(1) 11–21 4 text-figs, 1 pl. Frankfurt am Main, 30.06.2008
With 4 Text-figures and 1 Plate
Mark Williams, David J. Siveter, María José Salas, Jean Vannier,
Leonid E. Popov & Mansoureh Ghobadi Pour
The oldest assumed ostracods appear in the fossil record from the Tremadocian Paltodus deltifer cono-
dont Biozone. Although geographically widespread these early ostracods have no obvious Cambrian an-
tecedents. Their first appearance at ca. 485 Ma contrasts with molecular evidence that suggests a much
earlier (latest Proterozoic or Cambrian) origin for ostracods. Some Cambrian bivalved arthropods such
as Altajanella and Vojbokalina, conventionally referred to the Bradoriida, have carapace morphologies
that resemble Ordovician palaeocopid ostracods, though such a relationship is unproven without soft
part anatomy. Evidence from preserved soft anatomy demonstrates that Bradoriida, such as Kunmingella,
and Phosphatocopida, essentially the Cambrian ‘ostracod’ record of traditional usage, belong outside the
Eucrustacea. Early Ordovician ostracods appeared first in shallow marine, oxygenated environments on
shelf margins, in a similar setting to other elements of the ‘Paleozoic fauna’. Their biodiversity was low
(3 named genera and ca. 12 species), though some taxa such as Nanopsis and Eopilla achieved widespread
dispersal between major Ordovician palaeocontinents. As bradoriids were largely extinct by the Late Cam-
brian, ostracods do not appear to have directly competed with them for shallow marine environments.
The rapid colonisation of these settings by ostracods may have been facilitated by the available ecospace
vacated by Bradoriida.
Key words: Ostracoda, Bradoriida, Phosphatocopida, Tremadocian, biogeography
There are an estimated 65,000 living and fossil species of the
crustacean Class Ostracoda, divided in some schemes into
two major subclasses, the Podocopa and the Myodocopa (e.g.
Horne et al. 2002). Despite the abundance of ostracods in
the fossil record, their origin(s) remains enigmatic. Evidence
from molecular studies suggest that ostracods lie at or near
the base of the Pancrustacea (for definition see Mallatt
& Giribet 2006; Regier et al. 2005), suggesting an origin
perhaps during the latest Proterozoic (see Regier et al. 2005:
395, 400). The oldest undoubted ostracods – with their soft
anatomy preserved – are from 425 million year old deposits
in the Silurian of England (Text-fig. 1; Siveter et al. 2003a,
2007). The record of putative fossilized ostracod carapaces (the
term ‘carapace’ is used in the sense of Siveter et al. 2003)
suggests an older history for the group extending back at least
to the Early Ordovician (Tinn & Meidla 2004; Salas et al.
2007; see also Newman 2005). Two arthropod groups with
bivalved carapaces that have traditionally been suggested as
the forerunners of ostracods (e.g. Sylvester-Bradley 1961),
namely the bradoriids and phosphatocopids, are abundant in
Cambrian rocks worldwide. These two groups first appeared
in the Early Cambrian (Text-fig. 1; Williams et al. 2007) and
were largely extinct by the earliest Ordovician. Although pos-
sessing a bivalved carapace with inner and outer lamellae, evi-
Dr Mark Williams (Corresponding author <firstname.lastname@example.org>) & Prof. Dr David J Siveter, Department of Geology, University of Leicester,
Leicester LE1 7RH, UK. – Dr María José Salas, Centro de Investigaciones Paleobiológicas CIPAL, Facultad de Ciencias Exactas, Físicas y
Naturales, Universidad Nacional de Córdoba, Avenida Vélez Sarsfield 299, 5000 Córdoba, Argentina. – Dr Jean Vannier, Université Lyon 1,
UMR 5125 Paléoenvironnements & Paléobiosphère, Bâtiment GEODE - 2, rue Raphaël Dubois, 69622 Villeurbanne cedex, France. – Dr Leo-
nid E. Popov, National Museum of Wales, Department of Geology, Cathays Park, Cardiff CF10 3NP, UK. – Dr Mansoureh Ghobadi Pour,
Department of Geology, Faculty of Sciences, Gorgan University of Agricultural Sciences and Natural Resources, Gorgan 49138-15739, Iran.
© E. Schweizerbart’sche Verlagsbuchhandlung (Nägele u. Obermiller), 2008, ISSN 0037-2110
The earliest ostracods: the geological evidence
dence from soft-part anatomy indicates that phosphatocopids
are sister-group to the Eucrustacea and cannot be ancestors of
ostracods (Text-fig. 2; Siveter et al. 2001, 2003a; Maas et al.
