Hairy Museum of Natural History

A Paleozoic moon rises, hanging full and swollen on the horizon. Three hundred-million years ago the moon is closer, and its pull is stronger. At some distant ocean’s shore the tides rise high and horsehoe crabs clamber up onto the beach to spawn, drawn by the moon into a dance that they will continue, year after year, to the present day.

Farther inland, the moon climbs above a tropical forest of tree-sized horsetails and a different dance commences. Antennae probe the thick, humid air and the moonlight glints across multi-faceted eyes. Rivers of roughened cuticle eight feet long flow up from the frond litter and across the forest floor, and the air is filled with rustling as they snake around the gritty trunks of the horsetail jungle.

One by one, giant centipede-like forms emerge from the forest along the edge of an ancient lake. Their pace slows as they crawl across the lakeshore mud, but their stride never falters. Dozens of stout, spiny legs undulate in absolute precision, propelling the animals across the beach towards moonlit rendezvous. Their dance, unlike that of the horseshoe crabs, will falter after a few million years. The world will change, and their unparalleled invertebrate stature will prove impossible to maintain.

Year after year, the number of Arthropleura on the beach will diminish. None will remain at the close of the Carboniferous, but in certain places, the footsteps of their dance will be preserved.

—Matt Celeskey.

File under: Carboniferous, Permian.

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While temnospondyls seemed particularly suited towards making their way as aquatic ambush predators, during the Carboniferous and Permian periods they underwent a few experiments in alternative lifestyles. One of the most successful of these evolutionary excursions was undertaken by a temnospondyl family known as the Dissorophids.

The Dissorophids were more fully committed to a terrestrial lifestyle than most other temnospondyls. They developed a series of overlapping bony plates that covered their spinal column, which added strength to their backbones as they clambered across the Paleozoic landscape, and helped protect them from larger predators. Based on this feature, some workers have informally referred to the group as “armadillo toads.”

Big-headed Cacops is one of the best known dissorophids, but the showiest was undoubtedly Platyhystrix, shown here running from the varanopseid pelycosaur Ruthiromia. Platyhystrix (whose name means “flat porcupine”) had a sail along its back supported by dramatically curved and textured extensions of its vertebrae, and in lieu of bony plates along its back it had roughened bony armor stuck to its ribs. While this might not have been sufficient protection from the largest predators of the Permocarboniferous, it doubtless allowed Platyhystrix to cut quite a profile as it strutted through the swamps.

Dissorophids like Cacops and Platyhystrix went extinct at the end of the Permian, but they may have spawned a legacy more successful than any of their temnospondyl kin. Some researchers think that all modern amphibians—frogs, toads, salamanders, and the wormlike caecilians—can trace their ancestry back to these “armadillo toads.” Other scientists disagree, and conclude that temnospondyls ultimately left no descendants that are alive today. Whatever the case, temnospondyls were a particuarly successful group of early terrestrial vertebrates whose lineage stretched from the Early Carboniferous to at least the Middle Cretaceous—350 million to 100 million years ago. Few other vertebrate groups can claim such longevity or tenacity.

—Matt Celeskey.

File under: Carboniferous, Permian, Tetrapods.

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Tracks that were probably made by temnospondyls are known from many Pennsylvanian and Permian sediments. Track fossils are given their own scientific names because it is almost impossible to say for certain what specific animal made a particular track, although the age and shape of a footprint can often suggest what general type of animal made a certain type of track.

This image was based on two different types of tracks found in Permian sediments in southern New Mexico. A small temnospondyl leaving behind Limnopus tracks confronts a scorpion, who has made a series of Permichnium traces. This specific confrontation is not based on a particular trackway, although some paleoichnologists (scientists who study fossil footprints) have suggested that fine-grained rocks from the Permian of New Mexico show evidence of predatory interactions between vertebrates and invertebrates. The direct evidence for such confrontations is equivocal. But it seems certain that scenarios like this would have taken place along sandy shorelines towards the end of the Paleozoic, whenever two such disparate trackmakers scuffled over a few square feet of beachfront property.

—Matt Celeskey.

File under: Carboniferous, Permian, Tetrapods.

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The temnospondyls were a stubborn group of early amphibians who refused to part with portions of their anatomy that they had inherited from their aquatic ancestors. While their contemporaries were overhauling their fish-grade physiques to take advantage of opportunities available on dry land, the temnospondyls looked at their newly evolved hands, threw them up, and crawled back into the nearest river in an attempt to thwart evolution through sheer amphibious obstinacy.

