Lirainosaurus

Lirainosaurus astibiae was a small titanosaur from northern Spain. Unlike sauropods named from one bone, it is represented by braincase fragments, teeth, dozens of vertebrae, limb bones and two osteoderms from several individuals. Histology shows that at least some small animals were mature.

Quick facts

Discoveries at Laño quarry

The type series comes from continental beds L1A, L1B and L2 at Laño quarry, between the villages of Laño and Albaina in the Condado de Treviño area of Burgos Province. River channels and floodplains deposited these sediments during the late Campanian or early Maastrichtian. José Luis Sanz and colleagues named the species in 1999. The genus name combines Basque lirain, meaning slender, with the Greek word for lizard.

The holotype, MCNA 7458, is the foremost caudal vertebra. Paratypes include an incomplete braincase, teeth, dorsal and caudal vertebrae, parts of the shoulder and pelvic girdles, limb bones and two dermal plates. Later work added a second braincase fragment, a substantial tooth sample and many more elements from the axial and appendicular skeleton.

The material represents several animals rather than one articulated individual. Researchers must therefore compare size, anatomy and bed position when assigning isolated bones, and they cannot combine every element into a single ideal skeleton.

Small but mature sauropods

Early estimates put body length as high as 8–10 metres. More detailed scaling from limb bones suggested about four metres for some adults and up to six metres for the largest known individuals. Published masses around 2–4 tonnes depend on the proportions chosen for a reconstructed body and are not direct measurements.

Long-bone sections preserve fibrolamellar tissue interrupted by growth lines and extensive secondary remodelling. In mature individuals, newly formed bone replaced much of the original cortex. Small adult size did not result from growth simply switching off at an early age: deposition slowed toward the outer bone surface, while deep internal remodelling began relatively early.

This tissue record separates mature small-bodied animals from a chance sample made only of juveniles. It does not imply that every individual had the same dimensions. The Laño collection contains several growth stages and therefore preserves genuine variation through life.

Skull, teeth and feeding

Two incomplete braincases document the rear of the skull, but the whole snout is missing. Numerous teeth preserve differences linked to age and the repeated replacement of working crowns typical of titanosaurs. The complete lower jaws, tongue and other soft tissues of the mouth are unknown.

Herbivory is supported by the dentition and sauropod relationships. Feeding height would have depended on a neck that is not preserved as one complete series in any individual. Scenes showing Lirainosaurus grazing or browsing a particular tree are ecological reconstructions, not observations tied to fossil stomach contents.

Spine, limbs and armour

Anatomical revisions of the vertebrae and limb bones describe a slender quadrupedal sauropod with columnar legs and a long tail. The full neck and the tip of the tail still have to be completed from related titanosaurs.

Two fragmentary osteoderms prove that dermal ossifications were present. They do not demonstrate a continuous shell or orderly rows of shields. The number carried by one animal, their positions in the skin and the area they covered cannot yet be recovered.

Relationships and significance

Lirainosaurus is generally classified as a derived lithostrotian and is often recovered near Saltasauridae. Its anatomy and size invite comparison with other small European titanosaurs, but similar growth does not make those genera one species or direct ancestors of one another.

The combination of different ages, cranial remains, teeth, vertebrae, limbs, armour and microscopic bone structure makes it one of the most thoroughly studied European titanosaurs. It demonstrates that Late Cretaceous European communities supported persistent lineages of small sauropods as well as larger forms.

Island-related dwarfing is plausible in the palaeogeography of Late Cretaceous Europe, but the interpretation depends on both geography and histology rather than size alone. Colour, social organisation, reproduction and the cause of death of individual animals are not established. Ibirania provides a separate example in which adulthood of a small titanosaur was also tested microscopically.

Evidence, inference and reconstruction