The Lower Cretaceous marine reptile fossil Lagerstätte of Boyacá in the Ricaurte Alto Province (05° 37' 48'' N073° 33' 00'' W) of Colombia, South America is regarded as the most complete and globally important fossil record of the marine reptiles.
In 2022, the Ricaurte Alto Province lagerstätte was named among the “First 100 Geological Heritage Sites” designated by the International Union of Geological Sciences (IUGS).
Many Cretaceous Period fossils are displayed at the Paleontological Museum of the National University of Colombia in Bogotá and also at the local El Fósil Museum in Villa de Leyva, Boyacá.
The Cretaceous Period, a name first proposed in 1822 by Jean-Baptiste-Julien d’ Omalius d’Halloy a Belgian geologist, and derived from the Latin for “chalk”.
The Cretaceous Period followed the Jurassic and was both the last and longest period of the Mesozoic Era dating between 145.5 and 65.5 million years ago.
The Cretaceous Period was most certainly the age of reptiles where dinosaurs roamed the continents, ferocious marine reptiles swam the oceans and huge Pterosaurs patrolled the skies.
The Cretaceous Period was a “greenhouse” world with high levels of carbon dioxide, high global sea levels, high global temperatures, and high levels of plate tectonic activity.
Then, roughly 66 million years ago, at the end of the Cretaceous Period, an asteroid collided with Earth on the Yucatán Peninsula in Mexico.
This would result in one of Earth's five great mass extinction events that killed off all nonavian dinosaurs, pterosaurs, and many marine reptiles as well as many early mammals, amphibians, birds, reptiles, and insects.
At the start of the Cretaceous Period, the supercontinent of Pangea that had originally formed during the Late Carboniferous when the Rheic Ocean between Gondwana and Laurussia had closed was now beginning to re-open and split apart.
The splitting apart of Pangea was driven by the active forces pushing mantle plumes from the sub-continental mantle upwards towards the surface and the passive dragging forces of oceanic subduction retreat.
As the tectonic plates began to rift and converge at the plate boundaries so did the intensity build for orogenic or mountain-building events, volcanic activity, and climate change on Earth.
The subsequent splitting part of Pangea meant that the high sea levels were able to flood large low-lying continental areas producing numerous shallow epicontinental seas, especially in the tropical and subtropical regions of the world.
Extensive coastlines began to appear creating new habitats. As the rifting continued during the Cretaceous Period it triggered large-scale geographic isolation, radiation, divergence, and diversification in the evolution of both animals and plants.
The Ricaurte Alto Province of Colombia during the Lower Cretaceous epoch lay on a relatively shallow equatorial epicontinental seaway that ran between the northern continent of Laurasia and the southern continent of Gondwana bordering the proto-Pacific Ocean and Tethys Ocean.
The konservat-lagerstätten of the Lower Cretaceous Paja Formation of the Alto Ricaurte of the Eastern Cordillera of central Colombia was deposited in the warm waters of an epicontinental sea during Hauterivian – Aptian times.
Today the Paja Formation crops out almost continuously in the municipalities of Villa de Leyva, Sáchica, and Sutamarchán.
The geology of the Paja Formation is of a finely laminated shale with occasional clay or argillaceous limestones, fine–grained sandstones, and an abundance of pyrite, calcareous concretions, and special forms of depositional interbedding or intercalations of gypsum and calcite.
The Paja Formation preserves both marine and terrestrial fossils that are exceptional in terms of their quantity and quality. They include the skeletons of pliosaurs, plesiosaurs, ichthyosaurs, and marine turtles along with fish, ammonites, terrestrial plants, and dinosaurs.
The fossil record of the Paja Formation shows a combination of marine species that died where they lived (autochthonous) and other terrestrial species of animals that were displaced from the coastline or possibly by storms from other locations (allochthonous) to finally rest in the Paja Formation sediments.
Many of these fossils have helped the science of paleontology to fill in the Lower Cretaceous Gap for marine reptile species.
The Paja Formation is known for its abundance, variety, and exceptional preservation of ichthyosaurs (Platypterygius sachicarum, Muiscasaurus catheti), Plesiosauria including both the long–necked plesiosauromorphs (Callawayasaurus colombiensis) and the short–necked pliosauromorphs and marine turtles (Desmatochelys padillai, Leyvachelys cipadi) and their fossilized eggs.
It is uncommon in Colombia to find Mesozoic Era terrestrial vertebrates but the Paja Formation has revealed the sauropod Padillasaurus leivaensisnan an extinct genus of the Titanosauriform sauropod and is considered to be the first South American brachiosaurid ever discovered.
Among the most iconic fossil finds is the colloquially known as “El Fósil” or "The Fossil" which was found in 1977 near the Villa de Leyva region of Boyacá. It measures 7 meters without its tail and has a 2.8 meter-long skull and 30 cm teeth.
Until recently El Fósil was classified as Kronosaurus boyacensis but in 2021 it was designated a new genus and redescribed as Monquirasaurus boyacensis a large carnivorous pliosaurid.
Interestingly, the key to understanding why the Paja Formation fossils remained in such an exceptional state of preservation links to the properties of the water column of the equatorial epicontinental seaway.
The diversity of the sea floor fossil record is surprisingly lacking in benthic species for a shallow epicontinental seaway. Only low oxygen and hydrogen sulphide tolerant species and reducing bacteria seafloor mats were present.
There have been several hypotheses for the Paja Formations oxygen-poor epicontinental seafloor conditions. Including the presence of a halocline and thermocline that coincided to create a pycnocline zone, a feature of a much deep water environment. Alternatively, the epicontinental seaway experienced cycles of restricted access to the open sea.
The Paja Black shale was deposited in a low energy, oxygen-reducing environment with little or no bottom currents making it a perfect environment for the preservation of fossils.