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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.

The Abrupt Extinction

In 1991 Fastovsky and co-workers presented the results of a three year field study that looked at family-level diversity of dinosaurs in the Hell Creek Formation (HCF) of Montana (Figure 1).

The dig site: Map of the field area in Montana/Hell Creek Formation

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:

  • facies A: thalweg deposit
  • facies B: point bar deposit
  • facies C: floodplain deposit

    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.

    Albertosaurus Triceratops


    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.

    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:

  • Has the abrasion state of dinosaur fossil material been analysed in detail and what is the reliability of this method?

  • How has the varying REE signature of dinosaur bones been incorporated into this work? (For a state of-the-art approach to this problem have a look at Clive Trueman's and Mike Benton's new work on this subject)

  • The validity of citing the absence of mammalian material as evidence for the existence of Paleocene dinosaurs is questionable because this implies similar preservation potential of both groups.

    Last updated March 22, 1998