Worldwide distribution of the modiomorphid bivalve
genus Caspiconcha in late Mesozoic hydrocarbon seeps
ROBERT G. JENKINS, ANDRZEJ KAIM, CRISPIN T.S. LITTLE, YASUHIRO IBA,
KAZUSHIGE TANABE, and KATHLEEN A. CAMPBELL
Jenkins, R.G., Kaim, A., Little, C.T.S., Iba, Y., Tanabe, K., and Campbell, K.A. 2013. Worldwide distribution of the modio−
morphid bivalve genus Caspiconcha in late Mesozoic hydrocarbon seeps. Acta Palaeontologica Polonica 58 (2): 357–382.
Exceptionally well preserved specimens of the bivalve mollusc Modiola major were collected from a Lower Cretaceous(Barremian) hydrocarbon seep deposit in northern California. This material, together with the type series of M. major, andvarious other specimens from Upper Jurassic to Lower Cretaceous seep localities in California, is redescribed and re−ferred to the hydrocarbon seep−restricted modiomorphid genus Caspiconcha. We include also a description of Myo−concha americana because some previous reports have incorrectly synonymized Myoconcha americana with Caspi−
concha major. In addition, we report Caspiconcha sp. from a Lower Cretaceous (Albian) hydrocarbon seep from
Hokkaido, Japan, and we review all currently described species of Caspiconcha, and other species that probably belong tothis genus. We demonstrate that Caspiconcha had a widespread distribution in Late Jurassic to Early Cretaceous hydro−
carbon seeps, but became rare thereafter, with the last representative occurring in Upper Cretaceous strata of Japan. This
macroevolutionary pattern is similar to that observed in the seep−restricted brachiopods. After the decline of Caspiconcha
at the end of the Early Cretaceous and its last occurrence in the Campanian, the ecological niche of epifaunal tosemi−infaunal seep endemic bivalves was largely vacant and not reoccupied until the Eocene with the appearance of thevesicomyid and bathymodiolin bivalves. The formal placement of M. major into the genus Caspiconcha restricts the fos−
sil record of mytilids at seeps to post−Mesozoic times, and thus there is less discrepancy between the fossil record ofchemosynthetic mytilids and their divergence age estimates from molecular data.
Key wo r d s: Bivalvia, Modiomorphidae, Kalenteridae, chemosynthesis−based ecosystem, cold seeps, Mesozoic, Cali−
fornia, Japan.
Robert G. Jenkins [robertgj@staff.kanazawa−u.ac.jp], School of Natural System, College of Science and Engineering,
Kanazawa University, 920−1192, Japan;
Andrzej Kaim [kaim@twarda.pan.pl], Institute of Paleobiology, Polish Academy of Sciences, PL−00−818 Warszawa,ul. Twarda 51/55, Poland and Bayerische Staatssammlung für Paläontologie und Geologie, 80333 München, Germany;
Crispin T. S. Little [C.T.S.Little@earth.leeds.ac.uk], School of Earth and Environment, University of Leeds, Leeds LS2
9JT, United Kingdom;
Yasuhiro Iba [iba@mail.sci.hokudai.ac.jp], Department of Natural History Sciences, Hokkaido University, Sapporo060−0810, Japan;
Kazushige Tanabe [tanabe@um.u−tokyo.ac.jp], University Museum, University of Tokyo, Tokyo 113−0033, Japan;
Kathleen A. Campbell [ka.campbell@auckland.ac.nz], School of Environment, The University of Auckland, Auckland 1142,
New Zealand.
Received 4 March 2011, accepted 12 December 2011, available online 16 December 2011.
Copyright © 2013 R.G. Jenkins et al. This is an open−access article distributed under the terms of the Creative Commons
Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original
author and source are credited.

Introduction
Bivalve and gastropod molluscs are both abundant and diverse
in marine chemosynthesis−based ecosystems (hydrocarbon
seeps, hydrothermal vents, sunken wood, and vertebratebones), and a number of dominant species possess symbiotic
bacteria that obtain energy from the oxidation of hydrogen sul−
phide and/or methane. These chemosynthetic environments
are also characterized by a high level of endemism in the bi−
Acta Palaeontol. Pol. 58 (2): 357–382, 2013

valves and gastropods, with many taxa not known from more
typical marine environments. The fossil record of molluscs
from chemosynthetic environments has been traced back to
the Silurian (Little et al. 1997; Campbell 2006), but the abun−dance and diversity of bivalves and gastropods in chemo−
synthesis−based communities increased significantly during
the Jurassic and Cretaceous (Campbell and Bottjer 1995a; Lit−
tle and Vrijenhoek 2003; Kiel 2010). The earliest occurrencesof many modern seep− and vent−restricted bivalve and gastro−
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358

pod genera was in the late Mesozoic (Little and Vrijenhoek
2003; Campbell 2006; Kiel and Little 2006; Kaim et al. 2008a,
2009; Kiel 2010), a pattern also seen in sunken wood and ver−tebrate bone environments (Kaim et al. 2008b, 2011; Kiel etal. 2009). Although Upper Cretaceous chemosynthesis−based
communities have been relatively well studied recently (e.g.,
Amano et al. 2007; Jenkins et al. 2007a, b, 2008; Kaim et al.
2008a, b, 2009, 2010; Kiel et al. 2008b), their Jurassic to
Lower Cretaceous counterparts remain less well known (e.g.,
Sandy and Campbell 1994; Kiel and Campbell 2005; Camp−bell et al. 2008; Kiel et al. 2008a, 2010; Hammer et al. 2011;
Kaim 2011). A better knowledge of fossil seep communities
from this time period is therefore important in order to eluci−
date the pattern of bivalve and gastropod migration into thisunusual environment.
Of particular importance in the ecology of Cenozoic hy−
drocarbon seeps are the large bivalves that host symbiotic bac−teria in their gills belonging to the family Vesicomyidae (e.g.,
Calyptogena) and the mytilid subfamily Bathymodiolinae(e.g., Bathymodiolus), both of which appeared in the Eocene(Kiel and Little 2006; Amano and Kiel 2007). It is likely thatduring the Late Jurassic to Early Cretaceous their place was
occupied ecologically by species of the extinct modiomorphid
bivalve genus Caspiconcha, although it is not known if this bi−
valve possessed chemosymbionts. The type species of Caspi−
concha (C. whithami) was first described from seep carbon−
ates of East Greenland (Kelly et al. 2000) and another species(C. rubani) was subsequently proposed based on material
from the Ukraine (Kiel and Peckmann 2008; Kiel et al. 2010).
Kiel and Peckmann (2008) and Kiel et al. (2010) suggested
that the large modioliform bivalve Modiola major Gabb 1869,
widely distributed in Upper Jurassic to Lower Cretaceous hy−
drocarbon seeps in California, USA (Campbell et al. 1993),
may also belong to Caspiconcha. Furthermore, Kiel et al.
(2010) used the name Caspiconcha major for the species, al−though without proper systematic treatment. Due to poor pres−ervation of the type series of M. major, up until now it has
proved difficult to establish whether this species indeed be−longs to Caspiconcha (or to the genus Myoconcha, as sug−gested in 1930 by Stewart). In 2005 and 2006 we collected
new specimens of a Caspiconcha−like bivalve from the Eagle
Creek seep locality of California (Fig. 1), two of which wereextraordinarily well−preserved, including having original shellmineralogy. We compared this new material with Gabb’s(1869) original type material of M. major, historical speci−
mens of the species from Stanton (1895), and several other
newly collected M. major−like specimens from various Cali−
fornian seep sites, including the type locality (Table 1). This
work allowed us to conclude that the Eagle Creek specimens
are conspecific with M. major, and moreover that M. major isindeed a species of Caspiconcha. We use the new material toemend the diagnosis of Caspiconcha major (Gabb, 1869)
herein. We also checked the type material of Myoconcha
americana Stanton, 1895, which was considered as a juvenile
specimen of Caspiconcha major by Stewart (1930: 104) andKiel et al. (2010: 37), but conclude herein that this is a separate

ACTA PALAEONTOLOGICA POLONICA 58 (2), 2013

species which probably does not belong to Caspiconcha.
We also report a new occurrence of Caspiconcha from a
Lower Cretaceous (Albian) seep deposit from Hokkaido, Ja−pan. These data show that Caspiconcha had a wide geograph−ical distribution in seeps in the late Mesozoic, particularly dur−ing the Early Cretaceous, and was thus an important faunal el−ement within these communities.
Institutional abbreviations.—BSPG, Bayerische Staatsam−
mlung für Paläontologie und Geologie, München, Germany;
CAS, California Academy of Sciences, San Francisco, USA;
MCZ, Museum of Comparative Zoology at Harvard Univer−sity, Cambridge, USA; NMNS, National Museum of Natureand Science, Tokyo, Japan; SMUC, Sedgwick Museum,
University of Cambridge, Cambridge, UK; UCMP, Museumof Paleontology, University of California, Berkeley, USA;
USGS, United States Geological Survey, Reston, USA;
USNM, National Museum of Natural History in the Smithso−
nian Institution, Washington, USA; YNU, Yokohama Na−
tional University, Yokohama, Japan; ZPAL, Institute of
Paleobiology, Polish Academy of Sciences, Warsaw, Po−
land.

Material and methods
The material described in this paper comes from a number ofUpper Jurassic to Lower Cretaceous hydrocarbon seep de−posits in California and Hokkaido (Fig. 1, Table 1). Newly
collected specimens from California and Hokkaido were firstprepared in the laboratory by means of mechanical prepara−tion and some were then cast using silicone rubber. The re−sulting specimens were photographed either by the authors
or in the photo−labs of BSPG and ZPAL. The shell micro−
structure of the Eagle Creek specimen (CAS 72527−9) was
observed using a Phillips XL20 SEM at ZPAL. XRD analy−
sis on the same specimen was performed at the YNU using
Rigaku Rint 2500 equipment. In addition to the specimens
listed in Table 1 we examined the type specimens of Caspi−
concha whithami Kelly, 2000, which are now stored at the
SMUC. All figured specimens are stored at the MCZ, CAS,
USNM, UCMP, NMNS or SMUC.
Two of the localities with newly discovered specimens ofCaspiconcha, at Eagle Creek, California, and at Utagoesawa
Creek, Hokkaido, are still pending detailed description thatwill be published elsewhere. However, in Appendix 1 we
provide preliminary information about these sites pertinent
to our study.