2003). The only known bradoriid species with detailed soft
anatomy preserved, Kunmingella douvillei (Text-fig. 2; Hou et
al. 1996, 2002; Shu et al. 1999), has a morphology which
places it on the stem-line of the Crustacea. Appendages have
also been reported in a possible oepikalutid bradoriid, but have
not been described in detail (Hinz-Schallreuter 1993). An-
other group of putative ostracods that are well represented in
the Early Palaeozoic, the ‘Order Leperditicopida’, have large
bivalved carapaces typically 5-50 mm in length. However, as
their soft anatomy is unknown, their biological affinities are
uncertain (for a review of the group see Vannier et al. 2001).
The fossil record of small bivalved arthropods through the
Cambrian-Ordovician transition poses important questions
for the origins of the ostracods and even perhaps for other ar-
thropod groups (see Newman 2005). Were some bradoriids
ancestral to the Ostracoda? Where and when do the first podo-
cope or myodocope ostracods appear? What was their early
biogeographical distribution? What kinds of environments did
they evolve in? And, did they supplant earlier bivalved arthro-
Recognising ostracods in the fossil record
The phylogenetic position and classification of the Ostracoda
based on carapace and soft part morphology (e.g. Horne et
al. 2002, 2005) and molecular evidence (e.g. Yamaguchi &
Endo, 2003) is in a state of flux. Indeed, a monophyletic versus
polyphyletic origin for the Ostracoda is debated (e.g. Regier
et al. 2005; Newman 2005), and even the systematic relation-
ships of such major taxa as the Palaeozoic Order Palaeocopida
(placed by some within the ostracod Subclass Podocopa; e.g.
Horne et al. 2002) are still to be resolved (Yamaguchi &
Endo 2003). Three areas of investigation provide clues to the
earliest occurrence of ostracods: evidence from molecular biol-
ogy; recognition of fossilised soft anatomy; and recognition of
fossilised hard tissues (the carapace).
Molecular evidence supports the Ostracoda and Branchiu-
ra either as a paraphyletic clade at the base of the Pancrustacea,
or separates the Podocopa at the base of a clade that includes
cirrepedes and copepods, with the Myodocopa grouped with
the Branchiura at the base of the Pancrustacea (Regier et al.
Text-fig. 2. Figures 1–3. Phosphatocopid (1, 2)
and Bradoriid arthropods (3) preserved with soft
anatomy. Museum abbreviations are: OUM,
Oxford University Museum; RCCBYU, Re-
search Centre for Chengjiang Biota, Kunming
University, Yunnan, China. 1, 2, Klausmuelle-
ria salopiensis Siveter, Waloszek & Williams,
2003, anterior aspect (stereo-pair) and oblique
posterior views (RV to bottom) respectively,
× 260-OUM A.2209. 3, Kunmingella douvillei
(Mansuy, 1912), carapace in butterfly orienta-
tion, LV to bottom, × 7.5-RCCBYU 10258.
Text-fig. 1. Main events in the appearance of ostracods in the fossil
record. The dates of the events should be considered approximate. 1,
Origin of the Pancrustacea (Regier et al. 2005); 2, oldest Bradoriida
(Hou et al. 2002; Williams et al. 2007); 3, major extinction of the
Bradoriida during the latest mid Cambrian (age calibrated from
Shergold & Cooper 2004); 4, oldest ostracods (age calibrated from
Cooper & Sadler 2004); 5, youngest bradoriids (see Ghobadi Pour
et al. 2007, age calibrated with Cooper & Sadler 2004); 6, earliest
myodocopes (Gabbott et al. 2003, age calibrated with Cooper &
Sadler 2004); 7, oldest ostracods (myodocopes) with preserved soft
anatomy (Siveter et al. 2003b, 2007).
The earliest ostracods: the geological evidence
2005). Although molecular evidence suggests that Ostracoda
may be present in the latest Proterozoic or Early Cambrian
(cf. Regier et al. 2005), there is no fossil evidence to support
It is largely soft anatomy that defines the Ostracoda, and
its subdivision into the Myodocopa and Podocopa (see Horne
et al. 2002), but fossil ostracods with preserved soft anatomy
are extremely rare (Smith 2000; Siveter et al. 2003b, 2007).
Daunting for recognising the relationships of early fossil os-
tracods is the realisation that carapace design can be very mis-
leading: Siveter et al. (2007) have shown in Silurian mate-
rial that a myodocope body can reside in a palaeocopid-like
carapace. Indeed, modern myodocopes display a great variety
of carapace design although their morphology remains consis-
tently similar. The pre-Silurian fossil record of ostracods can-
not currently resolve these debates as there are no Cambrian
or Ordovician ostracods preserved with soft tissues. However,
some rock successions, such as the Early Ordovician Merevale
Shales of England, do yield ostracod-like bivalved arthropods
with soft-tissue preservation (Plate 1; Siveter et al. 1995) and
deserve further exploration.
