Photograph of fossil bivalve shells.

Bivalve Phylogeny and Classification

Overview

Efforts at bivalve classification and phylogenetic analysis have traditionally been based on prominent morphological features such as gill structure, stomach morphology, hinge teeth, siphons, etc. More recently, molecular data have begun to play a much more important role. This is a very active field of research today and hypotheses are still somewhat in flux, although the major features of the Tree of Life for bivalves have stabilized in recent years. A particular challenge is combining fossil and living taxa in a single phylogeny, because of the distinctive morphologies of many fossil groups and the large number of missing characters in fossil taxa (no soft bodies or molecules). The scheme used here is based on analyses of morphology and molecular data by Bieler et al. (2014), Combosch et al. (2017), and Ponder et al. (2020).


Image showing a phylogeny of bivalves.

Simplified phylogeny of Recent bivalves (based on Bieler et al., 2014).

Subclass Protobranchia

Nut Clams and other deposit-feeding bivalves with a wide, frilly “plantar” foot for shallow “plowing” through soft sediment; highest diversity in the deep sea; includes Nucula with unfused mantle margins, internally nacreous, taxodont teeth, smooth shells, enlarged labial palps with palp proboscides for deposit feeding, gills for respiration only.

Orders (with additional resources):


Examples of Protobranchia:


Photographs of Nuculana acuta.

Order Nuculanida. Nuculana acuta from the Early Pleistocene Waccamaw Fm. of Brunswick County, North Carolina (PRI 70436). Image from the Neogene Atlas.

Photographs of Yoldia laevis.

Order Nuculanida. Yoldia laevis from the Late Pliocene Yorktown Fm. of Suffolk County, Virginia (SDSM 136153). Image from the Neogene Atlas.

Photographs of Nucula proxima.

Order Nuculida. Nucula proxima from the Early Pleistocene Waccamaw Fm. of Brunswick County, North Carolina (PRI 70425). Image from the Neogene Atlas.

Photographs of Solemya bilix.

Order Solemyida. Solemya bilix from the Pierre Formation of Meade County, South Dakota (FHSMIP 1336). Image from the Cretaceous Atlas.

Subclass Autobranchia

Bivalves with gills in a W-shaped configuration on each side of the visceral mass, for respiration and feeding.

Infraclass Pteriomorphia

Mussels, Scallops, Pearl Oysters, and other non-burrowing bivalves with unfused mantle margins, large filter-feeding gills, and reduced foot. Most are monomyarian and byssate.


Pteriomorphia Example 1: Oysters (Ostrea)

Chalky, irregular shell; cementing.


Fossil specimen of the oyster Ostrea sp., purportedly from the Miocene of Spain (PRI 70602). Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Maximum length of specimen is approximately 11.5 cm (Sketchfab; CC0 public domain).


Photograph of a giant oyster.

Giant oyster (Ostrea sp.) from the Cretaceous of Peru (PRI 49890).


Pteriomorphia Example 2: Scallop (Argopecten)

Almost round, flat valves with dorsal auricles (“ears”); pallial veil with multiple pallial eyes and sensory pallial tentacles; ability to swim by clapping valves together.


Fossil specimen of the bivalve scallop Argopecten gibbus from the Neogene (Pliocene) of Sarasota County, Florida (PRI 50125). Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Width of specimen is approximately 6 cm (Sketchfab; CC0 public domain). Click here for a view showing both valves together.


Image showing an engraving of the fossil scallop Chesapecten jeffersonius, and a photograph of a specimen of the same species.
A scallop shell, Chesapecten jeffersonius, was the first American fossil to be scientifically illustrated: an engraving of this fossil was published in 1687 in Volume 3 of Historiae Conchyliorum by Martin Lister (see left image above; learn more here). The shell on the right is a specimen of Chesapecten jeffersonius from the Plio-Pleistocene of Sarasota County, Florida (UF 31888); image is from the Neogene Atlas.

"Swimming Bay Scallops Homosassa Florida" by Brian Dombrowski (YouTube).


Orders of Pteriomorphia (with additional resources):


Relationships of Pteriomorphia


Image showing the phylogeny of pteriomorph bivalves.

Simplified phylogeny of Recent pteriomorph bivalves (based on Combosch et al., 2017).


Additional Examples of Pteriomorphia


Photographs of Anadara scalarina.

