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Lewisuchus

Last week I mentioned the one oddball dinosauriform that had crocodilian-like osteoderm armor, so let’s take a look at that one too.

Lewisuchus admixtus lived in what is now northwest Argentina during the late Triassic, around 236-234 million years ago. About 1m long (3’3″), it was an early member of the silesaurids – a group of dinosauriforms that weren’t quite dinosaurs themselves, but were very closely related to the earliest true dinosaurs.

(They’ve also been proposed as instead being early ornithisichians, but we’re not getting into that today.)

Much like its later silesaurid relatives Lewisuchus had a long neck and slender limbs, and was probably mainly quadrupedal, possibly with the ability to briefly run bipedally to escape from threats. Its serrated teeth suggest it was carnivorous, likely feeding on both smaller vertebrates and the abundant insects found in the same fossil beds.

Uniquely for an early dinosauriform it also had a single row of bony osteoderms running along its spine. Although it lived at close to the same time as the similarly-armored Mambachiton their last common ancestor was at least 10 million years earlier, and no other early dinosaur precursors with osteoderms are currently known – so this was probably a case of Lewisuchus independently re-evolving the same sort of feature.

Mambachiton

Mambachiton fiandohana lived during the mid-Triassic, about 237 million years ago, in what is now Madagascar – which at the time wasn’t yet an island, still being connected to both east Africa and India as part of southern Pangaea.

It represents the earliest known branch of the avemetatarsalians, or “bird-line archosaurs”, a major group of the archosaur reptiles that also includes pterosaurs and dinosaurs/birds. 

It’s only known from a few fragments but it was probably around 2m long (~6’6″), and would have been a carnivorous lizard-like animal with a long neck and semi-erect quadrupedal limb posture.

Unexpectedly for a bird-line archosaur it also had a staggered double row of bony osteoderms along its back, suggesting that the very earliest avemetatarsalians had some crocodilian-like armor. This seem to have very quickly been lost, though – there’s no sign of osteoderms in the next branches to split off after Mambachiton, the aphanosaurs and pterosauromorphs – and although they occur again later in one dinosauriform and various non-avian dinosaurs, this appears to be multiple cases of independent re-evolution rather than retaining the original ancestral trait.

Triopus

Although Triopus draboviensis here might look like an isopod or a trilobite, this small arthropod was actually part of a rather rare group called cheloniellids.

Known from the early Ordovician to the early Devonian (~480-408 million years ago), only about 7 different species of cheloniellid have been described so far. Their evolutionary relationships were uncertain and controversial for a long time, but currently they’re thought to be distant cousins of trilobites within the Artiopoda.

Living in what is now Czechia during the late Ordovician, about 460-450 million years ago, Triopus is only known from two partial fossils. It was around 4cm long (~1.6″), and like other cheloniellids it had a body made up of wide radiating exoskeleton segments that fully covered its legs, and probably also a pair of whip-like appendages at the rear.

Its body was more domed than those of its relatives, who were generally very flattened, suggesting it was specialized for a slightly different lifestyle or habitat. Without any preserved appendages it’s not clear what its ecological role was, but since other cheloniellids had horseshoe-crab-like feeding structures it may have been a similar sort of generalist, preying on small invertebrates and scavenging carrion on the seafloor.

Wapitisaurus

Back in the 1980s, a fossil of a partial reptile skull was discovered in British Columbia, Canada, dating to the Early Triassic about 250 million years ago. Its triangular skull shape, large eye sockets, and what seemed to be distinctive spiky frills on the back of its head initially caused it to be identified as a relative of the gliding weigeltisaurids.

But the aptly-named Wapitisaurus problematicus would have had to be a very unusual member of this group. With an estimated length of up to 2m (6’6″) it was much larger than any other known weigeltisaurid, it was the only one known from the Triassic side of the “Great Dying” mass extinction event, it was found in a completely different part of the world, and its teeth seemed more like those of marine reptiles like thalattosaurs.

In recent years new discoveries and re-analysis of weigeltisaurid fossil material have resulted in much better modern understanding of their skull structure – and with that came the realization that Wapitisaurus really didn’t seem to match with them after all.

So a new study has finally identified what this problematic reptile really was… and it turns out the teeth didn’t lie! It was a marine thalattosaur all along!

Wapitisaurus had rather large eyes compared to most other North American thalattosaurs, and although the front parts of its jaws are missing it probably had a long slightly hooked snout similar to its close relative Thalattosaurus. It’s also now one of the oldest known members of the thalattosaur lineage, showing that some of their specialized skull features like retracted nostrils had actually appeared very quickly during their evolutionary history.

…Oh, and those “spiky frills” on the back of Wapitisaurus’ skull? They were actually all teeth from both the upper jaw and the palate, on broken shards of bone that had been displaced to just the right spot to muddle up its identity for over three decades.

Glossoceras

Although the only nautiloids living today have characteristic tightly coiled shells, earlier in their evolutionary history these cephalopods were much more diverse.

