The La Venta Konzentrat-Lagerstätte: a Neotropical moist forest biome of the middle Miocene

Description

The very fossiliferous Neogene rock successions that outcrop in an emblematic ‘badlands’ landscape, located in the Upper Magdalena Valley (Department of Huila, Colombia), constitute an exceptional natural site with many undeniable attributes of universal value. These sedimentary rocks host one of the richest assemblages of fossil vertebrate fauna ever unearthed in South America and, constitute the main geological exposure of the site The La Venta Konzentrat-Lagerstätte: a Neotropical moist forest biome of the middle Miocene.

The La Venta area extends from the eastern riverside of the Magdalena River, and topographically it constitutes a nearly flat terrain, gently westward inclined, with some small round hills. Its landscape is characterized by elongated escarpments, dipping slopes, and conspicuous gully systems, formed by almost continuously exposed and variegated bedrock. The absent or very limited development of regolith or of extensive soil over the bedrock and the presence of typical xerophytic vegetation confer to this region a remarkable ‘badlands’ scenery (Royo y Gómez, 1945; Fields, 1959; Villarroel et al., 1996; Guerrero, 1997; Montes et al., 2021). This landscape, known as the Tatacoa Desert, corresponds to an extensive tropical dry forest environment with the development of several distinctive ecosystems; and it has evolved through the interplay of lithology, long-term weathering, and the arid conditions induced by the orographic rain shadow of the Andean Mountain ranges (e. g. Garzón Massif), (Anderson et al., 2016; Dill et al., 2020; Montes et al., 2021).

The La Venta site is situated in the Neiva Sub-Basin of the Upper Magdalena Valley Basin: the southern segment of a longitudinal geomorphological and tectonic inter-Andean depression between the Central and Eastern Cordilleras of the Colombian Andes. The La Venta rock successions were accumulated in a tectono-stratigraphic setting that developed at beginning of the middle Miocene (van der Hammen, 1958; Mojica & Franco, 1990; van der Wiel, 1991; Villarroel et al., 1996; Sarmiento-Rojas et al., 2006). This accommodation space was generated by the subsidence of a large region that was previously constituted by the exhumation and the erosion both of Jurassic volcanic and volcaniclastic successions, as well of -the almost completely eroded before the middle Miocene- Cretaceous and Paleogene marine-, transitional- and continental- sequences (van der Wiel & van der Bergh, 1992; Vergara, 1997; Gómez et al., 2003; Sarmiento-Rojas et al., 2006; Montes et al., 2021).

The sedimentary rock succession at La Venta site is approximately 1,100 meters thick and corresponds to the Honda Group, a mid-Miocene siliciclastic sequence widespread throughout the Upper Magdalena Valley (Royo y Gómez, 1942; 1945; van der Hammen, 1958; Fields, 1959; Wellman, 1970; Villarroel et al., 1996; Guerrero, 1997). In this area, the Honda Group overlies Jurassic volcanic and volcaniclastic rocks unconformably, and it is covered unconformably by late Miocene and Pliocene coarse-grained siliciclastic sedimentary successions. At the La Venta site, the Honda Group is best exposed, and its most fossiliferous sections are substantially clustered. In this site, the Honda Group is divided in two stratigraphic fining-upward units: the La Victoria Formation and the Villavieja Formation (Fields, 1959; van der Wiel, 1991; Guerrero, 1997; Anderson et al., 2016; Montes et al., 2021; Mora-Rojas et al., 2023).

The La Victoria Formation and the Villavieja Formation consist -with variations in proportion, thickness, composition, geometry or internal structure-, of vertically and laterally alternating sequences of: medium to very thick tabular and lenticular strata of gray and reddish yellow to pale-brown very fine to coarse-grained lithic sandstones, which may be locally pebbly conglomeratic; and of medium- to very thick levels of reddish-brown and greenish-gray bioturbated mudstones with high fossil content, as well as red siltstones and claystones. The predominant fine-grained sedimentary rocks of the Villavieja Formation sharply contrast to the prevalent coarser-grained lithology of the older La Victoria Formation; whose uppermost segment is notable for its regionally extended suite of medium- to very thick lenticular beds of a clast-supported coarse-pebble conglomerates (Royo y Gómez, 1942; Fields, 1959; Wellman, 1970; Takay et al., 1992; van der Wiel & van der Bergh, 1992; Villarroel et al., 1996; Guerrero, 1997; Mora-Rojas et al., 2023).

