alphynix
alphynix
Nix Draws Stuff
Paleontology, science, and the general weirdness of nature. Also, feathering ALL the dinosaurs.
3458 posts
alphynix · a day ago
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New Ceratosaur Species Unearthed in Brazil
http://www.sci-news.com/paleontology/berthasaura-leopoldinae-10285.html
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alphynix · 2 days ago
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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.
Three different species have been described, with Skara anulata from the Swedish Orsten Lagerstätte (~497 million years ago) being the largest of them.
It was still minuscule even as an adult, just 1.5mm long (0.06"), and had an eyeless head with a small protrusion at the front tip that may have been some sort of sensory organ. It had five pairs of appendages on its head and one pair at the front of its body, with most of them bearing long dense spines along their edges, then the rest of its long body was limbless and ended in a forked "tail".
With its spiny limbs it would have been a filter-feeder, probably swimming just above the seafloor and either sifting out suspended particles from the water or brushing along the surface of the sediment to stir up organic matter and microbial biofilms from the mud.
The skaracarids exact evolutionary relationships are uncertain, although they've been proposed to be related to either the copepods or the oligostracan crustaceans, particularly the mystacocarids and the fish lice.
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There's still a lot of murkiness in the crustaceans' Cambrian fossil record, and disputes over the arthropod evolutionary tree in general, so there's plenty of room for future discoveries to give us some interesting surprises.
And one day, probably not even that far in the the future, everything I've written and illustrated for this series will be just as ridiculously outdated as the bizarre old reconstructions of Hallucigenia and the hypotheses about the Burgess Shale "weird wonders" all representing near-alien "failed" branches of life with no living relatives.
I can't wait to see what weird things we discover next.
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alphynix · 3 days ago
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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.
One of the earliest known of these crustacean larvae, and one of the oldest known true crustaceans, is Wujicaris muelleri.
Found in the Chinese Chengjiang fossil deposits (~518 million years ago), this microscopic larvae was just 270μm long (0.01"). It had a wide shallow head shield with a backwards-pointing spine, a pair of eyes, and another long spine projecting from its underside at the front of its body.
Like similarly-shaped modern larvae it probably lived on or just above the seafloor as meiofauna, using its developed head appendages for both locomotion and catching food particles.
For such an early example of a crustacean it's surprisingly similar to modern forms, resembling the "metanauplius" stages of some copepods and barnacles. Along with Yicaris, another larva from the same deposits, it was a probably a basal member of the major crustacean lineage that both those groups are part of: the altocrustaceans.
It suggests that some groups of crustaceans established their specific larval forms very early on in their evolution, and hit on a something that worked so well for them that they've barely needed to change it in over half a billion years.
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The pentastomids, or "tongue worms", are a very unusual group. Small worm-like animals that almost exclusive parasitise the respiratory tracts of vertebrate hosts, outwardly they don't even look like they're arthropods – but their true affinities are revealed by their chitinous "skin" and arthropod-like nervous system.
Their evolutionary affinities have been controversial, but they're now generally considered to be highly specialized and modified crustaceans and very closely related to the parasitic fish lice. An alternate hypothesis proposes them instead as being surviving early panarthropods, related to tardigrades or lobopodians – but this is based purely on morphology, while the crustacean placement is also supported by genetic evidence.
While their fossil record is very poor, there are several different larvae known from the late Cambrian, including Heymonsicambria scandica.
Found in the Swedish Orsten Lagerstätte (~497 million years ago), this larva was about 0.5mm long (0.02"). Like other tongue worms it had four limbs on its head ending in hooked grasping claws, but unlike modern forms it also had two pairs of vestigial legs further down on its body.
It's unclear if these very early pentastomids were actually parasitic, or what their hosts would have been at the time if they were. Conodonts have been proposed as potential hosts, but they also may have initially externally parasitized other types of arthropods – a Silurian-aged fossil shows an ancient tongue worm "caught in the act" of clinging on to an ostracod.
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alphynix · 4 days ago
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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.
Cambropachycope clarksoni is known from a small number of specimens from the Swedish Orsten Lagerstätte (~497 million years ago). And at first this stem-crustacean seems like it's easily one of the most bizarre of all Cambrian animals, looking more like a speculative alien creature design (or possibly a pokémon) than a real once-living animal.
Up to 1.5mm long (0.06"), it had a large pointed "head" bearing a single huge compound eye with up to 150 facets. Its long tapering body had one pair of simple appendages at the front, then three pairs of biramous appendages, and then four more pairs of paddle-like appendages that decreased in size further back.
…But its strange anatomy starts to make a little bit more sense once you figure out what's actually going on: that's not a head.
