Tải bản đầy đủ - 0trang
Chapter 23. Evolution of Amphicostella from the Middle Devonian in Europe (Metacopa, Ostracoda)
276 F.J. ADAMCZAK
analogous strategies in reduction of meshes and distribute the rest of them in long and regular
rows. Although it is assumed that the decrease in number of meshes has been without any prearrangement in the different taxa, the site where this process took place was not coincidental. The
change in number of meshes within a species occurs mainly in the posterior sector of the carapace.
It is also responsible for the most important variation, which may be as high as 80%. Another
aspect of this phenomenon seems to be related to the enlargement of meshes. Judging from the
areal size of the reticulum, it seems that natural selection favoured individuals with large reticular
elements. The arrangement of meshes in regular rows could have affected (1) the calcium carbonate
economy of the animal and (2) reinforcement of the carapace wall.
STRATIGRAPHIC OCCURRENCE OF
Amphicostella IN THE
The representatives of Amphicostella appear in calcareous clay rocks together with trilobites,
brachiopods, small rugose corals and bryozoans. They are not numerous and make up less than
one per cent of all ostracode forms in a sample. They have been found in three localities. At the
first locality (Wydryszow), besides the type species, there are two other forms, designated as
Amphicostella sp. 1 and Amphicostella sp. 2. At the second locality, there occurs Amphicostella
sp. 3, which seems to be quite closely related to A. prima Adamczak. Amphicostella sp. 3 has been
identified in only one sample (Text-fig. 1).
Although the four species appear suddenly in the Middle Devonian, it cannot be determined
whether they originated in this region or migrated to it during the latter part of the Early Devonian
transgression. The species mentioned above are presumably the earliest Amphicostella known
from Europe. The fifth species, A. sculpturata (Pokorn?), is represented in the Holy Cross Mountains
by one specimen, which occurs in the lower Givetian.
HOLY CROSS MOUNTAINS
H. ANTRI- P. RETICULATA
P. ABNORMIS ( X I
A. MAGNA ( 0 )
GRZEGORZOWICE ( X I
TEXT-no.I-Stratigraphicand sampling localities of the Middle Devonian in the Holy Cross Mountains where
Amphicostella has been found.
AmphicosteIIa from MiddEe Devonian in Europe 277
The diagrams of Amphicostella forms shown in Text-figs. 2 and 3 indicate the presence of
several sectors (outer, anterior, adductorial, posterior and caudal) which are parts of the ornamental pattern (meshes, carinae and the adductor muscle rosette) of the carapace and which have
been examined in left aspect. From the sectors mentioned, anterior, adductorial and posterior have
also been studied by pattern analysis (Pokorn9, 1969; Liebau, 1969; Benson, 1972). The remaining
sectors, because of the preservational state of the fossil material and the complexity of the valve
ornamentation, have not been studied by this method.
The meshes of the analysed sectors have been coded in Arabic and Roman numerals and
small and capital letters which are written below the reticular silhouette drawings of the different
species in a characteristicrecord (Text-figs. 3,5,7,8,9 and 10). The records simplify the analysis of
the reticulum and allow a particular pattern of meshes to be easily compared with the record of
another individual of the same or different taxon. In other words, they are homologized.
It seems logical to begin the analysis of the various ornamental details with the more stable sculptural elements of the carapace, i.e. the rosette of the adductors and the major carinal (ridge) system.
The rosette. The rosette of the adductors is situated in the central part of the carapace. It consists of six “petals” which are presumably impressions of the adductors (Text-figs. 2,4 and 6). Although the dimensions of the rosette are above the average, this does not mean that the adductors
were abnormally large. The rosette seems to be a very stable feature in Amphicostella. It has greatly
aided in the identificationof most individuals of this genus and is suggested to be an important
Amphicosteila sp. 3
A NTERl OR
2-Basic morphological features in Amphicostelfashowing the mura1 and carinae systems as seen in the
left valve ( X 140). Holy Cross Mountains, Grzegorzowice; Middle Devonian, Grzegorzowice Formation.
218 F.J. ADAMCZAK
The carinal system. The major ridge (carinal) system, considered as an enlargement of the muri
(Benson, 1977), consists of five carinae in the Eifelian forms (Text-fig. 2) and six ridges (carinae)
in the Givetian species. From the five ridges identifiable in the Eifelian forms, the ventral, outer and
adductorial ridges are less massive in Amphicostella sp. 1 and Amphicostella sp. 2 (Text-figs. 4, 6).
The remaining ridge elements, i.e. the posterior and anterior carinae seem to be much more stable.
Being massive and long, they connect the dorsal margin with the ventral carina. The sixth of the
major ridges is present in the posterior sector of A. sculpturata (Text-figs. 9,lO). It is long and stable
whereas its equivalents in the Eifelian forms are considered to be minor elements, which are less
firm and one or two in number. They appear in A. prima and Amphicostella. sp. 3.
