Makoiamya cotterallae, Grant-Mackie, John A., 2013

Grant-Mackie, John A., 2013, Makoiamya cotterallae, a new genus and species of bivalve (Ceratomyidae) from the latest Triassic of New Zealand and New Caledonia, Zootaxa 3741 (3), pp. 327-348 : 331-341

Makoiamya cotterallae n. gen. et sp.

Figures 11, 12A

1956. Anodontophora sp.— Campbell & McKellar: 700; Campbell: 48. 1959. Anodontophora sp.—Campbell: 201.

1975.? Anodontophora sp., cf. Anodontophora sp.—Martin: 919. 1981.? Ochotomya sp.— Grant-Mackie: 243, 246.

1985. Ochotomya sp.— Campbell et al.,? 26, 31.

1998. Ochotomya sp.— MacFarlan: 302, 304, 305.

2009. Ochotomya sp.— Campbell, in Gordon: 224.

Etymology. Named for Louise M. Cotterall, draughtsperson and photographer to the Geology section, who has been of enormous assistance in illustrating my publications over many years and has always maintained the highest of standards.

Diagnosis. Ceratomyid with moderately thick shell material, well inflated, with prominent umbo, subcircular to oval to subquadrate in outline, with rounded ridge running from umbo to posteroventral margin so that shell somewhat truncated posteriorly, slightly longer than high, hinge plate long, narrow; nymph of RV nearly full length of hinge plate; lacking ornamentation.

Holotype. L4576, internal cast of a RV with beak and part of the umbo detached to show the hinge line, from locality R16/f8528, coast at high tide line ca. 100 m west of south end of Kiritehere beach, and collection AU1427, by JAG-M 9/7/59; Ngutunui Formation, top Warepan (Late Norian).

Additional material. Paratypes: L4577 and L4578, from the same locality and collection as the holotype, consisting of large LV, smaller RV cast retaining some shell material; L4584, R15/f289, AU14973, partly open bivalved specimen, Otapirian; L4582, R16/f429, AU6559, undistorted steinkern, top Warepan or early Otapirian; L4585, R17/f8574, AU12169, small steinkern; L4583, R18/ f6562, AU8986, RV (with associated but disarticulated LV); L4579, L4580, R18/f6581, AU 1885, internal casts, RV, LV, all Otapirian; L4581, NC/f125, LV internal cast, top Warepan; L4586, NC/f167, AU5787, steinkern, Otapirian; L4612, R16/f429, AU6559, steinkern lacking anteroventral margin, may be top Warepan but more likely early Otapirian (for other details see Appendix 1). In addition, a further 38 specimens have been available for study in preparation of this report, as noted in Appendix 1.

Description. The holotype is an undistorted RV internal cast; dimensions are L= 51 mm, H= 45 mm, W= 18 mm, and L/H ratio is 1.13. Overall, the species is of moderate size (L up to ca. 60 mm), outline subcircular to oval to rounded subquadrate; equivalve, inequilateral, slightly longer then high (L/H ca. 1.1, mean 1.14; here and in the following description n = 15 specimens; see Table 1); moderately inflated (W/H ca. 0.215 to 0.5, mean = 0.35), with no posterior gape or dentate margins; beak prosogyrous, enrolled, variable in position but generally near anterior third (L-A/A ranges 1.6 to 11.5, mean 4.69)(Table 1); rounded posteroventral ridge passes from well inflated umbo to rounded-angular posteroventral margin, with slightly flattened or excavated area dorsal of ridge and slightly flattened posterior margin; hinge plate edentulous but RV has low boss immediately behind beak and long, low, nymph running nearly full length of hinge plate posterior to beak, and slightly curved to parallel hinge margin, with slight ventral emargination of hinge plate between boss and nymph; presence of nymph on LV uncertain; exterior unsculptured except for fine commarginal growth lines; shell material 2–3 mm thick, thickest dorsally; adductor muscle scars not certainly seen, but one RV internal cast ( Fig. 10 J) perhaps shows posterior scar and two steinkerns ( Fig. 11 I, J, M) may record anterior scars; possible posterior scar situated just inside posterodorsal corner of shell, obliquely oval, of moderate size, with low oblique ridge across scar; possible anterior scars small, crescentic, and of equal size on each valve, impressed into shell material just inside anterodorsal corner of valves; pallial line not seen.

A reconstruction is shown in Figure 12 with, for comparison, figures of Ceratomya , Unionites and Ochotomya .