The "head" is actually just an eyestalk with a pointy projection on the back. Cambropachycope's actual head encompasses the first third or so of its body, with the first appendages being antennae and its mouth located on its underside just behind them. The next three pairs of biramous limbs are other head appendages – probably corresponding to the second antennae, mandibles, and maxilluae of other pancrustaceans – and then the "paddles" are its thorax limbs.
With its huge cycloptic eye and swimming paddles it was probably an active predator preying on smaller planktonic animals, possibly convergently similar to some modern predatory water fleas.
A second very similar-looking stem-crustacean was also found in the same deposits – Goticaris longispinosa – hinting that there may have been an entire weird lineage of these little alien swimmers in the late Cambrian oceans, which the Orsten fossils are only giving us the smallest glimpse at.
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Meanwhile some of the most abundant animals in Orsten deposits are the phosphatocopids, with around 30 different species known from between about 518 and 490 million years ago.
These tiny arthropods were less than 1cm long (0.4") and had bivalved carapaces completely covering their bodies. They're mostly known just from their empty shells, but some specimens do preserve the rest of them, mostly represented by various early larval stages with the exact anatomy of later larvae and adults being less well understood.
They were traditionally regarded as being true crustaceans and part of the ostracods, and sometimes were lumped together with various other similar Cambrian bivalved groups like the isoxyids and bradoriids – but the similarities between them seem to have been superficial and convergent. More recently phosphatocopids have been considered to be stem-crustaceans instead, very closely related to the true crustaceans but not quite part of that lineage.
Klausmuelleria salopiensis is one of the earliest known phosphatocopids, found in the Comley Limestone deposits in England (~511 million years ago). Just 340μm long (~0.01") it's only known only from a single very early larval stage – often referred to as a "head larva" because it's essentially just a swimming head, with all four of its pairs of appendages corresponding to the head region of older individuals.
Like other phosphatocopids it would have lived just above the seafloor in oxygen-poor waters, and probably mostly fed on suspended particles of organic detritus. Some species had particularly strong spiny gnathobases at the bases of their appendages, however, suggesting they may have also been crushing and manipulating harder food items, possibly "micro-scavenging" on dead small planktonic animals.
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alphynix · 5 days ago
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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.
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Some of the earliest known pancrustaceans were the hymenocarines, a lineage of superficially shrimp-like mandibulates with a bivalved carapace covering their head and thorax. Although known only from the Cambrian, they were a diverse group during their existence and were some of the most abundant arthropods in some fossil sites.
Sometimes they're considered to be early or stem-mandibulates, branching off before the euthycarcinoids and the myriapod lineage, but generally they're placed as some of the earliest known pancrustaceans – and Ercaicunia multinodosa helps support that idea.
Known from the Chinese Chengjiang fossil deposits (~518 million years ago), Ercaicunia was about 1.1cm long (~0.4"). Micro-CT scanning of some of its tiny fossils has revealed much of its anatomy in high detail, showing features that identify it as one of the earliest known pancrustacean fossils – and the earliest that isn't microscopic.
It was either part of an early branch of the hymenocarines, or alternatively wasn't quite an actual hymenocarine itself, possibly being a very close relative or stem-member of their lineage.
It had a pair of large oval valves forming a carapace around the front of its body, with its long limbless abdomen and tail fan extending out the back. There were 16 pairs of biramous limbs on its thorax and several pairs of small spines on its back, and its head bore two pairs of antennae – one large pair in front and another much smaller pair hidden under its carapace – along with a pair of mandibles and maxilluae.
It also doesn't seem to have had any eyes, suggesting it had some sort of lifestyle where vision wasn't much use. Its mouthparts also indicate it was probably feeding on something that required manipulation and chewing up, but details of its diet and ecology are still poorly understood.
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Fibulacaris nereidis was one of the more unusual hymenocarines, known from the Canadian Burgess Shale deposits (~508 million years ago).
Up to 2cm long (0.8"), its narrow keeled carapace had a long backwards-pointing spine, with the front of its body curling over completely in a U-bend so that its head also faced backwards. It had stalked eyes, no obvious antennae or mandibles, and around 40 pairs of limbs running all the way along to its tail fan, indicating it had either a very short or a non-existent abdominal region.
It was probably a filter-feeder, using the shape of its carapace and the beating of its many legs to create a current drawing plankton and suspended organic particles towards its mouth. The combination of this lifestyle and its unusual anatomy suggests it probably swam around upside down, similar to some modern crustaceans like fairy shrimp.
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alphynix · 6 days ago
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Cambrian Explosion #57: Tuzoiida
What were tuzoiids?
We don't know.
Despite hundreds of specimens having been found, and around 20 different species being described, these arthropods are an ongoing puzzle.
