Date of Award


Degree Name

Biological Sciences


College of Science

Type of Degree


Document Type


First Advisor

Dr. F. Robin O’Keefe, Committee Chairperson

Second Advisor

Dr. Habiba Chirchir, Committee Member

Third Advisor

Dr. Herman Mays, Committee Member


Growing evidence indicates that elasmosaurid plesiosaurs from the Late Cretaceous Western Interior Seaway are members of a single clade, the Styxosaurinae. The styxosaurines are reported to be mostly Campanian in age, and taxa within the clade obtain the longest necks, by number of cervical vertebrae, of any known vertebrate. The styxosaurines are morphologically diverse and include taxa that exhibit a secondary reduction in neck length. Given the evolutionary plasticity of postcranial characters in plesiosaurs in general, and neck length in elasmosaurs, scrutiny of cranial osteology is pertinent to advancing understanding of Western Interior Seaway elasmosaurids. This study finds that an elasmosaurid specimen (UNSM 50132) from the Cenomanian of Nebraska is remarkably similar in cranial morphology to the Campanian Styxosaurus snowii (KUVP 1301). The phylogenetic affinity of UNSM 50132 was tested with a cladistic analysis with 94 Operational Taxonomic Units (OTU) and 270 anatomical characters, utilizing the Serratos et al. (2017) character matrix with changes and additions. The analysis supports five unambiguous synapomorphies for the genus Styxosaurus: (1) dorsomedian ridge of premaxilla located posteriorly (19.1); (2) dorsal portion of squamosal reflected anteriorly in lateral view (61.1); (3) posteromedian ridge on the supraoccipital (77.1); (4) a sharp ridge or keel located adjacent to the mandibular symphysis (114.1); (5) a retroarticular process that is shorter in anteroposterior length than the glenoid (116.0). Five additional ambiguous synapomorphies that support the monophyly of Styxosaurus include: lateral expansion of the maxilla that supports caniniform teeth, anisodont dentition, anterior embayment of the squamosal arch, an elongate posteromedian process of the premaxilla, a rugose boss on the ectopterygoid, parietals that form a sagittal crest that rises above the cranial roof, and elongate anterior to middle cervical centra. 67% of 100 bootstrap replicates support the monophyly of UNSM 50132, Styxosaurus snowii, Styxosaurus browni, and Styxosaurus sp. (SDSM 451). UNSM 50132 was previously referred to the genus Thalassomedon, a taxon considered to be outside of the Styxosaurinae. The recommended referral of UNSM 50132 to the genus Styxosaurus pushes back the earliest occurrence of Styxosaurinae in the Western Interior Seaway by over ten million years. Maximum parsimony analysis suggests that all Western Interior Seaway elasmosaurids belong to a single clade, including the genera Libonectes and Thalassomedon. Libonectes and Thalassomedon have been previously recovered as outgroup taxa to a clade composed of the sister relationship of Western Interior Seaway elasmosaurids and Aristonectinae. This study provides additional context for furthering understanding of the origins of Elasmosauridae in the Early Cretaceous.