An articulated cervical series of <i>Alamosaurus sanjuanensis</i> Gilmore, 1922 (Dinosauria, Sauropoda) from Texas: new perspective on the relationships of North America's last giant sauropod

The sauropod dinosaur <i>Alamosaurus sanjuanensis</i> Gilmore, 1922 has been known from Maastrichtian deposits of south-western North America for nearly a century. <i>Alamosaurus</i> is the youngest sauropod taxon known in North America. Originally described from an isolated scapula and ischium from New Mexico, more of <i>Alamosaurus</i> was revealed by an incomplete skeleton from southern Utah. Additional referred specimens from western Texas provided the first few examples of cervical, dorsal and sacral vertebrae known for the taxon, but these came from relatively small and immature individuals. Here we describe an articulated series of cervical vertebrae of a large, mature titanosaur from Big Bend National Park, Texas, and provide evidence that the specimen can be referred to <i>A. sanjuanensis</i>. The specimen represents the first articulated cervical vertebral series described for the taxon, which clarifies aspects of cervical vertebral anatomy and provides at least one new diagnostic character for <i>Alamosaurus</i>. Many previous cladistic analyses found <i>Alamosaurus</i> to be a saltasaurid titanosaur, sometimes closely related to the Asian taxon <i>Opisthocoelicaudia skarzynskii</i>. We present cladistic analyses incorporating new data from this and other specimens from Big Bend National Park. The first places <i>Alamosaurus</i> as a lithostrotian titanosaur outside Saltasauridae. The second analysis, with greater focus on South American titanosaurs, finds <i>Alamosaurus</i> allied to Lognkosauria, a clade of South American titanosaurs notable for giant size and exceptionally robust necks. This relationship may be more congruent with the fossil record than hypotheses of phylogeny that would require saltasaurid titanosaurs to inhabit northern Laramidia, Beringia and Central Asia through the late Campanian and Maastrichtian while leaving no record of their presence.