The modern sea spider had started to diversify by the Jurassic, study finds

They're not arachnids, but they do look like spiders: https://en.wikipedia.org/wiki/Sea_spider

That was my exact thought as well. They are not real spiders, apparently, but they sure resemble them.

It's interesting that I had not heard of these at all until about fifteen minutes ago.

I guess I’m not the only one!