Brachiopods from Eniwetok 

and Bikini Drill Holes 

By G. A. COOPER 

BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 260-FF 

Representatives of two genera 

described from Miocene to 

Recent sediments 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTO~ : 1964 

CONTENTS 

Abstract __________________________________________ _ Page 

1117 

1117 

1117 

1117 

1118 

Summary remarks _________________________________ _ Introduction ______________________________________ _ Reference _________________________________________ _ 

Systematic descriptions _____________________________ _ 

Argyrotheca _________ - _--------- _____ ----------- Thecidellina ___________ - ____________ ---- _______ _ 

ILLUSTRATIONS 

[Plate follows p. 1120] 

PLATE 301. Argyrotheca and Thecidellina. 

w 

678-878 0 - 64 - 5 

Page 

1119 

1120 
BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS 

BRACHIOPODS FROM ENIWETOK AND BIKINI DRILL HOLES 

By G. A. CooPER 1 

ABSTRACT 

Forty-five lots of brachiopods from deep drill holes on Eniwetok, Bildni, and Funafuti were submitted for study. In addition a survey wns made of brachiopod material from Bikini that 

wns received after publication of the "Recent Brachiopods" 

from Bildni (Cooper, 1954). A Recent specimen from Rongelap 

is introduced for comparison with those of Eniwetok and Bikini. 

Twenty-five lots were taken from Eniwetok, 19 from Bikini, 

nnd 1 from Funafuti. 

INTRODUCTION 

Two genera are represented, Argyrotheca and Thecidellina. Both of these were previously found living 

around Bikini Atoll (Cooper, 1954). Crania is another living brachiopod from Bikini, but it was not 

seen in any of the material from the drill holes. Unfortunately, the fossils are represented in most occurences by single specimens or lots too small to provide 

a clear idea of the species involved. It has thus proved 

impossible, except for a few, to identify the species 

represented, and the supply of specimens is inadequate 

for the establishment of new species. 

The specimens of Argyrotheca could not be identified with the living specimens. A few specimens of 

TheoidelUna are referred tentatively to a living species, 

but the remainder could not be identified as to species. 

SYSTEMATIC DESCRIPTIONS 

Argyrotheca sp. 

Plate 301, figures 1-11 

This genus is represented by 11 lots, all but 1 of 

them coming from Eni wetok. The range in depth from 

which the specimens come is from 290 to 1,110 feet. 

Specimens small, subquadrate in outline with the 

width slightly greater than the length, and the hinge 

slightly less than mid width. Sides gently rounded; 

anterior mn.rgin medially indented. Cardinal extremities rounded, obtuse. Valves unequal in depth, the 

1 U.S. NnUonnl Museum. This report Is publtshed by permlsslo,n of the 

Secrctnry of the Smithsonian Institution. 

brachial valve being the deeper. Anterior commissure 

rectimarginate. Surface with obscure irregular low 

radial plications. Shell substance coarsely punctate. 

Pedicle valve with lateral profile unevenly convex, 

the posterior part gently convex but the anterior part 

flattened. Anterior profile broadly but gently convex. 

Apical angle broadly obtuse, about 120° ; umbonal region moderately swollen; median region gently swollen 

but with a poorly defined sulcus extending from the 

anterior side of the umbonal regi.on to the anterior 

margin. Sulcus occupied by a low plication. Flanks 

bounding sulcus slightly convex and with low slopes. 

Flanks obscurely plica ted. 

Brachial valve with gently convex lateral profile but 

more convex than the pedicle valve and with the maximum convexity in the posterior third; anterior profile 

broadly and gently convex but more convex than the 

opposite valve. Umbonal region narrowly convex with 

1noderately steep slopes. Sulcus originating on the ·anterior side of the umbonal region, widening and deepening anteriorly to the margin. Sulcus moderately 

deep and bounded by moderately convex flanks. 

La:teral extremities slightly deflected. 

Figured speoimens: USNM 549695, 549696, 549701. 

Measurements, to millimeters 

USNM 

Length Brachial 

valve Htnge Mid valve Thickness 

549696 ________ 2. 3 2. 0 2. 1 2. 5 1.1 549701 ________ 1.9 1.5 1.6 2. 0 .9 

Stratigraphic occurrence and localities.-Eniwetok 

(F-1): Post-Miocene at levels 290-300 feet, 300-310 

feet, 320-330 feet, 330-340 feet, 450-460 feet, 500-510 

feet, 580-590 feet. Miocene (Tertiary f) at levels 1,050-

1,060 feet and Miocene (Tertiary e) at 1,100-1,110 feet. 

