Middle Ordovician Aparchitidae and Schmidtellidae: the significance of ‘featureless’ ostracods

Schmidtellidae and Aparchitidae form a common element of North American middle Ordovician ostracod faunas. Characterized by relatively simple morphology with few obvious diagnostic features they are conventionally assigned to the Leiocopa. Their simple morphology has led to numerous species being referred to as ‘bag gencra’ such as Aparchites or Schmidtella. Aparchitids differ markedly from schmidtellids in the nature and degree of valve overlap, shell thickness and development of dorsal and ventral valve modifications. Schmidtellids may be more closely related to leperditellaceans. Kayina, previously assigned to the leiocope Family Jaanussoniidae and recorded from the middle Ordovician of North America, does not conform to leiocope morphology and is probably a leperditellacean. Aparchitaceans (Aparchitidae and Jaanussoniidae) may represent the root-stock from which the Paraparchitacea evolved later in the Palaeozoic. In North America schmidtellids and aparchitids have potential for middle Ordovician stratigraphic correlation. They co-occur in normal marine shelf depositional environments but were absent from marginal marine depositional settings.


INTRODUCTION
Morphologically simple ostracods comprising a bivalved subrounded carapace with gently convex valves, apparently indistinct ventral structures, no sulcation and n o dimorphism are commonly documented in monographic studies of North American middle Ordovician ostracods. Often they form significant elements of such ostracod faunas (e.g. Harris, 1957;Kraft, 1962). Because of their simplicity they have inevitably been referred to a small number of genera, classic examples being Aparchites, to which more than a 100 species have been referred (see Kempf, 1986), and Schmidtella. Aparchitid and schmidtellid ostracods are conventionally assigned to the Leiocopa Schallreuter, 1973(see Williams 1990 which is envisaged to encompass three families: the Aparchitidae Jones (in Chapman), 1901 and the Jaanussoniidae Schallreuter, 1971 which are generally united in the Superfamily Aparchitacea, and the Schmidtellidae Neckaja, 1966. Utilizing two well preserved North American middle Ordovician ostracod faunas from the Simpson Group of Oklahoma and the Edinburg Formation of Virginia this paper aims to: establish criteria on which relatively featureless schmidtellid and aparchitid ostracods can be identified and distinguished; elucidate the relationships of aparchitid, schmidtellid and jaanussoniid ostracods; and indicate their stratigraphic and palaeoenvironmental significance in the North American middle Ordovician.

GENERAL MORPHOLOGY OF LEIOCOPES
By comparison with other Palaeozoic ostracods, particularly the majority of palaeocopes, leiocope ostracods generally have very simple carapace morphology. Typical leiocopes ( Fig. 1) have inequally sized gently convex valves and in lateral view overall carapace shape is generally subcircular or equilateral. Leiocopes are characterized by valve asymmetry, manifested by inequally sized valves and by ventral or dorsal modifications of one or both of the valves. Morphological features of the Aparchitidae, Schmidtellidae and Jaanussoniidae are summarized as follows. Aparchitidae. (See Swartz, 1969: Schallreuter, 1973Vannier, 1990: herein Fig. 1 A, D.) In lateral view Aparchitidae typically have a subcircular carapace. Both valves are gently convex and the larger right valve overlaps the left valve. A sulcus or sulcament is absent but a well defined adductor muscle spot is often evident on ornamented valves. Specialized structures are developed on the left or both valves and are usually situated ventrally at the junction between the lateral and ventral valve surfaces (Vannier, 1990, figs 1, 5). These generally consist of ridges, denticles or thickenings on the left valve, and grooves or bends on the right valve. Aparchitidae are widely documented from the Ordovician to Devonian in North America and Europe. Schmidtellidae. (See Warshauer & Berdan 1982;Berdan 1988; Fig. 1 B, E) Schmidtellidae are typically subcircular to slightly postplete in lateral view and inequivalved, the left valve overlapping the right valve. They have thick, convex valves often with distinct dorsal umbones and the margins of the lateral surface of valves are generally flattened or furrowed. Most forms are non-sulcate though forms with an internal adductorial ridge are reported. The Schmidtellidae are documented from the Ordovician to Devonian of North America and from the Ordovician of Europe.

