Dinosaur Jr. is an American rock band formed in Amherst, Massachusetts, in 1984, originally simply called Dinosaur until legal issues forced a change in name. The band was founded by J Mascis (guitar, vocals, primary songwriter), Lou Barlow (bass, vocals), and Murph (drums). After three albums on independent labels earned the band a reputation as one of the formative influences on American alternative rock, creative tension led to Mascis firing Barlow, who later formed Sebadoh and Folk Implosion. His replacement, Mike Johnson came aboard for three major-label albums.
Formation
Mascis and Barlow played together, on drums and guitar respectively, in the hardcore punk band Deep Wound, formed in 1982 while the pair were attending high school in western Massachusetts. After high school, they began exploring slower yet still aggressive music such as Black Sabbath, the Replacements, and Neil Young. Mascis' college friend Gerard Cosloy introduced him to psychedelic-influenced pop bands like Dream Syndicate, which Mascis in turn showed to Barlow. Barlow explained, "We loved speed metal...and we loved wimpy-jangly stuff".  Deep Wound broke up in mid-1984. Cosloy had dropped out of the University of Massachusetts Amherst to focus on running his independent record label, Homestead Records, and promised Mascis that if he were to make a record Homestead would release it. Mascis wrote a number of songs by himself and showed them to Barlow, to whom he offered the bassist position. Barlow said the songs "were fucking brilliant...They were so far beyond. I was still into two-chord songs and basic stuff like 'I'm so sad.' While I was really into my own little tragedy, J was operating in this whole other panorama." Mascis enlisted vocalist Charlie Nakajima, also formerly of Deep Wound, and drummer Emmett Patrick Murphy, otherwise known as Murph, to complete the band. Mascis explained the concept behind the group as "ear-bleeding country".  The band was initially named Mogo, and played their first show on University of Massachusetts Amherst campus in the first week of September 1984. However, Nakajima used the performance to launch an extended anti-police tirade. Mascis was so appalled by Nakajima's behavior at the show that he disbanded the group the next day. A few days later Mascis invited Barlow and Murph to form a new band without telling Nakajima. "I was kind of too wimpy to kick him out, exactly," Mascis later admitted. "Communicating with people has been a constant problem in the band." The trio named themselves Dinosaur, and Mascis and Barlow took over lead-vocal duties.

Q: When did the band form?
A: September 1984.

Q: What was significant about the formation?
A: The band was initially named Mogo, and played their first show on University of Massachusetts Amherst campus
Why did they change the name?
Mascis was so appalled by Nakajima's behavior at the show that he disbanded the group the next day.
Mascis was so appalled by Nakajima's behavior at the show that he disbanded the group the next day.