Dimensions. See Table 1. The largest specimen, a steinkern from AU12681, R16/f323, is incomplete, lacking about the posterodorsal third; what remains is H = 60+ mm, L = 50+ mm, A = 20+ mm, and W (2 Vs) = 35 mm, but slightly crushed.

Remarks. Of ratios listed in Table 1 the least variable is length vs height, with a mean value of 1.14 and a range of 1.03 to 1.29. Position of the beak is more variable, resulting in both L-A/A and L/A showing a much wider range (12.67 to 1.5, with a mean of 4.69 for L-A/A and 13.67 to 2.5, with a mean of 5.69 for L/A). On subcircular steinkerns lacking clearly defined adductor scars or hinge it is seldom easy to locate precisely the correct orientation of the hinge line, so a great deal of the variation suggested here is likely to be due to operator error. In fossil bivalves most parameters are subject to distortion during fossilisation but for Makoiamya cotterallae that least affected by sediment compaction is W, the thickness, or inflation, of the shell, so W/H has a much narrower range.

No adductor scars or pallial line are clearly seen on internal casts, but a few casts show what may be remnants of the adductor scars. One each LV and RV have faint depressions in the posterodorsal area, that would have been raised areas on the shell interior, in a position where the muscle is likely to have been attached. A RV (L4578, from R16/f8528) has a vague vertically oriented oval depression of ca. 13 x 9 mm2 ( Fig. 10 J) close to the posterodorsal margin with a slight ridge on its anterior and a few possible commarginal growth rings behind it that could represent a posterior adductor scar. It is crossed diagonally from the anterodorsal edge of the depression by a low straight narrow ridge. A steinkern, L4612, from R16/f429, shows very close to the anterodorsal margin on each valve an impression of a shallow oval depression of ca. 10 x 5 mm2 with its posterior margin raised as a low buttress ( Fig. 11 M). These are likely to be casts of the anterior adductor muscle scars. Possible anterior adductor scars are seen on a steinkern (L4582 also from R16/f429; Fig. 11 I) as small posteriorly concave, crescentic, raised areas (and thus depressed areas in the shell itself) immediately inside the margin of the steinkern in its anterodorsal area, with that of the LV clearer than that of the RV.

The hinge is not completely known, with the RV better preserved than the left in the specimens studied ( Fig.10 D, G). The nymph is clear in the RV ( Fig. 10 B, G) but its presence is not fully established for the LV. Nor is there indication of the presence of any equivalent or receptor in the LV for the boss seen in the RV.

The weak posteroventral ridge is usually obvious because compaction and lithification have apparently not greatly distorted the valves, perhaps attributable to lithology and shell thickness, but one indicator may be that not all valves show the faint surface excavation dorsal to the ridge even when the ridge itself is obvious. Apart from this ridge, the valve is smoothly rounded anteroposteriorly and dorsoventrally, with the anterior area more steeply rounded than the posterior. One collection from New Caledonia, NC/f248, however, consists of four bivalved specimens that have been significantly distorted post-mortem, and some show over-steepened posteroventral ridges and variably inflated valves.

Whilst Makoiamya cotterallae is known from 72 localities in the New Zealand Murihiku Terrane (Appendix 1), it is reported here from only six in the New Caledonian Téremba Terrane.

No congeners are known, but may include poorly known or inadequately described species from Late Triassic or Early Jurassic of any of the circum-Pacific or eastern Tethyan regions, as well as among material yet to be described. As the above discussion of the genus indicates, no evolutionary links can at this stage be suggested, but a relationship with Ceratomya seems possible and is implied by the family location of Makoiamya .

In early Aratauran (~Hettangian-Sinemurian; Early Jurassic) strata of the Awakino-Kawhia and New Caledonia areas an undescribed species of Pleuromya occurs, sometimes quite abundantly. In one New Caledonian Aratauran collection, AU7691 from NC/f389, on the northeastern coast of Uitoë Peninsula, with about 15 specimens of this Pleuromya , one steinkern approaches the shape of Makoiamya , with a short rounded outline and rounded ventral margin, rather than the elongate outline with straight ventral margin and very short anterior of Pleuromya n. sp. This one specimen clusters with the other steinkerns in the same collection without significant separation in plots of L/A and L/H statistics and can be seen as simply the endpoint in a continuum of morphs of the new Pleuromya . Given the difficulty in getting clear hinge details in specimens of both genera, a lone specimen of one might be superficially confused with the other, but a small collection of individuals clearly shows the differences even in the absence of the hinges.

