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Chapter 72. Is Neckajatia an ancestor of the platycope ostracodes?

Chapter 72. Is Neckajatia an ancestor of the platycope ostracodes?

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1052 R. F. LUNDIN



arrangement of eggs in it have occurred (Jaanusson, 1985, p. 81, 82). Through his analysis of the

modern cytherellid platycope Cytherella abyssorurn G. 0. Sars, Jaanusson explained, from the

standpoint of the functional morphology of the carapace, the reasons for the very conservative

phylogeny of this group once it came into existence. On the other hand, neither Jaanusson (1985)

nor other authors have discussed in detail the ancestors of the platycope ostracodes. It has been

generally supposed that the kloedenellacean ostracodes represent the roots of the platycopes (see

Guber and Jaanusson, 1964, p. 9; Adamczak, 1966, p. 13; and Schallreuter, 1968, p. 128).



REPOSITORIES

Ostracodes illustrated in this paper are housed in the type collections of the Department of

Geology at Arizona State University (ASU X). Sample numbers used in the plate explanations

have the prefix “MS” or “RFL”.



PURPOSE

In view of the above, the purpose of this paper is to evaluate the morphoiogy of Neckajatiu

and a related genus and compare that morphology to the morphology of the platycope ostracods.

This will provide a basis for the conclusion arrived at here, that Neckajatia is ancestral to the

Platycopa. It is not the purpose of this paper to undertake species level taxonomic revision or even

to describe and/or illustrate all species of the genera involved. Accordingly, my observations and

conclusions are based upon a few forms which demonstrate the important morphological characters.



THEPLATYCOPA

The platycope ostracodes have the following combination of shell characteristics which are

diagnostic for the group :

1) The right valve is larger than the left and overlaps it along all or nearly all of the free margin.

The overlap may be reduced anteriorly.

2) The contact structure along the free margin consists of a groove in the right valve into which

the edge of the left valve fits. The contact groove may be reduced or even absent anteriorly in some

species (Adamczak, 1968, p. 30; Jaanusson, 1985, p. 74).

3) The hinge is also an edge and groove arrangement, the groove being in the right valve into

which the edge of the left valve hinge fits. The detailed morphology of the hinge is somewhat

variable but the hinge groove of the right valve is, in all cases, to a greater or lesser degree a conPLATE1-Figs.



1-3, 5, 6,9. Neckujutiu modesfa (Neckaja). 1,2, 9. Right lateral ( x 71), ventral ( x 78) and slightly

oblique dorsal (anterior two-thirds of specimen, x 126) views of adult carapace (ASU X-73): note R/L overlap

and anterior straguloid process (fig. 9). 3. Dorsal view ( x 83) of adult carapace (ASU X-74). 5. Interior view

( X 92) of adult left valve (ASU X-75). 6. Interior view ( X 83) of adult right valve (ASU X-76) to show contact

groove and straight hinge. All specimens from the Silurian at Nyharnn, Gotland (MS 17). Figs. 4,7. Neckujutiu

h f u(Neckaja). Dorsal ( x 56, ASU X-77) and right lateral ( x 68, ASU X-78) views of adult carapaces from the

Silurian (early Wenlockian, Jaani Stage) of Estonia : note R/L overlap and anterior and posterior straguloid

processes. Fig. 8. “Primitiu” vuriolufu Jones and Holl. Ventral view ( x 72) of adult carapace (ASU X-81) from

the Silurian Much Wenlock Ls. Fm. at Lincoln Hill (RFL # 13), Welsh Borderland, showing R/L overlap

(anterior end abraded). Figs. 10, 11. Neckujufiu cf. N. lutu (Neckaja). Interior views of parts of adult left

( X 123, ASU X-79) and right ( x 132, ASU X-80) valves from the Silurian Mulde Beds at Dapps, Gotland

(RFL 1970) to show hinge details.



