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[dinosaur] Mosasauroid phylogeny (free pdf) + mosasaur fossils in concretions

Ben Creisler

New in PLoS ONE:

Free pdf:

Tiago R. Simões, Oksana Vernygora, Ilaria Paparella, Paulina Jimenez-Huidobro & Michael W. Caldwell (2017 ) 
Mosasauroid phylogeny under multiple phylogenetic methods provides new insights on the evolution of aquatic adaptations in the group. 
PLoS ONE 12(5): e0176773.
doi:  https://doi.org/10.1371/journal.pone.0176773

Mosasauroids were a successful lineage of squamate reptiles (lizards and snakes) that radiated during the Late Cretaceous (95–66 million years ago). They can be considered one of the few lineages in the evolutionary history of tetrapods to have acquired a fully aquatic lifestyle, similarly to whales, ichthyosaurs and plesiosaurs. Despite a long history of research on this group, their phylogenetic relationships have only been tested so far using traditional (unweighted) maximum parsimony. However, hypotheses of mosasauroid relationships and the recently proposed multiple origins of aquatically adapted pelvic and pedal features in this group can be more thoroughly tested by methods that take into account variation in branch lengths and evolutionary rates. In this study, we present the first mosasauroid phylogenetic analysis performed under different analytical methods, including maximum likelihood, Bayesian inference, and implied weighting maximum parsimony. The results indicate a lack of congruence in the topological position of halisaurines and Dallasaurus. Additionally, the genus Prognathodon is paraphyletic under all hypotheses. Interestingly, a number of traditional mosasauroid clades become weakly supported, or unresolved, under Bayesian analyses. The reduced resolutions in some consensus trees create ambiguities concerning the evolution of fully aquatic pelvic/pedal conditions under many analyses. However, when enough resolution was obtained, reversals of the pelvic/pedal conditions were favoured by parsimony and likelihood ancestral state reconstructions instead of independent origins of aquatic features in mosasauroids. It is concluded that most of the observed discrepancies among the results can be associated with different analytical procedures, but also due to limited postcranial data on halisaurines, yaguarasaurines and Dallasaurus.



Alysia S. Korn and Darrin C. Pagnac (2017)
Anatomically linked occurrence of concretionary preservation of mosasaurs from the Pierre Shale of South Dakota.
PALAIOS 32(5):330-336. 2017 
doi: http://dx.doi.org/10.2110/palo.2016.090

Fossils of mosasaurs, late Cretaceous marine reptiles, are frequently found partially preserved in concretions (i.e., hard mineral masses), but little is known about this mode of fossilization. Concretionary fossils are difficult to extract, so are often ignored in the field and laboratory. Accounts suggest that skulls, the most taxonomically diagnostic element, are most often preserved in concretions. The presence of concretions was documented in four anatomical regions (head, torso, tail, limbs) in 48 mosasaur fossils from the Pierre Shale of South Dakota. Specimens were compiled from a restricted lithostratigraphic interval to minimize the effects of lithologic variation on concretion formation. In this sample, the skull and torso were preserved more often in concretions than were limbs and tails. The occurrence of concretion formation was found to vary depending on anatomical region. The results suggest that concretion growth begins in the core of the organism with a higher volume of organic matter, and successively precipitates towards the periphery. The skull and torso contain many taxonomically diagnostic features; thus concretionary specimens should not be overlooked. Additionally, increased likelihood of preservation of specific anatomic regions was dependent on concretion formation, suggesting that our view of the fossil record is affected by concretions.