I am, of course, exaggerating. Temnospondyls were subjected to the same forces of modification and selection that shaped every other animal lineage, and throughout their long dynasty they did undertake some noteworthy evolutionary experiments. But they are unusual in that they retained the complex, multi-part vertebrae and intricately folded teeth that most other backboned animals modified shortly after making the transition from surf to turf. And looking at the flat, toothy, textured skull of a temnospondyl, one gets the impression that instead of wasting their time adapting to their environment, they had decided to hunker down and force their environment to deal with them.

Eryops, shown here lunging after a xenacanth shark, exemplifies the niche that temnospondyls returned to time and time again—that of the “crocamander” or “frogodile”. Long before the first alligators swam through any prehistoric bayou, Eryops and its kin were cruising the Carboniferous waterways. One hundred million years later, when dinosaurs first appeared, a group of temnospondyls called metoposaurs were still prowling the swamps, snapping at prey with heads so broad and flat that they resembled sharp-toothed toilet seats. And one hundred million years after that, when true crocodiles had muscled in on their territory in most of the world, the last of the”crocamanders” were still hunting in the relatively isolated waters of Australia.

Eryops is typically pictured as a squat, fat beast with a large, frog-like head and a somewhat insipid grin. This is probably not entirely inaccurate. However, restorations such as these fail to show the dozens of sharp, recurved teeth that line the edges of its mouth, or the extremely nasty-looking fangs that projected from its palate (a trait shared by all tempnospondyls). This portrait of Eryops, inspired by a skull collected from the Permocarboniferous of New Mexico, was drawn with those features in mind.

More temnospondyls to come…

—Matt Celeskey.

File under: Carboniferous, Permian, Tetrapods.

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Spiny-Brush Shark Edition

At the dawn of the Carboniferous period, around 350,000,000 years ago, great changes were taking place in the waters of the world. Many once-diverse and successful groups of fish hadn’t made it past the end of the previous period (the Devonian), so a plethora of aquatic opportunities had just become available for the fish that were fortunate enough to survive.

One of these fortunate groups were the sharks and their relatives, collectively known as chondrichthyans or “cartilaginous fish” for their most distinctive feature, a skeleton made largely of cartilage. Chondricthyans exploded into a fantastic diversity of forms during the Carboniferous, many of them odd and bizarre. Some developed tooth-rows like buzzsaw blades or pinking-shears, some grew winglike fins from the sides of their necks, some evolved long, antler-like pincers on their snouts, and some developed large clusters of spines atop their heads and over their dorsal fins. This last group is known as the stethacanthids, or spiny-brush sharks.

Stethacanthus (shown here in purple) is the most-familar of the group, and the most commonly illustrated. Its strange, wedge-shaped dorsal fin topped with dozens of sharp denticles must have cut quite a profile in the Carboniferous seas. Several suggestions have been made about the purpose of this unusual feature. Some have suggested that, from above, the two patches of tooth-like denticles on its fin and head might have appeared to be the gaping maw of a much larger shark. Others have thought that it might have served as a sort of velcro attachment point, allowing Stethacanthus to hitch a ride on larger fish, as remora do with sharks today. Unfortunately for these hypotheses, the spiny-brush of Stethacanthus and the closely-related Akmonistion (at left) were not particularly flexible and probably wouldn’t be of much use in those suggested situations.

One clue comes from the fact that all known stethacanthid specimens that bear the spiny-brush are male (this is easy to tell in most sharks, because males have prominent claspers behind their pelvic fins). Clasper-less (female) stethacanthid specimens are known, but none have been found with the dorsal spiny-brush. Some researchers have classified them as different animals, but it seems just as likely that they are female Stethacanthus or Akmonistion. Fossils of close stethacanthid relatives named Falcatus and Damocles support this idea.

Falcatus (“hook/sickle”) and Damocles (named after the king with a sword hanging over his head) have even more extensively-modified dorsal fins that curve forward over their heads like prongs, with the spiny denticles forming barbs near the tip. A spectacular fossil of two Falcatus from the Bear Gulch Limestones of Montana preserves a prongless, clasperless female was preserved biting the prong of a claspered (and presumably excited) male in what appears to be Paleozoic foreplay. The drawing shown here depicts a Damocles couple in a similar romantic moment.

Because the prongs of Falcatus and Damocles are modified versions of the same spiny-brush that adorns Stethacanthus and Akmonistion, it seems probable that all these sharks used their fins for similar purposes—as distinctive structures that males could use to advertise their prowess, and where females could focus some toothy affection during courtship and mating.

This artwork was used in PowerSharks!, “the Fintastic game for card sharks everywhere” developed by Ray Troll and myself.

—Matt Celeskey.

File under: Carboniferous, Sharks.

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