Buzz
The genuine genetic diversity within the animal kingdom is often hidden within small, easily overlooked creatures whose evolutionary history was filled with destruction and near-extinction events that left only a few lucky survivors. Many other large groups weren't so fortunate, with some being overtaken by more advanced successors, while others simply had poor luck.
What insights can we gain from these lost remnants of time? One of the enduring contributions of the late Stephen J. Gould is the concept of the ultimate futility of natural selection. Over the course of geological history, it doesn't truly matter how advanced your brain or circulatory system is. Catastrophic space rocks tend to ignore such factors.
10. Archaeocyathids
Archaeocyathids didn't last long on Earth, but they achieved remarkable things during their brief existence. They first appeared in the fossil record 542 million years ago, during the Precambrian Period. Much like modern sea sponges, with which they were likely related, archaeocyathids created hard, porous calcium-based structures, typically no larger than 15 centimeters (6 inches) in height. They formed cup-like and tubular shapes on the ocean floor.
Archaeocyathids likely fed in an even simpler manner than today's sponges, by passively filtering food particles from the water flowing through their porous forms. They also lived in a symbiotic relationship with blue-green algae, which they probably consumed as well.
This group evolved into hundreds of distinct species, becoming integral components of shallow marine ecosystems in tropical regions worldwide. They built massive structures atop their own accumulated remains, laying the foundation for the first reefs in many areas. In doing so, they created vital marine environments throughout the Lower Cambrian world, fostering the evolutionary development of more familiar animals.
Within just 20–25 million years, by the Middle Cambrian, they had vanished completely. The exact cause remains unknown, but it was likely due to competition from a new type of grazing predator or more advanced filter-feeding sponges or corals.
9. Helicoplacoids
Helicoplacoids are only known from the Lower Cambrian period, about 525 million years ago. They were among the first echinoderms, a group that includes today's starfishes and sea cucumbers. These creatures resembled tiny, 3- to 7-centimeter-long (1.2–2.8 inches) armored footballs capable of stretching and contracting their bodies. They had unique feeding grooves spiraling along their bodies and were among the earliest animals to develop skeletons.
It is believed that they lived upright in vertical mud burrows within the microbial mats that flourished in the shallow, silty bays surrounding Laurentia, the ancient continent that once stretched from British Columbia to California. They fed on plankton and organic debris from the water above.
In geological terms, helicoplacoids became extinct rather swiftly, surviving for only 15 million years. They disappeared by the end of the Lower Cambrian. It's possible that more mobile descendants replaced them, potentially evolving into the starfishes, sea urchins, brittle stars, and sea cucumbers we know today. Our current understanding suggests that helicoplacoids were simply too specialized to thrive in soft, static mud flats and couldn't adapt when burrowing animals emerged, disrupting their habitat and significantly altering the water currents.
8. Halwaxiids
A perplexing group of soft, yet scale-armored creatures emerged during the Early Cambrian period, and they remain a mystery to paleontologists today. This peculiar family is tentatively considered an ancestral form of mollusks, possibly an early relative of today's clams, squids, and snails.
The halwaxiids include the slug-like genus Wiwaxia and the associated worm-like genus Halkieria, along with several other isolated oddities. There is minimal solid evidence linking these animals together, as their distinctive armor could have evolved independently. An example of Wiwaxia is Wiwaxia corrugata, measuring 5.5 centimeters (2.2 inches) long, with two rows of long, blade-like defensive spines along its back.
The halwaxiids did not survive beyond the Middle Cambrian (497 million years ago). Some scientists suggest their extinction was caused by the decline of the bacterial seafloor mats they relied on for food, due to the rise of burrowing animals that altered the stability of Cambrian mud beds permanently.
7. Dinocaridids
The dinocaridids, or 'terror shrimp,' swam through the Cambrian seas around 515 million years ago. These sight-based predators possessed some of the most advanced eyes of any arthropod ever and inhabited oceans across the globe.
Dinocarids were true giants of the Cambrian seas. The 1-meter-long (3 ft) genus Anomalocaris had a strange whiskered mouth with two long feeding appendages. The exact method of feeding remains a mystery to scientists. It’s possible they hunted tiny trilobite species, but it seems more likely they swam through the waters like modern-day whale sharks, filtering plankton or browsing on soft-bodied worms.
