September 13. Monte San Giorgio lagerstätte

Update. During the Precambrian, last January, we talked about the origin of atmospheric oxygen, a development tied to the expansion of oxygen-excreting, photosynthesizing cyanobacteria. The point at which oxygen began to become plentiful in the atmosphere has been pegged at about 2.96 billion years ago, with levels becoming noticeably high by about 2.4 billion years ago or later. A new study by researchers at Trinity College Dublin, Ireland, and the Presidency University in Kolkata, India, pushes the onset of oxygenation back about 60 million years, to about 3.02 billion years ago. Here’s a link (and it's also on the January 19 post about the oxygen crisis). 

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Now back to the Triassic. In southern Switzerland, Triassic rocks record the remarkable variety of marine vertebrate life that lived in that region by middle Triassic time. Monte San Giorgio, overlooking Lake Lugano, was designated a World Heritage Site in 2003 because "is the single best known record of marine life in the Triassic period, and records important remains of life on land as well," according to the World Heritage designation by UNESCO. It is a lagerstätte, a fossil locality remarkable for the diversity of specimens as well as their state of preservation.

Photo of fossil in matrix by Tommy from Arad under GFDL. 

The area was a quiet, stagnant lagoon during the mid-Triassic, around 232 to 237 million years ago. It was similar to the Permian Basin and North Caspian Basin, but on a smaller scale. Reefs surrounded the lagoon so that little circulation resulted in an anoxic setting. Lack of oxygen at depth, no scavengers, and quiet conditions allowed for outstanding preservation of the shallow-water life as well as any skeletons that might have been washed into the lagoon. The richest fossil zone is only 5 to 16 meters thick, made up of thin laminations of alternating organic-rich black shale and organic-rich dolomite. These alternations may reflect sea-level fluctuations, with the dolomite depositing during high stands and the shale accumulating during low stands when the lagoon would have been even more stagnant than at other times.

The fossils in the thin layers at San Giorgio include aquatic reptiles similar to the nothosaurs we talked about yesterday, together with various fish and even a few terrestrial animals that must have washed into the lagoon. Tanystropheus was a 20-foot-long reptile, whose neck was more than half the total body length. Its skeletal structure suggests that it might have been a near-shore land dweller, using its long neck to snag fish from the shallows, or it might have been aquatic. Skin impressions of this animal have been found, showing it to be covered by rectangular scales.

Tanystropheus skeleton reconstruction (see below for credit)

A few embryos from the ray-finned fish Saurichthys have been found in the rocks at San Giorgio, retaining some soft parts. Their occurrence indicates that those fish bore their young alive rather than leaving unattended eggs.

—Richard I. Gibson

References and links:

Tanystropheus fossil in Milan Natural History Museum. Photo of fossil in matrix by Tommy from Arad via Wikimedia Commons, under GFDL. 

Restoration photo by Ghedoghedo under Creative Commons license.

Monte San Giorgio (in book, Exceptional Fossil Preservation)


Fish embryos