PAKUCARIS APATIS: A PAC-MAN-LIKE-CRAB OF THE GODDESS OF DECEIT

Pakucaris apatis. Illustration by Danielle Dufault, ROM

Meet Pakucaris apatis — the Pac-Man-like Crab of the Goddess of Deceit Apate — from the heart of the Canadian Rockies.

This fresh look at evolutionary trends during the Cambrian Explosion landed in my inbox — straight off the press from its June 15, 2021 publication in Papers in Palaeontology. 

In it, Alejandro Izquierdo-López and Jean-Bernard Caron untangle the evolutionary mysteries of Cambrian bivalved arthropods — a polyphyletic group of carapace-bearing arthropods that includes stem euarthropods, stem mandibulates and crustaceans. 

They describe Pakucaris apatis gen. et sp. nov., a new stem mandibulate bivalved arthropod from the middle Cambrian, Wuliuan Stage, Burgess Shale at Marble Canyon, Kootenay National Park, British Columbia, Canada. 

This new half a billion-year-old Burgess Shale mandibulate arthropod is the first we are seeing with a pygidium — an exciting case of convergent evolution. 

In Pakucaris, the pygidium — the plate-like region formed by the fusion of posterior body segments — has limbs similar to the preceding thorax/trunk. In this case, it convergently evolved between trilobites, mollisoniids and Pakucaris apatis — though Kylinxia also had a pygidium. 

Pakucaris apatis, a new Cambrian bivalved arthropod

Although similar structures appear in crustaceans, this is the first time we see a pygidium in a mandibulate (crustaceans + myriapods). What are we seeing in these Cambrian arthropods? 

The authors note that the number of segments in the thorax and pygidium of Pakucaris apatis increase at the same rate. 

If this occurs — as it does in trilobites — then the rate of segment generation in the pygidium must have exactly matched the rate of segment release. 

In our new friend, Pakucaris apatis only a few specimens have ever been found, so we have only two morphotypes of this wee arthropod from 11.6–26.6 mm long, which differ mainly in their size and number of segments — possibly reflecting sexual dimorphism (the differences between male and female of a species) or different anamorphic stages. 

Most specimens are around ~1 cm long but a single specimen is ~3 cm long. Given the limited number of specimens, we cannot yet speculate if we are seeing differences between males and females or post-embryonic growth and moulting stages within the species as seen in proturans and millipedes. Either possibility could be the case. We will hopefully find more of these lovelies within the Burgess Shale and elsewhere as a base of comparison.

Pakucaris apatis Alejandro Izquierdo @trichodes

The carapace is a dorsal crest that extends anteriorly into a small recurved rostrum and two anterolateral processes. 

Pakucaris also have some very interesting filiform structures at their head. These likely originate before the eyes and could be related to the labrum complex. The labrum is that flat extension of the head (below the clypeus), we see covering the mandibles of arthropods. Unlike other mouthparts, the labrum is a single, fused plate — though it originally was—and embryonically is—two structures.

The filiform structures we see in Pakucaris pop up in many arthropods, and their evolution provides an interesting puzzle.

Around 20% of the posterior-most body segments and limbs are covered by a large spine-bearing shield. The head bears a pair of eyes, a possible pair of unsegmented appendicular projections and two pairs of segmented appendages. 

The thorax is multisegmented, homonomous, with weakly sclerotized segments bearing biramous limbs, composed of a stenopodous endopod with c. 20 podomeres and a paddle-shaped exopod. 

Pakucaris is interpreted as a nektobenthic suspension feeder. Bayesian phylogenetic analysis implies a position within Hymenocarina as stem mandibulates. 

The posterior shield is regarded as a pygidium and represents a case of morphofunctional convergent evolution between mandibulates, artiopodans and mollisoniids. 

Trilobite Anatomy for Comparison

What is most interesting about Pakucaris apatis is it adds to a growing number of pygidium-bearing arthropods, potentially hinting at a common developmental pattern across early arthropod evolution. 

Many of our Cambrian arthropod friends faced similar living conditions and challenges — and adapted to them in similar means. Is this what we are seeing in Pakucaris? Maybe. 

Pakucaris does have a rather fetching posterior shield, which may be analogous to a trilobite pygidium — the plate-like fused segments used for protection and sometimes enrolment. Arthropods have evolved this feature multiple times convergently. And many crustaceans technically go through a pygidium phase — the oddity there being retaining it into adulthood.

This study not only increases our understanding of the early evolution of mandibulates but also illustrates a unique case of early evolutionary convergence during the Cambrian Explosion.

The name Pakucaris apatis means Pac-Man-like crab of the goddess of deceit Apate. Maryam A., the collection managers at the Royal Ontario Museum suggested it after noting the resemblance to the videogame character. She was a huge contribution to the team pulling this paper together.

Bivalved Cambrian Arthropods / Alejandro Izquierdo

Pakucaris belongs to a group of Cambrian arthropods termed bivalved arthropods. These have a carapace that covers their cephalothorax, in many cases, the only structure preserved. Exceptional preservation has allowed us to know more about these animals, albeit often only the hard parts of them.

Overall, Pakucaris shows us how different Cambrian bivalved arthropods can be — making our current phylogenies more difficult to clarify — and presents different features (frontal filaments, pygidium) which may be important as we look to understand early arthropod evolution.

Their paper was made possible by the University of Toronto and Royal Ontario Museum along with la Asociación de Becarios de la Caixa — funding research that expands our knowledge of nature. So far Pakucaris is classified in Hymenocarina — stem-crustaceans or mandibulates with bivalved carapaces — joining other Cambrian arthropods like Waptia fieldensis — but there is still much we do not know about this group and additional research — and research funding — will help us solve these mysteries.

Photos / Illustrations: Alejandro Izquierdo, University of Toronto. Art by Danielle Dufault, Palaeo-Scientific Ilustrator, Research Assistant at the Royal Ontario Museum, Host of Animalogic

References: A Burgess Shale mandibulate arthropod with a pygidium: a case of convergent evolution. https://onlinelibrary.wiley.com/doi/10.1002/spp2.1366