You have probably heard the story before. A meteorite/asteroid impacted a shallow Late Cretaceous sea where the Yucatan peninsula lies today. It has been seen as the cause for the iridium-rich ejecta that covered much of the earth at the Cretaceous/Tertiary (K/T) boundary. Mineral grains showing shock metamorphism have been found at Chicxulub. The K/T impact ejecta decreases in thickness further away from the impact crater and possible impact-generated tsunami wave deposits have been found in the Gulf of Mexico. Radiometric analyses of the Chicxulub impact melt and Haiti K/T tektites date this event to 65 million years ago.
In this scenario tsunamis followed the impact as did acid rain and the whole world was set on fire. Once these immediate effects had passed the whole world still lay in darkness due to particles blown up into the atmosphere during the impact and the ensuing forest fires. Not just dinosaurs, but a whole range of animals got extinct in the dark and cold aftermath of the impact. When plant eating animals died the carnivores soon followed. Through the remains of the Late Cretaceous flora and fauna the mammals could take over niches left once held by other animals. Mammals ruled the world from then onwards (despite the fact that there are far more bird [feathered dinosaur] species than mammals today).
Not all palaeontologists agree with this scenario. The tsunami has been questioned as has the forest fires and acid rain. The Chicxulub impact, according to some, predates the K/T boundary and its iridium layer by roughly 300,000 years. Gerta Keller suggests there was another impact responsible for the iridium layer, and one likely candidate would be the Shiva crater west of Mumbai.
However, the multi-impact model is just a variation of the first one, both rely on the assumption that sudden global catastrophes killed off the most specialized animals. Just to make one common misunderstanding clear: the dinosaurs did not go extinct because they were “primitive” and therefore predestined or doomed to failure. On the contrary, dinosaurs and mammals emerged more or less at the same time in the Triassic and it was not until after a possible global catastrophe that mammals had opportunities to diverge in greater intensity. it was because dinosaurs were well adapted and specialized to their niches that they could not survive possible disasters.
Still, there are signs that many animal groups were becoming fewer and fewer towards the Late Cretaceous before the impact(s). This has partly been blamed on gigantic volcanic eruptions (Deccan Traps) that created the Deccan plateau in India. However, the so-called Signor-Lipps effect has been used by the proponents of the impact theories to suggest that these earlier extinctions are not real, they only appear so because palaeontologists randomly excavate randomly preserved fossils. We simply have not found all fossils they argue. Proponents for the gradual extinction model suggest that all animals show this pattern, particularly among the marine ammonites for which there are much more fossils available.
It is a fascinating problem. Why is there no non-avian dinosaur in the early Tertiary? Our categorization of vertebrate animals into Classes down to species is only an invention of essentialist thinking of pre-Darwinian times (Linnaeus). In a temporal continuum there are no species, etc. Just because we decide to classify a very large group of animals as dinosaurs, because they share common ancestors and traits different from mammals (well, not if you go back to Perm), does not explain why this whole classified group disappeared since the variation was huge. Why did not some small, non-specialized non-avian dinosaurs, survive? Why did I not become a palaeontologist instead? It seems so much more interesting nowadays.
Whatever the answer to the extinction of dinosaurs is that is not the major issue in this short series of blog posts that deals with my current research. I shall focus on the Chicxulub impact crater itself and how this has affected the geology and human settlement in the northern Yucatan peninsula. As it turns out, the greater Chicxulub fracture zone may well have been a slayer of Colonial period Maya as well.