paraboloid colony bases in paleozoic stenolaemate bryozoans

Paraboloid colony bases in Paleozoic stenolaemate bryozoans FRANK K. McKINNEY

McKinney, F. K. 1977 07 15: Paraboloid colony bases in Paleozoic stenolaemate bryozoans. Lethaia, Vol. 10, pp. 209-217. Oslo. ISSN 0024-1164. Dendroid stenolaemate bryozoan colonies with paraboloid bases developed initially in the same manner as stenolaemate colonies with broadly encrusting bases. Their unique shape is related to the narrowly cylindrical shape of the encrusted surface (algal stipe?) and to radially differentiated rates of growth from the point of colony origin. The colony shape is interpreted as an adaptation to unconsolidated substrates in relatively quiet though not necessarily deep water. Frank K. McKinney. Department of Geology, Appalachian State University. Boone, North Carolina 28608, U.S .A. ; 17th September, 1976.

Most bryozoans are preserved in calcareous shales and limestones, a large proportion of which represent former unconsolidated substrates. Most branched stenolaemate bryozoans found in such rocks began growth on local solid substrates such as skeletal remains of other bryozoans, brachiopod shells, etc., while others apparently developed broadly encrusting colonies on algal substrates that were not preserved. The bases of attachment spread to overgrow much of the encrusted fragments, providing firm supports for the branched colonies above. A few anomalous species, such as Batostomella briareus (Nicholson) in the upper Middle Ordovician of the North American Midcontinent and Dittopora colliculata (Eich- wald) in the lower Middle Ordovician of Esthonia, had bluntly rounded, paraboloid basal portions (Fig. 1A-D). The term ‘paraboloid’ is used here in a generally descriptive sense. Close approach of several colony bases to a mathematically defined paraboloid sug- gests that that is the idealized shape, and the term is therefore adopted for general use in this paper.

The general nature of initial colony growth

in trepostome and other stenolaemate bryozoans with broadly encrusting bases raises ques- tions regarding the founding of colonies with paraboloid bases: (1) How do the early growth stages of such colonies compare with the early stages of stenolaemates with more typical, broad bases of attachment? (2) What was the nature of the substrate on which the colonies began growth? (3) What was the adaptive significance or advantage of such growth forms?

Several specimens of B. briareus with proximal ends preserved were selected for study of initial colony (astogenetic) development. Longi- tudinal and transverse thin-sections and serial acetate peel sections of the specimens were prepared for study as described by Boardman & Utgaard (1964). Because of the small size of the primary zone of astogenetic change, only one type of section, longitudinal or transverse, could be prepared from each specimen. Three- dimensional interpretations of the primary zone of astogenetic change therefore depend on serial peels and on correlation of longitudinal and transverse sections between colonies rather than on sections made at right angles to one

210 Frank K. McKinney LETHAIA 10 (1977)

Fig. I . 0 A-D. External views, X 1, of paraboloid bases of Bafosfomelfa briareus (Nicholson), Catheys Formation (upper Middle Ordovician), 3 km southeast of Mt. Pleasant, Tennessee. A, USNM 245196; B, USNM 245197; C, USNM 245198; D, 245199. 0 E. Founding individual and initially budded portions of trematoporid trepostome zoariurn with broadly encrusting base, X 100, shown in longitudinal section. Arrow points to point of reflection of exterior wall above initial portion of founding individual (a); base of encrustation and all other exterior wall is along lower margin of figure; other, vertical walls in illustration, are interior. USNM 245200, ‘Nemafopora bed’ (upper Middle Ordovician), 3-4 krn south of Cannon Falls, Minnesota. 0 F. Longitudinal acetate peel through initial portion of zoarium, XSO, of B. briareus with small exposed base of attachment; founding individual (a) has doubled clear exterior wall for short distance on right side. USNM 245201, same horizon and location as A-D above.

LETHAIA 10 (1977) Paraboloid bases in bryozoans 21 1

another within the same specimen. The specimens are housed in the US. National Museum of Natural History (USNM).

Initial colony development in stenolaemates Primary zones of astogenetic change in trepostome bryozoans, described for a relatively few species (Cumings 1912; Boardman & McKinney 1976), developed similar to primary zones of astogenetic change in radially spreading modem cyclostome bryozoans (Boardman & McKinney 1976:38-45). These bryozoans began colony growth by producing a founding individual (ancestrular zooid) whose earliest-formed skeleton is of apparently nonlaminated exterior wall, the €0- portion of which is closely appressed to the substrate. The initially formed skeleton was interrupted by a rounded orifice, from which distal growth proceeded to produce a skeletal cone of exterior wall. Asexually produced zooids were partitioned within the cone by compound (interior) laminated skeletal walls. After a short distance the portion of the cone of exterior wall that was not appressed to the substrate recurved as it continued growth so that the previously formed cone became covered by a second layer of exterior wall. As the reflected portion of exterior wall came into contact with the substrate along the margins of the cone it spread laterally over the substrate, to which it was closely affixed. (See Boardman & McKinney 1976, Text-fig. 2, for diagrams illustrating stages in primary zone of astogenetic change and radial spread of basal exterior wall.)

