Bryozoans in transition: The depauperate and patchy Jurassic biota
Introduction
Bryozoans are among the best-represented macroinvertebrates in the fossil record, reflecting the presence of a calcareous skeleton in the great majority of species. The pattern of changing bryozoan diversity through the Phanerozoic closely parallels that of marine invertebrates in general, as reflected by the similarity between the bryozoan family diversity curve (e.g. Taylor and Larwood, 1990) and the iconic Sepkoski marine invertebrate family curve (e.g. Sepkoski, 1981). Particularly striking features of the bryozoan family pattern are the rapid climb in diversity during the Ordovician to a level that is maintained as a plateau through most of the Palaeozoic, followed by a dramatic decline at the end of this era and renewed diversification in the post-Palaeozoic, scarcely perturbed by the KT event, that culminates in the diversity high of the Neogene. Available data on species numbers through time show a very similar pattern (Horowitz and Pachut, 2000, Fig. 1).
Recovery from the Permian mass extinction occurred more slowly in bryozoans than in many other invertebrate groups. Indeed, it took until the Late Cretaceous for the family-level diversity of the Palaeozoic to be regained. Both the Triassic and the Jurassic contain depauperate bryozoan biotas. Triassic bryozoans are considered to be particularly scarce and there has been a tendency to describe all newly discovered examples (Nakrem and Mørk, 1991, Schäfer et al., 2003a, Schäfer et al., 2003b, Zágorsek, 1993). Most Triassic bryozoans belong to Palaeozoic ‘holdover taxa’, especially trepostomes (see Schäfer and Fois, 1987). While there exist several papers reviewing Triassic bryozoans (Bizzarini and Braga, 1982, Morozova, 1969, Sakagami, 1985, Schäfer and Fois, 1987, Schäfer, 1994), Jurassic bryozoans have been conspicuously neglected. Nonetheless, the Jurassic was an important time in bryozoan evolution, witnessing a radiation of cyclostomes as well as the first appearance of cheilostomes, the bryozoan order that dominates at the present day.
This paper reviews the bryozoan biota of the Jurassic. We have compiled data from the literature and our own unpublished research records to document genus- and species-level patterns of bryozoan diversity through the stages of the Jurassic, assemblage species richness, the geographical distribution of Jurassic bryofaunas, and aspects of bryozoan palaeoecology. One hope is that this review will increase the awareness of Jurassic bryozoans among geologists and palaeontologists. The need for this is evident from the high incidence in the literature of inexact identifications of bryozoans as well as misidentification of other groups (especially sponges) as bryozoans. First, however, we describe the history of research and review the taxonomic composition of the Jurassic bryozoan biota.
Section snippets
History of research
Lamouroux (1821) was the first scientist formally to name bryozoan species from the Jurassic. Based on collections from the Bathonian of Normandy, he described 23 new species of ‘polypiers’ that subsequently became regarded as bryozoans. However, three of these species are now known to be sponges and one is an Upper Cretaceous bryozoan (Walter, 1970, pp. 214–215). Several of Lamouroux's Jurassic taxa are the type species of genera, either introduced by Lamouroux himself (Apsendesia, Berenicea,
Taxonomic composition of the Jurassic bryozoan biota
Three orders of marine bryozoans are represented unequivocally in the Jurassic: Cyclostomata, Cheilostomata and Ctenostomata. There are also records of statoblasts produced by the freshwater class Phylactolaemata, a dubious record of the Palaeozoic order Fenestrata, a trepostome and some ‘pseudobryozoans’.
Generic and specific diversity changes through the Jurassic
Critical appraisal of the literature on Jurassic bryozoans, together with unpublished records of occurrences, has allowed the assembly of a database of species occurrences at stage-level resolution. In total, this database contains 172 species, compared with the diversity estimate of 202 species quoted for the Jurassic by Horowitz and Pachut (2000, Table 1). Most species (102; 60%) in our database are known from only a single stratigraphical stage. However, 29 species (17%) occur in two stages,
Biogeography
Anecdotal observations suggest that Jurassic bryozoans are rare outside northern Europe. Most of the formally named species have been described using material from France, England or Germany (see historical review above), with just a few taxa based on Polish (Hara and Taylor, 1996, Reuss, 1867), Spanish (Taylor and Sequeiros, 1982) or Russian (Gerasimov, 1955, Viskova, 2006) material. Additional European records of Jurassic bryozoans have been made in Spain (Higazi, 1985, López, 1987, Reolid
Colony-forms
Several different schemes exist for classifying colony-forms in bryozoans, varying in the degree of subdivision as well as the terminology employed (Hageman et al., 1998, Nelson et al., 1988, Schopf, 1969). For the purposes of the current analysis of colony-forms in Jurassic bryozoans, a coarse subdivision is here adopted, based on the generalized geometric shapes recognised in benthic colonial animals by Jackson (1979). Colonies of Jurassic species can be classified into six main shape
Palaeoecology
As epibenthic suspension feeders, bryozoans in the Jurassic were dependent on hard or firm substrates for attachment, encrustation or boring, and a phytoplanktonic food resource, like their counterparts at the present day. While the details of the trophic preferences of bryozoans in the Jurassic are unknown, some information is available on the identity of the substrates they colonized, as well as biotic interactions with living substrates and other organisms.
Many Jurassic bryozoans are
Jurassic bryozoans and the Mesozoic Marine Revolution
The Mesozoic Marine Revolution (MMR) is a hypothesis of evolutionary changes driven by escalating biotic interactions, especially predation but also competition (Vermeij, 1977, Vermeij, 1987, Vermeij, 2008). Although the MMR intensified in the Cretaceous, it can be traced back to the Late Triassic, and Aberhan et al. (2006) provided evidence that it had an impact on Jurassic benthic animals. Is there any evidence for the impact of the MMR in Jurassic bryozoans? This question can be approached
Conclusions
- 1.
Bryozoans were very slow to recover from the Permian and Triassic mass extinctions and the known Jurassic bryozoan fossil record comprises fewer than 200 species (172 species are included in our database). This is significantly greater than the diversity of the Triassic but an order of magnitude less than that of the Cretaceous (Horowitz and Pachut, 2000).
- 2.
The rate of description of new Jurassic bryozoan species has continued to climb steadily over the last two hundred years, showing no sign of
Acknowledgements
This paper arose from presentations given at the 4th Polish Jurassica Symposium held in September 2004 at Baltów, as well as the Annual Meeting of the Geological Society of America held in October 2005 at Salt Lake City. Participation of PDT in the latter was made possible by a grant from the Royal Society. Rachel Prebble assisted with data compilation. The German Research Foundation (DFG) is thanked for the financial support to AE under DFG-Projekt Scha 355/22-1 “Radiation of bryozoans in
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