eumetazoa – Page 2 – Nix Illustration

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).

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”.

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.

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.

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.

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.

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.

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.

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.

Cambrian Explosion #46: Megacheira

The megacheirans were a grouping of Cambrian stem–euarthropods that had a distinctive pair of “short great appendages” on their heads with long finger-like spines. Their bodies were fully arthrodized into multiple hard-shelled segments, and their double-branched biramous limbs featured both legs and gill-fringed paddle-like flaps.

As small predators and scavengers they were important elements of some Cambrian ecosystems, and probably swam around just above the sea floor using their arm-like great appendages to grab and tear apart food items.

They weren’t really a distinct lineage, more of an “evolutionary grade“, usually considered to be in the “higer stem” close to the common ancestor of all the major euarthropod lineages, or possibly the very earliest actual euarthropods – but some studies instead place them as more “advanced” euarthropods closely related to chelicerates.

Part of this classification disagreement comes down to whether their great appendages were anatomically equivalent to the front appendages of radiodonts or whether they were convergently evolved, and whether one or both of those structures are also related to the development of antennae and chelicerae in later euarthropods or if they became highly modified into the labrum instead. It’s a subject of longstanding and ongoing debate so complicated that it’s known as the “arthropod head problem” – or sometimes “the endless dispute”.