Bryozoan skeletons: from crystallites to limestones
Bryozoans are a diverse phylum of invertebrates with a rich but poorly investigated fossil history. All bryozoan species are suspension feeders and colonial in habit. Although colonies may be up to a metre in diameter, the asexually produced individuals - zooids - are of millimetric proportions. The great majority of bryozoans live in the sea and secrete hard skeletons composed of the calcium carbonate minerals calcite and aragonite. These skeletons account for the good fossil record of the phylum, provide the characters used for recognising both fossil and living species, and can be present in such great abundance that their remains form limestones. The aim of this web page is to introduce some approaches, based largely on research co-ordinated by Dr Paul Taylor in the Department of Palaeontology at The Natural History Museum, for investigating bryozoan skeletons, from the smallest scale of crystallite ultrastructure to the largest scale of global distribution of bryozoan limestones.
Skeletal ultrastructure
Use of a high resolution scanning electron microscope is required to study the minute crystallites which are the fundamental constituents of bryozoan skeletons. These crystallites vary in shape, arrangement and crystallographic orientation. Several skeletal fabric types can be recognised, with individual walls often comprising a succession of these fabrics. Among cyclostome bryozoans, the calcitic skeleton is usually lamellar, consisting of tabular or lath-like crystallites stacked like tiles at a low angle to the wall surface. Cheilostome bryozoans may exhibit a similar ultrastructure but more commonly have fibrous skeletons consisting of needle-like or bladed crystallites oriented almost perpendicular to wall surfaces.
High magnification scanning electron micrograph of the minute crystallites forming the skeleton of the cyclostome bryozoan Cinctipora from New Zealand.
Although the
range of crystallite fabrics found in bryozoans and their
taxonomic distributions are now much better understood than
they were a decade ago (see references below), much
remains to be learnt about processes of biomineralisation
in bryozoans. For example, how does the animal
control crystallite growth and fabric type, and what is the
functional significance of different ultrastructural fabrics?
Taylor, P. D. and Weedon, M. J. 2000. Skeletal ultrastructure
and phylogeny of cyclostome bryozoans. Zoological
Journal of the Linnean Society, 128, 337-399.
Weedon, M. J. and Taylor, P. D. 2000. Skeletal ultrastructure
in primitive cheilostome bryozoans. 400-41. In:
Herrera Cubilla, A. and Jackson, J. B. C. (eds) Proceedings
of the 11th International Bryozoology Association Conference.
Smithsonian Tropical Research Institute, Balboa Republic of
Panama.
Zooidal microstructure
Microscopic examination of bryozoans is necessary for identifying species - the skeletons of the zooids which make up bryozoan colonies provide most of the morphological characters used in the taxonomy of both fossil and recent species. Although many of these features can be resolved using a hand lens or a low-powered binocular microscope, an ever greater reliance nowadays is being placed on the scanning electron microscope to observe and record zooidal microstructure. Environmental SEM has proved particularly useful in permitting type and rare specimens to be studied without the usual SEM routine of gold-coating. More than 10,000 SEM images of fossil and Recent bryozoans are held at the NHM and these form a vital resource for taxonomic and other research.
Scanning electron micrograph showing developing zooidal skeletons of the cheilostome bryozoan Reptadeonella. Recent, Pacific Coast of Mexico.
For some cheilostome
bryozoans it has been shown by Alan Cheetham and Jeremy Jackson
that small differences in skeletal morphology correspond exactly
with genetically distinct species. This knowledge is
leading to a re-evaluation of species formerly accorded erroneously
wide geographical and stratigraphical ranges. However,
a great many bryozoan species have yet to be studied using
SEM, and very few phylogenetic analyses have been attempted
using the information so gained.
Grischenko, A., Mawatari, S. F. and Taylor, P. D. 2000. Systematics
and phylogeny of the cheilostome bryozoan Doryporella.
Zoologica Scripta, 29, 247-264.
Colony structure
Colony structure varies considerably between bryozoan species. Some colonies grow as sheets or branching runners closely adpressed to a solid substratum such as a rock, a shell or a seaweed. Others grow erect from their substatum of attachment to form small tree-like or frondose colonies. In the modern 'lace coral' cheilostomes and in most Palaeozoic fenestrates, erect branches bifurcate and coalesce at intervals to give a regularly perforated colony form. Colony shape is fixed in some species but in others it varies between colonies.
Vertically fractured colony of the cyclostome bryozoan Meandropora from the Pliocene of Suffolk, England. The dome-shaped colony has long tubular zooidal skeletons arranged in bundles radiating from the origin (lower centre).
A basic understanding
of the functional morphology of bryozoan colony shapes has
emerged during the last few decades. Many colony structures
can now be interpreted in terms of different patterns of spatial
deployment of zooids, biomechanical properties, and the production
of efficient colony-wide feeding currents. One
of the most striking themes in bryozoan history has been the
convergent evolution of almost identical colony shapes by
different clades at different times. Analysis of colony
forms in fossil bryozoan assemblages has long promised to
be of value in palaeoenvironmental reconstruction but
much still remains to be learnt about the environmental factors
governing the distribution of different colony shapes in space
and time.
Taylor, P. D. 1999. Bryozoa. 623-646. In: Savazzi, E. (ed.). Functional morphology of the invertebrate skeleton. Wiley, Chichester, 706 pp.
Global distribution of bryozoan limestones
Bryozoans are major producers of biogenic carbonate sediments in shallow seas. For example, bryozoan-rich carbonate sediments blanket a considerable part of the continental shelves off southern Australia and to the south east of New Zealand. A striking feature of bryozoan-rich sediments forming at the present day is that they are almost entirely cool-water, non-tropical in distribution. Although bryozoans can be diverse in the tropics, including coral reef environments, they seldom contribute significant amounts of carbonate sediment. The reasons for this pattern are unclear but possible explanations include failure of bryozoans to compete with faster-growing corals and algae in the tropics, and increased predation pressure in the tropics preventing the growth of erect species cabable of generating significant quantities of carbonate.
Have bryozoan-rich deposits always been non-tropical? To answer this question a database has been compiled of the ages and palaeolatitudinal occurrences of bryozoan limestones, marls etc. Very different patterns are evident for Palaeozoic and post-Palaeozoic data, with the Palaeozoic showing a preponderance of bryozoan-rich deposits in the tropics whereas the post-Palaeozoic displays a modern pattern of non-tropical dominance. Reasons for this switch need investigating through a more detailed comparative analysis of facies in Palaeozoic and post-Palaeozoic bryozoan carbonates, and comparative studies of lataitudinal distributions through time in other carbonate producing organisms.
Palaeolatitudinal distribution of bryozoan-rich deposits from the Ordovician to the Holocene showing tropical concentration in the Palaeozoic (yellow region) and non-tropical distribution in the post-Palaeozoic (blue region).
Taylor, P. D. and Allison, P. A. 1998. Bryozoan carbonates in space and time. Geolog, 26, 459-462.
Paul D. Taylor, Department of Palaeontology