新的恐龍物種闡明了暴龍的演化歷程，以及對其臉部特徵的瞭解

原文網址：www.sciencedaily.com/releases/2017/03/170330103315.htm

新的恐龍物種闡明了暴龍的演化歷程，以及對其臉部特徵的瞭解

由澳洲和美國路易斯安那州、蒙大拿州、新墨西哥州和威斯康辛州的科學家組成的研究團隊，發現並命名了霸王龍家族中的一名新成員：Daspletosaurus horneri ，意指「霍納的懼龍」。

此物種以著名的恐龍古生物學家John "Jack" R. Horner命名，他曾擔任蒙大拿波茲曼洛磯山脈博物館的館長。研究人員以這種霸王龍的名字致敬他發現了無數恐龍化石，以及作為一名教師開啟並陪伴許多學生走過他們的學術職涯。模式標本目前為洛磯山脈博物館的研究館藏。

此研究由威斯康辛州迦太基學院生物科學系的教授Thomas Carr主持，他專門研究雷克斯暴龍(Tyrannosaurus rex)與其近親，統稱為霸王龍(tyrannosaurs)的演化和生長過程。

這種新的霸王龍發現於蒙大拿州，此處擁有的化石資源是恐龍演化的研究中心。蒙大拿州立大學的教授David Varricchio解釋：「這些樣本強調出蒙大拿州發現的恐龍化石紀錄有多麼卓越。它們的保存程度好到足以顯示這些巨型肉食動物活著時的外貌細節，且其全面性也可以讓我們瞭解霸王龍家族的演化過程。蒙大拿州至今仍然是探索白堊紀的絕佳地點。」

除了在霸王龍的家族樹上增添了一名新物種，團隊的研究也提供了霸王龍的演化模式及生活面貌—尤其是面部特徵的新資訊。這篇今日刊登於自然出版集團旗下的期刊《科學報導》(Scientific Reports)的最新研究中，發現的證據顯示霸王龍家族中有一條支系屬於無分支的稀有演化類型，並呈現出霸王龍的臉部覆滿鱗片且不具嘴唇，口鼻部對觸覺相當靈敏。

Carr說：「霍納懼龍是其譜系中關係最近的親戚，發現於加拿大亞伯達省的強健懼龍(D. torosus)之後第一位也是最後一位後裔。從兩者之間親密的演化關係，以及相近的地理分布位置和出現順序，我們得出牠們代表了在地質時間中一條單獨演化的譜系，其中強健懼龍逐漸演變成霍納懼龍。」

新墨西哥州立大學榮譽學院的教授Jason Moore進一步講解：「我們認為懼龍支系是線性演化(anagenetic)的例子，但要論證線性演化的困難度之一在於要確認問題中不同物種的生存時間並無重合。我們從雙麥迪遜組測出的放射性定年結果不只支持強健懼龍和霍納懼龍不曾同時生存，也有助於我們更加瞭解雙麥迪遜組紀錄的環境與生態變遷過程。」

澳洲詹姆斯庫克大學的地質科學教授Eric Roberts解釋：「放射性同位素定年法用於沉積物的長足進展，是檢測這則以及其他關於恐龍和別種古生物演化與生態議題的關鍵。此研究中呈現的新定年結果不過是冰山一角。在未來數年，持續進行的野外工作將會以前所未見的程度精進北美西部晚白堊紀恐龍的定年結果。」

Carr繼續說道：「當我們觀察這兩個物種生存的地質年代，懼龍的演化過程向我們指出發生在大型恐龍身上的演化速率可以有多慢，在此例中牠們的演化歷程經過了230萬年才完成。」

「此類種化過程稱作線性演化，跟較常見的分支演化(cladogensis)不同。分支演化意謂一個祖先物種分道揚鑣成兩個或以上的後代物種。雖然在關於演化的諸多研究中很少見到線性演化，但曾經有人發表在某些鴨嘴龍和角龍種類中有此現象出現。懼龍和其他類似的恐龍讓我們更加有能力挑選出化石紀錄中代表線性演化的證據。」

團隊的研究確實改變了對霸王龍面貌的認識，他們發現霸王龍戴著一張不具嘴唇，由平坦且寬大的鱗片以及如盔甲般的大片皮膚所組成的「面具」。此結論來自於霸王龍頭顱和鱷類、鳥類、哺乳類頭顱的比較結果，以及其他研究人員之前利用骨頭的結構來比對至不同種類的皮膚覆蓋物。

