FAMILY ORIGIN AND RELATIVES
Influenced by the earlier German researchers (e.g., Hayek, 1911; Schulz, 1936;
Janchen, 1942), some authors (e.g., Al-Shehbaz, 1973; Hauser & Crovello, 1982;
Takhtajan, 1997) believed that the Brassicaceae evolved in western North America.
Their evidence was based on the superficial similarities in floral morphology between
some species of Cleome Linnaeus (Cleomaceae) and the mustard tribe Thelypodieae.
However, this view was abandoned some 15 years ago following the first molecular work
on the family by Price et al. (1994) and the support it received in subsequent studies
(e.g., Hall et al., 2002; Koch, 2003; Koch et al., 2003; Beilstein et al., 2006, 2008;
Bailey et al., 2006; Franzke et al., 2009; Koch and Al-Shehbaz, 2009). The present belief
is that the family originated in the Old World, especially in Southwest Asia, where
Aethionema W. T. Aiton, the genus sister to the rest of the family, is centered and w
here the highest diversity of taxa is found.
There is a lack of agreement among students of the Brassicaceae about the age of the
split between Aethionema and remaining mustards and the split between the family
and sister Cleomaceae from their common ancestor. The lowest age estimates of 19 million
years ago (mya) were given by Franzke et al. (2009), but higher ages were calculated by
other scientists, including 40−50 mya by Koch et al. (2000, 2001), 20−40 mya
by Henry et al. (2006), 34 mya by Schranz & Mitchell-Olds (2006), and 37.6 mya
(24.2−49.4) by Couvreur et al. (2009). While the last age estimate included the
Capparalean fossil Dressiantha bicarpellata (ca. 89.3 mya; Gandolfo et al., 1998),
reliable fossil records of the Brassicaceae are rather rare. Nevertheless, some fossil
records, including the fruits of Thlaspi primaevum (ca. 29 mya) and others, are
needed to obtain a more accurate age of origin and for other divergences within the