It might be. Has something been chomping on it? Like your tortoise? Some of those leaves look suspiciously torn short at the edges. Or perhaps it got taken down by an edge trimmer.
If it's not a thistle, I'd say it's something in the same family and safe. But I'd have to see it grow out more leaves to be sure! Is there a full leaf at the bottom you can take a picture of on a counter top or something? So we can see the outline.