And Glossoceras gracile here is an example of one of the more unusual groups of nautloids: the ascocerids.

Living during the Late Silurian, about 422 million years ago, in wheat is now Gotland, Sweden, Glossoceras was only around 5cm long as an adult (~2″). Like other ascocerids it started out its life looking like a fairly standard early nautiloid, with a long straight shell that curved slightly upwards, but as it approached maturity things got weird – the front part of the shell grew out into a much more bulbous flask-like shape, and the old juvenile section broke off entirely.

The gas-filled buoyancy chambers of its adult shell were positioned directly above its body chamber rather than behind like in other nautiloids, giving it very good stability in the water. The shell walls were also very thin and lightweight, which would have made it a much more maneuverable swimmer.

Phosphatherium

Phosphatherium escuillei was one of the very earliest known members of the proboscideans, a lineage today represented only by the three living species of elephants.

Living in what is now Morocco during the late Paleocene and early Eocene, around 56 million years ago, it would have been about the size of a cat, roughly 30cm at the shoulder (~1′) and 60cm long (~2′). It had a fairly low flat head with a proportionally short snout, while the back end of its skull behind it eyes was elongated, supporting large powerful jaw muscles.

Wear patterns on its teeth suggest it ate a lot of tough vegetation, and it may have been a semiaquatic animal behaving somewhat like modern tapirs or pygmy hippos – spending a lot of the daytime lounging in water, and emerging onto land to forage during the night.

Spectember/Spectober 2023 #11: A Large Spider

An anonymous submission requested a “spider the size of a coconut crab”:

A shaded sketch of a speculative giant herbivorous descendant of jumping spiders. It's a big stocky spider covered in fuzz, with thick legs ending in cloven-hoof-like claws. It has the characteristic large main pair of eyes of jumping spiders, with the other three pairs more spread out around the front and sides of its wide head, and it also has two pairs of "horns". Its abdomen is wide and round, somewhat flattened on the top, with several spiky structures around the edge.

Ceratohispidus aspectus is a distant descendant of jumping spiders living on an Aotearoa-like landmass, isolated with no mammalian predators.

This particular lineage is notable for both their extreme gigantism (with their larger size and weight causing them to lose the ability to jump) and for having taken up herbivory in a similar manner to one modern species. Most of these big plant-eating spiders are around the size of wētāpunga, and occupy a similar ecological niche, but Ceratohispidus is the largest of them by far – rivalling the modern coconut crab with a body length of up to 40cm (~1’4″) and a legspan of almost 1m (~3’3″).

After reaching sexual maturity at 5-10 years old, adults grow very slowly, molting only once every year or two and taking several decades to actually get anywhere close to their maximum size.

Ceratohispidus’ thick legs end in hoof-like claws, and it selectively browses on vegetation by snipping off pieces with its pincer-like palps. A gizzard-like structure in its digestive system helps to grind up fibrous plant material with small gastroliths, and its wide abdomen houses both large book lungs and a tracheal system with air sacs that can contract and expand to provide a small amount of active ventilation.

While the “horns” and spikes ornamenting its body may provide some defense from the few avian and reptilian predators in its habitat, they’re mainly used as part of highly elaborate visual displays between individuals.

Spectember/Spectober 2023 #10: Tree Goat

An anonymous submitter asked for an “arboreal goat with grasping sloth-like claws”:

A shaded sketch of a speculative tree-climbing descendant of goats. It's a hairy sloth-like animal, clinging to a tree trunk with long chunky limbs that end in large hooked claw-like hooves. Its head is proportionally small, with fleshy giraffe-like lips, forward-facing eyes, and small stubby horns.

Cluraix cephalula is a distant descendant of feral goats in a tropical forest environment, representing a small tree-climbing offshoot of a specialized chalicothere-like lineage.

About 70cm long (~2’4″), it clambers around in the high tree canopies, with its forward-facing eyes providing good depth perception in this complex three-dimensional habitat. Its long hooked claw-hooves are used both to cling onto branches and to hook-and-pull clumps of foliage towards itself, stripping the leaves with its flexible fleshy lips.

Spectember/Spectober 2023 #09: Things With Wings

(Apologies for the abrupt absence – I’m okay, just having everything break down at once. This is fine.)

So— back to the speculative evolution request list!

TheBigDeepCheatsy requested a “cactus-dwelling/germinating evolution of introduced rosy-faced lovebirds”:

A shaded sketch of a speculative symbiotic relationship between lovebirds and saguaro cactus. The lovebird is shown on the left, a small parrot that looks very similar to modern rosy-faced lovebird except with hints of a more mottled color pattern. The cactus is shown on the right, a large fasciated saguaro with its top fanning out into a wide "crown", with several nest holes dug into it and multiple lovebirds occupying them.

While Agapornis cheatsyi is still quite physically similar to its introduced ancestors, this lovebird has developed a close symbiotic relationship with the cactus Carnegiea ornipolis, a descendant of the modern saguaro.

