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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