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Rhacheosaurus

Metriorhynchids were a group of fully marine crocodyliforms known from the mid-Jurassic to the early Cretaceous of Europe and the Americas. They were the most aquatic-adapted of all known archosaurs, with streamlined bodies, smooth scaleless skin, small front flippers, larger hind flippers, and shark-like tail flukes. They may also have been endothermic, and might even have given live birth at sea rather than laying eggs.

Rhacheosaurus gracilis here was a metriorhynchid that lived in warm shallow waters around what is now Germany during the late Jurassic, about 150 million years ago. Around 1.5m long (~5′), its long narrow snout lined with delicate pointed teeth suggests it fed on small soft-bodied prey, a niche partitioning specialization that allowed it to coexist with several other metriorhynchid species in the same habitat.

Unlike most other marine reptiles metriorhynchids didn’t have particularly retracted nostrils, which may have had a limiting effect on their efficiency as sustained swimmers since higher-set nostrils make it much easier to breathe without having to lift the whole head above the surface. The lack of such an adaptation in this group may be due to their ancestors having a single nasal opening formed entirely within the premaxilla bones at the tip of the snout, uniquely limiting how far it could easily shift backwards – other marine reptiles had nostrils bound by the edges of multiple different bones, giving them much more flexibility to move the openings around.

(By the early Cretaceous a close relative of Rhacheosaurus did actually evolve nostrils bound by both the premaxilla and the maxilla, and appeared to have started more significant retraction, but unfortunately this only happened shortly before the group’s extinction.)

Metriorhynchids also had well-developed salt glands in front of their eyes, but the large sinuses that accommodated these glands may have made their skulls ill-suited to deep diving, being more susceptible to serious damage from pressure changes and restricting their swimming to near-surface waters only.

Preserved skin impressions in some metriorhynchid fossils show several unusual “irregularities”, including curl shapes, small bumps, and cratering. It’s unknown what exactly caused these marks, but they may represent scarring from external parasites such as lampreys and barnacles.

Anthracodromeus

Anthracodromeus longipes was an early reptile* that lived in what is now Ohio, USA, during the late Carbonifeorus about 307-305 million years ago.

(*or possibly a very reptile-like stem–amniote)

Around 20cm in total length (~8″), it had a rather lizard-like shape with a long body and a short tail. The digits on all four of its limbs were highly elongated with hooked claws, which appears to have been an adaptation for climbing.

It inhabited a coal forest dominated by lycopsid trees and seed ferns, and as one of the earliest known tetrapods to develop climbing behavior it would have had some ecological advantages over its relatives, being better able to escape from predators and access new food sources.

Tauraspis

Osteostracans were an ancient group of jawless fish, closely related to early jawed vertebrates, whose fossils are known from the mid-Silurian to the late Devonian of what is now North America, Europe, and Asia.

They were heavily armored, with bony head shields and rows of large scales covering their bodies. While their flattened shapes and upward-facing eyes have resulted in them traditionally being interpreted as mud-grubbing bottom-dwellers, their paddle-shaped pectoral fins, dorsal fins, and strong tails indicate they were also quite good swimmers – and their diverse hydrodynamic head shield shapes suggest they probably had a much wider range of ecologies than previously thought.

Although many osteostracans had large flaring spines on the sides of their heads, or long snout-like spikes at the front, Tauraspis rara here was unique in having two long front-facing horn-like projections.

Around 7.5cm long (~3″), it lived in brackish and freshwater environments in what is now northern Siberia during the early Devonian, about 410-407 million years ago. Like other osteostracans it had a small keyhole-shaped “nostril” opening, and large patches of sensory organs known as “cephalic fields” on the sides and top of its head shield.

The fields were covered with a mosaic of small bony plates, and their exact function is still a mystery – but they may have been involved in sensing vibrations in the water, or possibly even been electric organs.

Similarly, what Tauraspis used its unusual pair of “horns” for is also unknown.

Romaleodelphis

Romaleodelphis pollerspoecki was a dolphin-like toothed whale related to the ancestors of both modern oceanic dolphins and beaked whales, living in coastal waters covering what is now Austria during the early Miocene about 22 million years ago.

Although only known from a single fossil skull, this cetacean was probably around 3m long (~9’10”). It had a long snout lined with over 100 small pointed uniformly-shaped teeth, and the bony walls of its inner ears were well-preserved enough to show that it was able to hear narrow-band high frequency sounds – a specific form of echolocation that has convergently evolved multiple times in various modern and extinct toothed whale lineages.

Based on the presence of ancient river-mouth deposits in the area where Romaleodelphis was found, it may potentially have been capable of traversing between marine, brackish, and freshwater environments similar to the modern franciscana.

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.