Revised Vertebral Count in the “Longest-Necked Vertebrate” Elasmosaurus platyurus Cope 1868, and Clarification of the Cervical-Dorsal Transition in Plesiosauria

Abstract

Elasmosaurid plesiosaurians are renowned for their immensely long necks, and indeed, possessed the highest number of cervical vertebrae for any known vertebrate. Historically, the largest count has been attributed to the iconic Elasmosaurus platyurus from the Late Cretaceous of Kansas, but estimates for the total neck series in this taxon have varied between published reports. Accurately determining the number of vertebral centra vis-à-vis the maximum length of the neck in plesiosaurians has significant implications for phylogenetic character designations, as well as the inconsistent terminology applied to some osteological structures. With these issues in mind, we reassessed the holotype of E. platyurus as a model for standardizing the debated cervical-dorsal transition in plesiosaurians, and during this procedure, documented a “lost” cervical centrum. Our revision also advocates retention of the term “pectorals” to describe the usually three or more distinctive vertebrae close to the cranial margin of the forelimb girdle that bear a functional rib facet transected by the neurocentral suture, and thus conjointly formed by both the parapophysis on the centrum body and diapophysis from the neural arch (irrespective of rib length). This morphology is unambiguously distinguishable from standard cervicals, in which the functional rib facet is borne exclusively on the centrum, and dorsals in which the rib articulation is situated above the neurocentral suture and functionally borne only by the transverse process of the neural arch. Given these easily distinguishable definitions, the maximum number of neck vertebrae preserved in E. platyurus is 72; this is only three vertebrae shorter than the recently described Albertonectes, which together with E. platyurus constitute the “longest necked” animals ever to have lived.

Reappraisal of Europe’s most complete Early Cretaceous plesiosaurian:Brancasaurus brancaiWegner, 1914 from the “Wealden facies” of Germany

“…16A) conform to the definition of plesiosaurian pectorals proposed by Sachs, Kear & Everhart (2013). They otherwise resemble the terminal cervicals in being shallowly concave and shorter than high with a deep central notochordal pit and rounded articular surface rims.…”

“…There are 35 cervical vertebrae ( sensu Sachs, Kear & Everhart, 2013) in addition to the atlas-axis complex (Fig. 15).…”

“…1A), which corresponds to the “lip” on the cranial articular surface described by Benson et al (2013a: 246) in Hastanectes valdensis . Irrespectively, a prominent central notochordal pit and rounded articular edges are present throughout (but more distinct cranially), and there is also no evidence of ventral notching otherwise indicative of elasmosaurids (see discussion in Sachs, Kear & Everhart, 2013; Sachs & Kear, 2015a; Sachs & Kear, 2015b). Wegner (1914: 257) reported that the notochordal pit in the third–eighth cervical (which is not observable due to the display armature) is 1–3 mm in diameter and deeply inset to about a quarter of the centrum length.…”

“…However, noticeable differences include the proportionately shorter ischia (Hampe, 2013 measured this relative to the pubis, which is ∼20% longer in B. brancai : ischium/pubis length = 152/167 versus 175/153 in G. wegneri ) and epipodials that are wider than long (see Wegner, 1914: 286); although the radius and fibula of G. wegneri are damaged and the ulna is equidimensional (see Hampe, 2013: 488). Differences are also evident in the dorsal/sacral vertebral counts (19/3 in B. brancai , 17/4 in G. wegneri , note that Hampe’s (2013) fourth pectoral is in fact the first dorsal according to the definition of Sachs, Kear & Everhart, 2013). However, Brown (1981) and Großmann (2007, Table 2) demonstrated that the number of vertebrae can vary within one plesiosaurian species (e.g., Seeleyosaurus guilelmiimperatoris shows 16–19 dorsals and 2–3 sacrals or Microcleidus ( Hydrorion ) brachypterygius 14–20 dorsals and 2–3 sacrals).…”

“…Sato (2002) also emphasized that osteological immaturity potentially masked the phylogenetic signal of the holotype (GPMM A3.B4). We concur with both these denouements, and underscore that the ontogenetic changes observed in GPMM A3.B4 versus the ‘adult’ G. wegneri GPMM A3.B2 have a particularly significant impact on topological arrangements (see Phylogenetic Definition herein)—especially relative fusion of the basicranial components, and prominence of the basioccipital tubera, notochordal pit and condylar groove (see Brown, 1981; Maisch, 1998; Kear, 2007); vertebral count, centrum proportions, articular facet concavity and neural spine morphology (which vary widely along the column: Brown, 1981; O’Keefe & Hiller, 2006; Sachs, Kear & Everhart, 2013; Sachs & Kear, 2015a); and shape of the girdle elements together with ossification of the pectoral/pelvic bar and proportions of the propodials (Brown, 1981; Carpenter, 1999; Kear, 2007). We therefore recommend that the taxonomic affinities of B. brancai (Figs.…”