New interior zooidal walls in trepostomes apparently could not be generated on the outer surface of exterior walls, but they could origi- nate from inner surfaces of exterior walls or from preexisting interior walls (see Boardman & Cheetham 1973 for summary discussion of why this is so). Reflection of the exterior wall over the free surfaces of the initial cone and its subsequent radial spreading over the substrate exposed the inner surface of the exterior wall, from which interior walls arose to define new zooids (Fig. 1E). Thus the founding individuals of the colonies came to be surrounded by the encrusting portions of the colonies, whether the colonies had a basically encrusting or massive habit, or whether they had en-

crusting bases from which extensive dendroid portions arose.

Trepostomes with paraboloid bases are anomalous in the lack of widely spread bases of exterior wall affixed to the substrate from which the remainders of the colonies grew. This led Anstey & Perry (1973:42) to speculate that such colonies had flexible, uncalcified joints at their bases.

Initial development of paraboloid bases Specimens of Batostomella briareus whose ex- teriors were examined have variously figured proximal surfaces. Some have small patches of exposed exterior wall (‘basal laminae’ of authors), others each have a skeleton-free circular area of variable size bu) larger than zooecial (zooidal skeleton) cross-sections, and yet others have no break in continuity of the surface of closely packed zooecia -‘across the entire basal area.

Specimens in which a small area of exterior wall is exposed at the base grew about the same as did trepostomes whose primary zones of change have been previously studied. In such specimens of Batostomella briareus, cones of exterior wall were initiated by the founding individuals, the free surfaces of the cones became recurved, and small radially spreading bases of attachment of exterior wall were formed, from which zooids were budded (Fig. 1F). The bases of attachment are separated from the substrate and in some cases have the margins of the former attachment covered by recurved zooids, indicating that detachment from the substrate occurred while the colony was still viable.

The primary zone of astogenetic change is buried within the paraboloid colony bases in the specimens of Batostomella briareus that have no exposed exterior wall at the proximal colony tip (Figs. 2A-D, 3A-D). In most specimens there is an oblique non-skeletal cylinder, up to approximately 0.3 mm in diameter, against which the founding individual of the colony is located. The non-skeletal cylinder apparently represents a cylindrical, non-calcified object that served as a substrate on which the colony began growth. The primary zone of astogenetic change of colonies that preserve the non-skeletal cylinder is basically similar

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212 Frank K . McKinney LETHAIA 10 (1977)

Fig. 2. Initial portions of maria of Batastornella briareus (Nicholson). 0 A, B. Longitudinal section, X 100 and X20, through founding individual (a). calcite spar-filled area representing substrate on which colony began (e), and reflected exterior wall over initial part of founding individual (distal point marked r). USNM 245202, Cynthiana Formation (upper Middle Ordovician), Lexington, Kentucky. O C . Transverse- acetate peel, X100. through zooecia of founding individual (a) and initially budded zooids almost completely encircled by doubled exterior wall (arrows), adjacent to subcircular spar- filled area representing substrate on which colony began (e). Same specimen as in Fig. 1B. OD. Longitudinal section, X20, through founding individual (proximal tip marked by arrow) of colony with no trace of encrusted substrate remaining. Same specimen as in Fig. 1A.

LETHAIA 10 (1977) Paraboloid bases in bryozoans 213

to that of other trepostomes; in each specimen studied there is a distally enlarged cone of zooecia bounded by exterior wall doubled back upon itself except along its base of attachment; the exterior wall is expanded along the substrate in all directions from the initial cone (Figs. 2A-C, 3A-C). The apparent differences between initial colony development in stenolaemates with paraboloid bases and those with broadly encrusting bases seem (1) to be related to the shape of the encrusted substrate rather than to a fundamentally different budding pattern or sequence, and (2) to result from a greater rate of colony growth in one direction from the circumferentially encrusted substrate, whereas a smoothly rounded surface was produced by less extended growth in all other directions radially from the point of colony origin.

Specimens in which no external evidence of a substrate exists apparently separated from the substrate early in colony development. Subsequent growth completely covered the basal exterior wall of attachment with zooids (Figs. 2D, 3A, C) and produced colony bases with external shapes similar to those that preserve internal and external evidence of a cylindrical substrate. In at least one colony (Fig. 3D) the lowermost portions of the colony lost the ability to continue growth as the colony extended distally. The specimen was penetrated by boring organism(s). The borings terminate at the surface along which growth was locally disrupted; this distribution implies that the borings may have caused the local cessation of growth. Lateral budding along the proximal side of the more distally located viable portion of the colony resulted in an intracolonial overgrowth over the abandoned zooecia, maintaining the paraboloid basal shape.

