Frank K. McKinney

NORTHERN ADRIATIC BRYOZOA FROM THE VICINITY OF ROVINJ, CROATIA

Abstract One hundred six species of Bryozoa collected from the northern Adriatic in the vicinity of Rovinj, Croatia, are distributed among the orders Ctenostomata (8 species), Cheilostomata (79 species), and Cyclostomata (19 species). Ctenostomes are underrepresented in the collections relative to the two orders with calcified colonies. Five of the cheilostome species are new: Hagiosynodos hadros n. sp., Schizomavella subsolana n. sp., Cellepora adriatica n. sp., Celleporina siphuncula n. sp., and Rhynchozoon revelatus n. sp. (previously referred to as Rhynchozoon sp. II Hayward). Seven species named by Heller (1867) are stabilized by selection of lectotypes (Beania hirtissima, Adeonella pallasii, Hagiosynodos kirchenpaueri, Exidmonea triforis, Crisia recurva) and neotypes (Mollia circumcincta, Schizomavella cornuta) from Hellers collection in the University of Innsbruck Institute of Zoology. Lectotypes are designated for the Adriatic species Hippoporina lineolifera (Hincks, 1886) and for Schizomavella mamillata (Hincks, 1880). Beania cylindrica (Hincks, 1886) and Schizoporella asymetrica (Calvet, 1927) are recognized as species rather than as subspecific units. The species-rich cheilostome genus Schizoporella Hincks, 1877, which contains some of the most widely known fouling bryozoans, is designated a nomen protectum. The species name Smittina cheilostoma (Manzoni, 1869) is preserved as established usage.

Bryozoan Monticules: Excurrent Water Outlets?

Monticules, regularly arranged modified areas on Paleozoic Bryozoa, may represent regions from which water currents produced by lophophores of adjacent feeding zoids escaped. Such circulation Patterns have been observed Recent forms.

Competition, clade replacement, and a history of cyclostome and cheilostome bryozoan diversity

One of the striking yet scarcely documented episodes of clade replacement in the post- Paleozoic fossil record is the decline of cyclostome Bryozoa and the corresponding, rapid diversi- fication of cheilostome Bryozoa. These clades are closely associated morphologically and phylo- genetically, and their ecological similarities have previously led to the inference that competition was a primary cause of the overt pattern of replacement. Alternatively, previous compilations of bryozoan families and genera have implied that extinctions at the Cretaceous/Tertiary boundary differentially affected cyclostomes, and thus were also an important factor in the transition. We first evaluated the ecological context for competition between the two clades, then updated and reexamined the history of absolute family diversity for bryozoans in consecutive geologic stages from Jurassic to Recent. The resulting trends echo the patterns shown in earlier family level com- pilations, but indicate a slight shift in the frequency of cheilostome family originations from Late Cretaceous to early Paleogene. The relative fall in cyclostome family diversity at the Cretaceous/ Tertiary boundary is significantly less than shown in earlier genus level compilations. We then assessed these various compilations of absolute diversity by analyzing species counts and percentages in 728 fossil assemblages, primarily from North America and Europe, over the same time interval. Cyclostome species overwhelmingly dominate assemblages from Jurassic through Cenomanian, then decline significantly in average percentage dominance through the Campanian. Cheilostomes are predominant in Campanian and later assemblages. Cyclostome species percentages do decrease overall through the Tertiary, but this decrease is small and non-uniform, varying around 30%, with a sharp drop in the Late Neogene. Our within-assemblage results indicate that as cheilostomes radiate, their mean species diversity, maximum diversity, and variance all increase, thereby ac- counting for much of the decline in average percentage of cyclostomes within assemblages. While this result does not exclude a role for competition, an hypothesis of relative decline in cyclostome species richness based on competitive extinction alone seems unlikely. Further, despite decreases in absolute species counts following end-Cretaceous extinctions, within-assemblage percentages of cheilostome or cyclostome species show only slight change relative to one another. Comparison of these and earlier diversity compilations indicates that the dynamics of bryozoan clade replacement may be perceived differently at different ecologic scales or taxonomic ranks.

Competitive displacement among post-Paleozoic cyclostome and cheilostome bryozoans.

Abstract Encrusting bryozoans provide one of the few systems in the fossil record in which ecological competition can be observed directly at local scales. The macroevolutionary history of diversity of cyclostome and cheilostome bryozoans is consistent with a coupled-logistic model of clade displacement predicated on species within clades interacting competitively. The model matches observed diversity history if the model is perturbed by a mass extinction with a position and magnitude analogous to the Cretaceous/Tertiary boundary event. Although it is difficult to measure all parameters in the model from fossil data, critical factors are intrinsic rates of extinction, which can be measured. Cyclostomes maintained a rather low rate of extinction, and the model solutions predict that they would lose diversity only slowly as competitively superior species of cheilostomes diversified into their environment. Thus, the microecological record of preserved competitive interactions between cyclostome and cheilostome bryozoans and the macroevolutionary record of global diversity are consistent in regard to competition as a significant influence on diversity histories of post-Paleozoic bryozoans.

