Mineralogy meets Paleontology #2

Favosites

Photo of the Great Square Cave of central Malawi. Well, no -- the opening is only about 3 mm across.

This is one space in a tabulate coral colony, the corallite, where the individual coral polyp lived. The space is now partly filled with scalenohedral calcite crystals that developed long after the animal was alive.

The layers in one direction are the tabulae - tables - upon which they lived, and which gives the name to the extinct coral group of Tabulates. The perpendicular layers are the septa (from Latin for wall, hedge, enclosure) that separated each corallite from adjacent cells. I don't know for sure where this example of genus Favosites is from, but it is probably from the US Midwest and probably Carboniferous in age, about 330,000,000 years ago.

Entire specimen for the photomicrograph above, showing both sides.

The rock containing the fossil was limestone, and as water percolates though limestone some of the rock often dissolves and re-precipitates calcite in open spaces – cracks, geodes, and in this case, the original openings within the fossil coral.

Drawing from H.A. Nicholson, 1877, The Ancient Life History of the Earth: A Comprehensive Outline of the Principles and Leading Facts of Palæontological Science.

Favosites was widespread and successful, occurring abundantly around the world in the warm shallow seas that prevailed during much of the Paleozoic near most continents. In its dozens of species, the genus had a long run, beginning in the Late Ordovician about 455 million years ago and ending at (or perhaps slightly before) the end-Permian extinction 251,000,000 years ago.

Another view. Some of the corallites are filled with reddish, iron-oxide-stained calcareous mud called “terra rosa”, a typical weathering product of limestones in Indiana and elsewhere in the US Midwest.

Favosites is sometimes called “honeycomb coral” because the vertical stacked corallites have hexagonal (or near-hexagonal) cross-sections in plan view. Hexagonal shapes are common in nature (for example, in columnar basalt, mud cracks, and bees’ honeycombs) in part because they yield a closest-packing geometry, a good volume-to-wall length ratio. Favosites is one of the best examples in the fossil world.

Top view of the specimen. Individual corallite “cells” are about 3 mm across.