Patterns and Timing of Dinosaur Extinction
Patterns and Timing of Dinosaur Extinction
Introduction
The following page intends to examine patterns and timings of dinosaur extinction. I
have tried to emphasise both concepts of gradual and sudden extinction at the
Cretaceous/Tertiary (K/T) boundary. I have also tried to introduce simple
stratigraphic concepts highlighting the possible causes and consequences
of the K/T event.
Patterns of Dinosaur Extinction
Did the dinosaur population dwindle gradually at the end of the Cretaceous as a result of
climatic change? Conversely, did an asteroid impact produce sudden environmental changes that
causing an abrupt extinction? There is evidence for both extinction
scenarios. The gradualists including Van Valenbelieve
that dinosaur diversity decreased over a period of 7 million years in the final stage of the Cretaceous.
However, Fastovskyand supporters of a catastrophic event
suggest that sudden environmental changes caused an abrupt extinction.
Their fieldwork focused on assessing ecological diversity rather than taxonomic diversity
and is therefore more useful in examining changes in extinction patterns. All taxa used for
this study were monophyletic and occured consistently throughout the three units of the HCF.
However, preservation of fossils in the HCF is strongly controlled by the
depositional processes that produce the different facies. For example, dinosaur bones found
in stream deposits resemble a death assemblage derived from different upstream
habitats. Hence, patterns of diversity are only reliable if identical facies are
compared. An important component of this field study was therefore to identify and
distinguish different facies in the HCF. Statistically significant fossil collections were
obtained from three facies:
Furthermore, taphonomic studies have revealed that in fluvial systems preservation of large
dinosaur bones is favored over small mammalian ones. Therefore, the degree to which
preferential preservation has modified the Cretaceous ecological diversity is assumed to be
constant.
However, their findings suggested that extinction was sudden and that there was no evidence
(probability < 0.05) of a gradual decline in dinosaur diversity. In fact, all eight dinosaur
families recorded in the study ranged into the upper third of the Hell Creek Formation.
Both species below, Triceratops and Albertosaurus, have been found in the HCF.
In summary: the dinosaur extinction followed a long period of decline (5-7 million years) in
both the species and numbers of individuals, which occurred near and after
the iridium boundary. However, the evidence for dinosaurs surviving the
impact event is questionable. Several questions remain unsolved:
Last updated March 22, 1998
The dig site: Map of the field area in Montana/Hell
Creek Formation
The Gradual Extinction Scenario - "Paleocene" Dinosaurs
Several palaeontologists including Sloan have joined the debate regarding
the extinction of dinosaurs with a provocative argument: Dinosaurs survived into the
Paleocene, therefore reducing the biologic magnitude of the K/T boundary extinction event.
They base this theory on stratigraphic and palynological evidence.
Sloan and co-workers postulate that dinosaur fossils contained in these
Paleocene channel deposits have not been reworked. They argue that if reworking had taken
place, the Hell Creek channel deposits should include mammalian as well as dinosaur fossils.
If reworking of the HCF is to contribute to younger Paleocene sediments, then large parts of
other Cretaceous fauna, not just dinosaurs, should appear in the Paleocene channel fillings.
According to Rigby and Sloan the most common elements, Cretaceous mammals should
appear in the reworked channel deposits. However, many mammals common in Cretaceous
sediments (e.g. Alphadon, Pediomys) do not appear in the younger deposits.
Albertosaurus
Triceratops