133845.pdf

(^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