Tải bản đầy đủ - 0trang
Chapter 66. The pathways of morphological evolution of Bythocytheridae
952 E. I. SCHORNIKOV
TEXT-FIG.1-Development of species richness of the Bythocytheridae fauna through time.
A, from Silurian up to present; B, in Devonian.
The earliest reliably identified Bythocytheridae are known from the Middle Silurian
(Ludlovian) : Berounella (1 species), Kirkbjjellina ( 3 species) and, probably, the genus Scaphium
with two species (BouEek, 1939; Jordan, 1964; Copeland, 1968). No bythocytherid species were
found in later deposits until the upper part of the Lower Devonian (the Emsian).
The Emsian contains the richest bythocytherid fauna represented by almost all the Devonian
genera. Only two of the described genera Paraberounella and Praebythoceratina, appear in the late
Devonian. A majority of the Devonian bythocytherid fauna is concentrated in the Upper EmsianLower Eifelian. Even apart from the Obisafit species complex, they comprise almost half of the
Devonian bythocytherid fauna. In its turn, the Devonian bythocytherid fauna is the richest in comparison with the fauna of other Palaeozoic systems. Text-fig. 1 shows the changes in species richness
of Bythocytheridae over the course of time. The list of superspecific taxa includes reference to
High indices of diversity of bythocytherids obtained for later geological epochs, as compared
with the Cretaceous, result primarily from the fact that these ostracods are better preserved and
well studied. For example, approximately half of Recent bythocytherids are represented by
Pathways of Morphological Evolution of Bythocytheridae 953
Sclerochilini which occur mainly on rocky substrates near the shore and are very seldom preserved in fossil material.
Of course, the picture of the development of the group, based on the examination of lists of
taxa established so far is provisional and can, to a greater or less extent, be changed by any new discovery of a rich bythocytherid complex (like the Obisafit one). Nevertheless, it is clear that the evolution of the Bythocytheridae had two partricularly flourishing periods, one in the Devonian
and the other in the Cretaceous, with the probable acme of its entire evolution during the Upper
Emsian-Lower Eifelian. The evolution of the Bythocytheridae survived two crises: in the late
Palaeozoic and at the Cretaceous-Tertiary boundary; the Permian crisis was the most severe. It
hardly seems possible that several forms known from the Ludlovian and obscure until the Emsian
gave rise to the richest and highly diversified bythocytherid fauna of the late Emsian. Probably,
the process of formation of this fauna was more gradual and the Pre-Devonian history of this
group lasted longer so that the origin of this fauna should be traced far back, perhaps as early
as the Ordovician, when the radiation of the basic stocks of Podocopida took place.
The following system of classification is proposed for the Bythocytheridae. The list includes
only those undescribed taxa, which are discussed herein ; the specimens depicted are deposited
in the Leningrad Mining Institute (LMI), and the Institute of Marine Biology (IMB).
Bythocytheridae Sars, 1926, Si1.-Quat.
Berounellinae Sohn et Berdan, 1960, Sil.-Dev.
Berounellini Sohn et Berdan, 1960, Sil.
Berounella BouEek, 1936, Sil.
N. trib., Sil.?-Dev.
N. gen. 1, Dev.; n. gen. 2, Dev.; n. gen. 3, Dev.; n. gen. 4, Dev.
? Scaphium Jordan, 1964, Sil.
N. subfam., Si1.-Carb.
Kirkbyellinu Kummerow, 1939, SiLCarb.
N. gen., Dev.
? Ranicella Grundel et Kozur, 1971, Carb.
Bythocytherinae Sars, 1926, Dev.-Quat.
Monoceratinini Szczechura, 1964, Dev.-Perm.
Fueloepicythere Kozur, 1981, Perm.
Monocerutina Roth, 1928, Dev.-Perm.
Pseudomonoceratinu Grundel et Kozur, 1971, Dev.?-Carb.-Perm.
Triceratinu Upson, 1933, Dev.-Carb.
N. gen., Dev.
Bythocytherini Sars, 1926, Dev.?-Quat.
Bythocerutinu Hornibrook, 1952, Cret.-Quat.
Bythocythere Sars, 1966, Quat.
