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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.

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.

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.

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.