science illustration – Nix Illustration

Hwiccewyrm

Hwiccewyrm trispiculum lived during the late Triassic, around 208-202 million years ago, in what is now England. It was one of the last known members of the procolophonid family, a lineage of small stocky lizard-like animals that had been widespread and abundant earlier in the Triassic.

(Traditionally procolophonids are classified as parareptiles, but some recent studies suggest this group is paraphyletic or polyphyletic, with some “parareptiles” potentially nesting within the diapsids instead.)

Measuring around 30cm long (~1′), Hwiccewyrm had wide flaring cheek bones ornamented with large spines, and like some other procolophonids it may also have had bony scute armor on its body. Its large blunt teeth suggest it was feeding on particularly tough foods such as fibrous vegetation or hard-shelled invertebrates.

Solenochilus

The distinctive pinhole eyes, leathery hood, and numerous tentacles of modern nautiluses were traditionally thought to represent the “primitive” ancestral state of early shelled cephalopods – but genetic studies have found that that nautiluses actually secondarily lost the genes for building lensed eyes, and their embryological development shows the initial formation of ten arm buds (similar to those of coeloids) with their hood appearing to be created via fusing some of the many tentacles that form later.

There’s a Cretaceous nautilidan fossil that preserves soft tissue impressions of what appear to be pinhole eyes and possibly a remnant of a hood, so we know these modern-style nautilus features were well-established by the late Mesozoic. But for much more ancient Paleozoic members of the lineage… we can potentially get more speculative.

So, here’s an example reconstructed with un-nautilus-like soft parts.

Solenochilus springeri was a nautilidan that lived during the Late Carboniferous, around 320 million years ago, in shallow tropical marine waters covering what is now Arkansas, USA.

Up to about 20cm in diameter, (~8″), its shell featured long sideways spines which may have served as a defense against predators – or possibly as a display feature since they only developed upon reaching maturity.

Mexidracon

Mexidracon longimanus was an ornithomimid dinosaur that lived during the late Cretaceous, about 72 million years ago, in what is now northern Mexico.

It was a fairly small ornithomimid at around 2.5-3m long (~8-10′), and its leg proportions suggest it was less specialized for fast running than some of its other relatives. But its most unique feature was its hands, with extremely elongated metacarpal bones – giving it palms that were longer than its upper arm!

It’s unclear what the function of these unusual limbs was. Possibly they gave Mexidracon extra reach to hook its claws onto foliage and pull it within reach of its head, in a similar manner to ground sloths, chalicotheres, and therizinosaurs.

April Fools 2025: When Pterosaurs Were Swimming Gryphons

When it was first described by Cosimo Alessandro Collini in 1784, Pterodactylus wasn’t originally interpreted as a flying reptile. The idea that species could go completely extinct wasn’t fully understood yet, so fossils were assumed to represent things that still existed somewhere in distant unexplored regions. And so, since the oceans seemed like the best place for undiscovered animals to hide, this strange little creature was initially speculated to be aquatic.

Although it was soon properly recognized as a flying animal with some surprisingly mammal-like early reconstructions, the aquatic idea persisted until at least 1830 when Johann Georg Wagler published a restoration of Pterodactylus with huge membranous paddle-like flippers. He even grouped pterosaurs together with ichthyosaurs, plesiosaurs, and monotremes in a proposed vertebrate class called “Gryphi” (literally “gryphons“) and considered them all to be transitional between birds and mammals.

Wagler’s whole classification system seems esoteric and improbable by modern standards, but it’s a fascinating look at a pre-Darwinian “chain of being” sort of mindset where all organisms were thought to exist in a fixed hierarchy with pre-set roles.

Incamys

Incamys bolivianus was a caviomorph rodent representing an early member of the chinchillid family, with its closest modern relatives being chinchillas and viscachas.

Living in what is now Bolivia and Argentina during the late Oligocene about 27 million years ago, it inhabited an arid open grassland at a time when the area’s climate had drastically cooled due to the formation of the Antarctic Circumpolar Current.

It’s estimated to have been similar in size to a large modern chinchilla – weighing around 700g (~1lb 8oz) and measuring about 25-30cm long not including the tail (~10-12″).

