The Cerrejón Formation, and Why It's so Important

Introduction

After the Cretaceous-Palaeogene Mass Extinction, ~66 million years ago, life on Earth began to recover. With non-avian dinosaurs no longer populating the Earth, placental mammals began to rapidly diversify. In contrast, aquatic environments provided a testing ground for the other major group of tetrapods: the diapsids. A coal mine in Colombia, which forms the 60–58-million-year-old Cerrejón Formation, provides an insight into just how reptiles survived and thrived in early Cenozoic South America, and even how the Amazon rainforest first arose.

Palaeontological Summary

The Cerrejón Formation was first mentioned in the technical literature by Thomas van der Hammen, though it was originally named the Septarias Formation. It wasn't until van der Hammen's 1958 study that it received its current name. The Cerrejón faded into obscurity, until it came to the attention of palaeontologists once again in 2009. A set of 28 fragmentary fossils of a massive diapsid were found, quickly identified as a snake. The snake was dubbed Titanoboa cerrejonensis—more on that later.¹

Titanoboa cerrejonensis, a large boid snake from the Cerrejón Formation of Colombia, swims towards a gloomy river's surface.
Author's own work.

At the time that Titanoboa was found, few Palaeocene vertebrates had been discovered in South American deposits. Though fragmentary, enough is known of Titanoboa and its relatives to figure out how it probably lived. Several traits unique among the Boidae, such as high palatal and marginal tooth counts and a reduced pterygoid, indicate that it was mostly (or entirely) piscivorous. This indicates a relatively high fish population, considering the presence of two crocodyliform piscivores.

The crocodylomorph fauna of the Cerrejón is dominated by the dyrosaurid tethysuchians. Three genera, Acherontisuchus guajiraensis, Cerrejonisuchus improcerus, and Anthracosuchus balrogus, were all described within five years of Titanoboa. Dyrosaurids were primarily fast-swimming forms, though may have occasionally come onto land. The exception to this rule was A. balrogus, which had a robust, short snout, and may have lived more like most crocodiles in the genus Crocodylus. Acherontisuchus and Cerrejonisuchus were more piscivorous, and probably lived more like the freshwater crocodile, Crocodylus johnstoni.²

The podocnemidid turtles Carbonemys cofrinii and Cerrejonemys wayuunaiki, as well as a bothremydid, Puentemys mushaisaensis, have also been found in the Cerrejón deposits. Carbonemys' immense size and powerful jaws have led to speculation that it fed on relatively large prey, hunting small crocodilians, and overall leading a lifestyle markedly different to modern podocnemidids. Finally, several unnamed lungfish have also been discovered in the Cerrejón. 

The palaeoflora of the Cerrejón is significant since it provides insight into the origin of neotropical rainforests. In 2009, Wing et al. noted numerous similarities with extant flora from the present-day Amazon rainforest, concluding that they may have been ancestral to some of those alive today, such as Stephania (represented in the Cerrejón by S. palaeosudamericana), and many more taxa covered in greater detail below.

Revised phylogenetic tree of the fauna of the Cerrejon Formation, based on Hastings et al. (2011) and Hastings et al (2013). Unlike the previous version, the indeterminate elopomorph reported from the Cerrejón has been included.
Author's own work.

Location, Stratigraphy and Palaeoenvironment

The Cerrejón Formation is situated within Cerrejón, an open-pit coal mine located in La Guajira Department, Colombia, near the town of Albania and 31 miles away from the city of Maicao. (see below). The Cerrejόn is subdivided into upper, middle and lower groups, based on the thickness and distribution of the coal beds it consists of. It is thought that, at some point in the geologically-recent past, some of the coal spontaneously combusted to form clinker, a type of red-and-black rock formed from scorched coal.

A map showing the location of the Cerrejón Formation of Colombia.

Map data ©️ Google Maps 2021

The depositional of the Cerrejόn gradually changed from a coastal plain influenced by estuaries to a river-influenced one. This can be determined through a thorough examination of lithofacies (different types of clastic or chemical sediments) and the composition of the Cerrejόn's palaeoflora. As of 2009, and representing all identifiable leaf morphologies as of that paper's publishing, a list of palaeoflora from the Cerrejón is as follows:

• 46 non-monocot angiosperm leaves (includes Stephania)
• 13 monocot angiosperm leaves
• 5 fern leaves (genera Salvinia, Stenochlaena, Lygodium, and Acrostichum)
• 1 conifer
• 33 types of fruits and seeds, 19–32 of which assigned to families or orders, primarily Araceae, Arecaceae, Fabaceae, Lauraceae, Malvaceae, Menispermaceae, and Zingiberales
• Montrichardia
• A palm of the Euterpeinae

