CORRELATION OF ZOARIAL FORM WITH HABITAT

LEO W STACH

Melbourne, Australia

ABSTRACT

A study of zoarial form in the cheilostomatous Bryozoa has shown that a definite relationship exists between the various zoarial types and their habitat. A percentage estimate of the various zoarial types present in an early Cenozoic bryozoan faunule gives a moderately accurate conception of the conditions under which the matrix bed was deposited.

INTRODUCTION

The bathymetric conditions under which their matrix bed was deposited may be deduced from a study of Cenozoic cheilostomatous bryozoan faunules. In later Cenozoic deposits the determination of the bathymetric facies is comparatively simple, since the majority of the Bryozoa present are still living and their present habitats are well known. But in early Cenozoic deposits the determination of bathymetric conditions is not so simple, since the percentage of Recent Bryozoa in these beds is much reduced. In fact, the persistent forms probably owe their survival to the present day because they are eneralized as to habitat and hence are of little use as bathymetric indicators.

Some general criterion is therefore necessary to obtain a bathymetric estimate in Lower Cenozoic formations. A study of zoarial form in present day groups has indicated a definite relationship between zoarial form and environment. The writer believes that this relationship can be applied with great facility, by a nonspecialist in the field, to determine some of the conditions of deposition of the sediments containing the zoaria.

The types of zoaria found in the Cheilostomata are reduced to the nine groups described in the succeeding paragraphs. The names given to these are derived from the principal groups in which the respective zoarial types are best represented.

It is true that within one genus, or even one species, there are zoarial types which fall into more than one of these groups; but in many cases this is due to adaptation of the species to different
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bathymetric facies. For instance, a common Victorian species, Caleschara denticulata (Macgillivray), ranging from early Miocene to the present, occurs in either the eschariform or vinculariform zoarial types. The former is the type represented in shore debris along the Victorian coast, whereas the latter is the form obtained at depths of 30-200 fathoms in Bass Strait and off the South Australian coast. From numerous sources of evidence it is recognized that the Victorian early Miocene has a facies of much deeper water than the comparatively shallow water early Pliocene deposits, and it is therefore not surprising that the vinculariform zoarium is exclusively found in the former series and the eschariform in the latter. Ref. 1

ZOARIAL TYPES

Membraniporiform.--Unilaminate zoaria whose entire basal lamina is attached to the substratum. This type is adapted for life in the littoral and sublittoral zones. Its form and situation tend to counteract the to-and-fro motion of the bottom water, where the depth is much shallower than the wave base. This is effected by the zoaria's incrusting flexible algae and loose shells, which sway with the motions of the bottom water. Though mainly limited to the littoral and sublittoral zones, many forms extend to deeper waters but are there numerically unimportant.

Petraliform.--The basal lamina of the unilaminate zoarium is attached to loose and irregular substrata, such as coralline algae and sponges, by means of chitinous radicles in all Petraliidae except Petralia Macgillivray (sensu stricto). This type is adapted for life in a particular habitat in the littoral zone, but occurs most abundantly in the sublittoral zone, since the space between the basal lamina and substratum weakens its resistance to wave action. The petraliform type is, in the littoral zone, a supplement of the membraniporiform type which can only incrust smooth, solid substrata. A specialized type of petraliform zoarium is seen in Petralia undata Livingstone, 1926 (non Macgillivray), where the zoarium is fenestrate and the proximal zooecia produce numerous radicles which unite to form an attaching filament fastening the zoarium to the sandy bottom, to
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which facies this form is restricted. Because of its structure, it has survived on sandy bottoms in fairly deep water affected by currents, the fenestrate zoarium offering less resistance to current action. Such a specialized case is the best type of bathymetric indicator.

Eschariform.--Zoarium bilaminate, foliaceous; attached to the substratum either by radicles or direct adherence. This type is adapted for life in sublittoral zones at depths of at least 10 fathoms. The zoarium is strongly calcified and usually has a rigid basis of attachment. It may extend to deeper water, but not to the littoral zone. A special adaptation for a sandy bottom habitat occurs in Parmularia Macgillivray, in which the zoarium develops a chitinous attaching filament from the center of its proximal margin and differs essentially from Petralia undata only in its lack of fenestration.

Reteporiform.--Zoarium attached, rigid, strongly calcified, and fenestrate. This type is adapted for life in regions where wave action and currents are strong, these factors being overcome by the rigidity and fenestration of the colony. The unilaminate convoluted zoarium of Reteporidae is heavily calcified and secondarily strengthened by successive layers of avicularia developed on the basal side, as shown by Buchner. The large bilaminate fenestrate Adeonae belong essentially to this group, but here the rigidity of the zoarium is due to heavy secondary calcification of the frontal wall. This type is most prolific in sublittoral regions.

Vinculariform.--Zoarium attached, consisting of erect, rigid, subcylindrical branches. The narrow branches of the nonarticulate colony are adapted for life in deep or sheltered waters where wave action is absent and currents scarcely active. This group typifies growth in quiet waters and should be distinguished from membraniporiform zoaria which incrust thin stems of algae and assume a superficial resemblance to the vinculariform type, as is particularly well seen in specimens of Thalamoporella californica Levinsen from Santa Monica, California. In transverse section, these pseudovinculariform types show remains of the algal stem or an axial cavity formerly occupied by it.
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Cellariform.--Zoarium articulated, with subcylindrical internodes consisting of numerous zooecia. This type is adapted for life in the littoral zone where algae usually form the basis of attachment and the effect of wave action is overcome by the articulation of the long, narrow internodes. This type extends occasionally to greater depths.

