Tải bản đầy đủ - 0trang
Chapter 22. Carapace sculpture in Amphissites (Kirkbyacea: Ostracoda)
260 P.J. JONES
(1973, 1976, 1980, 1982) for Beyrichiacea, and Schallreuter (1983) for Ordovician palaeocopes.
The last paper is important because two different types of sieve pores are described; the Klimphorestype consisting of many irregularly arranged micro-pores, and the Miehlkella-type consisting of
one ring of 5 of 8 micro-pores. Among the Kirkbyacea, Langer (1973) reported 9 to 12 normal
pore canals (“radiaporenkanale”) in each valve of Shleesha pinguis (Ulrich & Bassler, 1906); these
are mesh-pores, as they occur one per solum.
Benson (1975), in a discussion of the evolution of structural types, has suggested that modern
ostracods of the box-frame structural “morphotype” (e.g.. Trachyhleberididae) were represented
in Palaeozoic seas by the kirkbyaceans. This stimulating analogy needs to be pursued to determine whether the sculptural pattern of Kirkbyacea can be analysed by a methodology similar to
that used by Liebau (1971, 1977) on homologous sculpture patterns in Trachyleberididae.
This paper is an attempt to find such a parallel scheme in the Kirkbyacea by analysing the
carapace sculpture of three species of the genus Amphissites, the nominate genus of the family
Amphissitidae Knight, 1928. It is a pilot study because it is based on four specimens only, and
therefore lacks the numerical rigour that Liebau (1971) applied to his studies of the Limburginatype ornament. However, the material is diverse in that it contains three species, one of which is
some 40 million years younger, and is now separated from the other two by a distance of some
15,000 km. Despite these differences, homologous features can be recognized.
Amphissites is marine benthic ostracod, which was widely distributed during Middle Devonian
to Permian times. Sohn (1983) suggested that it was probably a swimmer, and that the marginal
frills and flanges would have inhibited a burrowing habit. More than 100 species have been described.
The material studied here consists of four specimens that are referable to three species. Dimensions-length (L), height (H) and width (W) are given in microns.
Amphissites centronotus (Ulrich and Bassler, 1906)
BMR reference collections of ostracod assemblage slides contain many well preserved specimens
of this species extracted from samples collected by the late Dr. Irene Crespin from the Upper Pennsylvanian (Virgilian) Graham Formation, Texas. A left valve (CPC 16641; adult, L=800, H=
450) taken from one of these slides (labelled-“Graham Formation; Plummer Stn 619 on Salt
Creek, 1 mile W. of Graham, Texas, U.S.A.”) is figured for comparison with the Australian
species. The median node on this specimen, and on some not figured here, possesses a short carina
on the ventral side, which appears to be characteristic of Amphissites carinodus Cooper, 1957
(=A. carinatus Cooper, 1946; not 1941). In all other respects the Texan specimens resemble A .
centronotus, where they probably belong.
Amphissites aff. A . centronotus (Ulrich and Bassler, 1906)
Carapace (CPC 16647; adult; L=860, H=460, W=400) collected from the Lower Carboniferous (late Visean) Bonaparte Beds, Bonaparte No.1 Well, core 6, 1,846 feet (561.4m), Bonaparte
Basin, northwestern Australia. Both valves of this carapace were compared for intervalve
differences, in order to check the intraspecific constancy of position of ornamental details (Liebau
1975). In other words they were used like two conspecific specimens to increase the data base.
Amphissites sp. B.
Right valve (CPC 16643; adult; L=980, H=630, W=380) collected from the same sample
Carapace Scu&ture in Amphissites 261
detailed above. Both the external and internal surfaces of this well preserved right valve were
studied; unfortunately the specimen was damaged after it had been photographed.
Right valve (CPC 16645; A-1; L=930, H=580, W=280) collected from the Lower Carboniferous (late Visean) Bonaparte Beds, Bonaparte No.1 Well, core 8,2404 feet (732.7 m), Bonaparte
Basin, northwestern Australia. Despite some abrasion of the surface, the polygonal fossae pattern,
and at least one important pore cone can be distinguished in this specimen. This right valve was
compared with that of the adult for ontogenetic differences.
