The endlessly repeated "there are no transitional forms" mantra of YECs is an interesting study. First, the term "transitional form" is a slippery YEC term that YECs define in such a way as to make it impossible to ever demonstrate (even if we were just talking about human history and your own family tree!). But that aside, what I find interesting is how this YEC mantra persists regardless of how many "transitional" forms are found; which begs the question of how many "transitional" forms will it take to make a YEC happy before they're willing to acknowledge, "Okay, maybe, just maybe, those things might be related to each other"? And the answer, of course, is that no amount of "transitional" forms will ever be enough, because they've already pre-decided (before any evidence) that they are not related, no-way, no-how, no matter what.
The example that comes to mind is the origin of tetrapods (incorrectly called the "fish-to-amphibian" transition by YECs). And specifically I recall a picture in YEC Duane Gish's 1970s/1980s book "Evolution: the Fossils Say No!" that criticized the "lack of transitional/intermediate forms" between fish and tetrapods:
"THERE ARE NO TRANSITIONAL FORMS!"
The Gish picture (figure 1) is still used by YECs today (figure 1 caption) to say "look, fully formed fins & feet with no intermediate transitional forms in between!"
Of course, Ichthyostega (discovered in 1932) was already old news, and before the days of the Internet information was harder to find, so perhaps Gish didn't know about Acanthostega (discovered 1952) which had characters intermediate between fish and Ichthyostega. Of course, once that discovery became more well known that still wasn't a good enough transitional form, so the YECs could still shout:
'THERE ARE NO TRANSITIONAL FORMS!"
And then, of course, more discoveries were made, but for whatever reason these weren't good enough, or didn't count, or still left large "gaps," so YECs felt they could still confidently shout:
"THERE ARE NO TRANSITIONAL FORMS!"
And then more discoveries were made...
"THERE ARE NO TRANSITIONAL FORMS!"
And still more discoveries were made (like the discovery of Tiktaalik in 2006)....
"THERE ARE NO TRANSITIONAL FORMS!"
*So evolution keeps changing, while young earth creationism stays the same ("THERE ARE NO TRANSITIONAL FORMS!"), and apparently no amount of "transitional forms" will ever be enough to get YECs to acknowledge, "Hey, you know, maybe, just maybe, those things might be related." OR EVEN JUST, "I still disagree, but I can see why everyone else concludes that they're related."