Where fossil material lacks soft tissue preservation, struc-
tures such as muscle scars, preserved on the inner surface of
the valves are of considerable value and have been used exten-
sively in taxonomy (e.g. Siveter & Vannier 1990). As there is
no soft tissue or muscle scar preservation in Early Ordovician
fossil material, recognition of the occurrences of ostracods is
based on carapace morphology alone. Even then, the earliest
Ordovician ostracods lack such distinctive carapace structures
as domicilar or extradomicilar dimorphism, or evidence about
hinge articulation, features that are significant for Palaeozoic
ostracod taxonomy (e.g. Henningsmoen 1953; Jaanusson
The characters used herein for recognising the earliest
(Tremadocian) ostracods invoke uniformitarian principles, that
is, morphological similarity of the carapace to post-Tremado-
cian ostracod faunas (Plate 1; see also Tinn & Meidla 2004).
On this basis these early Ordovician supposed ostracods ap-
pear to be assignable to the orders Palaeocopida and Binodi-
copida (some authors would subsume the Binodicopida into
the Palaeocopida, see ‘Superfamily Drepanellacea’ of Whatley
et al. 1993). The criteria we use to recognise the earliest ostra-
cods are: 1) a primary mineralised calcium carbonate carapace;
2) a bivalved carapace (typically around 0.5-1.5 mm in length,
and much smaller than most Bradoriida); 3) lobation in which
the lobes are oriented dorsal to ventral (like the palaeocopid
Tetradella), or which involve inflation of the ventral part of
the valve (characteristic for many Palaeocopida); and 4) devel-
opment of a distinct adductorial sulcus and associated post-
and pre-adductorial nodes. Some of these characters overlap
with those of Bradoriida and Phosphatocopida: the latter have
a number of carapace features that are not seen in ostracods,
particularly the development of an interdorsum in Phosphato-
copida, or relatively large size in Bradoriida (typically 0.25-1.7
cm long; e.g. see Siveter & Williams 1997 and Williams &
In the following discussion we focus on documenting the
earliest binodicopids and palaeocopids. We have not included
the Leperditicopida. Although conventionally treated as os-
tracods (e.g. Whatley et al. 1993), their systematic affinities
are unclear, compounded by the absence of post-Devonian
representatives of the group and of evidence from soft tissue
preservation (Vannier et al. 2001). Leperditicopids may have
appeared first in the Late Cambrian (Frederickson 1946),
overlapping with Bradoriida. Confirmed representatives of the
group appear in rocks of Floian age (Ordovician) in Spitsber-
gen (Williams & Siveter 2008).
The oldest ostracods
Most major groups of the ‘Paleozoic fauna’ first appear in the
Cambrian (Sepkoski, 1990). However, the record of ostracods
from this period is contentious. The Cambrian to Early Or-
dovician bradoriid and phosphatocopid bivalved arthropods
were part of the Cambrian radiation (e.g. Hou et al. 2002;
Williams et al. 2007). These two groups have traditionally
been considered as the earliest representatives of the Ostracoda
(e.g. Sylvester-Bradley 1961; Müller 1964, 1979; Jones
& McKenzie 1980) and some authors maintain that opinion
(Hinz 1993; Gozalo & Hinz-Schallreuter 2002). These ar-
guments centre on similarities between the ostracod, bradoriid
and phosphatocopid carapace (see Plate 1), all of which may be
bivalved, lobate, and possess an inner lamella (e.g. Williams
et al. 1994; Siveter & Williams 1997; Siveter et al. 2001).
However, it is generally accepted that a bivalved carapace has
been acquired convergently by a number of different malacos-
tracan, branchiopod and maxillopod crustaceans.
Bradoriids first appear in Lower Cambrian rocks, slightly
below the oldest trilobites (Williams et al. 2007). Like trilo-
bites, Bradoriida emerge highly differentiated into a number of
species and with a global distribution, suggesting a period of
evolution for which there is no fossil record. Bradoriida have
long been regarded as polyphyletic (Jones & McKenzie 1980),
and encompass a wide range of carapace designs. Nevertheless,
several species have been resolved that may be closely related
(Williams et al. 2007). These have a bivalved carapace, with
the dorsal portion of the two valves apparently connected by
a thin band of cuticle (without an articulating hinge as in the
palaeocopids or binodicopids), an amplete or postplete shape,
a concentration of lobes anteriorly or anterodorsally (though
other lobes can be developed more centrally or posteriorly),
and a lateroadmarginal ridge (see Plate 1, figs 1, 6, 8, 11). The
only known bradoriid with well preserved soft part anatomy is
Kunmingella and its anatomy, particularly the undifferentiated
2nd to 4th cephalic appendages, precludes its placement within
the Ostracoda or even the Eucrustacea (Hou et al. 1996, 2002;
Shu et al. 1999). Appendages of a possible oepikalutid bra-
doriid have also been figured but are not described in detail
(Hinz-Schallreuter 1993). Soft part anatomy is unknown
in the type bradoriid, Bradoria (see Plate 1, fig. 8).
