Indeed, there are fossil sea spiders from the Ordovician, which look much the same as the present sea spiders.
While there have been very abundant arthropods already several tens of millions of years before the first preserved sea spiders, almost all of them, for example the trilobites, were very unlike any present arthropods.
Sea spiders seem to be the first among arthropods who have reached a local optimum in body shape and structure, so they have not changed significantly after that, while also surviving various extinction events that have wiped out other groups.
Despite the name, terrestrial spiders are only very distant relatives of the sea spiders, and they have appeared only about one hundred and fifty million years later than the sea spiders. Terrestrial spiders are much more closely related to scorpions, than to sea spiders.
skullone · · focus · HN ↗
adrian_b · · focus · HN ↗
While there have been very abundant arthropods already several tens of millions of years before the first preserved sea spiders, almost all of them, for example the trilobites, were very unlike any present arthropods.
Sea spiders seem to be the first among arthropods who have reached a local optimum in body shape and structure, so they have not changed significantly after that, while also surviving various extinction events that have wiped out other groups.
Despite the name, terrestrial spiders are only very distant relatives of the sea spiders, and they have appeared only about one hundred and fifty million years later than the sea spiders. Terrestrial spiders are much more closely related to scorpions, than to sea spiders.