Эрдэмтэд 50 сая жилийн тэртээх экосистемийн нөхцөл байдлыг илтгэх олон зуун загасны чулуужсан яс бүхий ховор олдворыг илрүүлжээ.
Смитсоны байгалийн түүхийн үндэсний музейн палеонтологич Стюарт Эди цуглуулгын сан хөмрөгөөс 1.2 метр өргөн чулуун хавтанг олсон нь эртний Вайоминг мужийн нутагт амьдарч байсан “Knightia eocaena” хэмээх цэнгэг усны загаснуудын бөөнөөр үхсэн үеийн дүр зураг юм. Анх харахад энгийн чулуу мэт боловч нарийвчлан шинжилбэл загасны сэрвээ, хавирга, нурууны яс маш сайн хадгалагдан үлдсэн нь тодорхой болжээ. 1998 онд Свитватэр тойргоос олдсон энэхүү олдвор нь тухайн үеийн нуурын орчин бүрдэх үйл явцыг судлах чухал ач холбогдолтой юм.
Эдгээр загас 50-48 сая жилийн өмнө Гозиут нууранд амьдарч байжээ. Судлаачид усны химийн найрлага эсвэл температурын гэнэтийн өөрчлөлтөөс үүдэлтэй хортой замаг, улирлын чанартай өөрчлөлтүүд загасны бөөнөөр үхэхэд нөлөөлсөн байж болзошгүй гэж таамаглаж байгаа ч яг тодорхой шалтгаан нь одоогоор тогтоогдоогүй байна. Гэсэн хэдий ч загас үхсэнийхээ дараа шууд лаг шавраар бүрхэгдсэн нь тэднийг махчин амьтдаас хамгаалж, яс нь бутарч салгалгүйгээр ийнхүү төгс хадгалагдахад хүргэжээ.
Эртний Эоцений үеийн энэхүү олдвор нь өнөөгийн хуурай тал болон хувирсан Вайоминг муж нь тухайн үед далдуу мод бүхий өтгөн ургамал, 악어 (крокодил) зэрэг амьтад амьдардаг дулаан уур амьсгалтай бүс байсныг гэрчилж байна. Палеонтологич Стюарт Эдигийн үзэж буйгаар, ийм төрлийн олдворууд нь зөвхөн амьтны анатомийг хадгалаад зогсохгүй тухайн үеийн амьдралын хэв маягийг бүхэлд нь танин мэдэх “цаг хугацааны гэрэл зураг” болж өгдөг ажээ. Сүүлийн үеийн химийн шинжилгээний арга техникүүд ашиглан судлаачид эртний уур амьсгал, тунадасны хэмжээг нарийвчлан тодорхойлж, өнөөгийнхөөс тэс өөр байгалийн нөхцөлтэй “аналог бус” экосистемүүдийг нээн илрүүлсээр байна.
Дэлгэрэнгүйг эх сурвалжаас харах
↓Эх сурвалжийг нээх ↓
A four-foot-wide slab of Wyoming stone holds hundreds of tiny fish that died together roughly 50 million years ago. The fossils, a freshwater herring called Knightia eocaena, now sit in the Smithsonian’s National Museum of Natural History. They record a single mass die-off and hint at a Wyoming that has long since vanished.
At first, the slab looks like little more than a broad expanse of rock. Then a backbone appears, then a cluster of fin bones, then rows of delicate ribs, until the stone is crowded with skeletons that are still remarkably intact.
Paleontologist Stewart Edie, the museum’s curator of fossil molluscs, found it while searching the collections for “From These Lands: Sharing Our Natural and Cultural Heritage,” an exhibition of more than 600 specimens and cultural objects. The slab, collected by an amateur fossil hunter in Sweetwater County, was donated in 1998.
A School Caught in the Act
Knightia swam through Wyoming’s lakes between 50 and 48 million years ago. Rapid changes in temperature or lake chemistry periodically killed whole schools at once, and these so-called “mortality blocks” turn up often. As reported by the Smithsonian Institution, researchers have looked at toxic algal blooms and seasonal water changes as possible causes, but nobody can say what killed the fish on this particular slab. Edie remembers walking past it.
“I stopped, turned back, and looked more closely at it,” he recounts. “And once your eyes refocus, you start to see all these little fish emerge, their tiny little vertebrae and their other little bones. They’re exquisitely preserved.”
What the slab shows is the aftermath. Hundreds of fish sank to the lake floor. And dense Knightia layers at Fossil Butte National Monument point to die-offs that happened repeatedly.
“This slab is a snapshot of that mortality event some 50 million years ago,” Edie said, adding that fossils like it can preserve “more than just the anatomy of an extinct animal.”
Why Green River Fossils Last
The fish belong to the Green River Formation, a series of Eocene lake deposits famous for the abundance and exquisite preservation of their fossils. In Sweetwater County, they record ancient Lake Gosiute, which at its maximum extent covered nearly all of the county.
Burial came quickly, before scavengers could pick the fish apart. Oxygen-poor water slowed decay, and calm conditions kept the tiny bones from scattering. According to the article’s photo captions, Knightia is the most commonly found complete vertebrate fossil in the world, and it is also the Wyoming state fossil.
Fish are only part of the record. Delicate plants, insects and other organisms survived too, ones that might disappear under more ordinary conditions.
“You end up with just an incredible snapshot of what life was actually like back then,” Edie said.
A Wyoming You Wouldn’t Recognize
Today, southwestern Wyoming is a rugged expanse of weathered badlands and dry plains. Follow the fish back in time and the picture flips: lakes, lush vegetation, palms along the shore, crocodilians. The early Eocene was one of the warmest intervals of the past 66 million years. Among the mammals were tiny early horses, a fraction of the size of their modern descendants, and a skeleton of one, Protorohippus venticolum, is on display in the museum’s “Deep Time” fossil hall.
Scientists assemble this picture piece by piece. Plant fossils offer clues about vegetation and climate, animals reveal which creatures could live in those environments, and newer chemical techniques extract isotopic evidence about ancient temperatures and precipitation. According to writer Lily Rasmussen, rocks across the Greater Green River Basin preserve Eocene environments spanning parts of present-day Wyoming, Utah and Colorado.
Some of these ecosystems combined climate and biodiversity in ways unlike anything living together today. Edie calls them “non-analog” environments.
“Our landscapes really are just this tapestry of ancient environments,” he said. “When we start to peel back those layers, we start to appreciate how much life has changed through time.”
Enjoyed this article? Subscribe to our free newsletter for engaging stories, exclusive content, and the latest news.
