In 2014, at a site in Morocco that now borders
the Sahara Desert, a local fossil collector came across the remains of a big...something. It was dated to 480 million years old, near
the start of the Ordovician Period. The paleontologists who described the new
fossil named it Aegirocassis And they recognized that it was a member of
the group of strange creatures known as radiodonts. Radiodonts are long-extinct animals that looked
kinda like lobsters. They had partly segmented bodies and eyes
on stalks, like lobsters do, but they also had flaps hanging off of each body segment
that might’ve been used for swimming, and a pair of long head appendages that were used
for feeding. And their name comes from their distinctive
circular mouths, that were surrounded by overlapping tooth-plates, like a camera lens aperture. While fossils of these bizarre creatures were
first found in the late 1800s, it wouldn’t be until 1994 that paleontologists figured
out that radiodonts were early arthropods. Although they looked different from today’s
marine arthropods - like lobsters, crabs, and shrimps - radiodonts had some telltale
arthropod features, like a tough exoskeleton, a segmented body, gills, and appendages with
joints. But what was even weirder about this radiodont
from Morocco was its massive size. At more than 2 meters long, it was not only
the biggest radiodont ever, but it also may have been the biggest animal in the Early
Ordovician. And the reason it could get to be that huge
is pretty surprising. This bizarre marine giant may have only been
possible, thanks to a major revolution among some of the tiniest organisms in the world. The first radiodonts show up in the fossil
record in the early Cambrian period, about 40 million years before that big one from
Morocco. And some of these radiodonts were tiny. For example, one early radiodont from China
was about the size of a stick of gum. But it looks like it was a pint-sized predator. It had two appendages on its head that were
curved and had sharp spines, which it probably used for grabbing equally tiny prey. And the top predators of the Cambrian were
also radiodonts, with the biggest and best-known ones in the genus Anomalocaris. Anomalocaris canadensis was found in the famous
Burgess Shale of British Columbia and dated to 505 million years ago. Fossils of this species show that it grew
up to half a meter long. While it would’ve been small compared to
Aegirocassis, Anomalocaris was the largest predator in the Cambrian seas. It caught prey with its frontal appendages,
each equipped with trident-like spines. The jury's still out on whether its prey were
hard-bodied, like trilobites, or soft-bodied, like worms and jellyfish, but either way it
was still a formidable predator And with around 25 million years of evolution
between those two radiodonts, why wouldn’t the members of this group have just gotten
bigger and badder over time? But, weirdly enough, when paleontologists
examined the feeding apparatus of Aegirocassis, they were surprised to discover that it was
clearly not an active predator that went after large prey. Instead of grasping spines on its head appendages,
they found an intricate structure made up of fine, hair-like, parallel spines - like
the teeth of a comb. It looked like the kind of structure you’d
find on a plankton eater, like a modern baleen whale. So, instead of a fierce predator, paleontologists
concluded that this giant of the Ordovician seas was a suspension feeder - an animal that
feeds by straining plankton from the water rather than chasing larger prey. And it might be this feeding strategy that
allowed it to get so big. If you look back into the fossil record at
earlier radiodonts, there are a few other suspension feeders, but they started out a
lot smaller. For example, Tamisiocaris was about the size
of a shoebox and lived in the early Cambrian. Its appendages had evenly spaced, super-fine
spines on them that could filter out some of the smallest parts of marine food webs
- plankton. Now, plankton come in two basic groups. Phytoplankton make their own energy through
photosynthesis and Zooplankton that eat phytoplankton and algae. Phytoplankton are microscopic, while some
zooplankton are a bit bigger. And the delicate combs of Tamisiocaris would
have allowed it to filter out prey as small as half a millimeter, like copepods, which
are teeny-tiny crustaceans that are still around today. An even smaller suspension-feeding radiodont
showed up later in the fossil record, in the Middle Cambrian of Utah, about 502 million
years ago. Named Pahvantia, it was just under 24 cm long. Its head appendages had sharp spines in rows
that were so close together that it could have filtered not only zooplankton, but also
the smaller phytoplankton. So, being a suspension feeder doesn’t necessarily
make for large body size - and you don’t always need to be huge to subsist on plankton. That means that something else must have been
going on in the Late Cambrian to explain the rise of the biggest radiodont ever at the
start of the Ordovician. And that something else was the tiniest revolution
at the base of marine food webs - one that would have extra-large consequences. In modern ocean systems, phytoplankton live
in sunlit zones, where they photosynthesize and then supply energy to the darker, deeper
zones as they get eaten by zooplankton whose poop or decaying bodies sink to the ocean
floor. This is part of the process known as the biological
pump, which transfers biomass and energy from the surface of the ocean to the sea floor. And paleontologists thought that, during the
Cambrian, the surface was less connected to the dark ocean bottom, because the water column
in between was not full of plankton like it is now. The only fossil evidence of Early Cambrian
phytoplankton are microfossils called cysts. Many marine plankton today make cysts, which
are little hard casings that may provide defense from toxins in their environment, or to allow
them to go dormant for a while. The fossil record for zooplankton is a little
better. There are preserved mouth-parts from tiny
crustaceans and mineralized skeletons of radiolarians - single celled animals that are usually considered
to feed on phytoplankton. Both are similar to what you'd find today. Now, while plankton did become more complex
throughout the Cambrian, overall they seem to have been less diverse and abundant until
the end of the period. And as the Cambrian came to a close, a large
radiation event got started. This is known as the Paleozoic Plankton Revolution. And it continued into the earliest Ordovician,
with the whole event lasting less than 15 million years. While researchers are still debating exactly
what caused the Plankton Revolution, it looks to have been linked to a major change in climate. 500 million years ago, there were upheavals
in the Earth’s crust from tectonic activity. More rock was exposed to weathering, which
drew carbon dioxide out of the atmosphere. This caused what scientists call a reverse
greenhouse effect. The Earth cooled rapidly, in geologic terms,
as carbon dioxide levels plummeted over the span of 2 million years. The cooling created more suitable ocean temperatures
for plankton, which began to flourish. Photosynthesizing phytoplankton took in --more
carbon dioxide and released oxygen into the atmosphere, which made the temperature changes
happen even faster. As plankton grew denser and more diverse,
they may have ramped up the biological pump going from the sunlit ocean surface to the
deep sea floor - transforming oceans into more nutrient-rich, productive ecosystems,
like what we see today. And that is when the BIG radiodont Aegirocassis
showed up. More abundant plankton meant suspension feeders
could get much larger, because there was just more food available. Paleontologists count Aegirocassis as the
first of the giant  suspension feeders. These would eventually come to include members
of the agnathan fishes, nautiloids, placoderms, sharks, bony fishes, reptiles, ammonites,
and mammals. And, in fact, recent research links suspension-feeding
to large body size in whales. The biggest animal on Earth now...or ever...
is not an active predator, but a plankton eater: the Blue Whale. While the radiodonts didn’t make it beyond
the Devonian Period, the evolution of Aegirocassis was a preview of what was to happen, independently,
in many other groups of animals. It took a tiny explosion at the base of marine
food webs almost 500 million years ago to set the stage first for the largest radiodont
ever and, ultimately, for our modern oceans. Hey! It's Blake popping in. I just wanted to say
thanks for watching Eons, which is produced by Complexly. If you want to keep imagining the world complexly
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Interesting, great share!