A faster-paced world?: contrasts in biovolume and life-process rates in cyclostome (Class Stenolaemata) and cheilostome (Class Gymnolaemata) bryozoans

Zooids of cheilostome bryozoans are on average substantially more robust than are zooids of cyclostome bryozoans. The differences include greater number, length, and cross-sectional area of tentacles, plus a more extensively developed funiculus. Median values for mouth size and cilia- generated feeding current velocity are greater for cheilostomes than for cyclostomes so that cheilo- stomes have the potential for greater intake of nutrient energy per unit time, which may explain their apparently higher growth rates. For unit area of substrate occupied, the Cheilostomata (Class Gymnolaemata; members of the post-Paleozoic fauna) contain greater biomass and apparently gen- erate greater energy flow than do the Cyclostomata, which are the only extant order of the Class Stenolaemata (characteristic of the Paleozoic fauna).

Competitive interactions between related clades : evolutionary implications of overgrowth interactions between encrustin cyclostome and cheilostome bryozoans

Overgrowth interactions between encrusting cyclostome (eight species) and cheilostome (over 50 species) colonies were examined in subtidal communities in the northern Adriatic Sea off Rovinj, Croatia, in May–June 1988 and September–October 1990. Cheilostome colonies occupied ca. 80% of the available substrate space, whereas the cyclostome colonies occupied <5%. Out of 210 recorded interactions, cheilostomes overlapped cyclostomes in 164, while cyclostomes overlapped cheilostomes in only 16; 30 encounters resulted in mutual overlap or at least temporary growth termination along the line of contact. Most of the recorded interactions were for the cyclostomes Diplosolen obelia (Johnston) and Plagioecia patina (Lamarck). Both species elevated their colony margins on contact with a cheilostome in some instances, but D. obelia only overgrew competing cheilostomes in 12/120 encounters, and P. patina never prevailed. I propose that the cheilostomes have a key evolutionary innovation, lacking in cyclostomes, consisting of more rapid ontogenetic development of zooids along colony margins so that encrusting cheilostomes have substantially higher colony margins. This results in an outflow of filtered water along the edge of encrusting cheilostome colonies, while cyclostomes take water in at colony margins. These contrasting feeding currents apparently give a competitive advantage to the cheilostomes where colony margins approach and make contact with those of cyclostomes.

Evolution of Erect Marine Bryozoan Faunas: Repeated Success of Unilaminate Species

Twice, over era-scaled lengths of time, the relative diversity of branched unilaminate species among the erect bryozoan fauna increased by a factor of 10. In this duplicate natural experiment, a colonial form that offers the least resistance to the outflow of filtered water apparently replaced all the others. The data suggest long-term progressive evolution by an increase in the number of species and lineages that are hypothesized to be more hydrodynamically efficient in eliminating filtered water. Except for the Late Permian extinction event, the increase in the proportion of unilaminate species continued through, and perhaps was enhanced by, periods of mass extinction.

Functional interpretation of lyre-shaped Bryozoa

Colonies of Late Paleozoic lyre-shaped fenestrate bryozoans began growth on rigid substrates which were on the sea floor. After production of a basal disc, a fenestrate network developed in the shape of an erect bowed fan. The fenestrate fan was initially concave on the apertural side, but the curvature reversed a few millimeters above the base. Colonies were then separated along or just above the basal disc, and rested directly on the sea floor. The zooecial apertures were directed upward. The lateral margin of the fan grew to form the supporting struts of the soon-to-be lyre-shaped colony. An open vault formed below the upwardly bowed fan. Lyre-shaped colonies are interpreted to have lain on the sea floor so that each had the proximal end on the upcurrent side. Feeding zooids were on the upper surface and pulled water through their expanded tentacles. The filtered water passed through the fenestrules, into the vault, and out the open end of the vault, which was on the downcurrent side.

Taphonomic effects and preserved overgrowth relationships among encrusting marine organisms

Overgrowth relationships in the fossil record of encrusting organisms on marine hard substrata have been used to infer success in competitive interactions, particularly for modular organisms such as colonial invertebrates and coralline algae. However, this interpretation has been questioned to varying extenteven by those who have used the procedure-because of the possibility that any individual observation may represent growth over a senescent organism or dead skeletal remains. Where sufficient numbers of overgrowth relationships are available to demonstrate that the proportions of overgrowths between two modular taxa are not essentially equal, the competitive superiority of the more frequently overgrowing taxon should be accepted. The degree of success of the more frequently overgrowing taxon was at least as great as seen in the fossil record and was probably substantially greater, depending upon the percent of bare surface on the substratum. RESEARCH LETTERS 279

Historical Record of Erect Bryozoan Growth Forms

Information compiled from literature on marine bryozoans suggests that erect taxa have had a limited range of repeatedly evolved growth forms, each with characteristic branch widths or surface morphology, through the nearly 500 Ma history of the phylum. The range of erect growth forms among the Bryozoa has been limited to six (with only rare exceptions) that have co-occurred constantly throughout the history of the phylum. Ciliated tentacles of bryozoan lophophores are located just above the surface of colonies and draw a flow of food-bearing water. Outflow of the filtered water is confined to the space between the tentacles and the colony surface, until some point of escape is reached. In almost all erect bryozoans, lophophores are closely spaced and, in extant species, feed simultaneously. Therefore, for broad surface areas to develop, colony margins must be supplemented by other points of outflow (chimneys), commonly situated on surface disruptions, to keep local volume and velocity within manageable bounds. Presence of chimneys apparently permits branch widths to increase beyond certain size limits into less tightly constrained morphospace, and branch widths have been narrowly limited in the various growth forms except where regularly spaced chimneys can be demonstrated or reasonably inferred. It is hypothesized that the observed limits on form, size and surface morphology are derived primarily from the phylogenetic constraint of byrozoan lophophore size and attendant limitations on flow of filtered water across colony surfaces.