原文網址：http://news.unm.edu/news/did-teenage-tyrants-outcompete-other-dinosaurs
研究人員探討肉食性恐龍的後代如何減少物種多樣性
新墨西哥大學和內布拉斯加大學林肯分校的古生態學家，證明霸王龍這類巨型肉食性恐龍的後代可能會淘汰其他與之競爭的較小型物種，因此徹底重新改造了牠們所處的群集。
恐龍體型分布出現的空缺v.s.現代的肉食性哺乳類。圖片來源：UNM Newsroom
這篇「未成年恐龍對群集結構與多樣性的影響」(The
influence of juvenile dinosaurs on community structure and diversity)本週發表在期刊《科學》(Science)，是第一篇論文把未成年恐龍當作生態系中的獨立主體，探討牠們在群集尺度上對恐龍多樣性的影響。
「恐龍群集就像周六下午的購物中心一樣――擠滿了青少年，」主持這項研究的Kat
Schroeder解釋。她是新墨西哥大學生物系的研究生。「青少年在一個物種的組成個體當中佔有相當大的比例，因此牠們對於整個群集可用的資源來說會有非常實際的影響。」
由於恐龍是從蛋裡出生，因此霸王龍這樣的恐龍出生時體型勢必也很小，大概只跟家貓差不多大而已。這意味著在長成跟巴士一樣大的過程當中，這些重量介於一到八噸的巨型獸腳類(megatheropods)也會逐漸改變牠們的掠食模式與獵物種類。雖然古生物學家長久以來猜測這些巨大的肉食動物成長時行為也會跟著改變，但對牠們周遭的世界可能會產生什麼樣的影響還是有很多不清楚的地方。
「我們想要驗證恐龍成長過程中會擔任其他物種扮演的腳色，因此實際上會讓群集中可以共存的物種數目變少的說法，」Schroeder表示。
全世界已知的恐龍類型並不多，特別是體型較小的種類。
「恐龍的多樣性低得驚人。就算把化石造成的偏差算進來，恐龍的物種數目實在不多，」新墨西哥大學的生物學教授Felisa
Smith表示。她也是Schroeder的指導教授。
為了解開恐龍多樣性減少的謎題，Schroeder和共同作者從全世界知名的恐龍化石發現地點蒐集資料，總計超過了550種恐龍。他們接著根據重量和食性整理分類這些恐龍，並且檢視每個群集裡小型、中型、大型恐龍的數量。
他們驚訝地發現了一種明顯的模式。
「出現了一道空缺――在有巨型獸腳類的群集當中，很少有重量介於100到1000公斤的肉食性恐龍存在，」Schroeder表示。「而未成年的巨型獸腳類剛好能填補這些空缺。」
Schroeder也指出觀察恐龍多樣性隨時間的變化是另一道關鍵。侏儸紀(2億至1億4500萬年前)的恐龍群集空缺較小，白堊紀(1億4500萬年至6500萬年前)的群集則空缺較大。
「侏儸紀的巨型獸腳類變化沒那麼大，牠們青少年時期跟成年的樣子更加相近，這讓群集中有更多位置可以容納不同屬的巨型獸腳類，以及某些體型較小的肉食性恐龍，」Schroeder解釋。「反之，白堊紀時的恐龍群集幾乎全被成長過程中樣貌變化相當大的霸王龍和阿貝力龍給佔據了。」
為了分辨這些空缺是否真由未成年的巨型獸腳類造成，Schroeder和同事把青壯年恐龍列入考量來重建群集。透過骨頭橫剖面中的生長速率線，並且從生物大量死亡形成的化石群集來估計每年有多少恐龍幼兒存活下來，團隊可以計算出巨型獸腳類的個體組成當中未成年佔的比例是多少。
Schroeder解釋這項研究的重要之處在於他們釐清了(至少一部份)為什麼化石裡的恐龍族群多樣性比預期的還低。此外，研究也解釋了為什麼和預期的相反，體型非常大的恐龍的物種數目比小型恐龍還多出許多。不過Schroeder說最重要的是，結果呈現了當非常小的恐龍寶寶成長為十分巨大的成年恐龍時，會對生態系造成什麼樣的後果。
「恐龍是我一生的愛好。小時候的我是超級『恐龍兒童』，而現在依然如此。當我發現用來探討現今哺乳類和鳥類多樣性的方法，竟然沒有人用在恐龍身上時讓我萌生了對於恐龍多樣性的興趣。要把探討當代和古代生態系的方法套用到恐龍身上需要大量的資訊，幸好我們現在正處於恐龍研究的黃金年代，可以取得很多經過數位化的資訊，使得恐龍古生物學中需要大量數據才能回答的生態學問題可以得出更加合理的解答。」
Did teenage ‘tyrants’ outcompete other
dinosaurs?
