宣揚爪蟲
克爾凱郭宣揚爪蟲(學名:Kerygmachela kierkegaardi)是宣揚爪蟲屬(學名: Kerygmachela)[3]又稱齊氏揮螯蟲或是齊氏宣揚爪蟲[4],的模式種也是唯一一個物種。是在北格陵蘭皮里地的Buen Formation發現的,[2][5][6][7][8][9]與厭惡蟲屬和篩蝦都是西里斯帕斯特生物群。有著粗壯的附肢,每個附肢有四個長突刺,軀幹中分成十一個鰓狀側葉。每個胸節都有成對的鰓狀側葉和粗壯的行走附肢。 後部區域較小,呈梯形。有兩個長的、分段的尾刺。全身長約17.5公分。[10]而所宣揚爪蟲被歸類為“有鰓葉足動物“(Gilled lobopodians)(以宣揚爪蟲和厭惡蟲屬為代表)共有一個環狀的、帶刺的附肢,普遍認為它們是節肢動物幹群中的更多分支,[11]也稱為巨型葉足動物(“Large lobopodians”)(尖山葉足蟲屬、大網蟲屬、奧古斯鹿角蟲、宣揚爪蟲、厭惡蟲屬和西伯利亞蟲屬)。[9][12]在分類學上宣揚爪蟲是最外側的基幹物種與厭惡蟲屬和歐巴賓海蠍一樣。[13]
詞源學 编辑
宣揚爪蟲是由希腊语Kerygma是指宣告、宣揚的意思,希腊语chela是指爪子,種小名kierkegaardi 則是紀念一位丹麥學者索倫·奧貝·克爾凱郭爾(Søren Aabye Kierkegaard)。[3]
身體構造 编辑
它有一個清楚分明的頭部區域,有一對橫向皺紋、粗壯的前附肢(frontal appendages),每個附肢都有四個細長的突刺(process)。 後面有兩根長節狀的尾刺(tail spine),身長約17.5至20公分。身體呈類矩形,身體是由11 個扇鰭(flap)組成,每個節包括一對側向的扇鰭,這些扇鰭的背面叫做鰓(gill)的皺紋結構。[10][14]最前面的一對前附肢特化為一對擴大可以捕食生物的大小,沿其內緣具有堅固、細長的刺。 [10][11][15][16]
頭部 编辑
頭部形成一個朝前方的圓葉,在延伸至兩個附肢之間。面向前方的嘴巴的位置不是在末端,而是位於圓形前葉(circular lobe)下方的腹側,位於一對稱為嘴刺(rostral spine)的之間。[15]
眼睛 编辑
附肢底部的結構根據其位置和保存情況很可能是眼睛,具有特殊的起伏和高反射率。眼睛外形類似於三葉蟲的腎形或鐮刀形的眼葉。 將眼睛為位於腹外側,因為在標本中它們的外形最清晰,其中嘴部結構和口刺最明顯。眼睛的前半部位於附肢的下後方。 眼睛中有較明顯的起伏,[15]但是眼睛的起伏是有爭議的。[17]與寒武紀的葉足動物不同,寒武紀的葉足動物的眼睛不會擁有這種起伏。[18]眼睛是球形的。宣揚爪蟲眼睛的腹外側位置,兇猛爪網蟲(Onychodictyon ferox)的單眼狀眼睛也是在觸角的腹側。[15]宣揚爪蟲的眼睛也與奇蝦和麒麟蝦或是歐巴賓海蠍類似。[16]
宣揚爪蟲 Kerygmachela kierkegaardi | |
神經系統 编辑
在化石中有發現身經系統的痕跡,[19]位於附肢之間的應該為中樞神經系統(central nervous system)。 這些結構與抱怪虫科的有爪里拉琴蟲中描述的神經系統織相似,其拓撲學結構更類似於神經組織。與有爪里拉琴蟲不同的是,是從大腦的外側邊緣控制兩個前附肢。而有爪里拉琴蟲擁有提供相關神經節的神經纖維束[註 1](nervous tract)。宣揚爪蟲神經幾乎延伸到附肢的尖端,並可能可以控制最外面和最長的突刺。有從大腦向前引出一個神經前突(anterior neural projection;nap)(左圖的淺黃色部分),向後發出三對神經索,分別是前附肢神經束(frontal appendage nervous tract;nfa),眼神經(optic nerve;nop)以及通往其身體中段的神經索(nerve cord;nc)。宣揚爪蟲控制前圓葉的神經組織為源自第一個腦神經節——原腦的神經且宣揚爪蟲的大腦是原腦(protocerebrum;npc) [15]原腦包括主要的處理,例如身體和神經分泌細胞(neurosecretory cells)。[17]