The sedimentation of the La Victoria and the Villavieja formations records the widespread development of continental sedimentary settings. These environments were dominated by predominantly east-southeastward transport-directions fluvial systems, ranging from meandering to braided- and anastomosing systems. Additionally, extensive alluvial systems of fans and widespread floodplains were present (Savage, 1951; Stirton, 1953a; Stirton, 1953b; Fields, 1959; Wellman 1970; Takemura, 1986; van der Wiel, 1991, Takai et al., 1992, van der Wiel & van den Bergh, 1992, Hoorn et al., 1995; Guerrero, 1997, Mora-Rojas et al., 2023). The sediment deposition of these geological units is considered to have occurred predominantly between 13.8 Ma and 11.6 Ma. However, the maximum depositional calculated ages suggest that it could have extended from 16.0 Ma to 10.5 Ma. (Takemura & Danhara, 1985; Takemura et al., 1992; Flynn et al., 1997; Mora-Rojas et al., 2023).

The accumulation of these Middle Miocene units was contemporaneous with major landscape and tectonic reconfigurations that shaped the Northern Andes (Anderson et al., 2016; Montes et al., 2021; Carrillo et al., 2023). The depositional settings of the La Victoria and the Villavieja formations were almost entirely controlled by volcanism and tectonism in the Colombian Andes, developed into a large Andean foreland basin extended from the NW Andes to the Guyana Shield. These settings were part of the extensive fluvial and lake/wetland systems of the Miocene Western Amazonia: The Pebas Mega-Wetland System (Hoorn et al., 1995; Wesselingh et al., 2001; Wesselingh et al., 2006; Hoorn et al., 2010a; Hoorn et al., 2010b).

In the La Venta area, the prevalent sedimentological settings accumulated assorted hard parts of very diverse organisms, in characteristic ‘condensation’ and ‘placer’ deposits (sensu Seilacher & Westphal, 1971, and Seilacher et al., 1985). The abundant and diverse fossil remains found in the La Venta geological units make it an exceptional Konzentrat-Lagerstätte (Seilacher, 1970; Nudds & Selden, 2008). Although the predominant nature of the La Venta site fossil remains is disarticulated, its record provides valuable insights into Middle Miocene Neotropical ecosystems, and has been used to define biostratigraphic and biochronologic units that are applied to most of South America (Kay and Madden, 1997a; Madden et al., 1997; Carrillo et al., 2023).

The fossil remains of the La Venta Konzentrat-Lagerstätte reveal a diverse community of terrestrial and freshwater vertebrates, plants, and invertebrates. These organisms inhabited various biotopes in an ancient mosaic/riparian tropical moist evergreen forest that could have evolved into a tropical rainforest (Stirton, 1953a; Fields, 1957; Fields, 1959; Kay & Madden, 1997a; Kay & Madden, 1997b; Spradley et al., 2019). The site is characterized by the fossil remains of specialized faunas. These include crustaceans, freshwater fishes with bony and cartilaginous skeletons, amphibians, reptiles (such as lizards, snakes, aquatic and terrestrial crocodylomorphs, turtles, and tortoises), and various semiaquatic and forest-dwelling birds. Mammalian remains include native clades of Metatherians, Xenarthrans, and South American native ungulates, as well as toothed cetaceans, caviomorph rodents, sirenians, chiropterans, and platyrrhine primates (Savage, 1951; Stirton, 1953b; de Porta, 1969; Hoffstetter, 1971; Hirshfeld & Marshall, 1976; Kay and Madden, 1997, Carrillo et al., 2023).

The first known fossil finds at the La Venta Konzentrat-Lagerstätte dates to 1920. These remains enabled the subsequent scientific description of a new astrapotheriid, a type of South American native hoofed placental mammal, in 1929; and of a new giant caimanine alligatoriid, in 1941 (Cabrera, 1929; Mook, 1941; Savage, 1951). However, the first intensive fieldwork programmes for collecting vertebrate fossils at the site were developed during the scientific expeditions of the 1940s and early 1950s (Royo y Gómez, 1942; Royo y Gómez, 1945; Savage, 1951; Stirton, 1953b). Since then, the La Venta has played an increasingly important role in very significative and multidisciplinary studies on various geological and palaeobiological topics, including taxonomic and evolutionary studies of fossil and extant vertebrate groups, as well as the Cenozoic tectonic and paleogeography evolution of NW South America (e.g. Kay et al., 1997; Hoorn et al., 2010b; Montes et al., 2021).

