The textbook provides an example for the use of statistical methods in phylogeny inference in mammals. According to the Theria hypothesis, placental mammals (Eutheria) and marsupials (Metatheria) are sister groups, with monotremes (Prototheria) more distantly related. By contrast, the Marsupionta hypothesis places the marsupials and monotremes as sister groups, with the eutherian mammals more distantly related. Keith Killian and his coworkers used the DNA sequence data to test these hypotheses (Killian et al. 2001). What was their reasoning