A Burgess Shale mandibulate arthropod with a pygidium: a case of convergent evolution

Abstract

Cambrian bivalved arthropods are a polyphyletic group of carapace-bearing arthropods that includes stem euarthropods, stem mandibulates and crustaceans. Here, we describe Pakucaris apatis gen. et sp. nov., a new stem mandibulate bivalved arthropod from the middle Cambrian (Wuliuan Stage) Burgess Shale (Marble Canyon, Kootenay National Park, British Columbia, Canada). Two morphotypes of this small arthropod (11.6-26.6 mm long) are recognized, which differ mainly in their size and number of segments, possibly reflecting sexual dimorphism or different anamorphic stages. The carapace presents a dorsal crest extending anteriorly into a small recurved rostrum and two anterolateral processes. Around 20% of the posteriormost 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. Furthermore, Pakucaris adds to a growing number of pygidium-bearing arthropods, potentially hinting at a common developmental pattern across early arthropod evolution. 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 origin and early evolution of arthropods

“…Given the challenging preservation and the very anterior location of the mandibles in Waptia (Vannier et al ., 2018), there is also a possibility that these appendages are in fact mandibular palps. In any case, the limbless post‐antennular segment clearly remains a characteristic of other hymenocarines (Izquierdo‐López & Caron, 2021), and is accompanied by other unusual appendicular reductions in Odaraia and its allies, which seemingly also lack antennules altogether. More than simply showing a diagnostic absence or presence of development of the post‐antennular somite and limbs, hymenocarines are arguably characterized by an uncommon variability in expression of these frontal appendages (Izquierdo‐López & Caron, 2021), which may relate to early lability in the formation of the mandibulate head.…”

“…2). The lack of broad protecting carapaces in arachnomorphs is further associated with greater cuticular developments of post‐cephalic segments, in particular in the form of pleural extensions, fusion of posterior segments [for instance, the pygidium, but see Izquierdo‐López & Caron (2021)] and other ornamentations.…”

Extreme multisegmentation in a giant bivalved arthropod from the Cambrian Burgess Shale

“…Originally early euarthropods (Legg et al, 2012;Legg and Caron, 2014), the recent discovery of mandibles in multiple species (Aria and Caron, 2017;Vannier et al, 2018;Zhai et al, 2019), rather indicates an affinity with mandibulates (myriapods, crustaceans, and insects), most probably as early mandibulates (Aria and Caron, 2017) or stem-pancrustaceans (Vannier et al, 2018;Zhai et al, 2019). In most hymenocarines, though, the carapace obscures the cephalon, contributing to a lack of information regarding the cephalic conformation of most species (Izquierdo-Lo ´pez and Caron, 2021). Furthermore, certain traits are often not preserved in detail (e.g., legs, Izquierdo-Lo ´pez and Caron, 2019), which has similarly impacted our understanding of their phylogenetic placement.…”

A new euarthropod with large frontal appendages from the early Cambrian Chengjiang biota

“…A pygidium similar to the posterior shield-like structure in F. mamingae is also present in Bushizheia yangi (O'Flynn et al, 2020), and the recently described stem mandibulate bivalved arthropod, Pakucaris apatis Izquierdo-López and Caron, 2021. If the posterior shield of F. mamingae is a pygidium, it adds to a growing number of pygidium-bearing arthropods and supports a phylogenetic scenario in which a pygidium is not synapomorphic for a clade composed of xandarellids, naraoiids, helmetiids, tegopeltids, and trilobites, but rather is plesiomorphic, with the structure being retained in certain groups (e.g., B. yangi, F. mamingae, and Artiopoda; discussed in O'Flynn et al, 2020;Izquierdo-López and Caron, 2021).…”