cambrian explosion – Nix Illustration

Tuzoia

The tuzoiids were an enigmatic group of Cambrian invertebrates known mostly just from their spiny bivalved carapaces. Although hundreds of fossils of these arthropods were discovered over the last century or so, only vague fragments of the rest of their bodies have been found even in sites usually known for preserving soft tissue impressions.

…Until late 2022, when several new specimens from the Canadian Burgess Shale deposits (~508 million years ago) were described showing tuzoiid anatomy in exceptional detail, finally giving us an idea of what they looked like and where they fit into the early arthropod evolutionary tree.

Tuzoiids like Tuzoia burgessensis here would have grown up to about 23cm long (~9″). They had large eyes on short stalks, a pair of simple antennae, a horizontal fluke-like tail fan, and twelve pairs of appendages along their body – with the front two pairs at the head end being significantly spinier, and most (or all) of these limbs also bearing paddle-like exopods.

The large carapace enclosed most of the body, and was ornamented with protective spines and a net-like surface pattern that probably increased the strength of the relatively thin chitinous structure.

Together all these anatomical features now indicate that tuzoiids were early mandibulates (part of the lineage including modern myriapods, crustaceans, and insects), and were probably very closely related to the hymenocarines.

Tuzoiids seem to have been active swimmers that probably cruised around just above the seafloor, with their stout legs suggesting they could also walk around if they flexed their valves open. The arrangement of their spiny front limbs wasn’t suited to grabbing at fast-swimming prey, but instead may have been used to capture slower seafloor animals or to scavenge from carcasses.

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.

Paucipodia

Lobopodians were some of the earliest known panarthropods, closely related to velvet worms, tardigrades, and the ancestors of all the true arthropods. They were small soft-bodied worm-like animals with multiple pairs of fleshy legs, and some species also bore elaborate spikes, armor plates, and fleshy bumps all over their bodies – with the spiny Hallucigenia being the most famous example.

But unlike its more charismatic relative Paucipodia inermis here didn’t seem to have any ornamentation at all.

Known from the Chinese Chengjiang fossil deposits, dating to about 518 million years ago, Paucipodia lived in what was then a shallow tropical sea. Its 13cm long (~5″) tubular body had nine pairs of legs, with each foot tipped with a pair of hooked claws, and the inside of its mouth was ringed with tiny sharp teeth.

Several specimens have been found preserved in association with the weird gummy-disc animal Eldonia, which may indicate Paucipodia either preyed on them or scavenged on their carcasses.

Some Paucipodia fossils also have enigmatic tiny “cup-like” organisms attached to their legs. It’s currently unknown what exactly these were, or whether they were parasitic in nature or simply opportunistically “hitching a ride” similar to the Inquicus found on armored palaeoscolecid worms in the same fossil beds.

Stanleycaris

Radiodonts were early arthropods with specialized frontal appendages, disc-like mouths, complex compound eyes, and swimming flaps along the sides of their bodies. Once considered to be bizarre “weird wonders” of the Cambrian Explosion that represented a failed evolutionary experiment, we now know that they were actually a highly diverse and successful lineage that lasted for at least 120 million years.

While some radiodonts were the largest animals of their time periods, Stanleycaris hirpex here was one of the smallest known members of the group – although at around 10cm long (~4″) it was still respectably big compared to most other Cambrian animals.

Discovered in the Canadian Burgess Shale deposits (~508 million years ago), it was originally known only from isolated frontal appendages and mouthparts, and had been assumed to be a fairly typical member of the hurdiid family. But the recent discovery of over 200 new fossils, including some exceptionally well-preserved full body specimens, has catapulted it directly from being poorly-known into now being one of the most completely known of all radiodonts.

And it had a very big surprise for us, right in the middle of its face.

It turns out that Stanleycaris had a huge third eye, unlike anything ever seen in a radiodont before. A large unpaired eye was also part of the five-eyed arrangement in opabiniids and Kylinxia, and finding a similar example in radiodonts too raises the possibility that this sort of well-developed “median eye” may have been more widespread in early arthropods than previously thought.

