Fóssil de baleias com 17 milhões de anos de idade fornece a primeira data exata para o enigmático enigma da África Oriental

Uplift and aridification of East Africa causing changes in vegetation has been considered a driver of human evolution

Southern Methodist University

Fonte: https://www.eurekalert.org/pub_releases/2015-03/smu-1mw031715.php

IMAGE: A 17 million-year-old whale fossil stranded far inland in Kenya now sheds light on the timing and starting elevation of East Africa's puzzling tectonic uplift, says paleontologist Louis Jacobs, Southern... view more

Credit: (Southern Methodist University)

A elevação associada ao Grande Vale do Rift, na África Oriental, e as mudanças ambientais que produziu intrigaram os cientistas por décadas, porque o tempo e a elevação inicial têm sido insuficientemente restritos.   Agora, os paleontologistas têm explorado um fóssil da baleia de bico mais precisamente datada do mundo - e a única baleia encalhada encontrada até agora no continente africano - para identificar pela primeira vez uma data em que a misteriosa elevação da África Oriental começou.

O fóssil de 17 milhões de anos é da família de baleias-de-bico-ziphiidae. Descobriu-se 740 quilômetros para o interior a uma altitude de 620 metros na região do deserto do Quênia, disse o paleontólogo de vertebrados Louis L. Jacobs, da Southern Methodist University, em Dallas. No momento em que a baleia estava viva, estaria nadando muito para o interior de um rio com um baixo gradiente variando de 24 a 37 metros em mais de 600 a 900 quilômetros, disse Jacobs, co-autor do estudo.

The study, published in the Proceedings of the National Academy of Sciences, provides the first constraint on the start of uplift of East African terrain from near sea level.

"The whale was stranded up river at a time when east Africa was at sea level and was covered with forest and jungle," Jacobs said. "As that part of the continent rose up, that caused the climate to become drier and drier. So over millions of years, forest gave way to grasslands. Primates evolved to adapt to grasslands and dry country. And that's when -- in human evolution -- the primates started to walk upright."

Identified as a Turkana ziphiid, the whale would have lived in the open ocean, like its modern beaked cousins. Ziphiids, still one of the ocean's top predators, are the deepest diving air-breathing mammals alive, plunging to nearly 10,000 feet to feed, primarily on squid.

In contrast to most whale fossils, which have been discovered in marine rocks, Kenya's beached whale was found in river deposits, known as fluvial sediments, said Jacobs, a professor in the Roy M. Huffington Department of Earth Sciences of SMU's Dedman College of Humanities and Sciences. The ancient large Anza River flowed in a southeastward direction to the Indian Ocean. The whale, probably disoriented, swam into the river and could not change its course, continuing well inland.

"You don't usually find whales so far inland," Jacobs said. "Many of the known beaked whale fossils are dredged by fishermen from the bottom of the sea." 

 Identificada como um ziphiid de Turkana, a baleia teria vivido em mar aberto, como seus primos modernos e bicudos. Ziphiids, ainda um dos principais predadores do oceano, são os mais profundos mamíferos que respiram ar, mergulhando a quase 10.000 pés para se alimentar, principalmente de lula.

Determining ancient land elevation is very difficult, but the whale provides one near sea level.

"It's rare to get a paleo-elevation," Jacobs said, noting only one other in East Africa, determined from a lava flow.

Beaked whale fossil surfaced after going missing for more than 30 years
 

O fóssil da baleia de bico foi descoberto em 1964 por J.G. Mead no que é agora a região de Turkana, no noroeste do Quênia.   Mead, um estudante de graduação da Universidade de Yale na época, fez uma carreira na Smithsonian Institution, da qual se aposentou recentemente. Ao longo dos anos, o fóssil da baleia do Quênia desapareceu no armazenamento. Jacobs, que já foi chefe da Divisão de Paleontologia dos Museus Nacionais do Quênia, passou 30 anos tentando localizar o fóssil. Seu esforço foi recompensado em 2011, quando ele a redescobriu na Universidade de Harvard e a devolveu aos Museus Nacionais do Quênia.

The fossil is only a small portion of the whale, which Mead originally estimated was 7 meters long during its life. Mead unearthed the beak portion of the skull, 2.6 feet long and 1.8 feet wide, specifically the maxillae and premaxillae, the bones that form the upper jaw and palate.
The researchers reported their findings in "A 17 million-year-old whale constrains onset of uplift and climate change in East Africa" online at the PNAS web site.

Modern cases of stranded whales have been recorded in the Thames River in London, swimming up a gradient of 2 meters over 70 kilometers; the Columbia River in Washington state, a gradient of 6 meters over 161 kilometers; the Sacramento River in California, a gradient of 4 meters over 133 kilometers; and the Amazon River in Brazil, a gradient of 1 meter over 1,000 kilometers.

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Besides Jacobs, other authors from SMU are Andrew Lin, Michael J. Polcyn, Dale A. Winkler and Matthew Clemens.

From other institutions, authors are Henry Wichura and Manfred R. Strecker, University of Potsdam, and Fredrick K. Manthi, National Museums of Kenya.
Funding for the research came from SMU's Institute for the Study of Earth and Man and the SMU Engaged Learning program.

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