A closer look at an ancient sea beast is helping scientists connect the dots of the genetic lineage that led to today's spiders, insects, and shrimp. The 480-million-year-old Aegirocassis benmoulae, which was first reported in 2011 but is described in further detail in Wednesday's Nature, was one of the world's earliest arthropods. Today, that group is the most species-rich and diverse on Earth -- the number of beetles alone is, frankly, ridiculous.

That success is owed in large part to arthropods' highly adaptable bodies, which are segmented and have evolved to serve all manner of purposes. Their limbs, in the most basic form, are made of two connected branches. Each branch of the limb can adapt to serve a particular purpose (or be lost entirely) which lends itself to wide diversity across the group.

But until now, early arthropods didn't seem to have two-branched limbs.

Since its discovery in 2009 by fossil collector Mohamed Ben Moula, it's become apparent that Aegirocassis benmoulae was quite common in its day. And unlike previously discovered early arthropods, it had the makings of the double-branched limbs that arthropods have relied on: New fossils show that each segment of the large arthropod's body had two flaps, though they had not yet fused together or become true limbs.

"Until this discovery, we thought early arthropods had only one set of flaps, which was problematic," said lead researcher Peter Van Roy of Yale University. "But this upper flap we've found is the equivalent of the upper branch. It represents a state where you have these multiple branches, but they haven't yet fused together."

What's more, Van Roy and his colleagues were able to find signs of these secondary flaps in older arthropods when they went looking for them.

So Aegirocassis benmoulae is a fossil that conveniently fills a hole in the logical evolution of today's bugs and shrimp. But that's not the only reason Van Roy and his colleagues are excited about the new specimens they've examined.

At seven feet long, the arthropod is also one of the largest to ever live. Given how abundant its fossils are, that means that the ocean of 480 million years ago was able to support a massive and very common filter feeder. In other words, there must have been a ton of plankton.

"Of course this is a very big animal that needs a lot of food, and it’s a very common fossil, so it clearly had a big population," Van Roy said. "That indicates that you had a very diverse, complex, well developed plankton ecosystem that was already in place 480 million years ago."

Van Roy believes that these ancient shrimp cousins followed the same evolutionary trend that whales did: The species that adapted to filter feeding were able to grow to much more impressive sizes than those that had to actively hunt their prey.

"Our animal gives the oldest example of this overarching trend," he said. "And it seems to be something that's happened again and again."