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42 JOHN C. W. COPE

heteroconch genus to display a multiplicity of

muscle scars. Such features are common in other

genera of heteroconchs, including Cycloconcha

Miller, 1874, Celtoconcha Cope, 1996b and

Coxiconcha Babin, 1966. Pojeta & Runnegar

(1985) suggested that the multiple accessory

muscle scars in both Babinka and Coxiconcha

indicated derivation from Cycloconcha. This

suggestion seems untenable in view of the

fact that the former two genera have a well-

established Early Ordovician history, whereas

the latter does not appear until the Mid-

Ordovician. Coxiconcha is widely dispersed,

being known earliest from the Early Arenig of

the Montagne Noire (Babin 19820), but in the

Mid-Ordovician is known in France, Spain and

Portugal (Babin 1977), Morocco (Babin &

Destombes 1992), and Bohemia (Kriz 1995),

reaching Bolivia by the Late Llanvirn (Babin &

Branisa 1987).

Another group assigned to the heteroconchs

is the Superfamily Glyptarcoidea Cope, 1996b.

Cope (1996b) pointed out that these differ from

other heteroconchs in that their teeth radiate

dorsally, rather than ventrally as in other hete-

roconchs, and in their cladistic analysis of

bivalves Carter et al. (2000) concluded that

Glyptarca was a pteriomorph. The type species

of Glyptarca, G. primaeva Hicks, 1873,

described from the Lower Arenig of Ramsey

Island, South Wales, although abundant in the

type locality, is poorly preserved and little of the

dentition is known apart from a small sheaf of

anterior teeth and a blade-like posterior tooth;

no subumbonal dentition is visible. As a result of

this view of its dentition it was thought to have a

subumbonal lacuna in the dentition. The genus

was thus viewed as an ideal ancestor for the

cyrtodontids, which have curved dorsally flaring

anterior teeth, a subumbonal dental lacuna and

one or more straight posterior teeth (e.g. Morris

1978). Cope (19966) figured a new species, G.

serrata, from the Llangynog Inlier, some 60 km

east of Ramsey Island, but at the same horizon,

which showed perfectly preserved dentition,

demonstrating that the genus had in reality a

complex subumbonal overlap of the anterior

and posterior teeth and thus there was no direct

link to the cyrtodontids. Cope (1995, 19976)

showed that this dentition bears strong simi-

larities to that of cardiolarioids and these might

well have been ancestral to the glyptarcoids. The

glyptarcoids include the genera Glyptarca and

Celtoconcha, both known from the Lower

Arenig of Wales (Cope 19966), the former also

occurring in the Llanvirn of Mid-Wales (Cope

1999) where it occurs together with another

glyptarcoid, Camnantia Cope, 1999. The only

other glyptarcoid known hitherto was widely

reported as Glyptarca, but following Cope's

(19966) redefinition of the genus, is now known

to belong to a separate genus, Hemiprionodonta

Cope, 19966 (type species ?Dolabra lusitanica

Sharpe, 1853). This genus is known widely from

the Middle Ordovician of Spain (Babin &

Gutierrez-Marco 1991) and Armorica (Babin

1966) and has also been reported from the

Middle Ordovician of Saudi Arabia (Fortey &

Morris 1982). Hemiprionodonta is known from

both the Late Llanvirn (Llandeilian) and the

Caradoc of Morocco (Babin & Destombes

1992).

Pteriomorphia

Glyptarca has dentition closely related to that of

the genus Catamarcaia Sanchez & Babin, 1993

from the upper part of the Middle Arenig of

Argentina; this latter genus is the earliest form

that has a duplivincular ligament. Sanchez &

Babin (1993) believed that it was an early

pteriomorph lacking the central edentulous area

on the hinge-plate and Cope (1997a) decided

that it was the earliest arcoid sensu stricto that he

then included in the Neotaxodonta. More

recently Cope (2000) accepted that the Neo-

taxodonta could be included within the Pterio-

morpha, as they shared many features including

the multiple ligamental insertions, but he

retained the Order Arcoida in a restricted sense

and recognized the Cyrtodontoida as a separate

order. As Catamarcaia had a unique mix of

characters, the monotypic family Catamar-

caidae was proposed for it (Cope 2000). The

deficient early fossil record of the Order

Arcoida (sensu Cope 1997a) needs examination;

after Catamarcaia in the Middle Arenig, no

arcoids are known from the remainder of the

Ordovician. The next record is that of

Alytodonta Cope, 19976 from the Lower

Llandovery of Girvan, Scotland, on the Lau-

rentian margin, and then the Wenlock genera

Trecanolia and Uskardita (Ratter & Cope 1998)

to add to the previously known Freja Liljedahl.

1984. Judging from the occurrence of these later

genera, it appears possible that arcoids may have

inhabited some of the nearest inshore environ-

ments and this may explain their absence thus

far from the Mid- and Late Ordovician records,

which must be put down to collection failure.

In their cladistic review of early bivalve phy-

logeny Carter et al. (2000) preferred to derive

the glyptarcoids from Catamarcaia and assigned

them to the Pteriomorphia. Clearly Glyptarca

lies close to the point of divergence of the

heteroconchs and pteriomorphians and thus