paleoart – Page 5 – Nix Illustration

Ursactis

Soft-bodied annelid worms only very rarely fossilize, so the group’s origins during the Cambrian Period are still rather poorly understood. So far about thirteen different species have been found in sites of exceptional preservation, showing that even very early on in their evolution these worms had already diversified into a wide range of ecologies including bottom-feeders, carnivores, swimmers, tube-builders, and even symbiotes sharing living space with early acorn worms.

Ursactis comosa here adds a fourteenth species to the list. Found in a newly-discovered outcrop of the 508-million-year-old Burgess Shale fossil deposits in western Canada, it’s known from nearly 600 specimens clustered together in several large groups, making it the current best-known and most numerous of all Cambrian annelids.

Up to about 1.5cm long (~0.6″), it was a polychaete-like worm bearing bundles of long bristles. There was a pair of large sensory palps on its head, and its body was made up of an unusually small number of segments – just 10, with larger individuals just increasing the size of their segments instead of adding on additional ones like most modern annelids.

Unlike other Cambrian annelids it also shows some evidence of basic tagmatization, differentiating some of the rear segments of its body with much longer bristles.

The large numbers of Ursactis found preserved in one place suggests these worms were exhibiting some sort of swarming behavior. Since ages from juveniles to fully-grown adults are represented together, and their anatomy indicates they were crawling detritivores, they were probably all taking advantage of a particularly nutrient-rich patch of seafloor at the time they were abruptly buried in a mudslide.

Dunkleosteus

With its armored head and blade-like jaws, the placoderm fish Dunkleosteus terrelli is an iconic Paleozoic animal.

Living during the Late Devonian, about 375-359 million years ago, in subtropical waters covering parts of what are now North America and Europe, this species is known mostly just from the bony plates that covered its head and thorax. The rest of its skeleton was cartilaginous and rarely ever fossilized (only a few vertebrae and the pectoral fin are currently known), so its full body shape and size is poorly understood, and previous length estimates have ranged all the way up to 10m (33′).

…Except it turns out it wasn’t nearly that big.

Based on its head proportions, along with comparisons to more complete remains of other arthrodire placoderms, recent studies instead come up with a maximum length of about 4m (~13ft) – giving Dunkleosteus a much shorter-but-heavier chunky body shape, more like a tuna than a shark.

But even after this size revision Dunkleosteus would have still been one of the largest animals around at the time, with the ability to snap its jaws open at high speed and an incredibly strong bite force. It was probably specialized to mainly prey on other heavily-armored animals such as other placoderms and shelled cephalopods, and was likely a strong swimmer with a shark-like tail fin.

Preserved stomach contents in one fossil show remains of the fast-swimming cartilaginous fish Orodus – suggesting that much like the modern tuna it resembled, Dunkleosteus was also capable of bursts of high speed.

Rhombichthys

Ellimmichthyiformes were a group of ray-finned fish known from the early Cretaceous to the mid-Oligocene, about 140-30 million years ago. For much of that time they were quite widespread, found in various marine, estuarine, and freshwater environments across Africa, Eurasia, and the Americas.

Closely related to modern clupeiformes (herrings, sardines, and anchovies), and characterized by two rows of bony scutes – one in front of the dorsal fin and the other along the belly – they’re also known by the nickname “double‐armored herrings”.

Rhombichthys intoccabilis was a rather unusual-looking ellimmichthyiform from the mid-Cretaceous, around 95 million years ago. Living in shallow reef and lagoon waters covering what is now the West Bank in the Middle East, it was about 20cm long (~8″) and had a tall narrow dorsal fin along with incredibly elongated belly scutes that gave its body a rhombus-like profile.

Juveniles of this species seem to have lacked the extended belly scutes, instead having a much more rounded body shape. This may indicate that adults and juveniles occupied very different ecological roles, or that the distinctive scutes might have been a secondary sexual characteristic involved in displaying for courtship and reproduction.

