A new elasmosaurid (Sauropterygia: plesiosauria) from the Bearpaw Formation (late Cretaceous, Maastrichtian) of Montana and the evolution of neck length in elasmosauridae

Thesis (M.S.) University of Alaska Fairbanks, 2015Plesiosauria 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, interspecific and intraspecific variation affect character states. This is particularly true for elasmosaurids, a clade of long-necked plesiosaurians known from the Cretaceous. In 2010, a new, nearly complete skeleton, MOR 3072, was collected from the Late Cretaceous (Campanian-Maastrichtian) Bearpaw Shale of northeast Montana, and it provides morphological information rarely observed within Elasmosauridae. MOR 3072 consists of 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 hindlimbs, ribs, and gastralia. Here, I present a detailed description of the specimen and conduct the most complete phylogenetic analysis of Elasmosauridae to date. A new taxon is recognized on the basis of the following suite of autapomorphies and unique character combinations: a chordate bilobed external naris, a squared-off posteroventral margin of maxilla, the presence of a maxilla-squamosal contact, a deep anteroposterior-oriented cleft in the articular posterior to the glenoid, a reduced number of cervical vertebrae, proximal caudal vertebrae that are wider than dorsoventrally tall, and small facets for forelimb and hindlimb preaxial accessory ossicles. A phylogenetic analysis places MOR 3072 as the sister taxon to the long-necked, Western Interior elasmosaurids Hydralmosaurus serpentinus + Styxosaurus snowii. Being early Maastrichtian in age, MOR 3072 is the stratigraphically youngest elasmosaurid yet known from the Western Interior Seaway. It is also one of the smallest adult elasmosaurids ever recovered (4.5-5 m) and exhibits a reduced neck length due to a reduction in both the number of cervical vertebrae and centrum length, which is convergent with another clade of Maastrichtian elasmosaurids, Aristonectinae

A new elasmosaurid (Sauropterygia, Plesiosauria) from the Bearpaw Shale (Late Cretaceous, Maastrichtian) of Montana demonstrates multiple evolutionary reductions of neck length within Elasmosauridae

<p>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 species, <i>Nakonanectes bradti</i>. We present a description of <i>Nakonanectes bradti</i> and conduct an extended phylogenetic analysis of Elasmosauridae. <i>N. bradti</i> 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.</p> <p>http://zoobank.org/urn:lsid:zoobank.org:pub:85731F4A-EA70-4C9A-B595-792EDBC096C0</p> <p>SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at <a href="http://www.tandfonline.com/UJVP" target="_blank">www.tandfonline.com/UJVP</a></p> <p>Citation for this article: 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.</p