This geologic time scale (presented in typical fashion, i.e., vertical with the most recent time at the top) is designed to provide understanding of gymnosperm development and evolution and its relationship to plate tectonics, climate and time. The vertical space here is not proportional to the length of time but rather proportional to what is needed to record the events. The evolution of life on Earth is intricately interwoven with its geologic/physical evolution beginning with cooling and crust formation, movement of land masses, volcanism, development of atmosphere, extra-terrestrial collisions, etc. The names of the major time divisions, the Eras, are entirely defined by life stages (Proterozoic, Paleozoic, etc.) and further divisions such as Periods and Epochs are named for places, human inhabitants or physical characteristics of the exposed strata where fossil life was first discovered. The dates, given in millions of years ago (m.y.a.), are those of the International Commission on Stratigraphy (I.C.S.) which are of course slightly different from other sources including elsewhere on this website.
The borders between Eras and Periods have been created by Earth changes which significantly impacted life by forcing extinction on some organisms and creating opportunities for others. Some of these transitions were bigger than others. The most significant of these are described as “Worst Extinctions” based mainly on estimated extinction rates and we note the “Five Worst” on our chart and follow with a brief description of each of these. Of course there were many other dramatic extinction events and some were likely a series of different events over millions of years.
It is impossible to know exactly when an organism appeared or disappeared because our evidence is based on very small and localized samples of strata for any time period. This huge sampling error always permits the speculation that any life form may have appeared sooner or disappeared later than the fossil record indicates. This is especially true when the organism would leave a very faint “footprint” of its existence in rock strata, such as is the case with very small creatures with no shell or bones. For instance, the first convincing record of the hornworts (Anthoceratophytes) is in the Cretaceous, roughly 350 million years after the closely related mosses (Bryophytes) and liverworts (Marchantiophytes) emerged onto land in the Ordovician or Silurian Era. One would intuitively suspect the hornworts were here much earlier. Also, the discovery of a plant previously believed to be extinct, such as Metasequoia, illustrates the possible greater span for any organism than is represented by known fossil evidence.
Our very short personal existence makes it difficult to fully perceive how slow and yet how immense and dramatic Earth’s 4.6 billion year history has been. I hope that this chart shows those who study it that, as with our own lives, that biologic success is never permanent.
Earth History of Gymnosperms
(It may take a moment to upload)
THE FIVE GREATEST EXTINCTION EVENTS:
Please Note:
(a) that the usual suspects in the cause in nearly all of these major extinctions include asteroid/ comet impact and volcanism.
(b) the further back in time, the less certainty of ever knowing the cause.
(c) marine species generally take a greater beating than land species in extinction events.
(d) extinction rates estimated by John J. Sepkowski (University of Chicago).
Ordovician-Silurian: 425 m.y.a. loss of 60% of marine genera
Probable cause: As Gondwana drifted toward the south pole, there was a great cooling of this largest continent and extensive glaciation dramatically lowered sea level and exposed many shallow reefs where marine invertebrates, especially brachiopods, bryzoans, echinoderms (crinoids), corals and many other groups were left high and dry.
Late Devonian: 364 m.y.a. loss of 57% of marine genera
Possible causes: mostly likely not a single cause, rather a series of events over 15- 25 million years including ocean volcanism, ocean hypoxia, tectonic events, decreased atmosphere CO2 caused by lush Devonian Clubmoss, Fern, SeedFern, Quillwort and Progymnosperm forests, extraterrestrial impact. Primarily affects reef organisms, e.g., Brachiopods, Trilobites.
Permian-Triassic (“P-T”) : 251 m.y.a. loss of 95% of marine genera and 70% of land genera.
Possible causes: comet/asteroid impact, lava floods from Siberian Traps, ocean hypoxia, spike in sea fungal growth, sea floor release of methyl hydrates with CO2 and deadly temperature increase, shift in ocean circulation. “The mother of all extinctions.” A very long list of marine life became extinct and a less dramatic effect on terrestrial life but with destruction of tree forming species of Clubmosses, Horsetails, Seed Ferns, with a much drier climate in the early Triassic. No coal in the first 5 million years of the Triassic (the “coal gap”). But also an opportunity resulting in a rise of herbaceous plants including Club mosses, Spike mosses and Horsetails. Glossopteris, a Gondwana seed fern, is extensively replaced with another seed fern, Dicroidium. Cycads and Ginkgos are gaining ground. Podocarps, Cycadenoids and Voltziales appear. Cordiates become extinct.
End-Triassic : 200 m.y.a. loss of 52% of marine genera.
Probable cause: most likely massive lava floods from the central Atlantic magma province (separating North America and Europe) with resultant CO2 and methyl hydrate release and deadly global warming. Loss of certain land vertebrates, e.g., giant amphibians, some reptiles and therapsids (paving the way for dinosaurs). Also, a terrestrial vegetation shift from seed ferns and ferns toward gymnosperms, especially conifers such as Aruacariaceae, Cephalotaxaceae, Taxaceae, Pinaceae, Sciadoptaceae. Also, Cycadenoids and Ginkgos do well in the Jurassic. But the plants which benefited the most from this extinction event were the cycads which were able to form extensive tree forests and gave the Jurassic period the name “The Age of the Cycads.” The Voltziales become extinct.
Cretaceous-Tertiary (“K-T”): 65 m.y.a. loss of 47% of marine genera, 18% of land families (including the dinosaurs) (but some birds survive)
Probable cause: impact of a several mile-wide asteroid which created the Chicxulub crater now hidden in the Gulf of Mexico. Other possibilities include volcanic eruptions of basalt lava from the Indian Deccan Traps and, more recently, the possibility of an even greater impact in the ocean off the coast of Mumbai which may have caused the Deccan volcanic surge (the two impact theory). Ferns and Cycads suffer. Modern conifers and especially the angiosperms ultimately benefit (help not needed for the angiosperms since they had achieved late Triassic dominance on their own).