Of these, several are key to understanding the palaeoenvironment of the Cerrejón: Stenochlaena is a common climbing fern in Old World tropical swamps (though it was present in South America at one point); Montrichardia, now used as a healing plant, is present in tropical Guyana. Living members of the Euterpeinae are found in places like Yucatán, which has temperatures ranging from 23.5℃ (74℉) to 28℃ (82℉) (source). The fruits and seeds come from genera associated with tropical environments. The overall indication, and the theory favoured by Head et al, is that Cerrejón's average temperature was between 28℃ (82℉) and 30℃ (86℉). 

This is higher than the average temperature of the Amazon rainforest, which is between 25℃ (77℉) and 28℃ (82℉). The estimated palaeolatitude of the Cerrejón is 5° north of the equatorial plane, making it slightly lower than it is at present. This puts it at the same latitude as the Bay of Benin and Ilemi Triangle (both are in Africa, and have similar temperatures), and contributed to the high temperatures.³ Possibly contributing to the high temperature was the Palaeocene-Eocene Thermal Maximum, an event where global temperatures rose between 5–8℃.⁴

 

Anthracosuchus balrogus, a dyrosaurid mesoeucrocodylian from the Cerrejón Formation of Colombia, basks on a riverbank.
Author's own work.

Another possible source for determining the palaeoclimatology of the Cerrejón Formation is the most noteworthy of its palaeofauna, the prodigiously-sized snake Titanoboa cerrejonensis. The debate as to its method of thermoregulation, and as such its body temperature, has provided insights as to Cerrejón's environment. In 2009, it was suggested that Titanoboa would have required temperatures of between 30℃ (86℉) and 34℃ (93.2℉) simply to exist. It was then suggested that Titanoboa had a lower metabolic rate than present reptiles, and that it could survive in an equally hot environment without overheating. 

The opinions of another 2009 study, by Denny et al., differed from those of Head et al, with them suggesting that Titanoboa's use as a palaeothermometer is of uncertain accuracy. Additionally, the Denny et al. paper suggested that if Titanoboa reached the upper estimate of 34℃ (93.2℉), it would dangerously overheat when coiled. If this is correct, Titanoboa would have had to regularly and greatly change its body position to prevent overheating, more so than the green anaconda (Eunectes murinus). Since external temperatures of 34℃ would eventually prove dangerous for Titanoboa, the Cerrejón likely had a lower temperature. 

Overall, data from Titanoboa and the local palaeoflora suggests an average temperature of perhaps 30℃ (86℉),⁴ and this is supported in part by the euterpeines, the plants I mentioned earlier, and their preference for temperatures closer to 28℃ than 34℃.

Summary

The Cerrejón Formation of Colombia, South America, is primarily noted for its vertebrate palaeofauna, though for 51 years, little to no fossils were discovered. This changed in 2009, and with the recent progression of palaeontological methods, the stratigraphy and palaeoflora have a major role in understanding the climate of Palaeocene Colombia. 

Information as to Cerrejón's palaeoclimatology can, in part, be inferred through the immense boid snake Titanoboa cerrejonensis. The reliability of this data is hard to determine, with opinions differing between sources.⁵ It can also be determined more reliably by an analysis of the area's plant fossils. Overall, the Cerrejón Formation's palaeobiota provides a glimpse into the Palaeocene environment of Colombia.

References

Cadena, EA; Bloch, JI; Jaramillo, CA; 2010 "New podocnemidid turtle (Testudines: Pleurodira) from the middle-upper Paleocene of South America." Journal of Vertebrate Palaeontology. 30 (2): 367–382.

Cadena, EA; Bloch, JI; Jaramillo, CA; 2012a "New bothremydid turtle (Testudines, Pleurodira) from the Paleocene of Northeastern Colombia." Journal of Paleontology. 86 (4): 688.

Cadena, EA; Ksepka, DT; Jamarillo, CA; Bloch, JI; 2012b "New pelomedusoid turtles from the late Palaeocene Cerrejón Formation of Colombia and their implications for phylogeny and body size evolution." Journal of Systematic Historical Biology. 27 (8): 313.

Denny, MW; Lockwood, B; Somero, G; 2009 "Can the giant snake predict palaeoclimate?"

Halliday, TJD; Goswami, A; 2015a "Eutherian morphological disparity across the end-Cretaceous mass extinction." Biological Journal of the Linnean Society. 

