UNtVtRSlTv Cp ILLINOIS I ^ARY AT URBANA-CHAMPAIGN LlOLOGY CO CO l\Jt MAY 1 i 1978 University of Illinois ■ "rbana-ehwiwrttW Volume 39, No. 1 March 17, 1978 Internal Structures of Cyclocrinites dactioloides, a Receptaculitid Alga from the Lower Silurian of Iowa Matthew H. Nitecki Curator, Fossil Invertebrates Field Museum of Natural History and Markes E. Johnson Department of Geology Williams College wllliamstown, massachusetts ABSTRACT Cyclocrinites dactioloides is a marine, calcareous, receptaculitid alga which occurs as a common fossil in the Lower Silurian (Llandoverian Hopkinton Dolomite) of eastern Iowa and northwestern Illinois. The main axis and lateral branches, previously unknown in this species, are described. These internal parts, rarely known in other taxa of cyclocrinitids, were poorly calcified and hence were seldom preserved. The flattened lower parts of the thalli, previously considered resting surfaces, are now shown to result from post mortem, early diagenetic alterations. The lateral branches compare well with similar structures in both present-day dasyclads and in ischaditid receptaculitids and thus bridge the gap between these two separate orders of green algae. INTRODUCTION Receptaculitids constitute a large group of fossils that ranged from the Middle Cambrian to the Permian. Receptaculitids are characterized by a calcified, globose thallus consisting of a main axis bearing regularly arranged branches. The laterals (= meromes) generally terminate with a single head, below which are variously developed stellate structures. The two best-known groups of receptaculitids are ischaditids and cyclocrinitids. In is- chaditids, the always-cruciform stellate structures consist of four Library of Congress Catalog Card Number: 77-90348 US ISSN 0096-2651 Publication 1280 1 2 FIELDIANA: GEOLOGY, VOLUME 39 elements in one plane. In cyclocrinitids the stellate structures are absent or rarely developed. When present, as, for example, in Cy- clocrinites darwini, they consist of four to six elements. Heads are generally the most conspicuous elements of branches and in C. dactioloides they are distinct hexagonal globose structures. No stel- late structures are observed in C. dactioloides. In North America cyclocrinitids seem to have disappeared after the Silurian Period. The reports of the post-Silurian cyclocrinitids require further study. Typical examples of living dasyclads, fossil cyclocrinitids, and is- chaditids are shown in Figures 1 through 3. Cyclocrinites dactioloides, a typical cyclocrinitid, is a common fossil in the Lower Silurian (Llandovery Series) of eastern Iowa and northwestern Illinois. Although in the older American literature cyclocrinitids were considered to be either corals, sponges, proto- zoans, or algae, the present consensus in this country and abroad is that they are dasycladacean algae, or a group closely related to them (see, for example, Nitecki, 1970). There is little reason to doubt the similarities of cyclocrinitids to dasyclads. The arrange- ment and morphology of the laterals, the shape of the thallus, and the faceted surface of cyclocrinitids resemble the extant subfamily Bornetelloideae, particularly Bornetella sphaerica (fig. 1). How- ever, the close relationship of cyclocrinitids to receptaculitids is not generally accepted. Rietschel (1969) and Campbell et al. (1974) do not consider cyclocrinitids to belong with receptaculitids. Nitecki (1976 and earlier) is the main proponent of the receptaculitid nature of cyclocrinitids. He assigns cyclocrinitids together with ischaditids to the receptaculitid group primarily on the basis of the presence of stellate structures in Ordovician Cyclocrinites darwini and hexa- gonal facets in Silurian Ischadites koenigii. PREVIOUS WORK David Dale Owen in 1839 explored the Ordovician, Silurian, and Devonian strata in Iowa, Wisconsin, and Illinois and published his findings in 1840 and 1844. The first American illustration and description of a cyclocrinitid, Lunulites? dactioloides, appeared in Owen's 1844 publication (pi. 13, fig. 4, appendix, p. 69). Owen (1844, p. 97; pi. 7, fig. 5) illustrated the fossil with corals and referred the closely related Receptaculites also to corals. Eichwald (1840) was the first, shortly before Owen, to report and describe the cyclo- crinitids. His fossil Cyclocrinites spaskii was reported from the Fig. 1. Living Bornetella sphaerica (Zanardini, 1878) from Hawaii, a, thallus; b, cross-section through thallus; c, position of gametangia; d, lateral upon main axis and calcification (shaded). Based upon liquid preserved specimens. FIELDIANA: GEOLOGY, VOLUME 39 Fig. 2. Reconstruction of Cyclocrinites gregarius (Billings, 1866) from Gun River Formation (Silurian), Anticosti Island, Quebec. See Figure 1 for explanation. Baltic region. Later, in Germany, Stolley (1896) monographed cyclocrinitids as algae. Samuel Calvin redescribed Cerionites [= Cyclocrinites] dactioloides at about the same time. Calvin (1893, p. 15) concluded that Cyclocrinites dactioloides was a large, unattached protozoan colony capable of moving through the water with a "graceful rolling motion." His interpreta- tion that Cyclocrinites was a protozoan was not inconsistent with the prevailing scientific opinion of his day. Protozoa in the nineteenth century included many organisms now reassigned to other groups, including sponges, and size was not a limiting factor in their definition. Thus Calvin's assignment of cyclocrinitids to the NITECKI & JOHNSON: CYCLOCRINITES s .