Since the dawn of life on Earth, reefs have been hot spots of ocean biodiversity. As time went on,
early reef ecosystems came and went, with unique communities developing and thriving
before being wiped out by major extinctions. After the Devonian mass extinction wiped out most of the
ancient coral reefs 360 million years ago, even stranger reefs began to develop in their place,
culminating 100 million years later in the massive reefs of the middle Permian.
But soon a mysterious event would mark the beginning of the end for these ecosystems.
During the middle Permian, there was an inland sea on the western edge of Pangaea,
in what is now the Guadalupe mountain range of Texas and New Mexico.
This sea was home to a massive ancient structure known as the Capitan Reef.
Modern reefs are primarily formed by scleractinian corals, which are anthozoan cnidarians that form
colonies that mostly rely on a symbiotic relationship with algae to survive. However,
scleractinian corals would not appear until millions of years after the end of the Permian,
and the late Devonian extinction had reduced the abundance of rugose and tabulate corals that
were common in the earlier Paleozoic. So in the 100-million-year time between the mid-Paleozoic
coral reefs and the Mesozoic coral reefs, reefs were primarily made up of other organisms.
The Capitan reef was primarily built by sponges. These are much more primitive animals than corals,
with very few cell types and no specialized tissues, but they dominated the Permian reefs
nonetheless. Some sponges formed the massive shelves upon which the reefs were based,
while others formed branching structures and hollow cylinders,
similar to many of the sponges we see today. Other contributors to the reefs were bryozoans,
a more advanced group of colonial animals related to brachiopods,
ribbon worms, and mollusks. A bryozoan colony is made up of individual animals called zooids,
living within chambers in a shared structure. These zooids are connected to each other through
tubes running through the walls of their chambers, allowing resources to be shared and for different
zooids to specialize for different functions. Most modern colonies grow flat across surfaces,
but Paleozoic species more often grew into tall, frond-like structures as well.
Other sessile animals making up the reef included the crinoids, brachiopods, and bivalve mollusks.
Microbes also laid down reef material, as did a common encrusting organism
called Archaeolithoporella, which is thought to be formed by colonial algae.
Presumably swimming amongst these structures was an assemblage of late Paleozoic fish groups,
including strange shark relatives, the increasingly rare lobe-finned sarcopterygians,
and early ray-finned actinopterygians: the ancestors of almost 99% of modern fish species.
They shared the waters with Permian cephalopods, ranging from the straight-shelled orthoceratoids,
to the curly-shelled ammonoids, to the shell-less relatives of modern squids and octopuses.
Living both on the reefs and elsewhere on the ocean floor were many of the same invertebrate
groups that had existed since early in the Paleozoic, and continue to exist to this day.
But the arthropods roaming the Permian sea floor were quite different from
their early Paleozoic predecessors. The only trilobites left were a few remaining proetids,
and the eurypterids were now only found in freshwater.
Instead, a different group of arthropods had taken over: Crustacea.
The crustaceans had been around since the beginning of the Paleozoic,
with some even colonizing the land and giving rise to the insects. By the later Paleozoic,
they were also the most common marine arthropods, perhaps thanks to their possession of mandibles,
which trilobites and eurypterids lacked. The most abundant crustaceans in the Permian
fossil record were primitive pericarids known as pygocephalomorphs. Their modern relatives include
the amphipods as well as the isopods, whose fossil record had just begun prior to the Permian.
The other prominent Permian crustaceans were the syncarids, which today are represented by a few
dozen species common in extreme environments. Meanwhile, the ancestors of mantis shrimp were
already in possession of the formidable claws that mantis shrimp are known for today. Most of
the familiar crustaceans we have in modern times, like crabs, lobsters, prawns, and carid shrimp,
are decapods, and were the result of later radiations during the age of the dinosaurs.
But genetic evidence suggests the roots of many of these lineages were established by
this point in the Permian. ALL of these crustaceans form the group Malacostraca,
sharing a basic body plan of 20 to 21 segments making up a head, thorax, and abdomen. Outside of
Malacostraca lie the tiny ostracods, which thrived as plankton and as burrowers in the sediment
as they had since the early Paleozoic. Another ancient group, the copepods, are some of the most
abundant animals in the modern world. During the Permian, deep sea oxygen levels increased, and it
has been hypothesized that this is when copepods began their diversification in the deep sea.
But a threat to this ocean biodiversity was brewing
To the east of Pangea was China, still isolated as a series of islands while the rest of the
continents had united. Today, rocks in China preserve a long history of Permian reefs,
including middle Permian ones similar to those in Texas.
However, China was also the site of large, long-term volcanic eruptions,
releasing huge amounts of flood basalt and climate-altering gases. This appears to
coincide with extinctions all over the world that occurred before the end of the middle Permian.
The intensity of this Capitanian Mass Extinction
was only recently recognized, and the exact sequence of events is debated.
The increase in volcanic activity itself is only one of the proposed triggers for this catastrophe.
Regardless, the end result was the extinction of around 60% of marine species,
and many land organisms as well. Roughly 10 million years still remained
before the Permian biosphere would be dealt its final blow, and this would prove to be
insufficient time for the expansive sponge reefs to ever regain their middle Permian success.