#ptychopariida

The Ptychopariida were some of the earliest known trilobites, first appearing in the early Cambrian about 521 million years ago and surviving until the end of the Ordovician about 444 million years ago. They included some of the most numerous and common trilobite species, and were probably ancestral to multiple other major lineages – including the very last trilobites at the end of the Permian – making them incredibly important in understanding the overall evolution of trilobites as a whole.

…But this group is also one of the biggest wastebaskets in paleontology.

First established in the early 20th century, the ptychopariids seemed to have some fairly good defining characteristics based on their facial sutures, large thoraxes, and relatively small pygidia. But the group quickly became a dumping ground for a massive amount of Cambrian trilobites, eventually containing numerous different families, hundreds of genera, and many more individual species.

Actually figuring out their internal evolutionary relationships also turned out to be extremely difficult – so much so that some paleontologists working on them just gave up trying and arranged the genera names alphabetically instead!

Even cladistic studies from the 1970s onward struggled to make sense of these highly "problematic" trilobites, and any larger-scale analysis was a daunting task due to how huge and diverse the ptychopariid wastebasket had become over the years. Worse, some of the anatomical features the group had been based around were starting to look more like the result of a lot of convergent evolution across multiple lineages than any actual shared ancestry.

Efforts were still made at breaking up the mess, however, with better-understood sub-groups like the Proetida, Harpida, Asaphida, Trinucleida, and Olenida being gradually split off into their own separate orders over the course of the last few decades.

Ptychoparia striata

But even by the early 2010s what remained of the Ptychopariida was still paraphyletic at best, more of an "evolutionary grade" of early trilobites than a single lineage, with most of its constituent families also rather poorly defined. There was even a proposal to abandon the group entirely, stating that "it serves no scientific purpose" and that its orphaned contents should be considered "order uncertain" until their actual relationships can be untangled.

Today the "ptychopariids" are in dire need of a full revision – since they were the ancestors of many other major groups they're still crucial for understanding early trilobite evolution. There may be a salvageable single lineage somewhere in the remains of this wastebasket, even if it's restricted to just close relatives of the genus Ptychoparia, but until somebody tackles them properly they're stuck in taxonomic limbo with their name only being used in a loose sense.

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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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Source

Modocia weeksensis and Cedaria minor Trilobite Assemblage

Image (CC BY-NC 4.0)

Tricrepicephalus coria Cambrian Trilobite

Here is a truly beautiful trilobites: