* Markets volatile as investors look to Spain
* PetSmart to replace Sunoco in the S&P 500 index
* Mosaic shares tumble after weak earnings, sales
* Indexes down: Dow 0.6 pct, S&P 0.3 pct, Nasdaq 0.1 pct
By Ryan Vlastelica
NEW YORK, Oct 2 U.S. stocks fell on Tuesday after opening higher as uncertainty surrounded Spain's financial bailout and investors worried about disappointing third-quarter U.S. earnings.
The Dow was pressured by stocks closely tied to the pace of growth, including Caterpillar Inc and Boeing Co. A major headwind for the global economy has been falling demand from Europe, which has been drifting toward recession.
Spain was said to be ready to ask the euro zone for help, but Germany has signaled that Madrid should hold off on making its request, according to European officials on Monday.
"Spain being rescued would be good for risk assets and ultimately global growth, but while the benefits are largely priced in, we're still getting conflicting signals that understandably have investors apprehensive," said Brian Barish, president of Cambiar Investors LLC in Denver.
The S&P rose nearly 6 percent in the third quarter, lifted by accommodative actions by the U.S. Federal Reserve and European Central Bank, which market participants bet would boost flagging growth.
If Madrid were to seek a rescue, it would trigger European Central Bank buying of its bonds and help to ease U.S. investors' nervousness about the euro zone's impact on the U.S. economy.
"Until we get some kind of clarity, we should expect a lot of volatility and difficulty holding onto gains," said Barish, who helps oversee $7 billion in assets.
The Dow Jones industrial average was down 81.90 points, or 0.61 percent, at 13,433.21. The Standard & Poor's 500 Index was down 3.73 points, or 0.26 percent, at 1,440.76. The Nasdaq Composite Index was down 1.92 points, or 0.06 percent, at 3,111.61.
The Dow and S&P 500 extended losses and the Nasdaq turned negative, with weaker-than-expected results from Mosaic adding to worries about the upcoming third-quarter profit period.
Mosaic tumbled 4.7 percent to $55.27 as the S&P's biggest percentage decliner after reporting earnings and revenue that missed expectations.
JPMorgan Chase & Co was sued by New York's attorney general on Monday in the first action to come out of a working group created by President Barack Obama to go after wrongdoing that led to the financial crisis. The civil fraud lawsuit centered on mortgage-backed securities packaged and sold by Bear Stearns, which JP Morgan acquired.
JPMorgan shares edged down 0.8 percent to $40.65.
Major car companies reported September sales, with General Motors and Chrysler Group LLC posting gains while Ford Motor Co remained flat.
GM, the largest U.S. automaker, reported a 1.5 percent increase, while Ford reportred sales on par with its results from a year earlier. Chrysler showed a 12 percent jump in sales.
PetSmart Inc shares added 2.4 percent to $69.15 after S&P Dow Jones Indices said after Monday's close the company will replace Sunoco in the S&P 500 on Oct. 4.
Hedge fund manager David Einhorn again made critical comments about Green Mountain Coffee Roasters Inc, a stock in which he publicly unveiled a short position a year ago, but the stock rallied 1.9 percent to $23.77 as one of the biggest gainers on the Nasdaq 100.
Shares of ArQule Inc tumbled 57 percent to $2.14 after the biotechnology company and Japan's Daiichi Sankyo said they will end a late-stage trial of their experimental lung cancer drug.