Spineless Wonders of Evolution 197years old) rocks along the Gulf Coast of Texas, Louisiana, Mississippi, and Alabama (fig. 8.11).
They evolved from a simple nonornamented shell of Volutocorbis limopsis but quickly branched
into several very different lineages. In the Wilcox beds, there is the huge, broad shelled Athleta
tuomeyi, as well as the more normal lineage of Athleta petrosa. In the Weches beds, there are
side branches to the nonornamented Athleta dalli and the more bulbous Athleta lisbonensis. The
main Athleta lineage concludes in the upper Eocene Lisbon beds with the heavily ornamented
Athleta petrosa symmetrica.
Or how about everyone’s favorite fossils, the trilobites? Many different studies have been
published on their patterns of evolution. In most cases, they show trends that are due to very
subtle changes, such as the changing number of ribs in the thorax (fig. 8.12), as documented
by Peter Sheldon (1987) on a 3-million-year-long sequence of over 15,000 specimens from
eight trilobite lineages from the Ordovician of central Wales. Some lineages, such as Ogyginus,
show almost no net change in rib number (stasis), while the nileids, Ogygiocarella, and espe-
cially Nobiliasaphus show a dramatic increase in rib number over the interval. This example
FIGURE 8.11. Evolution in the marine snail lineage Athleta from the Eocene beds of the Gulf Coastal Plain. (From
Rodda and Fisher 1964; courtesy of the Society for the Study of Evolution)
MIDWAYSABINECLAIBORNE
(Fossiliferous Units)JACKSONAthleta petrosa
symmetricaAthleta petrosa
petrosaAthleta
dalliAthleta
lisbonensisAthleta petrosa
n. subsp.Athleta
tuomeyiVolutocorbis limopsisWilcoxReklawWechesCook Mountainand Stone City