Ontogeny and taphonomy: and experimental taphonomy study of the development of the brine shrimp Artemia salina

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 L4 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 Lagersta¨tten where preservation of cuticular detail can be astonishingly fine, but extends internally to muscles and viscera only rarely.