Chengjiang

One of the highlighted Konservat-Lagerstätten in my Rise of Animals post is Chengjiang (a.k.a. the Maotianshan Shales). While Burgess has the historical significance, in terms of importance and potential, Chengjiang is arguably more important (see Shu, 2008).

Chengjiang fossils are not the easiest to see, although they are admittedly abundant, at 40000+ specimens so far. Most are just faint impressions, and even body fossils are mostly the same colour as the surrounding rock (although exceptions exist: arthropods are darker, Yunnanozoon is grey). It’s therefore the rule to use polarised light when examining them (and all sorts of other photographic filters).

Despite all of this, they were known from very early on, from the start of the 20th century (Mansuy, 1912). Walcott was also aware of them and he even published on an arthropod from there (Walcott, 1911). Despite this, most probably due to geopolitical reasons, the faunas were more or less ignored, with only a couple of publications until the 1980s.

When I found the first fossil… I knew right away that it was an arthropod with paired appendages, extending forward, as if it was swimming on the moistened surface of a mudstone. But I realized that you could see the impression of the soft body parts. That night I put the fossils under my bed. But because I was so excited, I couldn’t sleep very well. I got up often and pulled out the fossils just to look at them.

It was rediscovered in 1984 by palaeontologist Hou Xianguang, quoted above (source: Hagadorn, 2002), and at ~525 Ma, it immediately attracted attention, as it rediscovery came when the Burgess redescriptions were in full swing. It got its call to fame with the description of the arthropod Naraoia (Zhang & Hou, 1985), putting it on the map as a perfect comparison with the Burgess Shale.

Its importance lies in the fact that it is older than the Burgess, nothing more. In terms of ecology and fauna, they are similar; but Chengjiang preserves animals from earlier, and so is more relevant to the study of animal origins (of course, this is not to say that the Burgess is useless!). For example, it is as of Chengjiang that the first evidence for large-scale sensory systems comes, as in the eyes of various arthropods, such as the anomalocaridids, Isoxys (Vannier et al., 2009) and Fuxianhuia (Budd, 2008).

First, I will list some of the more spectacular findings from there, in no particular order.

  • A definitive hemichordate is pictured above: Galeaplumosus abilus (Hou et al., 2011).
  • Lobopods, e.g. Diania cactiformis (pictured above; Liu et al., 2011)
  • Problematic palaeoscolecids (see Harvey et al., 2010).
  • Chondrophores (Sun, 1992).
  • Stem-group crustaceans, e.g. Ercaia minuscula (Chen et al., 2001).

Overall, the fauna comprised of just about every ecological tier, from primary-producing algae to the dominant (and non-dominant) arthropods, through sessile filter-feeding sponges, brachiopods and pterobranchs, carnivorous cnidarians, various burrying and ground-moving “worms” and fish-like animals (Chen & Zhou, 1997). Trilobites are exemplary of the ecological complexity that was already present, with benthic herbivores (Eoredlichia), carnivores (Naraoia – although its status as a trilo is in doubt!) and free-swimming ones co-existing (Shu et al., 1995).

The arthropods are by far the most abundant and diverse animals there (Leslie et al., 1996), followed by the sponges (Chen & Erdtmann, 1991).

Comparisons with the Burgess Shale are natural to make. They are overall extremely similar in faunal content, with the only notable differences being the lack of echinoderms and molluscs from Chengjiang.

Now for a bit of geology. Chengjiang rocks are grey, with a yellowish colour when weathered. The homogeneity is occasionally interrupted by ripples and storm-deposited beds. However, the bulk of the formation is homogeneous, very well-sorted and consistent with deposition on the continental slope several meters away from an estuary. The most fossiliferous horizons are a very characteristic yellow colour – this is a result of the shales having very little organic and carbonate in them.

The fauna itself shows no sign of disturbance. Unlike in the Burgess Shale or the Hunsrück Slates (on whose exceptional faunas I did some work on), the Chengjiang faunas are preserved in life position and show no sign of having been transported. It is possible that Chengjiang represents a single mass-death from volcanic ash – while none has been found, the fossiliferous clay is bentonitic, a tell-tale sign of volcanic activity.

