。」

發現3種「侏儒恐龍」骨頭

「侏儒恐龍」的骨頭由19世紀的化石獵人諾普喬所發現，他當時發現的恐龍骨特別細小，引起古生物學家的爭論，到底他發現的恐龍是全新的侏儒品種，又或只是大型恐龍的幼龍？直到現今，先進的分析技術容許科學家證實在島上發現的兩種或三種恐龍，都可能是侏儒品種。

研究發現了4腳的食草龍馬扎爾龍（Magyarosaurus），估計身長只有16到19呎，比起那些高100呎、重80噸的「遠親」阿根廷龍小近8倍，只比現代馬匹體積大一點。另一種叫沼澤龍（Telmatosaurus）的鴨嘴龍亦被發現只長13呎，比其22到32呎長的遠親短得多。研究發現，這些侏儒恐龍比起牠們在古大陸的近親更早性成熟，壽命亦較短。

星期日電訊報

＊　＊　＊

轉自：
http://www.telegraph.co.uk/scien … warf-dinosaurs.html

Found: ‘Jurassic Parkette’ – the prehistoric island ruled by dwarf dinosaurs
A prehistoric “lost world” ruled by miniature dinosaurs has been discovered by palaeontologists.

By Richard Gray, Science Correspondent Published: 9:00PM GMT 20 Feb 2010

Zalmoxes Photo: NATURAL HISTORY MUSEUM[/size]

The creatures lived on an island – a kind of pigmy Jurassic Park – and were up to eight times smaller than some of their mainland cousins.

One of the island-dwelling dinosaurs, named Magyarosaurus, was little bigger than a horse, but was related to some of the largest creatures to ever walk the Earth – gigantic titanosaurs such as Argentinosaurus, which reached up to 100 feet long and weighed around 80 tons.

Another of the dinosaurs was found to be a primitive dwarfed species similar to large duck-billed herbivores like Iguanodon, which could grow to be up to 10 feet long and weighed more than three tons.

Fossils from the dwarf dinosaurs were found in what is now modern day Romania, in an area known as Hateg, which, 65 million years ago – when the creatures were living there – was an island.

Professor Michael Benton, from the University of Bristol, who carried out the research with scientists at the Universities of Bucharest and Bonn, said the dinosaurs seemed to have evolved smaller bodies after becoming marooned there.

He said: “Most of the famous dinosaurs that we know about were living on big landmasses at the end of the Cretaceous period.

“The curious thing about Europe at this time was that it was largely covered by sea and much of Eastern Europe was a sort of archipelago of islands.

“If you are a big dinosaur on a small island with limited food and space, then the evolutionary pressure is either to go extinct or to get smaller.”

The findings will overturn some popular perceptions about dinosaurs, which are generally considered the largest animals to have ever stalked the Earth. Even their name, derived from the Greek for “fearfully great lizard”, implies that dinosaurs were gigantic creatures.

The dwarf bones were found by a 19th century fossil hunter called Baron Franz Nopcsa, who observed at the time that the species he discovered were unusually small.

His observations sparked debate among palaeontologists about whether the dinosaurs were entirely new dwarfed species or merely just juveniles of larger dinosaurs.

Most large dinosaurs were adapted to roaming across huge landmasses where their territories and food sources were plentiful. Their large size also offered protection against predators.

However, at the end of the cretaceous period, around 65 million years ago, much of Europe was under water and Hateg, which is now landlocked in central Romania, is thought to have formed a 30,000 square mile island – about half the size of modern Britain.

Palaeontologists believe that the rising sea levels in Europe cut many species of dinosaurs off the rest of the world, forcing them to adapt to their new, smaller habitats.

Modern analysis techniques have now allowed the scientists to confirm the remains of two common species of dinosaur found on the island, and possibly a third, were in fact dwarfed species.

The researchers found that the four legged herbivore Magyarosaurus had an estimated body length of just 16 to 19 feet and weighed just an eighth of its larger relatives such Argentinosaurus and the Paralititan, which grew up to 100 feet length and weighed up to 80 tons.

A hadrosaurid called Telmatosaurus was also found to be just 13 feet long, compared to the 22 to 32 feet of its closest relations. It is estimated to have weighed just an eighth of relatives such as the five ton Maiasaura.

The scientists say a third species, a two legged herbivore known as Zalmoxes, was a possible dwarf species related to the 26 foot long Tenontosaurus, and weighed around a quarter as much as its two ton relative.

The new findings will be published in the scientific journal Palaeogreography, Palaeoclimatology, Palaeoecology.

Professor Benton said many of the fossils were quite primitive in evolutionary terms for the late cretaceous period, lending support for the theory that they had become marooned on the island.

He said: “There is evidence that this island was tropical, just north of the equator, with rich vegetation and insects, but in order to support even several hundred animals, the evolutionary pressure would have been for them to get smaller or die out.

“There is very little evidence for large flesh eating dinosaurs, so the pressure for large body size to avoid being eaten was not the same.”

The fossilised dwarf dinosaur bones are some of the earliest examples of “island dwarfing” – where large species stranded on islands become smaller.

The evolutionary process has been a hotly debated topic among scientists following a series of discoveries of dwarf species of elephant, woolly mammoths and even human ancestors on islands.

Most recently an intense row broke out over whether the bones of an extinct species of human discovered on the island of Flores, in Indonesia – which became known as the Hobbit – were from a dwarfed species of human or a youngster.

Dr Paul Barrett, a palaeontologist at the Natural History Museum in London where reference fossils of the dwarfed dinosaurs are kept, said: “This certainly suggests that the evolutionary processes involved in island dwarfing have been operating over millions of years.

“Although dinosaurs are typically portrayed as being gigantic, this research also helps to emphasis that some of them were in fact quite small.”[/size]

[Author]

Posts