軀幹(身體) 编辑
軀幹有11對扇鰭,形成橢圓形輪廓,並有一個不分段的尾刺。身體有十一排小突節(tubercle),與每組側扇鰭相對應,每排有四個結節,越往尾部結節會變得更加顯眼。軀幹中央的兩個結節比外側的結節大,尾部間隔會更寬。 扇鰭向後逐漸變小,第四對和第五對側翼似乎最大,呈現橢圓形輪廓。 雖然宣揚爪蟲的標本只有發現到一根長尾刺,但通常都會理解成一對尾刺, 然而,標本在保存時都顯示出單一又長在軀幹末端的中間且未分段的尾刺。 尾刺如果不包括附肢的話,那幾乎與身體等長。[15]內部有包含圓形排列肌肉組織的結構。[2][10][15]
消化系統 编辑
宣揚爪蟲的消化系統擁有八個肝胰臟(Digestive glands),肝胰臟是與哺乳動物中分開的肝與胰相同,包括了消化酶和吸收被消化的食物。[9]
咽 部 编辑
習性 编辑
宣揚爪蟲的分類 编辑
宣揚爪蟲最初被Budd被描述為具有葉足動物[10],然而Chen對此宣揚爪蟲與葉足動物的親緣關係表示質疑,用了四個理由證明。特徵1,Budd所描述的動物中的「葉足」可能不是腿,因為這些結構在背部和腹側都以起伏形,表明葉足很僵硬;它們只跨越身體寬度的 30% 左右,太小而無法提供穩定性;它們可能位於身體兩側的側面,而寒武紀葉足動物的腿位於腹側相距 90°,所以這一位置與腿部不相容。 特徵2,「圓形的身體肌肉組織」結構太大且位置錯誤,無法與葉足動物的肌肉組織有差異。 特徵3,「有刺的附肢」顯然指的是宣揚爪蟲的附肢,但指的是葉足動物的葉足腿,這是兩種無與倫比的結構。 特徵4,「橫向皺紋」僅反映了類似的手風琴狀軀幹,這是外骨骼彎曲未分開的表面[16],之後Budd覺得毫無根據。Budd表示這群動物包含相對「節肢化(Arthropodization)」的代表(例如:奇蝦),具有類似節肢動物的附肢,以及更像「葉足動物」的動物(例如:宣揚爪蟲,歐巴賓海蠍)。[21]
宣揚爪蟲的出現代表了從祖先的葉足動物或有爪動物門的身體逐漸演化成節肢動物的第一步,許多其他下幹群節肢動物的定義特徵之一是扇鰭。 這些被為與節肢動物肢體同源(在放射齒目中,並延伸至其他下幹類群)。 除了葉足之外,還發現了這些扇鰭,位置更靠腹側。 [13]在所有類群中都會有不同的附肢, 如果葉足動物是並系群,那麼會有可能在奇蝦類群(宣揚爪蟲、厭惡蟲屬、歐巴賓海蠍和放射齒目)和節肢動物中保留這種特徵。 儘管沒有明顯的同源關係,但宣揚爪蟲、歐巴賓海蠍和奇蝦都具有共同的特徵。 在這些類群中,附肢內緣邊邊有刺,末端有一組較長的刺,附肢的主體有皺紋。 因此,這些身體結構之間存在一定程度的細節相似性。歐巴賓海蠍的附肢也很類似,但位於靈活的幹附肢(trunk)(是指歐巴賓海蠍的附肢連接到身體的區域)末端。奇蝦的附肢具有類似這種排列的刺,但高度硬化和分段。 由於它們的原始特徵此附屬物不能用於定義類奇蝦類群的單系群。[21]深度分化的節肢動物幹群(葉足動物,如尖山葉足蟲屬和大網蟲屬,以及有鰓的葉足動物)具有單節頭部,具有唯一對附肢。 這種頭部由單一原腦的情況在緩步動物中仍然存在。有鰓葉足動物的成對環狀附肢,到歐巴賓海蠍的融合幹附肢,再到放射齒目的硬化附肢。[11][17][22]
[23][24][25][26][27][28][29][30][31][32][33][19][34][35][36][37][38][39][34][36][40][41][42][43][5][44][45][46]
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注釋 编辑
- ^ (在白質中,起止、行程和功能基本上相同的神經纖維集合在一起形成的纖維束)