Controversies in Science/What killed the dinosaurs/A critique of Mass extinction of birds at the cretaceous-palogene (k-pg) boundary
(Review Paper) Cited in Controversies in Science/What killed the dinosaurs/A critique of Mass extinction of birds at the cretaceous-palogene (k-pg) boundary
- The extinction event that they suggest took place around this time period involves the very least one asteroid impact and likely an increase in volcanic activity. This would create a large amount of dust and ash which would linger in the atmosphere reducing the amount of sunlight able to reach the surface of the earth.
- Molecular studies cannot clearly identify which species of birds were present at the end of the Cretaceous period which is why these authors relied on fossil records to execute this study.
- The authors focus on 17 species of birds which were shown to have existed in the Cretaceous and none of them made the transition into the Paleogene era. Therefore, the authors believe that there was an extinction event (K-PG) near the end of the Cretaceous.
- An interesting point raised by the others was the large diversification of birds towards the end of the Cretaceous but a lack of the very small and vary large birds that are present today..
The authors studies the fossils of birds found in three different locations.
- Hell Creek Formation (Montana, North & South Dakota)
- Lance Formation (Wyoming)
- Frechman Formation (Saskatchewan)
These fossils were most likely deposited at these locations in the later part of the Cretaceous period, about 300,000 years before the Paleogene period.
Researchers examined the fossils of the birds found in the regions described above and focused their researched on the coracoid bone to prevent overlapping of birds into the same categories. They chose this bone because it is quite commonly preserved and also has very little variation within the same species. This allowed them to determine if any of the species that they have identified are also present after the K-PG extinction event.
Since this data is focused on the western part of North America they researchers cannot conclusively infer that the findings will hold true all over the world but, although centered in North America, this was likely a global event.
The studies found that the body size of birds varied greatly prior to the end of the Cretaceous period, though they didn't appear to represent the largest volant birds - larger species were found in the Mesozoic era. Larger birds (>10kg) and smaller birds (<200g) do not appear to be present immediately after the cretaceous period. There have been no recovered fossils of these species from the Paleogene era, but this only accounts for fossils found in North America. The study focused on 17 specific species of birds that were shown to exist in the Cretaceous period but didn't exist in the Paleogene period. These results indicate that an event occurred (at least one asteroid, if not more) that would lead to a mass extinction of these species during the Cretaceous era. 
- Longrich, N., R., Tokaryk, T., & Field, D., J. (2011). Mass extinction of birds at the cretaceous-palogene (k-pg) boundary. Proceedings of the National Academy of Sciences of the United States of America, 108(37), 15253-15257. doi:10.1073/pnas.1110395108 http://www.pnas.org/content/108/37/15253.full