The collection from NC/f389 also includes the brachiopod Herangirhynchia herangiensis MacFarlan, 1992 , who dated the locality as Ururoan. He gave the time-range of the species as late Aratauran–Ururoan (Hettangian– Pliensbachian). There is also in the fauna a bivalve identified as possibly the Ururoan (ca. Pliensbachian) marker genus Pseudaucella Marwick, 1926 , presumably the basis for the Ururoan age allocation, but its identity is uncertain; no other taxon in the small fauna indicates a Ururoan age and on the basis of the Pleuromya n. sp. the locality is accepted as of Aratauran age.

A solitary steinkern from NC/f388 on the northern coast of Uitoë Peninsula 50 m south of the last-mentioned locality, is slightly obliquely crushed and more similar to Makoiamya but is also interpreted as another shorterthan-normal specimen of the same Pleuromya that is widely present in other Aratauran outcrops nearby (e.g., NC/ f390, f391, f394).

Makoiamya cotterallae sometimes shows a superficial similarity to some specimens of Maorimonotis calvata (Marwick, 1953) with which it co-occurs in Late Warepan collections. They can be separated, however, because the monotid species has thinner shell material, a more obliquely elongate outline, often shows some evidence of radial ribbing, and lacks the posterodorsal ridge of Makoiamya .

Stratigraphic range. The new species is known so far from Murihiku strata of Kawhia and Southland Regional Synclines of New Zealand and from Téremba Terrane of Baie de St-Vincent–Moindou area, New Caledonia, in rocks of Late Norian ( Gnomohalorites cordilleranus zone; Warepan Stage) and Rhaetian (Otapirian) ages. In more detail, earliest specimens occur in the Late Warepan (Marokopan Substage; see Fig. 1 ) and Appendix 1 records them with Maorimonotis maniopotoi (Grant-Mackie 1978) in R15/f8953, with Pacimonotis gigantea (Avias 1953) in NC/f125 and NC/f248, with Entomonotis richmondiana (Zittel 1864) in R15/f8957, and with M. calvata at a few localities in both New Zealand and New Caledonia. Otapirian members are more abundant, occurring in both the informal early Otapirian (Campbell 1956), and from a greater number of localities higher in the stage. At one locality Makoiamya cotterallae has been found with a fauna including the bivalve Otapiria dissimilis (Cox, S.H. 1878) . An assemblage including this species has been characterised by Campbell (1956) as constituting a Late Otapirian fauna. Other Otapirian occurrences are in intermediate strata lacking indicators of either the early or Late Otapirian. Two specimens came from a single small package within the large Kiritehere slump (R16/f8747; see Grant-Mackie & Lowry 1964), the only fauna known from this package, so it is of uncertain Warepan–Otapirian age, although more likely top Warepan than Otapirian.

In New Caledonia Makoiamya cotterallae is found more frequently in Late Warepan than in Otapirian strata. Warepan occurrences are in the Entomonotis acutecostata and/or E. richmondiana zones (with Pacimonotis gigantea) and the Maorimonotis calvata zone. Otapirian occurrences include presence in the Late Otapirian.

The new species thus ranges from the Richmondiana Chronozone of the Marokopan Substage of the Warepan, through the informal early Otapirian into the Late Otapirian Stage, a duration of some 8 Ma (see Cooper 2004 for durations of local stages). It could also be expected to be found in suitable lithologies within this age range in the Nelson region at the northern end of South Island. There is no record of its presence in the Rakaia Terrane although the dominant fine-grained sediments there could perhaps have accommodated it. The genus appears to have been another victim of the T-J boundary extinction event, which it apparently failed to survive.

Associated fauna. Makoiamya cotterallae appears in Marokopan strata as part of a macrofauna including the very abundant monotid species and rare and generally inconspicuous brachiopods, bryozoans, gastropods, echinoderms and cephalopods. It occurs in Otapirian strata accompanied by various rare brachiopods which are more abundant in lithologies lacking Makoiamya ( Clavigera Hector, 1879 ; Rastelligera Hector, 1879 ; Psioidiella Campbell, 1991 ; Mentzelia kawhiana Trechmann, 1918 ; Sakawairhynchia Tokuyama, 1957 ; terebratulaceans) and bivalves, especially Kalentera marwicki Grant-Mackie 1960 , but also Lima Bruguière in Bruguière et al., 1797, Maoritrigonia leedae Fleming 1987, prosobranchs, and rare gastropods, cephalopods, crinoids, plant debris and the trace fossils Chondrites Sternberg, 1833 , and Zoophycus Massalingo, 1855.

