古生物學家找到了神祕的古代錐形海洋生物的分類地位

原始網址：www.sciencedaily.com/releases/2017/01/170111132815.htm

古生物學家找到了神祕的古代錐形海洋生物的分類地位

演化樹上的一根分枝在今天變得更加擁擠了一些。由多倫多大學20歲的學生Joseph Moysiuk領導的科學家團隊，終於確認了一種已經滅絕的奇特錐形動物族群的真實身分。

被稱為軟舌螺(hyolith)的這種海洋生物大約是在5.3億年前的寒武紀期間演化出來，牠們也是已知會形成礦物質外骨骼的首批動物之一。

雖然長久以來牠們被認為跟蝸牛、烏賊和其他軟體動物所屬同一家族，今日在科學期刊《自然》上刊登的一篇論文卻證實軟舌螺與腕足動物的關係更為密切。儘管腕足動物門存活至今的現生物種相當稀少，但牠們卻留下了相當豐碩的化石紀錄。

腕足動物擁有被上下外殼包覆的柔軟本體，這跟雙殼軟體動物的外殼呈現左右排列有所不同。腕足動物在攝食時會從前端打開牠們的外殼，其餘時刻則緊緊閉闔以保護他們的攝食構造以及身體的其他部分。

雖然化石紀錄中軟舌螺的骨骸為數眾多，但含有牠們軟組織構造中重要特徵的化石至今為止仍然付之闕如。

「我們最重要也最驚人的發現是找到了軟舌螺的攝食構造。這是一排從口部伸出的柔軟觸手，位在錐狀的下殼與蓋子般的上殼之間。」Moysiuk表示。「在現生動物當中只有一群動物擁有被雙殼包覆的類似攝食構造，那就是腕足動物。此發現顯示跟軟舌螺關係最密切的現存動物是腕足動物而非軟體動物。」

Moysiuk說：「這表示軟舌螺跟現今的腕足動物一樣以懸浮在水中的有機物質維生，牠們會利用牠們的觸手將食物掃進嘴裡。」

於多倫多大學主修地球科學和生態及演化生物學的Moysiuk完成的這項專題為該校研究機會培育計畫的一部份。此計畫為多倫多大學文理學院專門為大學生設立。

過往試著分類軟舌螺時都對牠們獨特的外型和構造相當棘手。所有軟舌螺都有著細長且兩側對稱的錐狀外殼，在開口處則有一小塊蓋狀甲殼(口蓋，operculum)可以將其封住。有些種類還有一雙堅硬的弧狀尖刺(海倫體，helens)從錐狀外殼和口蓋之間延伸出來。在其他動物族群當中完全沒有跟海倫體同樣的構造。

仔細檢查從伯吉斯頁岩中找到的不同軟舌螺樣品的海倫體延伸方向，研究人員認為這些尖刺可能是這些動物將牠們的身體從沉積物中撐高時所用的支架，牠們以此舉起攝食器官來增加捕食效率。

Moysiuk以及共同作者英國杜倫大學的Martin Smith，和多倫多大學與皇家安大略博物館的Jean-Bernard Caron，主要是依據從英屬哥倫比亞著名的伯吉斯頁岩中最新發現的化石來完成他們的論述。

「伯吉斯頁岩之所以獨一無二是因為它保存了在正常情況下通常難以留存下來的軟組織。」Moysiuk的指導教授Caron表示。Caron是皇家安大略博物館無脊椎古生物學的高級研究員，同時也是多倫多大學地球科學、生態學和演化生物學系的副教授。

「雖然有些論文的作者提出牠們跟軟體動物的親緣較為接近，但這些假說的依據仍然不夠充足。軟舌螺成為演化樹上無家可歸的一道分枝，成為古生物學家難以啟齒的存在。在第一次對軟舌螺做出記述約莫175年之後，我們終於從最新的野外研究成果中找到了破解牠們故事的關鍵。」

近期由Caron領導對伯吉斯頁岩進行的野外研究成果中，找到了許多成為此研究基礎的樣品。這些關鍵樣品發現於庫特尼國家公園中，史丹利冰河和大理石峽谷附近最新發現到的沉積物。該地點位於幽鶴國家公園最初的伯吉斯頁岩場址東南方40公里處。

要研究於5.42億年前開始的寒武紀，動物如何誕生及早期的演化過程時，伯吉斯頁岩是最重要的化石庫之一。在寒武紀大爆發誕生的生物群中，軟舌螺僅是眾多動物族群裡最具代表性的其中一類。牠們成為全球海洋生態系中一個成員眾多的家族長達2.8億年，卻在首批恐龍演化出現以前的2..52億年前步入滅絕。

「寒武紀大爆發是化石紀錄中最主要的動物族群迅速演化而成的時期，解開有關軟舌螺分類地位上的爭議有助於讓我們對這段時期有更多的理解。」Smith表示。他在牛津大學時開始這項研究，現在則為杜倫大學的古生物學講師。「對於要闡釋那些我們仍所知甚少的生物的演化史來說，我們的研究再次重申了伯吉斯頁岩類型的沉積物中保存的軟組織有多麼重要。」

從數個地點發現的這些樣品皆來自伯吉斯頁岩，其為加拿大洛磯山脈世界遺產的一部分。在研究於5.42億年前左右開始發生的寒武紀大爆發期間，動物的起源和早期演化過程時，伯吉斯頁岩是最重要的化石庫之一。

加拿大公園管理局保護這處享譽全球的場所，並且贊助同儕審核的科學研究以持續增進我們對這座豐富的古生物寶庫的認識。加拿大公園管理局出版的指南每年跟眾多遊客熱心分享這些早期動物演化的精采故事，而這項發現又為此更添一筆。

本研究的主要資金來源為皇家安大略博物館，以及加拿大自然科學暨工程研究委員會授予Caron的經費。

Paleontologists classify mysterious ancient cone-shaped sea creatures

One branch on the tree of life is a bit more crowded today. A team of scientists led by 20-year-old University of Toronto (U of T) undergraduate student Joseph Moysiuk has finally determined what a bizarre group of extinct cone-shaped animals actually are.

