寒武纪叶足动物:困惑与思考

doi:10.13745/j.esf.sf.2020.6.21

摘要/Abstract

摘要：

叶足动物在显生宙最早期的海洋中曾一度繁盛,但如今茫茫大海中它们已杳无踪影。不过,学术界主流认为它们并没有绝灭——早期叶足动物的一个分支成功演化出原始的节肢动物,成就了当今地球上最庞大的动物家族(六足类、甲壳类、多足类、螯肢类等);叶足动物另一个分支则离开海洋,保持基本躯体构型,变身为陆生叶足动物,然而历经沧桑后,仅余一两脉(有爪动物及缓步动物)残喘幸存至今。这个史诗般的精彩演化故事,深深吸引了众多学者竞先求索。近30年来,以我国云南澄江生物群为代表的一系列寒武纪特异埋藏化石库中发现了大量叶足动物化石。经研究,学术界迄今似已破解了早期叶足动物重重疑团,足以额手称庆。然而治学须严谨,考证不偏狭。作者认为,围绕早期叶足动物的迷雾尚未消散,许多问题还有待解答,部分观点仍需确证。本文对早期叶足类研究的部分主流观点提出一些不同看法,并对尚未破解的疑难问题提出尝试性的见解和猜想。

关键词: 叶足动物, 泛节肢动物, 蜕皮动物, 寒武纪, 澄江生物群

Abstract:

Marine lobopodians that boomed worldwide during the early Cambrian have been broadly considered as closely affiliated with living panarthropods, including onychophorans (velvet worms), tardigrades (water bears), and arthropods (insects, crustaceans, myriapods, chelicerates, and allies). Despite the great ecological gap, extant terrestrial onychophorans and tardigrades are widely considered as the surviving descendants of Cambrian marine lobopodians. Resemblances between early lobopodians and living onychophorans are not just caterpillar-like gross morphology but distinctive features, such as foremost the unjointed lobopodous limbs. Another lineage of early lobopodians is believed to have given birth to the earliest stem arthropods, forming the largest animal phylum on our planet today. This epic evolutionary story has attracted many scientists to speculate on their evolutionary scenarios and phylogenetic significance. However, conundrums around these enigmatic animals remain unsolved. Here, I tentatively propose some arguments against mainstream views and offer some hypothetical approaches to researches on Cambrian lobopodians.

Key words: lobopodians, Panarthropoda, Ecdysozoa, Cambrian, Chengjiang biota

中图分类号:

P52
P534.4

欧强. 寒武纪叶足动物:困惑与思考[J]. 地学前缘, 2020, 27(6): 47-66.

OU Qiang. Cambrian lobopodians: confusion and consideration[J]. Earth Science Frontiers, 2020, 27(6): 47-66.

图/表 8

图1 叶足动物谱系位置及部分寒武纪代表 A—泛节肢动物谱系树,主流观点认为干群叶足动物(非单系群)、现代叶足动物(有爪类+缓步类)、节肢动物(干群+冠群)构成泛节肢动物(单系群),“†”示意叶足动物的化石类群,椭圆淡蓝区域代表干群;B—中华微网虫(Microdictyon sinicum);C—长足啰哩山虫(Luolishania longicruris);D—无饰贫腿虫(Paucipodia inermis);E—强壮怪诞虫(Hallucigenia fortis);F—凶猛爪网虫(Onychodictyon ferox);G—云南火把虫(Facivermis yunnanicus);H—秀丽触角棘足虫(Antennacanthopodia gracilis)。箭头示身体前端。资料来源:照片B-G由马晓娅研究员提供。比例尺:5 mm(B-H)。

Fig.1 Phylogenetic position and some fossil representatives of Cambrian lobopodians

