Alamosaurus

Chapter • Meet the dinosaur

Overview

Alamosaurus was one of the very last giant sauropods on Earth and the only large long-necked plant-eater living in North America during the final few million years of the Cretaceous. Its barrel-shaped body rested on four pillar-like limbs, the long neck was carried roughly horizontally with a slight upward tilt, and the long tail was held clear of the ground — no sauropod dragged its tail. The head was small relative to the body, and the jaws carried slender, pencil-like teeth restricted to the front, ideal for stripping foliage rather than chewing it. Like several other titanosaurs, it probably had bony plates called osteoderms embedded in the skin; a large osteoderm from Texas has been referred to Alamosaurus. It had no feathers — those belong to the theropod side of the dinosaur family tree. Its size has been much debated. For decades estimates were based on incomplete skeletons and settled around 21 metres and roughly 30 tonnes. Newer finds of exceptionally large vertebrae and a huge femur from Texas have led researchers to suggest that fully grown individuals may have reached 26–30 metres and perhaps 40–70 tonnes, putting Alamosaurus in the same league as the biggest South American titanosaurs. The uncertainty remains large, however, because no complete skeleton of a giant individual is known and several of the largest bones come from different animals. Alamosaurus was a herbivore that probably browsed at several heights, taking conifer branches, ferns and the flowering plants that had become common by the Late Cretaceous. Its long neck let it sweep through treetops and across wide arcs of vegetation without moving its heavy body. Food was swallowed essentially whole and broken down in an enormous gut. An animal this large had to feed for many hours a day, and bone histology — the growth rings preserved in the bone — shows that titanosaurs grew rapidly through their first decades before growth slowed. The species was named in 1922 by the American palaeontologist Charles W. Gilmore, based on material from the Ojo Alamo Formation in San Juan County, New Mexico. The name refers to the Ojo Alamo locality ("cottonwood spring"), not to the Alamo mission in Texas, a common misunderstanding. Fossils have since been found in Texas (Javelina, Black Peaks and El Picacho formations), Utah (North Horn Formation) and New Mexico. Its very presence is a puzzle: sauropods were absent from North America for about 30 million years before Alamosaurus appears, suggesting the lineage immigrated from the south once land connections between North and South America allowed it.

Chapter • Where it lived

Habitat

Alamosaurus lived during the Maastrichtian, the final age of the Cretaceous, about 70–66 million years ago in what is now the south-western United States. The landscape was one of broad river valleys and floodplains, meandering channels, stands of conifers and early broadleaf trees, fern meadows and open ground, under a warm, seasonally humid climate near the shrinking Western Interior Seaway. Fossils come from the Ojo Alamo Formation (Naashoibito Member) in New Mexico, the Javelina, Black Peaks and El Picacho formations of Big Bend in Texas, and the North Horn Formation in Utah, with additional material reported from northern Mexico.

Chapter • Family life

Breeding

Chapter • Wild behavior

Behavior & Ecology

Did you know?

Chapter • Scientific place

Taxonomy