Bythocytheromorpha Mandelstam, 1958, Cret.
Crussacythere Griindel et Kozur, 1971, Cret.
Cuneoceratina Griindel et Kozur, 1971, Jur.?-Cret.
Cytherueison Hornibrook, 1952, Pa1eog.-Quat.
Dentibythere Schornikov, 1982, Quat.
Hunaiceratina McKenzie, 1974, Neog.-Quat .
Miracythere Hornibrook, 1952, Paleog.-Quat.
Nemoceratina Griindel et Kozur, 1971, Carb.?-Trias.
Neoberounellu Grundel et Kozur, 1972, Perm.?-Trias.
Nodobythere Schornikov, 1981, Quat.
Orientobythere Schornikov, 1981, Quat.
“Purabythocythere” Gou et Huang, 1982, Neog.-Quat. (non Kozur, 1981)
Puriceratina Griindel et Kozur, 1971, Cret.-Paleog. (= Cretaceratina Neale, 1975)
Putelucythere Griindel et Kozur, 1971, Trias.?-Pa1eog.-Quat.
Praebythoceratina Griindel et Kozur, 1971, Trias.-Jur.
Pseudoceratina, Bold, 1965, Neog.-Quat.
Retibythere Schornikov, 1981, Quat.
Rhombobythere Schornikov, 1982, Quat.
Tuberoceratinu Griindel et Kozur, 1972, Trias.-Cret.?
Veenicerutinu Griindel et Kozur, 1971, Cret.-Paleog.
Velibythere Schornikov, 1982, Quat.
? Puruberounellu Blumenstengel, 1965, Dev.-Perm.-Trias.?
Nagiellini Griindel et Kozur, 1971, Trias.
Nagiellu Kozur, 1970, Trias.
Protojonesiini Griindel et Kozur, 1971, Jur.-Cret.
Protojonesiu Deroo, 1966, Cret.
Suxellucythere Griindel et Kozur, 1971, Jur.
Tuxodiellu Kuznetsova, 1957, Cret.
Triebacytherini Griindel et Kozur, 1971, Perm.?-Trias.
Triebucythere Griindel et Kozur, 1971, Trias.
?Vulumocerutinu Kniipfer, 1967, Perm.
Vitjasiellini Schornikov, 1981, Pa1eog.-Quat.
Vitjusiella Schornikov, 1976, Paleog.?-Quat.
Bythocytherinae incertae sedis
Acvocuriu Gramm, 1975, Trias.
Covrucythere Gramm, 1975, Trias.
Rucvetinu Gramm, 1975, Trias.
Keijicytherinae Kozur, 1985, Dev.-Quat.
Striatobythoceratinini Kozur, 1985, Dev.-Perm.
Striutobythocerutinu Kozur, 1985, Dev.?-Carb.-Perm.
Toristu Kozur, 1985, Dev.
N. gen. 1, Dev.-Carb.; n. gen. 2, Dev.
N. trib. Dev.
Hercynocythere Blumenstengel, 1974, Dev.
N. gen., Dev.
Keijicytherini Kozur, 1985, Dev.-Trias.
Keijicythere Kozur, 1985, Perm.-Trias.?
Purubythocythere Kozur, 1981, Perm.
N. gen. 1, Dev.; n. gen. 2, Dev.;
Jonesiini Schornikov, 1981, Jur.-Quat.
Jonesiu Brady, 1866, Quat.
Kurilocythere Schornikov, 1981, Quat.
Neulocythere Schornikov, 1982, Quat.
Phlyctobythocythere Bonaduce, Masoli, Pugliese, 1976, Quat.
Plenocythere Swanson, 1979, Quat.
Ruggieriellu Colalongo et Pasini, 1980, Quat.
N. gen: 1, Quat; n. gen. 2, Quat.
Pseudocytherinae Schneider, 1960, Trias.?-Quat.
Pseudocytherini Schneider, 1960, Cret.-Quat.
Anturcficythere Neale, 1967, Quat.
Pseudocythere Sars, 1866, Cret.?-Quat.
Pteropseudocythere Schornikov, 1982, Quat.
Rostrocythere Schornikov, 1981, Quat.
