在北美洲的白堊紀(jì)時(shí)期,，生活著一種體型很小,，能夠啃霸王龍等大型恐龍腳踝的小恐龍，它們還會(huì)偷竊大型恐龍的蛋,。這就是目前科學(xué)家發(fā)現(xiàn)的北美洲迄今發(fā)現(xiàn)最小的食肉恐龍——“黃昏龍”（Hesperonychus）,。
科學(xué)家發(fā)現(xiàn)北美洲最小的食肉恐龍
黃昏龍鋒利的趾狀骨骼
這是一種最新發(fā)現(xiàn)的新物種恐龍，它是食肉性恐龍,，體型比現(xiàn)今的家貓還小,，喜歡獵捕昆蟲、小型哺乳動(dòng)物或陷在沼澤和森林中的動(dòng)物,，它們生活在0.75億年前白堊紀(jì)末期現(xiàn)今加拿大艾伯特東南部地區(qū),。
它們的體重大約是2公斤，站著的身高為0.3米,，它非常類似于微型版的迅猛龍（Velociraptor）,，它們之間的親緣關(guān)系很近。黃昏是兩腿直立行走,，長著像剃刀一樣的爪子,，在其第二個(gè)腳趾上長著鐮刀外型的爪子，它的身體很苗條,，頭部很小,，長著匕首般的牙齒。
加拿大卡爾加里大學(xué)生物科學(xué)系古生物學(xué)副研究員尼克－朗里奇（Nick Longrich）說：“它的體型還不及現(xiàn)代家貓的大小,，它們搜尋獵物和所吃食物主要受其體型限制,，只能吃一些昆蟲、小型哺乳動(dòng)物,、兩棲動(dòng)物,，或許還有一些恐龍幼體,。它一生中主要的時(shí)間都用于在沼澤和森林中搜尋食物。”
朗里奇指出,，小型食肉恐龍?jiān)谶h(yuǎn)古時(shí)期的北美洲環(huán)境非常罕見,，人們之所以產(chǎn)生這樣的觀點(diǎn)，主要是由于現(xiàn)今北美洲小型食肉恐龍數(shù)量遠(yuǎn)超過大型食肉恐龍,。遠(yuǎn)古時(shí)期小型食肉恐龍?jiān)谏鷳B(tài)環(huán)境中所具有的作用遠(yuǎn)超出我們對它們的認(rèn)識(shí),。這項(xiàng)由朗里奇和艾伯特大學(xué)古生物學(xué)家菲利普－柯里（Philip Currie）共同完成的研究報(bào)告發(fā)表在3月16日出版的《美國國家科學(xué)院學(xué)報(bào)》期刊上。
據(jù)悉,，早在1982年考古學(xué)家們就曾在加拿大恐龍省立公園等地收集到黃昏龍的骨骼化石,，由于無法識(shí)別它屬于哪種恐龍物種，就將其命名為“西部爪子”,。其中一塊最重要,、保存非常完整的骨盆化石是由加拿大德蘭赫德市皇家蒂勒爾博物館海洋爬行動(dòng)物館長伊麗莎白－尼科爾斯（Elizabeth Nicholls）發(fā)現(xiàn)的。這些骨骼化石都未曾被識(shí)別,，直到25年之后朗里奇將它們帶到艾伯特大學(xué)進(jìn)行深入研究分析,，朗里奇和柯里基于爪子化石和保存完好的骨盆，最終確定了這種恐龍的物種歸屬,。
朗里奇說：“起初我們認(rèn)為這是一種恐龍幼體,，雖然它的骨骼結(jié)構(gòu)都非常小。但是當(dāng)我們研究它的骨盆時(shí),，發(fā)現(xiàn)它的髖骨融合在一起,，這種情況只出現(xiàn)在恐龍發(fā)育成熟的時(shí)期。這種恐龍是迄今在北美洲地區(qū)發(fā)現(xiàn)體型最小的食肉恐龍,。”（生物谷Bioon.com）
生物谷推薦原始出處：
PNAS March 16, 2009, doi: 10.1073/pnas.0811664106
A microraptorine (Dinosauria–Dromaeosauridae) from the Late Cretaceous of North America
Nicholas R. Longricha,1 and Philip J. Currieb
aDepartment of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, AB, Canada T2N 1N4; and
bDepartment of Biological Sciences, University of Alberta, Edmonton, AB, Canada T6G 2E9
The fossil assemblages of the Late Cretaceous of North America are dominated by large-bodied dinosaur species. Associated skeletons of small dinosaurs are exceedingly rare, and small (<10 kg) carnivorous theropods have not previously been reported from these beds. Here, we describe a small dromaeosaurid from the 75-million-year-old Dinosaur Park Formation of Alberta, Canada. Hesperonychus elizabethae gen. et sp. nov. is represented by a pelvic girdle from an animal weighing ≈1,900 g. Despite its size, the pubes and ilia are coossified, indicating that the animal was somatically mature. This is the smallest carnivorous, nonavian dinosaur known from North America. Phylogenetic analysis of Hesperonychus reveals that it is not closely related to previously described North American dromaeosaurids. Instead, Hesperonychus is a member of the dromaeosaurid clade Microraptorinae, a group containing the 4-winged Microraptor and the feathered Sinornithosaurus, both from the Lower Cretaceous Jehol Group of China. Hesperonychus is the youngest known member of this lineage, extending the temporal range of the clade by 45 million years, and it is the first microraptorine known from North America, providing further evidence for an affinity between the dinosaur faunas of North America and Asia. Study of fossil collections from the Dinosaur Park and Oldman formations of Alberta has revealed numerous isolated bones of small, basal dromaeosaurids, which are tentatively referred to Hesperonychus. These fossils suggest that small dromaeosaurids were a significant component of the carnivore community in this Late Cretaceous biota.
