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.
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
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
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) .
Bug Creek channels and the Hell's Creek Formation indicate decline of the
dinosaurs population. The decline is rapid and increases further nearer the
boundary (Williams 1994).
Careful and detailed examination of stratagraphic sections at Zumaya, Spain
(Ward and others, 1986) and the Hell Creek formation, United States
(Sloan and others, 1986) show that extinction had begun well before the K/T
Even after accounting for reworking of sediments and mathematical corrections
to data sets, to remove the effects of biasing, Archibald and Bryant (1990),
still retain that observations cannot be explained by an asteroid impact, but
are a result of a geologically rapid, but non-catastrophic extinction causing a
loss in range and habitat diversity.