A number of Cambrian bivalved arthropods that have
been referred to the Bradoriida s.l. (for characterisation see
Williams et al. 2007) have carapace designs that strongly re-
semble post-Cambrian ostracods, particularly Vojbokalina and
Altajanella (Williams et al. 2007). These bradoriids possess a
straight dorsal outline, have multilobate valves, with the lobes
arranged dorso-ventrally like the Ordovician tetradellid pal-
aeocopids (see Plate 1, figs 10, 12; see also Henningsmoen
1953) and are small (millimetric) in scale. It is possible that
some taxa referred to the Bradoriida s.l. might be ostracods,
though without information from soft anatomy across a range
The earliest ostracods: the geological evidence
Plate 1. Early Palaeozoic bivalved arthropods. Bradoriida (1, 6, 8, 10–12), Ostracoda (2-5?, 7?, 9, 13–15) and Phosphatocopida (16). Museum
abbreviations are: MCZ, Museum of Comparative Zoology, Harvard University, USA; BGS, British Geological Survey, Keyworth, Nottingham,
UK; SM, Sedgwick Museum, Cambridge University, UK; PIN, Palaeontological Institute, Moscow, Russia; NHM, Natural History Museum,
London, UK; RCCBYU, Research Centre for Chengjiang Biota, Kunming University, Yunnan, China; CORD-MP, Museo de Paleontología,
Universidad Nacional de Córdoba, Argentina.
Beyrichona triceps (Matthew, 1903). An open carapace, RV to bottom × 16. Tremadocian, Arenaceous Beds, Shineton Shale
Formation, Shropshire, England. – BGS RX2277.
Rivillina? sp. An open carapace, LV to bottom × 18.5. Tremadocian, Shineton Shale Formation, Shropshire, England. – SM
The earliest ostracods: the geological evidence
of different bradoriid taxa, the relationships between the earli-
est Ostracoda and some ‘ostracod-like’ Bradoriida remain hy-
Phosphatocopids also appear in the Early Cambrian (e.g.
Siveter et al. 2001), and like bradoriids were global in their
distribution. Details of their soft anatomy, known from sev-
eral different taxa including falitids, hesslandonids and vestro-
gothiids (Müller 1964, 1979, 1982), clearly precludes them
from being ostracods (Siveter et al. 2001, 2003a; Maas et al.
The earliest occurrence of ostracods based on carapace
alone is during the Early Ordovician, (herein; see also Tinn
& Meidla, 2004; Salas et al. 2007), from rocks about 485
million years old. These ostracods are multi-lobate forms such
as the palaeocopids Eopilla and Nanopsis, and the binodicopid
Kimsella (see Salas et al. 2007). They have a straight dorsal
outline and a small (millimetre-scale) carapace (see Plate 1,
figs 9, 13). The geological record of myodocopes is somewhat
younger. The oldest myodocopes with preserved soft anatomy
are those of the Silurian Herefordshire Lagerstätte (ca. 425 Ma;
Siveter et al. 2003b), though records of myodocopes based
on carapace morphology extend back to the Late Ordovician
(Gabbott et al. 2003).
Stratigraphical occurrence of the earliest
ostracod faunas: the Tremadocian
The base of the Tremadocian Stage (and of the Ordovician
System) is defined at the level of the first occurrence of the
conodont Iapetognathus fluctivagus (Cooper & Sadler 2004).
Radiometric dates for the Stage place its lower boundary at
488.3 ± 1.7 Ma, and its upper boundary at 478.6 ± 1.7 Ma
(Cooper & Sadler 2004). The Tremadocian is characterised
by a global marine transgression, following a prolonged period
of a lowstand during the Late Cambrian (Artyushkov et al.
2000). Well developed graptolite and conodont biozonations
permit correlation of Tremadocian strata worldwide (Cooper
& Sadler, 2004). Where we use local Tremadocian stratigra-
phy we indicate how this correlates with the standard grapto-
lite and conodont biozonations (Cooper & Sadler 2004).