The two species from Australia will be formally described in a forthcoming publication. The
specimens illustrated in the present paper are deposited in the Commonwealth Palaeontological
Collection (prefix CPC) of the Bureau of Mineral Resources, Canberra.
Transparent overlays were drawn of SEM photographs (Pls. 1, 2) to determine the reticulation pattern and pore cone distribution of the four specimens to be examined. The overlays of
right valves were reversed in order to make valid comparison with left valves. The number of
options available in determining an ordered reticulation pattern in the adult carapace is limited by
the number of meshes that have developed during the ontogeny. Thus, by tracing the mesh pattern
in early instars, a “preferred” order of reticulation pattern may be detected in the adult stage. Even
with the present limited material, study of the changes from the (A-1) stage to the adult stage of
Amphissites sp. B proved to be rewarding. The (A-1) stage laid the foundation for the interpretation of the adult stage (Text-figs. 2a, b).
The lateral reticulation of the three species of Amphissites examined here is clearly ordered
(Text-figs. 1,2), and can be classified as macroreticulationin the sense of Liebau (1971,1977). Whether it is constant in the Amphissitidae to the same degree as the Limburgina-pattern is in modern
Trachyleberididae, can only be decided after examination of much more material.
The basic pattern consists of 13 areas (designated alphabetically M to 2)that lie between the
dorsal carina and the adventral structure (=outer marginal carina). It does not include the fossae
on the dorsum (e.g., see tilted carapace in Text-fig. lb). Apart from two mesh rows(M, N) that lie
parallel to the dorsal carina between the anterodorsal and posterodorsal ribs, the remaining 11
rows (P to Z; 0 is not used) are arranged in a U-shaped concentric pattern around the median
node, which is microreticulate.
The macroreticulation situated between the outer and the inner marginal carinae consists of
4 mesh rows (W to Z). Z-meshes are closer to the outer marginal carina. Single meshes of the W to
Z groups are difficult to distinguish near the anterior and posterior ends of valves, and therefore are
not individually numbered. In some species of Amphissites (e.g., A. carinatus Cooper, 1941) the W
to Z groups cannot be seen in lateral view, because the inner marginal carina is declined ventrally
(i.e., in cross-section; Henningsmoen, 1965, p. 337, fig. 5B) and obscures the outer marginal carina.
Individual carapaces in which this adventral structure is destroyed by dorsally applied compression (e.g., Amphissites aff. centrronotus; Text-fig. lb, P1. 1, fig. 5), also hide the W to Z meshes in
Macroreticulation situated between the inner marginal carina and the dorsal carina can be
individualized by the mesh row and number. The V-mesh row, situated closest to the inner marginal carina is perhaps the most distinctive, even when this structure is damaged (e.g., P1.2, fig. l).
TEXT-FIG.2a, b.-Mesh rows of the lateral reticulation, and sites of pore cones in Amphissites sp. B; Lower
Carboniferous, Visean, Bonaparte Basin, Western Australia: (a) (A-1) right valve; CPC 16645; (b) Adult
right valve (CPC16643); both illustrationsare reversed for comparisons with left valves. The epidermal cells
in rows Q to T of the A 4 stage are divided in the presumed adult stage. Particular cells (Rl9, S12. and T22) of
the A-1 stage are responsible for the ventralgrowth of the posterodorsal rib in the presumed adult stage. Others
are responsible for shell growth in the anterior (e.g., R1 to 9; Ql to 9) and the posterior (416 to 18) parts
of the carapace.
264 P.J. JONES
TABLEN NUMBER OF FOSSAEPER
Amphissites centronotus (ULRICH
In this example, an A-1 instar of Amphissites. sp. B, there are 33 V-cells, as compared with the 31
in the adult specimen (Table 1). The V mesh-row probably played an important role in the construction of the inner marginal carina, which appears as a calcified extension along the outer sides
of muri and of the intramural pores (Pl. 2, fig. 4). The epidermal cells, which the V-fossae now
represent, probably contained the genetic code that controlled the optimum amount of calcite
necessary for the construction of the inner marginal carina. The intramural pores may have
functioned as tegmental ducts to facilitate this celation process.
