Decay and Mineralization of Invertebrate Eggs

Abstract

Both the timing and nature of early Metazoan evolution remain controversial, with complementary, and sometimes conflicting, evidence from molecular data and fossil occurrences. Exceptionally preserved embryos from the Neoproterozoic and early Phanerozoic remain an important source of direct evidence: fossil embryos of complex organisms at a relatively advanced stage of development provide a test of hypotheses based on comparative embryology and the evolutionary development of living forms. Understanding how these fossils are preserved, and what morphological changes are induced by decay, is essential to interpreting the evidence that they provide for early metazoan evolution. A range of decay experiments was performed on eggs of living arthropods to explore the controls on their mineralization and likely fossilization. Surface mineralization occurred within one month, mainly in calcium carbonate or in a combination of calcium carbonate and calcium phosphate. There was no enhancement of mineralization with increased phosphate or calcium concentrations, or with changing oxic/anoxic conditions, and mineralization occurred both in the absence and presence of an associated carcass. The eggs showed remarkable resistance to decay, indicating that an extended time-period (more than a year) would be available for mineralization and fossilization in some settings. Where sediment was available, it could become attached to the egg surface, replicating the morphology in a fashion analogous to mineralization. The interior of the eggs was not mineralized in the experiments. The degree of mineralization was very variable, reflecting conditions both in the natural environment and in the fossil record.

Experimental mineralization of crustacean eggs: new implications for the fossilization of Precambrian–Cambrian embryos

“…The dry mass contained 120 ppm Ca and 1.3 wt.-% P, comparable to concentrations described for other invertebrate eggs (Martin et al, 2005). Scanning electron microscopic (SEM) mapping of P of an untreated egg in cross-section furthermore revealed an almost uniform distribution of P, however with higher initial P concentrations in the embryonic tissue of the germ area and in some areas of marginal embryonic tissue (Fig.…”

“…Experimental phosphatization of eggs was first reported from the shrimp Palaemon elegans, which was subjected to decay as a complete animal for 20 weeks (Briggs and Kear, 1993a). Later studies, which focused on the eggs, demonstrated that it was possible to maintain the external shape of Homarus gammarus, Limulus polyphemus, and Nephrops norvegicus egg cases for at least a year, including surface mineralization mainly as calcium carbonate or as a combination of calcium carbonate and calcium phosphate (Martin et al, 2003(Martin et al, , 2005. Laboratory decay experiments also achieved the replication of the outer morphology of H. gammarus eggs by coating the eggs with pre-existing clay minerals in the presence of metabolizing bacteria (Martin et al, 2004).…”

“…Whether changes in pH, the concentration of P or a combination of both caused the observed differences in Palaemon elegans, however, remained unclear. Partial mineralization was also described for decay experiments on invertebrate eggs of the European and Norwegian lobsters, Homarus gammarus and Nephrops norvegicus (Martin et al, 2005), in which mineralization of eggs has been closely associated with bacteria generating anoxic conditions and reduced pH. Mineralization of eggs occurred most commonly as calcium carbonate, or as a mixture of calcium carbonate and calcium phosphate.…”

Embryo fossilization is a biological process mediated by microbial biofilms

“…Our data provide direct evidence for microbial biofilms and identification of bacterial taxa involved in decay, pseudomorphing of cell structures, and potential for mineralization that can lead ultimately to exceptional fossilization. We recognize that even when microbes are present, mineralization of animal soft tissue may be produced abiogenically, for example, under conditions of high phosphate concentrations (26)(27)(28). The frequency of these mineralization modes remains to be determined for embryos.…”

Ammonoid Embryonic Development

“…Invertebrate eggs can be quite resistant to decay (Martin et al 2003(Martin et al , 2005) and various structures (mostly associated with ammonoid remains) interpreted as ammonoid eggs have been reported from the Carboniferous (e.g.,?Rhadinites or?Anthracoceras: Landman et al 2010), Triassic ( Ceratites: Müller 1969), Jurassic (Dreyfuss 1933;Wetzel 1959;Eleganticeras: Lehmann 1966, 1990Kachpurites: Baranov 1985;Etches et al 2009;Fig. 5.23) and Cretaceous (Baculitidae gen. et sp.…”