# Caspiconcha

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***Caspiconcha*** (ˌkæspiˈkɒŋkə) is an extinct [genus](/source/Genus) of [bivalve](/source/Bivalve) that lived in marine settings during the Late [Mesozoic](/source/Mesozoic). The *Caspiconcha* [molluscs](/source/Molluscs) lived in methane seeps in the deep ocean, for example near hydrocarbon [seeps](/source/Seeps). [Fauna](/source/Fauna) living in these extreme environments commonly rely on a [symbiotic](/source/Symbiotic) relationship with [chemosynthetic](/source/Chemosynthetic) [bacteria](/source/Bacteria).[1][2] Though not confirmed, *Caspiconcha* species are believed to have utilised chemosymbiosis like their relatives, by hosting the chemosynthetic bacteria in their [gills](/source/Gills). The finding sites of the *Caspiconcha* fossils in cold seeps, along with their close [phylogenic](/source/Phylogenic) relation to confirmed chemosymbiotic bivalves, are the main points of evidence for this.[2][3]

Characterised by their large size relative to other bivalves of the Mesozoic, *Caspiconcha* shells are made up of two [nacre](/source/Nacre) valves, hinged together by muscles and ligament.[4] They were typically modioliform (mussel-shaped)[2] and could reach over 300 mm in length.[3]

## Discovery

Bivalves of the genus *Caspiconcha* were first described under the proposed genus name *Modiola*. The species *Modiola major*, later changed to *Caspiconcha major*, was found in the late 1800s in marine limestones in California, USA.[5] The genus name *Caspiconcha* was first described much later, by Kelly *et al.* in 2000.[1] With this, the species declaration made by Gabb in 1869 was unaccepted, and his fossil findings were instead renamed.[6] *Caspiconcha* fossils were found in abundance in [mudstones](/source/Mudstones) on Northeastern [Greenland](/source/Greenland), in an otherwise biologically sparse environment. The specimen reached more than 300 mm in length and up to 28 mm in thickness. The type species *C. whithami* was based on the Greenlandic findings.[3]

### Recent findings

Since the discovery of *Caspiconcha* fossils in 2000 and the addition of a type species, seven additional [nominal species](/source/Nominal_species) and one uncertain species of *Caspiconcha* have been found. These are *Caspiconcha whithami* ([type species](/source/Type_species)), *C. major*, *C. rubani*, *C. basquensis*, *C. raukumaraensis*, *C. lastsamurai*, *C. yubariensis*, and the uncertain *Myoconcha aff. transatlantica*. The four species *C. lastsamurai*, *C. basquensis*, *C. yubariensis*, and *C. raukumaraensis* were the most recently discovered, described first by Jenkins *et al*. in 2018.[1]

### Nominal species

These are the seven nominal species and their respective discoverers. When synonyms are present, these are listed.

- *C. whithami* (Kelly *et al*. 2000)
- *C. major* (Gabb 1869) Also known as Modiola major.
- *C. rubani* (Kiel *et al*. 2010)
- *C. basquensis* (Jenkins *et al*. 2018)
- *C. yurabiensis* (Jenkins *et al*. 2018)
- *C. lastsamurai* (Jenkins *et al*. 2018)
- *C. raukumaraensis* (Jenkins *et al*. 2018)

### Uncertain species

*M. aff transatlantica* is believed to be a species of *Caspiconcha*, due to its similar anatomy and living environment.[1] Other than that, some bivalve specimens from USA were originally described as a distinct species, *Myoconcha americana*, by Stanton in 1895.[7] In 1930, Stewart instead suggested this to be the juvenile version of *C. major*, at the time known as *Myoconcha major*.[8] This statement has also been agreed on later, officially making *M. americana* and *C. major* different aged specimen of the same species.[9]

## Description

Like other bivalves, the *Caspiconcha* are molluscs with two [calcium carbonate](/source/Calcium_carbonate) shell valves, held together by [adductor muscles](/source/Adductor_muscles_(bivalve)) and [ligament](/source/Ligament). The two valves are [dorsally](/source/Dorsally) hinged, meaning the hinge is at the top or backside of the animal.[4]

Specimens of *Caspiconcha* are generally large compared to other bivalve genera found along the same formations. As fossils, they are generally facing upwards with their posterior. Finds suggest that they were byssally attached to rocks and other surfaces on the seafloor, much like modern mussels.[3]