?Triussocythere Grundel et Kozur, 1972, Carb.?-Trias.
Sclerochilini Schornikov, 1981, Cret.-Quat.
Convexochilus Schornikov, 1982, Quat.
Oviferochilus Schornikov, 1981, Quat.
Sclerochilus Sars, 1866, Cret.?-Quat.
Bythocytheridae? incertae sedis
Bytholoxoconchu Hartmann, 1974, Quat.
Gramm (pers. comm.) proposed that the Editiinae, a highly aberrant group previously referred
to the Bythocytheridae (Knupfar, 1967; Griindel and Kozur, 1971, 1973; Gramm, 1975), should be
Pathways of Morphological Evolution of Bythocytheridae 955
structure of Bythocytheridae, n. subfam. and Berounellinae.
A. Kirkbyelfina? sp., ventral view of valve; arrows indicate orifices of shell (Obisafit beds). B. Kirkbyellina
spinosu (Blumenstengel, 1962) (after Blumenstengel, 1977). C, D Berounellinae n. trib., n. gen. 1 , sp.
C. Dorsal view of valve. D. Inside view of valve (LMI 100/300, Obisafit beds). E. Berounellinae n. trib., n.
gen. 2, sp., inside view of valve (LMI 101/300,Obisafit beds). F. Berounellinae n. trib., n. gen. 3, sp., inside
view of valve (LMI 103/300, Obisafit beds). G. Berounellinae n. trib., n. gen 4, sp. 1, inside view of valve
(LMI 114/300, Obisafit beds). Designations: 1, anterodorsal border; 2, central dorsal (hinge) border; 3,
posterodorsal border; 4, internal elevation corresponding to S, ; 5, internal elevation corresponding to S,.
regarded as a separate family of the Cytheracea (Editiidae) with only two genera: Editia and
another new one established by Gramm. We agree with Gramm and place the remaining taxa
previously referred to the Editiinae in the Bythocytheridae.
As early as the Devonian, Bythocytheridae were represented by four of the five subfamilies
recognized. Each one seems to be evolutionarily more advanced from the hypothetical ancestral
group. The distribution of the basic morphological characters of these subfamilies is a mosaic and
suggests that the origin of these subfamilies was connected with radiation.
Berounellinae : The shells with very elongate anterodorsal margins, S1 and S , are usually conspicuous.
Pathways of Morphological Evolution of Bythocytheridae
Berounella and Kirkbyellina are not thought to be synonymous, but are regarded as belonging
to different subfamilies. Berounellinae n. trib. with 4 new genera from the Obisafit beds is assigned
to the subfamily Berounellinae (Text-fig. 2C-G, Plate 1, figs. 4-6). Scaphium may also belong to
this subfamily. The forms referred to this subfamily have very elongate, more or less horizontal
anterodorsal and posterodorsal margins, due to which they become similar to some Bairdiacea,
particularly to Acanthoscapha. However, this similarity is not evidence for the direct origin of this
group from Acanthoscapha-like ancestors. Acanthoscapha is one of a highly specialized group
adapted to life in semi-liquid mud. The elongate anterior and posterior ends of the shell function as
stabilizers similar to analogous formations in a number of pelagic Halocyprididae and swimming
Cyprididae (Hartmann, 1966). This similarity is obviously convergent.
Bythocytheridae n. subfam. have S, and S,. S , is V-shaped, narrow and deep in the region of the
adductor muscles and broad and shallow dorsally. This subfamily includes Kirkbyellina and a new
genus from the Obisafit beds, which is close to them and characterized by a strongly reduced caudal
process (Plate 1, figs. 2, 3). In external view Ranicella is similar to Kirkbyellina and may also belong to the new subfamily. However, the lack of information concerning the internal morphology
of the Ranicella shell makes this supposition questionable. Schallreuter (1968) and Griindel and
Kozur (1971, 1973) only considered the morphology of Kirkbyellina when they stated the similarity between Berounellinae and Tricornidae. At the same time, Kirkbyellina exhibits a number of
peculiar features that characterises it as a highly specialized form : a hypertrophied caudal process
and the presence of 3 gaping orifices in the closed shell (in addition to the orifice in the caudal
process). The orifices are located anterodorsally, anteroventrally and posteroventrally. Reynolds
(1978) observed only anterodorsal and posteroventral orifices. She assumed that they were filtrators and that these orifices served for circulation of water. However, in known Recent ostracods,
filtrators do not need these orifices. We believe that, very probably, they inhabited semi-liquid
sediments. Their anterodorsal and lateral spines and long caudal process would serve as stabilizers. The domicilial orifices allowed the extension of the antennulae, antennae and thoracopods.