They're known from between about 518 and 505 million years ago, in deposits associated with tropical and subtropical regions all around the world. They had large spiny bivalved carapaces up to 18cm long (7"), shaped like an upside-down domed taco shell, with a distinctive reticulated net-like surface ornamentation – but the rest of their ecology and anatomy is very unclear.
Most fossils are just empty carapaces, which appear to have been made of unmineralized chitin. Rare examples of soft-part preservation show they had a pair of stalked eyes sticking out the front, and a pair of short simple antennae, but impressions of the rest of their bodies are fragmentary and indistinct enough to not be particularly helpful.
They may have been related to the similar-looking isoxyids, or they might have closer affinities to the pancrustaceans or possibly even the equally enigmatic thylacocephalans. 
Unless we find some better fossils with clearer anatomy, they're going to remain a mystery.
Tuzoia canadensis was one of the pointier-looking tuzoiids, known from the Canadian Burgess Shale deposits (~508 million years ago). About 10cm long (4"), its carapace was lined with many long spines, with a more prominent spiky side frill than most other species.
Its reticulated ornamentation had fairly large polygonal facets, and like in other tuzoiids this is thought to have had a strengthening function – allowing it to create a large but very thin carapace with enough structural integrity to hold its shape.
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alphynix · 7 days ago
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Cambrian Explosion #56: Euthycarcinoidea
The euthycarcinoids were a group of euarthropods known from the mid-Cambrian to the mid-Triassic (~500-254 million years ago), surviving through multiple mass extinctions including the devastating "Great Dying" at the end of the Permian that finished off the trilobites. But despite an evolutionary history spanning around 250 million years they have a very sparse fossil record, extremely rare and known from less than 20 species across their entire time range.
For a long time their affinities were uncertain, and they've been variously suggested to have been crustaceans, trilobites, or chelicerates, or even to have been a lineage of earlier stem-euarthropods. But since the early 2010s better understanding of their anatomy has placed them in the mandibulates, probably as the closest relatives of the myriapods and helping to close the gap between the aquatic ancestors of that group and their earliest known terrestrial forms.
Most known euthycarcinoid species lived in brackish and freshwater environments, but some Cambrian species have been found in tidal flats associated with the terrestrial trace fossils Protichnites  and Diplichnites – indicating that they were amphibious and some of the very first animals able to walk on land.
Mosineia macnaughtoni is a Cambrian euthycarcinoid known from the Blackberry Hill fossil deposits in Wisconsin, USA (~500-489 million years ago).
At around 10cm long (4") it was fairly large for a Cambrian euarthropod, and like other euthycarcinoids it had a three-part body plan with a head bearing antennae, eyes, and mandibles, a thorax with pairs of uniramous legs, and a limbless abdomen that ended in a tail spine.
It probably ventured out of the water to feed on the rich microbial mats that covered the mudflats, taking advantage of an environment free from predators and competition. It may also have laid and fertilized its eggs on the shore, similarly to modern horseshoe crabs.
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alphynix · 8 days ago
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Cambrian Explosion #55: Fuxianhuiida 
In the final stretch of this month we finally come to the last of the major groupings of euarthropods: the mandibulates, which include the modern myriapods (centipedes and millipedes) and pancrustaceans (crustaceans and insects), along with several extinct groups.
Characterized by possessing mandible mouthparts, mandibulates are by far the largest lineage of arthropods and the most successful group of animals on Earth. Over a million living species are known (most of of which are insects, particularly beetles) and an estimated six-to-ten times more than that are probably still undiscovered.
Mandibulates probably diverged from their chelicerate cousins around the start of the Cambrian 540 million years ago. If the trilobites and their artiopodan relatives were early or stem-mandibulates then the earliest known fossils of the group are about 521 million years old, otherwise the first records come from a few million years later in the Chinese Chengjiang fossil deposits (~518 million years ago).
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The fuxianhuiids were a group known only from South China between about 518 and 512 million years ago, found in the Chengjiang, Xiaoshiba, and Guanshan fossil deposits. Such a limited regional distribution is particularly unusual for a major group of Cambrian euarthropods.
They've traditionally been considered to be basal or stem-euarthropods due to their fairly "primitive" overall appearance, but more recently a new idea has begun to emerge based on well-preserved head anatomy – that they're actually early mandibulates and possibly even closely related to the myriapods.
Fuxianhuia protensa is the eponymous species of this lineage, known from Chengjiang around 518 million years ago. Up to about 4cm long (1.6"), hundreds of specimens have been found representing varying life stages, showing that it underwent anamorphic development similar to some myriapods, gradually adding more and more segments to its body with each molt.
Its eyes were stalked, behind them was a pair of stout antennae, and on the underside of its head was a pair of specialized backwards-pointing appendages that may have been mandibles. It had a soft unmineralized exoskeleton, and like the xandarellid artiopodans it showed "segment decoupling" on its thorax, with multiple pairs of biramous limbs associated with each upper segment towards the rear of its body. Its long narrower abdomen was limbless and ended in a pair of fluke-like plates and a short tail spine.