Is Neckajatia an Ancestor of the Platycope Ostracodes? 1055



tinuation of the contact groove of the free margin. Accordingly, in its fullest development, the

contact relationship of the platycope valves is described as a holoselenic contact (Pokorny, 1957).

4) Domatial dimorphism (Jaanusson, 1985, p. 80) in which a portion of the posterior part of

the carapace forms a chamber for egg care. The extent and shape of the domatium and precise

arrangement and number of eggs placed in it is variable, but it is bounded anteriorly by a ridge,

the limen (Adamczak, 1966), which is variably developed but distinct in all platycopes.

Some representatives of the platycope ostracods have an anterior straguloid process (Guber

and Jaanusson, 1964, p. 3). This feature is variably developed on platycopes and is absent from

many of them (e.g., some species of Cavellina). I do not consider it to be an important diagnostic

character, but it appears to be reduced in those species in which the holoselenic contact condition

is well-developed. In such forms longitudinal movement (slippage) of the valves is minimized and

an anterior straguloid process serves no purpose in this regard. It is also possible that development

of straguloid processes is related to, or partly controls, the degree to which the valves can be opened.

In this way, development of the straguloid processes may be related to the position of posterior

termination of the intervalvar band (see Jaanusson, 1985). In any case, straguloid processes are

not consistently developed in Paleozoic platycopes.

The changes in the adductor muscle scars of the platycope ostracodes has been discussed at

length (e.g., Gramm, 1985 and elsewhere; Jaanusson, 1985). Adamczak (1966) has shown that the

early platycopes had an oval adductor muscle field with many scars. His illustrations of Nyhamnella musculirnonstrans, which is a platycope (Sarv, 1977), show that the scars have no special

regular arrangement as has been reported by Gramm (1985) for younger cavellinids. The adductor

muscle scar of Nyhamnella can be considered the primitive condition for platycopes. In any case,

the adductor muscle scar pattern is not significant to the issue discussed here because it is not

known for Neckajatia.

In summary, the characteristics indicated above (numbers 1-4) are the important diagnostic

characteristics of the platycope ostracodes. All of these seem to be stable characteristicswhich have

remained essentially unchanged, except for some details, since the late Llandoverian.



THEGENUSNeckajatia

Neckajatia modesta, N. lata, N. cf. N. lata and “Primitia” variolata Jones and Holl, 1865, are

four forms which demonstrate the platycope-like morphology of this group. The former is the type

species of Neckajatia and the latter should be placed in a new genus because it is distinctly

unisulcate. There is, however, no doubt about the close relationship between N. lata, N. cf. N. lata,

and “P.” variolata. In all regards, except for the distinct adductorial sulcus in “P.” variolata (Pl. 2,

figs. 5,7), these forms are similar. Furthermore, some specimens of N. lata and N. cf. N. lata (Pl. 2,

figs. 6, 8) have an incipient adductorial sulcus.

With regard to those characters discussed above which are considered important diagnostic

characters for the platycope ostracodes, the following can be stated for Neckajatia and the related

genus :

PLATE2-Figs. 1, 2, 4, 6, 8, 10. Neckjaria cf. N. lata (Neckaja), 1, 8. Interior ( x 66) and exterior lateral ( x 77)

views of adult right valve (ASU X-80). 2. Interior ( x 72) view of elongate adult right valve (ASU X-83). 4,6.

Interior ( x 77) and exterior lateral ( x 69) views of adult left valve (ASU X-79). 10. Interior ( X 72) view of adult

right valve (ASU X-82). All specimens from the Silurian Mulde Beds at Dapps, Gotland (RFL 1970). Figs. 3,

5,7, 9. “Prirnifia” variolata Jones and Holl. 3, 5. Interior ( x 66) and exterior lateral (x74) views of adult left

valve (ASU X-84). 7, 9. Exterior lateral ( x 73) and interior ( x 82) views of adult right valve (ASU X-85).

Both specimens from the Silurian Much Wenlock Ls. Fm. at Lincoln Hill (RFL #13), Welsh Borderland.



