One of the newly-discovered species, Jobaria tiguidensis, is a long-necked, broad-toothed creature that is "strikingly primitive" for a sauropod that appeared quite late - about 130 to 140 million years ago - in dinosaur history.

Paul Sereno is dwarfed by the Jobaria cast

An adult Jobaria would have weighed about 20 tonnes and grown to a length of 20 metres (65 feet).

University of Chicago palaeontologist Paul Sereno, whose team dug the creature out of the Sahara desert in the Republic of Niger, said the fossilised bones represent a particularly fine dinosaur specimen.

"With 95% of its skeleton preserved, the new species stands as the most complete long-necked dinosaur ever discovered from the Cretaceous period," he said.

Flash flood

Jobaria refers to "Jobar," a creature in the legends of the local Tuareg nomads; tiguidensis refers to a cliff near to where Paul Sereno and his colleagues made their discovery.

The desert we know today would not have existed in Jobaria's day - the animal would have populated an area covered by open forests and broad rivers.

The team endured intense heat during the dig

The team recovered a range of fossils from both juveniles and adults, which suggests Jobaria roamed in herds of mixed age. It looks as though the dinosaurs may have been caught in an ancient flash flood and buried quickly.

However, some may have died at the hands of the chief meat-eating dinosaur of the time, Afrovenator, an eight-metre (27-foot) long predator previously discovered in the same area by Sereno's team. Tooth marks are present on the ribs of one of the juvenile skeletons.

Ancient lineage

The team think Jobaria probably represents an ancient sauropod lineage that survived and flourished only in Africa during the Cretaceous.

An artist's impression of Jobaria tiguidensis

Unlike other Cretaceous sauropods, Jobaria has spoon-shaped teeth - well adapted for nipping the smaller branches of trees - and a relatively short neck composed of only 12 vertebrae. Jobaria's backbone and tail are also simple, compared with the complex vertebrae and whiplash tail of the older North American sauropods Diplodocus and Apatosaurus.

The other dinosaur announced by the team is one of the smallest sauropods ever found, with a maximum body length of about 15 metres (49 ft). The dinosaur has been given the name Nigersaurus taqueti. Despite its relatively small overall size, it still had a big mouth with over 600 teeth and an unusual snout.

The team had to rely on a number of incomplete specimens to build up a picture of this creature.