Paleoecology. The frequent occurrence of articulated specimens in growth position normal to bedding planes, anterior-down in the rock, indicates an infaunal mode of life. Its presence in fine sandstones and siltstones, and absence from coarser lithologies show that it lived in quieter conditions, possibly at about mid-shelf depths. This is supported by the prevalence of Makoiamya dominantly in the western limb of the Kawhia Regional Syncline and its near-absence from rocks of the more off-shore eastern limb ( Figs 2–6 ; fig. 13.15 of Stevens 1980). The apparent absence of fragmented shells, except in the basal beds of the Ngutunui Formation, and of bore holes in the shells indicates that the species lived deep enough to have generally avoided excavation by wave or current action or predation. Nor is there any evidence of epibiont attachment. It must be agreed that these last points offer only weak support for the conclusion because bore holes and epibiont attachments are rare globally in Triassic rocks.

These conclusions explain why Makoiamya cotterallae is found in strata generally lacking the otherwise almost ubiquitous Late Triassic brachiopods Clavigera and Rastelligera in this region—the brachiopods lived in shallower more vigorous conditions and are preserved in coarser lithologies, including occasional Clavigera ‘grit’ units, especially in Southland and Baie de St-Vincent, New Caledonia. Clavigera nevertheless is found in at least ten localities in New Zealand with Makoiamya (ca. 13% of the total), and Rastelligera at three (ca. 4%).

Kalentera marwicki is the only other bivalve frequently found articulated in the same strata as Makoiamya , also generally in growth position, and it has already been concluded to be part of this small infaunal community in fine to very fine sands of this region in the latest Triassic (Grant-Mackie 1960). This community included the much rarer protobranch bivalves mentioned above which are also frequently found articulated within strata interpreted as primary deposits, but at least some other forms (e.g. Maoritrigonia probably, and certainly the gastropods and cephalopods) must have been introduced into the fossil fauna from outside by redeposition.

Both the above trace fossils occur in fine sandstones at many levels in the late Warepan-Otapirian of the Kawhia-Marokopa-Awakino area and their paleoenvironmental implications remain to be elucidated. The Zoophycos is a relatively small type with a maximum diameter of 120-180 mm and laminae 4-5 mm thick; they clearly show the meniscus internal structure but collected specimens do not preserve the tubular margin (AU14977, R15/f285). The genus has already been reported from New Zealand Triassic rocks in both the Murihiku Terrane (Oretian, early Norian; Cave 1982) and Torlesse Terrane (Fordyce 1976) with specimens of a comparable size to ours. Much has been written about the possible paleoenvironmental significance of Zoophycos but there is little general agreement (see, e.g., Olivero 1996), although size may be an indicator (Olivero 2003), with smaller specimens such as those in the Warepan–Otapirian linked to outer shelf-upper slope deposits.

Figures

FIGURE 1. International and local chronostratigraphic divisions of late Triassic time (after Cooper 2004 and Grant- Mackie 1985). Some levels are given absolute ages (column headed Ma), after Cooper (2004); L = Late; M = Middle.

FIGURE 2. Localities within New Zealand that have yielded Makoiamya cotterallae. For more precise locations see Figures 3 – 8 and Appendix.

FIGURE 3. Part of the southwest Kawhia coast, southwest Auckland, showing localities listed in Appendix 1 (part of map sheet R 15).

FIGURE 4. Part of the Kawhia-Marokopa coast, southwest Auckland, showing localities listed in Appendix 1 (part of map Sheet R 16).

FIGURE 5. Part of the Marokopa-Kiritehere coast, southwest Auckland, showing localities listed in Appendix 1 (part of map sheet R 16).

FIGURE 6. Part of the Waikawau area, southwest Auckland, showing localities listed in Appendix 1 (part of map sheet R 17).

FIGURE 12. A. Restoration of RV of Makoiamya cotterallae, in internal view (based especially on Fig. 10 B, D, E, G, and J). B. Ceratomya aalensis (Quenstedt), Middle Jurassic, France; RV (upper) and LV internal views (from Cox 1969 b). C. Unionites rectus (Gümbel), Upper Triassic, Tyrol; RV internal; U. muensteri Wissmann, Upper Triassic, Tyrol; RV external (from Cox 1969 a). D. Dorsal view of generalised internal cast of LV of Ochotomya, showing, in negative relief, characteristic hinge features; a = low, broad groove, b = tooth-like projection, c = two shallow furrows (after Polubotko 1966, p. 14). Scale bars demonstrate the length of each valve.