Cephalopod egg fossil

Cephalopod egg fossils are the fossilized remains of eggs laid by cephalopods. The fossil record of cephalopod eggs is scant since their soft, gelatinous eggs decompose quickly and have little chance to fossilize. Eggs laid by ammonoids are the best known and only a few putative examples of these have been discovered. The best preserved of these were discovered in the Jurassic Kimmeridge Clay of England. Currently no belemnoid egg fossils have ever been discovered although this may be because scientists have not properly searched for them rather than an actual absence from the fossil record.

Ammonite eggs in well-aerated sea bottoms probably would have quickly been broken down by scavengers and aerobic bacteria. Fossil evidence supports this general idea since swarms of hatchling ammonitellae fossils are known although there are no associated egg fossils. One of the eggs preserved in the Kimmeridge Clay ammonite egg cluster K1486 bears crystalline phosphate on its surface. Since phosphate is mobile only in organic form this suggests the eggs were already decaying before fossilization. The fact that the ammonite eggs survived decomposition to become fossilized suggests two possibilities. The first is that the eggs were transported from the well-aerated location they were originally laid in to low-oxygen waters near the sea floor. The second possibility is that the eggs were laid in waters whose oxygen abundance varied, possibly by season.

Multiple instances of fossil ammonite eggs have been observed in the fossil record. However, prior to 2009 only two credible examples of ammonite eggs had been reported to the scientific literature. The first was an apparent clutch of eggs preserved in the sediment that filled in the living chamber of a harpoceratid dating back to the Toarcian age of the Jurassic period. This specimen was discovered in a concretion incorporated into glacial drift that came from the Baltic region. The ammonite itself was a fully grown individual with a macroconch shell. The second possible example was another adult macroconch of Ceratites from the Muschelkalk of Germany, which dated to the Upper Anisian of the Triassic period. An additional less plausible case has been reported from Kamchatka where an egg clutch was purportedly associated with a Desmophyllites dating back to the early Triassic. However a later summary of ammonite embryos from the same age and location does not mention any eggs being known and Desmophyllites is a Late Cretaceous genus, so this report is not reliable.

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