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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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.

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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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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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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.

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