Eniwetok (E-1): Miocene (Tertiary g) at 740-750 

feet. Bikini drill hole (2-B): Miocene (Tertiary g) 

from 873-883112 feet. 

1117 
1118 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS 

001nparison.-The only specimens to which direct 

comparison can be made, from the Pacific realm where 

A.rgyrotheca is rare, are those from Bikini Atoll described by Cooper (1954) as A.rgyrotheca sp. These, 

like tht1! Eni wetok specimens, are very small and, as 

previously noted, may not be adults. The small size, 

coarse pw1ctae, and poor development of the loop in 

the fossil specimens favor the interpretation that they 

may be young specimens, but the fact that no larger 

specimens have been found, either fossil or Recent, suggests that these may really be adults. 

The fossil specimens from Eniwetok differ from the 

specimens living in the waters around Bikini. They are 

slightly larger and thicker than the Recent shells and 

have plications, even though obscure, on the pedicle 

valve. Furthermore, the brachial valve is more con vex 

but the length to width indexes of the fossil specimens 

are smaller. 

The single specimen from the Bikini drill hole is regarded as belonging to the fossil species rather than 

the Recent one. 

Thecidellina 

The commonest brachiopod from the drill holes is 

Thecidellina, of which 34lots were taken: 1 from Funafuti, 11 from Eniwetok, and 22 from Bikini. In addition to these a Recent specimen from Rongelap was 

studied. Most specimens are poorly preserved or damaged detached brachial valves. Thecidellina is a cemented form, and its shape is usually controlled by the 

surface to which it attaches. This fact, taken with the 

generally poor preservation of the specimens, makes 

.it difficult to identify them with known species, and it 

is also difficult to establish more than one species from 

such a numerically poor supply of specimens. It proved 

impossible to identify any of the specimens of Thecidelli1Ut with T. congregata Cooper, which is fairly common in the waters over the steep slope around Bikini. 

A species strongly suggesting Thecidellina maxilla, 

another Recent species, appears in the cores. This 

species, which is more robust and elongate than T. 

congregata Cooper, was identified at Bikini and some 

specimens from the drill hole seem to be referable to it. 

The foHowing remarks are based on the best preserved of the ·fossil Thecidellinas : 

Thecidellina from Eniwetok 

Thecidellina cf. T. maxilla (Hedley) 

Plrute 301, figuves ·15, 16 

Thecidellina cf. T. mamilla (Hedley) is represented 

by two specimens, a pedicle valve and a brachial valve. 

The pedicle valve (USNM 549699) is fairly deep and 

the shell substance thick. The hemispondylium is sessile and the teeth large. The posterior is fairly small, 

but the anterior is strongly elevated. This valve appears to be thicker and deeper than T. congregata. A 

small entire specimen is lodged in one side of a larger 

pedicle valve interior. Both valves have large interareas, but the brachial valve of this youthful specimen 

has a much larger intera.rea than that ofT. congregata 

of comparable size, or even larger. Measurements in 

millimeters, length 5.0, width 4.2. 

A brachial valve (USNM 549698), probably of the 

same species as the foregoing pedicle valve, is thick 

shelled, moderately convex on the exterior, and well 

preserved. The interior of this valve is quite unlike 

that of T. congregata. The median septum is more 

slender and does not flare anteriorly 'like that of the 

modern Bikini species. The cardinal process is stouter, 

the ridge in the cardinal process is thicker, and the 

posterolateral extremities are somewhat hooklike and 

naked. The deposits on each side of the median septum 

on which the lophophore filaments lie are massive and 

thick in the Eniwetok species and entirely unlike those 

of T. congregata. Measurements, in millimeters: length 

3.9, width 4.5. 

Figured specimens: USN'M 549698, 549699. 

The figured specimens described above are from the 

Post-Miocene pa·rt of the Eniwetok core between 50 and 

70 feet. Severa'! brachial interiors from 30 to 40 feet 

are aiso referable here. 

Thecidellina cf. T. maxilla (Hedley) from Rongelap 

P1a.te 301, fi'gures 18, 19 

A single brachial valve (USNM 549697) was dredged 

from :a depth of 130 feet ·at Rongelap (RAP 348). The 

specimen is 'poorly preserved and is much worn. The 

bridge is missing and the brachial lamellae are greatly 

worn, as are the granules of the outer border which is 

almost smooth. Measurements, in millimeters : length 

4.0, width 4.0. 

