|R. B. J. Benson and P. S. Druckenmiller 2014||Cryptoclidian plesiosauroids with the following unique unambiguous synapomorphies: suborbital margin of temporal bar smoothly curved, not marked by a squared-off posterior termination of the maxilla (12.0; Figs 4 and 5), posteromedian process of premaxilla constricted by external naris (22.2; reversed in polycotylids and some leptocleidids, in which the premaxilla does not contact the external naris; although the dorsomedian processes of the premaxillae become narrower level with the anterior margin of the external naris in many other plesiosauroids, in xenopsarians they expand mediolaterally again posterior to the external naris), raised midline ridge present in region of pineal foramen (51.2; Fig. 4; the ridge is interrupted by the pineal foramen, when that is present), temporal bar not embayed ventrally and does not arch dorsally posterior to the postorbital bar (55.1; Figs 4 and 5), notochordal pit on basioccipital condyle, when present, located comfortably within the dorsal one-quarter of condyle (64.1; Hiller et al., 2005, fig. 6), foramen penetrating prearticular/splenialangular contact absent (119.1).
Additionally, a number of non-unique, unambiguous synapomorphies are also present: a long temporal fenestra, ratio of orbit length to temporal fenestra length 0.3–0.7 (3.0), premaxilla with dorsomedian ridge, crest, or eminence (17.1; Fig. 4; also present in Rhomaleosauridae and Tricleidus seeleyi), parietal sagittal crest forms a transversely compressed sheet (50.2), dorsal ramus of squamosal inflected abruptly anterodorsally seen in lateral view (61.1; Fig. 4), occipital condyle separated from basioccipital body and exoccipital facets by a completely encircling groove around its base (65.0), ventral process of basioccipital forming prominent, ventrally projecting ‘plate’ (66.1; also present in Hauffiosaurus and some microcleidids), three or more foramina in lateral surface of exoccipital body (69.2;also present in Kimmerosaurus langhami; reduced to two foramina in Brancasaurus brancai and one in Leptocleidus superstes;Wegner, 1914; Kear&Barrett, 2011), supraoccipital wider anteroposteriorly than tall dorsoventrally (76.0; also present in Kronosaurus queenslandicus; White, 1935), notch in posterior surface of basisphenoid body absent (81.0; also present in Picrocleidus beloclis), parasphenoid extends far posteriorly to underlap basioccipital ventrally, concealing basioccipital-basisphenoid contact in ventral view (84.2; also present in Picrocleidus beloclis, some rhomaleosaurids, and some pliosaurids), pterygoids make anteroposteriorly long contact posterior to posterior interpterygoid vacuity (99.2; also present in some microcleidids, rhomaleosaurids and pliosaurids), ventral surface of pterygoid lateral to posterior interpterygoid vacuity relatively broad mediolaterally and slightly concave or ‘dished’ (100.2; also present in some rhomaleosaurids), posterolateral portion of pterygoid forms squared lappet ventral to quadrate process (102.1; also present in some rhomaleosaurids; only weakly developed in Dolichorhynchops spp.), ventral margin of pterygoid/ectopterygoid anterior to subtemporal fenestra forms ventrally deflected flange homologous with but not identical to, an ectopterygoid/pterygoid boss (109.1), anterior cervical neural spines curve posterodorsally (157.0; a reversal to the primitive condition for Plesiosauria from the derived condition of anterior cervical neural spines inclined straight posterodorsally in Eretmosaurus rugosus and more derived plesiosauroids; the condition is transformed in some elasmosaurids and polycotylids).
Several characters are present uniquely or primarily in some xenopsarians, but have a homoplastic distribution and are not optimised as unambiguous synapomorphies of the clade. These include the presence of only a single, median subcentral foramen on the ventral surface of each caudal vertebra (character 191.1; e.g. Cope, 1869; Ketchum, 2011; reversed in polycotylids and some elasmosaurids), and a ‘sigmoidal’ morphology of the humerus (character 249.2; e.g. Wegner, 1914; Sato et al., 2006; O’Keefe, 2008; Ketchum, 2011; lost in derived elasmosaurids and only weakly developed in Leptocleidus superstes; Kear & Barrett, 2011). The presence of large numbers of gastroliths (‘stomach stones’) may also be characteristic of Xenopsaria, although it was not used as a character herein, and is generally absent in polycotylids. Small numbers of gastroliths (1–20) are occasionally found in association with Jurassic rhomaleosaurids and pliosaurids (Taylor, 1993; O’Keefe et al., 2009). However, greater numbers, forming a substantial gastric mass, are known in some leptocleidids [Umoonasaurus (Kear, Schroeder-Adams & Lee, 2006: see supplementary figures); >120 in Nichollssaura (Druckenmiller & Russell, 2008b)] and many elasmosaurids [˜200 in several specimens (Schmeisser & Gilette, 2009, table 4; Williston, 1893, 1904; Brown, 1904; Riggs, 1939; Welles&Bump, 1949; Everhart, 2000; Sato et al., 2006; Kubo et al., 2012)], including one specimen with more that 2000 gastroliths (Thompson, Martin & Reguero, 2007). However, most polycotylids, even those known from near-complete, articulated specimens lack gastroliths [reviewed by Taylor (1993) and Schmeisser & Gilette (2009)], with one exception being MNA V10046, the holotype of Dolichorhynchops tropicensis from the Cenomanian-Turonian of Utah (Schmeisser & Gilette, 2009; Schmeisser-McKean, 2012).