Systematic palaeontology
Class Bivalvia Linnaeus, 1758
Subclass Autobranchia Grobben, 1894
Superorder Heteroconchia Gray, 1854

JENKINS ET AL.—SEEP RESTRICTED BIVALVE CASPICONCHA REVISITED

359

Table 1. List of figured Caspiconcha and Myoconcha specimens treated in this paper.
Register number

Species

Locality

MCZ 10539

Caspiconcha major (Gabb, 1869)

East of Knoxville

MCZ 108540
MCZ 108538A
MCZ 108538B
MCZ 108538C
USNM 23041
CAS 72527−9
CAS 72530
CAS 72531.1

Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)

East of KnoxvilleWilbur Springs
Wilbur Springs
Wilbur Springs
Cold Fork of Cottonwood Creek
Eagle Creek
Eagle Creek
Eagle Creek

References
Gabb 1869;
Stewart 1930
Gabb 1869
Gabb 1869
Gabb 1869
Gabb 1869
Stanton 1885
this paper
this paper
this paper

CAS 72531.2

Caspiconcha major (Gabb, 1869)

Eagle Creek

this paper

UCMP 10225
UCMP 10226
UCMP 152077
CAS 72532
CAS 72533
CAS 72534
CAS 72535
CAS 72548
CAS 72536
CAS 71880
CAS 71882
CAS 71881
CAS 72537
CAS 71883NMNS PM25523
NMNS PM25524
NMNS PM25525
USNM 23042

Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha major (Gabb, 1869)
Caspiconcha sp.
Caspiconcha sp.
Caspiconcha sp.
Myoconcha americana Stanton, 1895

Cold Fork of Cottonwood Creek
Cold Fork of Cottonwood Creek
Cold Fork of Cottonwood Creek
Cold Fork of Cottonwood Creek
Cold Fork of Cottonwood Creek
Bear Creek
Lake Berryessa
Lake Berryessa
East of Berryessa
Wilbur Springs
Wilbur Springs
Wilbur Springs
Wilbur Springs
Wilbur Springs
Utagoesawa Creek
Utagoesawa Creek
Utagoesawa Creek
Cold Fork of Cottonwood Creek

Order Modiomorphoida Newell, 1969
Remarks.—The order Modiomorphoida was originally pro−
posed by Newell (1969) for extinct Paleozoic marine bi−
valves. The group was treated thereafter under the Pholado−
myoida (e.g., Fang and Morris 1997; Kelly et al. 2000, Grif−fin and Pastorino 2006) or Carditoida (e.g., Chavan 1954,
1969; Newell 1957; Nevesskaja 2009). Bouchet et al. (2010)
placed the superfamily Modiomorphoidea under “order un−
certain”, but following Bailey (1983) we here place thesuperfamilies Modiomorphoidea and Kalenteroidea in theorder Modiomorphoida, pending further investigations. As
here considered, the order Modiomorphoida was diverse in
the Paleozoic, but decreased in importance through the Me−
sozoic, with the last representatives reported from the Neo−gene (Kelly et al. 2000; Griffin and Pastorino 2006).

Superfamily Kalenteroidea Marwick, 1953
Family Kalenteridae Marwick, 1953
Remarks.—Originally classified as a modiomorphid bivalve
in the subfamily Myoconchinae (Kelly et al. 2000), Caspi−

Remarks

Figure no.

lectotypeFig. 5paralectotype
paralectotype
paralectotype
paralectotype

Fig. 6A
Fig. 6B
Fig. 6C
Fig. 6D
Fig. 7
Fig. 3Fig. 8A
Fig. 8B

counter part of
CAS 72531.1

this paper
this paper
this paper
this paper
this paper
this paper
this paper
this paper
this paper
this paper
this paper
this paper
this paper
this paper
this paper
this paper
this paper
Stanton 1885; Kiel et al. 2010

listedFigs. 9A, 10E
Fig. 10B
Fig. 9B
listedlistedFig. 9J
Fig. 10A
Fig. 9H
Figs. 9I, 10D
Fig. 9C
Fig. 9D
Fig. 9E
Figs. 9G, 10C
Fig. 9F
Fig. 11A
Fig. 11B
Fig. 11C
Fig. 12concha indeed shares many features with Myoconcha, includ−ing the deeply set anterior adductor muscle scars with myo−phoric buttress, modioliform shape, and extreme reduction of
the shell anterior (Fig. 2). Therefore, following Kelly et al.
(2000), we place Caspiconcha into the family Kalenteridae(an older synonym of Myoconchidae; see e.g., Bouchet et al.
2010). We also compare Caspiconcha to Myoconcha ameri−cana because of previous incorrect placements of the latter
species within the former genus (Stewart 1930; Kiel et al.
2010).

Genus Caspiconcha Kelly in Kelly et al., 2000
Type species: Caspiconcha whithami Kelly in Kelly et al., 2000; Upper
Barremian (Lower Cretaceous), NE Greenland.

Emended diagnosis.—Shell subtrapezoidal to cuneiform, oreven modioliform in shape with extreme reduction of its ante−rior margin; triangular depression running from the umbonal
area widening towards the mid flank; umbones located anteri−orly but not reaching the anterior end; exterior ornamentsmooth apart from commarginal growth lines; edentulous
hinge structure; anterior adductor muscle scar deeply set
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ACTA PALAEONTOLOGICA POLONICA 58 (2), 2013

B

A

D

geologic contact
major fault

C

F

E

Fig. 1. Locality map and outcrop photographs. A. Locality map of the
Utagoesawa Creek site, Hatonosu, Yubari City, Hokkaido, Japan. Also
shown is the location of the Omagari seep site. Solid pattern is the outcrop
area of the Cretaceous Yezo Group strata. B. Outcrop photograph of an
Utagoesawa Creek carbonate body showing large Caspiconcha sp. and/or
probable lucinid bivalve fossils. C. Locality map of hydrocarbon seeps in
California. Subpanel shows locality map of the Eagle Creek site, Ono, Cali−
fornia, USA. Solid circle with number indicates Caspiconcha bearing sites.
1, Eagle Creek; 2, Cold Fork of Cottonwood Creek; 3, Paskenta; 4, Bear
Creek; 5, Wilbur Springs; 6, east of Knoxville (exact place is unknown); ®

JENKINS ET AL.—SEEP RESTRICTED BIVALVE CASPICONCHA REVISITED

361

posterior pedal retractor muscle scarsinus-like connection line
internal ridge

ligamental area
pedal elevator muscle scar
hidden underneath hinge

posterior adductor
muscle scar

nymph

mantle muscle scars
anterior pedal
retractor muscle scarumbo

anterior adductor
muscle scar
caspiconchid process

myophoric
Myophoric buttresspallial line

Fig. 2. Schematic illustration of the right valve internal features of Caspiconcha major (Gabb, 1869).

within myophoric buttress; caspiconchid process present, par−tially covering anterior adductor muscle scar; entire shellaragonitic.
Species included.—Caspiconcha whitmani Kelly, 2000 fromthe Barremian of Greenland (Kelly et al. 2000), C. rubani
Kiel, Campbell, and Gaillard, 2010 from the Hauterivian of
the Crimean Peninsula, Ukraine (Kiel and Peckmann 2008;
Kiel et al. 2010), and C. major (Gabb, 1869) of northern Cal−
ifornia. These Californian localities largely constitute seep
carbonates within the Buchia bivalve−bearing mudstone of
the “Knoxville Beds”, lower Great Valley Group, which isLate Jurassic to Early Cretaceous in age. They include Tit−
honian (Paskenta), Valanginian (Bear Creek), Hauterivian(Wilbur Springs), Albian (Cold Fork of Cottonwood Creek),
and some poorly dated, probably Lower Cretaceous, sites(East of Knoxville, East Berryessa) (Gabb 1869; Stanton
1895; Sandy and Campbell 1994; Kiel et al. 2008a). In addi−tion, the newly recognized Barremian (Eagle Creek) seep de−posit near Ono, northern California, also contains C. major,
many lucinid bivalves, but no buchiids.
We also include in Caspiconcha a probable new species,
reported below, from the Albian Utagoesawa Creek, Yubariarea, central Hokkaido, and a single specimen that is proba−
bly a different species from the Campanian Omagari seep,
northern Hokkaido. Myoconcha aff. transatlantica (Ascher
7, East Berryessa. CRO: Coast Range Ophiolite. The map is modified from
Kiel et al. (2008a). D. Overview photograph of the Eagle Creek site. The
hydrocarbon−seep carbonates crop out near the junction between Eagle
Creek and North Fork of Cottonwood Creek. E. Photograph of typical seep
carbonate bodies at the Eagle Creek site. Right−top inset shows detail of car−bonate cements with stromatolite−like laminae. F. Photograph of strata at
the Eagle Creek locality yielding Caspiconcha specimens.

1906) from Lower Cretaceous (Hauterivian) strata of the
Czech Carpathians could also be a Caspiconcha species, and
Kelly et al. (2000) suggested that an undescribed bivalve
from the Cretaceous of New Zealand also might be related to
Caspiconcha (Kelly et al. 2000). The age of the latter is Late
Albian to mid−Cenomanian (Kiel et al. in press). Caspi−
concha sp. has been also reported recently from the Late
Albian Ispaster seep carbonates in northern Spain (Agirrez−
abala et al. in press). All occurrences listed above are from
confirmed or likely hydrocarbon seep deposits, apart from
Ascher’s (1906) locality, which seems to be no longer acces−
sible (Vašíček and Skupien 2004). Caspiconcha and Caspi−
concha−like species are listed in Table 2.
Remarks.—Caspiconcha shares a number of characters with
Myoconcha (see above), but lacks the external radial orna−
ment of that genus and, unlike it, possesses an edentulous
hinge. The shell of the type species (C. whithami Kelly,2000) was interpreted by Kelly et al. (2000) to be composedof aragonite. We can confirm this feature for the genus in
specimens of C. major (Gabb, 1869) from the Eagle Creek
locality. Kelly (in Kelly et al. 2000: 242) suggested that theoriginal inner shell layer of C. whithami was nacreous, al−though in all Greenland specimens the original shell had
been entirely replaced by silica. The Eagle Creek C. major
specimens (Fig. 3) clearly show that in this species the shelllayers are not nacreous, but have cross−lamellar and homoge−
nous microstructures (Fig. 4). According to Morris et al.
(1991: 273), the shell of Myoconcha is composed entirely ofhomogeneous structure, although Carter (1990: 271) argued
that further SEM observations are necessary to fully clarify
the microstructural composition of Myoconcha shells. The
differing shell microstructure of Myoconcha and Caspicon−
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ACTA PALAEONTOLOGICA POLONICA 58 (2), 2013

Table 2. Species list of Caspiconcha and Caspiconcha−like bivalves with locality information.
Locality

Area

Age

Stratigraphic unit

Capas con Pecten
bodenbenderi,
Neuquén

Argentina

Early Jurassic

Piedra Pintada Formation

Paskenta

Tehama County,
California, USA

Tithonian,
Late Jurassic

Bear Creek

Colusa County,
California, USA

Valanginian,
Early Cretaceous

Stony Creek Formation, Great
Valley GroupGrizzly Canyon Member, Crack
Canyon Formation, Great VallyGroup