### Anatomy

Further information: [Bivalves](/source/Bivalves) and [Bivalve shell](/source/Bivalve_shell)

The shell is generally wedge or barrel shaped (subtrapezoidal), with some specimen close to a modioliform, or typically "mussel shaped" shell.[2] The shells also display age through comarginal growth rings (i.e. radial circles), though these appear to be irregular rather than annual [growth rings](/source/Growth_rings), at least in some species.[1] Shell sizes are measured in height from the [hinge](/source/Hinge) line to the margin, with its length perpendicular to it. The width is then measured from the floor to its highest side.[10] The raised part of the valve, found externally above the hinge line, is called the [umbo](/source/Umbo_(bivalve)). The top part of the umbo is the [beak](/source/Beak), in which the growth lines connect.[11]

Among different *Capsiconcha* species, the [anterior](/source/Anterior) margin may be more rounded or more elongate. Prominent anterior muscle scars may be present.[1] Like many other bivalves of the same time, *Caspiconcha* are known to have [beaks](/source/Beaks) close to the anterior side of their body, between the hinge line and the anterior adductor muscle scar.[3] Internally, they also have hinge teeth below the hinge line. Based on their placement, the teeth may be categorised as [cardinals](/source/Cardinal_(anatomy)) or [laterals](/source/Lateral_(anatomy)), with the first type located closer to the hinge and beak of the valves.[12]

### Palaeoecology

Fossil records dating back to the [Silurian](/source/Silurian), around 444 to 419 million years ago (Ma), include molluscs in chemosynthetic environments. The diversity and abundance of chemosymbiotic molluscs greatly increased during the middle Cenozoic, especially from the Jurassic.[2] One of the major animal classes known to inhabit the otherwise hostile hydrothermal vents and seeps is the mollusc class Bivalvia, or bivalves. Several different bivalve families are known to depend on chemosymbiosis, to different degrees. Some families greatly reduced their gut, suggesting they relied exclusively on the symbiotic relationship, while others used a dual approach to nutrition.[13]

Chemosymbiotic relationships are mostly found around [hydrothermal vents](/source/Hydrothermal_vents) or cold [seeps](/source/Seeps), and include macroanimals in symbiotic relationships with chemosynthetic bacteria. The [chemolithotrophic](/source/Chemolithotrophic) bacteria utilise [redox reactions](/source/Redox_reactions) of [hydrogen sulfide](/source/Hydrogen_sulfide) released from underground vents and volcanoes. The inverterbrate benefits from receiving energy from the bacteria, which in turn makes use of the [invertebrate](/source/Invertebrate) surface being preferable to [mineral](/source/Mineral) surfaces. Especially important host animals are the large bivalves. During the late Jurassic and early Cretaceous, this [ecological niche](/source/Ecological_niche) is believed to have been occupied by members of the *Caspiconcha* genus.[14][2] While not yet proven, the *Caspiconcha* genus belongs to the bivalve family Kalenteridae, in which other genera are confirmed to have relied on chemosymbiosis in hydrothermal and seep environments.[13]

## Evolution

Further information: [Animalia](/source/Animalia) and [Molluscs](/source/Molluscs)

While generally accepted that the major mollusc classes, including Bivalvia, stem from a common ancestor, making them a [monophyletic](/source/Monophyletic) group, the precise origin is not clear.[15] Based on shell findings in the fossil record, the [nacreous](/source/Nacreous) shell associated with many bivalves appears to have emerged multiple times in the early Paleozoic in different bivalve classes.[4][15] This suggests a complex and diverse evolution of the bivalves in the Cambrian. The bivalvian family Kalenteridae, of which *Caspiconcha* is a genus, saw great diversity and abundance through much of the Paleozoic.[16]

The Paleozoic ended with the most critical [mass extinction](/source/Mass_extinction) yet, the [Permian mass extinction](/source/Permian_mass_extinction) at 245 Ma, in which an estimated 96% of all marine species went extinct.[17] One of the more strongly affected bivalve order is the [Modiomorphida](/source/Modiomorphida), of which *Caspiconcha* is included, although the specific [phylogeny](/source/Phylogeny) remains uncertain. After the Permian mass extinction, in the Early [Triassic](/source/Triassic), the superfamily [Kalenteroidea](/source/Kalenteroidea) may have been the only Modiomorphida survivor. The now extinct *Caspiconcha* family Kalenteridae went on to develop morphological features suggesting a byssally attached [habitus](/source/Habit_(biology)). Overall, the family had reached peak Mesozoic diversity during the late Triassic, though diversity was still low. With likely only one surviving, the family struggled to diversify within the remainder of the Mesozoic, and most likely went extinct by the end of the Cretaceous.[18]

