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Marine tetrapod evolution from Triassic to Anthropocene

Ben Creisler

A new paper:

Neil P. Kelley and Nicholas D. Pyenson (2015)
Evolutionary innovation and ecology in marine tetrapods from the
Triassic to the Anthropocene.
Science  348(6232):  aaa3716
DOI: 10.1126/science.aaa3716

More than 30 different lineages of amphibians, reptiles, birds, and
mammals have independently invaded oceans ecosystems. Prominent
examples include ichthyosaurs and mosasaurs during the Mesozoic (252
to 66 million years ago) and penguins and sea otters during the
Cenozoic (66 million years ago to the present). In today’s oceans,
marine tetrapods are ecologically important consumers with trophic
influence disproportionate to their abundance. They have occupied apex
roles in ocean food webs for more than 250 million years, through
major changes in ocean and climate, and through mass extinctions.
Major paleontological discoveries in the past 40 years have clarified
the early land-sea transitions for some marine tetrapods (e.g.,
whales, sea cows), although the terrestrial origins of many lineages
remain obscure. Incipient invasions appear frequently in marine
tetrapod history, but such early transitions account for only a small
proportion of the total fossil record of successful marine lineages,
which in some cases persist for hundreds of millions of years.

Marine tetrapods provide ideal models for testing macroevolutionary
hypotheses because the repeated transitions between land and sea have
driven innovation, convergence, and diversification against a backdrop
of changing marine ecosystems and mass extinctions. Recent
investigations across a broad range of scales—from molecules to food
webs—have clarified the phylogenetic scope, timing, and ecological
consequences of these repeated innovations. Studies of the physiology
and functional morphology of living species have illuminated the
constraints and tradeoffs that shape the pathway of initial marine
invasions. Comparative studies on muscle myoglobin concentration or
the evolution of sex determination mechanisms, for example, have
revealed rampant convergence for these adaptive traits in the marine
realm. Exceptionally preserved fossils have also revealed insights
into reproductive biology, soft tissue structures, and trophic
interactions. Fossils provide critical baselines for understanding
historical changes in marine communities and diversity through time,
and these baselines remain vital for evaluating the ongoing and severe
anthropogenic disturbance to marine tetrapod populations and marine
ecosystems as a whole.

Technological advances in remote sensing and biologging will continue
to provide crucial insights into the macroecology of marine tetrapods
below the water’s edge. Field data, when combined with extensive
vouchers represented in museum collections, provide the basis for
integrative models of the function and ecology of these logistically
challenging organisms. Placed in a phylogenetic comparative framework,
these data can enable tests of hypotheses about macroevolutionary
patterns. Although perpetually incomplete, new fossil discoveries
continue to improve our understanding of the early land-sea
transitions in lineages and reveal past ecologies that could not have
otherwise been predicted. Emerging imaging, molecular, and isotopic
techniques provide an opportunity to expand the investigational scope
for studying extinct taxa and to inform our understanding of how
living species evolved. Lastly, resolving the full evolutionary scope
of marine tetrapod history provides context for the origins of modern
ecological patterns and interactions, which are fundamentally being
altered by human activities.