Some 65 to 70 million years ago--shortly before their kind went extinct--a large herd of dinosaurs gathered by the sea to lay their eggs in the soft sand. Time passed, sediments from rivers covered the site and helped preserve it, the beach sand consolidated into sandstone, and the erstwhile shore became part of the southern Pyrenees in Spain. Several years ago paleontologists found a few fossil bone fragments and eggs. But no one knew just how rich in fossils this area was until José Luis Sanz, a paleontologist at the Autonomous University in Madrid, surveyed the nine- square-mile remains of the ancient shore. The site, says Sanz, is one of the largest dinosaur nesting sites known--perhaps the largest.
Sanz sampled the number of intact eggs and fragments distributed in the sandstone. He found that one 12,000-cubic-yard chunk of sandstone held the remains of about 300,000 eggs. Microscopic channels in the eggshells, which provided air for the embryos within, were unlike those in bird eggs and identified the fossils as dinosaur eggs. In addition to the eggshells, Sanz has also found tiny smashed bones that probably belonged to young dinosaurs, as well as the fairly intact remains of 24 nests, each containing between one and seven randomly placed eight-inch-wide spherical eggs. The nests are unlike other dinosaurs’ nests that have been found. At nests in Mongolia, for example, the eggs are laid in spiral patterns, not dropped randomly as at the Spanish site.
The well-preserved state of the nests, which lie within a few feet of the surface, leads Sanz to believe that newcomers to the rookery may have refrained from trampling on the nests of earlier habitués--which must not have been easy, given the overcrowded conditions. In some cases the nests--which are essentially just pits in the sandstone--lie only three feet apart. Based on the enormous number of eggs he’s found, Sanz thinks the dinosaurs were loyal to the site, returning to it repeatedly each reproductive season.
Unfortunately, Sanz hasn’t been able to identify the species of dinosaur that laid its eggs in this area: We just have some very fragmentary bone material, and from this evidence it is not possible to propose the kind of dinosaurs to which these eggs belonged.
Nevertheless, Sanz says, this is the first time researchers have ever uncovered unequivocal evidence that dinosaurs may have favored the seaside as a nesting site--perhaps because the yielding sands protected the eggs from breakage. Other paleontologists have unearthed young dinosaur bones from marine sediments, but those were probably the remains of individuals that had been washed out to sea. Says Sanz: This is the first unambiguous evidence of nesting behavior on a beach.