Tracing Competing Paradigms - Mapping Scientific Frontiers: The Quest for Knowledge Visualization - page 210

Fig. 6.8 Chicxulub's gravity field ( left ) and its magnetic anomaly field ( right ) (© Mark Pilkington
of the Geological Survey of Canada)
cluster, but also ones that challenged the paradigm. This would be a desirable
feature because scientists can access a balanced collection of articles from different
perspectives of a debate. Indeed, evidence strongly supporting the impact theory,
such as Hildebrand's 1991 paper on the Chicxulub crater and Keller's 1995 paper
on the conclusiveness of available evidence (Keller 1993 ) were found in the same
cluster. After all, when we debate about a topic, we are likely to cite the arguments
from both sides.
The KT impact cluster also included an article labeled as Signor. This is an article
by Signor and Lipps on what is later known as the Signor-Lipps effect. The Signor-
Lipps effect says if there were few fossils preserved, an abrupt distinction can look
like a gradual extinction. Because whether the KT event was a gradual extinction
or a catastrophic one is crucial to the debate, the high citation profile of Signor and
Lipps' article indicates its significance in this debate.
Tab le 6.3 shows the most representative articles of the KT impact cluster in terms
of their factor loadings. Walter Alvarez in his topic ( 1997 ) highly regarded Smit's
contribution to the impact theory: Alvarez found the iridium abnormally in Italy,
whereas Smit confirmed the iridium abnormally in Spain. Smit's 1980 article in
Nature , which topped the list, is located immediately next to the 1980 Science paper
by Alvarez et al. Both articles are strongly connected via a strong Pathfinder network
link. The table also includes Glen's 1994 topic Mass Extinction Debates .
Articles from the Gradualism camp are located between the KT Impact cluster
and the Periodicity cluster. Landmark articles in this cluster include ones from
Chunk Officer, a key opponent of the impact theory. The article by another anti-
impact researcher Dewey McLean is also in this cluster, but below the 50-citation
landmark threshold. McLean proposed that prolonged volcanic eruptions from the
Deccan Traps in India were the cause of the KT mass extinction.
Piet Hut's 1987 Nature article on comet showers, with co-authors such as Alvarez
and Keller, marked a transition from the KT impact paradigm to the periodicity
hypothesis. This article was seeking an explanation of the periodicity of mass
extinctions within the impact paradigm.