Order Arcida. Anadara scalarina from the Early Pleistocene Caloosahatchee Fm. of Okeechobee County, Florida (UF 116412). Image from the Neogene Atlas.

Photographs of Glycymeris americana.

Order Arcida. Glycymeris americana from the Early Pleistocene Nashua Fm. of St. Johns County, Florida (UF 216801). Image from the Neogene Atlas.

Photographs of Limaria caloosana.

Order Limida. Limaria caloosana from the Plio-Pleistocene (formation unknown) of Sarasota County, Florida (UF 34158). Image from the Neogene Atlas.

Photograph of Inoceramus barabini.

Order Myalinida. Inoceramus barabini from the Pierre Shale of Niobrara County, Wyoming (YPM 35475). Image from the Neogene Atlas.

Photographs of Perna conradiana.

Order Mytilida. Perna conradiana from the Late Pliocene Tamiami Fm. (Pinecrest Beds) of Sarasota County, Florida (PRI 70164). Image from the Neogene Atlas.

Photographs of Ostrea locklini.

Order Ostreida. Ostrea locklini from the Late Pliocene Tamiami Fm. (Pinecrest Beds) of Okeechobee County, Florida (UF 214103). Image from the Neogene Atlas.

Photographs of Interchlamys interlineata.

Order Pectinida. Interchlamys interlineata from the Late Pliocene Tamiami Fm. of Charlotte County, Florida (UF 39482). Image from the Neogene Atlas.

Photographs of Spondylus chipolanus.

Order Pectinida. Spondylus chipolanus from the Early Miocene Chipola Fm. of Calhoun County, Florida (UF 14050). Image from the Neogene Atlas.

Infraclass Heteroconchia, Cohort Paleoheterodonta

Freshwater Mussels and relatives. Shells thickly nacreous and aragonitic, with few hinge teeth.


Paleoheterodonta Example 1: Trigoniids

Trigoniids were diverse and widespread in the Mesozoic, but are today limited to a single genus, Neotrigonia, which is found only around Australia. Neotrigonia is considered a living fossil; learn more here.


Fossil specimen of the bivalve Trigonia costata from the Jurassic of Germany (PRI 55092). Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Longest dimension of specimen is approximately 8 cm (Sketchfab; CC0 public domain).


Paleoheterodonta Example 2: Freshwater Mussels

The exteriors of freshwater mussels are brown or yellow and have a thick, glossy periostracum. The inner surfaces of their shells are nacreous (a source of natural and cultured pearls, and pearl buttons historically) and are edentulous or feature heavy hinge teeth. Freshwater mussels have complex life cycles with parasitic (on fish) glochidia larvae.


Photographs of Lampsilis radiata.

Valve of the freshwater mussel Lampsilis radiata. Specimen is from Lake Kenka, N.Y. (PRI 70362).


Orders of Paleoheterodonta (with additional resources):


Additional Examples of Paleoheterodonta


Photographs of Trigonia clavigera.
Order Trigoniida. Trigonia clavigera from the Late Cretaceous Main Street Limestone Fm. of Denton County, Texas (UNM 5884). Image from the Cretaceous Atlas.
Photograph of Schizodus rossicus.

Order Trigoniida. Schizodus rossicus from the Coal Measures of Jackson County, Missouri (KUMIP 215378). Image from the Pennsylvanian Atlas.

Infraclass Heteroconchia, Cohort Heterodonta

The two subcohorts of this group are divided largely on the basis of molecular studies. Heterodonts generally have equally sized valves, heterodont hinges (few cardinal teeth, long lateral teeth), and lack nacre.

Subcohort Archiheterodonta

“Primitive Heterodonts.” Archiheterodonts are all marine and lack a siphon. This group consists of numerous families of extinct actinodonts plus the Recent order Carditida. Carditids include the diverse and abundant marine families Carditidae, Crassatellidae, and Astartidae.


Orders of Archiheterodonta (with additional resources):


Examples of Archiheterodonta


Photographs of Crassatella vadosa.

Order Carditida (Family Crassatellidae). Crassatella vadosa from the Maastrichtian Owl Creek Fm. of Tippah County, Mississippi (UNM 14864). Image from the Cretaceous Atlas.

