133845.pdf

(^44) JOHN C. W. COPE

superfamily Modiolopsoidea Fischer, 1887, for-

merly often classified with the heteroconchs, but

shown by Carter & Seed (1998) to include some

taxa that display multiply inserted non-parivin-

cular ligaments; they may thus be related to the

pteriomorphs. In the Treatise (Cox el al. 1969)

the modiolopsoids are included in the order

Modiomorphoida and the superfamily Modio-

morphoidea. Fang & Morris (1997) revised the

taxonomy of this problematic superfamily and

showed that it included two distinct groups of

bivalves: the modiomorphoids, typified by

the Devonian genus Modiomorpha, had well-

developed ligamental nymphs that allowed

them to be distinguished from the Ordovician

modiolopsoids, a group with which they had

frequently been confused, but which lacked the

ligamental nymphs. Fang & Morris (1997) also

showed that the family Permophoridae Van de

Poel, 1959 was a subjective junior synonym of

the family Modiomorphidae Miller, 1877 and

recommended that order Modiomorphoida be

abandoned and that the superfamily Modiomor-

phoidea Miller, 1877, as used in the Treatise

(Cox et al. 1969), should be replaced by the

Modiolopsoidea Fischer, 1887, in which they

recognized the families Modiolopsidae Fischer,

1887, Colpomyidae Pojeta & Gilbert-Tomlinson,

1977 and the Modiolodontidae Fang & Morris,

1997. Carter & Seed (1998) showed that some

modiolopsoids had multiply inserted non-

parivincular ligaments and suggested that their

affinities thus lay with the pteriomorphs (see

above). The modiomorphoids, on the other

hand, possessed a periostracum with calcified

spicules and Carter & Aller (1975) suggested

that this feature, widely found in the anom-

alodesmatans, indicated that their affinities lay

with that group.

Modiolopsids are recorded as far back as

the Early Tremadoc of Argentina, whence

Harrington (1938) figured a form he described

as Cosmogoniophorina tenuicostata sp. nov.

Harrington figured three syntypes, two of which

(Harrington 1938, p1. 3, figs 1, 5) show an almost

carinate shell and one (Harrington 1938, fig. 1)

also shows clear radial ornamentation on the

post-umbilical shoulder. There appears to be

little quarrel that these are modiolopsids, but

I find myself in agreement with Pojeta

(1971, p. 17) that Harrington's other syntype

(Harrington 1938, p1. 3, fig. 4) appears to show

taxodont teeth, and a less marked post-umbonal

shoulder and apparently lacks radial ornamenta-

tion, suggesting this specimen is a nuculoid. To

clarify matters I designate the specimen figured

by Harrington (1938, p1. 4, fig. 1) as lectotype of

the species Goniophorina (Cosmogoniophorina)

tenuicostata (Harrington), 1938. The same

species has been recently recorded from the early

Middle Arenig of Argentina by Sanchez (1997b)

who noted that as Harrington did not describe

the dentition, the generic and subgeneric

assignments are uncertain.

The next oldest species are two species of

Modiolopsis from the Early Arenig of South

Wales (Cope 19966) where the modiolopsids

Cosmogoniophorina and Parallelodus also

occur. Middle Ordovician modiolopsids are

recorded possibly from Spain (Babin &

Gutierrez-Marco 1991, pp. 123-124), from the

Middle and Upper Ordovician of Argentina

(Sanchez 1990, 1999b) and the Upper Ordo-

vician of Laurentia (Pojeta 1971), Kazakhstania

(Khalfin 1958) and Siberia (Krasilova 1970.

1979). The genus Corallidomns appears in the

Upper Ordovician; this genus is noteworthy

because it is the first boring bivalve (as first

reported by Whitfield 1893) and lived in crypts

excavated into colonial corals or bryozoans;

the same mode of life was followed by

Semicorallidomus Isberg, 1934.

Colpomyids normally have rather amorphous

bulbous-shaped teeth, referred to as articulating

devices by Pojeta and Gilbert-Tomlinson (1977

p. 27), that are not mounted on a hinge-plate.

The earliest forms are from the Late Tremadoc

of the Amadeus Basin of Australia, Colpantvx

and Xestoconcha (Pojeta & Gilbert-Tomlinson

1977); the latter genus also occurs in the Early

Arenig of South Wales (Cope 1996b).

Colpomyids are known from the Upper

Ordovician of Laurentia (Pojeta 1978). Siberia

(Krasilova 1979) and western Gondwana

(Sanchez 1999b) and survived into the Silurian.

The Modiolodontidae are modiolopsoids that

have a small number of cardinal teeth in each

valve. They are known from the Middle and

Upper Ordovician and younger rocks. One of

the earliest is the Modioliodon recorded from

the Llanvirn of Avalonia by Cope (1999).

Anomalodesmata

The earliest generally accepted anomalodes-

matan is Arenigomya from the Early Arenig of

South Wales (Cope 19966). Arenigomya shows

already several of the principal characters of the

group, including a finely pustulate shell orna-

ment and an edentulous hinge that sometimes

carries subumbonal articulating devices (see

Cope 19966, p1. 7, figs 12-14; text-fig. 7). These

characters are evident in the genera Cuneamya

and Rhytimya (the latter also possesses a pos-

terior gape), both of which are known from the

Late Ordovician of the United States, Canada