Some groups evolved away from predatory behavior to become specialized filter feeders, harvesting plankton much like a baleen whale, and grew to massive sizes on this abundant diet. The largest species discovered so far is Aegirocassis benmoulae, which reached a length of 2.1 meters (7 ft), making it one of the largest arthropods ever.
The 10-centimeter-long (4 in) Schinderhannes bartelsi, the last known dinocaridid, vanished 390 million years ago during the Early Devonian. It’s possible that dinocaridids couldn’t compete with the more modern predatory animals, such as squid and armored fishes, which rose to prominence after the end of the Cambrian Period.
6. Blastozoans
Blastozoans were a significant branch of primeval echinoderms that grew from elongated stalks, similar to modern-day sea lilies. Emerging in the Early Cambrian, they were among the first echinoderms, achieving great success and becoming far more common than helicoplacoids and other early echinoderm species.
One of the most prevalent Early Cambrian species was the flower-like genus Gogia, which thrived on archaeocyathid reefs and trilobite shells around Gondwana, an ancient supercontinent. Gogia was the tallest echinoderm of its time, reaching 10 centimeters (4 inches). Its growth was likely limited by the loose, shifting mud of its environment.
Blastozoans experienced a dramatic diversification during the Ordovician Period (490–434 million years ago), evolving into a variety of forms and becoming some of the most successful and iconic animals of their era. Some blastozoan species even survived a massive global glaciation, though only one group endured until the Late Devonian mass extinction 70 million years later.
These final survivors were known as 'blastoids,' a name with a charming, retro sci-fi feel. They clung to existence until the Permian Period, when a devastating event wiped them out for good. They perished 262 million years ago during the Capitanian Extinction, caused by a sharp drop in global sea levels, severe oxygen depletion, and marine acidification—factors that decimated shallow coastal ecosystems worldwide. Dr. David Bond of the University of Hull partially attributes this extinction to a massive volcanic eruption in Sichuan, China.
5. Homalozoans
Homalozoans were a peculiar group of ancestral echinoderms, known for their strange, often asymmetrical bodies. These creatures had flattened forms with a single armored appendage whose function remains a mystery. It might have been used for feeding, to anchor their bodies to the sediment, or perhaps as a tail-like structure for swimming. In truth, very little is known about how homalozoans lived.
Homalozoans are divided into two recognized orders: The Cornutans, which appeared in the Middle Cambrian, had boot-shaped bodies. Later, in the Early Ordovician, the more symmetrical Ankyroids emerged. The Ankyroids eventually overtook the Cornutans, surviving both the Ordovician glaciation and the mass extinction events of the Permian. They continued to exist until the Late Carboniferous Period (323–289 million years ago), but gradually became less common, eventually vanishing entirely.
The homalozoans were another group of diverse echinoderms from the Late Cambrian to Silurian periods, which resist easy categorization into existing classes. Their representation is limited to a scattered collection of specimens, some exhibiting intermediate features between groups, while others are entirely unique in structure. Early echinoderms generated vast biodiversity, leaving behind a complex array of peculiar fossils that future taxonomists would eventually seek to understand.
4. Graptolites
Graptolites were marine superorganisms, consisting of numerous microscopic creatures that formed branching colonies resembling bushy twigs. These colonies mostly grew on the seafloor of the Cambrian oceans, with each individual animal connected to others via a nerve cord.
Similar to the blastozoans, the Ordovician period was a golden age for graptolites, as they rapidly expanded and diversified into hundreds of new species. Some of them even adapted to floating near the ocean surface using inflated air sacs, filter-feeding from the water column or attaching to seaweed with filaments. These were among the first complex multicellular organisms to take advantage of the plankton-rich surface waters as a unique ecological niche, thriving globally.
Over the following 24 million years during the Silurian period, graptolites began to wane, with all floating varieties going extinct. Some paleontologists argue that the rise of fish, with their growing success and versatility as predators, made life too difficult for graptolites, eventually leading to their grazing-induced extinction.
