133845.pdf
tuis.
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(^38) JOHN C. W. COPE
Similodonta Soot-Ryen, 1964, is a character-
istic praenuculid genus of the Late Ordovician
of Laurentia and Baltica. It is characterized by
an abrupt 90° angle in the middle of the hinge-
plate, although Soot-Ryen (1964), in her original
diagnosis, claimed that the angle was near 80°.
The genus is well exemplified by the type species
S. similis (Ulrich, 1892) from the Richmond
Group of Minnesota. Other Laurentian species
include S. recurva (Ulrich, 1892) and S. costata
(Branson, 1909) together with the Ashgill
species S. magna (Lamont, 1946) and S. collina
(Reed, 1946) refigured by Cope (1996a) both
from Scotland, and a probable (but unidentified)
species figured from Northern Ireland by
Tunnicliff (1982). An as yet un-named new
genus, clearly related to Similodonta, was
figured by Pojeta (1978, p1. 2, figs 9-12). Late
Ordovician species of Similodonta are also
known from Baltica, and include S. spjeldnaesi
(Soot-Ryen & Soot-Ryen, 1960) from the Upper
Caradoc of the Oslo region; the same species
was also recorded from the Ashgill of the
Oslo region by Toni (1975). It could thus be
considered that Similodonta was a genus
characteristic of low-latitude areas (presumably
with warm water). The origins of Similodonta
are, however, probably Avalonian. Cope (1999)
has figured the earliest known species, S. ceryx,
from the Aurelucian Stage of the Lower
Caradoc of mid-Wales and suggested that it may
have evolved from the early Llanvirn praenu-
culid genus Arcodonta Cope, 1999, putting the
origin of the genus in Avalonia. Similodonta also
occurs in the Ashgill Series of North Wales
(collections of the British Geological Survey).
Another possible Gondwanan Similodonta was
figured by Steinmann & Hoek (1912) from the
Upper(?) Ordovician of Bolivia according to
Babin (1993), but the text seems to make it clear
that these specimens are distorted lingulid
brachiopods. From the Precordillera of
Argentina Sanchez (1999a) has figured what
appear very clearly to be genera closely related
to Similodonta. These include Villicumia and
Trigonoconcha, the latter with a hinge-plate
angle close to 65°; these clearly represent part of
a local Caradoc bivalve diversification that was
described by Sanchez (1999b); both these
new genera could have been derived from
Similodonta.
The Llanvirn Tironucula Morris & Fortey,
1976, from the Laurentian margin on Svalbard,
was possibly the first bivalve to reach Laurentia.
Its juvenile dentition is similar to that of many
praenuculids, but it develops blade-like teeth
ontogenetically. Babin (1982a) proposed the
family Tironuculidae to accommodate this
genus and the clearly related Ekaterodonta,
originally described from the Lower Arenig of
the Montagne Noire (Babin 1982a). This latter
genus is now also known from the Lower
Llanvirn of Spain (Babin & Gutierrez-Marco
1991) and from the Upper Llanvirn of Bolivia
(Babin & Branisa 1987) and, unlike Tironucula,
was a high-latitude genus. Another tironuculid,
Natasia, has been described from the Lower and
Middle Arenig of NW Argentina by Sanchez
(1996) and was subsequently assigned to a
separate subfamily within the Tironuculidae
(Sanchez 1997a).
The apparently wide geographical ranges of
some other nuculoid forms may in some cases
simply be due to inappropriate generic assigna-
tions. Thus the genus Ctenodonta. that has been
widely reported from the low-latitude Upper
Ordovician rocks, is based on the species C.
nasuta (Hall, 1847) from the Upper Ordovician
of the United States. However, many other
species have been assigned to this genus in the
past and, as remarked on by Pojeta (1971, p. 15),
all that this generic name effectively denotes is
that a nuculoid is being described. There do not
appear to be any definitive records of this genus
from the Early Ordovician although poorly
preserved material from the Tremadoc of
Argentina (Palaeoneilo iruyensis Harrington,
1938) was tentatively assigned to Ctenodonta by
Pojeta & Gilbert-Tomlinson (1977), and Babin
& Destombes (1992) figured two forms they
compared to Ctenodonta from the Middle
Arenig of Morocco. Definitive records of
Ctenodonta are from the Middle Ordovician of
low-latitude Gondwana (Australia; Pojeta &
Gilbert-Tomlinson 1977) and the Upper
Ordovician of Laurentia (Pojeta 1971) and
Siberia (Krasilova 1976, 1979). Other Ordo-
vician genera related to Ctenodonta include
Tancrediopsis, the earliest known of which
is from the Llanvirn of Mid-Wales (Cope
1999); the somewhat younger type species (T.
contracta) was originally described from
the Alumette Islands, Ontario, Canada, by
Salter (1859). Alococoncha Pojeta & Gilbert-
Tomlinson, 1977 is only known from the Middle
Ordovician of the Amadeus Basin, Australia.
Siberoctenia Krasilova, 1976, from the Upper
Ordovician of Siberia, has a large number of
small teeth.
Pojeta (1988, pp. 210-211) decided that the
monotypic superfamily Ctenodontoidea and the
family Ctenodontidae were poorly founded and
of uncertain diagnosis and suggested that, for
the time being, the few genera included in
the family could be incorporated into the
Nuculanoid family Malletiidae. Nuculanoids are