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Department of Paleobiology

  • Alan H. Cheetham
  • Senior Scientist, Emeritus
  • Phone:   202-633-1352 (JoAnn Sanner)
  • Fax:   202-786-2832
  • E-mail Address:   cheethamat SI dot edu
  • Mailing Address:
    Smithsonian Institution
    PO Box 37012, MRC 121
    Washington, DC 20013-7012
  • Shipping Address:
    Smithsonian Institution
    National Museum of Natural History
    10th & Constitution NW
    Washington, DC 20560-0121


Ph.D. Columbia University, 1959
M.S. Louisiana State University, 1952
B.S. New Mexico Institute of Mining and Technology, 1950

Research Interests

The evolutionary model known as punctuated equilibria posits that change is concentrated in the geologically brief intervals during which species originate, on the order of tens of thousands to hundreds of thousands of years, whereas the majority of a species' duration, typically extending for millions to tens of millions of years, is marked by little or no change. My research focuses on patterns of evolution in Cenozoic bryozoans at the finest scales of morphological and temporal resolution in order to test for consistency with the punctuated equilibria model versus the alternative model of phyletic gradualism, in which change is cumulative through a species' duration. The data with which my associates and I have tested this idea so far have come principally from cheilostome bryozoans in the Neogene of the Dominican Republic, Panama, Costa Rica, and Venezuela.

A major question in using data from the fossil record to study detailed patterns of evolutionary change is the fidelity with which "species" based strictly on the preserved features of skeletons mirror true biological species based on genetic differences. To explore this question, we have performed breeding experiments with living counterparts of the fossil bryozoans and examined their molecular genetics. The results have allowed us to estimate the relative contributions of genetics and environment to bryozoan skeletal features, as well as to substantiate the biological significance of species based on skeletal morphology. In addition, the modular organization of bryozoans permits a partial separation of the genetic and environmental components of variation, based solely on morphology, that closely approximates the values obtained from breeding experiments. Thus, changes in genetic variation can be estimated directly from morphological data in fossil populations.

Modular organization also provides bryozoans with an alternative life-history strategy in which cloning (production of genetically identical individuals) may come to predominate over the sexual reproduction of larvae that is typical of the group as a whole. Broken pieces of an individual (called a colony), each consisting of one or a few of the component modules (called zooids), may regenerate to the original colony form by budding new zooids in the right places. We are investigating whether populations in which cloning becomes predominant show a reduction in the genetic component of variation in a variety of Neogene and Paleogene cheilostomes from tropical America and elsewhere.

Assistants, Current, and Former Post-doctoral Fellows and Students


  • Beth Okamura (1984-1985) Currently at University of Reading, UK
  • Erik Thomsen (1977-1980) Currently at University of Aarhus, Denmark
  • Judith E. Winston (1975-1976) Currently Director of Research, Virginia Museum of Natural History
  • William C. Banta (1969-1970) Currently Professor of Biology, American University


  • Scott Lidgard (1981-1983) Currently Associate Curator, Field Museum of Natural History, Chicago
  • Eckart Håkansson (1970-1972) Currently at University of Copenhagen, Denmark
  • Reginald J. Scolaro (1967-1968) Currently retired oil company geologist


  • Research Associate, Panama Paleontology Project (PPP), Smithsonian Tropical Research Institute
  • Contributor, Neogene Marine Biota of Tropical America (NMiTA), Department of Geology, University of Iowa


  • The Paleontological Society Medal, 2001
  • The Raymond C. Moore Medal for Excellence in Paleontology, SEPM-The Society for Sedimentary Geology, 1997
  • Distinguished Achievement Alumni Award, New Mexico Institute of Mining and Technology, 1990

Editorial Service

  • Associate Editor, Paleobiology, 1982-1984, 1986-1988


Cheetham, A. H., J. Sanner, and J. B. C. Jackson.  2007.  Metrarabdotos and Related Genera (Bryozoa: Cheilostomata) in the Late Paleogene and Neogene of Tropical America.  Paleontological Society Memoir 67, 96 p. Supplement to Journal of Paleontology 81(1).

Cheetham, A. H., J. Sanner, P. D. Taylor, and A. N. Ostrovsky.  2006. Morphological differentiation of avicularia and the proliferation of species in mid-Cretaceous Wilbertopora Cheetham, 1954 (Bryozoa: Cheilostomata). Journal of Paleontology 80:49-71.

Cheetham, A. H.  2002.  Asexual propagation in the cheilostomes Metrarabdotos and Coscinopleura: effects on genetic variation and larval productivity. P. 73-80 in P. N. Wyse Jackson, C. J. Buttler, and M. E. Spencer Jones (eds.), Bryozoan Studies 2001, Proceedings of the Twelfth International Bryozoology Association Conference. A. A. Balkema Publishers, Lisse, 409 p.