Both the T and U meshes are aligned in continuous rows that are parallel to the V-row. The S
mesh-row is discontinuous in the area below the “kirkbyan pit”, where it is displaced by the R-mesh
row. There is a gradual reduction in the number of fossae per row from the V-group to the S-group
(Table 1). The sharp increase in the number of fossae in the R-row reflects the important role that
this mesh-row plays in cell-division from the (A-1) to the adult stage. This increase is already
apparent in the (A-1) stage of Amphissites sp. B, where the R-row develops from a single to a double
row of cells (following R20), immediately behind the posterodorsal rib (Text-fig. 2a, Table 1). Eight
R-meshes (Rl-8) arranged in a single row immediately behind the incipient anterodorsal rib in the
(A-1) stage are divided in the adult stage. A similiar development of cell division takes place in
the Q-mesh row (41-6, 16-18), where the net gain in fossae between the (A-1) and adult stages is
13 (as it is the R group). Cell division is not apparent within the P-meshes, but I have not studied
this group in detail.
Comparative studies of the reticulation patterns in the (A-1) and adult specimens of Amphissites sp.B show that the development of the posterodorsal rib is due to cell-division. The posterodorsal rib in the (A-1) specimen extends as far as the R19 cell (Text-fig. 2a). It is the division of this
cell, and of the S12 and T22 cells below it, that permits the posterodorsal rib to extend as far as the
U row in the adult specimen. Thus, cells R19, S12, and T22 of the (A-1) stage divide, and respectively become cells R29, 30, S14, 15 and T22, 23 in the adult stage(Text-fig. 2b). It would appear
then, that the posterodorsal rib is developed by the fusion of adjacent parts of the muri of twin
PLATEI-FiFs. 1-4,7. Amphissites centronotus (Ulrich and Bassler, 1906); Upper Pennsylvanian, Virgilian,
Graham, Texas, U.S.A. Adult left valve (CPC 16641); 1. Details of anterior end, showing pore cones B1,2 and
3, C3 and 4, and E3 (x240); 2. Lateral view. (x95); 3. Details of anterodorsal area of median node,
showing pore cones G1, and 2 ( x 240); 4. Details of posterodorsal area of median node, showing pore cones
E13, 14, F13, 14, G13, H11, 12, I11 ( ~ 2 4 0 ) ;7. Details of posteroventral area showing pore cones B10, 11
and 12 ( x 280). Figs. 5,6, 8. Amphissites alT. A . centronotus (Ulrich and Bassler, 1906); Lower Carboniferous,
Visean, Bonaparte Basin, Western Australia. Adult carapace (CPC 16647); 5. Left view ( X 60); 6. Ventral
view, with perforate conuli near the free margin (x60); 8. Details of anterodorsal area of median node,
showing pore cones H10, 12 and G13; cf. fig. 4 ( x 230).
266 P.J. JONES
Normal pore canals-both simple and sieve types-are present in the three species examined here.
The simple types occur as short pore cones, and the sieve types occupy the solae within fossae.
The distribution of pore cones (macrocones sensu Liebau, 1977) appears to be fairly constant in
all three species (Table 2; Text-figs. 3a, b; 4a,b).
OF PORE CONES IN Amphissites centronotus (Ulrich and Bassler, 19Oa AND ALLIED
AND THE 9 H-POINTS
SELECTED FOR THE CONSTRUCTION
AREAS SHADED ARE THOSE OCCUPIED BY THE MEDIAN
4a AND b.
A cmtronorus (left valve)
A sp. R (right valve; adult)
A aff c#nrronolus (left valve)
Asp. R (right valve; A-I stage)
A aff c~nlronolus(right valve)
Pore cones are best seen in Amphissites centronotus where 24 are confined to the macroreticulation between the dorsal and the inner marginal carina, and 14 (plus at least 10 intramural pores)
are situated between the inner and outer marginal carinae (Text-fig. 3a). There are no pore cones
on the microreticulate median node. The pore cones are distributed in concentric rows that approximately conform to the pattern made by the boundaries between mesh rows. These rows of pore
cones are designated here alphabetically A to I. A linear row of perforate conuli along the concave
2-Figs. 1-10. Amphissites sp. B; Lower Carboniferous, latevisean, Bonaparte Basin, Western Australia. 1,2,
Right valve, A-1 (CPC 16645); 1, lateral view, showing mesh rows (W-Z) between the inner and outer carinae
( x 65) ;2, detail of posterodorsalshoulder, showing one row of mesh between the H11 pore-cone and the posterolateral carina ( x 115); 3-10, right valve, adult (CPC 16643); 3, lateral view ( x 60); 4, detail of anterodorsal
shoulder, showing twin(G1) pore-cores( x 385); 5, detail of posterolateralcarina, separated from the H11 porecore by two rows of Q-mesh ( x 385); 6, detail of ventral part of inner marginal carina and V-mesh near the
porecone-note the carina is constructed from the muri, which contains intramural pores (tegmental ducts?)
