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A dyrosaurid, a marine crocodilian, swimming in the warm
surface waters during the end of the Cretaceous period. Illustration
credit: Guillaume Suan.
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The
ancestors of today's crocodiles colonized the seas during warm phases and
became extinct during cold phases, according to a new Anglo-French study which
establishes a link between marine crocodilian diversity and the evolution of
sea temperature over a period of more than 140 million years.
The
research, led by Dr Jeremy Martin from the Université de Lyon, France and
formerly from the University of Bristol, UK is published this week in Nature
Communications.
Today,
crocodiles are 'cold-blooded' animals that mainly live in fresh waters but two
notable exceptions, Crocodylus porosus
and Crocodylus acutus venture
occasionally into the sea. Crocodiles occur in tropical climates, and they are
frequently used as markers of warm conditions when they are found as fossils.
While
only 23 species of crocodiles exist today, there were hundreds of species in
the past. On four occasions in the past 200 million years, major crocodile
groups entered the seas, and then became extinct. It is a mystery why they made
these moves, and equally why they all eventually went extinct. This new study
suggests that crocodiles repeatedly colonized the oceans at times of global
warming.
Lead
author of the report, Dr Jeremy Martin said: "We thought each of these
evolutionary events might have had a different cause. However, there seems to
be a common pattern."
Dr
Martin, with a team of paleontologists and geochemists from the Université de
Lyon and the University of Bristol, compared the evolution of the number of
marine crocodilian fossil species to the sea temperature curve during the past
200 million years. This temperature curve, established using an isotopic
thermometer, is widely applied for reconstruction of past environmental
conditions and in this case, is based on the isotopic composition of the oxygen
contained in the fossilized remains of fossil marine fish (bone, teeth,
scales).
Co-author,
Christophe Lécuyer explained: "According to this method, it is possible to
calculate the temperature of the water in which these fish lived by applying an
equation linking the isotopic composition of the fossilized remains to the
temperature of mineralization of their skeleton. The seawater temperatures
derived from the composition of fish skeleton thus corresponds to the
temperature of water in which the marine crocodiles also lived."
The
results show that colonization of the marine environment about 180 million
years ago was accompanied by a period of global warming of the oceans. These
first marine crocodilians became extinct about 25 million years later, during a
period of global freezing. Then, another crocodilian lineage appeared and
colonised the marine environment during another period of global warming.
The
evolution of marine crocodilians is therefore closely tied to the temperature
of their medium, and shows that their evolution and their lifestyle, as in
modern crocodilians, are constrained by environmental temperatures.
Nevertheless,
one fossil lineage does not appear to follow this trend. Jurassic
metriorhynchoids did not go extinct during the cold spells of the early
Cretaceous, unlike the teleosaurids, another group of marine crocodilians.
Quite surprisingly, metriorhynchoids only disappeared a few million years
later. This exception will certainly provide grounds for new research,
particularly into how the biology of this group adapted to life in the pelagic
environment.
Professor
Michael Benton from the University of Bristol, another co-author of the study,
said: "This work illustrates a case of the impact of climate change on the
evolution of animal biodiversity, and shows that for crocodilians, warming
phases of our earth's history constitute ideal opportunities to colonise new
environments.
Citation
Jeremy
E. Martin, Romain Amiot, Christophe Lécuyer, Michael J. Benton. 2014. Sea
surface temperature contributes to marine crocodylomorph evolution. Nature
Communications, 5 DOI: 10.1038/ncomms5658.