Cheetham, A. H. 2002.  The founding and early history of the International Bryozoology Association (1965-1974).  P. 45-57 in P. N. Wyse Jackson and M. E. Spenser Jones (eds.), Annals of Bryozoology: Aspects of the History of Research on Bryozoans. International Bryozoology Association, Dublin, 381 p.

Cheetham, A. H. 2002. Response by Alan H. Cheetham for the Paleontological Society Medal, November 2001. Journal of Paleontology 76:784-785.

Cheetham, A. H., J. B. C. Jackson, and J. Sanner.  2001.  Evolutionary significance of sexual and asexual modes of propagation in Neogene species of the bryozoan Metrarabdotos in tropical America. Journal of Paleontology 75:564-577.

Cheetham, A. H. 2.1.3 2001.  Evolutionary stasis versus change.  P. 137-142 in D. E. G. Briggs and P. R. Crowther (eds.), Palaeobiology II. Blackwell Science, Oxford, 583 p.

Cheetham, A. H., and J. B. C. Jackson.  2000.  Neogene history of cheilostome Bryozoa in tropical America. P. 1-16 in A. Herrera and J. B. C. Jackson (eds.), Proceedings of the 11th International Bryozoology Association Conference, Allen Press, Lawrence, KS, 448 p.

Taylor, P. D., and A. H. Cheetham.  2000.  Gilbert Powell Larwood (1930-1997). P. vii-ix in A. Herrera and J. B. C. Jackson (eds.), Proceedings of the 11th International Bryozoology Association Conference, Allen Press, Lawrence, KS, 448 p.

Cheetham, A. H., J. B. C. Jackson, J. Sanner, and Y. Ventocilla.  1999.  Neogene cheilostome Bryozoa of tropical America: comparison and contrast between the Central American isthmus (Panama, Costa Rica) and the north-central Caribbean (Dominican Republic). P. 159-192 in L. S. Collins and A. G. Coates (eds.), A Paleobiotic Survey of the Caribbean Faunas from the Neogene of the Isthmus of Panama. Bulletins of American Paleontology no. 357, 351 p.

Jackson, J. B. C., and A. H. Cheetham.  1999. Tempo and mode of speciation in the sea.  Trends in Ecology and Evolution 14:72-77.

Cheetham, A. H., and J. B. C. Jackson.  1998.  The fossil record of cheilostome Bryozoa in the Neogene and Quaternary of tropical America: adequacy for phylogenetic and evolutionary studies. P. 225-240 in Donovan, S. K., and C. Paul (eds.), The Adequacy of the Fossil Record.  John Wiley and Sons.

Cheetham, A. H.  1997.  Response to presentation of the Raymond C. Moore Medal for Excellence in Paleontology. Journal of Sedimentary Research 67:1111-1112.

Cheetham, A. H., and J. B. C. Jackson.  1996.  Speciation, extinction, and the decline of arborescent growth in Neogene and Quaternary Bryozoa of tropical America.  P. 205-233 in J. B. C. Jackson, A. F. Budd, and A. G. Coates (eds.), Evolution and Environment in Tropical America, 425 p.  University of Chicago Press.

Cheetham, A. H., and J. B. C. Jackson.  1995.  Process from pattern: tests for selection versus random change in punctuated bryozoan speciation.  P. 184-207 in D. H. Erwin and R. L. Anstey (eds.), New Approaches to Speciation in the Fossil Record, 342 p.  Columbia University Press.

Cheetham, A. H., J. B. C. Jackson, and L. C. Hayek.  1995.  Quantitative genetics of bryozoan phenotypic evolution. III. Phenotypic plasticity and the maintenance of genetic variation.  Evolution 49:290-296.

Cheetham, A. H., J. B. C. Jackson, and L. C. Hayek.  1994.  Quantitative genetics of bryozoan phenotypic evolution. II. Analysis of selection and random change in fossil species using reconstructed genetic parameters.  Evolution 48:360-375.

Jackson, J. B. C., and A. H. Cheetham.  1994.  Phylogeny reconstruction and the tempo of speciation in cheilostome Bryozoa.  Paleobiology 20:407-423.

Jackson, J., and A. Cheetham.  1994.  On the importance of nothing doing: an exhaustive study of tiny bryozoans supports the idea of punctuated equilibrium.  Natural History 6/94:56-59.

Cheetham, A. H., J. B. C. Jackson, and L. C. Hayek.  1993.  Quantitative genetics of bryozoan phenotypic evolution. I. Rate tests for random change versus selection in differentiation of living species.  Evolution 47:1526-1538.

Jackson, J. B. C., and A. H. Cheetham.  1991.  Bryozoan morphological and genetic correspondence: what does it prove? Reply.  Science 251:318-319.

Jackson, J. B. C., and A. H. Cheetham.  1990.  Evolutionary significance of morphospecies: a test with cheilostome Bryozoa.  Science 248:579-583.