The last remaining, isolated graptolite colonies on the deep ocean seafloors vanished 315 million years ago during the Late Carboniferous Period, alongside the forgotten ankyroids. These lingering remnants failed to survive the ensuing glaciation period and the dramatic continental shifts that followed, which once again transformed marine environments on a global scale.
3. Conodonts
Conodonts appeared in the Late Cambrian, around 500 million years ago. Although they bore a resemblance to fish, they belong to a separate class as they never developed proper backbones. These worm-like, eel-shaped creatures were highly successful, inhabiting oceans around the world in a range of depths and temperatures. Most conodonts were quite small, with the largest reaching only a few tens of centimeters in length. Over 1,500 species of conodonts are known, with many likely being sluggish, bottom-dwelling scavengers, while others evolved to become more active predators.
Like trilobites, conodonts reached their peak during the Ordovician Period before beginning a long, gradual decline over millions of years. Remarkably, some managed to survive even longer than trilobites, enduring the Great Dying and lasting into the Triassic Period. The final conodonts went extinct 200 million years ago, during the Late Triassic. The cause of their extinction remains uncertain, as there was no singular catastrophe associated with this era. It’s likely that their decline was linked to continuous sea level fluctuations, repeated oxygen depletion events, and exposure to geothermal chemicals. Like the trilobites, the remaining conodonts were slowly eradicated by a series of persistent environmental challenges.
The last survivors of the conodonts were deep-water species known as gondollelids, which had evolved to become small, simple, and inconspicuous. Ultimately, these final conodonts too faded into extinction.
2. Trilobites
One of the most familiar species to meet its end during the Great Dying was the trilobite. With a staggering diversity of over 20,000 species known to science, trilobites were by far the most varied group on this list. They ranged in size from as tiny as 2 millimeters to an impressive 70 centimeters (28 inches). Emerging 521 million years ago in the Cambrian, they would thrive for almost 300 million years. Trilobites lived across oceans worldwide, taking on a variety of ecological roles from swimming plankton to crawling predators and deep-sea scavengers.
Following their peak during the Ordovician Period, when they dominated the oceans as the most abundant arthropods, trilobites began to experience a gradual but unstoppable decline, overwhelmed by a succession of apocalyptic events. By the Carboniferous Period, only a single trilobite order remained, and by the Permian, the relentless environmental and geological pressures in the dying oceans reduced this to just two families.
Trilobites were not inherently ill-suited for survival, but the Great Dying was as unforgiving as a relentless middle school PE teacher. Their tale is one of sheer adversity in the face of an inevitable, all-encompassing cosmic catastrophe.
1. Edrioasteroids
Edrioasteroids form another large group of extinct echinoderms. These creatures bore a striking resemblance to spineless sea urchins, fixed onto surfaces. The first known echinoderm, Arkarua, likely belonged to the edrioasteroid family, appearing 600 million years ago in the Late Precambrian and measuring about 1 centimeter (0.4 inches) across. By the Cambrian, 15 million years later, edrioasteroids had become major organisms, encrusting soft seabeds along most continental coastlines.
In the Ordovician, edrioasteroids experienced a significant expansion, adapting to a range of hard substrates like shell pavements and reefs as competition from blastozoans and similar species pushed them out of softer environments. After the Ordovician glaciation, some of these species had survived almost unchanged since the Cambrian, becoming the most primitive echinoderms of their time. By the Carboniferous Period, the surviving edrioasteroids had grown to 20 centimeters (8 inches) in height and entered a phase of rapid evolutionary development, unburdened by the competition from their blastozoan competitors.
Unfortunately, like many others in the prehistoric world, the edrioasteroids were not immune to the destructive forces of the Great Dying 251 million years ago. This global extinction event obliterated any remaining survivors within a mere 100,000 years, erasing this unique lineage from the planet forever.
The Great Dying was a catastrophic event that dwarfed even the dinosaur KT Extinction, wiping out nearly 90 percent of marine life. Surprisingly, our protomammalian ancestors survived, emerging into a radically altered world. While the exact cause remains unclear, a 2002 excavation unearthed evidence suggesting that a rock the size of Mount Everest may have struck Earth during this period, possibly in Australia. This impact likely worsened the already devastating ocean acidification, deoxygenation, and the shifting continents of the era.
God hates echinoderms.