These fossiliferous layers have yielded three ecological assemblages, collectable at different places within the formation: the Shankou, Haikou and Chengjiang biotas (altogether summed up as Chengjiang for ease).

Another comparison with the Burgess is useful to make here, this time taphonomical. Very broadly, Chengjiang and Burgess are lumped together as having Burgess Shale-type preservation. This is accurate insofar as they preserve the same ecologies and biotas to a similar level; the rocks themselves are very different: Burgess has organic-rich black shales, typical of anoxic deep-water mud, while the fossiliferous Chengjiang rocks are yellowish or grey. In the Rise of Animals post, I alluded to the ecological developments of the Cambrian Radiation as being responsible for the disappearance of this taphonomic window; this is unoubteably true, but it may not be the only reason; see Allison & Briggs (1993) or, uh, ask if you want more detail.
ResearchBlogging.org
Zhu, M., Babcock, L., & Steiner, M. (2005). Fossilization modes in the Chengjiang Lagerstätte (Cambrian of China): testing the roles of organic preservation and diagenetic alteration in exceptional preservation Palaeogeography, Palaeoclimatology, Palaeoecology, 220 (1-2), 31-46 DOI: 10.1016/j.palaeo.2003.03.001

I will just say a few words on taphonomy and exceptional preservation as it pertains to Chengjiang. Taphonomy in exceptional preservation is an incredibly complex subject, involving chemical interactions between bacteria, pore fluids, environmental chemical conditions and their feedback loops, and crystallography, so I will not go into the details here (you are free to ask though – this was part of my Bsc. work, so I’m trained in it). The Chengjiang fossils are made of carbon, pyrite, apatite and various irony minerals. The carbon is most noticeable, as is usually the case, in the gut; the body fossil itself has very little original carbon in it. The rest of the minerals show a characteristic diagenetic series that takes place in a very low-oxygen environment.

First, phosphatisation happens – the formation of apatite, replacing the soft tissues. This process is controlled – and ultimately dependent on – microbial mats, whose bacteria are feeding on the corpse. Right away, you see the ultimate paradox: In order for there to be exceptional preservation, there must be some loss of morphological detail where the bacteria were feasting. The phosphatisation can occur if there is enough P in the waters; in Chengjiang, this was the case. The pH must also be within the right range, so that the feedback between CO2 and H2S (produced by the bacteria) continues. Another prerequisite is the presence of iron. The H2S is a source of sulphur; through chemical wizardry (actually, just redox reactions), the iron hydroxides of the water react with the H2S and form pyrite; again, this is dependent on pH and limited by the availability of S and Fe. Differences in conditions lead to different sized crystals. Most often, the crystals end up really big and no detail can be seen. In the right conditions though, you get something like the Hundrück Slates, where you can X-Ray fossils to reveal tons of information. The iron-rich aluminosilicates can, under some circumstances, replace pyrite.

Remember, the apatite, pyrite and aluminosilicates are replacing the original soft tissue. The more diagenetic steps you have, the more information you lose. The Chengjiang fossils are mostly apatite, with pyrite being less common. This is why it is so difficult to study them (cf. second paragraph): you can’t X-Ray them, and apatite is pretty translucent – hence why you need all the lighting tricks to study Chengjiang fossils. But it’s worth it in the end.

There may also be chamosite crystals growing on the fossils, but they’re irrelevant and post-diagenesis. For more details on Chengjiang’s unique preservation, check out Gabbott et al. (2004).

My further reading suggestion is most definitely this gorgeous book: The Cambrian Fossils of Chengjiang: The Flowering of Early Animal Life (2004).

And finally, some pictures to round off the post. These are from Valentine (2001).

Fuxianhuia

Jiangfengia, stem-group arthropod.

Dinomischus, stalked mindfuck.

References:

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Chen J & Zhou G. 1997. Biology of the Chengjiang Fauna. Bulletin of the National Museum of Natural Sciences, Taichung, Taiwan 10, 11-105.

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Research Blogging necessities :)

Steiner, M., Zhu, M., Zhao, Y., & Erdtmann, B. (2005). Lower Cambrian Burgess Shale-type fossil associations of South China Palaeogeography, Palaeoclimatology, Palaeoecology, 220 (1-2), 129-152 DOI: 10.1016/j.palaeo.2003.06.001

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