表1 已报道的古生代海生叶足动物软躯体化石名录

Table 1 A list of Paleozoic marine lobopodians preserved with soft tissues

序号	属种	文献	古地理及岩性	年代地层
1	Xenusion auerswaldae	[16]	波罗的古陆;Kalmarsund砂岩	寒武系第2~3阶
2	Microdictyon sinicum	[17-18]	扬子板块;澄江泥岩 (帽天山“页岩”)	寒武系第3阶
3	Facivermis yunnanicus	[19-20]
4	Luolishania longicruris	[5,21]
5	Cardiodictyon catenulum	[14,22]
6	Onychodictyon ferox	[7]
7	Paucipodia inermis	[23-24]
8	Hallucigenia fortis	[25]
9	Megadictyon haikouensis	[26-27]
10	Miraluolishania haikouensis	[28]
11	Jianshanopodia decora	[29]
12	Onychodictyon gracilis (?)	[30]
13	Antennacanthopodia gracilis	[31]
14	Diania cactiformis (?)	[32-33]
15	Lenisambulatrix humboldti	[34]
16	Unnamed lobopodian	[35]	扬子板块;清江页岩
17	Collinsium ciliosum	[3]	扬子板块;小石坝泥岩
18	Tritonychus phanerosarkus	[36]	扬子板块;奥斯顿型(灰岩)
19	Hadranax augustus	[37]	劳伦古陆;Sirius Passet页岩
20	Kerygmachela kierkegaardi	[38-40]
21	Pambdelurion whittingtoni	[41-42]
22	Siberion lenaicus	[43]	西伯利亚古陆;Sinsk藻类透镜体	寒武系第4阶
23	Hallucigenia hongmeia	[44]	扬子板块;关山泥岩
24	Collinsiumsp.	[45]	扬子板块;关山泥岩
25	Unnamed “Collins’ monster”	[46]	冈瓦纳古陆;鸸鹋湾页岩
26	Microdictyonsp.	[47]	华南古陆;凯里页岩	寒武系乌溜阶
27	Acinocricus stichus	[48]	劳伦古陆;Spence页岩
28	Collinsovermis monstruosus	[49-50]	劳伦古陆;布尔吉斯页岩
29	Aysheaia pedunculata	[2,10]
30	Hallucigenia sparsa	[51-52]
31	Ovatiovermis cribratus	[4]
32	Orstenotubulus evamuellerae	[53-54]	波罗的古陆;奥斯顿型(灰岩)	寒武系鼓山阶
33	Unnamed luolishaniid	[55]	劳伦古陆;Fezouata泥岩	奥陶系特马豆克阶
34	Unnamed xenusiid (?)	[56]	冈瓦纳古陆;Soom页岩	奥陶系赫南特阶
35	Unnamed lobopodian (?)	[57]	劳伦古陆;Eramosa白云岩	志留系温洛克统
36	Carbotubulus waloszeki	[58]	劳伦古陆;梅逊溪菱铁矿结核	上石炭统