Its large number of segments and even larger number of limbs may have been an adaptation to fluctuating oxygen levels in the Changjiang environment, with greater numbers of flap-like gills providing a much larger surface area for respiration in hypoxic conditions.
It had complex brain anatomy comparable to some crustaceans, and may be one of the earliest known fossil examples of a species practicing extended parental care. Mature adults have been found associated with multiple juveniles, all about the same age and possibly from the same clutch, suggesting they were living together and being protected.
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alphynix · 9 days ago
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Cambrian Explosion #54: Trilobita – Transform and Roll Up
Most trilobites were able to roll themselves up into a protective ball – a behavior known as enrollment or volvation – exposing just their heavily armored backs to attackers. They're often found fossilized curled up like this, and rare preservation of soft tissues shows that they had a complex system of muscles to help them quickly achieve this pose while simultaneously tucking their antennae and all their limbs safely inside their enrolled shells.
Some species also developed sharp defensive spines and spikes that jutted out when they enrolled, making themselves even more daunting to potential predators in one of the earliest known examples of an evolutionary "arms race".
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The agnostids were a group that first appeared in the early Cambrian very soon after the redlichiids and ptychopariids, and lasted until the end of the Ordovician about 444 million years ago. They probably evolved from some of the first ptychopariids*, and were highly specialized compared to other early trilobites, with large head and tail shields that usually looked almost identical, sometimes lacking eyes entirely, and only having two or three thorax segments – showing some convergent similarities to their nektaspid cousins.
( * There's been some debate about whether agnostids are actually trilobites, or whether they're closely related "trilobitomorph" artiopodans or even stem-crustaceans that just happened to somewhat resemble trilobites. This is due to soft-tissue preservation revealing limb anatomy that was very different from trilobites and looked more like the legs of crustaceans. But despite some other oddities, like having six pairs of head appendages instead of trilobites' usual four, the rest of their anatomy still places them firmly as artiopods, and as either trilobites or almost-trilobites.)
Found all around the world mainly in deeper-water environments, agnostids probably spent most of their time crawling around on the sea floor, but they may also have been able to swim in a semi-enrolled posture. They're also sometimes found preserved sheltering inside the empty hard parts of other animals, such as hyolith shells or Selkirkia tubes.
Some agnostid fossils have been found clustered in large numbers around the remains of other animals, suggesting they may have been opportunistic predators and scavengers, sometimes even cannibalising each other.
Agnostus pisiformis was a typical member of this group, known from the mid-to-late Cambrian (~506-492 million years ago) from sites including Sweden, Siberia, England, Wales, and Canada – widespread and common enough that it can be used as an index fossil to help date rock layers.
It grew to about 1cm long (0.4"), and while its front and back ends looked very similar the tail shield can be identified by a small pair of spines on its outer edge. Despite having only a couple of articulated body segments to work with it was capable of rolling up so its head and tail shield met and fully enclosed its body, forming a pea-sized protective capsule.
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The corynexochids were another major group of trilobites to appear in the early Cambrian, descending from early redlichiids. They were one of the longest-lived trilobite lineages, lasting until the late Devonian about 376 million years ago, and often had large tail shields and spines adorning their bodies.
Olenoides nevadensis was a fairly rare species of corynexochid, known from the Wheeler Shale (~507 million years ago) and Marjum Formation (~505 million years ago) in Utah, USA 
Up to about 5cm long (2"), it was very spiky for a Cambrian trilobite, with long "cheek" spines, pointy side lobes on its body segments, a row of spines along its back, and four pairs of spines on its tail shield. Like some other early trilobites it wouldn't have quite been able to roll into a tight ball, instead forming more of a "cylinder" – with its bristling spiny segments protecting its partially-exposed sides, and its tail shield lining up with its head to present an intimidating row of spikes.
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alphynix · 10 days ago
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Cambrian Explosion #53: Trilobita – A Prolific Paleozoic Posse
The biggest stars of the Cambrian euarthropods, and most of the Paleozoic Era, were of course the trilobites. Known from literally tens of thousands of species spanning over 270 million years, they're some of the most recognizable and popular fossils.
Trilobites' exact evolutionary origins and transitional forms are unknown, but they're thought to have originated in Siberia in the very early Cambrian and their leg anatomy indicates they were a part of the artiopodan lineage. They made a sudden and dramatic entrance to the fossil record about 521 million years ago, appearing fully-formed and rapidly diversifying and spreading all around the world within just a couple of million years.