紐奧良路易斯安那州立大學健康科學中心的教授Jayc Sedlmayr解釋：「我們的研究中有許多部份不只是古生物學，而是於實驗室進行的比較解剖學成果。鳥類可視作現存的恐龍，而鱷類則為恐龍最近的親戚。我們將牠們解剖後深入血汙當中，根據同一類的面部神經和血管網絡在骨頭上留下的痕跡，我們可以利用相似之處來將其重建為一種新的霸王龍。」

「結果發現霸王龍的口鼻部和顎骨都很粗糙，除了沿著齒列的骨頭有一帶狹窄的光滑處之外。在鱷類，這種粗糙的質地深藏在體表平坦的寬大鱗片之下；由於霸王龍的骨頭具有同樣質地，因此牠們也具有相同的體表覆蓋物。」Carr如此解釋。「我們在霸王龍身上沒有找到一絲證據顯示牠們具有嘴唇：由鱗片覆蓋的粗糙質地一直延伸到將近齒列，而沒有容納嘴唇的空間。」

「然而，我們也找到證據顯示牠們的面部還覆有其他類型的皮膚，包括口鼻部及下顎兩側，此處的骨頭極度粗糙代表上方長有如盔甲一般的皮膚。這種跟盔甲一樣厚的皮膚或許可以保護霸王龍的這些部位在掠食及攝食的時候不受摩擦。」

他繼續說道：「驚人的是牠們眼睛後方的大角可以伸展出頭部兩側許多，代表這些角覆有角質層。此成分即為組成人類指甲堅硬且光亮的物質。」

鱷類和霸王龍的口鼻部與上下顎骨中有無數個微小的神經開孔穿梭其間，可以讓數百條三叉神經的分支通過而感知皮膚接收的訊息，使鱷類獲得有如人類指尖的敏感程度。Carr解釋：「有鑑於霸王龍也有同樣的小孔，我們認為牠們同樣具有超級敏銳的皮膚。」

Sedlmayr表示此種敏感度具有演化上的重大意義：「發現複雜的感官網路特別令人感興趣，因其源自三叉神經。由於三叉神經在不同脊椎動物中發展出各式各樣的『第六感』而具有與眾不同的演化史。這些第六感像是鳥類遷移時所用的感知磁場能力、鴨嘴獸的喙和海豚的毛囊具有電覺以利掠食、腹蛇利用偵測紅外線來辯別獵物、哺乳類透過觸鬚來幫助運動、短吻鱷可以偵測水傳來的震動、使象鼻成為靈敏的另一隻『手』，而霸王龍臉上則遍佈了可以達成這些作用的類似神經。」

New dinosaur species sheds light on evolution, provides facial makeover for tyrannosaurs

An investigation by a team of scientists from Australia, Louisiana, Montana, New Mexico, and Wisconsin has identified and named a new species of the tyrannosaur clan: Daspletosaurus horneri -- "Horner's Frightful Lizard."

The species is named for renowned dinosaur paleontologist John "Jack" R. Horner, formerly curator at the Museum of the Rockies (MOR) in Bozeman, Montana. The tyrannosaur's name honors his discoveries of numerous dinosaur fossils and his mentorship of so many students that launched them to accomplished scientific careers. The type (name-bearing) specimens are stored in the research collections of the MOR.

The research is led by Thomas Carr, a professor in Carthage College's Biology Department and an expert on the evolution and growth of Tyrannosaurus rex and its closest relatives, collectively called tyrannosaurs.

The fossil resources of Montana, where the new tyrannosaur was found, are central to studies of dinosaur evolution, explains Professor David Varricchio of Montana State University: "These specimens emphasize the excellent record of dinosaurs to be found in Montana. They highlight both the quality of the specimens, the preservation revealing the details of how these giant carnivores once looked in life, as well as the overall collection of specimens that provides insight into the evolution of the tyrannosaur group. Montana remains a wonderful place to explore the Cretaceous."

In addition to adding a new species to the tyrannosaur family tree, the team's research provides new information about the mode of evolution and life appearance of tyrannosaurs -- specifically the face. This latest study, published today in Nature Publishing Group's Scientific Reports, found evidence for a rare, nonbranching type of evolution in tyrannosaurs and that tyrannosaurs had scaly, lipless faces and a highly touch-sensitive snout.

Carr said: "Daspletosaurus horneri was the youngest, and last, of its lineage that lived after its closest relative, D. torosus, which is found in Alberta, Canada. The close evolutionary relationship between the species taken with their geographic proximity and their sequential occurrence suggests that together they represent a single lineage that changed over geological time, where D. torosus has morphed into D. horneri."