#*P1 "Y W Fig. 3. Reconstruction of Ischadites koenigii Murchison, 1839, from Racine Dolo- mite (Silurian), Chicago, Illinois. See Figure 1 for explanation. protozoa was a conservative judgement. The internal structure now recognized in this species, as well as in other cyclocrinitids, can be compared with similar features in living green algae, such as Bor- netella sphaerica. Nitecki (1970) pointed out the uncommon preser- vation of a main axis and lateral branches of Ordovician Cyclocri- nites welleri and C. pyriformis . Evidence for attachment by a pedicle in various cyclocrinitids was also discussed by Nitecki (1970, pp. FIELDIANA: GEOLOGY, VOLUME 39 Fig. 4. Cross-sections through thalli of large numbers of "swept together" Cyclo- crinites dactioloides (Owen, 1844). FMNH UC 59159, from Hopkinton Dolomite (Silurian), Eastern Iowa. 18-19). In the Silurian C. dactioloides, the axis and the lateral branches have not been previously demonstrated. PALEOECOLOGY Cyclocrinitids are found in rocks formed in the shallow seas covering midwestern North America during the Ordovician and Silurian Periods. They appear to have inhabited waters somewhat deeper than does the modern Pacific alga Bornetella sphaerica, which is found attached to rock fragments, shells, or corals in shel- tered, shallow pools. The substrate of C. dactioloides is less clearly defined and no attachment to any particular fossils or fragments has been observed. Based on the work of Johnson (1975), who recon- NITECKI & JOHNSON: CYCLOCRINITES Fig. 5. Thin, disc-shaped Cyclocrinites dactioloides (Owen, 1844) enclosed within its complete globose mold. FMNH UC 59064, from Hopkinton Dolomite (Silurian), Eastern, Iowa. structed three recurring communities in the Lower Silurian Hop- kinton Dolomite of eastern Iowa, we believe C. dactioloides to have lived below the level of wave action. These communities he found to be depth associated. They are 1) a shallow coralgal community; 2) a deeper pentamerid community; and 3) an even deeper stricklandid community. C. dactioloides is a common element in both the pen- tamerid and stricklandid communities, but is absent in the shallow coralgal community where the wave base was likely to cause dis- turbance on the sea floor. Even in the deeper water pentamerid community where Cyclo- crinites thrived, individuals were sometimes detached and swept together in large numbers (fig. 4). The depth range of modern, calcareous green algae may or may not indicate the kind of toler- ances experienced by Cyclocrinites and thus great caution needs to be exercised in assigning limits on the depth relationships of fossil communities. Taylor (1960, p. 30) states that presently, off Bermu- da, calcareous green algae flourish up to a depth of 90 m. It is likely .♦•Jte- 1cm Fig. 6. Lateral view ofCyclocrinites dactioloides (Owen, 1844) shown in Figure 5, demonstrating lateral branches and an axis. Fig. 7. Apical view of Cyclocrinites dactioloides (Owen, 1844) shown in Figure 5, demonstrating the branches and faceted surface. Fig. 8. Diagrammatic representation of Cyclocrinites dactioloides (Owen, 1844) based upon the specimen shown in Figure 5. 10 FIELDIANA: GEOLOGY, VOLUME 39 that differences in the depth range of the Iowa communities were on the order of tens of meters and that the platform seas covering the mid-continent during the Silurian Period rarely exceeded depths of 100 m. DISCUSSION The internal morphology of the cyclocrinitid thallus, although rarely preserved, has greatly aided in the identification of cyclo- crinitids as algae. The internal structures were undoubtedly poorly calcified in life, or at least at the time of burial, and hence had little chance of preservation. The specimens described in the present paper allow for a direct description of internal anatomy and for the confident reconstruction of missing morphological elements. Preservation : The hard parts of many Hopkinton organisms are silicified indicating that replacement occurred early, before dolomitization removed much of the original material. However, silicified cyclocrinitids have not been collected. Like other fossils from the Hopkinton Dolomite of Iowa the cyclocrinitids are pre- served as molds. The only exception is the non-silicified, calcified, preservation of the