Prior to the extinction at the end of the Cretaceous, members of the family began inhabiting vent and seep habitats, and developed chemosymbiotic relationships for survival. Due to the hostile nature of the hydrothermal vents and cold hydrocarbon seeps, the [endemic](/source/Endemic) fauna is very specialised. Certain adaptations are required, and it seems likely that these [taxa](/source/Taxa) diverged early on and evolved separately from their closest non-seep living relatives. Early animals may have lived on continental slope environments near vents and seeps, and with time moved downslope to eventually inhabit the seep or vent itself. Due to the harsh toxic environment, few [predators](/source/Predators) are present, and it would have offered the molluscs shelter.[19] It has also been stated that vent and seep fauna have not been as vulnerable to mass extinction events as animals occupying other habitats. The resilience stems mainly from their self-reliance, lack of need for photosynthesis, and in the case of vent fauna, their special adaptations to hot and toxic conditions.[19][20]

The earliest living *Caspiconcha* species is believed to be *C. major*, first discovered in the eastern [Pacific Ocean](/source/Pacific_Ocean) of the late Jurassic. The Early Cretaceous saw the emergence of *C. rubani* in the Tethys Ocean. During later Early Cretaceous, additional species evolved, and the peak diversity of *Caspiconcha* was reached in the late Early to Middle Cretaceous, after which the genus began to decline in diversity.[1]

### Distribution

*Caspiconcha* specimen have been found in the [fossil record](/source/Fossil_record) in the Atlantic Ocean, Pacific Ocean, and prehistoric [Tethys Ocean](/source/Tethys_Ocean), and in both the northern and southern hemispheres. Notable finding sites include East Greenland, the [Basque country](/source/Basque_Country_(greater_region)) in northern Spain, the Raukumara peninsula of New Zealand, [Hokkaido](/source/Hokkaido) in northern Japan, and southeast Ukraine.[9]

During the latter half of the Mesozoic, the [supercontinent](/source/Supercontinent) [Pangaea](/source/Pangaea) was breaking apart, beginning to form the modern continents and oceans. One of these major changes was the formation of the [Atlantic Ocean](/source/Atlantic_Ocean), one of the finding locals for *Caspiconcha* fossils.

## Classification

The *Caspiconcha* genus is generally accepted as belonging to the [Kalenteridae](/source/Kalenteridae) family within the superfamily [Kalenteriodae](/source/Kalenteriodae). On higher levels in the phylogeny, some place them within the [Carditida](/source/Carditida) order, while other sources use the [Imparidentia](/source/Imparidentia) order. Yet others prefer the [Modiomorphia](/source/Modiomorphia) order.[1][6][9] Below is a suggested [phylogenetic tree](/source/Phylogenetic_tree) based on multiple sources.

## See also

- [History of life](/source/History_of_life)
- [World Register of Marine Species](/source/World_Register_of_Marine_Species)
- [Molluscs](/source/Molluscs)
- [Bivalve](/source/Bivalve)
- [Heteroconchia](/source/Heteroconchia)

## References

1. Jenkins, R.G., Kaim, A., Hikida, Y. & Kiel, S. (2018). "Four new species of the Jurassic to Cretaceous seep-restricted bivalve Caspiconcha and implications for the history of chemosynthetic communities". *Journal of Paleontology*, 92(4). DOI: [https://doi.org/10.1017/jpa.2018.7](https://doi.org/10.1017/jpa.2018.7)

1. Jenkins, R.G., Kaim, A., Little, C.T.S., Iba, Y., Tanabe, K. & Campbell, K.A. (2013). "Worldwide distribution of the modio− morphid bivalve genus Caspiconcha in late Mesozoic hydrocarbon seeps". *Acta Palaeontologica Polonica* 58(2). DOI: [https://doi.org//10.4202/app.2011.0026](https://doi.org//10.4202/app.2011.0026)

1. Kelly, S.R.A., Blanc, E., Price, S.P. & Whitham, A.G. (2000). "Early Cretaceous giant bivalves from seep-related limestone mounds, Wollaston Forland, Northeast Greenland". In: Harper, E.M., Taylor, J.D. & Crame, J.A. (2000). *The Evolutionary Biology of the Bivalvia*. London, UK: The Geological Society.