Their hard calcified exoskeletons made them much more likely to fossilize than soft-bodied animals, with a distinctive three-part body plan consisting of a head shield, three-lobed thorax segments, and a tail shield. Each individual regularly molted their carapace throughout their life, meaning that most trilobite remains are actually empty discarded shells rather than actual carcasses.
Along with being heavily armored arthropod tanks, most species were also able to roll themselves up to defend against predators, and some developed additional elaborate spines and spikes.
…And some were just weird.
Many trilobites had well-developed complex compound eyes with crystalline lenses (and some had elaborate "hyper-compound eyes"), but much of the rest of their anatomy is still poorly known, with only about 21 species found with preserved soft body parts. We do know they had a pair of antennae, biramous limbs with walking legs and feathery gill branches, and at least one species had a pair of cerci at its rear end, but there was probably a lot of soft-part diversity in this massive group that we'll just never know about.
I could easily spend entire months on trilobites alone, but there's still a handful of other major groups of Cambrian arthropods to get through as we head towards the final week of this series. So we only really have time for the barest glimpse at their ridiculous variety.
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Some of the very earliest trilobites were the redlichiids, which ranged from about 521 to 500 million years ago during the early and mid Cambrian. They were more of an an "evolutionary grade" than a distinct lineage, giving rise to several other major groups during their time.
They were mostly fairly "standard"-looking trilobites with flattened bodies, although some had long spines or unusually large numbers of segments – up to 103 in one species.
Kleptothule rasmusseni was one of the odder-looking redlichiids, found in the Sirius Passet fossil deposits in Greenland (~518 million years ago). About 3cm long (1.2"), it had an elongated and relatively thin many-segmented body and a pointy head, giving it an almost snake-like appearance.
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The ptychopariids were another very early group of trilobites but were somewhat longer-lasting, surviving until the end of the Ordovician about 444 million years ago. Much like the redlichiids they were more of an "evolutionary grade" splitting off various other major lineages during their existence – including the proetids, the group that eventually became the last trilobites to ever live at the very end of the Permian.
They were also fairly standard in appearance (with a few eyeless forms and odd exceptions), but they also include one of the most common and familiar trilobites in the world: Elrathia kingii.
Up to around 5cm long (2"), this trilobite is known from massive numbers of specimens (possibly in the millions) from the Wheeler Shale in Utah, USA (~507 million years ago). It had a wide thorax and short spines on the "cheeks" of its head shield, and seems to have been unusually tolerant of low oxygen conditions on the seafloor. It was one the earliest known animals to exploit such an environment, avoiding predators and competition, and it probably either grazed on sulfur-oxidizing bacteria or had evolved a symbiosis with them similar to modern giant tube worms.
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alphynix · 11 days ago
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Cambrian Explosion #52: Artiopoda – Who Needs A Thorax Anyway?
The nektaspids were one of the most unique-looking groups of artiopodans, with soft-shelled unmineralized bodies, no eyes, and large head and tail shields with very few actual body segments in between – varying from 6 all the way down to none at all.
First appearing in the fossil record around 518 million years ago, only a few different species are known but they appear to have been abundant animals distributed in outer shelf waters worldwide during the Cambrian.
Their classification has traditionally been uncertain but specimens with well-preserved limbs show very trilobite-like leg anatomy, helping to place them in the artiopodans as potentially some of the closest "trilobitomorph" relatives to the actual trilobites.
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Naraoia compacta was one of the nektaspids with no thorax segments at all, having a semicircular head shield and a large oval tail shield. Up to 4cm long (1.6"), exceptional soft-part preservation shows it also had sideways-pointing antennae and biramous limbs with large paddle-like gill-fringed flaps.
It's known from various Cambrian sites around North America, most famously the Canadian Burgess Shale deposits (~508 million years ago). Additional fossils from the similarly-aged Chinese Kaili Biota and the older Australian Emu Bay Shale (~514 million years ago) represent either even more occurrences of Naraoia compacta or a very similar closely related species in the same genus.
It was probably a bottom-dwelling animal, possibly a burrowing deposit feeder eating organic matter in seafloor mud.
One Australian Naraoia specimen shows some of the earliest direct evidence of predation in the fossil record, with a distinct bite taken out of the carapace – thought to have been inflicted by a large radiodont.
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The evolutionary relationships of Aaveqaspis inesoni are unclear, but its general body shape suggests it may have been one of the few-segmented nektaspids.
Known from the Sirius Passet fossil deposits in Greenland (~518 million years ago), it was about 2.5cm long (1") and had a trilobite-like shape with an eyeless semicircular head shield, five thorax segments, and a tail shield bearing two huge backwards-pointing spines.
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Nektaspids were a surprisingly long-lived group of non-trilobite artiopodans, surviving through multiple mass extinction events and lasting at least until the late Silurian about 420 million years ago. The last known member was another Naraoia species in North America, Naraoia bertiensis.
