The rock formed in the Araripe Basin during the latest Aptian age of the Early Cretaceous, in what is reconstructed as a semi-arid lacustrine wetland - shallow standing water in a hot, dry landscape. The bottom of that water was where things stopped. Limestone accreted around dead organisms as nodules, sealing them before decay could finish its work, which is why the Crato preserves anatomy that almost never fossilises anywhere. Around twenty-five species of fossil fish have been recovered, and they are frequently found with their stomach contents still in place, so researchers can reconstruct who was eating whom in a lake that dried up before the Atlantic finished opening. The fish names read like a roll call of the Cretaceous freshwater world: Dastilbe, Cladocyclus, Calamopleurus, Lepidotes, Santanichthys, Placidichthys.

Insects are the Crato's quiet marvel. More than twenty orders are recorded, and the Odonata - dragonflies and damselflies - are unusually well represented: 384 specimens so far, 264 of them adults and 120 larvae. True bugs and grasshoppers and crickets are abundant too, in species and in sheer numbers of individuals, alongside plant remains that include some of the earliest flowering plants. Put those together and you have something rarer than a spectacular skeleton. You have an ecosystem: the insects, the plants they visited, the fish that ate the insects, the reptiles that ate the fish. Almost no other Cretaceous site lets you assemble that whole chain from a single quarry face.

Then there are the pterosaurs, which are the reason the Araripe Basin is known outside palaeontology. Tupandactylus, Ludodactylus, Arthurdactylus, Aymberedactylus, Lacusovagus and Brasileodactylus have all come out of these rocks, and the Crato preserves the parts of a pterosaur that ordinary fossilisation destroys - the soft tissue of the head crests, the sheets of skin that made those animals so strange to look at. Dinosaurs turn up too, though sparingly: isolated feathers, sauropod footprints, and one small feathered animal that would generate an international incident.

Fish fossils were noted in the area as early as 1823, but the Crato limestones were not methodically published until 1993, and for most of the twentieth century the unit was treated as the bottom member of the Santana Formation rather than a formation in its own right. That changed with a 2007 volume by David Martill, Günter Bechly and Robert Loveridge, which gave the Crato a formal type locality and separated it properly from the Santana, roughly ten million years younger. The age itself was argued over for decades, with most workers placing the beds on or near the Aptian-Albian boundary. The current reading puts them a little earlier, in the latest Aptian.

Beauty has a cost here. The same slabs that split so cleanly are quarried for cement and for paving stone, and the quarrying destroys sites. Worse, the fossils are so striking that an illegal trade grew up around them, large enough to count as a local industry, and palaeontologists working the region have been calling for an urgent preservation programme. The clearest case of what leaves is Ubirajara jubatus, a small feathered dinosaur from a chalk quarry between Nova Olinda and Santana do Cariri. Its describers stated it had left Brazil in 1995; a 2021 investigation found it had actually been removed in 2006 and acquired by the Staatliches Museum für Naturkunde Karlsruhe in 2009. The paper appeared in press in December 2020, was pulled within days, and was formally withdrawn in September 2021. Brazilian scientists ran a campaign under the hashtag #UbirajaraBelongstoBR, and on 12 June 2023 a German delegation led by foreign minister Annalena Baerbock handed the specimen back. It is now on display at the Plácido Cidade Nuvens Palaeontology Museum in Santana do Cariri, a few kilometres from the quarry it was taken from.