The Late Eocene – Recent Bradleya johnsoni Benson lineage (Crustacea, Ostracoda) in the Central Equatorial Pacific

Three stratigraphically successive species of Bradleya Hornibrook, 1952 from abyssal sediments of the central equatorial Pacific (Leg 85, Deep Sea Drilling Project) form an unbranching evolutionary sequence designated as the B. johnsoni Benson lineage. These taxa are B. sp. 1 (Lower Oligocene), B. johnsoni (Upper Oligocene – Middle Miocene) and B. thomasi n. sp. (Upper Miocene – Recent). A shared synapomorphic character, the posterior bridge complex, establishes the kinship of these species. Their stratigraphic ranges and a traceable series of changes in the organisation and mass of the reticulum are consistent with the hypothesis of direct ancestor/descendant relationships between them. Bradleya johnsoni is distinguished from B. sp. 1 by the appearance of new primary muri in the posteroventral region and by a general increase in reticular mass. In B. thomasi, the overall reticular pattern is maintained but it becomes more delicate and less rectilinear. Each species of this lineage shows similar variation in the posteroventral reticulum.


INTRODUCTION
Most of the approximately 35 described species of the trachyleberidid ostracode Bradleya Hornibrook, 1952 are strongly reticulate. Secondarily smooth carapaces have evolved in a few species through celation or mural loss (Benson, 1972(Benson, , 1982Whatley et al., 1984). The exoskeletal framework of Bradleya includes primary features, such as the bridge complex and ventrolateral and dorsal carinae (see Benson, 1972, fig. 9) and a latticework of raised muri. The pattern of the reticular grid and the relative prominence of its components have been emphasised by taxonomists in the differentiation of specieslevel taxa. Each fossa in the reticular grid is the surface expression of a secretory cell in the underlying, carapace-forming epidermis (Okada, 1981(Okada, , 1982. This one-to-one relationship allows the paleontologist to monitor evolution on the histological level by tracing the number and alignment of fossae over time (e.g. Benson, 1972Benson, , 1977aBenson, , 1982Benson, , 1983).
An undescribed species from the Paleocene of Chatham Island, New Zealand is the oldest taxon which can be assigned with confidence to Bradleya (Benson, 1972, pl. 1, fig. 9). Of shallow-water origin, it possesses a small, subquadrate carapace and a massively calcified reticulum. Similar morphologic attributes are also found in a diverse group of Cenozoic shelf species endemic to Australasian waters (see Benson, 1972Benson, , 1982Whatley et al., 1984).
Bathyal species first occur in the Early Eocene of the S.W. Pacific (Whatley, 1983(Whatley, , 1985Whatley et a l . , 1984). Migration into the abyssal realm occurred between 40 and 38 Ma as but one component of a major downslope colonisation of the sea-floor by shallowwater ostracode groups Deck, 1984, 1985). This event most likely took place in the S.W. Pacific, the site of the origin and initial radiation of Bradleya (Whatley, 1983(Whatley, , 1985. Once established on the sea-bed, the trans-Pacific dispersal of Bradleya was accomplished rapidly. The oldest known form outside the Indopacific is B. "dictyon" of van den Bold (1986) recovered from Upper Eocene sediments of bathyal origin (planktonic foraminifera1 zones P16-P17), Chapopote Fm., Tampico Embayment, Mexico. The existence of this population on the eastern rim of the Pacific implies that bradleyids were also present in the study area at this time. Unfortunately, ostracodes are absent from sediments of equivalent age in the Leg 85 cores. By Oligocene time, bathyal and abyssal species of Bradleya were widely distributed in the World Ocean (Benson, 1977b;Benson and Peypouquet, 1983;Steineck et al., 1984Steineck et al., , 1988Ducasse and Peupouquet, 1986). Such forms are typically blind, cosmopolitan in distribution and larger and more delicately ornamented than their shallow-water counterparts.