表1 已报道的古生代海生叶足动物软躯体化石名录

Table 1 A list of Paleozoic marine lobopodians preserved with soft tissues

序号	属种	文献	古地理及岩性	年代地层
1	Xenusion auerswaldae	[16]	波罗的古陆;Kalmarsund砂岩	寒武系第2~3阶
2	Microdictyon sinicum	[17-18]	扬子板块;澄江泥岩 (帽天山“页岩”)	寒武系第3阶
3	Facivermis yunnanicus	[19-20]
4	Luolishania longicruris	[5,21]
5	Cardiodictyon catenulum	[14,22]
6	Onychodictyon ferox	[7]
7	Paucipodia inermis	[23-24]
8	Hallucigenia fortis	[25]
9	Megadictyon haikouensis	[26-27]
10	Miraluolishania haikouensis	[28]
11	Jianshanopodia decora	[29]
12	Onychodictyon gracilis (?)	[30]
13	Antennacanthopodia gracilis	[31]
14	Diania cactiformis (?)	[32-33]
15	Lenisambulatrix humboldti	[34]
16	Unnamed lobopodian	[35]	扬子板块;清江页岩
17	Collinsium ciliosum	[3]	扬子板块;小石坝泥岩
18	Tritonychus phanerosarkus	[36]	扬子板块;奥斯顿型(灰岩)
19	Hadranax augustus	[37]	劳伦古陆;Sirius Passet页岩
20	Kerygmachela kierkegaardi	[38-40]
21	Pambdelurion whittingtoni	[41-42]
22	Siberion lenaicus	[43]	西伯利亚古陆;Sinsk藻类透镜体	寒武系第4阶
23	Hallucigenia hongmeia	[44]	扬子板块;关山泥岩
24	Collinsiumsp.	[45]	扬子板块;关山泥岩
25	Unnamed “Collins’ monster”	[46]	冈瓦纳古陆;鸸鹋湾页岩
26	Microdictyonsp.	[47]	华南古陆;凯里页岩	寒武系乌溜阶
27	Acinocricus stichus	[48]	劳伦古陆;Spence页岩
28	Collinsovermis monstruosus	[49-50]	劳伦古陆;布尔吉斯页岩
29	Aysheaia pedunculata	[2,10]
30	Hallucigenia sparsa	[51-52]
31	Ovatiovermis cribratus	[4]
32	Orstenotubulus evamuellerae	[53-54]	波罗的古陆;奥斯顿型(灰岩)	寒武系鼓山阶
33	Unnamed luolishaniid	[55]	劳伦古陆;Fezouata泥岩	奥陶系特马豆克阶
34	Unnamed xenusiid (?)	[56]	冈瓦纳古陆;Soom页岩	奥陶系赫南特阶
35	Unnamed lobopodian (?)	[57]	劳伦古陆;Eramosa白云岩	志留系温洛克统
36	Carbotubulus waloszeki	[58]	劳伦古陆;梅逊溪菱铁矿结核	上石炭统

表2 寒武纪叶足动物与现代有爪类、缓步类、节肢类性状对比[7,28,36,41,59-62,64,71-81]

Table 2 Similarities (homologous or analogous) and differences between modern onychophorans, tardigrades, arthropods and Cambrian lobopodians[7,28,36,41,59-62,64,71-81]

性状	寒武纪叶足动物 Cambrian lobopodians	有爪动物 Onychophorans	缓步动物 Tardigrades	节肢动物 Arthropods
蜕皮 (Ecdysis) ^a	√	√	√	√
α-几丁质的角皮 (Alpha-chitin cuticle)	?	√	√	√
局部硬化的角皮结构 ^b(Scattered cuticular sclerites)	√/×	×	√/×	√/×
体表环纹 (Body surface annulation)	√	√	×	×
铰合的硬化骨板 (Articulated sclerites)	×	×	×	√
内衬角皮的气管 (Tracheas lined with cuticle)	?	√	×	√/×
分节的附肢 (Jointed limbs)	×	×	×	√
叶足 (Lobopodous limbs)	√/×	√	√	×
附肢末端具爪 (Limbs with claws)	√	√	√	√
顶端口 (Anterior mouth)	√/×	×	√	×
硬化的口器 (Sclerotized mouthparts)	√/×	√	√	√
原脑附肢 (Protocerebral appendage)	√/×	√	×	×
单眼 (Ocelli)	√	√	√/×	√/×
复眼 (Compound eyes)	×	×	×	√/×
身体分段 (Segmented body)	√	√	√	√
血腔 (Haemocoel)	?	√	√	√
开放的循环系统 (Open circulatory system)	?	√	×	√
马氏管 (Malpighian tubules)	?	×	√	√
表达在胚胎分段条带的engrailed基因	?	√	√	√
1对腹神经管 (A pair of ventral nerve cords)	?	√	√	√
躯干腹侧神经节 (Trunk ventral ganglions)	?	×	√	√
头部体节数目 (Somite number of cephalon)	1~3(?)	3	1(?)	2~6
身体分部 (Tagmosis)	弱	弱	弱	强
躯干肌肉系统 (Trunk musculature)	环肌;纵肌	环肌;纵肌不分段	无环肌;纵肌分段	无环肌;纵肌分段
附肢肌肉系统 (Limb musculature)	?	外附肌+内附肌	外附肌+内附肌	外附肌+内附肌