Maghriboselache

Maghriboselache mohamezanei was a cartilaginous fish from the late Devonian Period, about 369 million years ago, living in the shallow marine waters that covered what is now the Anti-Atlas mountain range of Morocco in northwest Africa.

Up to around 2.5m long (~8′), it’s known from several exceptionally well-preserved and near-complete skeletons.

It had a streamlined body with large pectoral fins, small pelvic fins, and a strongly keeled crescent-shaped tail fin. And although it was superficially shark-like in appearance, it was actually part of a lineage known as cladoselachids, which were much closer related to modern chimaeras than to sharks.

It’s unclear if Maghriboselache had two dorsal fins like its close relative Cladoselache, but some specimens preserve evidence of a chunky spine where the front dorsal fin would have been. Others show no sign of a front dorsal fin or spine at all, suggesting there may have been some sexual dimorphism going on in this species, with males having a spine (and possibly also an associated front dorsal fin) and females only having a rear dorsal fin.

But the most unusual feature of Maghriboselache was its nose.

It had a very broad snout with large and unusually widely-spaced nostrils, which would have given it the ability to “smell in stereo” and determine the direction of scents carried through the water much more precisely – making it the earliest known example of that sort of sensory specialization.

Eons Roundup 13

I haven’t posted any PBS Eons commissions here for quite a while, so let’s catch up a bit of the backlog:

An illustration of the extinct mammal Repenomamus robustus. It's a low-slung, stocky, badger-like animal with a thick tail, depicted as colored brown and white. It's shown baring a mouth full of pointed teeth. — *Repenomamus robustus*

An illustration of the extinct mammal Repenomamus giganticus. It's a low-slung, stocky, badger-like animal with a thick tail, depicted as colored yellow-brown and dark brown. It's shown walking with its head held down as if stalking something. — *Repenomamus robustus*

The Cretaceous mammals Repenomamus robustus and Repenomamus giganticus, from “When Mammals Only Went Out At Night”
https://www.youtube.com/watch?v=JqZONKXWPfw

An illustration of a decaying carcass of the extinct whale Borealodon. Its skin is tattered and torn as if scavengers have been picking at it, and part of its lower jaw bone is exposed. — *Borealodon osedax* (carcass)

A carcass of the whale Borealodon, from “How Ancient Whales May Have Changed the Deep Ocean”
https://www.youtube.com/watch?v=1vb00-gcdtA

An illustration of the extinct early vertebrate Haikouichthys. It's a small eel-like animal with large eyes, a circular mouth, and seven gill slits. It's depicted as pale blue colored, and it's swimming in dark blue water. — *Haikouichthys ercaicunensis*

An illustration of the extinct early vertebrate Myllokunmingia. It's a small eel-like animal with fairly large eyes, a circular mouth, and six gill slits. It's depicted as pale green colored, and it's swimming in dark blue water. — *Haikouichthys ercaicunensis*

And the early vertebrates Haikouichthys and Myllokunmingia, from “Why Sour May Be The Oldest Taste”
https://www.youtube.com/watch?v=XXgd_cNZSvk

April Fools 2023: How Titanis Lost The Right To Bear Arms

Huge, flightless, and carnivorous, the phorusrhacids (or terror birds) were some of the largest apex predators in South America during its Cenozoic “splendid isolation” as an island continent – and they were possibly the closest that birds ever came to reclaiming the ecological roles of their extinct non-avian theropod dinosaur relatives.

And for a while in the late 1990s and early 2000s there was a hypothesis that they’d even re-evolved clawed hands.

This idea was based on the wing bones of Titanis walleri, the only terror bird known to have dispersed northwards during the Great American Biotic Interchange when North and South America became connected via the Isthmus of Panama.

