Las Hoyas

A wetland from 125 million years ago

Las Hoyas is in the municipality of La Cierva, around 20 km from Cuenca. Its rocks date from the Lower Cretaceous, around 125 million years ago. There was then a subtropical wetland here, a network of lakes and pools of lime-rich water resembling the Lagunas de Ruidera, with a wet season and a dry season. Land and water creatures lived together, groups that became extinct and ancestors of today’s species. It is a natural laboratory for seeing how the groups of plants and animals now dominating the planet emerged.

Exceptionally preserved fossils

What makes the site unique is how its fossils are preserved. There is a fern, Cladophlebis; an insect, Iberonepa romerali; a shrimp, Delclosia martinelli; a frog, Gracilibatrachus; the bird Eoalulavis; the dinosaur Pelecanimimus; the fish Rubiesichthys; the mammal Spinolestes; and shoals of young fish that died suddenly. It is compared with Yixian, in China, of the same age, and Messel, in Germany. In 2026 another bird, Gorgonavis alcyone, was described from an incomplete skull. A team from the Universidad Autónoma de Madrid has studied it for more than thirty years.

It has been a Bien de Interés Cultural, as a palaeontological zone, since 2016.

Some façade slabs

Las Hoyas was discovered through a wall. In the 1980s, a builder friend of the Cuenca geologist Santiago Prieto showed him fossil fish in limestone slabs he was using to clad houses; they came from a slope disturbed for pine planting, where machinery had left heaps of stone. Prieto went to the site and took a few samples to the university in Madrid. There, in Professor Nieves López’s office, he met palaeontologist José Luis Sanz, who took charge of the research. Armando Díaz Romeral also appears alongside him among those who brought the site to public attention. The first field season was in 1988, according to a radio interview with Sanz; another item from the same station refers to excavations since 1985. Some large remains of Concavenator were already among the samples from the 1980s, encased in rock, but the skeleton was not excavated until 2003 or described until 2010.

How the work is done

Each summer, students from the Universidad Autónoma de Madrid split the limestone layers with small cold chisels and examine what is inside each one. Anything of interest is recorded with its layer, because what matters is knowing how organisms change from one level to another, and taken to the museum to be cleaned, repaired and catalogued. Not everything is spectacular: a small mark may be a coprolite, fossilised excrement that may one day reveal what its producer ate. In a trench called Magenta, a long trackway was studied in 2021, left by a theropod dinosaur two or three metres tall with a slight limp: one toe was turned backwards. The footprints survived in such detail because they fell on microbial mats that hardened the mud almost immediately. Around thirty people from several countries worked in the 2026 field season; besides searching for fossils, they mapped the terrain to establish the extent of the ancient wetland, analysed samples at Barcelona’s ALBA synchrotron and began gathering data so that a doctoral thesis on artificial intelligence might one day help study them. The full story is told in Las Hoyas, from a wall to a fossil site.

Palaeontologist and illustrator Óscar Sanisidro, who chose Concavenator as the main subject of his Dinoguía de la península Ibérica (Anaya, 2024), explains how a dinosaur is drawn: like a dissection in reverse, from skeleton to muscles and then skin, grounding every decision in published data. Gaps in the fossil are filled by comparison with its relatives, and that is an inference, not something preserved: colour, for example, has only been studied in a few exceptional fossils from China. A drawing of Concavenator is not a photograph. Nor did it live alongside the animals of Lo Hueco: Las Hoyas is around fifty million years older.

Plan your visit

The fossils can be seen at the Museo de Paleontología de Castilla-La Mancha, in Cuenca.

Before you travel

The site cannot be visited independently. Only students and researchers may enter, by requesting access from the museum at least 15 working days in advance; entry is free. Nothing may be collected or excavated outside the scientific teams.

Combine with

The village of La Cierva and the landscape of Tierra Muerta.