MATERIAL AND METHODS
Most of the ostracods studied are from the middle Ordovician Simpson Group of Oklahoma, principally from the Oil Creek and Bromide formations, which contain rich ostracod faunas. The Simpson Group contains a prolific and well documented fossil fauna including trilobites (Shaw,1Y74), brachiopods (Cooper, 1956), echinoderms (Sprinkle, 1982), chitinozoans (Grahn & Miller, 1986), conodonts (Bauer, 1987) and ostracods (Harris, 1931(Harris, , 1957. The ostracod fauna is amongst the richest recorded from the middle Ordovician of North America with more than 130 species. The stratigraphy (Whiterockian-early Mohawkian, approximately equivalent to the Llanvirn-Caradoc interval in the British Series; see Ross et al., 1982;Sweet, 1984) and palaeoenvironments of the Simpson Group have been well documented (see Decker & Merritt, 1931;Longman, 1981Longman, , 1982. In the Simpson Group palaeocope ostracods are the dominant element of the ostracod fauna in terms of species. However, schmidtellid and aparchitid ostracods form a significant proportion, at least 23 species occuring. The Simpson Group provides an excellent faunal database from which to study these ostracods. Simpson Group ostracod carapaces (calcareous) were extracted from mudstones which were disaggregated in baths of 10% hydrogen peroxide, wet sieved, dried and then picked for microfossils. The ostracods often occur as complete carapaces and are relatively robust. Specimens have been thin sectioned in order to evaluate the nature of the valve margin.
In addition to specimens from the Simpson Group, silicified ostracods from the middle Ordovician Edinburg Formation of Virginia have also been included in this study (see Kraft, 1962
convex (ranging from gently to strongly convex) and form PI. 4, fig. 2). However, the smaller left valve can a dome shape (PI. 1, fig. 10; PI. 2, fig. 8). Rarely the overreach the larger right valve dorsally, a characteristic lateral surface of the valves may be more flattened.
Ontogeny. With the exception of size increase the general morphology of the carapace does not change considerably during ontogeny (Fig. 4). However, ornament does vary. Ontogenetic studies of Bultonotellu circulunfis (Harris, 1957) from Oklahoma show that the original figured specimens (Harris, 1957, pl (Kraft, 1962). Carapace, right valve lateral and dorsal views respectively, X35; IGR.5806. Edinburg Formation. Fig. 14. Balronotella parsispinosa (Kraft, 1962 Vannier, 1990), sometimes situated on a low ridge, or less commonly by thickenings, which are situated at the junction between the lateral and ventral valve surfaces (Pl. 1, figs 13, 14: P1. 2, figs 6, 9-11). Because of their small size, such structures are not always obvious in thin section. Excessive overlap is also prevented by a contact groove in the right valve, against which the overlapped portion of the left valve abutts (Pl. 1, fig. 4: see also Kraft, 1962, pl. 2, fig. 7; pl. 3, fig. 8). The overlap contact can also appear sinuous, with the larger right valve having a bow-shaped lappet (Pl. 2, fig. 8). Again, excessive overlap is prevented by denticles or thickenings of the left valve and by the presence in the right valve internally of a contact groove (see Vannier, 1990, figs 2, 3). The overlapping portion of the right valve (the frill or lappet) is usually demarcated from the ventral valve surface of the right valve by a groove or bend (PI. 1, fig. 10; P1. 2, figs 11, 14), or may be situated in a distinct channel (Pl. 2, fig. 7). Sometimes denticles are also situated on the right valve either at the junction between the lateral or ventral valve surfaces or sometimes within the demarcating groove of the lappet. The function of these structures is not clear. In thin section Kraft (1962, fig. 7c, i, j ) illustrated the overlap of two aparchitid ostracods (Aparchites fimbriatus and Aparchites suborbicularis = B. circulantis). In Kraft's illustrations the lappet of the right valve overlaps the flattened ventral surface of the left valve. The margin of the left valve is received internally by a shallow contact groove in the right valve formed by a marked thickening of the valve. The same features are evident in thin sections of aparchitid species from the Simpson Group (Pl. 4, figs 1, 2, 9, 10, 18: see also Fig. 5 D, E, G , H). The thickening of the right valve has marked relief in some species (Pl. 4, fig. 18) and forms a boss-like structure situated at some distance from the extreme tip of the right valve ventral margin (Pl. 4,figs 16,18). This is in contrast to schmidtellids where the contact groove of the overlapping valve, situated virtually at the ventral margin, is formed by a slight concavity in the valve and not by any related valve thickening (see P1. 4, fig.  11). For this reason the degree of overlap in aparchitids is generally much greater than in schmidtellids (see Fig. 5, compare D, E, G , H with M, P). In B. circulantis (see PI. 4, fig. 16) the thickened margin of the left valve appears t o enclose a 'hollow' area indicated by dark, possibly organic material. Similarly, the thickened margin of the right valve which forms the contact groove also encloses a 'hollow' space. A third 'hollow' area lies adjacent to the external marginal sculpture of the left valve. The significance of these features is unknown. They may be artefacts, though those of the left valve in particular resemble a calcified inner lamella (see also Kraft, 1962, fig. 7j). None of the other aparchitids investigated have structures resembling a calcified inner lamella. Hinge contact. Thin sections also clearly illustrate the nature of the dorsal contact. In the simplest case both valves meet at approximately the same height dorsally (Pl. 4, fig. 6) and the contact of the valves at the hinge appears to be simple (see Fig. 5 B). Commonly, particularly in the genus Baltonotella, the smaller left valve overreaches the larger right valve dorsally and may be strongly thickened adjacent the hinge (Pl. 4, figs 1, 5. 13). More rarely, complicated arrangements exist in which the hinge of the left valve clearly has a groove into which the modified margin of the right valve interlocks (Pl. 4, fig. 17; see also Fig. 5 A). Again the margin of the left valve adjacent the hinge is thickened, probably to strengthen the contact.
Features of the valve margin Ventral valve overlap. By contrast to the Aparchitidae in the Schmidtellidae the left valve is larger and overlaps the right valve (see Figs 1, 5). The overlap contact between the valves is always straight and the overlapped portion of the right valve is received by a contact groove in the left valve (Pl. 3, fig. 10; see also Berdan 1988, pl. 3, fig. 16). Unlike in aparchitids the contact groove is situated very close to the ventral valve margin and is formed from a shallow concavity in the valve (Pl. 4,figs 11,12,15,20).
The degree of valve overlap is therefore much less than in aparchitids (Fig. 5 M, P). Likewise there are no ridges, channels or denticles present at the junction between the lateral and ventral valve surfaces, although marked flattening of the ventral surface occurs in the right valve of some schmidtellids (PI. 3, fig. 12

