目前所知最古老的皮膚化石比之前的案例還老了2100萬年

原文網址： https://www.sciencedaily.com/releases/2024/01/240111113207.htm

研究人員最近鑑定出一片立體的皮膚化石，年代至少比之前描述過的還早了2100萬年。這片皮膚屬於一種古生代的原始爬蟲類，表面有顆粒狀的小突起，跟現在最相似的動物為鱷魚。此化石是保存至今的表皮(epidermis)中最古老的案例——表皮是陸生爬蟲類、鳥類和哺乳類皮膚的最外層，在生命登陸的過程中是個重要的演化革新。描述這具化石的文章1月11日發表在期刊《當代生物學》(Current Biology)，內容還有同樣採自美國奧克拉荷馬州Richards Spur石灰岩洞穴系統中的其他幾副樣品。

皮膚化石。圖片來源：Current Biology/Mooney et al.

「我們偶爾可以得到絕佳的機會來窺探許久之前的過去，」研究第一作者，多倫多大學的古生物研究生Ethan Mooney表示。他在讀大學時和多倫多大學的古生物學家Robert Reisz進行了這項計畫。

「這類型的發現可以大幅豐富我們對這些動物先驅的理解與認知。」

皮膚和其他軟組織鮮少形成化石，但研究團隊認為在此案例中，由於該洞窟系統具有某些特殊的性質使得皮膚可以保存下來。這些特性包括可以減緩分解速度的細粒黏土沉積物、有石油滲出以及該洞窟可能是個無氧環境。

「二疊紀早期的動物可能摔落到這個洞窟系統當中，被非常細的黏土沉積物埋藏起來而延緩了腐敗過程，」Mooney說。

「但引人注目的是此洞窟系統在二疊紀時也是經常有油氣滲漏出來的地方。石油和瀝青含有的碳氫化合物跟動物遺體之間的交互作用，可能是皮膚得以保存下來的原因。」

這片皮膚化石連指甲的大小都不到。經由共同作者，多倫多密西沙加大學的Tea Maho在顯微鏡下檢驗之後，顯示其為一塊表皮組織——此為羊膜動物(amniotes)的特徵之一。羊膜動物是一群陸生脊椎動物的統稱，包括了爬蟲類、鳥類以及哺乳類，牠們於石炭紀的時候從兩棲類祖先演化而來。

「我們看到它的時候都震驚不已，因為這完全不是我們預期會找到的東西，」Mooney表示。

「發現如此古老的皮膚化石是個絕佳的機會讓我們窺探許久之前的過去，並且觀察某些非常原始的動物可能擁有什麼樣的皮膚。」

這片皮膚有某些特徵跟古代以及現存的爬蟲類相同，包括類似鱷魚皮表面的顆粒狀突起，以及像是蛇與蚓蜥外皮的構造，表皮的鱗片之間具有絞鍊區。

然而，這片皮膚化石並沒有跟骨頭或者其他類型的遺骸一起出土，因此無法辨識它是屬於什麼動物或者是長在什麼部位。

這片古老的皮膚跟現生的爬蟲類有類似之處，顯示這些構造對於在陸地環境生存來說非常重要。

「它是一道極為重要的屏障，把身體內部進行的作用跟嚴酷的外在環境隔絕開來。」

研究人員表示這片皮膚或許可以代表早期羊膜動物中的陸生脊椎動物所擁有的原始皮膚構造，而這種構造最終演化出鳥類的羽毛以及哺乳類的毛囊。

這副皮膚化石以及其他樣本是由畢生都對古生物學極具熱忱的Bill May 和Julie May採集到的，地點為奧克拉荷馬州一座仍在營運的採石場——Richards Spur石灰岩洞穴系統。此處獨有的條件保存了許多早期陸生動物已知最古老的樣品，這些樣品目前存放在皇家安大略博物館。

 

Oldest known fossilized skin is 21 million years older than previous examples

Researchers have identified a 3D fragment of fossilized skin that is at least 21 million years than previously described skin fossils. The skin, which belonged to an early species of Paleozoic reptile, has a pebbled surface and most closely resembles crocodile skin. It's the oldest example of preserved epidermis, the outermost layer of skin in terrestrial reptiles, birds, and mammals, which was an important evolutionary adaptation in the transition to life on land. The fossil is described on January 11 in the journal Current Biology along with several other specimens that were collected from the Richards Spur limestone cave system in Oklahoma.

"Every now and then we get an exceptional opportunity to glimpse back into deep time," says first author Ethan Mooney, a paleontology graduate student at the University of Toronto who worked on the project as an undergraduate with paleontologist Robert Reisz at the University of Toronto.

"These types of discoveries can really enrich our understanding and perception of these pioneering animals."

Skin and other soft tissues are rarely fossilized, but the researchers think that skin preservation was possible in this case because of the cave system's unique features, which included fine clay sediments that slowed decomposition, oil seepage, and a cave environment that was likely an oxygenless environment.

"Animals would have fallen into this cave system during the early Permian and been buried in very fine clay sediments that delayed the decay process," says Mooney.

"But the kicker is that this cave system was also an active oil seepage site during the Permian, and interactions between hydrocarbons in petroleum and tar are likely what allowed this skin to be preserved."

The skin fossil is tiny -- smaller than a fingernail. Microscopic examination undertaken by coauthor Tea Maho of the University of Toronto Mississauga revealed epidermal tissues, a hallmark of the skin of amniotes, the terrestrial vertebrate group that includes reptiles, birds, and mammals and which evolved from amphibian ancestors during the Carboniferous Period.

"We were totally shocked by what we saw because it's completely unlike anything we would have expected," says Mooney.

"Finding such an old skin fossil is an exceptional opportunity to peer into the past and see what the skin of some of these earliest animals may have looked like."

The skin shares features with ancient and extant reptiles, including a pebbled surface similar to crocodile skin, and hinged regions between epidermal scales that resemble skin structures in snakes and worm lizards.

However, because the skin fossil is not associated with a skeleton or any other remains, it is not possible to identify what species of animal or body region the skin belonged to.

The fact that this ancient skin resembles the skin of reptiles alive today shows how important these structures are for survival in terrestrial environments.

"The epidermis was a critical feature for vertebrate survival on land," says Mooney.

「表皮對於生存在陸地的脊椎動物來說是關鍵性的特徵，」Mooney表示。

"It's a crucial barrier between the internal body processes and the harsh outer environment."

The researchers say that this skin may represent the ancestral skin structure for terrestrial vertebrates in early amniotes that allowed for the eventual evolution of bird feathers and mammalian hair follicles.

The skin fossil and other specimens were collected by lifelong paleontology enthusiasts Bill and Julie May at Richards Spur, a limestone cave system in Oklahoma that is an active quarry. The unique conditions at Richards Spur preserved many of the oldest examples of early terrestrial animals. The specimens are housed at the Royal Ontario Museum.

原始論文：Ethan D. Mooney, Tea Maho, R. Paul Philp, Joseph J. Bevitt, Robert R. Reisz. Paleozoic cave system preserves oldest-known evidence of amniote skin. Current Biology, 2024; DOI: 10.1016/j.cub.2023.12.008

引用自：Cell Press. "Oldest known fossilized skin is 21 million years older than previous examples."