Stocks fell by mid-afternoon Monday as a slide in crude oil prices picked up speed.

The S&P 500 was down 0.68%, the Dow Jones Industrial Average fell 0.41%, and the Nasdaq slid 0.6%.

Crude oil prices continued to spiral lower on Monday after a 9% jump to end last week. West Texas Intermediate crude oil fell 4.7% to $30.67 a barrel after spending the morning trading above $31. 

The turn lower on Monday was tied to comments from Saudi Arabia's state oil giant, Saudi Aramco, suggesting the company is continuing to invest in oil and gas projects despite global oversupply. Saudi Arabia has some of the lowest-costing oil in the world and production could be economically viable at as low as $15 a barrel.

The energy sector was the worst performer on markets Monday. Exxon Mobil (XOM) , Chevron (CVX) , BP (BP) and Schlumberger (SLB)  tradded lower, while the Energy Select Sector SPDR ETF (XLE) fell 3.2%.

Stocks closed out Friday with their first weekly gain in a month as a surge in crude oil prices triggered a rally in the energy sector and bets of more stimulus out of the eurozone boosted global markets. Crude enjoyed its best one-day gain since August.

McDonald's (MCD) shares traded near record highs after a better-than-expected end to 2015. The fast food chain reported fourth-quarter same-store sales up 5%, above estimates of 3.2%, while profit jumped 16% to $1.31 a share.

SunEdison (SUNE) climbed on reports David Einhorn's Greenlight Capital will appoint a director to the solar company's board. The company has suffered a slumping stock price and a number of C-suite departures recently. An announcement could be made this week. 

Yahoo!  (YHOO) shares were active after Verizon (VZ)  reportedly placed a bid for the company's core assets. Verizon has offered $8 billion for Yahoo!'s core assets, sources told New York Post reporter Josh Kosman, the publication reported.

Kimberly-Clark (KMB) fell 2.7% after a disappointing quarter. The company earned $1.42 a share, a penny short of estimates, while revenue of $4.5 billion missed forecasts of $4.6 billion. The consumer goods company expects 2016 sales to fall by as much as 3%.

Tyco International (TYC) jumped 10.4% after Johnson Controls (JCI) confirmed a merger. Johnson Controls shareholders will own 56% of the combined company and will receive $3.9 billion as part of the merger. Following the merger, the combined company will call Tyco's Irish headquarters home.

Caterpillar (CAT) shares fell 4.4% after Goldman Sachs downgraded the stock to to sell from neutral. Analysts forecast "sustained lower returns" on capital on weaker global infrastructure spending. Its price target was cut to $51 from $67.

Twitter ( TWTR) slid more than 3% after CEO Jack Dorsey confirmed the departure of four key executives late Sunday, including vice president of media Katie Stanton, vice president of product Kevin Weil and vice president of of engineering Alex Roetter. Shares have been under pressure for months as the social network fails to expand beyond its current user base.

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"We expect news of these departures will likely be met with skepticism in the market, at least in the short term, as investors continue to view Twitter as in a constant state of flux and unrest," Cramer wrote in a recent note for Action Alerts Plus. "Today's news will likely overshadow -- a least for the time being -- the numerous product innovations Dorsey and his team have implemented over the past several months."

D.R. Horton (DHI) shares fell 4% despite a better-than-expected quarter. The homebuilder earned 42 cents a share in its first quarter, a penny above estimates, while sales surged 12% to $2.4 billion, as analysts had expected.

Halliburton (HAL) fell 2.2% after swinging to a quarterly loss amid a severe downturn in the commodities market. The oilfield services provider said its bottom line was hit by severance payouts and asset write-offs during the quarter.

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