Living during the Pliocene and Pleistocene in Florida and Texas, between about 5 and 1.8 million years ago, Titanis stood around 1.5-1.8m tall (~5-6′) and was heavily built, with long strong legs and a massive hooked beak. Remains of its small wings were incomplete and fragmentary but had seemingly unusual joints, with what looked like a stiffer wrist and more flexible “fingers” than other birds, which led paleontologist Robert Chandler to propose in 1994 that this terror bird species had modified its wings into clawed grasping arms similar to those of dromaeosaurs, used to restrain prey animals while its beak tore them apart.

But the idea of a giant murder-bird with added meathook-hands only lasted about a decade. Further investigation in 2005 showed that Titanis‘ arms weren’t that weird after all – the same sort of joints are found in terror birds’ closest living relatives, the seriemas, and so Titanis really had the same sort of small vestigial wings as many other large flightless birds.

…However, there still could have been some claws on there. Many modern birds actually have one or two small claws on their hands that aren’t visible under their feathers, and terror birds like Titanis having something like that going on is completely plausible – they just wouldn’t have been using them for any sort of specalized predatory function.

Strange Symmetries #22: The Whalerus And The Twisted Tusks

Mammalian tusks usually grow in symmetrical pairs with only minor developmental asymmetry, but a few species have evolved much more uneven arrangements.

A colored line drawing of the extinct toothed whale Odobenocetops. Its body is beluga-like but it has a face more like a walrus than a whale, with a big fleshy bristly upper lip and a pair of protruding tusks. The right side tusk is much longer than the left. It's depicted with a mottled brown and white color scheme. — *Odobenocetops peruvianus*

Odobenocetops peruvianus was a small toothed whale that lived during the Miocene, about 7-3 million years ago, in shallow coastal waters around what is now Peru. Around 3m long (~10′), it was a highly unusual cetacean with binocular vision, a vestigial melon, muscular lips, and a pair of tusks – features convergent with walruses that suggest it had a similar lifestyle suction-feeding on seafloor molluscs and crustaceans.

In males the right tusk was much more elongated than the left, measuring around 50cm long (~1’8″) in this species and up to 1.35m (4’5″) in the closely related Odobenocetops leptodon. Since these teeth were quite fragile they probably weren’t used for any sort of combat, and they may have instead served more of a visual display function.

And despite being closer related to modern narwhals and belugas than to other toothed whales, Odobenocetops’ long right-sided asymmetric tusks actually seem to have evolved completely independently from the iconic left-sided asymmetric spiral tusks of narwhals.

An edited meme image using screenshots of Dr. Doofenshmirtz from "Phineas and Ferb". The text reads: "If I had a nickel for every time whales evolved asymmetric tusks, I'd have two nickels. Which isn't a lot, but it's weird that it happened twice."

A colored line drawing of an extinct woolly mammoth. It's an elephant-like animal covered in a thick coat of brownish hair, with a high domed forehead, small ears, and long curving tusks. The tusks are noticeably asymmetrical, one curving more downwards than the other. — Woolly Mammoth (*Mammuthus primigenius*)

The woolly mammoth (Mammuthus primigenius) lived across Eurasia and North America during the last ice age, mostly from the Pleistocene about 400,000 years ago to the early Holocene about 10,000 years ago – altohugh a few relict populations survived until around 4,000 years ago in isolated areas of Alaska, Siberia, and eastern Russia.

Around 3m tall at the shoulder (~10ft), these hairy proboscideans had very long curving tusks that were used for digging out vegetation from under snow and ice, scraping bark from trees, and for fighting.

The tusks showed a lot of variation in their curvature, and were often rather asymmetrical, a condition also seen in the closely related Columbian mammoth. Like modern elephants mammoths may have also favored using one side over the other for certain tasks, which over their lifetimes could result in uneven wear exaggerating the natural asymmetry even more.

Strange Symmetries #21: Uneven Ungulates

Asymmetry is commonly seen in the headgear of modern even-toed ungulates, with natural genetic variation, developmental stress, and injuries during life sometimes causing very wonky-looking horns or antlers.

No living species have asymmetry as a standard trait, however – but some fossil ungulates did.