Catenicelliform.--Zoarium of articulated internodes of few zooecia. This type is adapted for life in the littoral zone where wave action is strongly felt. The internodes consist of one or two zooecia, thus allowing the zoarium to follow the motion of the water with little risk of breaking the colony, because of its extreme flexibility. The earlier internodes of the zoarium are commonly attached by radicles to stems of red algae, the later branches becoming free.

Flustriform.--The chitinous zoarium, inherently flexible, is especially well adapted for a littoral habitat; but, having no calcareous supporting skeleton, it is incapable of preservation.

Lunulitiform.--Zoarium free, hollow conical, the zooecia opening on the outer face. The actual mode of life of this type of zoarium has not been emonstrated and is the cause of much controversy. Ref. 3 The examination of a series of nearly two hundred specimens of Lunularia and Selenaria from Bass Strait corroborates the fact that they are in no way attached to a fixed substratum in adult life, since no radicles were observed on any of the specimens, though it is probable that Conescharellina has some form of either temporary or permanent radicular attachment. Their free mode of life prohibits their existence in the littoral zone where wave action is strongly felt; and from their present day occurrences, they are restricted to sandy bottoms where current action is strong, their upper limit being about 15 fathoms.

Dartevelle has shown that many lunulitiform zoaria of the early Eocene (Ledien) of the Belgian Basin have undergone fracture due to excessive current action, the fragments each regenerating a complete zoarium. The regenerated portions can be distinguished readily under a hand lens, and thus their presence in a deposit constitutes a good indication of contemporaneous strongcurrent action.
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ESTIMATION OF THE FACIES

The various zoarial types overlap their prescribed zones both by natural migration and artificial dissemination; but the vast majority of each type conforms to its stated habitat, and a percentage estimate of the types present in a given faunule would not be affected by such sporadic exceptions.

FIG. I.--Illustration of the current trend in early Miocene times in the Geelong district (Victoria) as deduced from bryozoan faunules. The broken arrows indicate the current trends. The location of the area is indicated by the stippled portion in the inset map.

The stages in the estimation of the facies are: (1) the determination of the zoarial types present; (2) the determination of the predominating type or types; (3) consideration of any specialized types present. For work in the field, the first two alone could be employed and a conclusion could be reached in a short time by the examination of sievings taken from the deposit. The consideration of specialized types is essentially a laboratory study, and more specific data may
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then be added to the general conclusions drawn from numerical estimates. A numerical estimate in this sense refers to the relative number of specimens of each zoarial type and not to the number of species in the sample conforming to the particular types.

A few examples may here be cited from Victorian Cenozoic areas with which the writer is familiar. In the Geelong Cenozoic Basin, to the southwest of Port Phillip Bay, there is a remarkable development of Lower Miocene bryozoan limestones overlying Upper Oligocene marls. The latter, exposed in the Torquay dome, contain an assemblage which is predominantly lunulitiform with minor catenicelliform and cellariform elements, from which it is concluded that the conditions during deposition were fairly shallow water (about 15 fathoms) with current and wave action. The Lower Miocene bryozoan limestones surround the Jurassic outlier of the Barrabool Hills, and samples were taken from exposures occurring around it. On the southeast side, at Waurn Ponds, the faunule is predominantly eschariform with fewer cellariform and lunulitiform types, the suggested facies, subjected to wave and current action, being about 20 fathoms. On the western side, at Prowse's quarry, the faunule has eschariform, reteporiform, and vinculariform types in about equal proportions, suggesting a quieter and slightly shallower facies. On the northern side, in the Batesford quarry, the faunule is predominantly vinculariform with minor cellariform and eschariform elements, pointing to a similar bathymetric facies but quite placid water conditions. The inference to be drawn from these data is that the Jurassic buttress deflected a general northerly current in early Miocene times and protected the lee side where the quietwater facies is developed.

The early Pliocene faunule from Macdonald's locality at Hamilton, Victoria, recently investigated by the writer, Ref. 5 has lunulitiform, catenicelliform, and cellariform types in about equal proportion, with rarer eschariform and membraniporiform elements. From this it is inferred that the beds were deposited in fairly shallow water (about 1015 fathoms) under the influence of currents and wave action.

REFERENCES

  1. L. W. Stach, "Victorian Lower Pliocene Bryozoa, Part I," Proc. Roy. Soc. Victoria, N.S., Vol. XLVII, No. 2 (1935), p. 340.

  2. P. Buchner, "Studien uber den Polymorphismus der Bryozoen. I. Anatomische und systematische Untersuchungen an japanischen Reteporiden," Zool. Jahrb. Abt. Syst., Vol. XLVIII (1924), p. I55.

  3. S.F. Harmer, "Recent Work on Polyzoa," Presidential Address, Proc. Linn. Soc. London Sess. 143 (1931), p. 150.

  4. E. Dartevelle, "Contribution a l'etude des Bryozoaires fossiles de l'Eocene de la Belgique," Annales Soc. Roy. Zool. Belg., Vol. LXIII (1933), pp. 70-72, Pl. II, figs. 35.

  5. Op. cit., pp. 338-51, Pl..XII.

Main List of References
Stach. L.W., 1936. Correlation of Zoarial Form with Habitat. Journal of Geology, 44: 60-65.
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