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Prognathodon (Mosasauridae) had shark-like hypocercal tail fin

From: Ben Creisler

A new paper in Nature Communications:

Johan Lindgren, Hani F. Kaddumi & Michael J. Polcyn (2013)
Soft tissue preservation in a fossil marine lizard with a bilobed tail fin.
Nature Communications 4, Article number: 2423

Mosasaurs are secondarily aquatic squamates that became the dominant
marine reptiles in the Late Cretaceous about 98–66 million years ago.
Although early members of the group possessed body shapes similar to
extant monitor lizards, derived forms have traditionally been
portrayed as long, sleek animals with broadened, yet ultimately
tapering tails. Here we report an extraordinary mosasaur fossil from
the Maastrichtian of Harrana in central Jordan, which preserves soft
tissues, including high fidelity outlines of a caudal fluke and
flippers. This specimen provides the first indisputable evidence that
derived mosasaurs were propelled by hypocercal tail fins, a hypothesis
that was previously based on comparative skeletal anatomy alone.
Ecomorphological comparisons suggest that derived mosasaurs were
similar to pelagic sharks in terms of swimming performance, a finding
that significantly expands our understanding of the level of aquatic
adaptation achieved by these seagoing lizards.

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