Ostracods first appear at about the level of the P. deltifer
conodont Biozone. This appearance coincides with fundamen-
tal changes in marine biota following the demise of much of
the ‘Cambrian fauna’ through the interval of the C. andreis to
C. angulatus conodont biozones (Sturesson et al. 2005). Os-
tracods of Tremadocian age are known from southern Britain,
Scandinavia, Iran, Argentina, China and possibly Australia.
Despite having extensive deposits of Tremadocian age, no ear-
liest Ordovician, pre-Floian age ostracods have been reported
from North America or Russia.
A poorly preserved ostracod species (Plate 1, fig. 2) charac-
terises the upper part of the Shineton Shale Formation at the
level of the ‘Mignetian Stage’ C. salopiensis trilobite Biozone
in Shropshire (Williams & Siveter 1998; see Rushton et al.
1999, fig. 6.2 for stratigraphy). This species may be conspe-
cific with Shropshire material recorded as ‘primitiae’ (Jones
& Holl 1869) and Primitia spp. (Stubblefield & Bulman
1927). Much of the locally defined ‘Mignetian Stage’ strata are
non-graptolitic, but in the British sequence the C. salopiensis
Biozone immediately precedes an interval bearing the zonal
graptolite A. murrayi (see Cooper & Sadler 2004). Indeter-
Figs 3–5, 7. Ostracod-like arthropod, Tremadocian, central England (see Siveter et al. 1995).
3. Anterior part of carapace with two valves slightly displaced (close-up of anterior of Fig. 5), × 20. Southam Borehole.-BGS
4. Presumed LV lateral view, × 15.5. Berry Fields Farm Borehole.-BGS B7195.
5. Carapace with two valves slightly displaced and anterior appendages, × 15. Southam Borehole.-BGS BDM4841
7. BGS BDM4842 (counterpart of BDM4841), × 15.5. Southam Borehole.
Septadella jackmanae Stubblefield, 1933. LV lateral view, × 27.5. Tremadocian, Breadstone Shales, Gloucestershire, England.-
BRSMG Cs 1813
Bradoria scrutator Matthew, 1886. RV lateral view, × 7.7. Middle Cambrian, Dugald Brook Formation, Dugald Brook, Nova
Scotia, Canada. – USNM 483189.
Kimsella luciae Salas, Vannier & Williams, 2007. RV lateral view, × 46. Tremadocian, lower part of the Parcha Formation, Abra
de Sococha Section, Province of Salta, Argentina. – CORD-MP 11186.
Altajanella costulata Melnikova, 1992. RV lateral view (of silicon rubber mould), × 46. Upper Cambrian, Tandoshka Formation,
Gorny Altay. – PIN N4346/1.
Tsunyiella luna Zhang, 1974. Open carapace, LV to bottom, × 8.3. Lower Cambrian, Guizhou Province, China. – RCCBYU
Vojbokalina magnifica Melnikova, 1984. Carapace, left lateral view, × 36. Middle Cambrian, Leningrad Region, Russia – PIN
Nanopsis coquena Salas, Vannier & Williams, 2007. LV lateral view, × 39. Tremadocian, Upper Member of the Coquena
Formation, Quebrada Chalala Section, Cordillera Oriental, Argentina. – CORD-MP 11179.
Tetradellina henningsmoeni Harris, 1957. Carapace, right lateral view, × 118. Upper Ordovician, Bromide Formation, Oklahoma,
USA. MCZ 4643.
Fig. 15. ‘Bolbozoe’ sp. nov. A of Siveter, Vannier & Palmer, 1987. RV, lateral view, × 15. Upper Silurian, Long Mountain Silstone
Formation, Powys, Wales. – NHM OS13060.
Cyclotron sp. C of Williams & Siveter, 1998. LV lateral view, × 20. Upper Cambrian, Outwoods Shales Formation, Warwickshire,
England. – BGS BDA1261.
The earliest ostracods: the geological evidence
minate ‘ostracods’ spp. also occur in deposits of Tremadocian
age in boreholes in central England (Williams & Siveter
1998), as does an ‘ostracod-like’ bivalved arthropod with ap-
pendages preserved (Siveter et al. 1995). The latter is derived
from strata in the ‘R. flabellimormis Biozone’, suggesting an
early Tremadocian age. Although only the anntenules of this
arthropod are preserved, its carapace morphology is suggestive
of a myodocope (Plate 1, figs 3-5, 7).
Ostracods occur in the Bjorkasholmen Formation (previously
known as the Ceratopyge Limestone) of Norway and the Alum
Shale of Sweden at the level of the P. deltifer conodont Biozone.
The fauna includes one described palaeocopid, Nanopsis nanel-
la (Henningsmoen 1954; Tinn & Meidla 2004).