Lake Berryessa

Napa County,
California, USA

Koniakauer
Carpathians, Czech
Schloss (Hradiště)Republic
Crimean Peninsula,
Planerskoje
southern Ukraine
Wilbur Springs
Colusa County,(= Simmon's
California, USA
Spring according
to Durham 1998)

probably
Valanginian?, Early
Cretaceous
Hauterivian,
Early Cretaceous
Hauterivian,
Early Cretaceous

Great Valley Group
Tesin−Hradiště Formation
Planerskoje section

Species
Myoconcha
neuquena,
Myoconcha
neuquena var.
torulosa
Caspiconcha
majorCampbell and Bottjer 1993a;
Kiel et al. 2008

Caspiconcha
major

Kiel and Campbell 2005; Kiel etal. 2008; this paper

Caspiconcha
major

Kiel et al. 2008a; this paper

Myoconcha aff.
transatlantica
Caspiconcha
rubani

Reference
Leanza 1940; Griffin andPastorino 2006

Ascher 1906
Kiel and Peckmann 2008; Kiel
et al. 2010

Hauterivian,
Early Cretaceous

Great Valley Group

Caspiconcha
majorCampbell et al. 2002; this paper

Ono, Shasta County,
California, USA

Late Barremian,
Early Cretaceous

Lower Chickabally Mudstone
Member, Budden Canyon
Formation, Great Valley Group

Caspiconcha
major

Stanton 1895; this paper

Wollaston Forland,
Northeast Greenland
Cold Fork ofTehama County,
Cottonwood Creek California, USA

Late Barremian,
Early Cretaceous
Aptian–Albian,
Early Cretaceous

Utagoesawa Creek

Hatonosu, YubariCity, Hokkaido,
Japan

Albian,
Early Cretaceous

IspasterBasque−Cantabrian
Basin, Spain

Port Awanui

New Zealand

East of KnoxvilleNapa County,
California, USA

Omagari

Nakagawa Town,
Hokkaido, Japan

Eagle Creek
Kuhnpasset

Late Albian,
Early Cretaceous
Late
Albian–mid−Cenoman
ian, Cretaceous

Kuhnpasset Beds

Caspiconcha
whitmani
Caspiconcha
major

Lodoga Formation, Great
Valley Group
Hikagenosawa Formation in
Takashima et al. (2004); Main
Caspiconcha sp.
part Formation in Matsumotoand Harada (1964), Yezo Group

Kelly et al. 2000
Campbell and Bottjer 1993;
Kiel et al. 2008; this paper
Ogihara 2005; this paper

Black Flysch Group

Caspiconcha sp.

Agirrezabala et al. in press

Tikihore Formation

Caspiconcha sp.

Kiel et al. in press

Early Cretaceous

Great Valley Group

Caspiconcha
major

Gabb 1869; this paper

Campanian,
Early Cretaceous

Omagari Formation, YezoGroup

cha does not necessarily mean that they cannot be closely re−lated, because Morris et al. (1991: 53) inferred that an evolu−
tionary sequence from nacreous through homogenous tocross−lamellar microstructure is commonly observed in bi−
valves. Therefore, the shell microstructure of Caspiconcha
could be interpreted as an end−member of a similar shell
macroevolutionary sequence in the family Kalenteridae.
Kelly et al. (2000) incorporated Caspiconcha into thesubfamily Myoconchinae, identifying numerous charactersof that genus in common with Myoconcha. He also classified
the Myoconchinae as a subfamily of the Modiomorphidae
within the superfamily Modiomorphoidea. The Modiomor−
phidae was considered by Morris (1978), Fang and Morris(1997) and Carter et al. (2000) as a family of Anomalo−
desmata. However, many other ideas for its placement havedeveloped since then (see recent discussion in Griffin and

Hikida et al. 2003; Jenkins et al.
Caspiconcha sp. 2007a; Kiel et al. 2008; Kaim etal. 2008; Kaim et al. 2009

Pastorino 2006). The entirely aragonitic shell of C. major
supports placement of the Modiomorphidae among the ano−malodesmatans, which have wholly aragonitic shells (Carter1990), rather than with the Mytilidae, the majority of generaof which possess both calcitic and aragonitic shell layers(Carter 1990).
Stratigraphic and geographic range.—Tithonian (Upper Ju−
rassic) to Campanian (Upper Cretaceous). Barremian of NE
Greenland, Tithonian to Albian of California, Hauterivian of
Crimea and possibly Czech Republic, Albian–Cenomanianof New Zealand, Albian of Spain, and Albian and Cam−panian of Hokkaido.

Caspiconcha major (Gabb, 1869)
Figs. 2–10.
1869 Modiola major sp. nov.; Gabb, 1869: 191–192, 246, pl. 31: 88.

JENKINS ET AL.—SEEP RESTRICTED BIVALVE CASPICONCHA REVISITED

363
umbo

A

10 mm
pedal elevator muscle scar(see Fig. 3C)
external ligament
mantle muscle scars (Fig. 3D)
for microstructure anal.
(see Fig. 4)

ligament area
posterior adductor muscle scarBanterior pedal
retractor muscle scar
umbo

immuration trace
anterior adductor
muscle scar myophoric
buttressCpallial lineDpedal elevator muscle scar

5 mmgroove

groov

ed ar

ea

2 mm
myophoric buttressFig. 3. Modiomorphid bivalve Caspiconcha major (Gabb, 1869) CAS 72527−9 from Eagle Creek, California, USA, Upper Barremian (Lower Cretaceous).
Right valve. A. External view. B. Internal view. Details of pedal elevator muscle scar (C) and mantle muscle scars indicated by small arrowheads (D). Loca−tion of shell microstructure analysis (Fig. 4) is marked with a dotted white line. Black arrowheads point to the anterior.

1876 Modiola major Gabb, 1869; Whiteaves 1876: 74.
1894 Modiola major Gabb, 1869; Stanton (in Diller and Stanton 1894):
442.
1885 Modiola major Gabb, 1869; White 1885: 20.
1895 Modiola major Gabb, 1869; Stanton 1895: 48, pl. 3: 1.

1897 Modiola major Gabb, 1869; Cooper 1897: 84.
1902 Modiola major Gabb, 1869; Anderson 1902: 45.
1907 Modiola major Gabb, 1869; Crandall 1907: 34.
1909 Modiola major Gabb, 1869; Grabau and Shimer 1909: 521, fig. 701.
1914 Modiola major Gabb, 1869; Dickerson 1914: 128.
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ACTA PALAEONTOLOGICA POLONICA 58 (2), 2013
500 µm

50 µm

50 µm

C

C
50 µm

B

A

D

B

D

Fig. 4. Shell microstructure of modiomorphid bivalve Caspiconcha major (Gabb, 1869) CAS 72527−9 from Eagle Creek, California, USA, Upper
Barremian (Lower Cretaceous). External shell surface upwards in all figures. A.Vertical cross section through shell in the pallial region; see Fig. 3B for lo−
cation. B. Demarcation between middle (cross lamellar) and inner (complex cross lamellar) layers. C. Outer layer homogeneous structure. D. Lower part of
inner layer, note diagenetic alteration at base.

1914 Modiola major Gabb, 1869; Lawson 1914: 8.
1930 Myoconcha major (Gabb, 1869); Stewart 1930: 104, pl. 4: 1.
1993 Modiola major Gabb, 1869; Campbell et al. 1993: 39, 42–44,
fig. 2.
1993 Modiola major Gabb, 1869; Campbell and Bottjer 1993: 334,
table 1.
1995 Modiola major Gabb, 1869; Campbell and Bottjer 1995b: 470,
474, fig. 5.
2006 Modiola major Gabb, 1869; Campbell 2006: 394, Table 1.
2006 “Modiolus” major Gabb, 1869; Squires and Saul 2006: 121.
2008 Modiola major Gabb, 1869; Kiel and Peckmann 2008: 757.
2010 Caspiconcha major (Gabb, 1869); Kiel et al. 2010: 37, 43.
Type material: Gabb (1869) did not designate a holotype in his originaldescription. Here we designate the single specimen (MCZ 108539) fig−
ured by Gabb (1869: pl. 31: 88) to be the lectotype. Thus the four re−maining syntypes are paralectotypes. Lectotype: MCZ 108539, Fig. 5.
Almost complete large articulated specimen. Poorly preserved, with theumbonal part of the right valve missing. The specimen label states, “Eastof Knoxville, Lake County” as the collection locality, the site details of
which are further assessed in Kiel et al. 2008a. Paralectotypes: MCZ
108538 and 108540, Fig. 6. The MCZ 108538 includes three specimens,
which we designate as MCZ 108538A, B and C, from Wilbur Springs,
Colusa County, California. MCZ 108540 is from “East of Knoxville,
Lake County”.
Type locality: East of Knoxville in Napa County, California, USA.
Type horizon: “Knoxville Beds”, lower Great Valley Group.

Emended diagnosis.—Shell strongly elongated cuneiformwith nearly straight dorsal and ventral margins; umbo above
hinge line; weak internal ridge running from anterior pedal

elevator muscle scar towards ventral margin of the posterior
adductor muscle scar; rounded anterior adductor muscle scar
deeply set; posterior adductor muscle scar rounded with dor−sally located narrow posterior pedal retractor muscle scar
projected anteriorly; well rounded connected line between
ventral part of posterior pedal retractor muscle scar and pos−terior adductor muscle scar; shell entirely aragonite.
Description.—The description as follows is a composite offeatures seen in Gabb’s (1869) type material, Stanton’s(1895) material, three new specimens from the Eagle Creek
locality, and some other specimens collected from various
Cretaceous hydrocarbon seep sites in California (Table 1).
Because of the highly intraspecific and ontogenetic variabil−
ity of the species we also include detailed descriptions ofthese specimens separately in Appendix 2, in part to avoid
future confusion. A schematic drawing of the species is pre−sented in Fig. 2 showing the most important shell characters.
The shell is large (lectotype 131 mm long, 60 mm high,
34 mm wide) and strongly elongated cuneiform in shape withnearly straight dorsal and ventral margins. The valves areequivalved and strongly inequilateral, and the anterior mar−gins are moderately inflated near the umbones. The umbones
are close to the anterior end of the shell and are situatedslightly above or almost at the same height as the hinge line.
The shell thickness is variable, and is thickest at the myo−phoric buttress where the anterior muscle scar area is located.