THE GENUS KAYZNA
Although recorded from the middle Ordovician of Northern Europe (Schallreuter, 1971), jaanussoniid leiocopes in the middle Ordovician of North America have only been documcnted from the Bromide Formation of the Simpson Group, where the genus Kayina (originally described by Harris, 1957) was referred to the Jaanussoniidae by Schallrcuter (1971). Two species of Kayina are present in the Bromide Formation ( K . hybosa, the type-species, and K. porosa: see Harris, 1957) but only a small number of adult and juvenile specimens of these species are known. Kayina shares with other jaanussoniids the development of a distinct posterodorsally situated node (hollow in jaanussoniids; see Vannier, 1988). occurring only on the left valve (Fig. 6). However, Kayina does not conform to typical jaanussoniid morphology (see Williams & Vannier, 1993a; also compare with Fig. 1 C, F). European Ordovician jaanussoniids (see Vannier, 1988Vannier, , 1990) have typical aparchitacean mofphology, in lateral view appearing subcircular to amplete equilateral and in ventral or dorsal view having convex dome-like valves. By contrast Kayina has a typical leperditellid outline with the width and height of the carapace increasing posteriorly (Fig. 6). Additionally in Jaanussonia, the nominate genus for the Jaanussoniidae, the right valve overlaps the left valve (see Vannier, 1988). In Kayina however, the left valve overlaps the right valve in typical leperditellid fashion (see Fig. 5 N). Untypically for jaanussoniids, Kayina shows very slight valve overlap; the smaller right valve fitting into a shallow groove at the internal margin of the left valve (Fig. 5 a). As suggested by Berdan (1988, p. 287) Kayina appears closely related to the palaeocope Leperditella.