1056 R.F.LUNDIN



1) The right valve is larger than the left and overlaps it along all of the free margin. This is

clearly indicated for N. modestu (Pl. 1,figs. 1,2,9), for N. latu (Pl. 1 ,figs. 4,7), N. cf. N . lutu, (PI. 2,

figs, 1, 2, lo), and for “Primitia” vuriolutu (Pl. 1, fig. 8; P1. 2, fig. 9).

2) The contact structures of the free margin consist of a contact groove in the right valve into

into which the edge of the left valve fits. This is illustrated for N. modesta (Pl. 1, figs. 5, 6), N. cf

N. lutu (Pl. 2, figs. 1, 2, 4) and “P” vuriolutu (PI. 2, figs. 3, 9).

3) The hinge is generally not well-preserved. The shell of N . modestu is relatively thin and,

accordingly, fragile. Therefore, the hinge (and free edge) is commonly abraded and details of the

hingement which may have been present are not observable. Nevertheless, it is a long straight hinge

(Pl. 1, fig. 6). The nature of the hinge is rather well-shown for N. lutu (Pl. 1 , figs. 10, 11) and “P.”

vuriolata (PI. 2, figs. 3, 9). The hinge is straight, relatively long, and consists of an edge (left valve)

and groove (right valve) which is a continuation of the contact groove along the free margin (PI.

2, figs. 1, 2).

4) The species studied here show no evidence of domatial dimorphism, Although Neckaja

(1958) observed two forms with different height/length ratios among her specimens of N. lutu

for which she suggested sexual dimorphism as a possible explanation, no specimens of any species

of Neckujutiu or the related genus have a limen. N. cf. N. lutu shows the same variation in the

development of the posterodorsal portion of the carapace as Neckaja observed for N. lutu. Neither

elongate (Pl. 2, fig. 2) nor foreshortened (Pl. 2, fig. 1) specimens have a limen. Furthermore, I

have seen no evidence of bimodality in the shape of adults of N . cf. N. lutu. If the variation is sexsignificant, the expression of it in the shell is subtle or absent and dimorphism is of the Cyprideistype (Jaanusson, 1985, p. 80).



DISCUSSION

Similarities.-The species studied here are like typical platycope ostracodes in the following

respects :

1) Contact margin structures. The contact groove in the right valve of N. modestu (Pl. 1, fig. 6)

is reduced along the anterior margin just as indicated for some modern species of Cytherella

(Jaanusson, 1985, p. 74). In all other regards, the contact margin structures are virtually identical

to those of typical platycopes.

2) Valve relationships. Neckujatia and “Primitia” vuriolata exhibit right-over-left overlap, just

as is the case with all Paleozoic platycopes (I include here only those forms which have domatial

dimorphism and exclude forms which exhibit Cyprideis-type, or kloedenellid, dimorphism).

Numerous authors (e.g., Sohn, 1983) have reported reversal of overlap in species of Sunsubella

Roundy, 1926 and, therefore, discount the significance of the direction of overlap. Sunsubella is

not a platycope ostracod and therefore is not significant to the issue. Until evidence is presented

for common variation in direction of overlap among ostracods having platycope-like contact

margin structures and domatial dimorphism, I will consider the right-over-left overlap, in combination with the other characters discussed here, to be taxonomically important for the platycopes.

I conclude that direction of overlap is strongly controlled by genetics and that it is a very stable

character for the platycopes even though it may not be a stable character in some other groups.

3) The hinge. Although there are differences between the hinges of typical platycopes and the

hinges of the species studied here (see below), the basic structure of the hinges is the same. Both

groups have an edge (left valve) and groove (right valve) arrangement of the hinge and the hinge

groove is connected to the contact margin groove, at least at its posterior end (Pl. 2, figs. 1, 2).

