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Zoneait, new metriorhynchoid (Thalattosuchia) from Middle Jurassic of Oregon



Ben Creisler
bcreisler@gmail.com

A new online paper:

Eric W. Wilberg (2015)
A new metriorhynchoid (Crocodylomorpha, Thalattosuchia) from the
Middle Jurassic of Oregon and the evolutionary timing of marine
adaptations in thalattosuchian crocodylomorphs.
Journal of Vertebrate Paleontology (advance online publication)
DOI:10.1080/02724634.2014.902846
http://www.tandfonline.com/doi/full/10.1080/02724634.2014.902846#abstract


Metriorhynchid thalattosuchians represent the most extreme
archosaurian adaptation to the marine realm. Metriorhynchids possess
aquatic adaptations throughout the skeleton. These adaptations were so
extensive that some have suggested that they lost the ability to move
on land, yet their evolutionary timing remains unresolved. The closest
relatives of the metriorhynchoids, the teleosauroids, lack these
aquatic adaptations, and the earliest metriorhynchoids are known
exclusively from cranial material. Here I describe a partial skull
with associated forelimb elements of a new marine crocodylomorph,
Zoneait nargorum, gen. et sp. nov., of Aalenian–Bajocian age from the
Snowshoe Formation of east-central Oregon. Phylogenetic analysis
identifies Zoneait as the sister taxon to Metriorhynchidae. It
possesses a derived skull with orbits that are more laterally directed
and prefrontals that are more expanded than in other basal
metriorhynchoids. The preserved forelimb elements are less derived.
The humerus is elongate in comparison with that of other
metriorhynchoids. The ulna is slightly reduced in length and flattened
but resembles the teleosauroid condition more so than the plate-like
element of metriorhynchids. This suggests that marine adaptations in
metriorhynchoids were acquired in mosaic fashion, with modifications
of the skull preceding forelimb reduction, with this forelimb
reduction occurring first in the zeugopodial elements, prior to
reduction of the humerus. This evolutionary timing has important
implications for the transition from nearshore ambush predation to
pelagic open-marine predation in Thalattosuchia, suggesting that
adaptations related to prey detection and capture preceded the
locomotor adaptations that allowed these organisms to fully invade the
oceans.