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Falcatacaris

The enigmatic thylacocephalans were a group of bizarre little arthropods, found in marine deposits all over the world from the late Ordovician (~435 million years ago) to the late Cretaceous (~85 million years ago). They had shield-like bivalved carapaces, large compound eyes, three pairs of spiny grasping limbs, and multiple pairs of small paddle-like swimming limbs, but details of their internal anatomy are poorly known and their evolutionary relationships to other arthropods are still very uncertain.

Traditionally they’ve been classified as crustaceans, possibly as close relatives of remipedes or malacostracans – but they’ve also recently been proposed as instead being part of a much more ancient branch of arthropods, potentially related to stem–mandibulates like Acheronauta.

Falcatacaris bastelbergeri was a thylacocephalan living during the late Jurassic, about 150 million years ago, in what is now Germany. Around 2.5cm long (~1″), its carapace had tiny interlocking square “teeth” resembling a zipper along the hinge line between the two valves, a ridge along each side, and a long pointed knife-shaped spine at the front.

Like other thylacocephalans it was probably a swimming predator, likely nocturnal or hunting in murky conditions based on its enlarged eyes, and would have captured smaller aquatic prey using its raptorial limbs.

Avisaurus

Avisaurus darwini here lived at the very end of the Cretaceous, about 66 million years ago, in what is now the Hell Creek fossil beds in Montana, USA.

It was a member of a diverse group of Mesozoic birds known as enantiornitheans, which retained claws on their wings and often still had toothed snouts instead of beaks – and being part of the avisaurid family it was also one of the larger known examples of these birds, similar in size to a modern hawk at around 60cm long (~2′).

Although this species is only known from isolated foot bones, the remains have distinct enough anatomical features to show that Avisaurus had powerful gripping talons similar to those of modern hawks and owls, suggesting it had a similar lifestyle hunting small vertebrate prey in the ancient swampy Hell Creek ecosystem.

Tetracynodon

Therocephalians were a group of synapsids very closely related to – or possibly even ancestral to – the lineage leading to modern mammals. They were a diverse and successful group of carnivores during the latter half of the Permian, but suffered massively during the “Great Dying” mass extinction, with only a handful of representatives making it a few million years into the Triassic.

Tetracynodon darti was one of these rare Triassic therocephalian survivors, living in what is now South Africa around 251 million years ago. Only about 25cm long (~10″), it had slender limbs and strong claws that suggest it was a scratch-digger. Its long snout was lined with pointed teeth, and it was probably an active predator hunting by snapping its jaws at fast-moving prey like insects and smaller vertebrates.

Its combination of small size, burrow-digging habits, and unspecialized diet may be the reason it scraped through the Great Dying when most of its relatives didn’t – but unfortunately it seems to have been a “dead clade walking”, disappearing only a short way into early Triassic deposits.

Spectember/Spectober 2024 #10: Nosey Sea Monkey & Louse Mouse

Okay, let’s finish up this year’s round of speculative evolution concepts with one last post before we get back to normal paleoart content.

~~(…what do you mean it’s November 3rd? how did that happen?)~~

Belial Lyka asked for a “marine proboscis monkey with a somewhat buoyant nose”:

A shaded sketch of a speculative semiaquatic descendant of proboscis monkeys. It has a large trunk-like nose, a fat body, long arms ending in flipper-like hands, short legs with webbed feet, and a short thick tail. A second image shows its head with its nose inflated into a balloon-like display structure.

A descendant of modern proboscis monkeys, Phusarhinus beliallykae is a large semiaquatic primate found in shallow marshy coastal areas of what was once Southeast Asia. Around 4m long (~13′) it has a lifestyle somewhat similar to ancient early sirenians, feeding on soft aquatic plants and hauling out onto land to rest.

Its dense rib bones and long flipper-like grasping forelimbs make it rather front-heavy, allowing it to naturally float with its head and arms hanging down closer to the bottom for energy-efficient foraging. When it needs to resurface to breathe it shunts air from its lungs into its large inflatable nasal sacs, altering its buoyancy enough to tip its head back up towards the surface.

Unlike its ancestors the elaborate nasal structures are found on both males and females – although they’re more brightly colored in males and are also used for visual courtship displays and as resonating chambers for loud booming calls.

And somebody who only gave their name as “bunny” suggested a “parasitic rodent”:

A shaded sketch of a speculative parasitic descendant of semiaquatic rats. It has a wide flattened body with thick armor-like bands running down its back, a small head, stubby limbs tipped with long hooked claws, and a short fat tail.

Sanguichelonamys bunnyi is a highly unusual descendant of a rakali-like semiaquatic rodent that had a symbiotic relationship with early members of the Phusarhinus lineage. The rodents initially just removed algae and external parasites from the bodies of the increasingly bulky aquatic monkeys, but things have recently started to turn more parasitic.

At just 3cm long (~1.2″) Sanguichelonamys is one of the smallest mammals to ever exist, with a wide flattened body and sharp hooked claws used to cling onto its host monkey’s thick skin. Although it still does remove other parasites, during haul-out periods it will also use its sharp incisors to deliberately enlarge the wounds left behind – or even open up new ones – and directly feed on fresh blood from its host.

