Шинэ судалгаагаар 345 сая жилийн настай олдворыг урьд өмнө нь андуурч байсныг илрүүлж, уг амьтан нь хуурай газрын биш, харин усны амьдралтай холбоотой болохыг тогтоожээ.
1990 онд Шотландаас олдсон Westlothiana lizziae буюу “Лиззи” хэмээх чулуужсан олдворыг олон арван жилийн турш хуурай газарт дасан зохицсон хамгийн анхны амниот буюу мөлхөгч, хөхтөн, шувуудын өвөг гэж үзсээр ирсэн. Гэсэн хэдий ч чулуун хавтанд дарагдсан нарийн бүтцийг нь гэмтээхгүйгээр судлах боломжгүй байсан тул түүнийг зөвхөн гаднах төрхөөр нь дүгнэж байв. Америкийн Байгалийн түүхийн музейн судлаач Ксавье Женкинс болон түүний баг Европын Synchrotron Radiation Facility-д өндөр хүчин чадалтай рентген туяа ашиглан уг олдворыг 3D хэлбэрээр дахин шинжилжээ.
Шинжилгээний үр дүнд Лиззи нь гүрвэл төст толгойтой бус, харин загасныхтай төстэй ам, маш олон жижиг шүдтэй болох нь илэрсэн нь түүнийг усан дотор ангуучлах зориулалттай болохыг харуулж байна. Мөн амьтны дотоод бүтцээс илэрсэн заламгайн үлдэгдэл нь түүнийг усанд амьдарч, ууссан хүчилтөрөгчийг ашигладаг байсныг нотолж байна. Оксфордын их сургуулийн судлаач Бен Игельманы тайлбарласнаар, энэхүү олдвор нь мөлхөгчдийн ангилалд орохгүй, харин илүү эртний удам угсаанд хамаарах болох нь тодорхой болжээ.
Хэдийгээр Лиззи нь хүн болон үлэг гүрвэлийн шууд өвөг биш болох нь тогтоогдсон ч шинжлэх ухааны хувьд ач холбогдолтой хэвээр үлдэж байна. Түүний сарвуу болон хөлийн бүтэц нь дөрвөн хөлтөн амьтад хуурай газарт гарахаас өмнө ийм шинж чанарууд хөгжиж байсныг илтгэж байгаа юм. Судлаачид энэхүү олдворыг мөлхөгчид болон хоёр нутагтан амьтдын салбарлахаас өмнөх үед хамаарах эртний төрөл гэж үзэж байна.
Энэхүү шинэ дүгнэлт нь өмнө нь хуурай газрын амьтан гэж андуурч байсан бусад олдворуудыг ч дахин авч үзэх шаардлагатайг сануулж байна. Австралиас олдсон 359 сая жилийн настай мөрүүд ч мөн адил, амниот биш, харин ус болон хуурай газрын аль алинд амьдардаг байсан амьтдынх байж магадгүй юм. Профессор Рожер Бенсоны тэмдэглэснээр, хоёр нутагтан амьтдыг мөлхөгчдөөс илүү “энгийн” гэж үздэг боловч амьтад хуурай газарт дасан зохицох шинж чанараа хэсэгчлэн, удаан хугацааны туршид хуримтлуулсаар ирсэн байна.
Дэлгэрэнгүйг эх сурвалжаас харах
↓Эх сурвалжийг нээх ↓
A Scottish fossil long hailed as the earliest land-adapted amniote has been re-examined with high-powered X-rays, and it turns out not to be a reptile. Westlothiana lizziae, nicknamed “Lizzie,” has a fish-like mouth, a primitive skull and internal gills, all pointing to an amphibious animal.
Found in Scotland in 1990, the fossil was named for where it was found and for its lizard-like appearance. Nicknamed “Lizzie” almost at once, it was taken as the oldest evidence of an amniote, the group that unites reptiles, mammals and birds. If Lizzie wasn’t our direct ancestor, it was at least a member of the ancestral species that had, to put it loosely, plucked up the courage to leave the water.
A Judgment Made From The surface
The trouble was always the rock. The fossil lies within two crushed slabs of stone, and extracting the fossil would endanger the precious bones. The original assessment therefore rested only on what could be seen at the surface: four limbs, with digits ending in claw-like tips.
Dr. Xavier Jenkins, from the American Museum of Natural History (AMNH), and colleagues wanted a better look. They put the specimen through a high-powered X-ray at the European Synchrotron Radiation Facility to create a 3D reconstruction.
“Lizzie has been an icon for the early evolution of amniotes for decades,” Jenkins said in a statement. “But when we were finally able to see inside the fossil, we found an animal that looked very different from what we expected.”
What The X-rays Showed?
Instead of a lizard-like head, the scans revealed a fish-like mouth filled with an astonishing number of miniscule teeth, better suited to catching prey in water than on land. The study, published in Nature, stated that the skull looked very un-reptilian.
Most damning of all were the internal gills, which suggest Westlothiana spent enough time under water to benefit from extracting dissolved oxygen. If not fully aquatic, it was at least amphibious. As explained by Dr. Ben Igielman, who worked on Westlothiana for his PhD at Oxford University:
“The surprisingly primitive features of the skull show us that Lizzie was not only not a reptile but belonged to a much more ancient lineage.” He added a wry note: “It turns out looks can be deceiving, and Lizzie lived a completely different lifestyle than we previously thought.”
Still Significant, Just Differently
It may be disappointing that Westlothiana isn’t the ancestor of humans and dinosaurs alike, but that doesn’t make the specimen unimportant. Its claws and foot structure indicate that these features evolved before tetrapods, meaning amniotes plus amphibians, left the water.
The authors argue that Westlothiana precedes the split between reptiles and amphibians. They place it as a sister species to Utaherpeton, which they call “an enigmatic Late Carboniferous taxon previously regarded as one of the earliest microsaurs.”
Many other fossils once thought to represent early amniotes, they argue, more likely preceded the tetrapod family’s first great division. Professor Roger Benson, also from the AMNH, explained the wider point:
“It shows that, although we think of amphibians as being more ‘primitive’ than reptiles today, some of the traits of reptiles were already there in the ancestor of both amphibians and reptiles.”
Among those traits is the shape of the front limbs, which appear suited to bearing more weight than is necessary in water. The reappraisal also touches fossil trackways found in Australia, 359 million years old. Made by a four-legged creature with claw-like digits, they had been taken as evidence that amniotes existed at least that long ago. Now they could have been made by something Westlothiana-like, definitely a tetrapod but not necessarily an amniote.
“Terrestrial-looking traits, such as a reptile-like foot, a weight-bearing forelimb and claw-like phalanges, were not unique features of tetrapods, but rather accumulated piecemeal in their close relatives, in animals that were still living in and out of the water,” said Jenkis.
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