( x 385); 7, detail of internal view showing anterior terminal tooth, sieve-poresand the internal openings of the
twin (Gl) porecones ( x 125); 8, detail of posterior part of inner marginal carina, showing some sieve pores
exposed in fossa V33 ( x 385); 9, detail of anteroventral part of outer marginal carina, showing a B-row pore
cones (BS) behind a broken part of the inner marginal carina ( x 385); 10, detail of fig. 7, showing sieve pore
pattern, and the internal openings of twin pore cones G1. ( x 385).
TEXT-FIQ3a, b.-Coordinate-systems for the identification of homologous pore cones in Amphissites: (a) A.
centronotus (Ulrich and Bassler, 1906) and (b) A. sp. B.
TEXT-FIQ.4a, b.-Polygon and triangle system fitted on nine common H-points for (a) Amphissites centronotus
(Ulrich and Bassler, 1906) and (b) Amphissites sp. B.
270 P.J. JONES
area between the free margin and the outer marginal carina is referred to as row A. It is not seen
in external lateral view, and is not considered further.
The first row of distinct pore cones lies immediately under the inner marginal carina; this is row
B. Row C pore cones conform to the junction between the V and U meshes. Row D pore cones conform to the U/T junction; and the positions of successive rows of pore cones (Rows E to I) are
shown in Text-fig. 3a.
The positions of individual pore cones that are along these rows can be located by a coordinate
system that consists of 14 curved lines that radiate from a point near the dorsal part of the median
node. These lines connect the 14 B-cones with the cones of other rows, by logically following the
constraints of the mesh pattern (Text-figs. 3a,b).
By this coordinate system, homologous pore cones can be identified by notations e.g., C4, E4,
H11 and H12, which are common to both Amphissites centronotus, and A. sp. B. Text-fig. 4 shows
all such homologous points, (H-points; Benson, 1982) on the carapaces of the three species of
Amphissites studied here. Consideration of 9 of the H-points in the Late Pennsylvanian A. centronotus and the Late Visean A . sp. B, and of a grid composed of lines between these points, permits
assessment of the changes in phyletic allometry in these species over some 40 million years. (Textfigs. 4a, b). The changes have been slight, and the present evidence shows that the distribution of
normal pore canals in these species of Amphissites is remarkably constant.
In both Amphissites centronotus and A. sp. B, the sola areas of many fossae are obscured by
detrital grains. However, some sola areas are sufficiently exposed to reveal sieve pores (Pi. 1, fig. 1;
P1.2, fig. 8). Examination of the internal surface of A . sp. B shows definite sieve pores, and the
internal openings of pore canals, corresponding to the position of the twin GI pore cone
(PI. 2 fig. 10). Between the muri, the outline of which can be seen on the internal surface, the sola
areas are perforated by sieve pores (1-2 pm diameter; about 20-25 puncta per solum).
1. A system that analyses the sculpture pattern of the genus Amphissites is proposed, in terms of
the arrangment of fossae and the distribution of pore cones.
2. The constancy of the basic plan of the reticulation pattern, and of the distribution of normal
pores is demonstrated for three species of this genus over a period of some 40 million years.
3. In Amphissites sp. B, the changes in the reticulation pattern between the A-1 stage and the
adult stage, demonstrates the results of cell division in the formation of the posterodorsal rib,
and the increase in size of the carapace.
I am indebted to Mr. J. Mifsud for drawing the illustrations (Text-figs. 1 to 4), Mr. A.T. Wilson
for the SEM photography and printing, and Miss Sally Jones for typing the manuscript. This paper is published with the permission of the Director, Bureau of Mineral Resources, Geology and
Geophysics, Canberra, A.C.T., Australia.