** U.S.-listed shares of Mereo BioPharma MREO.O MAH0y.F plunge 36.7% to $1.86 and Ultragenyx RARE.O slip 27.7% to $29.95 in extended trading

** Ultragenyx and Mereo say their late-stage trial of setrusumab for a type of genetic disorder would progress to final analysis by year-end

** Co was expected to give interim data from the study in mid-July

** J.P.Morgan analysts said statistical bar for interim analysis was high, and more time will benefit the trial

** Brokerage called the sell-off "a material overreaction," saying after-hours trading "reflects a trial failure scenario"

** An independent safety committee found the experimental drug continues to be well tolerated -RARE

** Co was testing the drug to treat osteogenesis imperfecta, a genetic disorder that makes bones very fragile and easy to break, often from little or no injury, because the body doesn't produce collagen properly

** YTD, RARE down 1.5%, MREO down 16%

(Reporting by Kamal Choudhury in Bengaluru)

((Kamal.Choudhury@thomsonreuters.com;))