November 2024 – Nix Illustration

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.