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alphynix · 12 days ago
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Cambrian Explosion #51: Artiopoda – Surprising Lookalikes
The aglaspidid artiopodans were a major lineage of early Paleozoic euarthropods – one of the most diverse after their cousins the trilobites, although far far behind them in terms of actual species numbers.
But despite their diversity and worldwide range actual fossils of them are incredibly rare, and for a long time they were considered to be a "problematic" wastebasket group of uncertain affinities, mainly interpreted as being related to the chelicerates. More recently evidence from preserved limb anatomy has instead placed them within the artiopodans in a grouping known as vicissicaudatans, closely related to forms like Sidneyia and the later cheloniellids.
Unusually for euarthropods they had a phosphatic exoskeleton, and they experienced their main burst of diversification in the late parts of the Cambrian period, after most of the actual evolutionary explosion had already settled.
They mainly inhabited shallow near-shore environments, and may actually have been some of the very first animals to venture onto land. Some examples of the trace fossil Protichnites might represent aglaspidids scuttling over the Cambrian shorelines to mate and lay their eggs in a similar manner to horseshoe crabs.
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Kodymirus vagans was first discovered in the 1960s but was an enigmatic species for quite a while. It was interpreted as the earliest known sea scorpion in the mid-1990s, but more recently it's been reclassified as a vicissicaudatan artiopodan that wasn't quite a true aglaspidid, instead being a very closely related "stem" lineage.
Known from the Paseky Shale in Czechia (~514-509 million years ago), it reached about 8cm long (~3") and had a distinctive pair of large spiny appendages that resembled the "great appendages" of stem-euarthropods like megacheirans. However, unlike the disputed anatomical origins of those structures, Kodymirus' huge claws were instead modified from its first pair of legs in a striking case of convergent evolution.
(Some older reconstructions depict it with six pairs of enlarged limbs, but this is based on the outdated sea scorpion interpretation. A more recent study of specimens found individuals were only ever preserved with one pair each.)
It lived in very shallow brackish coastal waters in a calm lagoon, in what seems to have been a rather sparse ecosystem completely lacking many common Cambrian animals like trilobites and brachiopods. Possibly the environment was isolated and inhospitable enough that it allowed Kodymirus to take advantage of an ecological role usually occupied by other arthropods.
Scratch-mark trace fossils in the Paseky Shale match the size and shape of Kodymirus' raptorial appendages, but only ever show one side being used at a time. This suggests it was an active predator that swam along just above the seafloor, trailing the spines of one appendage through the soft surface of the sediment and "skimming" for hidden prey to grab.
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Glypharthrus trispinicaudatus was part of a lineage of Cambrian aglaspidids that were even more trilobite-shaped than most other artiopodans, convergently evolving a very similar appearance despite not being particularly closely related to them.
Known from the Guole biota in southern China (~490 million years ago), it was about 3cm long (1.2"). It had a large semicircular head shield with elongated eyes, 12 body segments, and a pair of furcae and a long tail spine at the rear of its body.
Like most aglaspidids it inhabited fairly shallow waters, with the Guole fossil assemblage probably representing an upper continental slope.
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The aglaspidids were successful and hardy enough to survive past the end of the Cambrian and into the Ordovician – although "Ordovician-type" members of the group had a distinct appearance compared to their earlier relatives, with more rounded head shields and their eyes either very reduced or missing entirely, suggesting that they took up a different lifestyle.
They eventually disappeared towards the end of the Ordovician, about 450 million years ago, possibly struggling to cope with increasing competition from the ongoing Great Ordovician Biodiversification Event.
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alphynix · 13 days ago
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Cambrian Explosion #50: Artiopoda – More Than Just Trilobites
The dominant group of Cambrian euarthropods were the artiopodans, a hugely diverse and long-lasting lineage that included the familiar trilobites along with all their close relatives.
They were some of the first euarthropods to appear in the fossil record, with fully formed trilobites seeming to "suddenly" appear about 521 million years ago and quickly spread worldwide. With the ancestral euarthropods estimated to have arisen between 550 and 540 million years ago, and the ancestral artiopodans not long after that, this means there must have been a lot of very rapid evolution and diversification in the space of just 20-30 million years.
Artiopodans were generally seafloor-crawling animals with flattened bodies and wide flaring segments in a trilobite-like shape. Different species could range from about 1mm (0.04") to around 70cm long (2'4") – with the largest Cambrian forms reaching as much as 55cm (1'10"), rivalling some of the bigger radiodonts in size.
Their exact position within the euarthropod family tree is still uncertain and varies from analysis to analysis, sometimes allied with the chelicerates in a group known as arachnomorphs, or sometimes instead placed as stem-mandibulates. Also sometimes the marrellomorphs are considered to be closely related to them.