The thickened keratinous skin along its head and back has a specialized hydrophobic surface that traps a layer of air while underwater, acting as a “rebreather” bubble similar to that of water anoles. Along with the ability to drastically slow down its metabolism and respiration rate, this allows Sanguichelonamys to survive being submerged during its host’s lengthy foraging dives.

Spectember/Spectober 2024 #09: Big Bad Wild Dogs

@poshtearexdoodles asked for “African wild dogs evolved to fill the larger pack animal niche of lions”:

Although in this particular timeline the endangered and fragmented wild painted dogs in Africa didn’t survive far past modern times, that wasn’t the final extinction of the species – captive groups persisted, and after the [REDACTED] of humans an escaped population managed to carve out a niche for themselves in the grasslands of southern North America.

A shaded sketch of a speculative descendant of painted dogs. It's a large wolf-like animal with short thick hyena-like jaws, a mottled shaggy coat of fur, and and small hoof-like nails on its feet.

Their most notable descendant is Megainolycos poshtearexi. At nearly 1m tall at the shoulder (~3’3″) it’s one of the largest canids to ever exist, comparable to present-day lions in size and general ecology, with short thick bone-crushing jaws convergent with those of hyenas.

It also retains the complex variable coat coloration of its ancestors, with each individual having a unique disruptive pattern of black, white, and brown blotches.

Much like its ancestors it’s a highly social pack predator specialized for endurance hunting. Along with some surprisingly horse-like limb ligament morphology, its feet now sport hoof-like nails rather than claws, and it pursues prey over long distances to the point of exhaustion – mainly targeting the larger ungulates across its prairie habitat such as hogs, horses, bovids, and deer.

Spectember/Spectober 2024 #08: Saberatel

Roy (@roygattero) requested “ratel becoming the new african big predator”:

A shaded sketch of a speculative descendant of honey badgers. It's a somewhat cat-like animal with a short snout, long saber-teeth, small ears, semi-digitigrade limbs, a mottled coat, and a short tail.

Perforictis royi is the latest in the long and venerable tradition of various synapsid lineages discovering the ecological niche of “big stabby saber-teeth“.

A descendant of the modern ratel/honey badger, it stands around 80cm tall at the shoulder (~2’8″), and convergently resembles the feline-like build of some of its more ancient relatives. Inhabiting the tropical forests of the rifted-off island continent of East Africa, it’s an ambush predator specializing in tackling larger prey – primarily ungulates and primates, but also occasionally giant rodents and hyraxes.

Usually cooperatively hunting in mated pairs, these mustelids stalk close to their targets before attacking, with one individual focusing on toppling and immobilizing their target while the other positions itself to deliver a swift precision killing bite to the throat with its saber-teeth and powerful neck musculature.

Perforictis scent marks its territory using extremely pungent secretions from its anal pouch rubbed onto vegatation, along with making loud scream-like vocalizations.

Spectember/Spectober 2024 #07: Mole Dino

Today’s spec creature is a combination of a couple of submissions – James P. Quick asked for “a post-K/Pg relict dinosaur from pre-glaciation Antarctica”, and an anonymous asked for “a subterranean (like, say, Talpa or Spalax) burrowing dinosaur”:

At the time of the K/Pg mass extinction some of the small ornithopods that inhabited Late Cretaceous Antarctica had been developing increasingly complex burrowing behavior and a more generalist omnivorous diet than most other ornithischians – and, along with their ability to endure the long dark cold polar winters, this was juuust about enough for them to survive while the rest of their non-avian cohorts vanished.

They were very briefly a fairly successful disaster taxon in the devastated polar forests, but they were quickly displaced by other diversifying survivors and never really got another ecological foothold to regain anything close to the non-avian dinosaurs former glory.

Instead the little ornithopods specialized even further for burrowing, spending more and more of their lives underground to avoid the increasing competition and predation from mammals and birds.

A shaded sketch of a speculative descendant of small ornithopod dinosaurs in Antarctica. It's a fuzzy little mole-like animal with a hoked beak at the tip of its toothy snout, no eyes, a keratinous shield on its head, large shovel-clawed forelimbs, a chunky body, stocky legs, and a fat bristly tail.

Now, well into the Cenozoic at the dawning of the Miocene, Cthonireliqua quicki is the very last representative of the non-avian dinosaurs. Small and stocky and mole-like, just 15cm long (~8″), it has muscular forelimbs with large shovel-like claws, a keratinous shield on its head, and a thick bristly tail where large fat reserves are stored.

Its eyes are almost completely absent, only vestigial remnants present under the skin of its face, and it navigates its extensive burrows using sensitive whisker-like filaments and its keen senses of hearing and smell. Still omnivorous like its ancestors, it feeds on whatever it comes across while tunneling – mainly worms, insects, smaller vertebrates, roots, and tubers.

