Бразилын Ботукату формацаас олдсон эртний мөрүүд нь тухайн үед амьдарч байсан хөхтөн амьтдын дунд хамгийн томд тооцогдох бодгалийнх байж болзошгүйг судлаачид тогтоосон байна.
Journal of South American Earth Sciences сэтгүүлд нийтлэгдсэн судалгаагаар, 130 сая жилийн тэртээх уг мөр нь үлэг гүрвэлийн үеийн хөхтөн амьтдын талаарх уламжлалт ойлголтыг өөрчилж байна. Урьд нь хөхтөн амьтдыг зөвхөн хэрэм эсвэл жижиг мэрэгч шиг хэмжээтэй байсан гэж үздэг байсан бол шинээр илэрсэн ул мөр нь үүнээс хамаагүй том, бүр үлэг гүрвэлээр хооллодог байж болохуйц биетэй амьтан байсныг харуулж байна. Доктор Эрминио Исмаэл де Араужо-Жуниорын удирдсан баг 11.4 см урт, 13.4 см өргөн мөрийг хэмжиж, уг амьтныг ойролцоогоор 1.55 метр урттай байсан гэж тооцоолжээ.
Энэхүү олдвор нь Chelichnopodidae овгийн хөхтөн амьтны мөр болохыг тогтоосон боловч одоогоор тухайн амьтныг тодорхой нэрлээгүй байна. Доктор Педро Бакын тайлбарласнаар, тухайн үед уг газар нутаг нь элсэн цөл байсан бөгөөд аварга том үлэг гүрвэлүүд байхгүй байсан нь хөхтөн амьтад илүү том биетэй болон хөгжих боломжийг бүрдүүлсэн байж болзошгүй аж. Гэсэн хэдий ч тухайн бүс нутгаас ямар нэгэн яс, гавлын ясны олдвор олдоогүй тул энэ нь зөвхөн ул мөр дээр үндэслэсэн таамаглал юм.
Судлаачид уг амьтныг орчин үеийн махчин амьтадтай адил омнивор буюу холимог идэштэн байсан гэж үзэж байна. Тэд үлэг гүрвэлийн зулзага эсвэл жижиг биетэй үлэг гүрвэлээр хооллодог байсан байж болох ч, ясны үлдэгдэл олдохгүй байгаа нь тухайн үеийн байгалийн нөхцөл байдал яс хадгалагдан үлдэхэд тааламжгүй байсантай холбоотой байж магадгүй юм. Энэхүү олдвор нь Мезозойн эриний хөхтөн амьтдын талаарх уламжлалт төсөөлөл болон тэдний биеийн хэмжээний талаарх ойлголтыг эргэн харах шаардлагатайг сануулж байна.
Дэлгэрэнгүйг эх сурвалжаас харах
↓Эх сурвалжийг нээх ↓
Footprints from Brazil’s Botucatu Formation appear to belong to the largest mammal yet identified from the era of the dinosaurs. The animal that made them is thought to have been twice the size of any known Cretaceous mammal, big enough to have hunted dinosaurs.
The standard tale goes like this: our mammalian ancestors were shrew- or squirrel-sized, forever dodging reptilian giants until an asteroid changed everything. Some mammals did prey on dinosaurs, but that was believed to be true only of the very smallest ones, about as big as modern birds.
Published in the Journal of South American Earth Sciences, the new study muddies that picture. The prints look like mammal paws, not dinosaur feet, yet the creature behind them was far bigger than anything found, or even suspected, from before the asteroid impact.
Reading the Prints
The largest tracks are 11.4 centimeters (4.5 inches) long and 13.4 centimeters (5.4 inches) wide, a ratio that helped convince Dr. Hermínio Ismael de Araújo-Júnior and his colleagues that a mammal was responsible. Their spacing suggests a stride of about 40 centimeters (16 inches).
Using species for which both bones and footprints are known, the team put the trackmaker at roughly 1.55 meters (5 feet) long. Nothing attributed to a dinosaur or crocodile looks quite like these prints, because they resemble scaled-up versions of tracks left by many mammals. The authors of the study haven’t named the animal, but they assign it to the Chelichnopodidae family of mammal footprints.
As the authors observe, the majority of vertebrate tracks in the Botucatu Formation were imprinted in low-cohesion sediment, a condition that generally results in simplified morphologies and the loss of fine anatomical detail.
A Desert With Room For Big Mammals
Two earlier candidates for the record, Aracoaraichnium leonardii and Brasilichnium anaiti, were reported in 2017. Both likely matched small dogs or large house cats in size, and both came from this same formation. The newcomer, around 130 million years old, is closer to a large dog or even a mountain lion.
So what was special about the place? For the moment, nobody has figured it out yet. According to first author Dr. Pedro Buck, in comments to Science, big dinosaurs may have been absent from this desert for reasons that remain unclear, leaving extra niches for mammals to fill.
Today, this part of Brazil is covered in greenery, but it looked nothing like that 130 million years ago. Back then, it was a huge desert, far from the nearest coastline. When Gondwana eventually broke apart, most of the Botucatu Formation ended up in South America, while a small part remained in Africa. Brasilichnium footprints were widespread across the region, including at the newly studied site.
No Skeleton, But A Likely Menu
Bones would settle a lot, yet paleontologists aren’t hopeful. Araújo Júnior, speaking to IFLScience, stated that no mammaliaform body fossils are known from Jurassic or Early Cretaceous rocks in Brazil, and the Early Cretaceous skeletal record across Gondwana is extremely scarce. In his words, the tracks reveal animals “simply absent” from the skeletal record.
He added that the conditions that preserved footprints in the desert may not have favored bones. Better odds may lie in deposits from friendlier local environments, where carcasses were more likely to be buried. He also suggested that:
“The traditional expectation that Mesozoic mammaliaforms were predominantly very small could potentially influence how unusually large tracks or fragmentary fossils are interpreted.”
Tracks can’t reveal diet the way teeth or jaws do, but the authors consider the maker an omnivore that may occasionally have preyed on juvenile or small-bodied dinosaurs. Smaller mammals, invertebrates, and whatever else it could catch were probably on the menu too, with plants a minor part at most.
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