JENKINS ET AL.—SEEP RESTRICTED BIVALVE CASPICONCHA REVISITED

dia

A

10 mm

gen

etic

she

triangular depr

ll d

efo

diag

en

n

B

edge of myophoric buttressligamental ridge

d efo

atio

essionpitted pedal elevator muscle scarhell
etic srm

365

anterior pedal retractor
muscle scar

rmat

ion

10 mm
sion
s
re
p
e
d
r
triangulaadductor muscle scar

D

C

E

diagenetic shell deformation
anterior pedal retractor
muscle scar

F

10 mm

adductor muscle scaredge of myophoric buttressdiagenetic shell deformation
pitted pedal elevator muscle scar
ligament ridge of left valve

G

5 mm

The shell is relatively thick in the antero−dorsal to antero−ventral area, thinning conspicuously in the posterior−ventral
to mid−flank of the ventral area.
The dorsal margin is straight in lateral profile for the full
extent of the nymph and ligament area and is then curved
ventrally at the postero−dorsal area. The posterior margin iswell rounded. The ventral margin is almost straight and isfeebly indented at the mid−flank. The anterior margin is shortand rounded. The shell is moderately inflated in dorsal as−
pect, with a wedge shape in both the anterior and posteriorFig. 5 Lectotype of modiomorphid bivalve Caspiconcha major (Gabb,
1869) MCZ 108539 from east of Knoxville, California, USA, Lower
Cretaceous. The pairs A and B, C and D, and E and F are the same views,
respectively with and without morphological interpretations. A, B. Leftvalve. C, D. Right valve. The shell of the specimen is missing forward of
the anterior adductor muscle scar, leaving an internal mould. E, F. Dorsal
view. G. Detail of ligament area of left valve. White arrowheads point tothe anterior.

margins. A triangular depression runs from the umbonal
area, widening towards the mid flank. There is an external or−
nament of commarginal growth lines, which are more pro−nounced on the shell anterior.
The shell interior is smooth apart from muscle scars andthe pallial line. The pallial line runs commarginally from theanterior adductor muscle scar towards the posterior and thenturns dorsally towards the posterior of the posterior adductor
muscle scar without a pallial sinus. The anterior adductor
muscle scar is rounded with pedal retractor muscle scars lo−
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ACTA PALAEONTOLOGICA POLONICA 58 (2), 2013edge of myophoric buttressA1

anterior pedal retractor
muscle scaranterior pedal retractor
muscle scaredge of myophoric buttressadductor muscle scarridgeinternal mould
ridge
shell

B
5 mm

depressionedge of myophoric buttressanterior pedal retractor
muscle scar

C

ridge

10 mm
specimen
distorted

adductor muscle scar

shell
5 mm

A2
adductor muscle scar
mould

D

adductor muscle scaranterior pedal retractor
muscle scar
10 mmedge of myophoric buttress

5 mm
shell

edge of myophoric buttressadductor muscle scar

Fig. 6. Paralectotypes of modiomorphid bivalve Caspiconcha major (Gabb, 1869). A. MCZ108540 from east of Knoxville, California, USA, Lower Creta−
ceous. Internal mould of left valve (A1), dorsal view (A2). B–D. Three specimens of Caspiconcha major (Gabb, 1869) from Wilbur Springs, California,
USA, Hauterivian (Lower Cretaceous). B. Left valve of MCZ 108538A. C. Internal mould of right valve of MCZ 108538B. D. Right valve of MCZ
108538C. White arrowheads point to the anterior.

cated dorsally. The posterior adductor muscle scar is roundedwith an antero−dorsally located narrow posterior pedal re−
tractor muscle scar, which projects towards the anterior. The
ventral connection between the posterior pedal retractor
muscle scar and posterior adductor muscle scar is depressed
posteriorly with well rounded line. A weak internal ridge
runs from anterior pedal elevator muscle scar towards theventral margin of the posterior adductor muscle scar, paral−
leled by an oblique groove on its ventral side.
The myophoric buttress is moderately steep but varies inshape among individuals. The hinge is stout and edentulous.
The nymph and ligament groove is very long and straight. The
external ligament is sturdy and triangular in cross−section, witha well−mineralized C−spring−type aragonite fibrous sublayer.
There are at least three shell layers. The outermost layer ishomogeneous, the middle layer is of cross lamellar structure,and the innermost layer is of complex cross lamellar struc−ture. The shell is composed entirely of aragonite.
Ontogenetic variation.—The shell anterior thickens signifi−
cantly as the individual grows and occurs mostly by growthin the inner surface of the shell. As a result the pedal eleva−
tor muscle scar becomes much deeper and stronger during
growth, although we observed some variation in this char−acter, with some juvenile specimens having deep and some
adults having shallow pedal elevator muscle scars.
Remarks.—Caspiconcha major was first described as Modi−ola major by Gabb (1869) based on poorly preserved speci−
mens. The species was later transferred to Myoconcha by
Stewart (1930) who stated that its shell is “… heavily but−
tressed and with a long posterior lateral [tooth]” (Stewart
1930: 104). He most likely confused the lateral [tooth] withthe ligamental groove, and, notwithstanding, his nomencla−

JENKINS ET AL.—SEEP RESTRICTED BIVALVE CASPICONCHA REVISITED

367

edge of myophoric buttress?umbo

possible muscle scar

tria

A

u
ng

lar

de

e
pr

ss

ion
10 mm

edge of myophoric buttress
?umbo

B

10 mm

Fig. 7. Modiomorphid bivalve Caspiconcha major (Gabb, 1869) USMM 23041 from Cold Fork of Cottonwood Creek, California, USA, Albian (Lower
Cretaceous), figured in Stanton (1895). A. Right valve. B. Dorsal view. White arrowheads point to the anterior.

tural change for Modiola major was not followed by later au−
thors (e.g., Campbell 2006) exclusive of Squires and Saul
(2006) who referred to this species as “Modiolus” major.
The shell shape of the lectotype is apparently much more
cuneiform than the shape of the Eagle Creek specimens.
However, the elongation of the lectotype is apparently due tothe diagenetic deformation of the specimen.
Caspiconcha major differs from C. whithami in generalshell shape that is cuneiform rather than subtrapezoidal, andpossesses a much thinner shell, particularly in the posterior
area. The anterior adductor muscle scar of C. major isrounded rather than elongated, and much smaller than that of
C. whithami. The caspiconchid process is short and narrowand possesses a small process−like undulation protruding

in−between the two anterior pedal retractor muscle scars. The
resilifer of C. major is short and narrow in contrast to that of
C. whithami, which is long and wide. The umbo of C. major
projects well above the hinge line, while in C. whithami it lies
below or at least level with the hinge line.
Kiel et al. (2010) stated that C. rubani can easily be distin−
guished from C. major by the presence of a strong internalridge in the former species which, according to Kiel et al.
(2010), is absent in C. major. However, the shells of C. majorfrom Eagle and Bear Creeks display a weak, but clearly visi−
ble internal ridge. Thus, C. major differs from C. rubani by
its much weaker internal ridge and general shell shape that ismore cuneiform in C. major rather than subtrapezoidal as inC. rubani. Furthermore, according to Kiel et al. (2010: fig.
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ACTA PALAEONTOLOGICA POLONICA 58 (2), 2013

A1

10 mm

A2

10 mm
pedal elevator muscle scar

B1

5 mm

B2

B3

5 mm

5 mm

Fig. 8. Modiomorphid bivalve Caspiconcha major (Gabb, 1869) from Eagle Creek, California, USA, Upper Barremian (Lower Cretaceous). A. CAS
72530, left (A1) and right (A2) valves. B. CAS 72531.1, left (B1) and right (B2) valves, dorsal view (B3). White arrowheads point to the anterior.

Fig. 9. Silicone rubber casts of modiomorphid bivalve Caspiconcha major (Gabb, 1869) from Cold Fork of Cottonwood Creek (A), Wilbur Springs (B–G), ®
East Berryessa (H, I) and Bear Creek (J), all California, USA. A. Internal surface of articulated small specimen UCMP 10225, right valve (A1), left valve(A2). B. Internal surface of left valve of small specimen UCMP 152077. C. Internal surface of right valve of small specimen CAS 71880. D. Internal surfaceof left valve of small specimen with some shell remains along the ventral margin CAS 71882. E. Internal surface of left valve of small specimen CAS 71881.
F. Internal surface of right valve of small specimen CAS 71883. G. Internal surface of right valve of small specimen with some shell remains in posteriorarea CAS 72537. H. Internal surface of left valve of small specimen CAS 72548. I. Internal surfaces of articulated specimen CAS 72536 with missing poste−rior margin, right valve (I1), left valve (I2). J. Internal surface of left valve of partial large specimen with internal shell details highlighted with dotted whitelines CAS 72534. White arrowheads point to the anterior.

JENKINS ET AL.—SEEP RESTRICTED BIVALVE CASPICONCHA REVISITED

A2

A1

369

B

2 mm
10 mm

E

D

C

5 mm
5 mm

2 mm

F

H

G

5 mm

5 mm

5 mm

I1

I2

10 mm

ligamental area

pedal elevator muscle scarJ

cavityal conintern

idge

nal r

inter

mantle muscle scars

pallial line

10 mm
posterior adductor muscle scar?
speculative shell margin

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ACTA PALAEONTOLOGICA POLONICA 58 (2), 2013

A1

B

10 mm

10 mm

C
A2
10 mm
10 mm

D1
D2

10 mm

E1

E2

10 mm

10 mm

E3

10 mm

Fig. 10. Modiomorphid bivalve Caspiconcha major (Gabb, 1869) from east of Berryessa (A and D), Cold Fork of Cottonwood Creek (B and E), and Wilbur
Springs (C), all California, USA. A. Right valve of large specimen CAS 72535 with missing posterior area, right valve (A1), dorsal view (A2). B. Right
valve of small specimen UCMP 10226. C. Internal mould of right valve of small specimen CAS 72537; see Fig. 9G for cast. D. Articulated specimen inter−
nal mould with missing posterior margin CAS 72536, left valve (D1), dorsal view (D2). See Fig. 9I for casts. E. Articulated small specimen internal mould
UCMP 10225, right (E1) and left (E2) valves, dorsal view (E3). See Fig. 9A for cast. White arrowheads point to the anterior.

7F), the pallial line of C. rubani converges with the ventraledge in its posterior part. In contrast, the pallial line of C. ma−
jor is almost parallel to the ventral edge.
Stratigraphic and geographic range.—Upper Jurassic (Titho−nian) to Lower Cretaceous (Albian) of California. The ca. 50
myr range of C. major matches some other seep−restrictedmolluscs, e.g., Conchocele bisecta (Conrad, 1849), whichranges from the Eocene to the Recent (e.g., Goedert et al.
2003; Amano and Jenkins 2007). However, further investiga−
tions of the morphology and ontogeny of additional C. major
specimens may reveal that the characters now considered to beof an interspecific variation actually reflect a presence of dif−ferent species.

JENKINS ET AL.—SEEP RESTRICTED BIVALVE CASPICONCHA REVISITED

371

A1

10 mm

C
A2

10 mm

10 mm
anterior muscle scar

B1

B2

10 mm

Fig. 11. Modiomorphid bivalve Caspiconcha sp. from Utagoesawa Creek, Hokkaido, Japan. A. NMNS PM25523, articulated large specimen, right (A1)and left (A2) valves. B. NMNS PM25524, right valve anterior−ventral fragment, external (B1) and internal (B2) views. C. NMNS PM25525, articulated
small specimen, anterior area missing. White arrowheads point to the anterior.

