作者 克爾-紐曼黑洞

盡管海百合Cyathidium foresti與 巨型牡蠣Neopycnodonte zibrowii的緊密共生并非必然（海百合也可直接固著于基巖上），但這種關(guān)聯(lián)可能帶來(lái)多重益處：首先，牡蠣殼的生長(zhǎng)持續(xù)為海百合提供新的固著空間，這對(duì)空間受限環(huán)境中的固著生物來(lái)說(shuō)至關(guān)重要；其次，二者雖都以濾食浮游生物為食，但攝食策略不同——牡蠣會(huì)主動(dòng)產(chǎn)生攝食水流，而海百合被動(dòng)依賴環(huán)境水流，因此可能受益于牡蠣創(chuàng)造的水流環(huán)境。

N. zibrowii屬于繁盛于白堊紀(jì)和古近紀(jì)，現(xiàn)已基本滅絕的硬牡蠣類（pycnodontine）；C. foresti是在侏羅紀(jì)至白堊紀(jì)期間多樣性高、數(shù)量豐富的海百合類群的孑遺。二者的共生關(guān)系同樣跨越了漫長(zhǎng)的時(shí)光——在位于丹麥法克瑟（Faxe）的距今逾 6000 萬(wàn)年的中丹尼階，即存在Cyathidium屬（該屬模式種C. holopus）和硬牡蠣（Pycnodontesp.）共生的化石。Nielsen 報(bào)道了法克瑟剖面大量原位保存在疑似硬底懸垂結(jié)構(gòu)下的群體，描述了海百合C. holopus常棲于牡蠣殼上，而牡蠣又附著于珊瑚形成的崖壁，且C. holopus本身常被新生牡蠣覆著。Steenstrup 在描述C. holopus時(shí)即記載其常見于厚牡蠣殼上，Ravn 與 Hennig 均注意到海百合偏好以倒置姿態(tài)附著于牡蠣鉸合區(qū)最大凹面處。6000 萬(wàn)年前的法克瑟牡蠣-海百合組合可能和現(xiàn)今的亞速爾群島組合一樣棲息于冷水深海珊瑚礁相伴的硬底懸崖與懸垂結(jié)構(gòu)下。

丹麥法克瑟中丹尼階的Cyathidium holopus與牡蠣化石。A：成百個(gè)C. holopus化石；B：位于可能的懸垂結(jié)構(gòu)下的大量C. holopus化石；C：固著于牡蠣殼內(nèi)表面的海百合（示意圖）；D、E：硬牡蠣殼內(nèi)表面固著的處于不同個(gè)體發(fā)育階段的C. holopus/ Wisshak 等，2009b

這些事實(shí)對(duì)研究海百合和牡蠣演化具有重要意義——這些海百合為逃避“中生代海洋革命（the Mesozoic Marine Revolution）”期間增強(qiáng)的捕食壓力而向深海遷移。固著海百合在晚中生代至新生代早期幾乎從淺水完全消失，被無(wú)柄的羽星類海百合取代，后者具有活動(dòng)能力且常嚴(yán)格夜行，因而能避開晝行性捕食者。最可能的潛在捕食者是具碎殼能力的真骨魚類，這類捕食者在馬斯特里赫特期至古新世經(jīng)歷了重大輻射演化。從Cyathidium在塞諾曼階至馬斯特里赫特階的記錄來(lái)看，其生境遷移始于晚白堊世[巴黎盆地與英國(guó)南部盆地（Anglo-Paris Basin，法國(guó)庇卡底和英國(guó)普利茅斯一帶）坎帕階的新發(fā)現(xiàn)即為證據(jù)]，而法克瑟的材料表明該過(guò)程到中達(dá)寧階已完成。這一時(shí)間線與真骨魚類輻射演化完美吻合。而牡蠣亦在逃避固著競(jìng)爭(zhēng)或捕食壓力。此外，白堊紀(jì)末危機(jī)可能也促進(jìn)了這一“生境逃離”過(guò)程，該事件最終導(dǎo)致了白堊紀(jì)-古近紀(jì)大滅絕。在捕食壓力相對(duì)較小的穩(wěn)定深水環(huán)境中，Cyathidium-(Neo)pycnodonte組合不僅躲過(guò)了這次大滅絕事件，更在未留下任何已知化石記錄且未顯著改變形態(tài)的情況下延續(xù)了整個(gè)新生代，最終成為今天可供原位研究的“活化石群落”。[完]

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