In Iran, the oldest ostracods occur in the Lashkarak Formation
of the Alborz region (Text-fig. 3) and are associated with the
trilobite Psilocephalina lubrica, indicating a mid Tremadocian
age, and likely below the level of the ‘Ceratopyge limestone’
of Sweden. Ostracods also occur at the level of the lowermost
subzone of the P. proteus conodont Biozone (Drepanoistodus
aff. amoenus subzone, approximately equivalent to the lower
A. murrayi graptolite Biozone), indicating a late Tremadocian
age (Ghobadi Pour 2006). These ostracods include possible
palaeocopids and podocopids but are too poorly preserved to
permit a firm identification.
Ostracods are recorded from the Salta and Jujuy provinces,
Cordillera Oriental, northwestern Argentina (Salas et al.
2007). They are preserved in rocks from the southern exten-
sion of the Central Andean Basin and include Nanopsis, Eo-
pilla and Kimsella. The ostracods come from three different
localities in the Lower Ordovician. In the Quebrada Chalala
section (Purmamarca area) the ostracods are from the Coquena
Formation (Benedetto & Carrasco 2002) and are associated
with the trilobite Notopeltis orthometopa, and the brachiopods
Tarfaya? brachymyaria, Lipanorthis andinus, and Astraborthis
quebradensis. The N. orthometopa Biozone is now under revi-
sion, but a late Tremadocian age for the ostracod-bearing levels
is widely accepted. In the Abra Santa Laura (Sierra de Mojoto-
ro) region the ostracods are from the Floresta Formation asso-
ciated with bivalves and trilobites (Sánchez & Vaccari 2003).
The presence of the trilobite Parabolinella sp. (Vaccari, pers.
comm., 2005 in Salas et al. 2007), which occurs elsewhere in
the Rupasca Formation within the upper part of the P. deltifer
Biozone (P. deltifer pristinus Subzone; Zeballo et al. 2003; Ze-
ballo & Tortello 2005), indicates an early late Tremadocian
age. Ostracods also occur in the Abra de Sococha (Parcha-In-
camayo area) within the Parcha Formation. The ostracod-rich
levels belong to the upper part of the A. murrayi graptolite
Biozone and the P. proteus and A. deltatus conodont biozones
Text-fig. 3. Ostracod valves of Tremadocian age dispersed over a bedding surface of the Lashkarak Formation in the Simeh-Kuh Section, 13
km north-west of Damghan, Simeh-Kuh, eastern Alborz Mountains, Iran (see Ghobadi Pour 2006). National Museum of Wales collection
NMW2004.22G.207. Scale bar 1 mm.
The earliest ostracods: the geological evidence
(Ortega & Albanesi 2003; Salas & Albanesi 2003) and can
reasonably be attributed to the late Tremadocian (Salas et al.
The palaeocopid Eopilla ingelorae Schallreuter, 1993a, from
the lower part of the Emanuel Formation of the Canning Ba-
sin of Northwest Australia, is associated with the ostracods Eo-
dominina and Conchoprimitia (see Schallreuter 1993b, and
pers. comm., 2005 in Salas et al. 2007). Although these rocks
were earlier assigned a late Tremadocian age (e.g. Schall-
reuter 1993a), more recent stratigraphical evaluation of the
Emanuel Formation suggests an early Floian age (late Lance-
fieldian – early Bendigonian Australian regional stages; Brock
& Holmer 2004). We include discussion of this material here,
as it indicates the persistence of the late Tremadocian ostracod
fauna into the Floian.
Apparently diverse assemblages of ostracods have been recov-
ered from the Fenhsiang Formation (= Ichang Formation,
Hou 1953a) of western Hubei, and the Yehli Formation (Hou
1953b) of Liaoning Province, NE China, both of possible late
Tremadocian age (see summary in Salas et al. 2007). The ex-
act age of these ostracod-bearing horizons requires confirma-
tion from new field studies, and the fauna itself needs revision.
However, the illustrations of Hou (1953a) indicate possible
palaeocopids (“Sinoprimitia” hupeiensis Hou, 1953a resembles
Nanopsis), binodicopids (“Sinoprimitia” sinensis Hou, 1953a,
“Primitia” tumiduformis Hou, 1953a), leiocopids (“Bythocy-
pris” subcircularis Hou, 1953a), and eridostracans (“Primitia”
ichangensis Hou, 1953a resembles Conchoprimitia). Schall-
reuter (1993b) synonymized Ctenobolbina sinensis and C.
taitzehoensis and tentatively assigned them to Eopilla.