Researchers examine how carnivorous
dinosaur offspring reduced species diversity
Paleo-ecologists from The University of
New Mexico and at the University of Nebraska-Lincoln have demonstrated that the
offspring of enormous carnivorous dinosaurs, such as Tyrannosaurus rex may have fundamentally re-shaped their
communities by out-competing smaller rival species.
The study, The influence of juvenile dinosaurs on
community structure and diversity, was released this week in the journal Science and is the first to examine
community-scale dinosaur diversity while treating juveniles as their own
ecological entity.
“Dinosaur communities were like shopping malls on a
Saturday afternoon ̶ jam-packed with
teenagers” explained Kat Schroeder, a graduate student in the UNM Department of
Biology who led the study. “They made up a significant portion of the
individuals in a species, and would have had a very real impact on the
resources available in communities.”
Because they were born from eggs, dinosaurs like T.
rex necessarily were born small ̶ about the size of a house cat. This meant as
they grew to the size of a city bus, these “megatheropods,” weighing between
one and eight tons, would have changed their hunting patterns and prey items.
It’s long been suspected by paleontologists that giant carnivorous dinosaurs
would change behavior as they grew. But how that might have affected the world
around them remained largely unknown.
“We wanted to test the idea that dinosaurs might be
taking on the role of multiple species as they grew, limiting the number of
actual species that could co-exist in a community,” said Schroeder.
The number of different types of dinosaurs known from
around the globe is low, particularly among small species.
“Dinosaurs had surprisingly low diversity. Even
accounting for fossilization biases, there just really weren’t that many
dinosaur species,” said Felisa Smith, professor of Biology at UNM and
Schroeder’s graduate advisor.
To approach the question of decreased dinosaur
diversity, Schroeder and her coauthors collected data from well-known fossil
localities from around the globe, including over 550 dinosaur species.
Organizing dinosaurs by mass and diet, they examined the number of small,
medium and large dinosaurs in each community.
They found a strikingly clear pattern:
“There is a gap
̶ very few carnivorous dinosaurs
between 100-1000kg [200 pounds to one ton] exist in communities that have
megatheropods,” Schroeder said. “And the juveniles of those megatheropods fit right
into that space.”
Schroeder also notes that looking at dinosaur
diversity through time was key. Jurassic communities (200-145 million years
ago) had smaller gaps and Cretaceous communities (145-65 million years ago) had
large ones.
“Jurassic megatheropods don’t change as much ̶ the
teenagers are more like the adults, which leaves more room in the community for
multiple families of megatheropods as well as some smaller carnivores,”
Schroeder explained. “The Cretaceous, on the other hand, is completely
dominated by Tyrannosaurs and Abelisaurs, which change a lot as they grow.”
To tell whether the gap was really caused by juvenile
megatheropods, Schroeder and her colleagues rebuilt communities with the teens
taken into account. By combining growth rates from lines found in
cross-sections of bones, and the number of infant dinosaurs surviving each year
based on fossil mass-death assemblages, the team calculated what proportion of
a megatheropod species would have been juveniles.
Schroeder explained that this research is important
because it (at least partially) elucidates why dinosaur diversity was lower
than expected based on other fossil groups. It also explains why there are many
more very large species of dinosaurs than small, which is the opposite of what
would be expected. But most importantly, she added, it demonstrates the results
of growth from very small infants to very large adults on an ecosystem.
“Dinosaurs have been a life-long passion. I was, and
still very much am a ‘dinosaur kid.’ My interest in dinosaur diversity came
about when I realized that no one was really looking at dinosaurs the way we
look at modern mammals and birds. There’s a ton to be gained from applying the
methods of modern and paleo-ecology to dinosaurs. Fortunately, we’re now in an
age of dinosaur research where a lot of information is available digitally, so
the big data-intensive questions of ecology are now becoming more plausible for
dinosaur paleontology,” Schroeder said.
原始論文：Katlin
Schroeder, S. Kathleen Lyons, Felisa A. Smith. The influence of
juvenile dinosaurs on community structure and diversity. Science,
2021; 371 (6532): 941 DOI: 10.1126/science.abd9220
引用自：University of New Mexico. "Did teenage
'tyrants' outcompete other dinosaurs? “
沒有留言:
張貼留言