Along with the third eye, some of the Stanleycaris specimens preserve fine internal details of its nervous system and show that its brain was made up of two segments instead of the three seen in modern arthropods. It also had gills positioned on its underside, unlike most other radiodonts which had them on their backs.

Retro vs Modern #09: Hallucigenia sparsa

If just one single species had to represent how our reconstructions of prehistoric animals can drastically change, it would have to be Hallucigenia sparsa.

1970s

First discovered in the 1910s in the Canadian Burgess Shale fossil deposits, specimens of Hallucigenia were initially categorized as being a species of the early polychaete worm Canadia. It wasn’t until the 1970s that they were recognized as being something else entirely, and the first reconstruction of this tiny animal was bizarre.

It was depicted as a long-bodied creature with a single row of tentacles along its back, and several pairs of long sharp spines that were interpreted as being stilt-like “legs” used to walk. The tentacles were thought to catch food from the water and pass it forwards to the bulbous “head” – and at one point it was even proposed that all the tentacles had their own additional “mouths” at their tips!

It’s easy to look back on this version now and laugh at how ridiculous and obviously wrong it was, but it’s important to remember the historical context here. This was coming from a point when the incredible animal diversity of the Cambrian Explosion was only just starting to be understood, revealing a range of poorly-understood bizarre and alien-looking forms like Opabinia – “weird wonders” that were considered to be representatives of previously unknown ancient branches of life.

At the time, Hallucigenia‘s utter weirdness and impractical body plan seemed to almost make sense as a unique evolutionary “failed experiment” that had left no living relatives.

1990s

Discoveries of legged-and-armored lobopodian “worms” in the Chinese Chengjiang fossil deposits during the 1980s prompted a re-interpretation of Hallucigenia in the early 1990s. Speculatively reconstructing it as a lobopodian with the spines on its back and with the tentacles as a set of paired clawed legs started to make it seem a lot less alien and a lot more like a real velvet-worm-like animal – and just a year later the “missing” other half of the leg pairs was confirmed to be present in some of the fossil specimens.

But it was still unclear which end was actually the head, and whether the large blob-like structure was a real part of Hallucigenia‘s anatomy or just an artifact of the fossilization process.

2020s

New research in the mid-2010s finally settled the head problem and clarified a lot of Hallugicenia‘s anatomy, discovering that the slender elongated end had a pair of simple eyes and a mouth with a throat ringed with tiny teeth.

We now know Hallucigenia sparsa lived all around the world during the mid-Cambrian, about 518-508 million years ago, with body fossils known from Canada and China and isolated spines found in numerous other similarly-aged locations. Instead of an evolutionary dead-end “weird wonder” it was actually an early member of the vast arthropod lineage, just one of a highly diverse collection of successful Cambrian lobopodians, and its closest living relatives are probably velvet worms and tardigrades.

It grew up to about 5cm long (2″) and had seven pairs of long sharp defensive spines along its back, covered with a microscopic surface texture of tiny triangular “scales”. It had seven pairs of clawed walking legs, with most of its feet tipped with two claws each but the final two pairs having just one, and its body ended right at the final pair of limbs – the “blob” structure in some fossils was actually just an artifact the whole time, formed by Halligenia‘s innards being forcefully squeezed out during its burial in the seafloor sediment.

Its neck region bore three pairs of long delicate tendril-like limbs, which may have been covered in feathery hair-like structures for filter-feeding similar to some other lobopodians. A small pair of velvet-worm-like antennae may also have been present on its head, and could have been a sexually dimorphic feature.

Utaurora

Ever since the bizarre anatomy of Opabinia was first recognized in the 1970s, it’s been a persistently unique “weird wonder” of the Cambrian period. Over the decades we’ve figured out that it was an early type of arthropod in an evolutionary position between lobopodians and radiodonts, but this whole time it’s still been sitting there alone as the only known representative of a weird stem-lineage with no other known close relatives.