An endocast of the shape of its brain from a near-complete fossil skull shows that it had a well-developed sense of hearing, particularly in vocalization processing, suggesting it may have been a social animal living in groups communicating with complex calls similar to modern chinchillids. It was probably a ground-dweller less agile than its modern relatives, but still capable of fast movements.

Rhyniella

Rhyniella praecursor was an early springtail that lived during the early Devonian, about 410-400 million years ago, in what is now Scotland. Discovered in the exceptionally well-preserved Rhynie chert fossil site, it’s one of the earliest known hexapods.

It was around 2mm long (~0.08″) and closely resembled some of its modern relatives – with distinctive anatomical features like a collophore and a furca – showing that springtails were already well-established in such an early terrestrial ecosystem.

It probably had a similar sort of ecological role to modern springtails, too, being involved in the breaking down of organic matter and the formation of soils.

Greenwaltarachne

Greenwaltarachne pamelae was an orb-weaver spider that lived in what is now Montana, USA, during the mid-Eocene, around 46 million years ago.

Known from a single fossil of an adult female, it had a body length of about 2mm (~0.08″) and a legspan of around twice that. The specimen is even well-preserved enough to show banded markings on the legs resembling those of some modern orb-weaver species.

It would have lived in what was then a rift valley with a tropical climate, along the shoreline of the ancient 160km long (~100 miles) Lake Kishenehn. It was part of a highly diverse ecosystem full of numerous other invertebrates – including miniscule fairyflies, and even mosquitoes with evidence of blood preserved inside their bodies – and a wide variety of mammals ranging from tiny rodents to large brontotheres.

Hassianycteris

Hassianycteris messelensis was an early bat that lived in what is now Germany during the mid-Eocene, about 47 million years ago.

It’s generally considered to be very closely related to the common ancestry of modern bats – but a recent study suggests that the stem-bat evolutionary tree is actually quite a bit more complicated than previously thought.

It had a 35-40cm wingspan (~14″-16″), and thanks to the exceptional preservation of the Messel Pit fossil site we actually know some details about its external life appearance. One specimen preserves a soft-tissue impression of its ear shape, and fossilized melanosomes suggest that its fur was colored reddish-brown.

Its wing proportions indicate it was adapted to fly high and fast in open spaces, and its strong jaws and preserved gut contents show it mainly preyed on tough-shelled insects like beetles.

Miosiren

Miosiren kocki was a sirenian (sea cow) that lived during the early Miocene (~20-15 million years ago) in what is now the North Sea basin in northwestern Europe.

Similar in size to the very largest modern manatees, about 4-4.5m long (~13-14’10”), it has traditionally been classified as an early member of the manatee lineage – but a study in 2022 suggested it may instead represent a much earlier stem of the sirenian evolutionary tree, with its ancestors potentially having diverged around 34 million years ago.

It had unusually thickened bones in its skull, especially around the roof of its mouth, which would have given its jaws a very strong chewing force. Isotope analysis of its teeth show it was part of a marine algae-based food web, unlike the seagrass-based diets of other sirenians, so it may have been specialized to feed on a much tougher diet. Possibly it was eating something like calcareous algae, or more speculatively it might even have been crunching on hard-shelled algae-consuming marine invertebrates.

Eons Roundup 15

It’s time for a little bit more recent PBS Eons work this week:

An illustration of Arctodictis, an extinct predatory mammal related to modern marsupials. It's a vaguely wolverine-like animal with a boxy snout, a low-slung body, stout limbs, and a long tail. It's depicted in a crouched stalking pose, and it's colored grey-brown with darker spot and stripe markings. — Arctodictis sp.

An illustration of Thoatherium, an extinct hoofed mammal. It resembles a small horse or deer, with a long face, high-set pointed ears, a boxy body, a short tail, and long slender legs each ending in a single hoofed toe. It's depicted with its head lowered to browse on some leaves, and it's colored reddish-brown with a darker back and limbs and a lighter belly — Arctodictis sp.

• The metatherian predator Arctodictis and the litoptern ungulate Thoatherium from “The Mystery of South America’s False Horses”