sequence here designed as the Bradleya johnsoni Benson lineage. A review of previously published work and re-examination of specimens from Jamaica and Barbados revealed that these taxa occur in the Atlantic and Caribbean provinces. Access between these regions and the Pacific was provided by a deep oceanic corridor in the region of the present day Isthmus of Panama which remained an important pathway for microfaunal interchange until the Upper Miocene (Keller & Barron, 1983 (Fig. 2). These taxa possess a unique morphologic feature, the posterior bridge complex (PBC- Fig. 3) which is not found in other known species of Bradleya. We interpret this character as a synapomorphic (derived, homologous) trait establishing the kinship of these species. An explicit hypothesis of the ancestor/descendant relationships of these taxa is based on: 1) their stratigraphic succession without overlap; and 2) a traceable series of changes manifest in the organisation and mass of the reticulum (Fig. 4). Thus, chronistic and phenetic data from the Pacific suggest that the sequential appearance of B. sp. 1, B. johnsoni and B. thomasi is the product of anagenetic speciation within a clade designated the B. johnsoni lineage. This lineage is distinguished by the following morphologic characters: 1) the PBC centred over the muscle-scar node ; 2) two horizontal muri connecting the PBC to the ocular ridge; and 3) the basic arrangement of muri and fossae in the posteroventral quadrant. This arrangement consists of an arcuate column of 4 ( B . sp. 1) or 8 ( B . johnsoni, B. thomasi) fossae between muri 1 and 2 (see Fig. 4), bordered posteriorly by two groups of fossae, a dorsal triplet (EE' E" of Fig. 4) and a box-like ventral cluster of four (FGHI of Fig. 4). The evolution of B . johnsoni was marked by longitudinal cell division in the region between muri 1 and 2 to form the couplets A' A, B'B etc. (compare Fig. 4 a , b with 4 c, d) and by a general increase in the mass of the reticulum. In contrast, the emergence of B. thomusi did not entail discrete changes in the reticular matrix. Instead, within a basic reticular pattern duplicating that of B. johnsoni, the sculptural relief diminished and the grid of intersecting muri became less rectilinear.
species of the B. johnsoni lineage, we have observed minor intraspecific variation. This variation, represented by the rotation of fossae peripheral to the triplet EE' E , will be described in a succeeding section. Present data place the earliest known occurrences of B. johnsoni and B. thomasi in the central Pacific. Consequently, these species are considered to have arisen there and shortly thereafter, to have dispersed to other regions. However, Fig. 2 shows that the stratigraphic record of Bradleya in the central equatorial Pacific contains gaps which preclude the in situ indentification of the B. sp. 1 -B. johnsoni and B. johnsoni -B. thomasi speciation events. Therefore, the possibility that B. thomasi and B. johnsoni evolved elsewhere and subsequently emigrated to the central Pacific cannot be completely ruled out.
The origin and early development of the Bradleya johnsoni lineage cannot be established at the present time. The oldest taxon referred to this clade is the undescribed form reported by van den Bold (1986) from Upper Eocene sediments of the Tampico Formation, Mexico (see Plate I, fig. 4). We consider these specimens as a stratigraphically early variant of B. sp. 1 from the Pacific but they may prove to represent a distinct species. Nevertheless, the PBC and arrangement of fossa in the posteroventral quadrant indicate a close relationship to the johnsoni clade. Comparative studies are needed to clarify the phylogenetic affinities between these taxa and Eocene forms described from the S.W. Pacific by Whatley et al. (1984). It is perhaps significant that constituent species of the B. dictyon (see Whatley et al., 1984) lineage are characterised by orthogonal muri of equal height in the posterior reticulum. If this shared morphocharacter proves to be synapomorphic, then both groups can be considered sister-taxa derived from a common but as yet undiscovered ancestor in the early Paleogene.

