Argument #1: Transitional Fossils

[Anomalocaris]

I just wanted to bring one point here since the trilobytes are really getting thrown under the bus. Do you know how many major site there are where you can find 525 million year old fossils? Fucking 2 on the whole goddamn planet. 2! Of course you havent found the damn ancestor to trilobytes, the rocks it was fossilized in are in the damn mantle, or buried so deep we likely never find them.

Actually, there are many sites around the world that offers large arrays of Cambrian fossils. But You are unlikely to find the ancestress of trilobites in Cambrian fossils because at the beginning of Cambrian, trilobites are already clearly distinguished from their arthropod relatives and have the full array of distinguishing trilobite features. This indicates the common ancestor of all Arthropods considerably predate Cambrian, and trilobite lineage, as well as other arthropod lineages, already had a long independent heritage at the start of Cambrian.

The Cambrian explosion isn't so much an sudden explosion of life without known ancestors. It is actually the sudden explosion of fossils of calcium based, mineralized shells. It is thought that prior to Cambrian, ocean water chemistry did not support the formation of hard, mineralized calcite shells. Ancestors of those trilobites that left the earliest fossils of trilobite shells were still trilobites, just trilobites with shells that didn't contain calcite and so is much harder to preserve.

This doesn't mean pre-Cambrian trilobites can't fossilize, just fossilization of trilobites without mineralized shells are much more rare, those that fossilize are more likely to be in poorer state of preservation, and what is preserved would be more difficult to recognize as trilobites.

But nonetheless there are fossil evidence to show there were earlier lineage of indisputable trilobites that didn't look like trilobites because they lacked the distinctive, three lobed, segmented calcite exoskeletons of the iconic trilobite of popular science literature. Instead they looked instead like nondescript giant beetles with round soft chitin shells unreinforced by calcium carbonates.

They were not recognized for what they were for a long time because they looked superficially so unlike the classical trilobite. But in the late 1980s their fossils were systematically dissected under microscopes with dental drills. When the nondescript chitin carapace was ground away, what was revealed below was an arrangements of segmented limbs, mouth parts, and breathing gills highly distinctive to the trilobite lineage.

In other words, a lineage of trilobites existed that didn't have the iconic trilobite calcium shells which so easily and commonly fossilized in Cambrian era and later. Instead this lineage had unreinforced soft chitin shells that were hard to fossilize, hard to recognize when fossilized, but which could exist in the water chemistry prior to Cambrian, when hard mineralized calcite shells were not supported by water chemistry.