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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 family.

 
 
 
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