Towards an understanding of cosmopolitanism in deep time: a case study of ammonoids from the middle Permian to the Middle Triassic

Cosmopolitanism occurred recurrently during the geological past,
especially after mass extinctions, but the underlying mechanisms remain
poorly known. Three theoretical models, not mutually exclusive, can lead
to cosmopolitanism: (1) selective extinction in endemic taxa, (2) endemic
taxa becoming cosmopolitan after the extinction and (3) an increase in the
number of newly originated cosmopolitan taxa after extinction. We analyzed
an updated occurrence dataset including 831 middle Permian to Middle
Triassic ammonoid genera and used two network methods to distinguish major
episodes of ammonoid cosmopolitanism during this time interval. Then, we
tested the three proposed models in these case studies. Our results
confirm that at least two remarkable cosmopolitanism events after the
Permian–Triassic and late Smithian (Early Triassic) extinctions,
respectively. Partitioned analyses of survivors and newcomers revealed
that the immediate cosmopolitanism event (Griesbachian) after the
Permian–Triassic event can be attributed to endemic genera becoming
cosmopolitan (Model 2) and an increase in the number of newly originated
cosmopolitan genera after the extinction (Model 3). Late Smithian
cosmopolitanism is caused by selective extinction in endemic taxa (Model
1) and an increase in the number of newly originated cosmopolitan genera
(Model 3). We found that the survivors of Permian–Triassic mass extinction
did not show a wider geographic range, suggesting that this mass
extinction is nonselective among the biogeographic range, while late
Smithian survivors exhibit a wide geographic range, indicating selective
survivorship among cosmopolitan genera. These successive cosmopolitanism
events during severe extinctions are associated with marked environmental
upheavals such as rapid climate changes and oceanic anoxic events,
suggesting that environmental fluctuations play a significant role in
cosmopolitanism.