Ramoceros osborni was a relative of the modern pronghorn living during the mid-Miocene, about 13 million years ago, in open plain habitats of what is now the Midwest and Mountain states of the USA.

It was smaller than modern pronghorns, around 70cm tall at the shoulder (~2’4″), and males had long antler-like horns with three tines. Bizarrely, one of these horns was always at least twice the size of the other, with “left-horned” and “right-horned” individuals seeming to occur in equal numbers.

It’s not clear how this asymmetry affected combat between males. Could they only properly lock horns with “opposite-sided” rivals, or did this uneven arrangement actually prevent physical fights and restrict them more to just visual displays?

An illustration of the head of Tsaidamotherium, an extinct hoofed mammal distantly related to modern giraffe and okapi. It has a vaguely moose-like head with a bulbous fleshy snout. Its left ossicone "horn" is above its eye and very small, while the right ossicone is much larger and positioned towards the middle of its forehead, forming a wide blunt helmet-like structure like a very stubby fat unicorn horn. — *Tsaidamotherium hedini*

Meanwhile in China another Miocene ungulate known as Tsaidamotherium hedini also had strange headgear, with an enlarged right “horn” forming a helmet-like dome on top of its head. This species was featured here on the blog just year, so check out that post for more details about it.

Strange Symmetries #20: The 16 Million Year Fiddler Crab Rave

Many decapod crustaceans have slightly asymmetrical pincers, often with one claw being chunkier and specialized for “crushing” while the other is more slender and used for “cutting”.

But fiddler crabs take this sort of asymmetry to the extreme as part of their sexual dimorphism – males have one massively oversized claw, which is used for both visual display to potential mates and for physical fights against rivals.

Some of the earliest fiddler crabs are known from the Miocene of what is now northern Brazil. Although the fossils have been given several different taxonomic names since their discovery in the 1970s (including Uca maracoani antiqua, Uca antiqua, and Uca inaciobritoi) they’re currently considered to be indistinguishable from the modern Brazilian fiddler crab, Uca maracoani, meaning that these crabs have remained externally unchanged for the last 16 million years.

Up to about 4cm in carapace width (~1.6″), modern Uca maracoani are found in coastal mangrove swamps and tidal mudflats around the northern and eastern coasts of South America – and some of these environments have also undergone little change since the Miocene. Males of the species can develop their enlarged pincer on either side of their bodies, with lefties and righties seeming to occur in equal numbers.

Strange Symmetries #19: Wonky Whales

Toothed whales – the branch of cetaceans that includes modern dolphins, porpoises, beaked whales, and sperm whales – have surprisingly asymmetrical skulls, with some of the bones skewed to one side and just the left nostril forming their blowhole.

Some of the most obvious external manifestation of this lopsidedness can be seen in sperm whales, which have their blowhole at the front left side of their head, and in male narwhals, which usually have a single left-side tusk.

This sort of asymmetry first appeared in the skulls of early toothed whales around 30 million years ago. And since the highest amounts of wonkiness have gone on to develop in lineages that hunt in dark, cluttered, or murky waters, this suggests that the trait is somehow linked to the evolution of complex echolocation.

Some ancient members of the river dolphin lineage also had some additional unusual asymmetry, sometimes having slightly sideways-bending snouts.

Ensidelphis riveroi was one of the weirdest of these, living around the coasts of what is now Peru during the Miocene, about 19 million years ago. Around 3m long (~10′), it had a very long narrow toothy snout that curved distinctly off to the right along its length.

A sketch showing Ensidelphis' bizarre side-curving snout. A hypothetical straight snout is shown outlined in blue, while the actual curvature is overlaid in red. — Expectation vs reality

It’s not clear what the function of this bend was, or even if the only known skull actually represents the normal condition for this species. But Ensidelphis’ bendy snoot might have been used to probe around in muddy seafloor sediment or to extract prey from crevices, possibly like an underwater version of the modern wrybill.