Sci-Day 12: The K-T Extinction Event

Happy Sci-Day, everyone! This week, I will be writing about one of the most tragic events in Earth's history. This event spelled the doom of many creatures, with non-avian dinosaurs among the casualties. Reading about this catastrophic time in history always makes me feel a bit sad - as a herpetology enthusiast, the Mesozoic represents a golden age for reptiles. In the Mesozoic, you could go to any ecosystem and almost universally the largest animal present would be a reptile. In the sky, you had pterosaurs - in the sea, there were the plesiosaurs and mosasaurs, and of course there were dinosaurs on the land. All of these groups were completely extinguished by this event - I would give almost anything to be able to travel back and study such creatures in the same way that I can study extant taxa.

The K-T extinction event marks the end of the Cretaceous period and the close of the Mesozoic Era. It also marks the beginning of the Paleocene period and Cenozoic Era, the latter of which continues today. While older estimates date this event at 65 million years ago, more recent estimates have revised this to around 66 million years (Renne et al., 2013).

One of the most common hypotheses for the cause of the K-T extinction is that it was triggered by a large comet or asteroid impact. Such an impact would have caused devastation on a global scale - the effect would be like nuclear winter on steroids. With all the debris and dust from the collision blocking out much of the sun's light and warmth, plants and phytoplankton would find it all but impossible to undergo photosynthesis (Alvarez et al., 1980), leading to widespread plant death and subsequent food chain collapse. In the 1990s, this hypothesis was further supported by the discovery of a 180-km wide impact crater (dubbed the Chicxulub crater) in the Yucatan peninsula (Hildebrand et al., 1991). Additionally, there is a thin layer of sediment marking the KT event (known as the 'KT boundary) present in all sedimentary rocks of the relevant age. This sediment shows high concentrations of iridium - this metal is rare in the Earth's crust, but is common in asteroids. This fact could mean that the KT boundary layer represents deposit of debris from the impact (Schulte et al., 2010). Furthermore, the fact that there is the fact that the extinctions seem to have happened around the same time as the impact, which is interpreted by some as strong situational evidence for this hypothesis.

While this hypothesis is generally accepted as the event that caused the demise of the non-avian dinosaurs [as well as many other groups of organisms], it is still a somewhat controversial issue. Some have actually argued that the extinction of non-avian dinosaurs was more gradual than some might claim, and both sides of the debate have support from the fossil record. A study of 29 fossil sites in Europe revealed that dinosaurs had significant diversity up until the KT event, with over 100 species present across the sampled sites (Riera et al., 2010) - this appears to support the hypothesis of a sudden extinction. Additionally, further research suggested that global non-Avian dinosaur diversity was significantly higher, with somewhere between 678 and 1078 species existing up until the event (Le Loeuff, 2012). However, there is evidence of a gradual decrease in non-avian dinosaur species richness at some fossil sites - a study of fossil-bearing rocks along the Red Deer River in Alberta shows that the number of species declined from roughly 45 to around 12 over the course of 10 million years (Ryan et al., 2001). If this is indeed true and is not due to differing preservation potentials of the sediment with age, it seems to support the gradual extinction hypothesis of non-avian dinosaurs. One possibility is that dinosaurs were gradually on the decline in some parts of the world, while they continued to thrive in other regions. However, without more data/evidence, we will not know for sure.

Dinosaurs were not the only group affected by the KT event. Many groups of squamates such as monstersaurs and polyglyphanodonts were nearly wiped out by the event, taking 10 million years to recover (Longrich et al., 2012). Additionally, both mosasaurs and plesiosaurs died out (Chattergee and Small, 1989). Mosasaurs and plesiosaurs were the apex marine predators of their time, growing to truly immense proportions. It is truly a pity that they are no longer with us.

