What parts of the fossil record are most lacking in specimens? That is, if you were to trace the evolution of a modern mammal (humans, for example) from abiogenesis to now, which periods are the most lacking in fossils? Where are the biggest gaps in our evolutionary history.
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$\begingroup$ Remember that you need to distinguish the types of fossils. For example, I don't think that someone is missing forams from the Cretaceous, but on the other hand some land mammals from the same period might be quite scarce. $\endgroup$– GimelistCommented Nov 4, 2014 at 9:48
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1$\begingroup$ @Michael: I would have though "if you were to trace the evolution of a modern mammal (humans, for example) from abiogenesis to now" would have narrowed that down enough. $\endgroup$– naught101Commented Nov 5, 2014 at 3:32
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The biggest temporal gap would be (IMO) the Precambrian, specifically pre-Ediacarian. Accordingly the biggest gap in the evolutionary history is the origin of eukaryots, both because of the paucity of pre-Ediacarian formations but also because few of the early protists were likely to be fossilizable.
From the probable apparition of life ca. 3.8-3.5 Ga (e. g. 1 for a review of Archean fossils) to the Ediacarian (ca. 600 Ma) in which metazoans (i. e. "animals") are already diverse (e. g. 2), this is more than 3 Ga of life history for which we have very little fossils (though our knowledge of this fossil record increases currently thanks, in part, to our better understanding of the "chemical" fossil record).
1: J. W. Schopf, 2006. Fossil evidence of Archaean life. Philosophical transactions of the Royal Society, B., 361: 869-885.
2: S.Conway Morris, 1993. The fossil record and the early evolution of the Metazoa. Nature, 361: 219-225.
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3$\begingroup$ Nice answer! It might be worth explicitly mentioning that this is due to the difficulty in preserving fossils from anything that doesn't have hard body parts. (And not, say, from unusual environmental conditions in the Proterozoic/Archean.) $\endgroup$ Commented Apr 16, 2014 at 17:05
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1$\begingroup$ Indeed but I hesitated mentioning it since it makes an assumption on which eukaryots appeared first (which is not a consensus, it seems, amongst molecular biologists). I would assume them to be Excavates in which case they would be unlikely to have any "shell" (based on what we know of modern Excavates) but if the basal eukaryots were Rhizarians (which is the group that includes Forams and Radiolarians but also many many thecate amoebas) it is a possibility that they would have fossilizable parts. But I guess I'm nitpicking :) $\endgroup$ Commented Apr 17, 2014 at 6:05
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1$\begingroup$ For historical interest there's Romer's Gap, an apparent ~15 million year-long gap in the early Carboniferous fossil record for tetrapod evolutionary history (identified by the american palaeontologist Alfred Sherwood Romer in the 20th century - en.wikipedia.org/wiki/Romer's_gap). Even though sites with fossil tetrapods have since been found from that period (see the Wikipedia page References), there are various ideas presented on the same Wikipedia page suggesting other factors in the observation of the gap's existence. $\endgroup$– JimboCommented Oct 31, 2015 at 12:39
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$\begingroup$ you also have the fact that sedimentary rock (aka rock that would have fossils in it) from those periods is just very rare, it gets rarer the further you go back, becasue there has been more time for other things to destroy it. $\endgroup$– JohnCommented Feb 16, 2018 at 18:58
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$\begingroup$ @John Indeed, that's what I meant by
because of the paucity of pre-Ediacarian formations. $\endgroup$ Commented Feb 19, 2018 at 8:42