BRADLEYA SP.1
In the central equatorial Pacific, Bradleya sp. 1 ranges from zones CP16C/P18 to C18/P20 in the biostratigraphy of Barron er al. (1985) and   (Fig. 2). This interval is equivalent to the time span of 34.8 to 31.5 Ma according to the geochronometric scale of Berggren et al. (1985, fig. 5). Outside the Pacific, B. sp.1 occurs in the Upper Eocene of Mexico (van de Bold, 1986; see Plate I, Fig. 4) and in the Lower Oligocene of Barbados (Steineck et al., 1984; see Plate I, Fig. 5), Jamaica (Steineck, 1981, p. 355) and the south Atlantic (Benson, 1977b, plate 1, Fig. 4). The Mexican occurrence, represented by three specimens from the type locality of the Chapapote Fm., Tampico Embayment may represent the earliest occurrence of Bradleya in the Caribbean region. Abyssal ostracode faunas of Late Middle Eocene age from the Oceanic Fm., Barbados do not contain this genus (Steineck et al., 1984).

BRADLEYA JOHNSON1 BENSON
In the central equatorial Pacific, this species ranges from CP19A/P21 to N12 (Fig. 2). Interpolation from biostratigraphic and sedimentation-rate data constrains its origin to the interval from 31.5 to 30.4 Ma. Absence of bradleyids in Leg 85 samples correlated with zones N13 to N15 precludes accurate determination of its true extinction level. The morphology of B. johnsoni is stable over its extended stratigraphic and geographic ranges. Several morphologic innovationsnew macroreticular elements in the posteroventral quadrant, increased thickness and height of muri, secondary reticulation in the anterior and anteroventral regions, massively calcified rampart of the PBCreadily distinguishes this form from B. sp. 1.
Pacific assemblages of B. johnsoni show considerable variation in reticular mass. End-members of this apparently gradational morphocline are illustrated in Plate 1, fig. 10 (delicate) and Plate 2, fig. 4 (massive). Similar variation in south Atlantic populations can be inferred from a comparison of the holotype from the Rio Grande Rise (Benson & Peypouquet, 1983, pl (Benson, 1972, pl. 8, fig. 7) recovered from the central portion of the South Atlantic. The geographic and stratigraphic significance and causation of this variation not yet clear.

BRADLEYA THOMAS1 N. SP.
This species occurs from zone N16 to the Recent in the central equatorial Pacific. Morphologic divergence between Bradleya thomasi n. sp. and its ancestor becomes more pronounced with time. Although early specimens of B. thornasi (e.g. N 16, N 17) can be readily differentiated from late examples of B. johnsoni, some characteristics of the latter recur in the former but in a reduced or incomplete state. An analogous case of interspecific evolution in an Eocene lineage of the related genus Echinocythereis Puri, 1954, has been described by Reyment (1985, p. 187). He speculated that weak directional selection was responsible for the gradual divergence from the ancestral form, an ex-  145x. B, B. johnsoni, pl. 2 , fig. 4, 145x. C, B. thornasi, pl.  3 , fig. 7, 260x.
planation that might also apply to B. thomasi. Both the evolutim of B. thomasi and a decrease in the diversity of the ostracode fauna occupying the central equatorial Pacific took place in synchrony with the growing dominance of frigid, corrosive waters in the abyssal circulation of this region (Steineck et af., 1988; for paleoceanographic background see Mayer, Shipley & Winterer, 1986). However, present-day occurrences in the World Ocean (e.g. Cronin, 1983) suggest that B. thomasi has subsequently developed broader environmental tolerances. The delicate, low reticulum of B. thomasi may be an adaptation to central Pacific bottom-waters which during the Late Miocene were becoming progressively depleted in dissolved calcium carbonate.

