你是否注意過大象厚重的掌上長著幾個(gè)腳趾,？許多博物館里的標(biāo)本都是五個(gè)，但近日英國一項(xiàng)最新研究說,，對(duì)一些大象腳掌的仔細(xì)分析顯示，它們都長有“六趾”,，與中國的國寶大熊貓一樣長有“六趾”,。
英國皇家獸醫(yī)學(xué)院等機(jī)構(gòu)研究人員在新一期美國《科學(xué)》雜志上報(bào)告說,，分析了許多大象腳掌的標(biāo)本，確認(rèn)了“第六趾”的存在,，不過這不是一個(gè)像其他五趾那樣真正的趾頭,，而是由籽骨變異而成的具有腳趾功能的結(jié)構(gòu)。
籽骨是許多動(dòng)物體內(nèi)都有的一種小骨頭,，可以起到支撐肌腱的作用,。但本次研究發(fā)現(xiàn)，許多大象腳后跟處的籽骨變大變長,，已經(jīng)形成了類似其他五個(gè)腳趾骨的結(jié)構(gòu),，它伸出的方向與五個(gè)腳趾骨相反，在腳后跟的位置幫助支撐大象身體重量,。
領(lǐng)導(dǎo)這一研究的約翰·哈欽森教授說,，其實(shí)在很久以前就有研究人員在解剖大象時(shí)發(fā)現(xiàn)了籽骨結(jié)構(gòu)，但都把這當(dāng)作一塊奇怪的軟骨,，并沒有多加注意,，很多博物館在設(shè)置大象標(biāo)本時(shí)也沒有加上這個(gè)“第六趾”。
大象的“第六趾”被長期忽視還因?yàn)榇笙竽_掌難以研究,?；铙w大象不易麻醉，它厚厚的腳掌也不容易用X光等手段來透視,。哈欽森從動(dòng)物園獲得了多頭大象去世后的腳掌樣本,，經(jīng)過比對(duì)分析才能夠確認(rèn)。
他還指出,，這是進(jìn)化中反復(fù)出現(xiàn)的一個(gè)現(xiàn)象,，即可以將籽骨變異成一個(gè)具有手指或腳趾功能的結(jié)構(gòu)，大熊貓的“六趾”現(xiàn)象也是如此,。大熊貓的腳掌上有一個(gè)乍看像人手大拇指的結(jié)構(gòu),，但仔細(xì)觀察就會(huì)發(fā)現(xiàn)，它的五個(gè)腳趾都并排長在一起,，這個(gè)“拇指”是由其腳掌邊緣的籽骨變異形成的,，用于抓握竹子。(生物谷Bioon.com）
doi:10.1126/science.1211437
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From Flat Foot to Fat Foot: Structure, Ontogeny, Function, and Evolution of Elephant “Sixth Toes”
John R. Hutchinson, Cyrille Delmer, Charlotte E. Miller, Thomas Hildebrandt, Andrew A. Pitsillides1, Alan Boyde
The enlarged radial sesamoid bones of giant panda forefeet (1, 2) are classic examples of evolutionary exaptation (3, 4): co-option of old structures for new functions. It is less widely recognized that such “sixth toes” or “false thumbs” have evolved convergently in numerous tetrapods, such as moles and frogs (5, 6). They exist in numerous mammals in a less enlarged state, variably called the prepollex/prehallux (here called predigits), radial/tibial sesamoids, or other terms (such as falciform, accessory scaphoid, or navicular). Whether these sesamoids are ancestrally or convergently evolved in various tetrapod clades remains to be determined. The latter seems likely, given the absence of similar sesamoids in most fossil outgroups, yet a cartilaginous nodular precursor cannot be excluded. Regardless, enlarged sesamoids are quite prominent in both the manus (forefeet) and the pedes (hindfeet) of elephants, where they have been mistaken for sixth digits or otherwise presumed to play a role in foot support (7–9). Indeed, the recent discovery that moles have developmentally switched their radial sesamoid (prepollex) to a digit-like identity (10) intimates that elephants and other species may have done the same. Here, we report a multidisciplinary anatomical, histological, functional, and phylogenetic analysis (11) of the predigits in elephant feet. We hoped this would illuminate how elephants evolved their characteristic subunguligrade (nearly “tip-toed,” with only distal toes contacting the ground) foot posture and function, as compared with the plesiomorphic plantigrade (“flat-footed,” with wrists/ankles contacting the ground) foot posture in many other tetrapods.
