讓人難得有機會一窺留有羽毛的恐龍尾巴樣貌的琥珀樣品
中國、加拿大和布里斯托大學的研究人員發現一條恐龍尾巴和上頭的羽毛被完整地封存在一塊琥珀當中
發表於《當代生物學》( Current
Biology)上的這篇研究有助於填補恐龍羽毛構造及演化上的細節，這是難以從化石證據中推論出來的。
研究人員表示雖然並非首度在琥珀中發現羽毛，但之前的樣本卻很難十分肯定地說明它們來自於何種動物。
加拿大薩斯喀徹爾皇家博物館的Ryan McKellar表示：「這塊新樣本保存了一條動物幼體尾巴中的八塊脊椎骨，分佈於周遭的羽毛保留了立體構造與細微結構。」
「我們可以肯定地說這條尾巴來自於哪種動物，因為內部的脊椎骨並未像現代鳥類或者牠們關係最近的親戚一樣融合成一根骨頭，也就是尾縱骨(pygostyle)。反之，這條尾巴長度很長且可以自由彎曲，兩側垂有成排的羽毛。換句話說，這些羽毛必定來自於一種恐龍，而非史前鳥類。」
2015年，來自北京中國地質大學的研究第一作者Lida Xing，在緬甸密支那的琥珀市場中發現了這件獨一無二的樣品。
原本認為這塊琥珀中包裹的是某種植物，並預計要把它拿來打造成一具珠寶或珍玩，但Xing認出它可能具有重大的科學價值，而建議德煦古生物研究所買下這具樣品。
研究人員表示這具樣本為9900萬年前一隻獸腳亞目的恐龍，其留有羽毛的尾巴保存在白堊紀中期的琥珀當中。雖然起初很難辨認琥珀內含物質的細節，但Xing和他的同僚借助電腦斷層掃瞄，以及顯微鏡觀察而得以看見細部構造。
羽毛顯示出這條尾巴的表面呈現栗棕色，而底面則為灰白色。此樣品也對羽毛的演化有所啟發。這些羽毛缺乏一根發育完整的主軸，或稱羽軸(rachis)。它們的構造也指出現代羽毛當中最細微的兩級構造，也就是羽枝(barb)和小羽枝(barbule)，形成的時間早於羽軸。
布里斯托大學地球科學院的教授Mike
Benton補充：「這項研究的驚人之處在於能夠看到一條恐龍尾巴上的所有細節，包含骨頭、肌肉、皮膚以及羽毛；並且可以想像這隻小傢伙的尾巴被樹液黏住，之後可能因為無法脫困而死亡。」
「我們不認為恐龍跟現在的某些蜥蜴一樣能夠斷尾求生。」
研究人員也檢視了包覆在其中的尾巴暴露在琥珀表面處的化學性質。分析結果顯示骨頭周遭的軟組織層含有些微的亞鐵離子，此為同樣包在樣品中的血紅素遺留下來的痕跡。
這項發現顯示琥珀具有可以用來補足化石紀錄的重要價值。Ryan McKellar補充：「雖然琥珀碎片僅能保存古代生態系的隻字片語，但是它們可以記錄生物的細微構造、其立體排列方式以及容易腐敗的部分，這些是很難用其他類型的樣品來研究的。」
「琥珀這種新資訊來源值得人們加以大力研究，而且應該被視為一種化石資源而多加保護。」
研究人員表示他們現在「十分期待看到這個地區將來還會有更多發現，可以重新塑造我們對恐龍以及其他脊椎動物身上羽毛和軟組織的認識。」
Amber specimen offers rare glimpse of feathered
dinosaur tail
Researchers
from China, Canada, and the University of Bristol have discovered a dinosaur
tail complete with its feathers trapped in a piece of amber.
The finding reported today in Current Biology helps to fill in details of the
dinosaurs' feather structure and evolution, which can't be surmised from fossil
evidence.
While the feathers aren't the first to be found in amber,
earlier specimens have been difficult to definitively link to their source
animal, the researchers say.
Ryan McKellar, from the Royal Saskatchewan Museum in Canada,
said: "The new material preserves a tail consisting of eight vertebrae
from a juvenile; these are surrounded by feathers that are preserved in 3D and
with microscopic detail.
"We can be sure of the source because the vertebrae are not
fused into a rod or pygostyle as in modern birds and their closest relatives.
Instead, the tail is long and flexible, with keels of feathers running down
each side. In other words, the feathers definitely are those of a dinosaur not
a prehistoric bird."
The study's first author Lida Xing from the China University of
Geosciences in Beijing discovered the remarkable specimen at an amber market in
Myitkyina, Myanmar in 2015.
The amber piece was originally seen as some kind of plant
inclusion and destined to become a curiosity or piece of jewellery, but Xing
recognized its potential scientific importance and suggested the Dexu Institute
of Palaeontology buy the specimen.
The researchers say the specimen represents the feathered tail
of a theropod preserved in mid-Cretaceous amber about 99 million years ago.
While it was initially difficult to make out details of the amber inclusion,
Xing and his colleagues relied on CT scanning and microscopic observations to
get a closer look.
The feathers suggest the tail had a chestnut-brown upper surface
and a pale or white underside. The specimen also offers insight into feather
evolution. The feathers lack a well-developed central shaft or rachis. Their
structure suggests that the two finest tiers of branching in modern feathers,
known as barbs and barbules, arose before a rachis formed.
Professor Mike Benton from the School of Earth Sciences at the
University of Bristol, added: "It's amazing to see all the details of a
dinosaur tail -- the bones, flesh, skin, and feathers -- and to imagine how
this little fellow got his tail caught in the resin, and then presumably died
because he could not wrestle free.
"There's no thought that dinosaurs could shed their tails,
as some lizards do today."
The researchers also examined the chemistry of the tail
inclusion where it was exposed at the surface of the amber. The analysis shows
that the soft tissue layer around the bones retained traces of ferrous iron, a
relic left over from haemoglobin that was also trapped in the sample.
The findings show the value of amber as a supplement to the
fossil record. Ryan McKellar added: "Amber pieces preserve tiny snapshots
of ancient ecosystems, but they record microscopic details, three-dimensional
arrangements, and labile tissues that are difficult to study in other settings.
"This is a new source of information that is worth researching
with intensity, and protecting as a fossil resource."
The researchers say they are now "eager to see how
additional finds from this region will reshape our understanding of plumage and
soft tissues in dinosaurs and other vertebrates."
原始論文：Lida Xing,
Ryan C. McKellar, Xing Xu, Gang Li, Ming Bai, W. Scott Persons IV, Tetsuto
Miyashita, Michael J. Benton, Jianping Zhang, Alexander P. Wolfe, Qiru Yi,
Kuowei Tseng, Hao Ran, Philip J. Currie. A Feathered Dinosaur Tail with
Primitive Plumage Trapped in Mid-Cretaceous Amber. Current Biology,
2016; DOI: 10.1016/j.cub.2016.10.008
引用自：University of Bristol. "Amber specimen
offers rare glimpse of feathered dinosaur tail." ScienceDaily. ScienceDaily,
8 December 2016.
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