The Truckee Formation is a

lacustrine A lake is often a naturally occurring, relatively large and fixed body of water on or near the Earth's surface. It is localized in a basin or interconnected basins surrounded by dry land. Lakes lie completely on land and are separate from t ...

geologic formation A geological formation, or simply formation, is a body of rock having a consistent set of physical characteristics ( lithology) that distinguishes it from adjacent bodies of rock, and which occupies a particular position in the layers of rock exp ...

Nevada Nevada ( ; ) is a landlocked state in the Western United States. It borders Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the seventh-most extensive, th ...

, USA. It preserves

fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

dating back to the late

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

. Part of the

Lahontan Basin Lake Lahontan was a large endorheic prehistoric lake during the Pleistocene that occupied modern northwestern Nevada and extended into northeastern California and southern Oregon. The area of the former lake is a large portion of the Great Basi ...

, it was deposited in the prehistoric Lake Truckee, a predecessor to

Lake Lahontan Lake Lahontan was a large endorheic prehistoric lake during the Pleistocene that occupied modern northwestern Nevada and extended into northeastern California and southern Oregon. The area of the former lake is a large portion of the Great Basi ...

Paleoecology

The formation is notable for its preservation of articulated vertebrate skeletons in

diatomite Diatomaceous earth ( ), also known as diatomite ( ), celite, or kieselguhr, is a naturally occurring, soft, siliceous sedimentary rock that can be crumbled into a fine white to off-white powder. It has a particle size ranging from more than 3 ...

, which is mined near

Hazen, Nevada Hazen is an unincorporated community in Churchill County, Nevada, United States. The community is approximately southeast of Fernley and northwest of Fallon, on U.S. Route 50 Alternate. History Hazen was founded in 1903 as a station on th ...

. The diatomite deposition, which would have occurred seasonally, provides a snapshot of the ecological dynamics that would have occurred in the lake ecosystem every year. Despite the exquisite preservation, only a total of eleven vertebrate taxa are known from the diatomite beds. However, the two most abundant vertebrate taxa of the formation provide crucial data about eco-evolutionary dynamics within a prehistoric ecosystem. Among the most famous taxa from the formation is the

stickleback The sticklebacks are a family of ray-finned fishes, the Gasterosteidae which have a Holarctic distribution in fresh, brackish and marine waters. They were thought to be related to the pipefish and seahorses but are now thought to be more close ...

Gasterosteus doryssus ''Gasterosteus doryssus'' is an extinct species of freshwater stickleback fish that inhabited inland freshwater habitats of the North American Great Basin during the Miocene. It is known from thousands of articulated fossil skeletons, comprising ...

'', which provides the majority of fossil fish specimens from the formation. Fossil specimens of this stickleback form a

time series In mathematics, a time series is a series of data points indexed (or listed or graphed) in time order. Most commonly, a time series is a sequence taken at successive equally spaced points in time. Thus it is a sequence of discrete-time data. ...

spanning many generations over thousands of years, in which they can be observed developing a number of adaptations to their lake environment and diverging into two distinct

ecotypes Ecotypes are organisms which belong to the same species but possess different phenotypical features as a result of environmental factors such as elevation, climate and predation. Ecotypes can be seen in wide geographical distributions and may event ...

, exemplifying the rapid evolution that their modern relative, the

three-spined stickleback The three-spined stickleback (''Gasterosteus aculeatus'') is a fish native to most inland and coastal waters north of 30°N. It has long been a subject of scientific study for many reasons. It shows great morphological variation throughout its ra ...

, is well known for. The other most frequent taxon in the formation is the

killifish A killifish is any of various oviparous (egg-laying) cyprinodontiform fish, including families Aplocheilidae, Pantanodontidae, Cyprinodontidae, Fundulidae, Nothobranchiidae, Profundulidae, Aphaniidae and Valenciidae. All together, there ar ...

Fundulus nevadensis ''Fundulus'' is a genus of ray-finned fishes in the superfamily Funduloidea, family Fundulidae (of which it is the type genus). It belongs to the order of toothcarps (Cyprinodontiformes), and therein the large suborder Cyprinodontoidei. Most of ...

''; despite its abundance, they are very rare up until the youngest sections, very few of its fossils co-occur with sticklebacks, and when the killifish do become abundant in the fossil record, the sticklebacks essentially disappear from the lake within a period of less than 50 years. This suggests that following their colonization of the lake, the killifish may have driven the sticklebacks to extinction on a rapid timescale comparable to modern

biological invasions ''Biological Invasions'' is a peer-reviewed scientific journal on invasion biology published by Springer Science+Business Media. Abstracting and indexing The journal is abstracted and indexed in Scopus and the Science Citation Index Expanded. Ac ...

. Uniquely, the paleofauna of the formation shares many elements with the native freshwater fauna found in lowlands along the

Pacific Coast Pacific coast may be used to reference any coastline that borders the Pacific Ocean. Geography Americas North America Countries on the western side of North America have a Pacific coast as their western or south-western border. One of th ...

. It is thought that the

Sierra Nevada Mountains The Sierra Nevada ( ) is a mountain range in the Western United States, between the Central Valley of California and the Great Basin. The vast majority of the range lies in the state of California, although the Carson Range spur lies primarily ...

were lower when the formation was deposited, allowing for connectivity between the Pacific lowlands and the rivers draining out of Lake Truckee, and in turn allowing for fishes from these habitats to colonize the lake. The rise of the mountains would have eventually cut off this water source, drying out the rivers and driving the ancestors of these fish to extinction. The exception is the

catfish Catfish (or catfishes; order (biology), order Siluriformes or Nematognathi) are a diverse group of ray-finned fish. Catfish are common name, named for their prominent barbel (anatomy), barbels, which resemble a cat's whiskers, though not ...

'' Ameiurus hazenensis'', which belongs to a lineage found in eastern North America; it is thought that Lake Truckee also had some connectivity with the

Mississippi Basin The Mississippi River is the primary river of the largest drainage basin in the United States. It is the second-longest river in the United States, behind only the Missouri. From its traditional source of Lake Itasca in northern Minnesota, ...

, explaining the catfish's presence there.

Vertebrate paleobiota

Based on Cerasconi ''et al.'', 2024:

Ray-finned fish

Amphibians

Reptiles

Birds

Mammals

References

* {{cite web, title= Fossilworks: Gateway to the Paleobiology Database, author= ((Various Contributors to the Paleobiology Database)), url= https://www.fossilworks.org, access-date= 17 December 2021 Neogene geology of Nevada Miocene United States Tortonian Lahontan Basin Lacustrine deposits Geologic formations of Nevada