Rocks and Dinosaurs
Where is the evidence from ?
Maastrichtian dinosaurs are known from all continents except Australasia. The western interior of North America is the only area where a continuos terrestrial sequence encompasses the K-T boundary. Therefore, this area has been the focus of several detailed studies, 20 of the total 26 known localities for late Maastrichtian are located in the western interior of North America (Archibald, 1992).
A thorough a study of non-marine vertebrate fauna across the K-T boundary produced an estimated 36% extinction rate (Archibald and Bryant 1990).
How did dinosaurs fare in this ?
Dinosaur extinction in Montana, Alberta, and Wyoming was a gradual process that began 7 million years before the end of the Cretaceous and accelerated rapidly in the final 0.3 million years. Could this be a result from increasing competition from ungulates (mammals)? Sloan others(1986) believe this to be an important factor.
Diversity of dinosaurs leading up to the extinction
Dinosaurs showed greatest diversity in the final 15 million years of the Cretaceous with more than 400 genera present (Russell,1995). Upper Campanian facies in Canada, showed that the dinosaur fauna was diverse with 45 genera present from 13 families.
By the upper Maastrichtian diversity had declined, to 24 genera in 12 families (Weishampel 1990) and 30 genera to 7 at the K-T boundary Sloan others(1986).
Bill Clemens (1992) also reported that there was a decrease in diversity from the Campanian to the Maastrichtian , a time period of over 10 Ma, where the number of genera decreased from 32 -19.
The actual number of genera to make it to the boundary is still uncertain which highlights the limitations of the fossil record, with respect to, incompleteness and absolute certainty of the evidence.
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| Fig 4:Graph indates the rate of decline of the dinosaurs over the last geological stages befor the K-T boundary |
Death and Duration; The Extinction
Strata at the Fort Peck Reservoir, Montana, contain fossil assemblages of Tyrannosaurus and Amatosaurus, in the lower levels, and Triceratops and Thecelosaurus in the upper levels. The top of the formation is marked by a mass death assemblage containing a greater number of large dinosaurs than smaller specimens.Facies at Hell's Creek were reported to contain 7 species of dinosaur, Palaeocene pollen and mammalian teeth, a controversial topic when it was believed to be located 1.3m (40,000 yrs) above the iridium anomaly of the K-T boundary Sloan and others (1986).
Can this be true ?
A barren zone of 3m is present below the boundary where dinosaur bones and teeth become increasingly rare Sloan and others (1986). The last unreworked teeth specimens occurs 60cm below the clay layer (Sheenan and others, 1991). If dating, by the magneto-stratagraphic method Lerbekmo and others (1996) for the final 1m of Cretaceous sedimentation in this area, is correct the 60cm to the boundary could represent a time span of 15,000 years or more.
This figure is considered an instant in geological time, however, with respect to question of gradual extinction,it provides ample time for natural causes ...... without necessity for a catastrophic explanation, such as an asteroid.
What happened to other animals ?
Some large land animals and mammals were greatly affected, while very large crocodiles and very small amphibians were not. A total a survival rate of terrestrial species at the K-T boundary to be 53 % Archibald & Bryant (1990) .Summary
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