This enabled the animals to move with their shells closed. It is possible that these orifices were
surrounded by dense setae (similar to the rostra1 incisure in Myodocopina). They prevented the
sediment particles from entering the domicilium. It seems impossible that the remaining bythocytherids could originate from Kirkbyellina-like forms. On the inside surface of the sulcus in Kirkbyellina, apodemes have been found to which the muscles were attached. The apodeme top, to
which the adductor was attached, is oblong vertically and has a small area to which a limited
number of muscle bundles (apparently 5) can be joined (Plate 1, fig. 1).
Bythocytherinae originally had only S, and usually a well pronounced lateral sculpture. The
adventral inflation is frequently present. In contrast to the ventrolateral inflation of the Keijicytherinae and Pseudocytherini, this inflation does not occupy the adductor area but lies below it. In
the Palaeozoic specimens the hinge was adont, in the Post-Palaeozoic it was lophodont or more
PLATE1-Fig. I . Kirkbyellina sp. Interior view of the right valve, arrow pointing to adductor apodeme, X 300 (Obisafit beds). Figs. 2, 3. Bythocytheridae n. subfam., n. gen., sp. 2. Exterior view of the right valve X 180. 3.
Caudal process, interior view, x 300 (LMI N 222/300, Obisafit beds). Fig. 4. Berounellinae n. trib., n. gen.
3, sp. Exterior view of the left valve, x 175 (LMI N. 103/300, Obisafit beds). Fig. 5. Berounellinae n. trib., n.
gen. 1 , sp. Exterior view of the left valve, x 210 (LMI N 100/300, Obisafit beds). Fig. 6. Berounellinae n. trib.,
n. gen. 4, sp. 2. Exterior view of the left valve, x 130 (LMI N 112/300, Obisafit beds). Fig. 7. Striatobythoceratinini n. gen. 1, sp. 1. Exterior view of the left valve, x 225 (LMI N 234/300, Obisafit neds). Fig. 8.
Striatobythoceratinini n. gen. 1 , sp. 2. Anterior part of the left valve, x 290 (LMI N 134/300, Obisafit beds).
Fig. 9. Striatobythoceratinini n. gen 2, sp. Exterior view of the right valve, x 250 (LMI N 139/300, Obisafit
beds): surface sculpture is false, formed as a result of diagenesis. Fig. 10. Stsiatobythoceratinini n. gen. 1, sp.
3. Exterior view of the left valve, x 155 (LMI N 235/300,Obisafit beds).
3-Muscle scars of Bythocytheridae and musculature forming them.
A, Jonesia simplex (Norman, 1865); B, Pseudocythere moneroni Schornikov, 1981 ; C, Sclerochilus (Sclerochi1us)firmulus Schornikov, 1981 ; D, Nodobythere nodosa Schornikov, 198 1 ; E, Retibythere bialata Schnornikov, 1981 ; F, Cytheralison sp. G-K, Rhombobythere mica Schornikov, 1981. G, normal adductor scar;
H-K, different variations of the adductor muscle scar. (A, after Schornikov, 1980; B-E, after Schornikov,
1981; F, after McKenzie, 1974; G-K, after Schornikov, 1982a).
Pathways of Morphological Evolution of Bythocytheridae 959
complex. The adont hinge may be formed secondarily as the result of reduction of some elements
(Rhombobythere). Seven tribes are recognised in this subfamily (see list).
Monoceratinini S, is V-shaped, weakly developed, occasionally reduced (Pseudomonoceratina).
In Keijicytherinae and Pseudocytherinae, there is a “diffuse” dorsal muscle field (Text-figs. 3A-C).