…Until now!

A fossil from the Wheeler Shale in Utah, USA (~507 million years ago) that was originally thought to be a tiny radiodont has been re-studied, and now we finally have another member of the opabiniid family: Utaurora comosa.

Only about 3cm long (1.2″), Utaurora had 15 pairs of swimming flaps along the sides of its body, and a tail region with a 7-part fan and a pair of serrated spines. Hair-like gill blades covered both its back and the bases of its swimming flaps, and although its head region was poorly preserved it probably had an arrangement of 5 eyes and a long flexible claw-tipped proboscis similar to that of Opabinia.

Its discovery extends both the geographical and temporal known range of opabiniids, and suggests that their continued scarcity in other Cambrian fossil sites compared to other soft-bodied arthropods may simply be because they were just incredibly rare animals in those habitats at the time.

Cambrian Explosion #61: Crustacea – Little Wigglers

We’re finally at the end of this series, and to finish off let’s look at one of the few types of Cambrian true crustaceans that are known only from fully mature adults: the skaracarids.

These tiny soft-bodied meiofaunal animals are known from late Cambrian areas of “Orsten-type preservation” in Sweden and South China, with a possible additional fragmentary occurrence in Poland – suggesting that they had a global distribution.

Cambrian Explosion #60: Crustacea – Larvae Larvae Everywhere

One of the characteristic features of the crustacean lineage are their larval forms, passing through various tiny larval stages. They often look nothing like their eventual adult forms and historically weren’t even recognized as being the same species, with their complex lifecycles not being properly recognized until the late 1800s.

A lot of Cambrian crustaceans are only known from their larvae, preserved in exquisite microscopic detail in sites of “Orsten-type preservation”. Only disarticulated fragments of larger-bodied forms have been found in a few places, and it isn’t until much later in the Paleozoic that fossil crustaceans actually seem to become abundant in marine ecosystems.

It’s not clear why there’s such a bias in their early fossil record compared to most other arthropods, but possibly they were just very very rare animals early on. Adult forms may have mostly lived in places where they just didn’t fossilize, while their tiny larvae sometimes dispersed into different environments with a better chance of preservation.

Cambrian Explosion #59: Stem-Crustacea – Actual Ancient Aliens & Bivalved Buddies

The majority of known fossils of Cambrian crustaceans are in the form of minuscule microfossils with “Orsten-type preservation” – formed in oxygen-poor seafloor mud and exceptionally well-preserved in three-dimensional detail. They can only be discovered and studied after dissolving away the rock around them with acid and picking through the residue under a microscope, then they’re scanned with an electron microscope to see their fine details.

And it turns out some of these tiny early crustaceans looked really weird.

Cambrian Explosion #58: Hymenocarina

The pancrustaceans are a grouping of mandibulates that contains all of the crustaceans and hexapods (insects and their closest relatives) along with their various stem-relatives.

They’re critical components of most ecosystems on the planet, and are major parts of the nutrient cycle. In aquatic environments the crustaceans dominate, with modern copepods and krill being some of the most abundant living animals and making up enormous amounts of biomass providing vital food sources for larger animals. On the land springtails and ants are especially numerous, and the air is full of flying insects, the only invertebrates to ever develop powered flight. Some groups of insects have also co-evolved complex mutualistic partnerships with flowering plants and fungi.

Hexapods and insects don’t appear in the fossil record until the early Devonian, but they’re estimated to have first diverged from the crustaceans* in the early Silurian (~440 million years ago), around the same time that vascular plants were colonizing the land.

(* Hexapods are crustaceans in the same sort of way that birds are dinosaurs. They originated from within one of the major crustacean lineages with their closest living relatives possibly being the enigmatic remipedes.)

But crustaceans and their pancrustacean ancestors go back much further into the Cambrian, and we’ll be finishing off this month and this series with some of those early representatives.