Paleoecolog ical interpret at ions Batostomella briareus has been reported (as Eridotrypa briareus) from the upper Middle Ordovician of central Tennessee by Wilson (1949) in (1) the Cannon Facies of the Bigby- Cannon Limestone, the typical aspect of which is ‘uniformly bedded fine-to-medium-bedded blue limestone’; (2) the Constellaria bed of the Shaly Facies of the Catheys Formation, described as thin, irregular beds of limestone with

‘partings of gray shale that vary greatly in thickness but that are ordinarily very thin’.

Anstey & Perry (1973:42) reported specimens with tapered bases assigned to Eridotrypa mutabilis from the ‘Clays Ferry Tongue at the base of the Eden’ Formation near the .Middle- Upper Ordovician boundary in the Ohio Val- ley. The Clays Ferry Tongue is limestone that is intermediate between the clay shale of the Eden Shale and the predominantly micritic limestone of the lowermost Dillsboro Forma- tion (Anstey & Perry 1973:9).

The fine-grained sediments in which branched bryozoans with paraboloid bases were en- tombed represent relatively quiet-water deposition. Substrate selection is suggested because the Paleozoic stenolaemates with paraboloid bases grew preferentially from soft substrates even though a diversity of shells that would have provided adequate ,hard substrates are preserved with them. Such bryozoans are interpreted as having begun growth on flexible substrates such as algal stipes. The colonies eventually separated from the substrates (zoaria with exposed basal exterior wall or with no cylindrical tube in the base) or incorporated a segment of the substrate within the paraboloid base (zoaria with skeleton-free cylindrical tubes incorporated within the base).

Pronounced selection for specific organic substrates is known for several living bryozoan species (e.g. Rogick & Croasdale 1949; Ryland 1962; Cook 1964; Moore 1973) and has been suggested for some fossil bryozoans (Rohlich 1952, cited in Duncan 1957; Fischer & Buge 1970; Palmer & Hancock 1973). The substrates selected are interpreted to have been algal, in part from the negative evidence that they completely deteriorated, and in part from analogy with common occurrence of modern bryozoans on algae. Shape of the encrusting basal exterior wall in Batostomella briareus varies from cylindrical to planar, suggesting that several algal taxa may have formed substrates or that the specific substrate was com- posed of stem and leaf-like blade.

Stability of the colonies on the sea floor was presumably achieved by the tapered bases gradually settling deeper into the relatively soft sea floor as the colony grew in size. This is an example of the ‘iceberg’ adaptation of some benthic invertebrates to soft substrates discussed by Thayer (1975), in which the sub- merged portion of sedentary invertebrates


serves to stabilize the portion extending above the substrate.

Authors of a series of works on bryozoan ecology (most notably Stach 1936; Lagaaij & Gautier 1965; Schopf 1969) have delimited the general distribution of diminuitive rigid, branched (vinculariiform) bryozoans in modern seas. Schopf (1969:236), working in the North Atlantic off New England, determined that erect, rigid, heavily calcified bryozoans pre- dominate over erect flexible forms in collections from depths greater than 35 m, and that ‘. . . nearly all of the collections in shallower water include only flexible forms’. There is a tendency among many paleontologists to direct- ly transfer Schopf’s 35 m boundary to accumu- lations of Paleozoic bryozoans, interpreting branched bryozoan fossils as representative of deep water and ignoring the implications of size difference between robust branched Paleo- zoic bryozoans and the delicate modem vincu- lafiiform bryozoans. Branched cryptostome bryozoans, with branches typically less than 2 mm in diameter, are closer Paleozoic analogues of modern vinculariiform colonies (noted also by Cuffey 1%7:24). In determining the paleoecologic setting of Paleozoic bryozoans with paraboloid bases, I believe that branched examples of modern Porites corals provide a more appropriate comparison.

In the reef tract of the Florida Keys, the branched coral Porites divaricata grows prolifi- cally in shallow near-shore environments, along with calcified and non-calcified algae and the marine grass Thallassia. Distribution of P.