Apparently groups with originally V-shaped S,, including Monoceratinini, had the same muscle
field. Representatives of this tribe are reliably established only from the Palaeozoic deposits.
Other tribes of Bythocytherini originally had a furrowed S,, formed as vertical furrow. Muscle
scars in the middle of the dorsal muscle field are arranged in a vertical row (Text-figs. 3D, E). In the
case of reduction of the furrowed S,, the scars of the middle part of the dorsal muscle field ranged
into a vertical row indicate that earlier the S, was furrowed. This has been established for Cytheralison, Miracythere, Hanaiceratina, Taxodiella, Vitjasiella and some other genera.
Keijicytherinae are characterised by the presence of S, (occasionally reduced) and the absence of
S,. The dorsal muscle field is “diffuse” and does not form a vertical row of muscle scars (Text-fig.
3A). Here 3 Palaeozoic tribes with adont hinge are included: Striatobythoceratinini (Plate 1, figs.
7-10) having a lateral spine or ventrolateral inflation occupying the adductor, n. trib. (Plate 2, figs.
1-4) lacking the lateral spine and ventrolateral inflation; the genus Hercynocythere and a n. gen.
are included herein. The representatives of both tribes have a distinctly pronounced flange and
usually well expressed lateral sculpture. Keijicytherini (Plate 2, figs. 5-7) is characterised by a trend
towards reduction of the flange and lateral sculpture as a consequence of adaptation to life in
sediments. The Mezozoic-Cenozoic tribe Jonesiini is distinguished from the Palaeozoic tribes by
its lophodont hinge and characterised by a trend towards reduction of the flange, the absence of
lateral sculpture or the presence of secondarily formed lateral sculpture, such as longitudinal
striae, furrows, carinae and plicae. The shells of the representatives of Jonesiini and Keijicytherini
are homeomorphic. Both tribes exhibit a trend towards the formation of internal transverse ribs
(see Schornikov, 1981, p. 33). Jonesiini probably originated from the least specialised representatives
of Keijicytherini though it is possible that they originated from some other less specialized group
such as, for example, n. trib. In spite of considerable differences in their shell morphology, the
appendages of the Recent Jonesiini are strikingly similar to those of the Bythocytherini. This led
to their unification in one subfamily (Schornikov, 1981). It is now obvious that the Bythocytherinae
and Keijicytherinae were separate as early as the Devonian. The evolution of the Keijicytherinae
proceeded by the way of adaptation to life in sediments which is reflected in the morphology of
the shell. Both Devonian and Recent Bythocytherinae live on the sediment surface. The shells of
different groups of Bythocytherinae underwent considerable, differently directed transformations,
but in many features remained similar to those of archaic forms. The appendages of the ancient
Bythocytheridae and Keijicytherinae were strongly articulated and armed with a greater number of
setae. This is evident from a number of ancestral features mosaically distributed in different groups
of Bythocytheridae and other Cytheracea (Schornikov, 1981, 1982b). The similarity in the appendages of Bythocytherinae and Keijicytherinae is homeomorphic. In these animals, fusion of
some parts of the appendages and reduction of their elements proceeded in parallel. In their
morphology it was not possible to find any noticeable feature that could be regarded as new
compared to their ancestral state. Special transformations in the structure of appendages involved
only small details such as slight changes in the proportion, form and plumage of some elements.
Pseudocytherinae reliably established from the late Cretaceous onwards are characterised by
the absence of sulci or the presence of a slight vestige of S1 (in some Pseudocytherini). The dorsal
muscle field is “diffuse” (Text-figs. 3B, C). Organs connected with food intake are subject to strong
transformation, together with considerable reduction of ancestral elements. Muscles forming the
peripheral scars of the central muscle field are also reduced. Other appendages are also considerably
reduced as compared with the ancestral type, but have a number of archaic features as compared
Pathways of Morphological Evolution of Bythocytheridae
with the Jonesiini and Bythocytherinae. Pseudocytherini are characterized by destabilization of the
hinge structure and interposition of the free border of the right and left valves. In Sclerochilini, the
interposition of the valves significantly changed and resulted in formation of a two-listed hinge
with the right valve overlapping the free border (Schornikov, 1981). The lateral sculpture is either
absent or formed secondarily, such as several costae arming the borders (in some Pseudocytherini).