The reticulum. Although the pattern of meshes of a particular species of Amphicostella is quite
distinct, the number of meshes, especially in the posterior sector, is considerably variable. A review
of the different parts (sectors) of the valve given below begins with the anterior sector, which may
be divided into dorsal and ventral sections. The dorsal part starts at the level of mesh I of the adductorial sector (Text-fig. 3). It may be biserial, coded: A], B1; Az, B2, etc., which means that the
meshes are arranged in two rows, as for example in A. prima (Text-fig. 8) or three rows (Al, B1, C1;
Amphicostella sp. 3
ADDUCTOR I A L
ANTE R IOR
c5 85 As
CI, 01, A 4
C3 83 A 3
C2 B2 A2
CI 81 A I
1 I f
13 12 1 1
23 22 21
33 32 31
1 3 42
C' 6' A'
Lh 3h 2h lh
4 39 29 19
L f 3f 2f I f
Le 3e 2e le
Ld 3d 2d Id
5c Lc 3c 2c 1c
5b L b 3 b 2 b Ib
5a La 3a 20 la
D C B A
B EXTRA MESH
TEXT-no.34chematic drawing of reticulation patterns traced from a SEM micrograph and record of meshes
( x 160). Holy Cross Mountah, Grzegorzowice; Middle Devonian, Grzegoaowice Formation.
Taxr-pro. QSEM micrographs. 1. Left lateral view of carapace( x 155). 2. Fragment of reticulationwith adductor
muscle rosette (to the left) and mural system. 3. Inside view of a mesh showing a surface of sieve-type pores(?)
Holy ,Cross Mountains, Wydryszow; Middle Devonian, Grzegorzowice Formation.
280 F.J. ADAMCZAK
Az, Bz,Cz; A3, etc.) as in Amphicostella sp. 2, Amphicostella sp. 3 and A . sculpturata (Text-figs. 3,
7 , 9). Often the triserial arrangement of meshes can dorsally pass into a biserial one, similar to
that visible in A. prima. In Amphicostella sp. 1 (Text-figs. 4 and 5), this part of the sector shows an
intermediate character indicating that it starts at the height of mesh I with three meshes (A1, B1,
C,) and passes into a biserial design (Az,Cz;A3,C3),and again demonstrates a triserial arrangement
with the reticular elements coded &, B,, C,; AS,etc. The ventral part of the anterior sector of the
Amphicostella sp. 1
5 i @ Li 3 i 2 i
L h 3 h 2h
Lg 3 9 2 9
5 f Ig 4 f 3 f 2 f
Le 3 e 2 e
Ld 3d 2d
Lc 3 c 2c
L b 3 b 2b
La 3 a 2a
D C B
IEl l e
~ - j DIMINISHED MESH
0 REDUCED MESH
5-Schematic drawing of reticulation patterns traced from a SEM micrograph and record of meshes
( x 185). Holy Cross Mountains, Wyoryszow; Middle Devonian, Grzegorzowice Formation.
micrographs of Amphicoe
tella as seen in left lateral view ( ~ 1 5 5 ) .
Holy Cross Mountains, Wydryszow; (Amphicostelfa sp. 2. A . prima), GRegorzQwi~
(Amphicostelfu sp. 3); Middle Devonian,
282 F.J. h m c z m
Eifelian forms is as a rule triserial(11, 12, 13; 21, 2,, &; 31, 3,, etc.), whereas in the Givetian species it
begins biserially and terminates with one mesh. In Amphicostella sp. 2 the lower part of the anterior
sector shows a Iarge and united mesh coded in 1 1, Z1, X* and the presence of another mesh of X
situated between the meshes, 41 and 4, (Text-fig. 7).
Although the adductorial sector may show a quite differentiated pattern of meshes, the number
of meshes remains quite stable within species. In the dorsal part of this sector, i.e. above the
adductor muscle rosette which has been coded with small letters (a, byc, d, e, f), there are 1-9 or
Amphicostella sp. 2
. 31. 2i l i
G3 84 A4
GI 8 3 A3
Ci 8 2 A2
Lh 3h 2h l h
5 9 m 9 39 29 1g
F ' E' D' C' 8' A'
F E D C B A
A t 3tmZf 11
ie 4e j e - i e le
M Ld 3d Zdmld
5c Lc 3c 2 c a c
605b L b 3b 2 M b
5aPoLa 3a 2a la
F E D C B AA,
4; 4; Po 4;
74khematic drawing of reticulation patterns traced from a SEM micrograph and record of meshes
( x 190). The same specimen is pictured in Text-@. 6.
Mesh X, as interpretedin the present paper, is an element which docs not fit very well in therecord in question.
Amphicostella from Midde Devonian in Europe 283
more meshes. They are biserially arranged with distinct horizontal muri and less massive vertical
elements. Occasionally, a mesh X may appear in this part of the valve (Amphicostella sp. 3, Textfig. 3). In front of the adductor muscle rosette there are two (I, 111) or three (I, 11, 111) meshes.
