Friday, March 3, 2023

[Paleontology • 2022] Fengzhengia mamingae • A New Euarthropod with Large frontal Appendages from the early Cambrian Chengjiang Biota, Chengjiang Lagerstätte, Southwest China

Fengzhengia mamingae 
O’Flynn, Williams, Yu, Harvey & Liu, 2022. 

We describe Fengzhengia mamingae gen. et sp. nov., a new euarthropod from the lower Cambrian (Series 2, Stage 3) Chengjiang Lagerstätte, Southwest China. Fengzhengia mamingae possesses prominent frontal appendages, stalked, circular eyes, a simple, sub-triangular head shield, and a trunk with 15 tergites, the anterior nine each bearing a single medial axial spine. Limited evidence suggests biramous trunk appendages with paddle-shaped exopods. At the posterior end is a sub-triangular region, possibly a pygidium, articulated with a tail fan. The frontal appendage of F. mamingae resembles those of certain ‘great appendage’ arthropods and Isoxys. We test the affinities of F. mamingae by parsimony and Bayesian analyses and tentatively suggest that it is an early branch of Deuteropoda. We suggest that F. mamingae may have been a nektobenthic scavenger or predator, and its dorsal exoskeleton is notable for exhibiting defensive spines.
Keywords: lower Cambrian; Chengjiang; Euarthropoda; new genus; new species

Phylum Euarthropoda Lankester, 1904

Class, Order, and Family incertae sedis

Genus Fengzhengia new genus
  Fengzhengia mamingae gen. et sp. nov. 

Diagnosis. Euarthropod possessing pair of upward-orientated frontal-most appendages, each consisting of stout shaft and distal articulated region composed of at least six podomeres, five of which clearly bear elongate triangular endites. Two anterior and no posterior stalked compound eyes. Head shield with rounded genal angles. Trunk consisting of 15 tergites—anterior-most nine bearing medial axial spines. Biramous trunk appendages with paddle-shaped exopods fringed with setae. Posterior shield-like structure merged from ≥ three somites articulated with tail fan.

Etymology. Genus name from 风筝 fēngzhēng, Mandarin, kite, i.e., a quadrilateral whose four sides can be grouped into two pairs of equal-length sides that are adjacent to each other, with reference to the overall shape of the fossil. Species from 马明 Mă Míng, the wife of 杨志鑫 Yáng Zhìxīn of Kunming, the man who recovered and prepared the fossil.

Fengzhengia mamingae gen. et sp. nov. (YKLP 11431a).
A-B, lateral view (stereo pair) of the part (stereo images have a tilt of 20˚ to emphasise topographic differences); C, composite line drawing of the same.
All scale bars are 5 mm. 
Abbreviations: ist, indeterminate soft tissue; cw, compression wrinkle(s); en?, putative endopod podomere(s); es, eye stalk(s); ex?, putative exopod(s); gr, groove; hs, head shield; lae, left eye; ps?, putative pygidial segment; pyg?, putative pygidium; rae, right eye; tf, tail fan; tfm?, perceived margin of tail fan; t1-15, tergites 1 through 15. 

Fengzhengia mamingae possesses a fully arthrodised body and limbs and consequently rests comfortably within Euarthropoda (Aria, 2019). It has large frontal appendages which are otherwise typical of, e.g., radiodonts, megacheirans, and isoxyids, and biramous trunk appendages with short paddle-shaped exopods. It possesses a subtriangular head shield and a trunk bearing 15 tergites, the first nine with a medial spine not unlike those found on some trilobites (Fortey and Owens, 1999) and fuxianhuiids (Chen et al., 2020). Fengzhengia mamingae also has a possible pygidium articulated with a tail fan that resembles the posterior anatomy of Kylinxia zhangi. This structure in F. mamingae adds further evidence that pygidia were not uncommon in early branching euarthropods (see Izquierdo-López and Caron, 2021 who corroborate this).

Although the phylogenetic position of Fengzhengia mamingae is unstable with the available preserved morphology, our analysis suggests it is an early branch of the Deuteropoda, assuming a deutocerebral identity for the frontal appendage.

The results of phylogenetic analyses where the frontal appendage of Fengzhengia mamingae is coded as protocerebral, ipso facto placing it outside Deuteropoda, provides two scenarios: in parsimony analysis F. mamingae occupies a lower branch relative to Kylinxia zhangi and all other deuteropods, as would be expected. However, in Bayesian analysis is it resolved as a later branch to K. zhangi within Deuteropoda. It is beyond the scope of this article to attempt to solve the discrepancies stemming from different analytic tools, but the difficulties here stem from how ancestral state reconstructions vary. New datasets in combination with better material will form key tools in helping us to further understand the phylogenetic relationships within the euarthropod stem.

The dorsoventrally flattened exoskeleton, tail fan, and paddle-shaped exopods may be indicative of a nektobenthic mode of life for Fengzhengia mamingae. It likely used its stalked eyes and frontal appendages, by analogy with other Cambrian euarthropods, either to scavenge or find prey, and its spines may have performed a defensive role.

Robert J. O’Flynn, Mark Williams, Mengxiao Yu, Thomas H.P. Harvey and Yu Liu. 2022. A New Euarthropod with Large frontal Appendages from the early Cambrian Chengjiang Biota. Palaeontologia Electronica. 25(1):a6. DOI: 10.26879/1167

Plain Language Abstract: We describe a new fossil arthropod (an invertebrate animal with an exoskeleton and jointed limbs), Fengzhengia mamingae, which lived in the seas of South China about 518 million years ago. At the front of its body, Fengzhengia mamingae possesses long, stout appendages and two stalked, circular eyes. Its head is covered by a sub-triangular shield and its trunk has 15 subdivisions, the first nine with spines. At the back is a possible tail. Fengzhengia mamingae is important for identifying the range of morphologies in early Cambrian arthropods. It may have lived as a scavenger or predator and the spines on the trunk subdivisions may have functioned to protect it from attack.