Figured specimen: U:SNtM 540097. 

Direct comparison with better preserved specimens is 

difficult. In lateral profile the specimen is ·fairly 

strongly convex in the posterior third and is thus unlike 

T. congregata Cooper. In this respect it is similar to 

Thecidelli1Ut cf. T. maxilla from Eniwe.tok (F -1, 30-40 

ft., 50-70 ft.), but is still more convex. The Rongelap 

specimen on the interior has a shorter but stouter median septum, a broader lateral and anterior margin, 

less thickened and less elaborate brachial lamellae, 

deeper sockets, and a thicker ridge in the concavity of 

the cardinal 1process than the Recent T. congregata 

Cooper. The Rongelap specimen is more elongate than 
BRACHIOPODS FROM ENIWETOK AND BIKINI 1119 

T. congregata, and it seems to be related to T. marJJilla 

rather than toT. congregata. 

Other Eniwetok species of Thecidellina 

Pl•a•te 801, figure 17 

Post Mincene.-E-1, 270-280 feet: A small complete 

specimen with valves sprung apart but not separated. 

This is ·a deep-shelled ·form with sessile hemispondylium in the pedicle valve and a strongly elevated 

septum in the brachial valve. Loop deposits are headed 

and quite unlike the specimen described above. This 

a·ppears to be ·a species different from the specimens 

higher in the wen at ·50-70 feet. 

E-1, 470-480 feet: One brachial valve too poor for 

determination or comparison. 

E-1, 630-640 feet: A small brachial valve, well preserved but incomplete ·and unlike those above. 

F -1, 45-55 feet: Incomplete brachial valve, elongate 

in form, possibly related toT. marJJilla Hedley, a Recent 

species from Funafuti and Bikini. 

F-1, 160-170 ·feet: A thick-shelled widely triangular pedicle valve (USNM 549702) with a thick cailosity 

81lmost obscuring the hemispondylium. It cannot be 

identified s·atisfactorily. Measurements, in millimeters: 

length 4.0, width 4.3. (See pl. '301, fig. 17.) 

Fi{fltrea specimen: USNM: 549702. 

F -1, 0-150 ·feet above 3,5'62 feet: This is an interior 

filling with ·all the shell dissolved :away. The specimen 

is difficult to interpret and impossible to identify. 

Miocene (g) .-E-1, 690-:-700 ·feet: Small incomplete 

brachial va'lve unlike any o.f the ·above. 

E-1, 720-7-30 ·feet: Small brachial valve, incom•plete 

but with septum similar to preceding specimen and 

probably the same. 

Miocene (f) .-E~1, 990-1,000 feet: Brachial valve 

different ·from an the preceding in being fairly strongly 

transverse. 

E-1, 2,120-2,130 feet: Sm·all brachial valve with a 

high-median septum and unlike any of the foregoing. 

Thecidellina from Bikini 

Theoidellina sp. A 

Plate 801, figures 12-14 

Thecidellina sp. A (USNM 549700) .-The shell is of 

medium size for the genus, elongate triangular in outline, with a moderately deep pedicle vaJve and fairly 

convex brachial valve. The maximum convexity is 

near midvalve. The cicatrix of 'attachment is variable, 

one specimen having a small area and another was attached by fully half the pedicle valve. Pedicle valve 

interior not known. Measurements, in millimeters: 

length 3.4, with 2.7, thickness 1.8. 

The brachial valve (USNM 549703a) interior has a 

thick median septum and narrow pustulose margin. 

The brachial lamellae are thickened and have thick 

nodes f.rom anterior to posterior. The bridge is thick. 

Measurements, in millimeters : length 3.5, width 4.0. 

Figured specimens: UlSiNIM 549700, 549703a. 

This species is uhlike T. congregata Cooper in its 

elongate outline and details of the brachial valve. Externally the brachial valve is more convex than that of 

the Recent genus. Inside, the median septum is 

stouter, the granulose margin narrower, and the brachial lamellae are more uniformly and more strongly 

thickened. 

The brachial valves are different from that from 

Rongelap in having a more transverse outline and 

stronger granulose ·margin. Thecidellina cf. T. marJJilla 

from the Eni wetok well is a wider and differentiy 

shaped species and its brachial valve has a more slender 

median septum and much more strongly :and massively 

thickened brachial lamellae. 

Age: Miocene (g). 