Furthermore, some specimens of N. cf. N. lata and “P.” variolata show evidence of a reduction in



Is Neckajatia an Ancestor of the Platycope Ostracodes? 1057



in development of the inner hinge (contact) ridge near the anterior end of the hinge (PI. 1, fig. 1 1 ;

P1.2, fig. 9). Although the evidence isn’t overwhelming, this condition seems to be like that reported

by Jaanusson (1985, p. 77, fig, 1) for Cytherella abyssorum and by Adamczak (1966, figs. 1, 2) ,for

Nyhamnella musculimonstrans and Gotlandella martinssoni.,

4 ) Straguloid processes. N. modesta (PI. 1, figs. 3, 9) and N. lata (PI. 1, fig. 4) have anterior and

posterior straguloid processes the former of which is less-well developed than that on Nyhamnella

musculimonstrans Adamczak, 1966, the earliest known true platycope. Nevertheless, the morphology of the anterior straguloid process of Neckajatia modesta and N. lata, except for size, is

virtually identical to that of Nyhamnella musculimonstrans. Furthermore, on the species studied

here in which the contact groove is well-developed (Neckajatiacf.lata and “P. variolata), straguloid

processes are absent. In short, although I place no great taxonomic value on the presence of

straguloid processes, in those species of Neckajatia which have them, they are similar to those of

the earliest platycopes.

Diferences.-The primary differences between the species studied here and true platycope

ostracods involves the following characters which are considered significant to the definition of

the Platycopa:

1) The hinge. Similarities in the hinges of Neckajatia, “P.”variolata and the platycopes have

been pointed out above. The primary difference is that in Neckajatia and “P.”variolata the hinge

is relatively long (compared to the length of the carapace), straight and parallel to the longitudinal

axis of the valve. In the early true platycopes, the entire hinge structure (Adamczak, 1966) is arched,

but the hinge proper is straight, short and may be inclined to the longitudinal axis of the valve.

The orientation of the hinge relative to the longitudinal axis of the valve apparently is not very

important because it is quite different in males and females of the same species (see Adamczak,

1966, P1. 1, figs. 1, 2; P1. 3, figs. 1 , 2). In short, the hinge structure of Nyhamnella, Gotlandella and

many other Silurian platycopes (see Sarv, 1977) is differentiated into a short, straight posterior

part (the hinge proper) and an anterior pert, the two of which together form a curved (arched)

line along which the valves articulate. No such differentiation can be seen in Neckajatia and “P.”

variolata.

2) Dimorphism. As indicated above, there is no evidence in either Neckajatia or “P.” variolata

for domatial dimorphism, a characteristic which is diagnostic for platycopes. I have seen no

evidence of a limen in any specimens of either Neckajatia or “P.” variolata. If these forms are

dimorphic, the dimorphism is very subtle and could be of the Cyprideis-type only.





CONCLUSIONS

Because of the similarities of Neckajatia and the true platycope ostracodes, I conclude that the

former is an ancestor of the latter. Neckajatia modesta is more platycope-like in general appearance

than N. lata, N. cf. N. lata or “P.” variolata. Therefore, of those species of Neckajatia presently

known, N. modesta is the most likely immediate ancestor of the platycopes. Considering the

present state of knowledge, I propose a phylogeny like the one shown in Text-fig. 1.

Text-figure 1 implies that the earliest true platycope (Nyhamnella musculimonstrans) was derived

from Neckajatia modesta. The changes which took place, according to this hypothesis, are as

follows :

1) The hinge proper (the straight portion) was shortened (relative to valve length) through

posteriorward extension of the anterior portion of the contact margin structures. In this case, there

was also an associated elaboration and strengthening of the straguloid processes, especially the

anterior one.



1058 R.F. LUNDIN



IRD.



SILURIAN

LLANDOVERIAN



1



System



W ENLOCKIAN



Series



Nyhamnella and the Platycopa



*

-I

!3



X



0



3



0



3

5



Neckajatia lata 8 N. cf. N. lata



3

is



(0



z!



Y



TEXT-FIG.

1-Proposed ancestor-descendant relationships among the Leperditellacea, Neckajatia, “Primitia” varioIata, Nyhamnella and other platycope ostracodes. The “Nyhamnellaand the Platycopa” branch is generalized.