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Sidneyia inexpectans was an artiopodan from the Canadian Burgess Shale deposits (~508 million years ago). Up to about 12cm long (5"), it had a somewhat boxy shape with three clearly defined body regions – a head shield with small stalked eyes protruding from the sides, a segmented thorax, and a tail with a fluke-like tail fan.
It was probably a bottom-dwelling predator and scavenger, and preserved gut contents show it mainly fed on hard-shelled prey like small trilobites.
It seems to have had a fairly widespread distribution, with fossils also known from similarly-aged deposits in North China. Possible additional fossils have also been found in Greenland and the United States, and a potential second species Sidneyia sinica in the 10-million-year-older Chengjiang deposits in southwest China, but these aren't widely accepted as being valid.
Living alongside Sidneyia in the Burgess Shale ecosystem was the enigmatic Burgessia bella. About 4cm long (1.6"), it resembled a tiny horseshoe crab with a rounded carapace, a pair of antennae, and a long tail spine. It had no eyes and was probably a seafloor scavenger, feeding on carrion and organic detritus.
Its evolutionary affinities are uncertain, but it's often placed as a close relative of either the marrellomorphs or the artiopodans.
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The xandarellids were a group of rare artiopodans characterized by unmineralized shells, large semicircular head shields, and "segment decoupling" – having each plate on their backs associated with multiple pairs of legs instead of just one, with the number of legs per plate increasing further towards the rear of their bodies.
They're known mainly from the Chinese Chengjiang fossil deposits (~518 million years ago), but a single species in the younger Moroccan Tatelt Formation (~508 million years ago) indicates they had a wider and longer range than previously thought.
Cindarella eucalla from Chengjiang is one of the better-known members of this group, with both its exoskeleton and some of of its soft-part anatomy preserved in some specimens.
It was a fairly big euarthropod for the time at about 12cm long (5"), and had nearly 40 pairs of legs but only about 23 body plates, some of which were completely covered by its large head shield. Its stalked eyes stuck out from under the sides of its head and had around 2000 lenses each, suggesting it had excellent vision.
It was probably an actively mobile animal living on or near the seafloor, either hunting smaller animals or scavenging, with its keen eyesight allowing it to both locate food in low-light conditions and avoid larger predators like radiodonts.
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alphynix · 14 days ago
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Cambrian Explosion #49: …Some Sort Of Euarthropod?
The major groups of the euarthropods are the chelicerates, mandibulates, and the extinct artiopodans, but there were some Cambrian species that still can't be easily fitted in to any of those lineages.
Kiisortoqia soperi is a fairly common species in the Sirius Passet fossil deposits in Greenland (~518 million years ago), with nearly 200 specimens discovered. About 5cm long (2"), its anatomy indicates it was definitely a euarthropod with a fully segmented arthrodized body and limbs – but it can't be confidently associated with anything more specific than that.
It had a large pair of spiny grasping appendages on its head with a surprising resemblance those of radiodonts. It's unclear if they were anatomically equivalent to those front appendages and inherited from an ancestral euarthropod with great appendages, or if they were convergently modified from antennae or another pair of limbs.
It's been proposed as having some sort of relation with the panchelicerates, or with the artiopodans and trilobites. But it may also have just been sort of its own thing, part of a separate lineage of very basal euarthropods that were the "sisters" to all the rest of the group.
A very similar-looking species named Bushizheia was recently described from the similarly-aged Chinese Chengjiang fossil deposits, but sadly it doesn't preserve any additional anatomical details that would actually help with figuring out these arthropods' evolutionary affinities.
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The marrellomorphs were a group of euarthropods that had unmineralized exoskeletons, making them unlikely to fossilize except in areas of exceptional preservation. They were striking-looking animals often sporting large elaborate head shields, and while only a few species are known due their very patchy fossil record, in some places they seem to have actually been incredibly common.
They were traditionally thought to be unusual trilobite-like animals and an early branch of the artiopodans or arachnomorphs, but some studies have instead recently placed then as stem-mandibulates. Their anatomy doesn't fully match them to any of the major lineages, though, and it's possible they were their own basal branch of weird euarthropods, very closely related to the ancestors of the other groups but not quite actually part of them.
The marrellomorph Marrella splendens, nicknamed the "lace crab", is one of the most numerous fossils found in the Canadian Burgess Shale deposits (~508 million years ago), with over 25,000 specimens found.
About 2cm long (0.8"), it had large backwards-pointing spikes on its head, long antennae, a pair of stout paddle-like appendages, and around 25 body segments with biramous limbs. Preserved diffraction gratings on its outer spikes indicate parts of it were iridescent.
It may have used its paddle-like appendages for propulsion, swimming around in what must have been abundant colorful swarms in the then-tropical shallow Burgess sea. Its not clear if it had eyes – there may have been a pair located under the bases of the front spikes – but its later relative Mimetaster is at least known to have had small stalked eyes on its head.