Locality: Bikini (B-2) '8Jt 715lh-786lh 1ft. 

Uniden tiftable Thecidellina 

Single and damaged specimens of Thecidellina from 

Bikini that could not be identified are as follows: 

Bikini 1: Post-Miocene at 253-258 feet. 

Bikini (2-A) : Post-l\fiocene at 3681;2--.3731;2 feet and 

6571;2-663 feet; Miocene (Tertiary g) at 925-9351;2 feet. 

Bikini (2-B) : Post-Miocene at 4421;2-453 feet and 

6941;2-705 feet. Miocene (Tertiary g) at 7361;2-7 4 7 

feet, 7781;2-789 feet, 8201;2-831 feet, 831-8411;2 feet, 

8831;2-894 feet, and 9251;2-936 feet. Miocene (Tertiary 

e) at 1,167-1,1771;2 feet and 1,461-1,4711;2 feet. 

Theoidellina from Funatu ti 

Thecidellina fro·m Funafuti in a drill hole at 70 

feet.-A small brachial interior, badly damaged and 

not identifiable. 

SUMMARY REMARKS 

The living species of Argyrotheca was not identified 

in the drill holes at Eniwetok and Bikini. In the living 

fauna this genus was found ~;mly on the outer slope of 

the atoll. Thecidellina was found on the slope and in 

the lagoon as well. Two species were identified at 

Bikini and both of them have a considerable range of 
1120 BIKINI AND NEARBY ATOLLS, MARSHALL ISLANDS 

depth. Thecidellina maxilla ranged down to 450 feet 

on the outer slope and was found in the lagoon at 180-

200 feet. T hecidellina congregata was taken not only 

on the slope but from depths down to 800 feet. Inasmuch as T. maxilla occurs in the lagoon as well as on 

the sl-ope, it is impossible to be sure whether the spedmens from the drill holes come from the lagoonal· or 

some other facies. 

REFERENCE 

Cooper; G. A., 1954, Recent Brachiopods, in Bikini and nearby 

Atolls, Marshall Islands. Pt. 2, Oceanography (biologic) : 

U.S. Geol. Survey Prof. Paper 260-G, p. 315-318, pls. 80, 81. 
GEOLOGICAL SURVEY 

8 

10 

ARGYROTHECA 

17 

13 

18 

14 

THECIDELLINA 

PROFESSIONAL PAPER 260 PLATE 301 

PLATE 301 

FIGURE 1-11. Argyrotheca sp. (p. 1117). 

1-5. Respectively, dorsal, anterior, side, ven- tral, and posterior views of a small speci- men from 1,050 to 1,060 ft, Eniwetok F-1, 

X 6. USNM 549701. 

6-10. Respectively, anterior, side, posterior, 

dorsal, and ventral views of a large com- plete specimen from 290 to 300ft, Eniwetok 

F-1, X 6. USNM 549695. 

11. Interior of a brachial valve, same as preceding, showing large median septum and 

anterior ring of nodes, from 300 to 310 ft, 

Eniwetok, F-1, X 8. USNM 549696. 

12-14. Thecidellina sp. A (p. 1119). 

12. Interior of the brachial valve showing 

median septum, bridge, and cardtnal process, X 6, from 726 to 736).il ft, Bikini 2-B. 

USNM 549703a. 

13, 14. Side and dorsal views of a complete 

specimen showing interareas on both valves, 

X 8, fro!Il 715)-f to 726 ft, Bikini 2-B. USNM 549700. 

15, 16. Thecidellina cf. T. maxilla (Hedley) (p. 1118). 

15. Interior of a large pedicle valve with a smaller one lodged within, X 4, from 60 to 

70ft, Eniwetok E-1. USNM 549699. 

16. Interior of a brachial valve, possibly the same species as the above pedicle valve, 

showing septum and papillos anterior 

border, X 6, from 50 to 60 ft, Eniwetok 

E-1. USNM 549698. 

17. Thecidellina sp. from Eniwetok (p. 1119). 

17. Interior of a large pedicle valve showing 

thick shell and part of hemispondylium, 

X 6, from 160 to 170 ft, Eniwetok F-1. 

USNM 549702. 

18, 19. Thecidellina cf. T. maxilla (Hedley) (p. 1118). 

18, 19. Respectively, exterior, X 3, and interior, X 6, of the same specimen showing 

median septum and cardinal process but 

minus the bridge; dredged from depth of 

130 ft, RAP 348, Rongelap. USNM 

549697.