Decay and preservation of polychaetes: taphonomic thresholds in soft-bodied organisms

Abstract

A series of experiments was carried out to investigate the nature and controls (oxygen, microbial populations, agitation) on the degradation of soft tissues. Decay was monitored in terms of morphological change, weight loss, and change in chemical composition in the polychaete Nereis virens. Polychaetes include a range of tissue types of differing chemical composition and preservation potential: muscle, cuticle, setae, and jaws. Regardless of conditions, all the muscle had broken down and fluid loss through the ruptured cuticle had reduced the carcass to two dimensions within 8 days at 20°C. In most cases some cuticle, in addition to the jaws and setae, remained after 30 days. Where oxygen was completely eliminated, the rate of decay of the more volatile issues was significantly reduced. The degree of both osmotic uptake of water by the carcass and changes in water pH differed depending on whether the system was open or closed to oxygen diffusion. Autolytic and chemical processes are not sufficient to fully degrade the carcass in the absence of bacteria. Where internal bacteria are present, the presence or absence of water column bacteria made little difference to decay rate. Initial degradation (in the first 3 days) affects mainly the lipid fraction and the collagen of the cuticle. Later decay reduces the nonsoluble protein and increases the relative proportion of refractory structural components (tanned chitin and collagen) to more than 95% by day 30. Thus, only the sclerotized tissues are likely to survive beyond 30 days in the absence of early diagenetic mineralization. The sequence of degradation predicted from the relative decay resistance of macromolecules in the sedimentary record (protein → carbohydrate → lipid) is not, therefore, a consistent indicator of the preservation potential of structural tissues which incorporate them.The experiments reveal five stages in the decay of polychaete carcasses; whole/shriveled, flaccid, unsupported gut, cuticle sac, jaws and setae. All are represented in the fossil record. This allows an estimation of how far decay proceeded before it was halted by the fossilization process. The most complete preservations occur in the Cambrian where the Burgess Shale preserves evidence of muscle tissues. Traces of the gut and cuticle are more widely preserved, as at Mazon Creek, Grès à Voltzia, Solnhofen, and Hakel. Preservation varies within Konservat-Lagerstätten. The most common whole body preservation includes only the more recalcitrant tissues, jaws (where present) and setae, with an impression of the body outline. The stage of decay can be used as a taphonomic threshold, to provide an indication of how significantly the diversity of an exceptionally preserved biota is likely to have been reduced by taphonomic loss.

Sediment Effects on the Preservation of Burgess Shale-Type Compression Fossils

“…2 hours prior to burial. For Nereis (body length 125-250 mm, supplied by Seabait Ltd., Northumberland, UK), one series of specimens was killed by immersion in freshwater (with a potential for associated osmotic cell rupture) and a second by exposing their heads to a stream of hot water (see Briggs and Kear [1993]). …”

“…The cleaned organic residues were dried, powdered, and chemically analyzed for (1) non-water-soluble protein, (2) chitin, (3) water-soluble protein, (4) carbohydrate, and (5) phospholipid, using the various hydrolysis and colorimetric techniques employed by Briggs and Kear (1993). As a result of the obscuring effects of the sediment it was not possible to document absolute amounts, so these data have been recorded as percentages.…”

“…Despite reports of accelerated carcass decay associated with some sediments (e.g., Plotnick 1986;Allison 1988;Briggs and Kear 1993), most models for BST preservation invoke mineralspecific diagenesis as an essential factor, either by suppressing normal enzyme-microbial-based decay processes (e.g., Butterfield 1990Butterfield , 1995Gaines et al 2005Gaines et al , 2012 or secondarily by enhancing the recalcitrance of relatively labile substrates (e.g., Orr et al 1998;Petrovich 2001). Surprisingly, there has been little attempt to test these various models, though Martin et al (2004) have shown that sedimentary particles will adhere to lobster eggs in the presence of bacteria, and Naimark et al (2013) have documented substantially enhanced preservation of Artemia when they are buried in kaolinite.…”

Soft‐Bodied Fossils Are Not Simply Rotten Carcasses – Toward a Holistic Understanding of Exceptional Fossil Preservation

“…[51] The same decay experiments allowed the chevron-shaped structures in Conopiscius, a Carboniferous chordate, to be interpreted as myomeres rather than external scales, and also indicated that a decay-resistant cuticle was not necessarily present in Pikaia from the Burgess Shale. [51,55] Decay in seawater has now been monitored in a range of taxa in laboratory experiments (see Table S1, Supporting Information): anthozoans, [56] annelids, [48] chaetognaths, [57] priapulids, [18] onychophorans, [17] pterobranchs, [58] enteropneusts, [59] nonvertebrate chordates, [20] and cyclostomes. [60] Thus the sequence of character loss has been determined for taxa representing most clades of eumetazoans.…”

“…Sufficient calcium and phosphate ions must be available and pH must drop in order for calcium phosphate to precipitate instead of calcium carbonate (i.e., the calcium carbonate/phosphate switch). [24] Such a decrease is a normal result of bacterial decay, [26,31,48] but phosphatization tends to favor the preservation of particular tissues and taxa. [81,83] Decay experiments have shown that phosphatization occurs on a laboratory time scale and is not necessarily restricted to a few unusual settings.…”