Some of Marrella's rear legs had a mesh of spiny inwards projections on their undersides that might have functioned as a filter-feeding sieve, catching food particles and then later passing them up to its mouth using its other limbs.
Much much rarer in the Burgess ecosystem was Skania fragilis, known from less than 30 specimens.
It had a superficial resemblance to the enigmatic Ediacaran animal Parvancorina, and for a while the two were thought to potentially be closely related – but more recently Skania has been determined to have actually been another type of marrellomorph.
About 1cm long (0.4"), it was covered in a large carapace that gave it a trilobite-like shape, with the tips of a pair of antennae and several additional pairs of elongated antenna-like appendages sticking out at the front of its body, and many pairs of shorter legs hidden underneath.
Despite their incredibly patchy fossil record the marrellomorphs were a successful bunch for quite a while, and they lasted until at least the early Devonian (~400 million years ago). It's likely they were wiped out later in that period by a series of extinction events that badly affected marine life at the time.
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alphynix · 15 days ago
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Cambrian Explosion #48: Panchelicerata
Now, at just over halfway through the month, we've finally reached the proper euarthropods!
One of the major branches of this group are the chelicerates, which include modern horseshoe crabs, arachnids, and (probably) sea spiders, and a few extinct groups like the sea scorpions. Characterized by having chelicerae mouthparts, they're a very diverse and successful group of animals, with spider and mite species being especially numerous.
Their lineage is estimated to have diverged from other euarthropods around the start of the Cambrian 540 million years ago, and along with their ancient stem-chelicerate relatives they make up a slightly larger grouping known as panchelicerates. Sometimes these are also combined even further with trilobites and their close relatives to make up the arachnomorphs.
Sea spiders are usually classified as the earliest branch of the chelicerates, but they've also been suggested to possibly be a completely separate lineage of the very earliest-diverging euarthropods instead. Late Cambrian microfossils of what may be sea spider larvae have been found in the Swedish Orsten Lagerstätte (~497 million years ago), and if they are actually sea spiders (and sea spiders are actually chelicerates) then these would represent the earliest known true chelicerates in the fossil record.
But the earliest known panchelicerates come from slightly earlier in the Cambrian.
Known from the Canadian Burgess Shale deposits (~508 million years ago), Habelia optata was about 2cm long (0.8") and had a multi-segmented armored carapace covered in long spines and tiny bumps, and a long two-part tail. Its head was especially complex, with numerous appendages making up an intricate set of mouthparts specialized for crushing and processing small hard-shelled prey – an arrangement convergently resembling the jaws of mandibulates.
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Urokodia aequalis is known from the Chinese Chengjiang fossil deposits (~518 million years ago). It's thought to have been slightly more closely related to true chelicerates than Habelia was – possibly being an unusual-looking mollisoniid.
About 2.5cm long (1"), it superficially resembled a millipede, with a spiny head shield, many body segments, and a paddle-like tail. What may have been stout antenna-like head appendages are preserved in one specimen, but otherwise little else is known about the rest of its anatomy.
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alphynix · 16 days ago
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Cambrian Explosion #47: Bradoriida
The tiny bradoriids first turn up in the fossil record just before the earliest known trilobites, about 521 million years ago, and very quickly became some of the most abundant euarthropods in the mid-Cambrian. Found all around the world, they were clearly important components of many Cambrian food webs and probably had varying lifestyles from species to species, ranging from living on the seafloor to actively swimming around in the water column.
Less than 2cm long (0.8"), they're mostly known just from fossils of their bivalved carapaces, but some specimens preserve evidence of a pair of antenna and varying arrangements of biramous and uniramous limbs.
They were traditionally thought to be crustaceans closely related to ostracods, but some studies have instead shifted them towards being considered stem-crustaceans or stem-mandibulates instead. And more recently rare high-detail preservation of the soft anatomy of a few species have suggested they actually belong even further down the arthropod evolutionary tree, as "higher stem" euarthropods positioned between the megacheirans and the earliest actual euarthropods.
Kunmingella douvillei is one of the best-known bradoriids, with its fossils being incredibly abundant in the the Chinese Chengjiang fossil deposits (~518 million years ago). About 5mm long (0.2"), it had two different types of biramous limbs and a couple of pairs of uniramous limbs at the back.
Some female specimens have been found preserved with eggs, carrying up to 80 on their  rear biramous limbs, representing some of the oldest evidence of brood care in the fossil record.
After their initial success the bradoriids gradually declined through the late Cambrian, but a few species persisted into the Ordovician and they finally went extinct sometime around the middle of that period – which was also around the time that the ostracod crustaceans began to appear and diversify.
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