Pseudocytherini still have slight vestiges of the flange, which is absent in Sclerochilini. Very probably, the Pseudocytherinae originated in the Post-Palaeozoic from the Keijicytherinae group specialised for living in sediments. Their ancestors had reduced lateral sculpture and a strongly reduced
flange. The transformation of the mandible and maxillula and masticatory apparatus apparently
occurred with the development of symbiotic relationships with sessile benthonic organisms
(commensalism and probably, parasitism). In the Pseudocytherini and Sclerochilini, parallel
reduction of the anterior endites of the maxillulae occurred and the terminal setae of the first endite
and the palp transformed into hooks that serve for fixing the animal on food objects.
For the sake of brevity only some most interesting items of the morphological evolution of
Bythocytheridae are considered below.
Adductor Muscle Scar: In the present state of knowledge we can only guess at the early history
of formation of the adductor muscle scar in the Cytheracea. This scar originated, presumably,
from the archaic rounded and multi-element scar as a result of reduction of peripheral muscle
bundles through rudimentation, as was the case with the formation of the oblong vertical scar in
the Platycopa (Gramm, 1985). It is very probable that some scars fused, since in Recent Cytheracea
each stigma of the adductor is formed by several muscle bundles. There is some correlation between
the degree of vertical extension of the scar and the parallelity of the lateral sides of the shell where
the adductor is attached (Schornikov, 1982a). One of the prerequisities of the formation of the
vertically extended few-element adductor in Cytheracea is the presence of flattened-parallel areas
in the region of the adductor attachment. They may result from the lateral compression of the shell,
as in Cytherellidae, the formation of a deep S2, the formation of a transverse “internal elevation”
as the result of considerable thickening of the shell wall in the middle part as in Berounellinae n.
trib., n. gen. 1 (Text-fig. 3D), or the formation of apodemes with flattened-parallel tops as in
Kirkbyellina (Plate 1, fig. 1) and Vitjasiella beliaevi Schornikov, 1976 (Schornikov, 1976, 1981).
It is still not clear which of the pathways or probably, several pathways, were realized in the
course of evolution. In any case, our studies have shown that as early as the Devonian, the adductor muscle scar of the Bythocytheridae was vertically extended and consisted of few elements
(presumably 5). This construction of the adductor muscle scar is one lcrf its most stable states, which
has been maintained in the Bythocytheridae right up to the present day, in spite of considerable
transformations of the shell during the evolution of this group. Only the form and inclination of
the row has changed in connection with the spatial arrangement of the planes of the opposite sides
of the shell to which the adductor was attached. However, further evolution of the adductor muscle
scar in the Cytheracea resulted in one more stable state consisting of 4 elements, and further
2-Fig. 1. Hercynocythere sp. 1. Exterior view of the left valve, x 160 (LMI N 145/300,Obisafit beds). Fig. 2.
Hercynocythere sp. 3. Posterior part of the left valve, x 165 (LMI N 164/300, Obisafit beds). Fig. 3. Hercynocythere sp. 2. Exterior view of the left valve, x 150 (LMI N236/300, Obisafit beds). Fig. 4. Keijicytherinae n. trib.,
N 144/300, Obisafit beds). Fig. 5. Keijicytherini n.
n. gen., sp. Exterior view of the right valve, ~ 2 1 (LMI
gen. 1, sp. Exterior view of the left valve, x 110 (Obisafit beds). Fig. 6. “Monocerutina” sublimis Polenova,
1952. Male shell, view from the left, x 150 (Holotype). Fig. 7. Keijicytherini n. gen. 2, sp. Exterior view of
the right valve, x 105 (Obisafit beds). Fig. 8. Monoceratinini n. gen., sp. Posterior part of the left valve,
~ 3 2 0(Obisafit beds). Fig. 9. Jonesiini n. gen. 1, sp. Exterior view of the right valve, X 8 5 (IBM N 1796,
Australian waters of the Indian Ocean, Recent). Fig. 10. Jonesiini n. gen. 2, sp. Left valve of the A-1 Instar
stage, x 100 (IBM N 1777, Australian waters of the Indian Ocean, Recent). Fig. 11. “Pseudocythere” cf. P.
