<p id="d1895203e258">There has been long-standing debate about the relative roles
of intrinsic biotic interactions
vs. extrinsic environmental factors as drivers of biodiversity change. Here, we show
that, relatively early in the history of complex life, Milankovitch “grand cycles”
associated with astronomical rhythms explain between 9 and 16% of variation in species
turnover probability (extinction probability plus speciation probability) in a major
Early Paleozoic zooplankton group, the graptoloids. These grand cycles would have
modulated climate variability, alternating times of relative stability in the environment
with times of maximum volatility, which influenced oceanic circulation and structure
and thus, phytoplankton populations at the base of the marine food web.
</p><p class="first" id="d1895203e261">Periodic fluctuations in past biodiversity,
speciation, and extinction have been proposed,
with extremely long periods ranging from 26 to 62 million years, although forcing
mechanisms remain speculative. In contrast, well-understood periodic Milankovitch
climate forcing represents a viable driver for macroevolutionary fluctuations, although
little evidence for such fluctuation exists except during the Late Cenozoic. The reality,
magnitude, and drivers of periodic fluctuations in macroevolutionary rates are of
interest given long-standing debate surrounding the relative roles of intrinsic biotic
interactions vs. extrinsic environmental factors as drivers of biodiversity change.
Here, we show that, over a time span of 60 million years, between 9 and 16% of the
variance in biological turnover (i.e., speciation probability plus species extinction
probability) in a major Early Paleozoic zooplankton group, the graptoloids, can be
explained by long-period astronomical cycles (Milankovitch “grand cycles”) associated
with Earth’s orbital eccentricity (2.6 million years) and obliquity (1.3 million years).
These grand cycles modulate climate variability, alternating times of relative stability
in the environment with times of maximum volatility. We infer that these cycles influenced
graptolite speciation and extinction through climate-driven changes to oceanic circulation
and structure. Our results confirm the existence of Milankovitch grand cycles in the
Early Paleozoic Era and show that known processes related to the mechanics of the
Solar System were shaping marine macroevolutionary rates comparatively early in the
history of complex life. We present an application of hidden Markov models to macroevolutionary
time series and protocols for the evaluation of statistical significance in spectral
analysis.
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