表2 寒武纪叶足动物与现代有爪类、缓步类、节肢类性状对比[7,28,36,41,59-62,64,71-81]

Table 2 Similarities (homologous or analogous) and differences between modern onychophorans, tardigrades, arthropods and Cambrian lobopodians[7,28,36,41,59-62,64,71-81]

性状	寒武纪叶足动物 Cambrian lobopodians	有爪动物 Onychophorans	缓步动物 Tardigrades	节肢动物 Arthropods
蜕皮 (Ecdysis) ^a	√	√	√	√
α-几丁质的角皮 (Alpha-chitin cuticle)	?	√	√	√
局部硬化的角皮结构 ^b(Scattered cuticular sclerites)	√/×	×	√/×	√/×
体表环纹 (Body surface annulation)	√	√	×	×
铰合的硬化骨板 (Articulated sclerites)	×	×	×	√
内衬角皮的气管 (Tracheas lined with cuticle)	?	√	×	√/×
分节的附肢 (Jointed limbs)	×	×	×	√
叶足 (Lobopodous limbs)	√/×	√	√	×
附肢末端具爪 (Limbs with claws)	√	√	√	√
顶端口 (Anterior mouth)	√/×	×	√	×
硬化的口器 (Sclerotized mouthparts)	√/×	√	√	√
原脑附肢 (Protocerebral appendage)	√/×	√	×	×
单眼 (Ocelli)	√	√	√/×	√/×
复眼 (Compound eyes)	×	×	×	√/×
身体分段 (Segmented body)	√	√	√	√
血腔 (Haemocoel)	?	√	√	√
开放的循环系统 (Open circulatory system)	?	√	×	√
马氏管 (Malpighian tubules)	?	×	√	√
表达在胚胎分段条带的engrailed基因	?	√	√	√
1对腹神经管 (A pair of ventral nerve cords)	?	√	√	√
躯干腹侧神经节 (Trunk ventral ganglions)	?	×	√	√
头部体节数目 (Somite number of cephalon)	1~3(?)	3	1(?)	2~6
身体分部 (Tagmosis)	弱	弱	弱	强
躯干肌肉系统 (Trunk musculature)	环肌;纵肌	环肌;纵肌不分段	无环肌;纵肌分段	无环肌;纵肌分段
附肢肌肉系统 (Limb musculature)	?	外附肌+内附肌	外附肌+内附肌	外附肌+内附肌

图2 寒武纪典型叶足动物形态及同源对比 A,B,C—多毛柯林斯虫(Collinsium ciliosum);A—模式标本(YKLP 12127),示前6对特化具长刚毛的叶足、背-侧部环状分布的棘刺状骨板、头部结构等特征[3];B—A中聚焦框内放大,示躯干表面密集的短刚毛;C—另一标本前端,示头部的骨板、触角、可能的眼睛(YKLP 12131);D—凶猛爪网虫(Onychodictyon ferox)完整标本(ELEL-SJ101888A),示12对叶足及10对背骨板,箭头指示顶端口[7];E—该标本的前端放大,示顶端口、吻突、头部环状骨板、触角、眼睛、咽球、肠道等;F—凶猛爪网虫单个背骨板(棘刺未保存);G—现代有爪动物(Tasmanipatus barretti),产于塔斯马尼亚,箭头示叶足的外附肌在背部内侧的附着点;H—现代缓步动物(Macrobiotus sp.)前端的共焦激光照片(对比E),示顶端口、口道、口针、咽球、等端口;I—凶猛爪网虫另一标本(ELEL-SJ100307)前端放大,箭头指示透镜状单眼;J—现代有爪动物(Euperipatoides rowelli)幼体,示触角、透镜状单眼(箭头指示)、黏液乳突及叶足;K,L—分别为凶猛爪网虫及现代有爪动物的钩爪。资料来源:柯林斯虫照片(A-C)由杨杰研究员提供;现代有爪类及缓步类图像(G,H,J,L)由Georg Mayer教授提供。