Phyletic transition is not intended for all Silurian platycopes. Jones and Holl(I865) described “P.” variolata

from the Woolhope Limestone near Malvern, England. This occurrence is stratigraphically lower than for

any of my collections, and needs to be checked. The dashed line and question mark indicates the uncertainty

of the lower limits of the range of this species. N. cf. N. lata is presently known from only one locality in

the Mulde Beds of Gotland. Accordingly, the limits of the range of this form are not firmly established, but

it is clear that N. Iata ranges lower than N. cf. N. lata.



2) Introduction of domatial dimorphism through the development, in the female, of a posterior

chamber for egg care, bounded anteriorly by a limen.

Present evidence indicates that these changes occurred in late Llandoverian time. Jaanusson

(1985) has emphasized that once the changes occurred, the combination of characters became

stable and only minor changes have occurred since the Silurian.

Finally, it is interesting, under this hypothesis, to speculate about the Ordovician ancestors

of Neckujutiu. The morphology of N. modestu suggests that it was derived from a morphologically

rather generalized group of ostracods with a relatively long, straight hinge. The Leperditellacea

qualify in this regard. Indeed the Neckujutiu species studied here, as well as “P.” vuriolatu, are

reminiscent of the Ordovician Millerutiu and Schmidtellu (Adamczak, pers. commun., 1985). On

the other hand, Schallreuter (pers. commun., 1985) has suggested that Kurinututiu Schallreuter,

1978, a monotiopleurid, could be ancestral to Neckujutiu. Although the hinge of Kurinututiu is

long and straight and similar to that of Neckujutiu, Kurinututiu seems to have left-overright

overlap (not entirely certain from Schallreuter’s 1978 material) and is distinctly dimorphic.

I favor an interpretation which derives Neckujutiu and the platycope ostracodes from a morphologically more generalized group than is represented by the monotiopleurids.



ACKNOWLEDGEMENTS

I express my gratitute to the following people who have provided materials which have been

important to the completion of this study: A. F. Abushik, L. Sarv, David J. Siveter; L. E. Petersen

and the late A. Martinsson. In addition, I have had several stimulating discussions about platycope ostracods with Valdar Jaanusson, who understands this group of ostracodes better than anyone



Zs Neckajatia an Ancestor of the Platycope Ostracodes? 1059



I know. For materials he has provided and the encouragement he has given me to pursue this

problem, I express my sincerest thanks. This study is part of a project supported by the National

Science Foundation (Grant No, EAR-8200816).



REFERENCES

1966. On kloedeneuds and cytherellids (Ostracoda Platycopa) from the Silurian of Gotland. Stockholm Contrib. Geol., 15(2), 1-21.

- 1968. Palaeocopa and Platycopa (Ostracoda) from the middle Devonian rocks in the Holy Cross Mountains,

Poland. Ibid., 17, 1-109.

GRAMM, M.N. 1985. The muscle scar in cavellinids and its importance for the phylogeny of platycope ostracodes.

Lethaia, 18, 39-52.

GUBER, A.L. and JAANUSSON, v. 1964. Ordovician ostracodes with posterior domiciliar demorphism. Publ. Palueont.

Inst. Univ. Uppsala, 53, 1-43.

JAANUSSON, VALDAR, 1985. Functional morphology of the shell in platycope ostracodes-a study of arrested evolution. Lethaia, 18, 73-84.

JONES, T.R. and HOLL, H.B. 1865. Notes on the Palaeozoic bivalved Entomostraca; No. VI-Some Silurian species

(Primitia). Ann. Mag. Nut. Hist.. Lond., ser. 3, 16, 414-425.

NECKAJA, A.I. 1958, New species and genera of Ordovician and Silurian ostracodes from the northwestern Russian

Platform. VNZGRI, Trudy, n.s., vypusk 115, sbornik 9, 349-372. [In Russian].

