Primeval 9 – Silurian Period

 

The Silurian Period existed between 443 and 419 billion years ago, and enjoyed relatively stable and warm temperatures, in contrast with the extreme glaciations of the Ordovician Period before it, and the extreme heat of the ensuing Devonian Period.(1)Axel Munnecke et al. (2010) “Ordovician and Silurian sea–water chemistry, sea level, and climate: A synopsis” Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 296, Number 3–4, Pages 389–413 During this period, the Earth entered a long, warm greenhouse phase, and warm shallow seas covered much of the equatorial land masses. Early in the Silurian, glaciers retreated back into the South Pole until they almost disappeared in the middle of Silurian. The period witnessed a relative stabilization of the Earth’s general climate, ending the previous pattern of erratic climatic fluctuations. Layers of broken sea shells provide strong evidence of a climate dominated by violent storms generated by warm sea surfaces. Later in the Silurian, the climate cooled slightly, but by the Silurian-Devonian boundary the climate had become warmer.

Earth 430 Million Years Ago

Earth 430 Million Years Ago

The Silurian was the first period to see large fossils of wide spread terrestrial animals, believed to have generally lived in the moss forests and giant Prototaxite fungi along the edge of lakes and streams. However, the land animals did not have a major impact on the Earth until they diversified in the Devonian. There were a series of minor extinction events during the Silurian Period, although the causes are unknown. The Silurian has a higher concentration of isotopic excursions than any other period; the Ireviken event, Mulde event and Lau event each represent isotopic excursions following a minor mass extinction(2)Christian Samtleben et al. (2000) “Development of facies and C/O-isotopes in transects through the Ludlow of Gotland: Evidence for global and local influences on a shallow-marine environment” Facies, Volume 43, Page 1 and associated with rapid sea-level change. Each one left a similar signature in the geological record, both geochemically and biologically; coastal fish were particularly hard hit, as were brachiopods, corals and trilobites. These extinction events are considered minor because of the few species that were affected by the events, although they each left an impact on the planet’s ecology. The Ireviken event is best recorded at Gotland (Sweden), where over 50% of trilobite species went extinct, additionally 80% of the global conodont species also become extinct during this event.

Prototaxites growing along the Silurian coast.

Prototaxites growing along the Silurian coast.

The Lau Event came at the end of the Silurian Period, and was a larger extinction event, although not considered a major extinction event. Major changes are observed worldwide, with a “crisis” observed in populations of conodonts and graptolites.(3)Adam Urbanek (1993) “Biotic crises in the history of Upper Silurian graptoloids: a palaeobiological model” Historical Biology, Volume 7, Pages 29–50 Specifically conodonts suffered in the Lau event, and graptolites in the subsequent isotopic excursion.(4)David K. Loydell (2007) “Early Silurian positive δ13C excursions and their relationship to glaciations, sea-level changes and extinction events” Geological Journal, Volume 42, Number 5, Pages 531–546 The exact nature of the Lau Event is a mystery, and it does appear as if the event was extreme in certain locations, while other regions might have been minimally impacted, as some regions of the planet took on anachronistic traits, where stromatolites, which rarely form in the presence of abundant higher life forms, are observed during the Lau event and, occasionally, in the overlying Burgsvik beds.(5)Mikael Calner (2005-04-01) “A Late Silurian extinction event and anachronistic period” Geology, Volume 33, Number 4, Pages 305–308 Microbial colonies of Rothpletzella and Wetheredella became abundant; this suite of characteristics is common to the larger Ordovician–Silurian and Permian–Triassic extinctions.

The details of the Lau Event appear reminiscent of the past few thousand years, as species and ecosystems throughout the world have been destroyed by humanity. The obvious difference it that their doesn’t appear to be an advanced species on the planet at the time capable of inflicting this damage. It is possible that the damage was inflicted by a temporary colony. If their star system was only nearby for under a million years, it might not have mattered what happened to the indigenous ecosystem. Naturally these colonists would also have to have been from a compatible ecosystem. If there was an ancient civilization that caused the Great Oxidization and seeded terrestrial life, then they would have likely seeded many planets as star-systems passed them. The resulting number of worlds with compatible life could be in the millions throughout the galaxy. If one of these worlds was nearby 419 million years ago, Earth’s oceans may have provided a new and interesting selection of foods, and a new breading ground for their own aquatic species.

References   [ + ]

1. Axel Munnecke et al. (2010) “Ordovician and Silurian sea–water chemistry, sea level, and climate: A synopsis” Palaeogeography, Palaeoclimatology, Palaeoecology, Volume 296, Number 3–4, Pages 389–413
2. Christian Samtleben et al. (2000) “Development of facies and C/O-isotopes in transects through the Ludlow of Gotland: Evidence for global and local influences on a shallow-marine environment” Facies, Volume 43, Page 1
3. Adam Urbanek (1993) “Biotic crises in the history of Upper Silurian graptoloids: a palaeobiological model” Historical Biology, Volume 7, Pages 29–50
4. David K. Loydell (2007) “Early Silurian positive δ13C excursions and their relationship to glaciations, sea-level changes and extinction events” Geological Journal, Volume 42, Number 5, Pages 531–546
5. Mikael Calner (2005-04-01) “A Late Silurian extinction event and anachronistic period” Geology, Volume 33, Number 4, Pages 305–308