Justification of Outstanding Universal Value

The La Venta Konzentrat-Lagerstätte: a Neotropical moist forest biome of the Middle Miocene stands out as a unique window into Earth's past to study Neotropics terrestrial ecosystems during the Neogene. In fact, it offers exceptional opportunities to understand the relationship between climatic/environmental change and biotic evolution in the tropics (Carrillo et al., 2023). For instance, the sedimentary and paleontological record of the La Venta site demonstrates the evolution of diverse ecosystems, whose very rich communities (e. g. Kay & Madden, 1997; Carrillo et al., 2023; Mora-Rojas et al., 2023), lived during part of the globally significative warming and cooling periods, the Miocene Climate Optimum and the subsequent Miocene Climatic Transition (Flower & Kennett, 1994; Zachos et al., 2001; Hamon et al., 2013; Holbourn et al., 2015; Steinthorsdottir et al., 2021). These phenomena provide one of the best analogues for understanding the near-future climate changes and the biotic responses that these shifts could induce (Steinthorsdottir et al., 2021; Carrillo et al., 2023; Mora-Rojas et al., 2023).

With a remarkable fossil preservation, the La Venta paleontological site demonstrates the rich biodiversity of endemic faunal groups prior to the Great American Biotic Interchange (Kay et al., 1997). Beside invertebrates and woody plants, more than 190 different faunal fossil forms have been described, including 93 new species, and 52 new genera.

The paleo-ichthyofauna from La Venta elucidates the origins of the rich extant fauna of the Orinoco and Amazon River basins. In fact, it stands as one of the most extensively studied Miocene freshwater fish assemblages within the ancient hydrographic system of the Amazonas (Lundberg, 1997; Ballen & Moreno-Bernal, 2019; Carrillo-Briceño et al., 2021a, 2021b, 2023; Ballen et al., 2022; Schwarzhans et al., 2022).

What is more, the reptilian fossil remains from La Venta provide evidence of specialized adaptations. Notable fossil records include the most complete skeleton of a small caimanine identified to date and the oldest record of anacondas (Carrillo et al., 2023).

A notable fossil group from La Venta are the mammals. In particular, its characteristic mammal assemblage allowed to propose the Laventan Stage (13.5 to 11.8 Ma) and the corresponding biochronologic unit: Laventan South American Land Mammal Age (Madden et al., 1997). The Laventian mammal fauna comprise: 44 new fossil species of rodents, primates, and bats, 29 new species of extinct of native ungulates and xenarthrans, and 15 new taxa of metatherians, including the oldest representatives of some extant (Kay et al., 1997; Defler, 2019a; Spradley et al., 2019; Mora-Rojas et al., 2023; Carrillo et al., 2023). The following are some of the outstanding mammal findings at La Venta.

The site boasts one of the most diverse metatherian fossil assemblages of South America (Goin, 1997; Suárez, 2019). The high diversity, potentially linked to the Miocene Climatic Optimum (Suárez, 2019), includes some species only recorded in La Venta, though some of the genera are present in other South American fossil assemblages (Carrillo et al., 2023). The fossil record of metatherians includes the most complete skeleton of the thylacosmilid Anachlysictis gracilis ever found and one of the most complete for Sparassodonta (Suárez et al., 2023).

Notably, La Venta stands out for its remarkable representation of Xenarthra, offering a significant fossil diversity within this group in tropical South America (Carrillo et al., 2023).

Moreover, the site is particularly crucial for studying the biogeography and evolution of South American native ungulates (SANUs), since it records the three existing clades Astrapotheria, Litopterna, and Notoungulata (Defer, 2019b), including the last representatives of Astrapotheria (Johnson & Madden, 1997; Croft et al., 2020).

In relation to rodents, the four main caviomorph clades (Cavioidea, Chinchilloidea, Erethizontoidea, Octodontoidea) are represented in La Venta by abundant material (Walton, 1997; Defler, 2019a; Mora-Rojas et al., 2023).

In addition, the La Venta site hosts one of the most diverse assemblages of extinct bats in South America (Czaplewski, 1997), encompassing some of the oldest records of bat families in the southern cone (Czaplewski et al., 2003). Considering the generally sparse bat fossil record in South America (Lim, 2009), La Venta fauna have yielded crucial insights into the evolutionary history of South American bats. The fossil record includes the earliest specimen of a plant-visiting bat (Palynephyllum antimaster), whose diet was centered on nectar and insects (Yohe et al., 2015).

Furthermore, the site yields one of the most diverse extinct primate assemblages (Stirton & Savage, 1950; Stirton, 1951; Luchterhand et al., 1986; Kay et al., 1987; Setoguchi & Rosenberger, 1987; Rosenberger et al., 1991; Fleagle et al., 1997; Kay & Meldrum, 1997; Takai et al., 2001). As one of the few tropical localities with platyrrhine fossils in South America, the La Venta site is key to understanding the evolutionary history of this group (Carrillo et al., 2023).