WKORNY, v. 1957. The phylomorphogeny of the hinge in Podocopida (Ostracoda, Crustacea) and its bearing on

taxonomy. Acta Univ. Carolinae. Geol.. 1957, 1-22.

PRANSKEVICHIUS, A. 1972. South Baltic Silurian Ostracoda. Lithuanian Scientific-Research Geol. Surv. Instit., Trans.

15, 1-280. [In Russian].

SARV, L. 1977. Early cavellinids in the Silurian of the East Baltic. In KALJO, D. (ed.). Facies and Fauna of the Baltic

Silurian. Inst. Geol. Acad. Sci. Estonian SSR, Tallinn, 259-283. [In Russian with English summary].

SCHALLREUTER, R. 1968. Ordovizische Ostracoden mit geradem Schlossrand und konkavem Ventralrand. Wissensch.

Zeitsch. Ernst Moritz-Arndt-Univ. Hreifswald, 17, 127-1 52.

- 1974. New names for ostracode genera. Geol. Foren. i Stockholm Forh., 96, 278.

- 1975. Aniluciter nom. nov. for Reticulina Neckaja 1973 and remarks on the genera Beecherellita and Altha

(Ostracoda). Ibid., 97, 387.

- 1978. On Karinutatia crux Schallreuter gen. et Sp. nov. Stereo-Atlas of Ostrucod Shells, 5(6), 45-48.

SOHN, I.G. 1983. Ostracodes of the “Winifrede Limestone” (Middle Pennsylvanian) in the region of the proposed

Pennsylvanian System stratotype, West Virginia. Bull. Amer. Paleont., 84 (316). 1-53.

ADAMCZAK, F.



DISCUSSION

Sohn: Are the pits on Neckajatia lata artifacts or real? If real, they suggest that Nakajatia may

not be related to Carboniferous to Recent Platycopida.

Reversal of overlap is common in upper Paleozoic and younger ostracodes in several stocks

(Sansabella).

Lundin: The pits on Neckajatia lata are real. I would not place any major taxonomic value on

that characteristic because pits are absent from N. modesta.

My experience with other groups of ostracodes indicates that direction of overlap is strongly

controlled by genetics, It is not common for reversal of overlap to occur and where it does it seems

to be a genetic accident within a species. If you know of platycopes in which direction of overlap

seems to be commonly variable then we must change our diagnosis of the group. In that case, domatial dimorphism, the contact margin features and the hinge are the only characters which distinguish this group. For additional comment see the section of text entitled “Discussion”.

Neale: I can add something to this question of valve overlap in the Platycopa with respect to

the higher deposits in the Mesozoic. Here valve reversal has been shown to occur on occasion

in the Cretaceous of the Middle East. I would certainly not regard valve reversal in this instance



1060 R. F. LWNDIN



as being of more than specific significance. Anderson recorded a similar concluson on the significance of valve reversal in the genus Cypridea although these are, of course, members of a very

different group.

Lundin: It seems clear that the entire topic of reversal of overlap needs additional study. It

is equally clear that the vast majority of kloedenellaceans (forms with Cyprideis-type dimorphism)

have left-over-right overlap and the vast majority of Paleozoic cytherellaceans (forms with domatial dimorphism) have rightzover-left overlap. For the present I must consider this to be a significant

characteristic for higher level taxonomy.

Adamczak: The genus Neckajatia, it seems to me, has some relation with Schmidtella and

Milleratia in which (in the Ordovician) the left or the right valve is the longer and the interior of

the valves very much resembles the Neckajatia, which is a Leperditellid ostracod.

Lundin: For my reply see the section of text entitled “Conclusions”.

Schallreuter : Which taxon is the ancestor of Neckajatia?

Lundin: 1 do not know the answer to that question, but judging from comments just made by

Dr. Adamczak, certain leperditellacean genera may qualify. The other obvious option would be

to look toward the monotiopleurids for an ancestor. For additional comment see the section of

text entitled “Conclusions”.



XI



Biostratigraphy and

Exploration



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