陸地上最早的四足動(dòng)物或許并非是勇敢的探險(xiǎn)者,，它們有可能僅僅是依沼澤而生的食腐動(dòng)物,。它們并非未經(jīng)“思考”便冒險(xiǎn)踏進(jìn)干旱的沙漠，而后才開始隨環(huán)境而進(jìn)化的,。一塊來自波蘭東南部古瀉湖中的,、3.95億年前的、長(zhǎng)約8英尺的四足動(dòng)物化石表明,，人類遠(yuǎn)古的祖先,，有可能并不是那些勇敢的、需要從一個(gè)池塘“躍”向另一個(gè)池塘的魚類,，而是生活在林區(qū)的具四附肢生物,，因?yàn)樗鼈儾恍枰?ldquo;冒險(xiǎn)”。
最近,，發(fā)表在The Journal of Geology雜志上的一項(xiàng)研究"Woodland Hypothesis for Devonian Tetrapod Evolution"顯示,，魚類與兩棲類的過渡形式存在于布滿樹的洪泛平原里。該研究的主要科研人員俄勒崗大學(xué)的Gregory Retallack博士說,，來自馬里蘭,、紐約和賓夕法尼來的諸多化石都顯示它們并非生存于池塘或沙漠中，而是與潮濕的林地土壤相伴生,，池塘或沙漠土壤中能夠找到許多魚類化石,，但都不是我們的遠(yuǎn)古祖先。
基于化石的研究,，魚類與兩棲類的過渡形式存在于布滿樹的洪泛平原里,。Retallack博士繼續(xù)說，我們的遠(yuǎn)古祖先并不笨,，它們是典型的機(jī)會(huì)主義者,，它們利用了充滿樹木的洪泛平原的優(yōu)勢(shì)得以繁衍生息。附肢能夠幫助繞開諸多樹木的阻礙,，而韌性的脖子則能使它們?cè)跍\水中取食,。因此，蠑螈（salamanders）與魚類最主要的區(qū)別——附肢與脖子,，進(jìn)化出來的目的并非是為了在沙漠中“探險(xiǎn)”,，而是在潮濕洪泛林地中的適應(yīng)性特征,。
Retallack博士說，對(duì)過渡形式的化石產(chǎn)出地周圍的古土壤與沉積物的研究,，是理解魚類何時(shí)何地在何種狀態(tài)下開始行走的關(guān)鍵,。許多汽車上的“達(dá)爾文魚”（Darwin fish，一條具腿的漫畫魚）裝飾紋其實(shí)代表了生命進(jìn)化的一個(gè)重要階段,。這一研究還挑戰(zhàn)了之前四足動(dòng)物生活于淺海中的理論,。（生物谷Bioon.com）
doi:10.1086/659144
PMC:
PMID:
Woodland Hypothesis for Devonian Tetrapod Evolution
Gregory J. Retallack
AbstractThe rarity of Devonian tetrapods and the absence of tetrapods during the first 14 million years of the Mississippian (Romer’s Gap) have inspired hypotheses of fish-tetrapod evolutionary transition as an escape from difficult habitats such as deserts or stagnant waters. These hypotheses and Romer’s Gap are tested here using depth to calcic horizon in paleosols of the northern Appalachians as a proxy for precipitation and vegetation changes during the Devonian and Mississippian. All tetrapod bones and trackways in this region, as well as fossil tree remains, were found at times of high precipitation (indicated by deep calcic paleosols), when subhumid woodlands expanded at the expense of semiarid shrublands in alluvial lowlands. No tetrapods or tree fossils were found in abundant paleosols with the shallow calcic horizons of aridland soils. The mean annual precipitation requirement of Devonian-Mississippian tetrapods and trees was at least mm. Global distribution of prototetrapods and aquatic tetrapods also shows that they lived earliest (Eifelian) in coastal lagoon or estuary margin soils of humid regions rather than in aridlands or intertidal flats. Tetrapods later spread to a variety of habitats, but continued preference for noncalcareous soils of woodlands and forests may explain Romer’s Gap as a preservational artifact. A woodland hypothesis of tetrapod evolution is presented here: limbs and necks were selected for by scavenging and hunting in shallow-flooded woodlands and oxbow lakes during a unique period in Earth history, after evolution of flood-ponding trees and before effective terrestrial predator resistance.