Naturally fasciated, this cactus grows a splaying fan-like crown which the lovebirds excavate their shallow nest burrows into. Feeding on the cactus’ fruit in early summer, the lovebirds then disperse the seeds via their droppings – a process that significantly improves propagation chances, both due to the birds commonly foraging and defecating around suitable nurse plants and the passage through their gut speeding up germination.

Someone calling themself “LB” asked for some “flying afrotherians”:

A shaded sketch of a speculative flying tenrec. It's a bat-like animal with membranous wings supported by three elongated fingers, and a large shrew-like head with long toothy jaws and an elephant-shrew-like nose.

Elbeitandraka venenifer is a descendant of tree-climbing Malagasy tenrecs that developed gliding membranes – and its lineage is now just about achieving true powered flight.

About 25cm long (~10″), its proportionally short broad wings require it to fly very fast to generate enough lift for its weight. It mostly only actively flies when traveling between roosts and feeding sites (or when escaping from threats), alternating between gliding to save energy and flapping to recover altitude.

It’s an opportunistic omnivore, crawling around in the tree canopy foraging for vegetation, fruits, fungi, invertebrates, and the occasional smaller vertebrate, using its flexible sengi-like nose to probe around in crevices.

Much like modern common tenrecs it’s capable of hibernating for months at a time through periods of scarce food availability. It also accumulates alkaloid toxins in its body from its arthropod prey, advertising its unpalatability to predators with bold contrasting warning coloration on its wing membranes.

And here’s a combination of a couple of anonymous requests for both “flying heterodontosaurs” and “dragons with hind leg wings, a la sharovipteryx”:

A shaded sketch of a speculative flying predatory heterodontosaurid dinosaur, show both on the ground in a quadrupedal posture and in flight. Its hind legs form its main wings, with elongated outer toes supporting large pterosaur-like flight membranes. It also has a hooked beak at the front of fanged jaws, an s-curved neck, a compact fuzzy body, short forelimbs with taloned hands and small stabilization membranes, and a long vaned tail.

Inversodraco rapax is a highly specialized Jurassic descendant of heterodontosaurids that took to climbing and gliding, developing delta-wing-like membranes on their hindlimbs convergently similar to those of the earlier sharovipterygids.

Around 75cm long (~2’6″), it has unusually flexible hip joints for a dinosaur, able to splay its legs out to the sides to deploy wings supported by an elongated outer toe on each foot. Its arms form small forewings for stability, and its long tail ends in a vane of stiffened feathers that aid in steering.

Unlike its herbivorous-to-omnivorous ancestors it’s primarily a carnivore, swooping down onto small prey and grabbing it with its talon-like forelimbs.

Spectember/Spectober 2023 #08: Various Filter-Feeders

Admantus asked for a “freshwater baleen whale”:

A shaded sketch of a speculative freshwater baleen whale. It has a very wide duck-like snout with whisker-like bristles, short baleen inside its mouth, very small reduced eyes, and broad paddle-like flippers.

Rostrorutellum admantusi is descended from small cetotheres that became isolated in a large inland body of water (similar to the modern Caspian Sea), eventually becoming landlocked and gradually reducing in salinity towards fully freshwater.

Highly dwarfed in size, just 2-3m long (~6’6″-9’10”), they’re slow swimmers with broad duck-like snouts that are used to scoop up mouthfuls of sediment and strain out their invertebrate prey in a similar feeding style to gray whales.

Due to the murkiness of the water, and the lack of large predators in their environment, they have poor eyesight and instead use sensory bristles and electroreceptors around their snouts to navigate and detect prey.

And an anonymous submission requested a “whale-like filter-feeding marine crocodile”:

A shaded sketch of a speculative filter-feeding crocodile. It has spatula-like jaws lined with many delicate closely-spaced needle-like teeth, flipper-like limbs, and a long paddle-like tail.

Sestrosuchus aigialus is a 6m long (~20′) crocodilian closely related to the modern American crocodile, living in warm shallow coastal waters.

It’s adapted for an almost fully aquatic lifestyle convergently similar to the ancient thalattosuchians, swimming with undulations of its long tail and steering with flipper-like limbs. But unlike other crocs it’s specialized for filter-feeding, with numerous delicate needle-like teeth in its jaws that interlock to sieve out small fish and planktonic invertebrates from the water.

A couple more suggestions also asked for “fully aquatic pinnipeds” and “future crabeater seal evolution”:

A shaded sketch of a speculative filter-feeding fully aquatic crabeater seal. It has four wing-like flippers, a streamlined body, and elongated jaws with many lobed teeth used to sieve krill.

Euphausiolethrus volucer is a fully aquatic descendant of the crabeater seal. About 5m long (~16’4″), it occupies the ecological niche of a small baleen whale in the krill-abundant Antarctic waters that lack most actual baleen whales.

Its jaws contain numerous finely-lobed teeth that are used to strain krill from the water, and it utilizes all four of its wing-like flippers to swim in an “underwater flight” motion similar to that of plesiosaurs.

Highly social, it tends to congregate in pods that cooperate to herd swarms of krill for easier feeding.