Known as hyoliths, these marine creatures evolved over 530 million years ago during the Cambrian period and are among the first animals known to have produced mineralized external skeletons.

Long believed to belong to the same family as snails, squid and other molluscs, a study published today in the scientific journal Nature shows that hyoliths are instead more closely related to brachiopods -- a group of animals which has a rich fossil record, although few living species remain today.

Brachiopods have a soft body enclosed between upper and lower shells (valves), unlike the left and right arrangement of valves in bivalve molluscs. Brachiopods open their valves at the front when feeding, but otherwise keep them closed to protect their feeding apparatus and other body parts.

Although the skeletal remains of hyoliths are abundant in the fossil record, key diagnostic aspects of their soft-anatomy remained critically absent until now.

"Our most important and surprising discovery is the hyolith feeding structure, which is a row of flexible tentacles extending away from the mouth, contained within the cavity between the lower conical shell and upper cap-like shell," said Moysiuk. "Only one group of living animals -- the brachiopods -- has a comparable feeding structure enclosed by a pair of valves. This finding demonstrates that brachiopods, and not molluscs, are the closest surviving relatives of hyoliths.

"It suggests that these hyoliths fed on organic material suspended in water as living brachiopods do today, sweeping food into their mouths with their tentacles," Moysiuk said.

Moysiuk, who studies Earth sciences and ecology & evolutionary biology, completed this project as part of the Research Opportunity Program at U of T, a special undergraduate research program in the Faculty of Arts & Science.

The distinctive appearance and structure of the hyolith skeleton has obstructed previous attempts to classify these animals. All hyoliths had an elongated, bilaterally symmetrical cone-shaped shell and a smaller cap-like shell which covered the opening of the conical shell (known as an operculum). Some species also bore a pair of rigid, curved spines (known as helens) that protruded from between the conical shell and operculum -- structures with no equivalents in any other group of animals.

Examination of the orientation of the helens in multiple hyolith specimens from the Burgess Shale suggests that these spines may have been used like stilts to lift the body of the animal above the sediment, elevating the feeding apparatus to enhance feeding.

Moysiuk and coauthors Martin Smith at Durham University in the United Kingdom, and Jean-Bernard Caron at the Royal Ontario Museum (ROM) and U of T were able to complete the descriptions based mainly on newly discovered fossils from the renowned Cambrian Burgess Shale in British Columbia.

"Burgess Shale fossils are exceptional because they show preservation of soft tissues which are not usually preserved in normal conditions," said Caron, Moysiuk's research supervisor, who is the senior curator of invertebrate palaeontology at the ROM and an associate professor in U of T's Departments of Earth Sciences and Ecology & Evolutionary Biology.

"Although a molluscan affinity was proposed by some authors, this hypothesis remained based on insufficient evidence. Hyoliths became an orphaned branch on the tree of life, an embarrassment to paleontologists. Our most recent field discoveries were key in finally cracking their story, around 175 years after the first description of a hyolith."

Caron led recent fieldwork activities to the Burgess Shale which resulted in the discovery of many specimens that form the basis of this study. The key specimens came from recently discovered deposits near Stanley Glacier and Marble Canyon in Kootenay National Park, about 40 kilometres southeast of the original Burgess Shale site in Yoho National Park.

The Burgess Shale is one of the most important fossil deposits for studying the origin and early evolution of animals that took place during the Cambrian period, starting about 542 million years ago. Hyoliths are just one of the profusion of animal groups that characterize the fauna of the 'Cambrian Explosion'. They became a diverse component of marine ecosystems around the globe for more than 280 million years, only to go extinct 252 million years ago, prior to the evolution of the first dinosaurs.

"Resolving the debate over the hyoliths adds to our understanding of the Cambrian Explosion, the period of rapid evolutionary development when most major animal groups emerge in the fossil record," said Smith, who started this research at the University of Cambridge and who is now a lecturer in paleontology at Durham University. "Our study reiterates the importance of soft tissue preservation from Burgess Shale-type deposits in illuminating the evolutionary history of creatures about which we still know very little."

The Burgess Shale, from which the specimens were recovered from several locations, is part of the Canadian Rocky Mountain Parks World Heritage Site. It is one of the most important fossil deposits for understanding the origin and early evolution of animals that took place during the Cambrian Explosion starting about 542 million years ago.

Parks Canada protects this globally significant site, and supports peer-reviewed scientific research that continues to enhance our understanding of these rich paleontological deposits. This discovery adds another element to the dramatic story of early animal evolution that Parks Canada guides share enthusiastically with hundreds of park visitors every year.

Funding for the research was provided primarily by the Royal Ontario Museum and a Natural Sciences and Engineering Research Council of Canada Discovery Grant to Caron.

原始論文：Joseph Moysiuk, Martin R. Smith, Jean-Bernard Caron. Hyoliths are Palaeozoic lophophorates. Nature, 2017; DOI: 10.1038/nature20804

引用自：University of Toronto. "Paleontologists classify mysterious ancient cone-shaped sea creatures." ScienceDaily. ScienceDaily, 11 January 2017.