Coloured this sketch of Bikini Timeline too.
But there will also be second Topless timeline, so topless mermaids aren't gone, don't worry!

It is a prequel to this picture
https://www.furaffinity.net/view/47552784/
which marks the exact moment when Bikini Era began.Catherine Brown was one of the surface girls who was transformed into mermaid by mermaid Princess Agatha.
It took her some time to adapt to mermaid life but one moment that gave Catherine most discomfort was... toplessness
https://www.furaffinity.net/view/29209796/
All mermaids were topless, and Cathy felt uncomfortable swimming with exposed breasts as well as always seeing bare breasts around. She got to used to it somehow eventually but after starfish accidently stuck to her tit once
https://www.furaffinity.net/view/36569956/
she soon remembered how much does she miss covering her bust. Cathy took her bikini top from her surface home and also decided to try to convince some other mermaids to try wearing such cloth too. A bit surprising but Princess Agatha herself got cuirious by this idea. She didn't want to wear tops, but wanted to learn why does Catherine like them so much. So princess asked her if she could bring her a bra to try it on. Agatha only wanted to put it on once out of curiosity and then put it away. At least that's how she thought it would be...CATHERINE: "Your Highness?"
AGATHA: "Oh, hi Catherine! Swim in!"
CATHERINE: "Thanks! I know your photo session begins soon, so if..."
AGATHA: "Oh, don't worry, I have plenty of time before it. Also, please, not need to be so formal. Call me just "Agatha" or at least "Princess Agatha", but not "Your Highness".^^"
CATHERINE: "Okay, Princess Agatha!^^"
AGATHA: "That's better! So what's on your mind?"
CATHERINE: "Oh, I brought what you asked me about." (gives her bikini top)
AGATHA: (takes it)"Aha, so here is this thing you put on your chests for some reason... hmm.... so how do I use it?"
CATHERINE: "I'll help you!" (helps Agatha put bikini on and ties it behind your back and neck)
AGATHA: "Hmm..."
CATHERINE: "So? What do you think?"
AGATHA: "Hmm, it feels... feels... surprisingly pleasant! Now let's see how I look..." (swims towards mirror and looks into it. Then she "stands" as if frozen)
CATHERINE: "Agatha?"
AGATHA: "WOW!!! I look amazing!!!"
CATHERINE: "See? I knew you'll love it!^^"
AGATHA: "I do!!! Oh my gosh, I looks so gorgeous in it!!!"
CATHERINE: "Here, I took something else for you" (gives princess magazine)
AGATHA: "What is it?"
CATHERINE: "Bikini catalogue", made of water-proof paper!"
AGATHA: "Catalogue?! You mean there are many types of bikinis?!"
CATHERINE: "More than you think! My friend Fiona has dozen sexy bikinis back home on the surface - she likes to seduce guys with them."
AGATHA: (turns the pages of catalogue, getting more and more excited) "Why didn't we think of decorating our breasts like this..."
CATHERINE: "What did you say?!" (looks excited too now)
SERVANT: (from another room) "Princess Agatha, photographer is here!"
AGATHA: "Oh, it is time then." (with a sad face she prepares to take off bikini)
CATHERINE: "Agatha, wait!"
AGATHA: (stops)"Yes?"
CATHERINE: "Maybe I sound bold, but... why don't go to photo session like this?"
AGATHA: (sly smile appears on he face) "I like the way you are thinking, Catherine. Let's shock my subjects a little!" (swims to photo session, still wearing bra)
CATHERINE: "I can't believe it really happened!:-D"That's how it began! Agatha and Catherine couldn't imagine that this photo session will become a turning point in the history of merfolk that will change their life forever. Soon bare-breasted mermaids will become a thing of the past and brand new Bikini Era will arrive!