Alamosaurus sanjuanensis

Alamosaurus sanjuanensis was the dominant giant sauropod of latest-Cretaceous southwestern North America. Its fossils occur from New Mexico and Texas into Utah, showing that enormous long-necked herbivores returned to the continent after a long gap in the local sauropod record. Charles Gilmore named it in 1922 from a left shoulder blade and a right ischium collected near Ojo Alamo, New Mexico. Later discoveries added articulated neck vertebrae, limbs, pelvis, tail, skull fragments and small armour plates. These remains reveal a broad-bodied titanosaur roughly 20 to 26 metres long, although the largest isolated bones sometimes assigned to Alamosaurus could indicate still bigger animals. Because those giant bones do not always overlap the diagnostic type material, the most extreme estimates remain uncertain.

Its fossils occur between approximately 70 and 66 million years ago. Values shown here are approximate and reflect the current curated seed dataset.

Alamosaurus had the column-like limbs, wide torso and long neck typical of titanosaurs. Articulated cervical vertebrae from Big Bend preserve extensive internal air spaces that lightened the neck, while the shoulder and pelvic bones were built to support great mass. Referred skull elements indicate a relatively small head with narrow teeth suited to cropping vegetation. Small bony osteoderms occurred in the skin. The animal is reconstructed from multiple individuals because no complete articulated skeleton is known.

• Long pneumatic neck vertebrae with complex internal air spaces
• Broad, robust shoulder girdle and columnar weight-bearing limbs
• Wide-bodied titanosaur proportions
• Small dermal osteoderms embedded in the skin
• Relatively small skull with narrow cropping teeth
• Large adult bones from several southwestern localities

Geologist John B. Reeside Jr. collected the type shoulder blade and nearby paratype ischium from Barrel Spring Arroyo, about one mile south of Ojo Alamo in San Juan County, New Mexico, in June 1921. Smithsonian palaeontologist Charles W. Gilmore described Alamosaurus sanjuanensis the following year. Later finds from the Javelina and Black Peaks formations of Big Bend, Texas, greatly expanded knowledge of the neck and body, while additional remains across the Southwest established Alamosaurus as a characteristic latest-Cretaceous dinosaur.

Discovery credit: John B. Reeside Jr..

Naming authors: Charles W. Gilmore.

Alamosaurus occupied coastal plains, river systems and seasonally dry inland habitats across the southwestern United States near the end of the Cretaceous. In the San Juan Basin and Big Bend region it lived among hadrosaurs, ceratopsids, ankylosaurs, tyrannosaurids and giant azhdarchid pterosaurs. The map marker represents the broad Ojo Alamo type region rather than the precise arroyo exposure.

As a giant herbivore, Alamosaurus probably cropped large quantities of vegetation and relied on gut fermentation rather than chewing. Its long neck expanded the feeding area available without moving the entire body. Trackways or nesting sites have not been securely assigned to the species, so herd structure, migration and parental behaviour remain unknown. The small osteoderms may have provided limited protection or stored minerals, but their precise function is unresolved.

• Alamosaurus was not named after the famous Alamo mission; its name comes from the Ojo Alamo Formation.
• It was one of the last non-avian sauropods in North America.
• Its holotype is a single left shoulder blade housed by the Smithsonian.
• An articulated Texas neck helped confirm its position among derived titanosaurs.
• Some enormous bones referred to Alamosaurus suggest individuals larger than the conservative range used here.

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1. 01
   A new sauropod dinosaur from the Ojo Alamo Formation of New Mexico

   Charles W. Gilmore · Smithsonian Miscellaneous Collections 72(14) · 1922

   Open source
2. 02
   An articulated cervical series of Alamosaurus sanjuanensis Gilmore, 1922 (Dinosauria, Sauropoda) from Texas: new perspective on the relationships of North America's last giant sauropod

   Ronald S. Tykoski, Anthony R. Fiorillo · Journal of Systematic Palaeontology 15(5), 339-364 · 2017

   Open source
3. 03
   Alamosaurus sanjuanensis Gilmore, 1922 holotype USNM 10486

   Smithsonian National Museum of Natural History

   Open source