The Mesozoic Park: A snapshot in time – the Cretaceous Pompeii…
We have all seen the amazing fossils of feathered dinosaurs and proto/early-birds that have emerged from north-eastern China and rewritten evolutionary history over the last couple of decades. The rocks in which they were entombed were deposited in a marshy, lake dotted landscape that was episodically buried by catastrophic flows of incandescent gas and volcanic ash (very convenient for dating, using uranium-lead ratios). Aged 133-120 million years, from the early Cretaceous, the entire temperate climate ecosystem has been very well preserved, and named the Jehol biota (after a mythical land in Chinese legends). It represents the best resolved picture of life of the time when the earliest birds had just diverged from feathered dinosaurs.
The ecosystem includes a mix of ‘modern’ and 'ancient’ species, some endemic and others found all over the world. The region may have been isolated at the time from Europe by a stretch of open sea, but either way this stretch of late Jurassic-early Cretaceous time between 160 and 130 million years ago seems to have been a time of widespread diversification. It includes primitive dinosaurs harking back to the Triassic, along with the earliest members of the families that culminated in triceratops, t-rex and oviraptor. The trees were mostly conifers, monkey puzzles, gingkoes and cypresses, all old timers on the planty bush of life. The earliest advanced birds, snails, flowering plants, pollinating insects and clams have also turned up here along with the largest known Mesozoic mammals.
Along with the wide diversity in species of a near complete ecosystem, many examples of common fossils have been found, giving sufficient baseline data for statistical analysis of populations in terms of size, growth sequences and disease. Many soft tissues have been found preserved, including skin, fur and feathers, colour patterns and flowers hanging off twigs. Most skeletons are articulated, indicating rapid burial where they died, and no scavenger activity. Together these have revealed many details of the appearance and lifestyles of many creatures, including the colours of early birds and feathered dinosaurs. The organisms were preserved when large ashfalls sealed the remains into the muds at the bottom of freshwater lakes, sealing them away from oxygen and scavengers.
The fossiliferous rocks definitely agreed to be pure 'Jehol’ are the lake bottom sandstones and shales of the Yixiang and Jiufotang formations, and both have been given the status of official Lagerstatte, a German word used to denote a fossil site of exceptional quality of preservation or evolutionary importance (in this case both). They also include coal beds, oil shales and volcanic rocks. While first 'discovered’ in 1910, their true potential to revolutionise our understanding of this important epoch in the history of life has only become apparent over the last 20 years, as research into the fossils has mushroomed.
Many of the fossils come from mass mortality events caused by the eruptions that preserved the remains. They were mostly Plinian, like the eruption that devastated Pompeii in 79CE. A huge column of hot ash and gas is expelled from the vent, and collapses in when the thermal energy fades, dumping tonnes of hot rock and ash in a flow down the mountain. Many fossils are partly carbonised, in death poses similar to victims of pyroclastic flows (see the Pompeii dog we covered at http://on.fb.me/1IP4DnH), and horizons containing many corpses of diverse species seem to record individual moments of long gone devastation amongst these peaceful tree lined lakes. The fine grained ash helped preserve high levels of detail as the organisms slowly turned into rock.
A complex (and ongoing) argument has emerged over whether these life forms were the descendants of the Daohugou ecosystem (filled with feathered dinosaurs, but no advanced birds), a late Jurassic assemblage some 30 million years older found in the same geographical area, that will be the subject of my next post. Some place the whole sequence together, others divide it in the time honoured geological tradition of lumpers versus splitters (see http://on.fb.me/1PpsZnU for my disquisition on the topic). Five main beds representing individual snapshots of this era seem to be recognised by most researchers with a reasonable degree of consensus.
Each year brings new discoveries, papers and wonderful insights from this iconic fossil site, whose true scientific value is only just beginning to be properly exploited. More exciting news is certain to follow. Just like Pompeii and Herculaneum, what was a catastrophe for the denizens on the moment has created a time capsule of buried knowledge for us to enrich our understanding of the world we live in and where we came from.
Loz
This article is a part of our “Mesozoic Park” series that is leading up to the release of Jurassic World. For more information, please see our introductory post at: http://on.fb.me/1ELwHW5
Image credit: 1 Confuciusornis: Naturhistorisches Museum Wien/Tommy from Arad 2: Sinosauropteryx, Sam / Olai Ose / Skjaervoy 3: Hyphalosaurus, Paleozoological Museum of China.: Captmondo 4: Jeholornis: M.violante http://bit.ly/1cn38Rl
http://www.earth-of-fire.com/page-8882087.html
Original -paper free access: http://bit.ly/1c6b9cw
(Source: facebook.com)