Biogeography of the earliest ostracods
The wide geographical occurrence of ostracods and their al-
most simultaneous appearance on several palaeocontinents
suggests rapid dispersal, tolerance of a range of different cli-
matic belts, or a much reduced latitudinal temperature gradi-
ent from low to high southern latitude, that is consistent with
interpretations of Tremadocian climate as being ‘warm’ (Coo-
per & Sadler 2004, fig. 12.3). The earliest record of ostracods
is in the mid Tremadocian of Baltica with the occurrence of the
palaeocopid Nanopsis (Tinn & Meidla 2004), and contem-
poraneously in Gondwana (Cordillera Oriental of Argentina)
with Nanopsis, Eopilla and Kimsella (Salas et al. 2007). The
Text-fig. 4. Distribution of ostracods in the late Tremadocian (palaeogeography modified from Popov et al. , relative position of Baltica
and Gondwana after Cocks & Torsvik ). The icon indicates the occurrence of ostracods but does not denote a particular group. Ostracods
are widely distributed from their early appearance in the P. deltifer Biozone. Genera such as Nanopsis range between Baltica and Gondwana,
whilst Eopilla appears in Gondwana and possibly North China. 1, Coquena, Floresta and Parcha formations, Argentina (Salas et al. 2007); 2,
Bjorkasholmen Formation of Norway and Alum Shale of Sweden (Tinn & Meidla, 2004); 3, Shineton Shales Formation, England (Williams
& Siveter 1998); 4, Lashkarak Formation, mid-late Tremadocian of Iran (this paper); 5, Fenhsiang Formation (= Ichang Formation) of western
Hubei (Hou 1953a); 6, Emanuel Formation, northern Western Australia (Schallreuter 1993a, b) – this record is now considered earliest
Arenigian; 7, Yehli Formation, Liaoning Province, NE China (Hou 1953b).
The earliest ostracods: the geological evidence
presence of indeterminate ostracods in Avalonia and Alborz at
about the same time indicates the widespread distribution of
the group (Text-fig. 4). If the faunas of China are included in
this analysis, a latitudinal range from the tropical zone (North
China palaeoplate) to the southern high latitude Avalonia mi-
croplate is indicated (Text-fig. 4).
Individual genera are widespread in shallow marine litho-
facies, with Nanopsis present in Baltica and Gondwana (Argen-
tina; Text-fig. 4) during the P. deltifer Biozone, whilst Eopilla
appears to have distributed rapidly through Gondwana occur-
ring in the late Tremadocian of Argentina, and shortly after-
wards in Australia. Eopilla may even have extended to North
China (Text-fig. 4).
Ostracods of Tremadocian age are not recorded from the
major palaeoplates of Laurentia or Siberia (see Text-fig. 4). In
these areas ostracods become common in rocks of Dapingian
age (= ‘Third Stage’ sensu Cooper & Sadler 2004) in North
America (e.g. Harris 1957; Berdan 1988) or become abun-
dant in strata of Darriwilian age in Siberia (see Abushik et al.
1990): there are records of ostracods from earlier horizons in
Siberia, but these are post-Tremadocian (e.g. Kanygin 1971;
see also Tinn & Meidla 2004). In other Ordovician palaeo-
geographical terranes the earliest ostracods so far documented
appear to be post-Tremadocian, for example in Kazakhstan
(Melnikova 1986), Novaya Zemlya (Schallreuter et al.
2001), and Armorica (e.g. Vannier 1983). Notwithstanding
the patchy record, the present known distribution of Tremado-
cian ostracods hints at an earliest occurrence of the group cen-
tred on Gondwana/Baltica (Text-fig. 4).
Biogeography of terminal bradoriids compared
with the earliest ostracods
Bradoriida had global distribution during the early and middle
Cambrian, but their biodiversity was curtailed by a major ex-
tinction event at the end of the middle Cambrian (Williams
et al. 2007). The distribution of remnant early Tremadocian
bradoriids shows stratigraphical overlap with ostracods only in
Avalonia (Williams & Siveter 1998). In Baltica the bradoriid
‘Eremos bryograptorum’ is reported from an early Tremadocian
level in Scania (Hinz-Schallreuter 1993: 432), but the pres-
ervation of the material is poor. Even in Avalonia, only one
species of bradoriid, Beyrichona triceps, persisted into the late
Tremadocian. Here bradoriids and ostracods may have co-ex-
isted in the shelf marine environments of the Shineton Shales
Formation. There are records of rare bradoriids post-dating the
Tremadocian (e.g. Ghobadi Pour et al. 2007), but these are
sporadic, might be reworked, and do not suggest the persis-
tence of widespread Bradoriida in the Ordovician.