Photographs of Eucrassatella speciosa.
Order Carditida (Family Crassatellidae). Eucrassatella speciosa from the Late Pliocene Tamiami Fm. (Pinecrest Beds) of Sarasota County, Florida (PRI 70076). Image from the Neogene Atlas.
Photographs of Astarte concentrica.
Order Carditida (Family Astartidae). Astarte concentrica from the Early Pleistocene James City Fm. of Beaufort County, North Carolina (PRI 69878). Image from the Neogene Atlas.
Photographs of Carditamera arata.
Order Carditida (Family Carditidae). Carditamera arata from the Late Plicoene Tamiami Fm. (Pinecrest Beds) of Sarasota County, Florida (UF 207359). Image from the Neogene Atlas.

Subcohort Euheterodonta

“True Heterodonts.” This hyperdiverse group includes all remaining bivalves. Most have a siphon.


Image showing a phylogeny of euheterodont bivalves.

Simplified phylogeny of Recent euheterodont bivalves (based on Combosch et al., 2017).

Megaorder Anomalodesmata

Highly specialized bivalves lacking hinge teeth (or with secondarily derived teeth), mostly-fused mantle, periostracum with calcareous spicules, and prismatonacreous shells. Clade includes the carnivorous bivalve genus Poromya, which has very reduced “septibranch” gills and a septum that creates a pump to suck prey into an enlarged, eversible incurrent siphon; a reduced, cuticle-lined stomach is modified for crushing thin-shelled prey.


Photographs of Poromya houbricki.

Holotype of Poromya houbricki Bernard, 1989 (USNM 222378). Image from the Smithsonian Institution (GBIF; Creative Commons 0 license).


Orders of Anomalodesmata (with additional resources):


Examples of Anomalodesmata


Photograph of Goniomya americana.

Order Pholadomyida. Goniomya americana from the Fox Hills Formation of Dewey County, South Dakota (YPM 24148). Image from the Cretaceous Atlas.

Photograph of Chaenomya.

Order Pholadomyida. Chaenomya sp. from the Drum Limestone of Montgomery County, Kansas (KUMIP 215036). Image from the Pennsylvanian Atlas.

Photograph of Sanguinolites.

Order Pholadomyida. Sanguinolites sp. from the Keechi Creek Fm. of Palo Pinto County, Texas (KUMIP 260902). Image from the Pennsylvanian Atlas.

Photograph of Cardiomya anaticepsella.

Order Poromyida. Cardiomya anaticepsella from the Eocene Keasey Fm. of Oregon. Length is 10.5 mm. Image from Hickman (2014), provided courtesy of Carole Hickman.

Photograph of Cardiomya pectinata.

Order Poromyida. Cardiomya pectinata from the Recent of Whatcom Bellingham Bay, Pt. Frances (Portage Is.), Washington. Image is from the Florida Museum of Natural History Division of Invertebrate Zoology (GBIF; Creative Commons Attribution-NonCommercial 4.0 International license; image slightly widened).

Megaorder Imparidentia

The largest group of living bivalves, most of which are infaunal.


Imparidentia Example 1: Mercenaria.

See the first page of this chapter to learn more about the morphology of Mercenaria.


Fossil specimen of the bivalve Mercenaria mercenaria from the Quaternary of St. Mary’s County, Maryland (PRI 76728). Specimen is from the collections of the Paleontological Research Institution, Ithaca, New York. Width of specimen is approximately 10 cm. Click here for a view showing both valves together. Model from the Digital Atlas of Ancient Life (Sketchfab; Creative Commons 0 license).


Imparidentia Example 2: Teredo (Shipworm)

Bivalves in the genus Teredo have a long, worm-like body and live within a secreted calcareous tube in wood; they have a small shell modified for rasping and are one of the few invertebrates that are able to digest cellulose.


Photograph of the shipworm Teredo clappi removed from its tube.

The shipworm Teredo clappi removed from its tube. Image is from the Museum of Comparative Zoology, Harvard University (GBIF; Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported license; image trimmed).


Photograph of the tube of Teredo navilis.

Tube of Teredo navalis from Spain, Canarias Lanzarote, Puerto del Carmen; NMR 37462. Total length of tube 200 mm. Image by Joop Trausel and Frans Slieker, Collection of the Natural History Museum of Rotterdam (GBIF; Creative Commons Attribution 4.0 International license; image dimensions changed and label transcribed here).

Photograph of Teredo borings in wood.