Mesh I1 is often smaller, or very small.
The ventral part of the adductorial sector is coded with capital letters from A, A', A" to F".
It has a maximum of seventeen (Amphicostella sp. 1, Text-fig. 5 ) and a minimum of twelve meshes
(A. sculpturata, Text-fig. 10). In the latter species, which comes from Czechoslovakia and shows
the right valve, reticular elements A', B, D", E' and E" have disappeared. The posterior sector demonstrates not only the largest diversity in the pattern of meshes, their number, dimensions
5h Lh 3h 2h lh
39 29 lg
B5 A 5
8 3 A3
0 2 A2
2s 2? 21
$ 3; 3;
8" A '
3f 21 If
C B A
EXTRA M E S H
8sChematic drawing of reticulation patterns traced from a SEM micrograph and record of meshes
( X 190). The same specimen is pictured in Text-fig. 6.
284 F.J. ADAMCZAK
and size among the different species of Amphicostella, but it also reveals an immense individual
variability. There are, at the base of this sector, just above the ventral carina, up to seven meshes
coded A,, A, ByC,D, E, F, as in Amphicostella sp. 2 (Text-fig. 7) or six meshes A, ByC, D, E, F, as in
Amphicostella sp. 1 (Text-fig. 5). In A . prima four meshes occur, coded: A, ByC, D, whereas Amphicostella sp. 3 and A . sculpturata (Text- figs. 9,lO) have five meshes (A, €3, C, D, E) in this part of the
valve. Besides the mentioned reticular elements, the majority of species of Amphicostella have five,
more or less vertically arranged, rows of meshes which are, beginning from behind, coded: la, lb,
lc, Id, le, lf, etc. to Say5b, 512, etc. In Amphicostella sp. 1 and Amphicostella sp. 2 there occur some
additional but incomplete rows of meshes coded as X-es. These are situated between rows 1 and 2,4
and 5, and in front of row 5. The mentioned species have as many as over ten X meshes present
in the posterior sector of the carapace (Text-figs. 5, 7). Furthermore, in A . prima the number of
X meshes is also important though not to such a degree as in the previous species. The largest
number of X meshes found in the form mentioned is five. Generally, A . prima, Amphicostella sp. 3
and A. sculpturata have only one X mesh, which may be situated any place in the posterior sector
of the valve, or none. Moreover, specimens provided with several X meshes frequently show that
Amphicostella sculpturata (Pokorn9)
I l e 2e13e
; I d 2d13d
IIc 2 ~ 1
I l b 2bI3b
[ l a 2a13a
Le 5 e
3 ~ Sc
A s 0"
A3 6 3 c3
0 B2 Cz
TEXT-FIG.9-Schematic drawing of reticulation patterns traced from a SEM micrograph and record of meshes
( x 190). Holy Cross Mountains, Skaly; Middle Devonian, Skaly Formation.
Amphicostellafrom Midde Devonian in Europe 285
rows, 2, 3 and 4 are slightly curved anteriorly (Text-figs. 5, 7) whereas forms lacking such meshes
are straighter (cf. Text-fig. 8).
Another aspect of the appearance of meshes in the posterior sector is the occurrence, particularly
in row 5 and between rows 5 and 4, of increased muri. Often, these acquire the shape and size of
a mesh and are interpreted here as vestiges to them (Text-fig. 8).
A further set of features, which are worthy of mention, concern the size of meshes. These disclose a wide spectrum of dimensions among species and important variability on an intra-species
level in space and time (Text-fig. 11). They are round to polygonal in shape. Their shape is also a
function of size. It seems that it is the size of meshes which has had a significant selective value, as
indicated in the diagram of Text-fig. 11. On the other hand, changes in mesh size also influence the
size of the reticulum, which has been analysed with help of a bild analyser. The reticulum becomes
smaller (Text-fig. 12). This probably resulted in a more economical management of calcium carbonate for the animal and, perhaps, some behavioral advantage in which the valve wall becomes
strengthened, i.e. more resistant to wave action.
Amphicostella sculpturata (Pokornjr)
C3 8 3 A3
C2 8 2 A2
C' 8' A '
L f 31
Le 3e ~e le
Ld 3d 2d Id
LC 3c 2c l c
L b 3b Zb l b
La 30 2 0
EXTRA M E S H
1O-Schematic drawing of reticulation patterns traced from a SEM micrograph and record of meshes
160). Czechoslovakia; Middle Devonian.
AMPHICOSTELLA SP. 1
A. PRIMA AMPHICOSTELLA SP. 3
of mesh indices (mesh diameterlvalve length X 100) in Amphicostelk.
AMPHICOSTELLA SP. 1
AMPHICOSTELLA SP. 3
12-Dimensions of reticulation area in % in adductorial and posterior sectors in Amphicostella.