Placoderms

Picture source: http://www.devoniantimes.org/who/pages/phyllolepis.html

The Placoderms were the most diverse and successful group of fish in the Devonian period (420-360 Mya). Placoderms were aquatic vertebrates, which had characteristic platy armour. Their habitat varied extremely, being found from the Arctic to the Antarctic, high altitudes in lakes, and depths around 3km deep in the ocean, found in both marine and non-marine settings. There were nine subgroups, all evolving at different periods of the Devonian, however no subgroups survived through the Devonian Carboniferous extinction (Young, 2010).

The Hangenberg event took place at the late Devonian period. It was this event in which the Placoderms became extinct. The several events which happened in the late Devonian affected 70% of marine life, however terrestrial species were hardly affected (Bond & Wignall, 2008).

The exact cause of the event is yet to be proved, however there are several hypotheses which are commonly argued amongst scientists. The most viable hypotheses focus on the idea that the spread of terrestrial plants (Murphy, 2005). The increase in terrestrial vegetation meant that more organic material and nutrients were being deposited into waters, causing the waters to become Eutrophic (nutrient rich) (McGhee, 2012).  

Eutrophication process.

 Diagram source: https://sites.google.com/a/owu.edu/lake-erie-eutrophication/what-is-eutrophication/the-nutrients

Eutrophication occurs as a result of a water body becoming enriched in nutrients, enhancing photosynthesis in the shallow waters through the formation of algae blooms, however creating anoxic waters at the bottom of the water body as a result of decomposition taking place. In addition to the eutrophication of shallow tropical waters, wide spread forests speed up the process of soil formation, and in addition, also sped up silicate-carbonate cycle. The process as a whole consumes CO2 and releases it in the form of dissolved silicate rocks into oceans in which it the precipitates and forms marine sediments.  The transferring of CO2 from the atmosphere to the hydrosphere meant that there was loss of a greenhouse gas, and therefore resulted in cooling (Bond & Wignall, 2008).

In short, the terrestrial widespread of plants and forests resulted in reduction of CO2 in the atmosphere, and lead to a global cooling event. Eutrophication would be a viable hypothesis as to how bottom dwelling species in the tropics became extinct – they could not survive in the anoxic conditions (McGhee, 2012). Global cooling resulting in a short glaciation may be to blame to for the extinction of the remainder species which lived at the poles – particularly the South Pole where ice caps rapidly formed. While there is no extensive evidence for a glaciation event in terms of a sedimentary deposit, the sudden drop in sea level and temperature indicates it was highly possible (Bond & Wignall, 2008).

To compare the extinction of Placoderms to Humans is extremely difficult. The extinction of the species was the result of climatic and atmospheric changes due to the widespread of terrestrial plants. The short term prediction for climate in our lifetime is for it to continue to increase, and seeing as we are coming out of a glacial event, cooling and glaciation is one thing we won’t have to worry about. However adjusting to a new climate is something we should be concerned about considering how fast our global temperature on average is increasing, and should the rapid glaciations be the sole reason which caused the extinction of the Placoderms, then the need to rapid adapt to a new climate should be a concern for humans in the future.

References

Bond, D., & Wignall, P. (2008). The role of sea-level change and marine anoxia in the Frasnian–Famennian. Palaeogeography, Palaeoclimatology, Palaeoecology, 107-118.

McGhee, G. (2012). Extinction: Late Devonian Mass Extinction. Chichester: John Wiley & Sons Ltd.

Murphy, D. (2005, July 9). Late Devonian Mass Extinctions. Retrieved from Devonian Times: http://www.devoniantimes.org/opportunity/massExtinction.html

Young, G. (2010). Placoderms (Armored Fish): Dominant Vertebrates of the Devonian Period. The Annual Review of Earth and Planetary Sciences , 523-550.