The earliest ostracods colonised the same kinds of benthic
marine shelf environments as bradoriids (for a summary of the
habitats of Bradoriida see Williams et al. 2007). However,
there is no evidence of ostracods displacing bradoriids into
deeper marine facies, an ecological shift that is possibly the
case with other Ordovician groups (e.g. Barnes et al. 1994:
145). Indeed, the rapid, widespread global and latitudinal dis-
tribution of ostracods at their first appearance may reflect an
availability of ecospace following the demise of the bradoriid
faunas. Wide distribution may also have been facilitated by
the ‘warm’ climate of the Tremadocian (Cooper & Sadler
Environmental distribution of the earliest
The ‘Paleozoic fauna’ appears to have originated in shelf ma-
rine environments (Sheehan 2001) and ostracods followed
this pattern. The earliest ostracods occupied well oxygenated
marine shelf sequences, both in carbonate- and clastic-rich
settings. Nanopsis occurs in marine carbonate-rich sequences
of the Bjorkasholmen Formation which formed in a shallow
epicontinental sea (Dronov & Holmer 1999). The strong
association of certain palaeocopids with carbonate platforms
is a pattern that would continue throughout the Ordovician
(see Vannier et al. 1989). Nanopsis is also present in clastic-
rich environments, in the Coquena Formation in Argentina
(Benedetto & Carrasco 2002), where grey, bioturbated
mudstones were deposited in an assumed inner-shelf setting
(Salas et al. 2007). Given the wide latitudinal distribution
and varied lithofacies, early ostracods like Nanopsis may have
been environmental generalists. Ostracods like Kimsella and
Eopilla also occur within shallow marine settings of the lower
part of the Parcha Formation of Argentina, in calcarenitic lev-
els (Astini 2002, 2003; Salas et al. 2007). A few early os-
tracods may have colonised somewhat deeper shelf settings,
as for example those in the Shineton Shales Formation (Wil-
liams & Siveter 1998). Myodocope-like bivalved arthropods
are also reported from the laterally equivalent Merevale Shales
Formation of England (Siveter et al. 1995). In Iran, ostracods
occur in mudstones at two stratigraphical levels in the Lash-
karak Formation, both interpreted as being deposited in outer
shelf settings, and associated with the trilobites Psilocephalina
lubrica and Asaphellus, respectively in nilied and raphiophorid
trilobite biofacies (Ghobadi Pour 2006).
Post-Tremadocian Ordovician ostracod
Ostracod diversity remained low during the Tremadocian,
with just three named genera (Kimsella, Eopilla and Nanopsis)
and a handful of species globally (perhaps 12 depending on the
affinities of British, Iranian and Chinese material). Ostracods
underwent a major diversification after the Tremadocian. In
the best studied faunas, those of Baltoscandia, some 33 spe-
cies are reported from the post-Tremadocian Billingen through
Kundan regional stages, including many geographically wide-
spread Ordovician genera such as Laccochilina, Euprimites and
Ogmoopsis (Tinn & Meidla 2004). This post-Tremadocian
increase in ostracod biodiversity tracks that of other benthic
marine organisms (Hammer 2003), though is uneven in its
development. In southern Britain (palaeocontinental Ava-
lonia) for example, ostracod biodiversity remained low until
the Darriwilian (e.g. Botting 2002) and in North America
(Laurentia) until a level in the Dapingian or early Darriwilian
(‘Whiterockian’ ostracod faunas, e.g. Harris 1957; Berdan
1988). Many post-Tremadocian ostracod morphologies can be
traced back to ancestors like Nanopsis (Tinn & Meidla 2004),
The earliest ostracods: the geological evidence
but also form clear lineages that continue through the Ordovi-
cian (Williams et al. 2003), resulting in an ostracod biodiver-
sity with tens to hundreds of species in individual formations
(e.g. Meidla 1996) by the Late Ordovician. Although there is
local evidence for a link between ostracod diversification and
regional volcanism (Botting 2002), the global nature of this
biodiversification event and its implications for the develop-
ment of early Palaeozoic benthic marine communities remains
to be fully investigated.
Mark Williams thanks the University of Leicester for study
leave. We are grateful to Ewa Olempska (Warsaw) and An-
dreas Maas (Ulm) for their constructive reviews of this paper.
Leonid Popov acknowledges support from the National Mu-
seum of Wales. Mansoureh Ghobadi Pour was supported by
the University of Agricultural Sciences and Natural Resources,
Gorgan. We thank Ludmila Melnikova (Moscow), Hou
Xianguang (Kunming) and Mark Florence (Smithsonian
Institution, Washington DC) for access to specimens in their
care. This is UMR 5125 contribution number UMR5125-
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Manuscript received: 05 September 2007
Reviewed manuscript accepted: 21 February 2008