Teredo borings in wood. Image by Terje O. Nordvik (GBIF; Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International license; image trimmed).


Photograph of wood bored by Teredo.

Wood bored by Teredo sp. from the Pliocene of San Benito Co., California (PRI K21009).


Individuals of the trace fossil Teredolites, which are borings made in fossil wood by Teredo (PRI 61842); specimen is from the Paleocene Lopez de Bertodano Fm. of Seymour Island, Antarctica. Fossil is part of the research collections of the Paleontological Research Institution, Ithaca, New York. Length of rock is approximately 16.5 cm. Model from the Digital Atlas of Ancient Life (Sketchfab; Creative Commons 0 license).


Imparidentia Example 3: Family Galeommatidae

The family Galeommatidae includes many species that are commensal with other invertebrates. Members of the family have a mantle that envelops an internalized shell and that is adorned with sensory papillae and tentacles. Members also have a crawling foot and many brood larvae in their gills.


Photograph of Scintilla violescens.

Scintilla violescens, a member of the family Galeommatidae. Specimen is from Hong Kong. Photograph by the Florida Museum of Natural History Division of Invertebrate Zoology (GBIF; Creative Commons Attribution-NonCommercial 4.0 International license).


Imparidentia Example 4: Rudists (Extinct)

Rudist bivalves are extinct (Jurassic-Cretaceous) bivalves with inequally-sized valves that formed a tube and cap. They were important reef-building animals during the Cretaceous, temporarily filling a niche that had instead been occupied by corals during much of the Phanerozoic.


Photographs of the rudists Hippurites radiosus and Dictyoptychus persica.

Rudist bivalves. Left: Hippurites radiosus from the Late Cretaceous of France. Specimen in the collection of the National Museum of Natural History, Smithsonian Institution. Specimen is about 10 cm tall. Photo by Warren Allmon. Right: Dictyoptychus persica from the Late Cretaceous of Iran (PRI 41794). Photo courtesy of Warren Allmon.


Fossil specimen of the rudist bivalve Hippurites radiosus from the Cretaceous of Dordogne, France (PRI 80129). Specimen is from the research collections of the Paleontological Research Institution, Ithaca, New York. Length of specimen is 21 cm. Model by Digital Atlas of Ancient Life (Sketchfab; Creative Commons 0 license).


Illustration showing the parts of a rudist shell.

Diagram of rudist morphology, based on the genus Radiolites. The left (free) valve is shown above the right (attached) valve. Drawing by Christi Sobel (based on Skelton, 1979).


Illustration showing the three rudist morphotypes.

The three rudist morphotypes: left, elevator; middle, clinger; and right, recumbent. Drawing by Christi Sobel (based on Gili and Götz, 2018).


Photographs of rudist fossils preserved in airport tiles.

Airport paleontology: Rudists in Cretaceous limestone tile flooring in the airport at Orlando, Florida. Photos by Warren Allmon.


Orders of Imparidentia (with additional resources):


Additional Examples of Imparidentia


Photographs of Lucina pensylvanica.
Order Lucinida (Family Lucinidae). Lucina pensylvanica from the Early Pleistocene Caloosahatchee Fm. of Glades County, Florida (UF 145123). Image from the Neogene Atlas.
Photograph of Pseudomiltha megameris.

Order Lucinida (Family Lucinidae). Pseudomiltha megameris from the Eocene of Jamaica (PRI 49883). This internal mold represents one of the largest known members of this family.

Photograph of living Tridacna.

Order Cariida (Family Cardiidae). Live individual of the giant clam Tridacna sp. on display in an aquarium. Photograph by Jonathan R. Hendricks.

Photograph of Tridacna gigas shell.

Order Cariida (Family Cardiidae). Recent specimen of the giant clam Tridacna gigas from Bikini Atoll on display at the Museum of the Earth, Ithaca, New York.

Photographs of Arcinella cornuta.

Order Venerida (Family Chamidae). Arcinella cornuta from the Late Pliocene Tamiami Fm. (Pinecrest Beds) of Sarasota County, Florida (UF 217954). Image from the Neogene Atlas.

Photographs of Chione burnsii.
Order Venerida (Family Veneridae). Chione burnsii from the Early Miocene Chipola Fm. of Calhoun County, Florida (UF 73140). Image from the Neogene Atlas.