Fig.2 Morphology and homologous correlation of typical Cambrian lobopodians

图3 寒武纪叶足动物与侏罗纪及现代六足类节肢动物幼虫对比 A,B,C—侏罗奇异虫(Qiyia jurassica)模式标本(STMN65-1),腹-斜侧压,示胸部腹侧吸盘、前6对粗短的原足、第7对细长的原足、肛突等结构[88];B—原足放大视图,示表面刚毛及末端的两排钩爪;C—身体末端放大视图,示2对肛突和1对末突(具纵列的气孔及棘刺),箭头指示气孔;D—现代肉蝇(Cuterebra sp.)完全发育的第三期幼虫(前视图),示体表离散分布的硬化棘刺;E,F,G,H—秀丽触角棘足虫(Antennacanthopodia gracilis)模式标本(ELEL-EJ081876B)[31];F,G,H—叶足放大视图,示表面环状、错落分布的棘刺(箭头指示)。资料来源:侏罗奇异虫照片(A-C)由王博研究员提供。AP:肛突;AT:触角;HK:钩爪;L1-L8:第1~8对叶足;LP:足垫;MG:中肠;P1-P7:第1~7对原足;SK:吸盘;TK:躯干;TP:末突。比例尺:2 mm(A);1 mm(B-E);200 μm(F-H)。

Fig.3 Comparisons between Cambrian lobopodians and larvae of Jurassic and extant hexapods

图4 寒武纪叶足动物与现代叶足动物的肌肉系统 A—寒武纪叶足动物秀丽触角棘足虫模式标本(Antennacanthopodia gracilis, ELEL-EJ081876B),示叶足内部肌肉组织[31];B,C—现代栉蚕(Euperipatoides rowelli)共焦激光图像,示腿肌、躯干纵肌及环肌;D-I—有爪动物的纳米层析图像,分别示提肌、按肌、启动肌、后动肌、前旋肌及后旋肌[78];J—现代水熊(Hypsibius exemplaris)扫描透射电镜图像[79]。彩色代表腿肌(叶足1~4),灰色代表躯干纵肌等结构。Bc:体腔;Cm:环肌;Crt:牵爪肌;He:头;Lgm:纵肌;Lm:腿肌;Mg:中肠;Om:斜肌。比例尺:A中1 mm;J中20 μm。资料来源:图像B-J由Georg Mayer,Vladimir Gross,Ivo Oliveira,Henry Jahn提供。

Fig.4 Muscular system of Cambrian and modern lobopodians

图5 寒武纪叶足动物与现代有爪动物的头部构造 A—凶猛爪网虫头部构造;B—链状心网虫头部构造;C—强壮怪诞虫头部构造;D—云南火把虫头部构造;E—长足啰哩山虫头部构造;F—秀丽触角棘足虫头部构造;G—现代有爪动物(Euperipatoides)胚胎共焦激光图像(颚的胚芽由人工染黄),头部分段性状可与凶猛爪网虫对比。图片来源:照片C由陈均远研究员提供,D、E由马晓娅研究员提供,G由Georg Mayer教授提供。

Fig.5 Head structures of Cambrian lobopodians and modern onychophorans

图6 节肢动物分节性状起源的两种假说 A—覆盖薄层角皮并具环纹的叶足动物(如寒武纪贫腿虫及温和步行虫,具静水骨骼系统);B—局部出现背骨板(如寒武纪爪网虫及微网虫);C—背骨板增大且叶足出现骨板(潜在的过渡类型);D—演化出关节铰合的外骨骼(见于节肢动物的杠杆式外骨骼系统)。图A-D据文献[108]修改。蓝色区域代表骨板或外骨骼。A’—覆盖薄层角皮并具环纹的叶足动物;B’—角皮整体逐渐增厚,环纹凹褶演化出一系列原始关节膜;C’—角皮继续整体增厚及硬化,演化出原始的外骨骼,同时原始关节膜数量逐渐减少;D’—演化出关节铰合的外骨骼。

Fig.6 Two hypotheses on the origin of articulation in arthropods

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