Noteworthy is the preservation state of the fossils, which are in excellent condition not only due to the suitable conditions and processes during their burial and diagenesis but also because the climatic characteristics of the region have favored the preservation of rocks and its associated fossils. The fossil discoveries made so far indicate that, generally, the remains are quite complete, facilitating the identification and characterization of specimens. As a matter of fact, it is possible to find fossil pieces that may not be preserved as easily in other deposits, such as teeth or tusks of small mammals. The preservation of fossils is outstanding not only at a macroscopic level but also at a microscopic level, as demonstrated by studies conducted on turtle bones where microstructural components, such as osteocyte cells, blood vessels, and potentially remnants of their original biomolecular components, have been identified (Cadena, 2020).

Besides, the geological formations comprising the La Venta site have resulted in an iconic "badlands" landscape, featuring extensive and well-developed outcrops. This distinctive scenery is a result of the complex interplay among lithology, prolonged weathering, and the arid conditions influenced by the orographic rain shadow of the Andes Mountain range (Dill et al., 2020; Montes et al., 2021).

On the other hand, the comprehensive geologic record of La Venta offers a valuable glimpse into the paleobiogeographic and tectonostratigraphic evolution of northern South America (Montes et al., 2021). This is particularly significant as the Miocene deposition of the Honda Group coincided temporarily with major landscape and tectonic reconfigurations that played a pivotal role in shaping the Northern Andes (Carrillo et al., 2023). In this regard, the La Venta site provides an opportunity to clarify the relationships between flora and fauna to topography, soil composition, and sediment supply (Carrillo et al., 2023).

Although this extraordinary natural site is still in the early stages of exploration, the unparalleled fossil richness of La Venta in the context of tropical South America has been studied over a century, being one of the most studied fossil assemblages of the continent (Carrillo et al., 2023). To date, there are 224 publications related to this paleontological site (Carrillo et al., 2023), many involving international and highly regarded researchers and institutions. The scientific expeditions such as the 1944-1950 Commission of Vertebrates, the 1977-1987 Kyoto University-Ingeominas, the 1985-1992 Duke University-Ingeominas, and several 21st century explorations demonstrate the scientific tradition of the La Venta paleontological site.

Criterion (viii): The La Venta site qualifies for criterion viii under theme 1, "History of planet Earth and the evolution of life," as a property that records the co-evolution of the planet and life during the Miocene Epoch. The fauna that inhabited northern South America approximately 13 Ma uniquely reflects how organisms adapted not only to a global climatic event known as the Miocene Climate Optimum but also to the changes in tectonic and landscape configuration in the northern Andes. This information holds significant value in the contemporary context, as La Venta stands as one of the most pertinent analogues for comprehending ongoing climatic changes and the potential biotic responses to these shifts.

The extraordinary richness of the La Venta fauna, leading to its designation as a Konzentrat-Lagerstätte, is particularly noteworthy. Within the diverse fossil groups present, the mammal record stands out for its representativeness, enabling the delineation of the Laventan Age/Stage dating back 13.5-11.8 Ma. Its uniqueness is further accentuated by the presence of species found nowhere else in the world.  Additionally, the scientific tradition surrounding La Venta spans over a century, positioning it as a benchmark location for knowledge acquisition and invaluable research opportunities.

Statements of authenticity and/or integrity

The Honda Group, the geological unit hosting the fauna and flora of La Venta, outcrops in various locations along the Upper and Middle Magdalena River Valley (Guerrero, 1997). However, it is in the La Venta area where the best outcrops are found, featuring the most complete exposed sections and the most productive fossiliferous levels within this unit. 

In terms of size, the paleontological site covers a surface area of about 300 km², which includes the attributes (host rocks of the Honda Group and in situ fossils) that give the site its value. In this regard, the conservation state of the site is favorable.

It should be noted that a considerable number of fossils collected from this site are accessible through local museums, such as La Venta Natural History Museum (Museo de Historia Natural La Tatacoa) (Oviedo et al. 2023). This museum currently houses the largest collection of La Venta fossils, comprising over 4000 specimens, among which are exceptionally complete and well-preserved specimens (Carrillo et al., 2023). In turn, the latest information regarding the paleontological site suggests that the site still holds substantial fossiliferous potential, which is why it requires future prospective efforts in the stratigraphic intervals across the La Venta site (Carrillo et al., 2023).

