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New plesiosaur records from Lower Cretaceous of Neuquén Basin, Argentina

Ben Creisler

A new paper:

José P. O'Gorman, Dario G. Lazo, Leticia Luci, Cecilia S. Cataldo,
Ernesto Schwarz, Marina Lescano &  María Beatriz Aguirre-Urreta (2015)
New plesiosaur records from the Lower Cretaceous of the Neuquén Basin,
west-central Argentina, with an updated picture of occurrences and
facies relationships.
Cretaceous Research 56: 372–387

In this work new records of elasmosaurid plesiosaurs are described
from the Lower Cretaceous Agrio Formation of the Neuquén Basin
(west-central Argentina) and their bearing facies and taphonomic modes
are analyzed. New records from both members of the upper
Valanginian–Hauterivian Agrio Formation are described along with a
careful revision of previous records from the Pilmatué Member. These
records include the first elasmosaurids reported from the upper
Hauterivian of South America. Even though the studied materials
comprise solely fragmentary postcranial elements, they suggest the
presence of at least two different elasmosaurid taxa. Additionally,
the rarity of cervical centra with a ventral notch in the articular
faces among pre-Cenomanian elasmosaurids is confirmed. In terms of
plesiosaur taphonomy and bearingfacies the studied plesiosaurs from
the Agrio Formation were recorded in two different taphonomic modes,
1) preservation of disarticulated but closely associated postcranial
elements within dark-grey shales, that probably resulted from
bloating, drifting and eventually sinking of carcasses to a muddy and
calm seafloor under oxic to suboxic conditions, without further
transport or physical reworking; and 2) preservation of isolated
plesiosaur bones in shell beds, including mostly weathered and
encrusted isolated vertebrae. These taphonomic modes are not randomly
distributed in the studied marine succession, but closely follow
cycles of changes in the relative sea level and can be linked to
fourth-order high frequency cycles. Taphonomic mode 1 includes better
preserved specimens deposited in highstand systems tract intervals
subjected to higher sedimentation rate while taphonomic mode 2 is
related to transgressive systems tract intervals subjected to a
reduced sedimentation rate and higher time of exposure of remains on
the seafloor, resulting in poorly preserved specimens.