Background. Xinjiangchelyidae are a group of basal cryptodires from the Middle Jurassic – Early Cretaceous of Asia and Early Cretaceous of Europe. Recent studies show that xinjiangchelyids are rather uniform and conservative in their shell morphology and comparatively more variable in skull shape. Morphology and variability of cervical vertebra(e) (CV) of xinjiangchelyids remain poorly known. More or less complete series of CV were described for Xinjiangchelys qiguensis and X. tianshanensis, whereas for other xinjiangchelyids they are alternately unknown or often incomplete, deformed and need additional preparation. Methods. We examined several hundred isolated CV of a xinjiangchelyid Annemys sp. from the Middle Jurassic (Bathonian) Itat Formation of Berezovsk Quarry, Krasnoyarsk Territory, West Siberia, Russia. Some of these CV are well preserved and three-dimensional and allow reconstruction of the complete series of CV of Annemys. Results. The formula of CV is (2(3()4()5()6()7()8), although C8 is rarely amphicoelous. Previously, xinjiangchelyids were thought to have only amphicoelous CV. All CV are relatively short (similar to other forms of Annemys, shorter than in X. tianshanensis and much shorter than in X. qiguensis). The anterior and posterior central articulations are approximately on one level. The anterior central articulations form a wide triangle (C2), equilateral triangle (C3, C4), or wide oval (C5–C8); the posterior central articulations form a narrow oval (C2–C5, C8), roundish (C6) or wide oval (C7). The ventral keel is low on the anterior CV and high on C7 and C8, a condition similar to some sinemydids/macrobaenids and more advanced cryptodires. The parapophyses are located at the posteroventral edge of the centrum in C2–C5, and at the anteroventral edge of the centrum in C6–C8. The articular surfaces of the prezygapophyses face dorsolateral in C2 or dorsomedial in more posterior CV, forming an angle of about 20° from the horizontal in C3 and C4 and about 30–35° in C5–C8. The postzygapophyses are widely separated, unlike X. qiguensis and similar to other forms of Annemys and X. tianshanensis. Isolated cervical ribs are present in the material, but their associations with CV are unclear. Discussion. Our study shows that xinjiangchelyids were more variable in the morphology of their CV than thought previously. They demonstrate three neck morphotypes: the short-necked (Annemys), medium-necked (X. tianshanensis) and long-necked (X. qiguensis). In addition to length of CV, these morphotypes differ in cervical formula, shapes of central articulations, and patterns of cervical rib attachment. Morphology of the CV of Annemys suggests the possibility of both vertical and lateral movements of the neck that agrees with recent studies on neck mobility of primitive turtles.
A method for deducing neck mobility in plesiosaurs, using the exceptionally preserved
Nichollssaura borealis