On the other hand, a substantial portion of the La Venta site is encompassed by the designation of a protected area called La Tatacoa Regional Integrated Management District (Distrito Regional de Manejo Integrado La Tatacoa) in charge of the regional environmental authority (Coorporación Autónoma Regional del Alto Magdalena - CAM). Within the management plan of this district, the paleontological richness of the area is recognized as one of the values to be conserved.

It is worth mentioning that in the proposed paleontological area there are no mining or petroleum titles that could jeopardize the integrity of its geological attributes. However, there are anthropic pressures on the site, mainly due to tourism and the increasing construction of hotels in the area. Anthropic activities such as these could have a significant impact not only on the landscape, but also directly on the integrity of the site due to degradation factors such as waste disposal, plundering and trafficking of paleontological material. Faced with these situations, the Colombian Geological Survey (Servicio Geológico Colombiano), the national authority in charge of the protection and conservation of Colombia's geological heritage, has been collaborating closely with local communities and authorities in the area to mitigate the impact of anthropogenic activities on the site. Initiatives include plans to integrate geological heritage as an environmental variable of the territory by incorporating the site's boundaries into land-use planning instruments and the protected area management plan already mentioned. Moreover, the entity has conducted workshops for local tour guides, aiming to raise awareness about the paleontological significance of the region. Collaborative efforts extend to involve other replicators of good conservation practices, such as local museum managers, to encourage the study of the fossil pieces that have already been collected and deter the extraction of additional paleontological material from the site.

Moreover, the Colombian Geological Survey is actively spearheading the designation of the La Venta site as a protected geological heritage zone. Consequently, it is facilitating the coordination of the associated Geological Management and Protection Plan. It is important to highlight that the complementary declaration of the La Venta site as a geological protection zone is considered timely, as the Colombian Geological Survey has detected that the current protection figure of the area in charge of the country's environmental authority has not proven sufficiently effective for the geological heritage conservation.

The effort to protect this area from a geological perspective has been a collaborative effort involving local communities. The relationships between geological heritage and administrative, local, and scientific communities are essential considerations that the Colombian Geological Survey has been exploring. The interaction between local and scientific knowledge is one of the most interesting conditions that gravitate around paleontological elements of the area and that, finally, has transformed them from features to paleontological heritage, as there is a relationship established between people, the stories and meanings that are built around them. This is finally shaped in a process of appropriation not only of scientific knowledge but also of the territory, and how communities build affective and identity bonds around it.

Thus, the Colombian Geological Survey plays a pivotal role in fostering and facilitating these intricate processes, ensuring a harmonious and mutually beneficial relationship between geological heritage and the diverse communities involved.

Comparison with other similar properties

The La Venta site reflects the life forms that inhabited a specific context in South America before the Great American Biotic Interchange (GABI), making it unique on a global scale. As a matter of fact, the La Venta fauna and flora document endemic fossil species, many of which lack present-day descendants.

Currently, of the 96 properties registered with the World Heritage List under criterion VIII, there are at least 11 properties related to paleontology. Of these, only one bears a close resemblance to La Venta: the Australian Fossil Mammal Sites (Riversleigh / Naracoorte). Although this site covers a greater temporal range than La Venta, its relevance is centered on the richness of the fossil record of mammals, while the La Venta site encompasses representatives not only of mammals but also of other taxonomic groups. Something similar happens with the Miocene primates paleontological site of Napak (Uganda), currently included in the International Union of Geological Sciences - IUGS Geological Heritage Sites. This site stands as the richest source of early Miocene well-preserved primates. However, once again the La Venta site excels due to its high palaeodiversity.

On the other hand, the Tentative List of UNESCO includes at least four (4) proposals with paleontological interest that align temporally with the fauna of La Venta. Among these, only one is also situated in South America: the Paleontological Sites of Pisco and Camana Basins in Peru. This site yields insights into a diverse fauna, encompassing vertebrates (fish, reptiles, birds and mammals), invertebrates (molluscs, crustaceans, annelids and echinoderms), and microfossils, spanning from the Eocene to the Upper Pliocene, a record that covers 40 Ma. In this regard, the geological window covered by the fossil record of the Pisco and Camana Basins is considerably broader than that of La Venta. The high fossil content of Pisco and Camana Basins confirms the existence of an ancient subtropical coastal ecosystem and represents a unique terrestrial fauna adapted to coastal conditions. On the contrary, the great abundance of fossils from La Venta provides evidence of an entirely distinct ecosystem: a purely continental environment with a tropical climate dominated by freshwater currents. Hence, the paleontological insights provided by the La Venta site complement the reconstruction of South America Miocene ecosystems and contribute significantly to our understanding of past life during that epoch.