1. Gosling, E. (2015). *Marine Bivalve Molluscs*. Oxford, UK: Wiley Blackwell. pp. 14-16.

1. Gabb, W.M. (1869). *Geological Survey of California. Palaeontology*. Philadelphia: Caxton Press of Sherman & co.J. Doi: [https://doi.org/10.5962/bhl.title.2131](https://doi.org/10.5962/bhl.title.2131)

1. Bouchet, P. (2018). "*Caspiconcha major* (Gabb, 1869)". *Molluscabase*. [https://www.marinespecies.org/molluscabase/aphia.php?p=taxdetails&id=1264536#sources](https://www.marinespecies.org/molluscabase/aphia.php?p=taxdetails&id=1264536#sources)

1. Stanton, T.W. (1895). *Bulletin of the United States Geological Survey no 133*. Washington: Government Printing Office. pp. 14.

1. Stewart, R.B. (1930). *Gabb's California Cretaceous and Tertiary Type Lamellibranchs*. Special Publication No. 3, The Academy of Natural Sciences of Philadelphia. pp. 104.

1. Kiel, S., Campbell, K.A. & Gaillard, C. (2010). "New and little known mollusks from ancient chemosynthetic environments". *Zootaxa*, 2390(2010).

1. Kentucky Geological Survey. (n.d.). "Bivalve parts". *Kentucky Geological Survey, Earth Resources*. [https://www.uky.edu/KGS/fossils/fossil-pelecypoda-Bivalves-parts.php](https://www.uky.edu/KGS/fossils/fossil-pelecypoda-Bivalves-parts.php) (accessed 2025-11-22).

1. Dall, W.H. (1901). "The morphology of the hinge teeth of bivalves". *The American Naturalist*, 35(411), pp. 175-182. URL: [https://www.jstor.org/stable/2454330](https://www.jstor.org/stable/2454330)

1. Amano, K., Kiel, S., Hryniewicz, K. & Jenkins, R.G. (2022). "Bivalvia in ancient hydrocarbon seeps". In: Kaim, A., Landman, N.H. & Cochran, J.K (eds). *Ancient Hydrocarbon Seeps*. Berlin: Springer, pp. 267-321.

1. Kiel, S. (2010). *The Vent and Seep Biota: Aspects from Microbes to Ecosystems*. Springer: Topics in Biology, vol 33.

1. Jackson, D.J., McDougall, C., Woodcroft. B., Moase, P., Rose, R.A., Kube, M., Reinhardt, R., Rohjsar, D.S., Montagnani, C., Joubert, C., ´Piquemal, D. & Degnan, B.M. (2009). "Parallell Evolution of Nacre Building Gene Sets in Molluscs". *Molecular Biology and Evolution*, 27(3), pp. 591-608. Doi: [https://doi.org/10.1093/molbev/msp278](https://doi.org/10.1093/molbev/msp278)

1. Damborenea, S.E. (2004). "Early Jurassic Kalentera (Bivalvia) from Argentina and its palaeobiogeographical significance". *Revista de la Asociación Paleontológica Argentina*, 41(2), pp. 185-189.

1. Erwin, D.H. (1990). "The End-Permian Mass Extinction". *Annual Review of Ecology and Systematics*, 21(1990), pp. 69-91. Doi: [https://www.jstor.org/stable/2097019](https://www.jstor.org/stable/2097019)

1. Tunnicliffe, V. (1992). "The Nature and Origin of the Modern Hydrothermal Vent Fauna". *PALAIOS*, 7(4), pp. 338-350. Doi: [https://doi.org/10.2307/3514820](https://doi.org/10.2307/3514820)

1. Kiel, S. & Little, C.T.S. (2006). "Cold-Seep Mollusks Are Older Than the General Marine Mollusk Fauna". *Science*, 313(2006).

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Adapted from the Wikipedia article [Caspiconcha](https://en.wikipedia.org/wiki/Caspiconcha) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Caspiconcha?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.