Jason Moore, a professor in the Honors College at the University of New Mexico, elaborated: "One of the difficulties in demonstrating anagenetic change, as we suggest occurred in the Daspletosaurus lineage, is establishing that the different species in question don't overlap in time. The new radiometric dates we measured from the Two Medicine Formation not only help support that D. torosus and D. horneri did not live at the same time, but also help us refine the timeline of environmental and ecological changes recorded by the Two Medicine Formation."

Eric Roberts, a professor in geosciences with the College of Science and Engineering at James Cook University, explained: "Advances in radioisotopic dating of sedimentary deposits is key to testing this and many other evolutionary and ecological questions about dinosaurs and other ancient organisms. New age dates presented in this study are just the tip of the iceberg. Ongoing work in this field will provide unprecedented improvements in the dating of Late Cretaceous dinosaurs from western North America over the next few years."

Continued Carr, "When we consider the geological ages of the two species, the evolution of Daspletosaurus gives us an indication of how slowly evolution can act on large dinosaurs, which in this case happened over a span of 2.3 million years.

"This type of speciation is called anagenesis, which is different from the more common type called cladogensis, where an ancestral species splits into two or more descendant species. Although uncommon in many evolutionary studies, anagenesis has been reported in some duck- billed dinosaurs and horned dinosaurs. Daspletosaurus and these other dinosaurs point the way forward in picking out the evidence for anagenesis in the fossil record."

The team's work literally changes the face of tyrannosaurs, which they found was covered by a lipless "mask" of large flat scales and extensive patches of armor-like skin. This conclusion results from comparison of tyrannosaur skulls with those of crocodilians, birds, and mammals, and earlier work by other researchers who had matched bone texture with different types of skin covering.

Jayc Sedlmayr, a professor at the Louisiana State University Health Sciences Center New Orleans, explained, "Much of our research went beyond field paleontology: it was generated from lab based comparative anatomy, where you get arms deep in "blood and guts" dissecting birds as living dinosaurs and crocodilians as their closest living relatives and based on the similarities of the facial nerves and arteries we found in those same groups that left a trace on the bones, we were able to then reconstruct them in the new tyrannosaur species."

"It turns out that tyrannosaurs are identical to crocodilians in that the bones of their snouts and jaws are rough, except for a narrow band of smooth bone along the tooth row. In crocodilians, the rough texture occurs deep to large flat scales; given the identical texture, tyrannosaurs had the same covering," explained Carr. "We did not find any evidence for lips in tyrannosaurs: the rough texture covered by scales extends nearly to the tooth row, providing no space for lips.

"However, we did find evidence for other types of skin on the face, including areas of extremely coarse bone that supported armor-like skin on the snout and on the sides of the lower jaws. The armor-like skin would have protected tyrannosaurs from abrasions, perhaps sustained when hunting and feeding."

"Strikingly, the large horn behind the eye is elevated beyond the side of the head, indicating a covering of keratin, the hard and shiny material that makes up human fingernails," he continued.

In crocodilians and tyrannosaurs, the snout and jaws are penetrated by numerous small nerve openings, allowing hundreds of branches of the trigeminal nerve to innervate the skin, producing a sensitivity that, in crocodilians, is as sensitive as human fingertips. "Given that the foramina are identical in tyrannosaurs indicates that they had super-sensitive skin as well," explained Carr.

This sensitivity is part of a bigger evolutionary story, explained Sedlmayr. "Our findings of a complex sensory web is especially interesting because it is derived from the trigeminal nerve, which has an extraordinary evolutionary history of developing into wildly different 'sixth senses' in different vertebrates, such as sensing magnetic fields for bird migration, electroreception for predation in the platypus bill or the whisker pits of dolphins, sensing infrared in pit vipers to identify prey, guiding movements in mammals through the use of whiskers, sensing vibrations through the water by alligators, and turning the elephant trunk into a sensitive 'hand' similar to what has been done to the entire face of tyrannosaurs."

原始論文：Thomas D. Carr, David J. Varricchio, Jayc C. Sedlmayr, Eric M. Roberts, Jason R. Moore. A new tyrannosaur with evidence for anagenesis and crocodile-like facial sensory system. Scientific Reports, 2017; 7: 44942 DOI: 10.1038/srep44942

引用自：Carthage College. "New dinosaur species sheds light on evolution, provides facial makeover for tyrannosaurs." ScienceDaily. ScienceDaily, 30 March 2017.