Caspiconcha sp.
Fig. 11.

Material.—NMNS PM25523 (Fig. 11A): One incomplete
adult shell with two articulated valves lacking the posterior

372

part and much of the ventral part of the right valve; NMNS
PM25524 (Fig. 11B) partially preserved anterior part of theleft valve of large specimen; NMNS PM25525 (Fig. 11C) ju−
venile mould of a shell with two articulated valves in butter−fly position preserving some shell material in the umbonal
areas of both valves. All specimens from the Utagoesawa
Creek locality, Yubari City, Hokkaido, seep carbonate in
Main Part Formation, Upper Albian, Lower Cretaceous.
Description.—The shell is large (the largest specimen NMNS
PM25523 is 247 mm long) and elongated, subtrapezoidal inlateral aspect, with strong inflation in the dorsal part of thecentral flank running obliquely towards posterior part of theventral margin. The valves are equivalve and strongly inequi−lateral. The umbones are close to the anterior end, situated at,or slightly below, the hinge line. The shell is thick, especiallyin the anterior part where the growth lines become more
densely packed. A triangular depression runs from the um−
bonal area widening towards the mid flank. The dorsal margin
is slightly curved around its whole extent. The posterior mar−gin in the adult shells is not preserved, while in the juvenile itis rounded, but strongly curved at the junction with ventral
margin which, in turn, becomes slightly rounded starting fromthis point. The exterior ornament consists of commarginal
growth lines, more pronounced on the shell anterior.
The shell interior is well preserved only in NMNS
PM25524 and represented by a generally smooth surface
with a large and elongated anterior adductor muscle scar bor−dered posteriorly by a stout myophoric buttress. Obliquegrooves are visible on the mould surface of the juvenile
NMNS PM25525 corresponding to the oblique ridge of theshell. The hinge is stout with very long ligamental groove.
The other features of the hinge are obscured by diagenesis.
Remarks.—The specimens are partially silicified including
some parts of adjoining sediment what hampered appropriate
preparation. We could not observe the internal surface of themost completely preserved specimen (NMNS PM25523);
thus, we decided to leave this species in open nomenclature.
The species is similar to C. whithami in having a very thickshell, subtrapezoidal shape, and elongated anterior adductor
muscle scar, but differs in having a curved hinge line andstrong inflation of the lateral flanks. It differs from C. major
in having very thick subtrapezoidal shell, with strongly in−flated lateral flanks and elongated anterior adductor muscle
scar. The Utagoesawa Caspiconcha is probably a new spe−cies, but we feel this needs confirmation from better pre−served material.

Genus Myoconcha Sowerby 1824
Type species: Myoconcha crassa Sowerby, 1824; Bajocian (Middle Ju−
rassic), near Bristol, United Kingdom.

Myoconcha americana Stanton, 1895
Fig. 12.
1895 Myoconcha americana sp. nov.; Stanton 1895: 48, pls. 2–11.
1930 Myoconcha americana; Stewart 1930: 104.
2010 Myoconcha americana; Kiel et al. 2010: 37.

ACTA PALAEONTOLOGICA POLONICA 58 (2), 2013
Holotype: USNM 23042, moderatly preserved almost complete rightvalve.
Type locality: Stephenson’s, Cold Fork of Cottonwood Creek, Tehama
County, California, USA. <1 km northeast of Stevenson Peak, and ~1.5
km due west of Woodyard Flat (Jones and Bailey 1973). Detailed map
can be found in Campbell et al. (2002).
Type horizon: Lower Cretaceous Knoxville Beds, Albian according toKiel et al. (2008a).

Dimensions.—The holotype is 40.4 mm long, 21.2 mm high,and 4.7 mm wide.
Description.—The specimen is a right valve only and is pre−served in a dark grey mudstone with abundant skeletal detri−tus, including crinoid brachials, echinoid spines and someother unidentified shell fragments. The right valve is almost
complete apart from the most anterior part that is partiallybroken. It is slender cuneiform in shape with fine, regularly
increasing growth lines. The hinge is almost straight and ex−
tends over 28 mm. The umbonal part is ornamented by faintradial and commarginal ribs. The commarginal ribs areslightly angulated at the crossing with the radial ribs (Fig.
12D), a pattern typical for Myoconcha. Unfortunately the
specimen surface is covered by some kind of glue that pre−vents further observations. The internal surface is covered by
sediment apart from the most anterior part where a small andelongated lateral tooth is visible on the anterodorsal margin(Fig. 12C).
Remarks.—Observations of the holotype of Myoconcha
americana confirms that it belongs to the genus Myoconcha
rather than to Caspiconcha. Most diagnostic features are
the umbonal ornament and presence of a probable tooth. We
are not sure, however, whether the presence/absence of the
tooth is a stable character during ontogeny, or a feature ofjuvenile individuals. Stewart (1930) and Kiel et al. (2010)
interpreted Myoconcha americana as a juvenile of C. major
and synonymized this species with the latter. Here we rec−ommend a return to the original concept of Stanton (1895),
retaining the species in Myoconcha.

Discussion
Vermiform grooves in Caspiconcha
In the description of Caspiconcha whithami, Kelly (in Kelly et
al. 2000: 243) noted that “the interior surface of the type speci−
men shows evidence of irregular vermiform grooves, ca. 1.5mm wide and up to 51 mm long, and with a semi−lunularcross−section”. We examined the type material of C. whithami
at SMUC and we found that apart from the holotype (K8318)
these grooves also are present in one of the paratypes (K8433).
The healed hole in the mid−ventral area of the holotype(K8318) of C. whithami seems to be the end of an abandoned
trace. This hole displays deflection of the growth lines,
strongly suggesting syn−vivo interaction between bivalve andthe bioeroder. Similar grooves occur in the shells of C. major
specimens from Eagle Creek, although they have a moreJENKINS ET AL.—SEEP RESTRICTED BIVALVE CASPICONCHA REVISITED
rectangular crings
rginalA

mma
ular coomma

rginalinvisib

le becregrings

ause o373

rectangularDcommargina

l rings

f glue1 mmbroken umbo
5 mmbroken umboB

tooth?

C

5 mm
1 mm

Fig. 12. Modiomorphid bivalve Myoconcha americana Stanton, 1895 (USNM 23042), right valve. A. External view. B. Dorsal view. C. Detail of the hingearea showing a possible tooth. D. Detail of shell surface ornamentation. Growth lines show rectoangular shape of shell. Black arrowheads point to faintradiaxial ribs mostly obscured by the glue in this image. White arrowheads point to the anterior.

strongly meandering morphology than those in C. whithami
shells. These grooves in the Eagle Creek C. major shells arelocated both in the pallial region and the shell periphery, andthey generally extend from inside the shell towards its periph−ery, and end at the shell margin (Fig. 3B). Similar trace fossils
are also known from internal shell surfaces of juvenile C.
rubani specimens from Ukraine (Kiel and Peckmann 2008:
fig. 6C), and the probable Caspiconcha specimen from theOmagari seep site, Japan (Hikida et al. 2003: fig. 9−3).
We suspect that all these Caspiconcha specimens were set−tled by the same type of animal and we consider this to be most
likely a polychaete worm. Many Recent seep bivalves (e.g.,
Adula, Calyptogena, Acharax, Bathymodiolus, Conchocele,
Lucina, Solemya, and Thyasira) are known to be infested byparasitic polychaetes from the family Nautiliniellidae (Miuraand Laubier 1989, 1990; Miura and Hashimoto 1996), and anarabellid polychaete has been reported by Dean (1992) as in−festing wood−boring pholadid bivalves. Although Miura andLaubier (1989, 1990) and Miura and Hashimoto (1996) re−
ported several nautiliniellids living in bivalve mantle cavities,
there is no indication that they may have caused any traces.
Nevertheless, several polychaetes do produce such traces inthe skeletons of other marine invertebrates (e.g., Korringa
1952; Voigt 1965; Bromley and D’Alessandro 1987;

Rodrigues et al. 2008), although these are usually U−shaped
and are attributed to polydorids or spionids. The Caspiconcha
inhabiting worm apparently lived in the space between mantle
and shell interior surface while the host bivalve was alive, in−hibiting its shell growth. Based on the principle of actualism,
we hypothesize that the possible polychaete worm was a para−site on the fossil bivalve. The infestation could have happened
to Caspiconcha at any time because of its sessile ecology (seebelow). The presence of traces in most Caspiconcha species
known so far might indicate that the Caspiconcha/worm inter−action had a worldwide distribution.

Palaeoecology of Caspiconcha
A common feature of chemosynthesis−based ecosystems is
the occurrence of abundant specimens of large bivalves. For
example, Calyptogena magnifica living at hydrothermal
vents on the Galápagos Rift reaches 241 mm in length (Bossand Turner 1980), and Calyptogena sp. from Late Pliocene
seeps of the Boso Peninsula, Japan grew as long as 234.5 mm(Majima et al. 1992). Large sized shells also are found inother bivalve groups inhabiting these ecosystems, e.g., luci−
nids (Bouchet and von Cosel 2004; Taylor and Glover 2009)and solemyids (Kanie et al. 1999). This gigantism results
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ACTA PALAEONTOLOGICA POLONICA 58 (2), 2013

from the abundance of nutrition available to the bivalves via
their chemosymbiotic bacteria, and is similar to that seen in
the photosymbiotic giant clams (tridacnids). Some bivalve
clades possessing both symbiotic and non−symbiotic taxa,
e.g., thyasirids, display clear correlation between body size
and the presence of symbiotic bacteria (Dufour 2005). The
large body size in Caspiconcha strongly suggests the animals
also hosted chemosymbiotic bacteria.
The presence of a gape at the anterior ventral shell marginof C. whithami suggests that this species was equipped with a
byssus (Kelly et al. 2000). A similar gape can be also observedin C. major and the Utagoesawa Caspiconcha sp. The gape in−dicates that Caspiconcha was a sessile animal using its byssus
for attachment, as suggested by Kelly et al. (2000). C.
whithami occurs in densely−packed clusters of shells in thecore of carbonate mounds (Kelly et al. 2000: 236). This type ofoccurrence is known also from C. major specimens from theCold Fork of Cottonwood Creek and Wilbur Springs localities(Campbell et al. 1993, 2002). In addition, specimens of C.
whithami and C. rubani are frequently found at angle of 30°
relative to the bedding plane (Kelly et al. 2000; Kiel andPeckmann 2008). Based on these lines of evidence, we sug−gest that Caspiconcha, like modern bathymodiolin mussels,
could live both attached onto hard substrates and also semi−
infaunally, partially buried in sediment.

