Archaeocyathids

1. Archaeocyathids • Early – Middle Cambrian, marine. Shallow current-washed water, high tolerance to turbidity. Passive filterer? Cambrian reef builders, symbiotic algae?

2. D’Arcy Wentworth Thompson 1860-1948 Scottish naturalist, mathematician and classicist. Wrote “On Growth and Form” (1917) pointing out that the morphology of organisms was related to the mechanisms of growth required to achieve the form and the physical forces acting on the organism. Initially viewed his approach as an alternative to Darwinian selection. Now seen as aspects of Darwinian fitness.

3. Of Size and Small Things • Physical forces do not scale linearly. Surface area and strength usually scale as the square of size increase, mass and metabolic demand increase as the cube of size. • Small, single-celled organisms exist at a scale where gravity is relatively unimportant, but surface forces and viscosity are primary • This is why most cells are spherical – surface tension • This is why most simple aggregations of cells are like chains of beads • Shells, tests, etc. can be viewed as responses to the dominant forces at these scales, operating over the organism’s life span.

4. Sponge Spicules • Complex morphology often viewed as indicating phylogeny • D’Arcy Thompson saw them as defined by boundaries between geometric packing of spherical bodies. • Suggests that spicule morphology may be a secondary effect of colonial growth and not as closely tied to heredity as thought

6. Holistic approach and Transform Grids • D’Arcy Thompson also became famous for suggesting studying the whole form using transform grids – an approach not really feasible until the latter 20th century