INNER LAMELLA
With the exception of B. circulantis there is no evidence of a calcified inner lamella in any of the specimens observed in thin section (see also Gramm, 1984, pl. 3, figs 10, 11).
Although clearly thickened in areas near the contact margin and hinge area, n o bilamellar features (Gramm, 1984(Gramm, , 1988Adamczak, 1991, figs 3, 4) or remains of a line of concrescence were recognized within the shell. The basic structure of the carapace integument described by Okada
(1982; epicuticle, procuticle, epidermal cells in both outer and inner lamella) from Recent podocopid species is likely to be applicable to most extant and extinct ostracods, including Lower Palaeozoic schmidtellids and aparchitids. If so, the procuticle of the inner lamella was probably not calcified in Ordovician aparchitids and schmidtellids and therefore not preserved in the fossil record. The external thickenings of the shell are interpreted herein as positive features of thc calcified procuticle of the outer lamella; they seem to occur in places where the carapace needed to be strengthened (ventral contact, near ligament attachment). The internal thickenings observed on the inner surface of the valves near the ventral contact margin may be the attachment area of a thin, flexible non-calcified inner lamella, similar to the vestment (see Kornicker, 1969) of Recent ostracods. Berdan (1988), primarily based on absence of sulcation, considered the Schmidtellidae to be Leiocopa and to belong to the Superfamily Aparchitacea. However, the morphological features of typical Aparchitacea Cjaanussoniids and aparchitids) show important differences from schmidtellids. Principally jaanussoniids and aparchitids are relatively thin shelled and have the right valve overlapping the left valve ( Fig. 5 D-I). The degree of overlap can be marked and in the Aparchitidae the contact groove of the right valve is situated some distance from the valve margin. Aparchitaceans show no marginal flattening of the valves and are characterized by dorsal or ventral valve modifications usually limited to the left valve. By contrast, schmidtellids are thick shellcd with the left valve overlapping the right valve. The degree of overlap is slight with the contact groove of the left valve situated near to the valve margin ( Fig. 5 P). In addition schmidtellids lack the valve modifications which are so characteristic of jaanussoniids and aparchitids. These fundamental differences indicate that the Schmidtellidae should be classified separately from the Aparchitacea. In addition, many schmidtellids closely resemble leperditellids (see also discussion in Warshauer & Berdan, 1982). Leperditellids (palaeocopes) are likewise thick shelled, have the left valve with a distinct contact groove for receiving the overlapped portion of the right valve and show similar flattening or furrowing of the valve margins. The relationship of the Schmidtellidae to the Leperditellidae needs to be clarified, particularly by invcstigation of valve margin morphology in the Leperditellidac. It is possible that the Schmidtellidae are more closely related to leperditellacean palaeocopes than to leiocopes. The paraparchitaceans and pseudoparaparchitaceans form a vast group of smooth, non-sulcate, subovoid ostracods with external features similar to aparchitaceans. They occur worldwide from the Devonian through to the Permian and are well represented in North America (Sohn, 1971a(Sohn, , b, 1983. The question of their origin has not been addressed. Paraparchitaceans differ from aparchitaceans in having a ventral left over right valve overlap (Fig. 5 0, R). Although neglected in the revision of the group (Sohn, 1971a, b ) , this constant pattern constitutes a common feature shared by all paraparchitaceans. Paraparchitaceans show variation in size and shape of adult carapaces, interpreted (Sohn, 1971a, p The presence of a calcified inner lamella in paraparchitaceans, claimed by Sohn (1971~) as an indicator of podocopid affinities, is not clearly demonstrated either from sections or from views of single valves (e.g. Shishaella, Sohn, 1971b, pl. 9, fig. 16) and is therefore debatable. Paraparchites hurnerosus (tranverse section, Sohn, 1971a, pl. 2, fig. 17) shows internal thickenings of the shell on both valves at the contact margin but no distinct calcified inner lamclla. It is also true for pseudoparaparchitaceans as exemplified by Microparaparchites (Sohn, 1983, pl. 3, fig. 24). Gramm (1984, pl. 10, figs 6, 7) shows bilamellar structures in Paraparchites suborbiculatus from the Carboniferous, where the shell develops inwards a dark, probably organic rich layer close to the ventral contact margin. Even though this layer is a possible inner lamella, it is extremely faint in comparison with the calcified inner lamella of true podocopes (Schallreuter, 1979), which exhibit a large vestibule. The origin of paraparchitaceans from the aparchitacean lineage (during the Devonian?) is a strong possibility.