internal organs of C. dactioloides , showing the main axis and lateral branches (figs. 5-8). Body shape: Cyclocrinites dactioloides and allied species have been described as completely globose or as flattened cushion-shaped organisms (Nitecki, 1970). Although intermediate shapes have been noted, as well as rare, evenly flattened, thin, disc-shaped specimens, the majority of these fossils from Illinois and Iowa are cushion-like (fig. 9). Since attachment in these has not been ob- served, Nitecki (1970) concluded that the flat base (almost always present in all other than globose forms) functioned as a resting surface. Now, we believe that the flattened bases of cushion-shaped specimens, the cushion-shaped forms themselves, and even the flat, disc-like fossils are not ecologically controlled, but rather are the result of post-mortem alterations. We base this conclusion upon the finding of a thin, disc-like specimen enclosed within a complete mold of a globose individual (figs. 5-8). This remarkable find may explain shapes previously misinter- preted as representing actual life conditions. We do not know what geological processes were responsible for this preservation, which reflects a diagenetic alteration of the organism; neither are we sure whether or not the reduction in size (flattening) occurred im- NITECKI & JOHNSON: CYCLOCRINITES 11 Fig. 9. Lateral view of cushion-like Cyclocrinites dactioloides (Owen, 1844). FMNH PP 17709 from Hopkinton Dolomite (Silurian), Eastern Iowa. From Nitecki (1970). mediately after burial. Although the flat fossil is found within a globose mold, the well-preserved sides of the individual are flat, while the mold is concave. Thus it is difficult to explain the reduc- tion of the thickness of the fossil, the flattening of the surface, and the accompanying faithful reproduction of the faceted surface on the spherical mold. The shrinkage and flattening were probably due to some loss of structural cohesion of the organism, after its original globose form was preserved as a mold. Alternately, the thallus could have been open enough to receive a partial filling of lime mud. The unsupported portion could then collapse during diagenesis, while retaining the external form. The flattened base may thus represent the sedimentary interface. This evidence forces us to revise our ideas about the shape and structures of these fossils; certainly the flatness of specimens need not be ascribed to ecological factors. Cyclocrinitids appear to have been subject to post-mortem altera- tions uncommon among other organisms found associated with them. 12 FIELDIANA: GEOLOGY, VOLUME 39 Facets: The shape and arrangement of facets are observed in the free globose specimens, in cushion-shaped fossils and in molds. Although the regular arrangement of facets is characteristic of most receptaculitids, the cyclocrinitid facets are among the most uni- form. They are very regular, hexagonal structures arranged in symmetrical and overlapping rows. The facets are impressions of very closely packed heads forming six-sided units. These are clearly shown in Figure 9. The tiny perforations sometimes seen at the center of the facets represent the insertion of lateral branches. Attachment: We have not observed any attachment scars in this species. In those cyclocrinitids in which attachment scars are pres- ent, they consist of an extension of the main axis. In C. pyriformis and in C. welleri the thallus is elongated and pyriform, and the main axis may protrude in the manner of a recent dasyclad such as Neomeris . In C. darwini and C. spaski, where the thallus is globose, the attachment is represented by a pronounced scar. These have been previously discussed by Nitecki (1970, figs. 12, 13). In specimens of C. dactioloides which are completely preserved a small opening is frequently found on the bottom side. This is comparable with the preservation of the recent B or netella sphaerica from the Philippines and from Hawaii, where this alga is often collected without the holdfast. In the specimens from the Philip- pines, because the holdfast remains firmly attached to the sub- strate, only about one in three has a holdfast preserved (Gilbert, 1943). Egerod (1952) has pointed out that in the past, because of the ease with which the main axis is detached, there was a widespread belief that B. sphaerica had no holdfast at all. This belief was reinforced by examination of museum specimens, invariably with- out holdfasts, and frequently with the areas where the stipes may have protruded poorly preserved. We believe that inC. dactioloides the lack of attachment scars is also a peculiarity of preservation. Just as the internal organs have not until now been observed (in spite of decades of collecting, yielding hundreds of specimens), so the attachment scar will eventually be recognized in this species when better preserved specimens are found. Rosette: Our