Tuesday, April 18, 2017

[Paleontology • 2017] Nakonanectes bradti • A New Elasmosaurid (Sauropterygia, Plesiosauria) from the Bearpaw Shale (Late Cretaceous, Maastrichtian) of Montana Demonstrates Multiple Evolutionary Reductions of Neck Length within Elasmosauridae


Nakonanectes bradti 
Serratos, Druckenmiller, & Benson, 2017 


ABSTRACT
Plesiosauria is a diverse clade of marine reptiles that have been studied since the early 19th century. However, phylogenetic relationships within the group have been contentious due to limited taxon sampling and a misunderstanding of how ontogeny and interspecific and intraspecific variation affect character states. This is particularly true for elasmosaurids, a Cretaceous clade of long-necked plesiosaurians. In 2010, a new, nearly complete skeleton, MOR 3072, was collected from the Late Cretaceous (Campanian–Maastrichtian) Bearpaw Shale of northeast Montana. The specimen provides detailed morphological information rarely observed within Elasmosauridae, including a complete skull, the anterior 23 cervical vertebrae, a partial dorsal and caudal vertebral column, incomplete pectoral and pelvic girdles, elements of both fore- and hind limbs, ribs, and gastralia. Being early Maastrichtian in age, MOR 3072 is the stratigraphically youngest elasmosaurid known from the Western Interior Seaway and is recognized as a new genus and speciesNakonanectes bradti. We present a description of Nakonanectes bradti and conduct an extended phylogenetic analysis of Elasmosauridae. N. bradti is found to be deeply nested within the clade of large-bodied, long-necked, Western Interior Styxosaurinae. However, MOR 3072 is one of the smallest adult elasmosaurids ever recovered (5.1–5.6 m) and exhibits a reduced neck length compared with other North American elasmosaurids, resulting from a reduction in both centrum length and number of cervical vertebrae (39–42 were originally present). These features are convergent with the Southern Hemisphere clade of short-necked Maastrichtian elasmosaurids, Aristonectinae, and demonstrate multiple origins of short-necked body proportions from longer-necked ancestors within Elasmosauridae.

The newly named short-necked elasmosaur, Nakonanectes bradti, swims through an ancient sea in this artist’s reconstruction.
Illustration by James Havens, @alaskapaleoproject

 

Laser Scan of Cranium of MOR 3072, holotype specimen of Nakonanectes bradti, gen. et sp. nov.   

SYSTEMATIC PALEONTOLOGY
DIAPSIDA Osborn, 1903
SAUROPTERYGIA Owen, 1860

PLESIOSAURIA de Blainville, 1835
XENOPSARIA Benson and Druckenmiller, 2014

ELASMOSAURIDAE Cope, 1869
STYXOSAURINAE Otero, 2016

Phylogenetic Definition:— The clade including all taxa more closely related to Styxosaurus snowii than to Aristonectes parvidens (revised from Otero, 2016).


NAKONANECTES, gen. nov.
Type and Only Species:— Nakonanectes bradti.
Horizon:— Bearpaw Shale, lower Maastrichtian, Upper Cretaceous.


NAKONANECTES BRADTI, gen. et sp. nov. 
Holotype and Only Specimen:— MOR 3072, including a complete skull, articulated anterior 23 cervical vertebrae, dorsal and anterior caudal vertebrae, partial coracoids, much of the forelimbs including partial humeri, complete epipodial and mesopodial rows and associated phalanges, both ilia and portions of the pubis and ischium, portions of the hind limb including one complete and one partial femur with portions of the epipodial row, and numerous ribs and gastralia.

Etymology:— The generic name is derived from Nakona, the name for native Assiniboine people of northeastern Montana, which means ‘the friendly people.’ The specific name honors David Bradt of Florence, Montana, who found the specimen.

Nakonanectes bradti Serratos, Druckenmiller, & Benson, 2017



Serratos, D. J., P. Druckenmiller, and R. B. J. Benson. 2017. A New Elasmosaurid (Sauropterygia, Plesiosauria) from the Bearpaw Shale (Late Cretaceous, Maastrichtian) of Montana Demonstrates Multiple Evolutionary Reductions of Neck Length within Elasmosauridae. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2017.1278608.

Alaska scientists help name new elasmosaur species from Montana - UAF News and Information http://news.uaf.edu/alaska-scientists-help-name-new-elasmosaur-species-from-montana/


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