Fig. 3. Initial portions of zoaria of Batostomella briareus (Nicholson). 0 A. C. Longitudinal acetate peel and section, respectively, both X 100, through skeletons of founding individual (a) and early budded mids. Exterior wall along encrusted base (b) may be. clearly seen in A, and point of reflection (r) of exterior wall along side of founding individual is illustrated in C. Same specimen as in Figs. 1A and 2D. O B . Longi- tudinal acetate peel, X50, through founding individual (a) and tangential to base of encrustation on roughly cylindrical substrate. USNM 245203, Occurrence as Fig. IA-D. 0 D. Longitudinal section, XZO, through initial portion of colony penetrated by boring organism early in development of colony (boring indicated by b); damaged portion of colony was later covered by overgrowth spreading toward base from more distal portions of the colony; arrow to lower left is situated within the spar-filled area representing the substrate on which the colony was established, and points to base of founding individual. USNM 245204; same occurrence as Fig. 2A-B.

divaricata around Rodriquez Key, a backreef island near Key Largo, has been documented by Turmel & Swanson (1964, 1976). P. divaricata occurs in thickets approximately one meter deep along the eastern and southern windward bank margin of Rodriquez Key. Sediment along this bank margin, as the result of abundant skeletal growth and fragmentation, consists of skeletal sand and gravel, whereas both in shallower water toward Rodriquez Key and in deeper water away from the island, a mixture of lime mud and skeletal sand pre- dominates.

The lower portions of colonies of Porites divaricata are buried in the substrate, which generally covers more than half the colonies. A pronounced discontinuity in distribution of living tissue marks the depth to which colonies of P . divaricata are buried: living tissue covers colonies distal to the sediqent-water interface and is absent over proximal (buried) surfaces. Proximal ends of P. divaricata colonies are variously shaped: some are attached to broken fragments of earlier colonies or small shell fragments and some have broken free from their base of attachment yet continued growth. The latter type has closest analogy with bryozoans with paraboloid bases; their partial submergence in the sediment results from the ‘iceberg’ effect, from active sedimentation enhanced by the baffling effect of Porites and Thallassia, or most likely by a combination of both effects.

There is some dissolution of buried portions of Porites divaricata skeletons, but similar dissolution is not conspicuous in specimens of Batostomella briareus. This may be related to lack of organic protection over skeletons of P . divaricata and former presence of con- tinuous organic protection around proximal ends of B. briareus. The interface between living marine bryozoans and their environment is a cuticular membrane that is the outermost layer of the body wall (e.g. Borg 1926:191). The outer cuticle is maintained in buried portions of such species as Bugula purpurea and Amathia convoluta, bryozoans that main- tain attachment to progressively more deeply buried shells in silt and fine-grained sand substrates in the Neuse Estuary, North Carolina (personal observation). Similar duration of the outer cuticle on B. briareus would have pre- vented dissolution of skeleton on the proximal end of the colony. With potential for transfer


of nutrients through the colony-wide coelom between skeleton and outer cuticle, the con- tinuous cuticle may have allowed continued skeletal growth to increase diameter of buried paraboloid bases and branch segments.

Another possible adaptation of paraboloid bases may have been for densely populated thickets, similar to the Porites-Thallassia thickets mentioned above, populated in part by algae on which growth of bryozoans with paraboloid bases began. Weight of a developing bryozoan colony may have slowly bent the host algal stipe so that the bryozoan colony slowly settled into the thicket matrix, the paraboloid base allowing easy settlement into the thicket but not requiring burial in sediment. No specimens were observed in growth position, so neither alternative can be chosen at present.

Wilson (1962) interpreted the paleogeo- graphic and paleoecologic setting of Middle Ordovician rocks of central Tennessee that contain Batostomella briareus. During the Middle Ordovician the Central Tennessee Bank was a belt of shallow water, at some times exposed land, extending north-south through the Nashville area. It was paralleled on the east and on the west by bands of somewhat deeper water. B. briareus apparently occurred in very shallow water facies capping the Cen- tral Tennessee Bank, and, when the bank crest was exposed, in very shallow shore- parallel facies. Although the sediments in which B. briareus is preserved contain abundant calcified algae (L. P. Alberstadt, personal communication) and are interpreted as representing ‘very shallow water’ deposits, no pre- cise depth of deposition has been determined.

General analogy between environments of growth of B. briareus and P . divaricata is suggested by rough correspondence in size of branches in colonies, similarity of sediment textures, abundance of calcified algae associated with both, observed partial submergence of P . divaricata into sediment, and the inter- pretation of paraboloid bases of B. briareus as an adaptation to settling into the substrate whether it be unconsolidated sediment or a thicket framework such as that produced by aggregations of P. divaricata.

Acknowledgements. - Early drafts of this paper were discussed and criticized by Leonard P. Alberstadt, Daniel B. Blake, Judith E. Dudley, and Philip A. Sandberg; their criticisms greatly improved the paper.

Research on Batostomella briareus was begun while the author was studying under a Smithsonian Post- doctoral Fellowship and was completed with assistance of Appalachian State University Faculty Research Grants.

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paraboloid colony bases in paleozoic stenolaemate bryozoans

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