ORDOVICIAN BIVALVE BIOGEOGRAPHY 43
they show affinities to both groups and it could
be that their placement is correct within the
latter clade; there are many early arcoids and
cyrtodonts that apparently do not have multiple
ligamental insertions, including the earliest
cyrtodontids known from the Tremadoc of
Australia (Cyrtodonta and Pharcidoconcha;
Pojeta & Gilbert-Tomlinson 1977), and from the
Lower Arenig of Wales (Cyrtodonta and
Cyrtodontula; Cope 1996b), together with the
strongly ribbed genus Falcatodonta (Cope
1996b).
Cyrtodonta itself ranges from the Tremadoc
Series of Australia through to the Silurian. In the
Early Ordovician, cyrtodontids seem to have
been little constrained by latitude; they appear
first in the Tremadoc of Australia (Pojeta &
Gilbert-Tomlinson 1977) and they also occur in
the Lower Arenig of Avalonia (Cope 1996b).
They have been recorded with a query from the
higher latitudes of the Mid-Ordovician of both
Spain (Babin & Gutierrez-Marco 1991) and
Morocco (Babin & Destombes 1992). In the last
two cases no dentition is known and the generic
assignment was made on the basis of shell shape.
Cyrtodontula was also recorded from the Middle
Ordovician of Argentina by Sanchez (1990).
However, it is on the Late Ordovician low-
latitude carbonate platforms that cyrtodonts
became most diverse (Cope & Babin 1999) and
by then clearly showing a preference for low
latitudes. In Laurentia Cyrtodonta occurs
together with such genera as Cyrtodontula,
Ortonella and Vanuxemia. The earliest Laurent-
ian occurrence of cyrtodontids appears to be the
shallow water St Peter's Sandstone fauna of
Minnesota first described by Sardeson (1896)
from the late Chazyan ( = Early Caradoc). This
fauna includes two species each of Cyrtodonta
and Vanuxemia (Sardeson 1939a, b). The latter
genus is also found on the Late Ordovician
carbonate platform of Baltica, whence Isberg
(1934) also described the genera Thorslundia
and Warburgia. Soot-Ryen & Soot-Ryen (1960)
also record Cyrtodontula from the Oslo area.
Cyrtodonta and Vanuxemia were recorded
from Kazakhstania by Khalfin (1958) whilst
Cyrtodonta, Cyrtodontula, Plethocardia,
Ortonella and Vanuxemia were recorded from
Siberia by Krasilova (1970, 1979)
Accompanying the cyrtodontids are a variety
of genera of ambonychiids; these are pterioid
bivalves that have their umbones at the anterior
end of the shell and commonly have a byssal
sinus. They are strongly anisomyarian or mono-
myarian in the adult stage and the posterior
adductor is often displaced towards the centre of
the valves. Such clearly epifaunal forms are,
unsurprisingly, common in the low-latitude
carbonate platforms. However, the earliest
ambonychiid recorded hitherto is from Ava-
lonia, from the Middle Arenig of South Wales
(Cope 19966), where it occurs in a sandstone;
this is an undetermined species of Cleionychia,
which is also the earliest ambonychiid genus to
occur in the Upper Ordovician of North
America (Pojeta 1966). Middle Ordovician
ambonychiids are known from Australia whence
Pojeta & Gilbert-Tomlinson (1977) described
species of Glyptonychia, Leconychia and
Pteronychia. Pojeta (1966) illustrated something
of the large variety of North American ambony-
chiids and figured Late Ordovician species of
Allonychia, Ambonychiopsis, Anomalodonta,
Ambonychia, Cleionychia, Eridonychia, Mary-
onychia, Opisthoptera and Psilonychia; to these
may be added Claudeonychia Pojeta, 1997.
Some of these genera are also known from the
Baltic Upper Ordovician carbonate platforms
(Isberg 1934) where the fauna also includes
genera such as Anomalocoelia, Paramytilarca
and Praeanomalodonta. The ambonychiid
faunas from around the continents of Kaza-
khstania and Siberia are not as varied as those of
Laurentia and Baltica but include species of
Ambonychia and Cleionychia (Khalfin 1958;
Krasilova 1979).
Pterineids are also particularly characteristic
of the Upper Ordovician, but the earliest is a
Palaeopteria from the Lower Arenig of South
Wales (Cope 19966) only known from a single
right valve. The next oldest genus is Gond-
wanan, in this case from the Amadeus Basin of
Australia, the genus Denticelox Pojeta &
Gilbert-Tomlinson, 1977, which, unlike later
pterineids is biconvex. Carotidens Foerste, 1910
and Palaeopteria occur in the Upper Ordovician
of the United States and Canada; both these
genera have asymmetrical valves, with a flatter
right than left valve.
The last group of pteriomorphian bivalves
to evolve in the Ordovician was the limids.
Tunnicliff (1987) described the genus Myo-
dakryotus from the Caradoc of North Wales.
This genus has characteristics in common with
both cyrtodontids and later limids, suggesting
that the group was derived from the cyrtodon-
tids. Pojeta & Runnegar (1985, fig. 17) figured a
limid species from the Late Ordovician of
Canada and the United States, which they
assigned with a query to the genus Prolobella
Ulrich, 1894, regarded by Cox et al. (1969) as a
palaeoheterodont. Ordovician limids lack the
internal ligament of more recent forms (Pojeta
& Runnegar 1985).
A further group of the pteriomorphians is the
