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Tuesday, January 23, 2007

First Doushantuo, now Changzhougou

Grypania Spiralis

One of the most interesting aspects of evolutionary biology is the origin of eukaryotic organisms. The oldest bonafide eukaryotic fossil was discovered in the Negaunee iron formation (Michigan, 1875 million years old). The body fossil is named Grypania spiralis. Eukaryotic fossils have a rich record in the Neoproterozoic (1000-542 million years ago), a sparser record in Mesoproterozoic rocks (1600-1000 million years ago) and vert few bonafide Paleoproterozoic body fossils (2500-1600 million years ago. In 2000 Zhu and colleagues from Tianjin Institute described some of the largest 'eukaryotic' impressions that were touted as one of the richest finds in the Paleoproterozoic. The analysis by Zhu and colleagues relied heavily on gross structural and morphological comparisons to known (younger eukaryotic organisms such as Tawuia and Chuaria

Chuaria Circularis
Compression structures (Lamb et al, 2007). Copyright Elsevier Publications.

A week or so ago, much was made of the 'embryo' fossils from the 580 million year old Doushantuo Formation in China. While not specifically dismissing all the fossil finds from Doushantuo, the fossilization of embryos was found to be contentious. In a forthcoming paper in Precambrian Research, Lamb et al. take on the supposed eukaryotic fossils from the Paleoproterozoic aged Changzhougou Formation (1800 million years old) in North China.
Lamb and colleagues conducted a number of detailed tests on these compression like structures to determine if they were biogenic. The compressions lack a distinctive morphology, no distinguishing 'marker' features such as walls or ornamentation, they lack an internal structure, contain no evidence of tissue or cells, they show little evidence for carbon and have shapes and features consistent with clay-rich clasts. Rather than a collection of the oldest and largest eukaryotes, these impressions are non-biogenic sedimentary structures. The search for the oldest and largest eukaryotic fossils continues.

In other news, a new study from the University of Florida shows how rare earth elements can be used to trace the movement of birds into North America....

GAINESVILLE, Fla.: A University of Florida-led study has determined that Titanis walleri, a prehistoric 7-foot-tall flightless "terror bird," arrived in North America from South America long before a land bridge connected the two continents.

UF paleontologist Bruce MacFadden said his team used an established geochemical technique that analyzes rare earth elements in a new application to revise the ages of terror bird fossils in Texas and Florida, the only places in North America where the species has been found. Rare earth elements are a group of naturally occurring metallic elements that share similar chemical and physical properties.....continue


Joe Meert


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