Palaeogeography and evolutionary history
Table 2 lists the occurrences of Caspiconcha and Caspi−
concha−like bivalves. The Jurassic species Myoconcha neu−
quena Leanza, 1940 from Argentina could be an ancestor of
Caspiconcha, based on its shared characters with Caspi−
concha, e.g., edentulous hinge with probable caspiconchid
process (Leanza 1940: pl. 1A; Griffin and Pastorino 2006:
figs. 4−1, 4−2), although this species needs further study.
Caspiconcha itself ranged from the Jurassic to Late Creta−(Ma)

Caspiconcha major

ceous and had a worldwide distribution (Figs. 13, 14). Most
occurrences of Caspiconcha are from Lower Cretaceous
strata, indicating that this genus was characteristic of Early
Cretaceous hydrocarbon seep environments. By the Late
Cretaceous, however, Caspiconcha is found only in theCampanian Omagari hydrocarbon seep locality (Hikida et al.
2003: fig. 9−3). Kelly et al. (2000) suggested that a muchyounger (Maastrichtian) species of modiomorphid might bepresent in New Zealand. This species, identified as Caspi−
concha sp. is actually of Late Albian–mid−Cenomanian age(Kiel et al. in press).

Caspiconcha and the macroevolutionary historyof chemosynthetic communities
Seep palaeoecology.—Caspiconcha grew to over 300 mm inlength (Kelly et al. 2000) and apparently was the largest bi−
valve genus inhabiting Early Cretaceous chemosynthesis−
based ecosystems. The only other bivalves comparable in size
to Caspiconcha in the Cretaceous are the lucinid Nippono−
thracia from the Albian Ponbetsu and Cenomanian Kanajiri−sawa seep localities, both from Hokkaido (Kanie et al. 1993,
Kanie and Kuramochi 1996; Kiel et al. 2008b), and unidenti−
fied lucinid bivalves from the Albian Utagoesawa seep local−
ity (RGJ and AK, unpublished data). After the rapid decreaseof Caspiconcha occurrences at the end of Early Cretaceous,large−sized bivalves in chemosynthesis−based ecosystems
were rare for the rest of the Mesozoic, with the exception of
the lcunids from Kanajirisawa (middle Cenomanian; Kanie
and Kuramochi 1996) and New Zealand (Campanian; Kiel etal. in press), and the Campanian Caspiconcha sp. from Oma−gari. Bivalves larger than 10 cm were excessively rare in seepsites until the early Miocene and the occurrence of large
vesicomyids clams and bathymodiolin mussels (e.g., Amanoand Kiel 2007), although a large lucinid bivalve has been re−ported from an Eocene non−seep environment in Jamaica

Caspiconcha whithami

Caspiconcha rubani

MAASTRICHTIAN 66.0
72.1

CAMPANIAN

CRETACEOUSLATE

SANTONIAN
CONIACIAN
TURONIAN
CENOMANIAN

83.6
86.3
89.8
93.9
100.5

Cold Fork of Cottonwood
Creek, California, USA
Caspiconcha sp.
from Utagoesawa,
Japan

East of Knoxville andeast Berryessa,
California, USA?

ALBIAN

Caspiconcha sp.
from Spain

113.0

EARLY

126.3

BARREMIAN
HAUTERIVIANVALANGINIANBERRIASIANLATE JURASSIC

Eagle Creek,
California, USA

APTIANTITHONIAN

130.8
133.9
139.4
145.0
152.1

Others
Caspiconcha sp.
from Omagari, Japan

Bear Creek,
California, USA

Caspiconcha sp.
from New Zealand?

Wilbur Springs,
California, USA

Kuhnpasset,
Greenland
Crimea, Ukraine?

Paskenta,
California, USA

Fig. 13. Stratigraphical range of Caspiconcha and Caspiconcha−like species. See text and Tables 1 and 2 for details of species and localities.

JENKINS ET AL.—SEEP RESTRICTED BIVALVE CASPICONCHA REVISITED

Caspiconcha major (Gabb, 1869)
Tithonian–Albian

Caspiconcha sp.
AlbianCaspiconcha whithami Kelly, 2000
Barremian

E

D

C

B

375

?Caspiconcha sp.
Campanian

Myoconcha aff. transatlantica Burckhardt, 1903
Hauterivian

F
A
Caspiconcha sp.
AlbianMyoconcha neuquena Leanza, 1940
Early Jurassic

G

Caspiconcha sp.
Albian–CenomanianH

Caspiconcha rubani Kiel,
Campbell, and Gaillard, 2010
Hauterivian

Fig. 14. Palaeobiogeographical distribution of Caspiconcha and Caspiconcha−like species in the late Mesozoic world’s oceans. A. Palaeomap at 120 Ma fromhttp://jan.ucc.nau.edu/~rcb7/index.html. B. Caspiconcha major (Gabb, 1869), Late Jurassic (Tithonian) to Early Cretaceous (Albian) from California, USA.
The specimen is from the Eagle Creek site. C. Caspiconcha sp., Lower Cretaceous (Albian) of Basque, Spain (image from Agirrezabala et al. in press).
D. Caspiconcha whithami Kelly, 2000, Lower Cretaceous (Barremian) of Greenland (SMUC K 8318, holotype). E. Possible Caspiconcha, described as
Calyptogena sp. in Hikida et al. (2003) from the Upper Cretaceous (Campanian) Omagari site, Hokkaido, Japan. F. Caspiconcha sp., Lower Cretaceous(Albian), Utagoesawa Creek, Hokkaido, Japan. G. C. rubani Kiel et al. (2010), Lower Cretaceous (Hauterivian) of Ukraine (image from Kiel et al. 2010).
H. Caspiconcha sp., Lower Cretaceous to Upper Cretaceous (Upper Albian to middle Cenomanian) of New Zealand (image from Kiel et al. in press).
145.0

Jurassic

100.5

65

Cretaceous
Early

Late

0 (Ma)

23

Paleogene

NeogeneBathymodiolins
Nucinella

Lucinidae

Infauna

Solemyidae

Bivalves (family or genus)

Vesicomyidae

Epifauna/semi-infauna

Caspiconcha

Epifauna

Terebratulida

Brachiopods(order)Rhynchonellida

Thyasiridae
Fig. 15. Ranges and palaeoecological interpretations of major group of chemosynthetic bivalves and brachiopods from Late Jurassic to Recent hydrocarbon
seeps. The epifauna and semi−infauna almost vanished at the end of Early Cretaceous and did flourish again from the Eocene with the appearance ofvesicomyids and bathymodiolins. Caspiconcha was common until the end of the Early Cretaceous after which there was only one occurrence in the Late
Cretaceous. In contrast, infaunal bivalves were present continuously from the late Mesozoic to the Recent.

http://dx.doi.org/10.4202/app.2011.0026

376

ACTA PALAEONTOLOGICA POLONICA 58 (2), 2013

(Taylor and Glover 2009). We shall stress, however, that thislarger seep bivalve occurrence gap could be an artefact result−ing from the general paucity of Maastrichtian and Paleocene
seep deposits known to date.
Substantial faunal changes in chemosynthesis−based com−munities took place during the late Mesozoic to early Paleo−
gene time period (Campbell and Bottjer 1995a). Brachiopods
dominated Palaeozoic to Jurassic chemosynthesis−based com−munities (Campbell and Bottjer 1995b; Sandy 1995, 2010;
Kaim et al. 2010; Peckmann et al. 2011) and then started to de−crease in importance through to the Late Cretaceous, although
the rhynchonellide brachiopod Peregrinella was extremely
abundant in Early Cretaceous hydrocarbon seep communities(Campbell and Bottjer 1995a, b; Kiel and Peckmann 2008).
The abyssochrysoid gastropod genus Paskentana had a simi−
lar pattern of occurrence as Caspiconcha and Peregrinella,being widely present in seeps of the Early Cretaceous, but be−
coming extinct at the end of this time period (Kiel and Peck−mann 2008). From the late Mesozoic to the Paleogene, bi−
valve−dominated chemosynthesis−based communities increa−
sed in number (Campbell and Bottjer 1995a), and many mod−ern−type chemosynthetic molluscs appeared, e.g. vesicomyid
and bathymodiolin bivalves and provannid gastropods.
Having a semi−infaunal and/or epifaunal ecology, Caspi−
concha may have been a competitor to rhynchonellide bra−
chiopods in seep environments during the time period theyco−existed, although the sessile rhynchonellide brachiopods
had less mobility than the bivalves, which, although byssate,
would have been able to move if conditions became sub−opti−
mal. The seep−restricted rhynchonellide brachiopods became
extinct at the end of Early Cretaceous (Kiel and Peckmann
2008), while Caspiconcha survived to the Late Cretaceous in
Japan (Hikida et al. 2003; Kiel et al. 2010), although known
so far from a single specimen at a single locality. After their
extinction the niche for epifaunal and semi−infaunal organ−
isms in hydrocarbon seeps was apparently unoccupied fromthe Late Cretaceous until the appearance of bathymodiolinbivalves in the Eocene (Fig. 14), the reasons for which arepresently unclear.

Conclusions

Caspiconcha major is not a mytilid.—Because Caspiconcha
major was originally considered a mytilid bivalve there was anapparent temporal conflict between the fossil record of seep
restricted mytilid bivalves and the estimated age of divergenceof the subfamily Bathymodiolinae based on molecular analy−
ses, with the oldest “mytilid” (i.e., C. major) specimens being
Tithonian (~150 Ma) and the divergence age estimate being
22–94 Ma (Little and Vrijenhoek 2003). Now, however, withthe taxonomic shift of M. major into the genus Caspiconcha,
seep restricted mytilids no longer range into the Mesozoic
fossil record. Thus the oldest seep restricted mytilid bivalve
is Bathymodiolus willapaensis (Squires and Goedert, 1991)
from lower Middle Eocene to Oligocene strata in Washington
State (Kiel 2006). As a result the fossil record of this group be−comes much more compatible with the estimated moleculardivergence age of the chemosynthetic mytilids.

Acknowledgements

Detailed study of Gabb’s (1869) type series of Modiola ma−
jor Gabb, 1869, other Modiola major specimens from sev−
eral localities in California, and new exceptionally well−pre−served specimens from the Eagle Creek locality, allowed us
to formally transfer this species to the genus Caspiconcha
and family Kalenteridae. Species of this genus had a world−
wide distribution in hydrocarbon seeps of Jurassic to Early
Cretaceous age, but thereafter were rare, with the last occur−
rence from an Upper Cretaceous hydrocarbon seep locality
in Japan. A similar pattern is seen in the seep−restrictedbrachiopods. It remains unclear why these two major epi−
faunal/semi−infaunal groups disappeared in the Early to Late
Cretaceous while the infaunal bivalves were unaffected. The
niche for epifauna/semi−infauna in seeps was then unoccu−
pied until appearance of bathymodiolin and vesicomyid bi−
valves in the Eocene.

