Caspiconcha (ˌkæspiˈkɒŋkə) is an extinct genus of bivalve that lived in marine settings during the Late Mesozoic. The Caspiconcha molluscs lived in methane seeps in the deep ocean, for example near hydrocarbon seeps. Fauna living in these extreme environments commonly rely on a symbiotic relationship with chemosynthetic bacteria.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 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 Though not confirmed, Caspiconcha species are believed to have utilised chemosymbiosis like their relatives, by hosting the chemosynthetic bacteria in their gills. The finding sites of the Caspiconcha fossils in cold seeps, along with their close phylogenic relation to confirmed chemosymbiotic bivalves, are the main points of evidence for this.
Characterised by their large size relative to other bivalves of the Mesozoic, Caspiconcha shells are made up of two nacre valves, hinged together by muscles and ligament. They were typically modioliform (mussel-shaped) and could reach over 300 mm in length.
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.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 The genus name Caspiconcha was first described much later, by Kelly et al. in 2000. With this, the species declaration made by Gabb in 1869 was unaccepted, and his fossil findings were instead renamed.Bouchet, P. (2018). "Caspiconcha major (Gabb, 1869)". Molluscabase. https://www.marinespecies.org/molluscabase/aphia.php?p=taxdetails&id=1264536#sources Caspiconcha fossils were found in abundance in mudstones on Northeastern 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.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.
Recent findings Since the discovery of Caspiconcha fossils in 2000 and the addition of a type species, seven additional nominal species and one uncertain species of Caspiconcha have been found. These are Caspiconcha whithami (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.
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. Other than that, some bivalve specimens from USA were originally described as a distinct species, Myoconcha americana, by Stanton in 1895.Stanton, T.W. (1895). Bulletin of the United States Geological Survey no 133. Washington: Government Printing Office. pp. 14. In 1930, Stewart instead suggested this to be the juvenile version of C. major, at the time known as Myoconcha major.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. This statement has also been agreed on later, officially making M. americana and C. major different aged specimen of the same species.
Description Like other bivalves, the Caspiconcha are molluscs with two calcium carbonate shell valves, held together by adductor muscles and ligament. The two valves are dorsally hinged, meaning the hinge is at the top or backside of the animal.Gosling, E. (2015). Marine Bivalve Molluscs. Oxford, UK: Wiley Blackwell. pp. 14-16.
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.
Anatomy
The shell is generally wedge or barrel shaped (subtrapezoidal), with some specimen close to a modioliform, or typically "mussel shaped" shell. The shells also display age through comarginal growth rings (i.e. radial circles), though these appear to be irregular rather than annual growth rings, at least in some species. Shell sizes are measured in height from the hinge line to the margin, with its length perpendicular to it. The width is then measured from the floor to its highest side. The raised part of the valve, found externally above the hinge line, is called the umbo. The top part of the umbo is the beak, in which the growth lines connect.Kentucky Geological Survey. (n.d.). "Bivalve parts". Kentucky Geological Survey, Earth Resources. https://www.uky.edu/KGS/fossils/fossil-pelecypoda-Bivalves-parts.php (accessed 2025-11-22).
Among different Capsiconcha species, the anterior margin may be more rounded or more elongate. Prominent anterior muscle scars may be present. Like many other bivalves of the same time, Caspiconcha are known to have beaks close to the anterior side of their body, between the hinge line and the anterior adductor muscle scar. Internally, they also have hinge teeth below the hinge line. Based on their placement, the teeth may be categorised as cardinals or laterals, with the first type located closer to the hinge and beak of the valves.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
Palaeoecology Fossil records dating back to the 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. 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.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.
Chemosymbiotic relationships are mostly found around hydrothermal vents or cold seeps, and include macroanimals in symbiotic relationships with chemosynthetic bacteria. The chemolithotrophic bacteria utilise redox reactions of 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 surface being preferable to mineral surfaces. Especially important host animals are the large bivalves. During the late Jurassic and early Cretaceous, this ecological niche is believed to have been occupied by members of the Caspiconcha genus.Kiel, S. (2010). The Vent and Seep Biota: Aspects from Microbes to Ecosystems. Springer: Topics in Biology, vol 33. 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.
Evolution
While generally accepted that the major mollusc classes, including Bivalvia, stem from a common ancestor, making them a monophyletic group, the precise origin is not clear.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 Based on shell findings in the fossil record, the nacreous shell associated with many bivalves appears to have emerged multiple times in the early Paleozoic in different bivalve classes. 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.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.
The Paleozoic ended with the most critical mass extinction yet, the Permian mass extinction at 245 Ma, in which an estimated 96% of all marine species went extinct.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 One of the more strongly affected bivalve order is the Modiomorphida, of which Caspiconcha is included, although the specific phylogeny remains uncertain. After the Permian mass extinction, in the Early Triassic, the superfamily 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. 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.