DISTRIBUTION OF APARCHITIDAE AND SCHMIDTELLIDAE IN THE MIDDLE ORDOVICIAN OF NORTH AMERICA Stratigraphic distribution
Aparchitids and schmidtellids are widespread and often abundant in middle Ordovician sequences of North America (Fig. 3). Based primarily on the faunas of the Simpson

Palaeoenvironmental distribution in the Simpson Group
In the Simpson Group aparchitids and schmidtellids do not occur in deposits of very shallow marine aspect such as the Birdseye micrites of the Oil Creek, Mclish or Bromide formations (see sedimentological data of Longman, 1981Longman, , 1982Fay et al, 1982). However, in sediments deposited in a marine subtidal setting they are diverse and numerically abundant. In the Oil Creek Formation schmidtellids are widespread in subtidal facies considered by Lewis (pers. comm.) to have been deposited in water depths of up to a maximum of 20m. Species such as P. pauciperforata commonly occur in such facies but are absent from birdseye micrites in the same Formation. Aparchitids such as B. elegans are also common in subtidal facies of the Oil Creek Formation.
In the Bromide Formation sedimentary facies representing subtidal deposition contain aparchitid species such as S. mitlipunctatu, H. nodosimarginata, B. circuluntis and B. parsispinosa and schmidtellids such as 'Schmidtellu' minu fa in profusion. Longman (1981Longman ( , 1982 estimated that in some areas of the depositional basin of the Bromide Formation (the Southern Oklahoma Aulacogen: see Shatski 1Y46) sedimentation depths of 8Om were achieved. As with the Oil Creek Formation aparchitids and schmidtellids do not occur in sediments thought to be the product of very shallow marine deposition (i.e. birdseye micrites) but are widespread in subtidally deposited normal marine sediments.
In the Simpson Group aparchitid and schmidtellid ostracods appear to occur exclusively in sediments indicative of subtidal deposition. Berdan (1988) added no palaeoenvironmental data to her description of faunas from the early middle Ordovician of Utah, which includes rich schmidtellid faunas like those of the Oil Creek Formation of Oklahoma. However, the associated ostracods and macrofauna (Berdan, 1988, p. 274-275) suggest that the Kanosh Shale and lower part of the Lehman Formation were deposited subtidally.

Palaeoenvironmental distribution in North America
Similar aparchitid and schmidtellid faunas to those of the Bromide Formation are widely documented from other North American middle Ordovician sequences (see Fig. 3) but largely without reference to palaeoenvironment. However, Copeland (1982) in defining ostracod biofacies for the Lower Esbataottine Formation of Canada recognized that the occurrence of taxa he referred tentatively to Aparchites were characteristic of his deeper marine platform biofacies. Likewise it seems from the associated ostracods that aparchitid rich faunas in the middle Ordovician Edinburg Formation of Virginia (Kraft, 1962; pls 1, 2) and the Decorah Shale of Minnesota (Swain et al., 1961;pls 46, 47) also occur in sediments subtidally deposited in normal marine shelf environments.