Stratophenetic Study of Evolution in Turborotalia (Eocene Planktonic Foraminifera)

The genus Turborotalia includes several morphospecies of Eocene planktonic foaminifera that are commonly used in biostratigraphy. Since the 1950s, taxonomists have speculated that there is a chronocline beginning with relatively globular forms in the early middle Eocene (Turborotalia frontosa) and ending with keeled and compressed forms in the late Eocene (T. cerroazulensis and T. cocoaensis), although some speciation and extinction has also been postulated. To investigate this, total of 52 populations of 200 individuals were picked from ODP Site 865 (equatorial Pacific Ocean), the samples spanning about 12 million years. Populations were mounted in a standard orientation, photographed, and various measurements made on each specimen, including important lengths, angles and shell chirality. The large data set - over 10,000 curated specimens and 100,000 measurements – allowed population overlaps to be investigated and potentially rare morphotypes to be identified. It was necessary to develop novel clustering statistics to test whether populations consist of one or more morphoclusters. Adjacent populations were investigated to test the hypothesis of stratophenetic linkage (i.e. that one population is ancestral to the next). The data reveal a pattern of gradual evolution with two identified branching events and three extinctions. The hypothesis that early middle Eocene populations are directly ancestral to the upper Eocene ones cannot be rejected. Two branching points were identified, leading to morphospecies Turborotalia altispiroides and T. cunialensis respectively, and are interpreted as speciation events. The study demonstrates the power of stratophenetic approach to phylogeny reconstruction when large populations of fossils can be sampled continuously from long stratigraphic records.