Unfortunately for Cthonireliqua, and the rest of its Antarctic ecosystem, time is running out. Over the last few million years Antarctica’s climate has been steadily cooling and drying, the continent has become fully isolated, and the Antarctic Circumpolar Current has formed. Glaciation is well underway in the continental interior, and the once-lush forests are shrinking away and being replaced with tundra.

Soon all evidence of these dinosaurs’ existence will be buried under the ice.

Spectember/Spectober 2024 #06: Death Ray

Yeah, I’m keeping this stuff going again for another round of Spectober!

An anonymous request asked for a “big macropredatory ray”:

A shaded sketch of the head of a speculative descendant of pelagic stingrays. Its a large ray with triangular wing-like fins, a slightly shark-like pointed snout, protrusible sharp-toothed jaws, and a vaguely skull-like pale marking on its underside.

Speirobatis thanasima is a large ray found in open oceanic waters, reaching sizes comparable to the modern giant manta ray at around 3m in length (~10′) with a wingspan of over 5m (~16’5″).

But despite its body shape closely resembling that of mobulids or myliobatids, its closest present-day relative is actually the pelagic stingray. Already being active hunters with mouths full of sharp pointed teeth, these ancestral rays gradually evolved bigger body sizes and began occupying an apex predator niche similar to that of large sharks and toothed whales.

Speirobatis is a strong swimmer, using flapping motions of its triangular wing-like pectoral fins to travel at high speed and make acrobatic jumps out of the water. Highly intelligent and social, it lives in family groups that hunt cooperatively – encircling and herding schools of fish tightly into bait balls, taking turns stunning prey with slaps of their fins, and then grabbing incapacitated individuals with snaps of their protrusible jaws.

Groups will also tackle larger prey such as marine mammals and sharks, harassing and chasing targets to exhaustion while taking quick opportunistic bites out of them.

Spectember 2024 #05: Most Weasel

GayCoonie suggested a “future legless mustelid”:

A shaded sketch of the head of a speculative descendant of the least weasel, shown in both side view and bunched up clinging to a branch. It's a very elongated animal with no obvious limbs, having just small hooked spurs where its legs should be. It has a small weasel-like head with front-facing eyes and small ears, a long neck, a long slender chest and torso, and a thicker fatter back end with a tapering semi-prehensile tail.

Descended from an arboreal offshoot lineage of the modern least weasel, Maximagale gaycooniei is a bizarre mustelid that appears to have converged on the lifestyle of ambush-hunting tree snakes, evolving in northern latitudes where actual snakes are largely absent.

Growing to about 1m long (3’3″), it’s not truly legless but its limbs are all reduced down to tiny vestigial single-clawed spurs, which are used to help anchor its body while climbing and as claspers during mating. It moves around with a distinctive inchworm-like looping gait, alternating grasping and releasing with its front and hind spurs.

Its build is bottom-heavy, with most of its mass concentrated in its thicker back end, and its tail is semi-prehensile. It clings to trees with its body bunched up, camouflaged with cryptic coloration, and rapidly whips its long flexible front half out to snap its powerfully-muscled jaws at prey – such as insects, birds, lizards, frogs, small mammals, and pretty much anything else that comes within its reach.

It will also opportunistically raid the nest of birds and arboreal mammals.

Due to the less frequent meals its ambush-hunting tactics provide, it has a much slower metabolism than its ancestors, and it conserves energy with daily periods of torpor and longer hibernation during the colder months of winter. It has also retained its ancestor’s tendency to seasonally shrink its brain size to reduce energy requirements even more.

Spectember 2024 #04: Forest Gelada

Someone who identified themself only as Pendrew asked for a “ruminant-like Old World Monkey”:

A shaded sketch of a speculative descendant of geladas. It's a baboon-like quadrupedal monkey with front limbs longer that the hind limbs giving it a sloping back. It has a long almost horse-like snout, small high-set eyes, small ears, a rather long neck for a primate, and a short tufted tail. Its front limbs are digitigrade and have large curved claws, while its back limbs are plantigrade with smaller nail-like claws.

After much of East Africa rifted off into a separate continent, shifting climate turned the alpine grasslands of what was once the Ethiopian Highlands into into warmer subtropical forests – and the highly terrestrial grass-eating geladas that inhabited the region adapted to new sources of food.

Yedenigelada pendrewsii is a large quadrupedal herbivorous monkey, about 1.5m tall at the shoulder (~5′). It has a specialized pseudoruminant digestive system with a three-chambered stomach, similar to that of camelids, and it occupies an ecological niche convergent with the ancient chalicotheres, selectively browsing on trees and shrubs while sitting upright and using its long clawed forelimbs to pull branches within reach.

Unlike its highly social ancestors this species is mostly solitary, although during the breeding season groups of males come together in leks to compete for female attention. Displays consist of inflating large colorful throat pouches to make loud resonating calls, and flipping upper lips to bare teeth and gums.

A shaded sketch of the head of a speculative descendant of geladas, showing its upper lip flipped up to display its teeth and gums, and an inflated red throat pouch.