Halliday, TJD; Upchurch, P; Goswami, A; 2015b "Resolving the relationships of Paleocene placental mammals." Biological Reviews.

Hastings, A; Bourque, JR; Bloch, JI; Rincon, AF; Jaramillo, CA; 2013 "New fossil lungfishes (Dipnoi, Lepidosirenidae) from the Paleogene of northern South America and new methods for toothplate identification." Society of Vertebrate Palaeontology 73rd Annual Meeting. 140.

Hastings, AK; Bloch, J; Jamarillo, CA; 2011 "A new longirostrine dyrosaurid (Crocodylomorpha, Mesoeucrocodylia) from the Paleocene of north-eastern Colombia: biogeographic and behavioural implications for new-world Dyrosauridae." Palaeontology. 54 (5): 1095–1116.

Hastings, AK; Bloch, JI; Cadena, EA; Jaramillo, CA; 2010 "A new small short-snouted dyrosaurid (Crocodylomorpha, Mesoeucrocodylia) from the Paleocene of northeastern Colombia." Journal of Vertebrate Palaeontology. 30 (1): 139–162.

Hastings, AK; Bloch, JI; Jaramillo, CA; 2014 "A new blunt-snouted dyrosaurid, Anthracosuchus balrogus gen. et sp. nov. (Crocodylomorpha, Mesoeucrocodylia) from the Palaeocene of Colombia." Historical Biology. 27 (8): 998–1020.

Head, J; Bloch, J; Bernal, JM; Rincon Burbano, AF; Bourque, J; 2013 "Cranial osteology, body size, systematics and ecology of the giant Paleocene snake Titanoboa cerrejonensis." Society of Vertebrate Paleontology. 140–141.

Head, JJ; Bloch, JI; Hastings, AK; Bourque, JR; Cadena, EA; Herrera, FA; Polly, PD; Jaramillo, CA; 2009a "Giant boid snake from the Paleocene neotropics reveals hotter past equatorial temperatures." Nature. 457: 715–718.

Head, JJ; Bloch, JI; Hastings, AK; Bourque, JR; Cadena, EA; Herrera, FA; Polly, PD; Jaramillo, CA; 2009b "Head et al reply" (to Denny et al., 2009). 

Jaramillo, CA; Pardo Trujillo, A; Rueda, M; Torres, V; Harrington, GJ; Mora, G; 2007 "The palynology of the Cerrejon Formation (upper Paleocene) of northern Colombia." Palynology. 31: 153–189.

Makarieva, AM; Gorshkov, VG; Li, BL; 2009 "Re-calibrating the giant snake paleothermometer."

McInherney, FA; Wing, S; 2011 "A perturbation of carbon cycle, climate, and biosphere with implications for the future." Annual Review of Earth and Planetary Sciences. 39: 489–516.

Quintero, JA; Candela, SA; Rios, CA; Montes, C; Uribe, C; 2009 "Spontaneous combustion of the Upper Paleocene Cerrejón Formation coal and generation of clinker in La Guajira Peninsula (Caribbean Region of Colombia." International Journal of Coal Geology. 80 (3): 196–210.

Scavezzoni I; Fischer V; 2021 "The postcranial skeleton of Cerrejonisuchus improcerus (Crocodyliformes: Dyrosauridae) and the unusual anatomy of dyrosaurids." PeerJ. 9.

Snyderman, JMK; 2009 "Biased reptilian palaeothermometer?" 

Tewalt, SJ; Finkelman, RB; Torres, IE; Simoni, F; 2006 "World Coal Quality Inventory: Colombia." United States Geological Survey. 132–157.

Van der Hammen, T; 1958 "Estratigrafía del Teciario y Maestrichtiano continentales y tectogénesis de los Andes colombianos." 

Wing, SL; Herrera, F; Jaramillo, CA; Navarro, CG; Wilf, P; Labandeira, CC; 2009 "Late Paleocene fossils from the Cerrejón Formation, Colombia, are the earliest record of Neotropical rainforest." Proceedings of the National Academy of Sciences. 106: 18627–18632

¹ The green anaconda (Eunectes murinus) has been proposed as an extant analogue for Titanoboa cerrejonensis.

² A recent study (Scavezzoni & Fischer, 2021) found that Cerrejonisuchus was more terrestrial than its relatives.

³ This is only speculation based on the palaeolatitude of the Cerrejón.

⁴ The exact cause of this event is unknown, but has been ascribed to intensified methane release from hydrate reservoirs.

⁵ Please note that this is only a median estimate, and formal publications may contain an entirely different result.

⁶ See the section directly above for more information on this debate.