Эртний Девоны үед хамаарах уг загасны анатомийн онцлог нь түүнийг элсэнд хагас шигдэн амьдардаг байсан гэх таамаглалыг үгүйсгэж байна.
Хятадын Юньнань мужийн Шишаньчун ордоос олдсон Asioaspis brachyotus хэмээх энэхүү шинэ төрлийн хуягт, эрүүгүй загасны олдвор нь сээр нуруутны гарал үүслийг судлахад чухал ач холбогдолтой юм. Galeaspida бүлэгт багтах энэ загас нь 419 сая жилийн өмнө амьдарч байсан бөгөөд эрүүт сээр нуруутан болон бусад эрүүгүй загасны хоорондох эволюцийн холбоосыг тайлбарлахад түлхүүр болдог байна. Судалгааны үр дүнг Vertebrata PalAsiatica сэтгүүлд нийтэлжээ.
Хятадын Шинжлэх ухааны академийн Сээр нуруутан амьтдын палеонтологи ба палеоантропологийн хүрээлэнгийн судлаач Жи-Күн Гай болон түүний багийнхан олдвор бүхий зургаан толгойн дуулгыг шинжилсэн байна. Олдворын толгойн дуулга нь ойролцоогоор 4.7-6 см урттай, ирэрхүү ирмэгтэй бөгөөд дор хаяж 12 хос заламгайн ууттай болохыг тогтоожээ. Эдгээр шинж чанар нь уг загасыг Polybranchiaspiformes багт хамааруулж, түүнийг примитив болон дэвшилтэт шинжүүдийг хослуулсан завсрын төрөл болохыг баталж байна.
Судлаачдын үзэж байгаагаар, уг загасны нүд болон толгойн дуулганы нүх сүвүүд нь хоорондоо өргөн зайтай байрласан нь түүний амьдралын хэв маягийг тодорхойлдог. Өмнөх төрлийн загаснууд элсэнд хагас шигдэн амьдардаг байсан бол Asioaspis brachyotus-ын анатомийн байршил нь ам болон заламгайн хөндийд лаг шавар орохоос сэргийлдэг байжээ. Иймд эрдэмтэд энэхүү загас нь далайн ёроолын гадаргуу дээр амьдардаг байсан бөгөөд энэ нь тус бүлгийн доторх эволюцийн шилжилтийн үе шатыг харуулж байна хэмээн дүгнэжээ.
Дэлгэрэнгүйг эх сурвалжаас харах
↓Эх сурвалжийг нээх ↓
Paleontologists have described a new armored, jawless fish from the Early Devonian of Yunnan, China. Named Asioaspis brachyotus, it sits between primitive and advanced members of its group. Its anatomy suggests it lived on the seafloor surface rather than half-buried in sand.
The fish lived in what is now southwestern China about 419 million years ago, during the Lochkovian stage of the Early Devonian. It belongs to the Galeaspida, an extinct group of bony-headed, jawless fishes found only in China and northern Vietnam.
Galeaspids matter because they are more closely related to jawed vertebrates, humans included, than to living jawless fishes such as lampreys and hagfish. The discovery is reported in the journal Vertebrata PalAsiatica.
A Group Central to Vertebrate Origins
According to Dr. Zhi-Kun Gai of the Institute of Vertebrate Paleontology and Paleoanthropology at the Chinese Academy of Sciences, and colleagues, galeaspids have long been central to understanding where key jawed-vertebrate traits came from, such as jaws, the spiracle and paired appendages. The reason is their closer phylogenetic affinity with jawed vertebrates than extant jawless fishes have.
Despite being restricted to China and northern Vietnam, these armored fish were remarkably varied during the Siluro-Devonian. To date, 19 families, 74 genera and 100 species have been described. Cladistically, the group splits into two basal orders, Hanyangaspidiformes and Dayongaspiformes, and three major orders: Eugaleaspiformes, Polybranchiaspiformes and Huananaspiformes.
Asioaspis brachyotus falls within Polybranchiaspiformes. The earliest known member of that order, Platylomaspis, dates to about 438.6 million years ago, yet the group did not diversify widely until the Early Devonian. The Xishancun Formation alone produced nine genera and 15 species.
What the Fossils Reveal?
The material was collected from the Xishancun Formation near the Miandian Reservoir in Qujing, Yunnan province. The collection includes three nearly complete headshields and three incomplete ones.
The oval headshield measured roughly 4.7 to 6 cm (2 inches) in maximum length. It had serrated edges, a coarse granular surface and a very broad rim on its underside. The researchers counted at least 12 pairs of gill pockets, although incomplete preservation prevented an exact count.
Based on these diagnostic features, the team assigned the species to Polybranchiaspiformes. Their phylogenetic analysis then placed it between the primitive and the advanced relatives, since it combines features of both.
A Shift From Buried to Surface Life
The most telling difference lies in the openings on the headshield. Earlier primitive relatives had eyes and a central opening clustered near the top of the headshield, a setup suited to life partly buried in sand. In the new species, those openings are widely spaced.
As reported by the authors, this rules out a half-buried lifestyle like that of Platylomaspis and Nanningaspis, because silt and mud would have flooded the mouth and gill chamber. The openings are instead widely separated and close to the margin of the ventral rim, as in the Pentathyraspidae, Duyunolepididae and Polybranchiaspidae. So where did it live? The scientists conclude that:
“It probably lived in a normal epibenthic habit like most of galeaspids,” they noted. “Considering its phylogenetic position, Asioaspis brachyotus probably represents an evolutionary transition from the semi-infaunal benthic to epibenthic lifestyle within Polybranchiaspiformes.”
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