Note added in proof
In a recently published paper Kaim and Schneider (2012) in−
vestigated juvenile specimens of Myoconcha crassa (thetype species of Myoconcha) from Middle Bathonian (Middle
Jurassic) clays of southern Poland. Very well preserved
shells of this species displayed original microstructure com−prising two layers: inner complex cross−lamellar and outer
simple cross−lamellar. These microstructure types are thesame as the inner and middle layers of Caspiconcha major
investigated herein. Kaim and Schneider (2012) use this in−formation to argue that both Myoconcha and Caspiconcha ,and by inference the family Kalenteridae, belong to the order
Carditida, as earlier suggested by e.g., Chavan (1954, 1969);
Newell (1957, 1969) and Nevesskaja (2009), rather than the
Pholadomyoida or Palaeoheterodonta (see also discussion in
Damborenea 2004).

We thank Steffen Kiel (University of Göttingen, Germany) and Simon
Kelly (CASP, University of Cambridge, UK) for helpful information
about the studied specimens of Caspiconcha, and for their critical re−views on this paper. We also thank to Manuela Schellenberger (BSPG)
and Grażyna Dziewińska (ZPAL) for photographing some of the speci−
mens, Michael Jollands (University of Leeds, UK) for making some ofthe silicone rubber casts, and Tomoki Kase and Yasunari Shigeta (bothNMNS) for sharing Utagoesawa Caspiconcha specimens. Field study
in California by RGJ, AK, YI, and KT was facilitated by Michael A.
Murphy (University of California, Riverside, USA) and was financially
supported by the Japan Society for the Promotion of Science grant(JSPS, nos. 14255001 and 18403013). The research of RGJ was finan−cially supported by the Nippon Foundation−Hadal Environmental Sci−
ence Education Program (HADEEP) in Atmosphere and Ocean Re−
search Institute, University of Tokyo, a TAWNI award by ChEss andresearch grant for JSPS fellow. AK was supported by ZPAL, JSPS, andJENKINS ET AL.—SEEP RESTRICTED BIVALVE CASPICONCHA REVISITED
the Humboldt Foundation. YI was supported by JSPS. KC was sup−ported by the US National Research Council and NASA Ames Re−
search Center during field collection of specimens.

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Appendix 1
Geographical and geological setting of new Caspiconcha localities in California and Japan.
Eagle Creek, California (Late Barremian).—The locality with
Caspiconcha is exposed along Eagle Creek approximately 10 metres
upstream from its confluence with the North Fork of Cottonwood
Creek near Ono village, Shasta County, California (Fig. 1C–F) (GPScoordinates: N 40°27’55.3”, W 122°36’41.1”). The locality exposes
the upper part of the Lower Chickabally Mudstone Member, Budden
Canyon Formation, Great Valley Group. The age of the Lower
Chickabally Mudstone Member is assigned to the Late Barremian,
indicated by the ammonites Eulytoceras phestum, Ancyloceras ele−phas, Heteroceras jeletzkyi, Shasticrioceras poniente (Murphy 1956,
1975; Murphy et al. 1969). This age determination is also confirmedby recent investigations using nannoplankton fossils (Fernando et al.
2011). The locality was mentioned by Stanton (1895: 48), but has un−
til now not been recognized as a hydrocarbon seep deposit.
Approximately 1−m−sized in diameter carbonate bodies arescattered along 40 metres of Eagle Creek and the North Fork of Cot−
tonwood Creek (Fig. 1D). The carbonate rocks display several tex−tures typical of hydrocarbon seep deposits, i.e., isopachous fibrous
rim cements with stromatolite−like laminae and clotted micrite. The
carbonates and surrounding mudstone contain specimens of Caspi−
concha major, solemyid and lucinid bivalves, unidentified gastro−
pods and numerous decapod crustaceans. The carbon isotopic com−positions of the carbonates display negative values as low as −46‰
VPDB (RGJ, AK, YI, and KT unpublished data). Both the textures
and the carbon isotope composition show the carbonates wereformed under the influence of the anaerobic oxidation of methane,a biogeochemical process typical of hydrocarbon seep activity(Campbell et al. 2002; Campbell 2006). This site is the northern−most Mesozoic seep deposit recognized in the Franciscan−Great
Valley subduction system (Fig. 1c). The Great Valley Group in thisnorthernmost region laps onto the Klamath Mountains terranerather than upon Coast Range Ophiolite basement. Wright and

Wyld (2007) have suggested a different tectonic history for these
northern Great Valley strata.
Utagoesawa, Japan (Late Albian).—The Utagoesawa site is lo−cated along Utagoesawa Creek in the Hatonosu area of Yubari City,
Hokkaido (Fig. 1A, B)(GPS coordinates: N 43°04’88.8” and E
141°55’80.1”). The strata exposed here belong to the Yezo Group,part of the post−Aptian fore−arc basin deposits distributed along thecentral axis of Hokkaido, and extending towards Sakhalin (Taka−shima et al. 2004). In the Hatonosu area, the Yezo Group is repre−
sented by relatively well exposed Upper Albian to Lower Santonian(Cretaceous) deposits (Matsumoto and Harada 1964; Futakami1982).
Our specimens (Table 1) come from an accumulation of carbon−
ate bodies (ca. 1 m in diameter each), distributed along approxi−
mately 10 metres of the southern bank of Utagoesawa Creek (Fig.
1A, B). The carbonates are surrounded by Upper Albian (Lower
Cretaceous) Yezo Group sediments and, in addition to Caspiconchasp., contain numerous specimens of large lucinid and solemyid bi−
valves, and provannid−like gastropods (Fig. 1B; RGJ and AK un−
published data). The Utagoesawa Creek carbonates display textures
typical of hydrocarbon seeps, i.e., isopachous fibrous rim cements,
clotted micrite, and numerous peloids. The carbonates also are char−acterised by depleted carbon isotopic compositions to −47‰ VPDB(RGJ and AK unpublished data). Biomarker analysis performed by
Ogihara (2005) on carbonate samples from this locality revealed
crocetane and pentamethylicosane (PMI) with highly depleted in
13
C carbon isotopic compositions as low as −121.7‰ VPDB, indica−
tive of the presence of anaerobic methane oxidizing archaea. Theselines of evidence strongly suggest that the Utagoesawa Creek car−bonate deposits were also formed by the anaerobic oxidation of
methane in a hydrocarbon seep setting.

Appendix 2
Detailed descriptions of specimens of Caspiconcha major (Gabb 1869)
Gabb’s (1869) type series. Lectotype MCZ 108539 (Fig. 5).—The specimen was collected from East of Knoxville, Lake County,
California (Loc. #1063; according to a label with the specimen), in
the “Knoxville beds” of the lower Great Valley Group, Lower
Cretaceous, and figured by Gabb (1869: pl. 31: 80) and Stewart(1930: pl. 4). The shell is elongated cuneiform to mytiliform inshape. The left valve retains original shell; the right valve ismostly an internal mould apart from the most posterior portion andventral parts where shell material is preserved. The left valve con−tains a deep triangular depression extending from umbonal region
to the mid−ventral part. The shell is strongly inflated in the area
immediately posterior to the triangular depression. However, thismight be diagenetically enhanced as the shell appears to beslightly compressed in the vertical plane. Regular commarginal
growth lines are visible on the anterior part. There is a shallow in−dentation in the shell outline in the middle part of the ventral mar−gin, which also could result from vertical diagenetic compressionof the shell.
The myophoric buttress is deep. The anterior adductor muscle
scar is small and deep, seemingly slightly delimited from the anterior
pedal retractor muscle scar, although this feature might be obscured
by excessive preparation. The imprint of a caspiconchid process isclearly visible anterior to the adductor muscle scar. The hinge isstraight and extends over 51.3 mm dorsal to the umbonal area. Poste−rior to the hinge there is a long groove visible parallel to the dorsal
shell margin, which apparently is an imprint of the ligamental ridge(thickened shell margin). This structure was apparently interpretedby Stewart (1930) as a “long posterior lateral” and consequently citedby Griffin and Pastorino (2006) as a lateral tooth. The shell is 131.1
mm long, 59.7 mm high, and 33.9 mm wide.

JENKINS ET AL.—SEEP RESTRICTED BIVALVE CASPICONCHA REVISITED

Gabb’s (1869) type series. Paralectotype MCZ 108540 (Fig. 6A).
—This specimen was collected from East of Knoxville, Lake
County, California (Loc. #1063; according to a label with the speci−men), in the “Knoxville Beds” of the lower Great Valley Group,
Lower Cretaceous. The anterior part of the shell is composed of origi−
nal shell (poorly preserved) of the right valve and an internal mouldof the left valve. The left valve displays characters of the internal
shell morphology. The myophoric buttress is deep and steep. The
adductor muscle scar is deep and delicately separated from the ante−rior pedal retractor muscle scar. A pitted pedal elevator muscle scar iswell expressed immediately below the dorso−anterior end of myo−phoric buttress. The hinge and umbonal area poorly preserved.
Gabb’s (1869) type series. Paralectotype MCZ 108538 (Fig.
6B–D).—Three juvenile specimens A, B, C (A and B on a single
piece of rock) were collected from Wilbur Springs, Colusa County,
California (Loc. #1046), Great Valley Group, Hauterivian (Lower
Cretaceous). Specimen MCZ 108538A (Fig. 6B) is an almost com−plete juvenile left valve. It is an internal mould with shell material
remaining in the anterior and middle of ventral part and dorso−pos−terior part. The shell is associated with a brachiopod coquina (Pere−grinella whitneyi). The shell is cuneiform and the myophoric but−
tress is deep. The anterior adductor muscle scar is deep and thehinge line is straight. There is a concavity in the mid−part of ventral
shell region. Specimen MCZ 108538B (Fig. 6C) comprises an in−
ternal mould of a juvenile (but larger than MCZ 108538A) rightvalve with anterior part preserved. The myophoric buttress is deep
but not steep. The anterior adductor muscle scar is deep and some−what separated from anterior pedal retractor muscle scar. The um−
bonal and hinge area is poorly preserved but the hinge line appar−
ently straight and an impression of the ligamental ridge is partiallypreserved. Specimen MCZ 108538C (Fig. 6D) constitutes a rightvalve internal mould, with shell material still present in the poste−rior and mid−ventral parts. The myophoric buttress is deep but notsteep. The anterior adductor muscle scar is deep but the tip of its
mould is broken.
Stanton’s (1885: pl. 3: 1) Caspiconcha major (Gabb, 1869).
USNM 23041 (Fig. 7).—The specimen was collected from Cold
Fork of Cottonwood Creek, Tehama County, California. Lodoga
Formation, Great Valley Group, of Aptian–Albian (Early Creta−
ceous) age. It is an internal mould of a right valve with very few
remnants of recrystallised shell material in the anterior part. In sev−
eral places the shell is not completely eroded so the internal featuresof the shell are obscured. The most anterior part of the shell is bro−ken. The left valve appears to be present in the matrix. A cross−sec−tion of the most posterior part of the left valve is visible. The matrix
is a solid beige coloured carbonate with some admixture of broken
bivalve and gastropod shells, including a few other small specimensof Caspiconcha. These specimens would need further preparationin order to be described adequately.
The shell is wide cuneiform in shape and strongly inflated in theanterior part. It becomes much flatter in the posterior and dorsal
parts that comprise the adult portion of the shell. The triangular de−pression running from the umbonal area and widening towards themid flank is well−developed. The myophoric buttress is poorly pre−served, but it seems to be rather shallow (similar to the specimen
CAS 72527−9 from Eagle Creek). The anterior muscle scar is notclearly visible due to the poor state of preservation. The posterior