fueguensis Brady, 1880. External view of male left valve x 105 (IBM N 1776, Australian waters of the Indian
962 E. I. SCHORNIKOV
reducing to merely 2. The 4-element adductor muscle scar was formed iteratively. The study
of the Recent bythocytherids (Schornikov, 1982a) indicated that a decrease in the number of
elements in the adductor scar may be associated with the formation of a more or less spherical
shell (ix. the most robust, other conditions being equal). In Cytheralison and Rhombobythere
(Text-figs. 3F, G) and in the Terrestricytheracea, which possess strongly convex shells, the middle
adductor scars are triangular in shape and shifted horizontally with respect to each other. At
the same strength of musculature, vertical shortening of the adductor attachment zone thus became possible, which sets fewer limitations for the formation of the convex shell. With strongly
convex walls such an adductor also appears insufficient for optimal function. A contradiction
arises here between the value of strengthening of the shell through optimisation of its form and the
limitation for vertical extension of the area where each of the five adductor elements could be
separately attached. It is not, therefore, by chance that Rhombobythere mica Schornikov, 1982,
which possesses an almost round shell in cross-section, exhibits strong variation in the form of the
adductor scars (Text-figs. 3G-K). Sometimes, this species forms a 4-element scar because of the
fusion of the two lower of the middle stigmae. In this case, the strength of the adductor
increases. In the case examined, the area of the 4-element scar (Text-fig. 3K) proved to be 18 %
more than that of the 5-element scar common to the genus (Text-fig. 3G).
Lateral sculpture: The problem of the primary sculpture in this group is similar to the familiar
problem of the “hen or egg”. Reticular structures are mostly widespread in nature. This is primarily
associated with “granulation” of natural objects. Undoubtedly, throughout the evolutionary
history of the Bythocytheridae ancestors, retiform relief of integuments appeared repeatedly.
This relief was transformed, reduced and re-appeared. A majority of the ancient Bythocytheridae
have a relatively uniform lateral reticulation of the shell. This is assumed to be the original primary
sculpture in this group.
Bythocytheridae exhibit various transformations of the archaic lateral reticulation: reduction,
hypertrophy and more frequently, simultaneous reduction of some elements and hypertrophy of
the remainder. Hypertrophied elements are subjected to various transformations. Sometimes stable
states are observed when the sculpture produced conforms perfectly to the established relationships between the environment and organism. Some characteristics of such sculpture can extend to
a great number of the descendent taxa of the group. However, a possibility (and hence a tendency)
always persists toward changing these relationships and then the former characteristics, once
stable, prove to be no longer optimal. As a consequence, the structures which have achieved a
morphofunctional peak for this group are again subjected to reduction.
A new, secondary sculpture usually appears on the shells with completely reduced lateral sculpture. This new sculpture may be similar to, or different from, the previous one. Later on it undergoes transformations analogous to those suffered by the primary sculpture.
Sets of these parallel and successive transformations, complete to a greater or less extent, are
frequently observed in each subfamily or frequently even within smaller taxa. They are most
completely represented in the Keijicytherinae.
Striatobythoceratinini n. gen. 1 (Plate 1, figs. 7,8, 10) : Most species of the genus have a pronounced, relatively uniform reticulation. Sometimes, it undergoes reduction so that it appears almost
indistinguishable. Completely smooth forms were not found in the group, but in a species of the
genus closest to it the shell surface is smooth (Plate 1, fig. 9). The latter species apparently inhabited
semi-liquid mud and possesses, beside the lateral spine, a large posterodorsal spine. The caudal
process is completely reduced. Thus, it exhibits a convergent similarity with a Recent inhabitant
of Tanhanika Lake, Kuvalacythereis braconensis Wouters 1979. N. sp. 1 (Plate 1, fig. 7) has a
somewhat hypertrophied lateral sculpture. This hypertrophy is irregular. Mesh walls oriented along
the border of the valve are most conspicuous. Analogous hypertrophy of some elements and reduc-