Las Hoyas, from a wall to a fossil site

Fish in a wall

In the mid-1980s, in La Cierva, machines disturbed a hillside to plant pines and left piles of limestone slabs. A builder used them to clad houses, and some held fish. He showed them to a friend, the Cuenca geologist Santiago Prieto, a caver and later a teacher training lecturer, who went to the site and took a few samples to his former lecturers in Madrid. In the office of palaeontologist Nieves López he met José Luis Sanz, a Mesozoic specialist. Sanz first thought the rocks were Jurassic; then he saw they were younger, Cretaceous, and took charge. The first campaigns lasted a week, with two or three lecturers and a few students, geological maps and no very clear idea where to look.

Prieto does not claim the discovery of Concavenator. He remembers that among those early samples were large pieces enclosed in rock that suggested a bigger animal, but preparing them took a great deal of work, and small, clean fossils were then more appealing. The skeleton was excavated in 2003 and described in 2010. Today a room in the Museo de Paleontología de Castilla-La Mancha bears his name.

Opening the book

About 125 million years ago Las Hoyas was a subtropical wetland of pools and little islands of vegetation. The water and mud coming in left thin layers that now open like the pages of a book. Each summer, students from the Universidad Autónoma de Madrid separate them with small cold chisels. Anything of interest is recorded with its layer, because what matters is seeing how organisms change from one level to another: many fish here, crayfish there. Then, in the museum, it is cleaned, repaired, registered and studied.

It is not all dinosaurs. A little mark may be a coprolite, fossilised dung that one day, once analysed, will reveal what the animal that left it ate and who else lived in the wetland. There is a small shark with two spines and bipedal dinosaur footprints about forty-five centimetres long. In 2021, in the Magenta trench, a long trackway was studied, left by a theropod two or three metres tall that limped: one toe remained turned backwards. It was preserved in such detail because the footprints fell on microbial mats that hardened the mud almost immediately.

What comes next

About thirty people from Spain, Italy and France worked in the 2026 campaign, and there were hundreds of applications to take part. Besides excavating, they mapped the terrain to establish the wetland’s extent, were processing data from a synchrotron and were beginning to gather information for an artificial intelligence thesis to help study it. They are looking not just for the next striking animal but to understand how the whole wetland worked.

The questions are no longer limited to Las Hoyas. In 2024, at the initiative of villagers from Landete, Algarra and Casas de Garcimolina, who paid part of the cost through crowdfunding, study began on another layer of laminated limestone between the two villages: the Proyecto Chikuto, named after an amateur from Landete who showed palaeontologists where there were fossils. With Las Hoyas and Buenache de la Sierra, it is another piece in the puzzle of the Serranía’s Cretaceous wetlands.