The K-T extinction also spelled the end for the last pterosaurs. By the end of the Cretaceous, the only family definitely present was the Azhdarchidae; while there is some evidence of other families, the remains are far too fragmentary to assign them to any specific groups (Barrett et al., 2008). Evidence seems to suggest that pterosaurs were on the decline at the time, while modern families of birds were simultaneously increasing in diversity. While it was originally thought that this increase was indicative of birds 'replacing' pterosaurs due to interspecific competition or by filling niches left empty by the disappearance of pterosaur species (Robertson et al., 2004), the correlation between pterosaur diversity decline and bird diversity increase is simply not conclusive to the competition hypothesis (Butler et al., 2009). Additionally, there were small pterosaurs during the Late Cretaceous (Prondvai et al., 2014), further disputing the idea of direct competition.

I hope that this has given you a bit of a better understanding of the K-T extinction! While many groups of taxa were devastated in addition to the non-avian dinosaurs, it would take far too much time and space for me to go into any great depth on all of them. If you are interested in learning more, I encourage you to find resources online such as Google Scholar to read more about this subject!

Acknowledgements:
Renne, Paul R.; Deino, Alan L.; Hilgen, Frederik J.; Kuiper, Klaudia F.; Mark, Darren F.; Mitchell, William S.; Morgan, Leah E.; Mundil, Roland; Smit, Jan. 7 February, 2013. Time Scales of Critical Events Around the Cretaceous-Paleogene Boundary. Science 339 (6120): 684-687.
Alvarez, Luis. W.; Alvarez, Walter; Asaro, Frank; Michel, Helen V. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208 (4448): 1095-1108.
Schulte, Peter. March 5, 2010. The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary. Science (American Association for the Advancement of Science) 327 (5970): 1214-1218.
Riera, V.; Marmi, J.; Oms, O.; Gomez, B. March 2010. Orientated plant fragments revealing tidal palaeocurrents in the Fumanya mudflat (Maastrichtian, southern Pyrenees): Insights in palaeogeographic reconstructions. Palaeogeography, Palaeoclimatology, Palaeoecology 288 (1-4): 82-92.
Le Loeuff, J. 2012. Paleobiogeography and biodiversity of Late Maastrichtian dinosaurs: how many dinosaur species went extinct at the Cretaceous-Tertiary boundary? Bulletin de la Société Géologique de France 183 (6): 547-559.
Ryan, M. J.; Russell, A. P.; Eberth, D. A.; Currie, P. J.. 2001. The taphonomy of a Centrosaurus (Ornithischia: Ceratopsidae) bone bed from the Dinosaur Park Formation (Upper Campanian), Alberta, Canada, with comments on cranial ontogeny. PALAIOS 16 (5): 482-506.
Longrich, Nicholas R.; Bhullar, Bhart-Anjan S.; Gauthier, Jacques A. 2012. Mass extinction of lizards and snakes at the Cretaceous-Paleogene boundary. Proceedings of the National Academy of Sciences of the United States of America 109 (52): 21396-21401.
Chatterjee, S.; Small, B. J. 1989. New plesiosaurs from the Upper Cretaceous of Antarctica. Geological Society, London, Special Publications 47 (1): 197-215.
Barrett, P. M.; Butler, R. J.; Edwards, N. P.; Milner, A. R. 2008. Pterosaur distribution in time and space: an atlas. Zitteliana 28: 61-107.
Robertson, D. S.; McKenna, M. C.; Toon, O. B.; Lillegraven, J. A. 2004. Survival in the first hours of the Cenozoic. GSA Bulletin 116 (5-6): 760-768.
Butler, Richard J.; Barrett, Paul M.; Nowbath, Stephen; Upchurch, Paul. 2009. Estimating the effects of sampling biases on pterosaur diversity patterns: implications for hypotheses of bird/pterosaur competitive replacement. Paleobiology 35 (3): 432-446.
Prondvai, E.; Bodor, E. R.; Ősi, A. 2014. Does morphology reflect osteohistology-based ontogeny? A case study of Late Cretaceous pterosaur jaw symphyses from Hungary reveals hidden taxonomic diversity. Paleobiology 40: 288-321.