DISCUSSION
Several investigators have concluded that reticular patterns in advanced cytheraceans are stable in, and diagnostic of species (Benson, 1972;1977a;1982;Liebau, 1975Liebau, , 1977Al-Furraih, 1977;Okada, 1981Okada, , 1982. In accord with these findings, species of the Bradfeyu johnsoni lineage display remarkably consistent arrangements of muri and fossae. This regularity is clearly demonstrated in the posteroventral reticulum ; Figs. 4 and 5 depict silhouettes of the surface sculpture in this region, on which homologous elements are identified by an alphanumeric code. However, close inspection of these diagrams reveals, that within the basic design, there exists in each species similar variation in the relative positions of fossae adjacent to E". In qualitative terms, the intraspecific variation can be modelled as a gradational morphocline in which fossae E and H "migrate" in an arc relative to E . The series of morphotypes so produced terminates at two configurations termed the B (Basal)morph and the L (lateral) -morph. In the B-morph (Figs. 4a, c, Fc; Plate 1, fig. 3; Plate 3, fig. 6), H is located beneath the posteroventral border of E , and the ridge separating F and H and the ventral ridge of E intersect at a Y-shaped junction. In the L-morph (Fig. 4b, d; Plate 2, fig. l l ) , H is positioned laterally along the posterior edge of E , and its apex is in contact with the ridge forming the base of EE'. In the intermediate morph, H occupies a partly lateral and partly basal location with respect to E" (Fig. 5a, b).
From another perspective, the intraspecific variation present in species of the Brudleya johnsoni lineage can be described in relation to a prominent mural pore here designated as pore conulus alpha. Because the number and location of pore conuli are commonly thought to be the most constant heritable feature on the ostracode carapace, they provide useful reference points for plotting variation in the position of fossae (e.g. Benson, 1977aBenson, , 1982Benson, , 1983. Pore conulus alpha has been observed to occur in the following positions: 1) at the posteroventral corner of E (Figs. 4a,c;5a,b); 2) at the posteroventral corner of E' (Fig. 4b, d); and 3) on the vertical murus forming the posterior boundary of E (Fig. 5c). Within the morphotypes defined previously, there is no consistent placement of fossae relative to the position of pore conulus alpha. This observation suggests that the fossae under discussion (E', E" and H) not only change their relative positions but that they also "pivot" on the carapace surface relative to pore conulus alpha.
With the limited data provided by the Leg 85 samples, we have demonstrated the temporal coexistence of different variants and the presence of similar variants in successive species of the B. johnsoni lineage. Nevertheless, it would be rash to draw far-reaching conclusions at the present time about the causative mechanisms underlying this variation. Clearly, further quantitative analysis of larger and geographically widespread assemblages is needed to establish the stratigraphic and geographic distribution of this variation and to investigate its correlation with genetic, developmental or ecologic factors.  (Benson, 1972) and Florida-Hatteras Slope (Cronin, 1983). Description. Carapace large, robust; elongate-oval in shape with a broadly rounded anterior margin with 12-15 short denticles and a truncate posterior margin with prominent spines in dorsal and ventral corners. Dorsal margin straight except for anterior projection. Greatest width at central margin. Ventral margin gently concave in lateral view, broad and flat. Valve surface reticulate containing thin, sharp-edged muri enclosing shallow, flat to weakly concave sola. Muri and sola densely foveolate ; foveolae stellate in outline. A single row of foveolae is present on the lateral surfaces of muri. Horizontal and vertical muri equally emphasised. Posterior reticulum composed of a honeycomb arrangement of polygonal cells. Posteromedian ridge subdued and disrupted. Ponticulate ventrolateral carina prominent and flaring, continuing anteriorly into a subdued ocular ridge. Bridge present; PBC prominent, typical in form with low rampart. Internal space of mural loop closed dorsally by weak horizontal partitions but tending to be open in ventral portion. Sensillum pores of two types:
Remarks. The PBC and the arrangement of posteroventral reticular elements differentiate B. thomasi from B. dictyon (Brady). The bridge of B. dictyon is subdivided internally by horizontal and vertical partitions; its posterior termination consists of a straight or gently curved ridge of variable height which supports a single horizontal mums projecting into the lumen of the mural loop (Benson, 1972, fig. 13b;Whatley et a f . , 1984, p. 287, pl. 1, figs. 1-3). In B . dictyon, the posteroventral reticulum is most often complex and non-rectilinear (Ducasse & Peypouquet, 1979, pl. 3, fig. 9;Whatley et af., 1984, pl. 1, figs. 2, 3). However, Benson (1972, fig. 13b) figured a specimen from the Recent of the eastern Pacific with three rows of verticially aligned fossae. This pattern, although reminiscent of B. thomasi, nevertheless, lacks the characteristic triplet of fossae enumerated as EE'E (see Plate 3, fig. 4 ; Fig. 5c) of that species.
Bradfeya fordhowensis Whatley , Downing, Kesler & Harlow from the upper Neogene of the Lord Howe Rise resembles B. thomasi in size, outline and in the low, delicate reticulum. In the former taxon, the bridge is absent or suppressed in contrast to its prominent development in the latter. These forms also differ in the number of fossae in the region posterior to the mural loop and immediately below the posteromedian ridge.
Bradfeya johnsoni Benson is more heavily calcified and has higher, excavate, distally rounded muri with multiple rows of lateral foveolae. In addition, in this species, the PBC is enclosed by an elevated, flat-topped rampart which contrasts with the more subdued and rounded rampart of B. thomasi. Most specimens of B.

Bradfeya johnsoni
Description. Elongate, subrectangular species with a dense, orthogonal reticulum. Sculptural elements often massive in character. Raised, linear posteromedian ridge present. PBC with great relief; internal triangular fossae surrounded by an elevated, flat-topped rampart. Muri high, excavate with bluntly rounded to gently angled tops. Foveolae present on distal and lateral (multiple rows) surfaces. Concave sola set in deep, steep-walled fossa. Subdued secondary muri, contrasting with primary reticulation, present in anterior and anteroventral regions. Remarks. This widespread Oligo-Miocene species ranges from zones P21 to N21 in the Leg 85 collections.

Remarks.
Left in open nomenclature because of the limited number of specimens available in the Leg 85 samples. Bradleyu "dictyon" of van den Bold (1986) is smaller and more quadrate than Pacific examples (see PI. I, fig. 4) but the bridge structure and robustness and pattern of the reticulum suggests placement here. Plate I, fig. 5 illustrates a corroded and compressed specimen from the Lower Oligocene of Barbados. This carapace appears to be an instar of B. sp. 1. fig. 4.