The Precambrian-Cambrian Boundary (PCB) was recently interpreted within the Ara Group of the Huqf Supergroup of Oman (Figures 1 and 2), and dated at 542.0 million years before present (Ma) ± 0.3 million years (my) (Amthor et al., 2003; Geology, May 2003, p. 431–434). The Ara Group is believed to be coeval with the Hormuz Series salt of the Arabian Peninsula and southwest Iran (Figure 1). In just over two decades, the estimated age of the global PCB has changed by almost 50 million years: 590 Ma (Harland et al., 1982), 570 Ma (Harland et al., 1991), and 545 Ma (Gradstein and Ogg, 1996). This new PCB age estimate from Oman is close to other estimates in Siberia (older than 543.8 + 5.1 to -1.3 Ma) and Namibia (younger than 543.3 ± 1 Ma).