381

muscle scar is not preserved, or very faint (Fig. 7). The caspi−
conchid process is invisible under the overlying shell material. The
hinge is almost straight and extends over the 74.5 mm of the dorsal
margin of the shell. No hinge characters are preserved. The growthlines accumulate gradually, apart from the most ventral part where
the growth lines occur more densely. The shell is 201 mm long,
112.7 mm high, and 32.5 mm wide.
Caspiconcha major (Gabb, 1869) from Eagle Creek.—Specimen
CAS 72527−9 (Fig. 3) is an almost complete adult shell lacking theposterior part of the left valve. Specimen CAS 72530 (Fig. 8A) is an
adult shell with articulated valves imperfectly preserved, and alarge part of dorsal margin missing. Specimen CAS 72531.1 (Fig.
8B) is an almost complete juvenile shell with articulated valves.
The anteriormost part of the shell is broken; the right valve shellmaterial has peeled off; and the counterpart of the shell preserved asa separate specimen CAS 72531.2.
The best preserved shell CAS 72527−9 (Fig. 3) is large andstrongly elongated. In lateral aspect it changes during ontogeny
from subtrapezoidal to cuneiform–modioliform shape, with moder−ate inflation in the anterior near the umbo. The valves are equivalve
and strongly inequilateral. The umbones are close to anterior end
and are slightly elevated above the hinge line. The shell thickness isvariable, being thickest at the myophoric buttress (anterior adductor
muscle scar area), relatively thick in the antero−dorsal to antero−ventral area, and thinning conspicuously in the posterior−ventral to
mid−flank of the ventral area. In the antero−dorsal to ventral area theshell thickens, and the lateral shell growth is limited in comparison
to the posterior part. A triangular depression runs from the umbonal
area widening towards the mid flank. The dorsal margin is straightin lateral profile for the extent of the nymph and ligament area, andthen curved ventrally at the postero−dorsal area. The posterior mar−gin is well rounded. The ventral margin is almost straight, and fee−
bly indented at the mid−flank, probably for a byssal attachment. The
anterior margin is short and rounded. The shell is moderately in−flated in dorsal aspect, with a wedge shape in both anterior and pos−terior margin. The exterior ornament is commarginal, and enhancedin the shell anterior. At least four shell layers are preserved, with theoutermost and innermost layers composed of brown coloured dia−
genetic calcite. The two inner layers are aragonite and whitish.
The shell interior is smooth apart from a pitted mantle muscle
scar area in the anterior part of the pallial region. The pallial line iswell expressed running comarginally from the anterior adductor
muscle scar towards the posterior, and then turns upward to the pos−terior of the posterior adductor muscle scar, with no pallial sinus.
An internal ridge runs from the anterior pedal elevator muscle scar
towards the ventral margin of the posterior adductor muscle scar,paralleled by an oblique groove on its ventral side. the anterior
adductor muscle scar is rounded with two pedal retractor muscle
scars located dorsally and subdivided by a narrow but conspicuous
undulation of the caspiconchid process. This process is narrow andweakly expressed, with the posterior tip broken in the available
specimens. The myophoric buttress is moderately steep. The poste−rior adductor muscle scar is rounded, with a dorsally located narrow
posterior pedal retractor muscle scar projected anteriorly. The hinge
is stout and edentulous. The nymph and ligament groove are very
long and straight. The resilifer is very narrow and elongated. It is
sharply pointed both to the anterior and posterior, and widest in its
mid−part immediately after the end of the anterior adductor muscle
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382

scar. The external ligament is sturdy and triangular in cross−section,with a well−mineralized C−spring−type aragonite fibrous sublayer.
The small, presumably juvenile, specimen CAS 72531 is
25 mm long, 16 mm high, and 8 mm wide. The shell is water−dropshaped and is very thin and generally much less elongated than theadult shells. The hinge is narrow and long extending at least up to
the 2/3 of the dorsal shell margin, and covered along the wholeextent by a corresponding long and narrow exterior ligament. The
inner surface of the hinge is visible on the counterpart (CAS72531.1), including a circular anterior adductor muscle scar with nomyophoric buttress developed.
Shell mineralogy and microstructure have been observed from
specimen CAS 72527−9 (Fig. 4). The shell can be divided into threesublayers (Fig. 4A). The outermost layer is homogeneous (Fig. 4C).
The middle layer displays cross lamellar structure (Fig. 4A, B)
while the inner layer is complex cross lamellar (Fig. 4A, B). XRD
analysis shows that the original shells are composed entirely of ar−agonite. The ligament is also biomineralized by aragonite. Someparts of the shell underwent partial recrystallisation to calcite (seedark spots on Fig. 4A).
Caspiconcha major (Gabb, 1869) from Cold Fork of Cotton−
wood Creek.—Specimen UCMP 10225 (Figs. 9A, 10E) is the in−ternal mould of a small articulated shell. Specimen UCMP 10226(Fig. 10B) is an internal mould of a small right valve. Specimen
CAS 72532 is an internal mould of an articulated adult shell (shellmaterial partially preserved with the posterior half missing). Speci−
men CAS 72533 comprises an internal mould of a left valve, with
its hinge and anteriormost parts broken.
The shells from Cold Fork of Cottonwood Creek are strongly
elongated, cuneiform to modioliform in both young and adult stages,
with a moderate inflation in the anterior near umbonal area. The
shells are equivalve and strongly inequilateral; the umbones are close
to the anterior end and slightly elevated above, or equal to, the hingeline. The shell thickness is uncertain. A triangular depression runs
from the umbonal area and widens towards the mid flank. The dorsal
margin is straight in lateral profile for the extent of the ligamental
area and is then curved ventrally at the postero−dorsal area. The pos−terior margin is well rounded while the ventral margin is almost
straight, and feebly indented at the mid−flank. The anterior margin isshort and rounded. The shells are moderately inflated in dorsal as−
pect, with a wedge shape in both anterior and posterior margin.
The shell interior is smooth with no internal ridge visible. The
anterior adductor muscle scar is rounded with the pedal retractor
muscle scars located dorsally. The myophoric buttress is moder−ately steep. The ligament groove is very long and straight.
Caspiconcha major (Gabb, 1869) from Bear Creek (CAS 72534;
Fig. 9J).—The specimen is an articulated internal mould of a large
specimen with the anteriormost and posteriormost parts missing.
The shell is large and strongly elongated. Being an internal mould
the shell thickness is unknown. The shell interior is smooth apartfrom pitted mantle muscle scar area in the anterior part of the pallial
region. The pedal elevator muscle scar is well visible. The pallial
line is well expressed, running comarginally from the myophoric

ACTA PALAEONTOLOGICA POLONICA 58 (2), 2013

buttress towards the posterior. An internal ridge runs from anterior
pedal elevator muscle scar towards ventral margin of the posterior
adductor muscle scar, parallel to the pallial line. The anterior
adductor muscle scar and caspiconchid process are not preserved.
The myophoric buttress is moderately steep. The probable posterior
adductor muscle scar is oval to rounded. The nymph and ligamentgroove is very long and straight.
Caspiconcha major (Gabb, 1869) from East Berryessa.—Speci−men (CAS 72535; Fig. 10A) is a right valve with anteriormost andposterior parts missing. The shell is almost entirely preserved.
Specimen CAS 72548 (Fig. 9H) is an internal mould of a young left
valve. Specimen CAS 72536 (Figs. 9I, 10D) is an internal mould of
the anterior half of an articulated shell.
The shell is moderate to small in size, elongated modioliform,
with moderate to strong inflation in the anterior near umbonal part.
The valves are strongly inequilateral. The dorsal margin is straightin lateral profile for the extent of the ligament area and then curved
ventrally in the postero−dorsal area. The posterior margin is wellrounded, while the ventral margin is almost straight. The anterior
margin is short and rounded. The shell is moderately inflated in dor−sal aspect with a wedge−shaped anterior margin. The shell interior issmooth. The pallial line and internal ridge are not seen. The anterior
adductor muscle scar is rounded with two pedal retractor muscle
scars located dorsally. The myophoric buttress is moderately steep
for CAS 72535 and 08, and is steep for CAS 72536. The pedal ele−vator muscle scars are well visible on CAS72536. The posterior
adductor muscle scar is not seen. The hinge is stout and edentulous.
The ligament groove is very long and straight. The shell is very
thick in its anterior part while it is much thinner in the posterior part.
Caspiconcha major (Gabb, 1869) from Wilbur Springs.—Speci−
men CAS 71880 (Fig. 9C) is a right valve. Specimen CAS 71882(Fig. 9D) is an internal mould of left valve. Specimen CAS
71881(Fig. 9E) is internal mould of left valve. CAS 72537 (Fig. 9G,
10C) is an articulated specimen. Specimen CAS 71883 (Fig. 9F) isan internal mould of a right valve. Specimen UCMP 152077 (Fig.
9B) is an internal mould of a minute left valve.
The shells are small and elongated, cuneiform to modioliform,
and moderately inflated in the anterior (adjacent to umbonal) part.
The valves are equivalve and strongly inequilateral. The umbones arelocated close to the shell anterior. A triangular depression runs fromthe umbonal area, widening towards the mid flank. The dorsal mar−gin is straight in lateral profile for the extent of the nymph and liga−
ment area, and then curved ventrally in the postero−dorsal area. The
posterior margin is well rounded while the ventral margin is almost
straight, feebly indented at the mid−flank. The anterior margin isshort and rounded. The shell is moderately inflated in its dorsal aspect
and wedge−shaped at both anterior and posterior margins. The exte−rior ornament consists of commarginal growth lines. The shell inte−rior is smooth. The pallial line is poorly seen. An internal ridge is notpresent. The anterior adductor muscle scar is rounded with two pedal
elevator muscle scars located dorsally. The myophoric buttress ismoderately steep. The hinge is stout and edentulous. The nymph andligament groove are very long and straight.