Eocoelia was a brachiopod about a centimeter long.
The data is from fossils gathered at thirteen different depths. As is usual in geology, the diagram gives the data for the deepest (oldest) fossils at the bottom, and the upper (youngest) fossils at the top. The diagram covers about ten million years.
Each shell is ribbed. For each shell, the ratio of rib height to rib width was measured. For each depth, the average ratio was computed. Each little horizontal box shows the average, and also the statistical distribution about the average.
Notice that the boxes at the top have no overlap at all with the boxes at the bottom. That is why the shells at the top are the species Eocoelia sulcata, but the shells at the bottom are the species Eocoelia hemisphaerica.
Notice that each box overlaps the box above it, and the box below it. As you go from bottom to top, there are no breaks or sudden jumps. There is a smooth transition that starts at one species and ends at a different species.
Eocoelia is a small brachiopod found in lower Silurian rocks, specifically the Upper Llandovery and lower Wenlock. The animal didn't fossilize, but its shell is found world-wide, including Britain, Nova Scotia, Pennsylvania, Iowa, Siberia, Norway, and South America. In several of these locations, a succession of 4 species has been recognized. That succession, shown in the diagram, is found consistently over all of North America and Europe.
Brachiopods appeared in the early Cambrian, almost 600 million years ago. They are still with us, although the number of orders slimmed down a bit during the Jurassic. The diagram goes from 415 million years ago to 405 million years ago.
The data is from:
Zeigler, A.M., 1966. The Silurian Brachiopod Eocoelia hemisphaerica (J. de C. Sowerby) and related species. Palaeontology, v.9, part 4, p.523-543.and see also
Johnson, M.E., 1979. Evolutionary brachiopod lineages from the Llandovery series of eastern Iowa. Palaeontology, v.22, part 3, p.549-567.
Baarli, B.G., 1986. A biometric re-evaluation of the Silurian brachiopod lineage Stricklandia lens/S. laevis. Palaeontology, v.29, part 1, p.187-205.