specimens, as well as those described by earlier workers, show an unusual arrangement of randomly distributed facets of the type Nitecki (1969, fig. 8c; 1970, fig. 18) has termed a rosette. The typical hexagonal cyclocrinitid head is surrounded by NITECKI & JOHNSON: CYCLOCRINITES 13 only six other heads. A departure from this arrangement is an addition of one or two facets, causing a seven- or eight-sided facet or head. The eight-sided rosette, because of its regularity, is highly suggestive of an anatomical entity and we have considered that it may represent a scar of a morphological structure. The number of facets in rosettes is the same as the number of facets in the lower- most "nucleus" of other receptaculitids; however, the nuclear facets are always smaller than surrounding ones. In the rosette of C. dactioloides , the facets are of various sizes and they frequently depart from the hexagonal shape. Therefore, we conclude that the rosette is only an accident of packing of branches, in which eight, instead of six, facets surround a single head. Main axis : All our information about the central axis comes from a single specimen. A part of the internal mold is shown in two different views. A lateral view (fig. 6) illustrates lateral branches extending obliquely from a small section of the main axis. An apical view (fig. 7) shows the lateral branches radiating from the centrally located main axis. Only an upper portion of the main axis is pre- served where it is very thin. The ratio of the thickness of the axis to the length of the branches is 5 to 1. Lateral branches: Characteristically, the internal morphology of cyclocrinitids includes thin, straight branches, terminating with heads. The preservation of our specimen is not good enough to determine whether or not a constriction below the heads was pres- ent. Neither does preservation allow for the reconstruction of the exact nature of the branches, except that an internal canal appears to have been present. The branches expand gradually. CONCLUSIONS Cyclocrinites dactioloides is a typical representative of the cy- clocrinitids, a group of Paleozoic algae related to dasyclads and ischaditids. The relationship of cyclocrinitids to dasyclads is well established and accepted and is based upon the similarities of the shape of the thallus, the main axis, and the branches. The facets, calcification, and reproductive structures are also comparable. Is- chaditids, a small, well-defined receptaculitid group, are similar to cyclocrinitids in most respects, even in the shape of their facets and in the development of stellate structures. It seems likely, therefore, that ischaditids are also related to dasyclads. The riddle of the taxonomic placement of all receptaculitids is still unsolved because 14 FIELDIANA: GEOLOGY, VOLUME 39 it is not yet known whether the large group of receptaculitids other than ischaditids is a coherent taxon. The similarities of cyclocri- nitids and ischaditids to the dasyclads warrant the assignment of cyclocrinitids and ischaditids to the complex of siphonous green algae. The differences between them, however, necessitate placing cyclocrinitids and ischaditids in a separate order from Dasy- cladales. MATERIALS AND OCCURRENCES The specimens cited in this paper are from the Cyclocrinites beds which occur at the middle of the Hopkinton Dolomite in eastern Iowa (see Johnson, 1975). The equivalent stratigraphic interval was called the Cerionites beds by Calvin and Bain (1900, p. 445). All materials are deposited at Field Museum of Natural History. FMNH UC 59064 (figs. 5-8) is from a horizon 3.75 m. below the top of the Cyclocrinites beds, Clarence Martins Quarries, about 7.25 km. NW of Cascade in Dubuque County (SE& NEV4 and NEV4 SE% Sec. 16, T87N, R2W). FMNH UC 59159 (fig. 4) is from the upper 4 m. of the Cyclocrinites beds, Krapfl Quarry, about 4 km. NW of Dyersville in Delaware Co.(SE% SEV4 Sec. 23, T89N, R3W).FMNHPP 17709 (fig. 9) was found near Clinton, Iowa, exact locality unknown. ACKNOWLEDGEMENTS Most of the specimens cited in this paper were collected by M. E. Johnson during the course of sampling conducted in the study of Silurian communities from Iowa. We are grateful for financial sup- port to Nitecki from National Science Foundation grant DEB 77- 11129. Financial aid for the summer field work of Johnson in 1975 was provided through a Sigma Xi grant and a Penrose Grant (Geological Society of America) through the Chevron Oil Company. Funds for joint field work by the authors were made possible through Field Museum of Natural History. Our colleagues in Field Museum critically read the manuscript. REFERENCES Calvin, S. 1893. On the structure and the affinities of Cerionites dactylioides Owen. Proc. Iowa Acad. Sci., 1, pp. 13-15. Calvin, S. and H. F. Bain 1900. Geology of Dubuque County, Iowa. Iowa Geol. Surv. Ann. 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