ONTOGENY AND TAPHONOMY: AN EXPERIMENTAL TAPHONOMY STUDY OF THE DEVELOPMENT OF THE BRINE SHRIMP ARTEMIA SALINA

Abstract

Although the relationship between ontogeny and phylogeny has been of long-standing interest to palaeontologists, the fossil record has provided little insight into the development of long extinct organisms. This has changed with the discovery of numerous assemblages of fossilized invertebrate embryos and larvae, but realising their evolutionary significance is hampered by a paucity of data on the relationship between ontogeny and taphonomy. We describe the results of an experimental taphonomy study of the development of the anostracan brine shrimp Artemia salina, which show that in conditions of aqueous aerobic and anaerobic autolysis and microbial decay, the developmental stages exhibit differential preservation potential. The most decay resistant developmental stage is the diapause cyst, encapulsating the gastrula, in which the gross morphology of the embryo can be maintained for 18 months or more in simple anaerobic conditions. Otherwise, the embryo shrinks within the cyst and cellular and tissue detail of breaks down as lipid droplets coalesce. Postembryonic excysted larvae decay more rapidly. The rate of decay is similar among all larval stages with the exception of the L 4 larva, which resists cuticle failure for longer than later developmental stages. The larvae decay leading to liquefaction of the muscles and viscera, leaving an intact but empty and progressively shrunken and distorted cuticle that eventually loses structural integrity and collapses. Our experimental results provide an explanatory model for the phenomenal abundance of putative diapause stage embryos, in the absence of postembryonic stages, as seen in the Ediacaran Doushantuo Formation of South China and the incompleteness of fossilized developmental sequences of embryos and larvae more generally. It also cautions against the association of developmental stages in fossil deposits without additional evidence. Finally, the pattern of decay seen in larvae provides an explanation for the preservation style of Orsten-type Lagerstätten where preservation of cuticular detail can be astonishingly fine, but extends internally to muscles and viscera only rarely.

Experimental taphonomy of giant sulphur bacteria: implications for the interpretation of the embryo-like Ediacaran Doushantuo fossils

“…This container was sealed and maintained at a temperature of 158C continuously before the experiments. Attempts to kill the bacteria in a consistent manner using strongly reducing conditions induced with beta-mercaptoethanol (BME) [17,[19][20][21] were ineffective. Preliminary trials placing Thiomargarita in oxic, anoxic and reducing conditions to assess preservation potential under different environmental conditions using standard protocols [19] produced no discernible difference between samples.…”

“…However, as fossils have invariably undergone death, post-mortem decay and diagenetic alteration, it is not meaningful to compare them directly with living organisms [18]. Although patterns of decay in animal embryos are now being studied [17,[19][20][21][22], nothing is known about decay processes or preservation potential in giant sulphur-oxidizing bacteria. Previous studies on decay of bacteria (see earlier studies [23,24] for reviews) are of limited relevance because of the comparatively small size, lack of central vacuole and lack of successive reductive division in the species studied.…”

The Weng'an Biota (Doushantuo Formation): an Ediacaran window on soft-bodied and multicellular microorganisms

“…In the case of decay, carefully designed experiments can help constrain which features can feasibly be preserved, elucidate likely preservation pathways, and identify biases introduced by the decay process (Sansom 2014). Experiments have shown that animal embryos have a relatively high preservation potential, particularly when enclosed in a fertilization envelope, whereas primary larvae have an extremely low likelihood of being preserved (Raff et al 2006;Gostling et al 2008Gostling et al , 2009). In addition, these experiments have identified the likely mechanism for Doushantuo-type preservation: 3D replication of cells by robust bacterial pseudomorphs followed by phosphate mineralization (Raff et al 2008).…”

Large spinose microfossils in Ediacaran rocks as resting stages of early animals

“…Based on available evidence from size, morphology, ultrastructure, and internal contents, the most compelling interpretation of LOEM taxa is that they are metazoan resting stages. We do not claim that all early animals had a resting stage in their life cycle; taphonomic experiments (58) indicate that cyst-forming animals should be differentially wellrepresented in the fossil record. Nor do we claim that all LOEMs must be metazoan cysts.…”