Ventral Morphology of the Non-Trilobite Artiopod Retifacies abnormalis Hou, Chen & Lu, 1989, from the Early Cambrian Chengjiang Biota, China

Abstract

The artiopodans represent a diverse group of euarthropods with a typically flattened dorsal exoskeleton that covers numerous pairs of biramous ventral appendages, and which are ubiquitous faunal components of the 518-million-year-old Chengjiang Lagerstätte in South China. Despite their abundance, several Chengjiang artiopodans remain poorly known, such as the large euarthropoda Retifacies abnormalis, Hou, Chen & Lu, 1989, which is distinguished by the presence of mesh-like ornamentation on its dorsal exoskeleton. Although only a few ventral details were described in a single study in 25 years, it has been frequently featured in phylogenetic analyses that explore the relationships between Cambrian euarthropods. Here, we employ micro-CT and fluorescent microphotography to investigate the exceptionally preserved ventral morphology of R. abnormalis and explore its phylogenetic implications through maximum parsimony and Bayesian inference. Detailed morphology revealed here better supports R. abnormalis as a sister group to the diminutive artiopod Pygmaclypeatus daziensis, also known from Chengjiang, and strengthens the close relationship of these taxa that have been suggested by previous studies as early-branching representatives of Trilobitomorpha. Cephalic appendages suggest this animal might be a scavenger, possibly feeding on soft-bodied organisms. Different pairs of pygidial appendages suggest an anamorphic post-embryonic ontogeny, which adds to the understanding of the developmental mode of Cambrian artiopods, and further supports the statement that post-hatching segment addition occurred in the ancestor of Euarthropoda.

Keywords: Cambrian artiopoda; Trilobitomorpha; micro computed tomography; ventral morphology.

Figure 1

Macro-photographic (A,C,E), fluorescent (B,F–I), and tomographic (D) images of Retifacies abnormalis. (A) Dorsal view of YKLP 13927. (B) Close-up of the proximal eight annuli of right antenna in (A). (C) Dorsal view of YKLP 11436. (D) Head region shown in (C) demonstrates the morphology and position of the hypostome relative to the cephalic appendages. (E) Dorsal view of YKLP 11430. (F) Distal portion of the first left post-antennal appendage. (G) Close-up of left antenna in (E). (H) Close-up of the fourth left thoracic appendage (LTh4) with endopodite and exopodite. (I) Close-up of two podomeres of the fifth right thoracic appendage (RTh5). Note the prominent enditic spines. Abbreviations: ant—antenna (e); ann—nth annulus of the antenna; Cn—nth post-antennal cephalic appendage; exo—exopodite; hs—head shield; ls—lateral spines on the posterior margin of the pygidium; pdn—nth podomere of the aapendages; pg—pygidium; s—spine (s)/seta (e); Tn—nth thoracic tergite; Thn—nth thoracic appendages; tc—terminal claw; R—right; L—left. Scale bars: 1 cm in (A), 5 mm in (C,E), 2 mm in (D), 1 mm in (B,H), 0.5 mm in (G,I), and 0.2 mm in (F).

Figure 2

Images of ventral view rendered from 3D tomographic models of Retifacies abnormalis (YKLP 11430, Figure 1E). (A) Overview. (B) Close-up of right cephalic appendages (head shield removed). (C) Close-up of left cephalic appendages, with head shield false-coloured in blue, and the posterior edge of head shield marked with a blue–dashed line. (D) Close-up of second left thoracic appendage (LTh2 in (A,C)) with exopodite false-coloured in green and enditic setae false-coloured in red. (E) Close up of fifth right thoracic appendage (RTh5) with enditic setae false-coloured in red. Other abbreviations; pt—protopodite. Scale bars: 1 cm in (A), 2 mm in (B–D), and 1 mm in (E).

Figure 3

Images rendered from 3D tomographic models of Retifacies abnormalis (YKLP 11426, Figure A1D) showing details of thoracic and pygidial appendages. (A) Ventral view of appendages from the right side of the animal. (B,C) Ventral and dorsal views of second (Th2) and fourth (Th4) right thoracic appendages with exites (false-coloured in purple) proximally attached to their respective protopodites. (D) Ventral view of endopodites of all preserved pygidial appendages, with pygidial shield and exopodites digitally removed for clarity. Each endopodite false-coloured differently. Other abbreviations: en—endopodite; exi—exite; pyan—nth pygidial appendage. Scale bars: 5 mm in (A) and 2 mm in (B–D).

Figure 4

Macro-photographic (A,C), fluorescent (B), and tomographic (D) images of Retifacies abnormalis showing details of the tailspine and pygidium. (A) Dorsal view of CJHMD00025. (B) Close-up of tailspine in (A). (C) Dorsal view of YKLP 11432 showing mesh-like ornaments on the dorsal of pygidium. (D) Dorsal view of YKLP 11432 showing pygidial appendages and specialized accessories on the anterior end of tailspine. Other abbreviations: tsn—nth tailspine segment; sa—specialized accessories. Scale bars: 5 mm in (A,C,D) and 2 mm in (B).

Figure 5

Images rendered from 3D tomographic models of Retifacies abnormalis ((A,B) rendered from YKLP 11430, see Figure 1E; (C–E) rendered from YKLP 11432, see Figure 4C). (A) Ventral view of pygidial appendages (each shown in a different false colour). (B) Ventral view of the exopodite and exite on the first pygidial appendage. Note the different directions and positions of the exite (purple) and the exopodite lamellae. Exopodite lobe (el) false-coloured in cyan. (C) Dorsal view includes all pygidial appendages from the left side of the animal. Lamellae of each appendage false-coloured differently. Pygidial shield completely removed for clarity. (D) Dorsal view of the second left pygidial appendage (pya2 in (C)) with exopodite lobe (el) false-coloured in red. (E) Dorsal view of the fifth left pygidial appendage with exites (exi) with lamellae false coloured in cyan and exopodites (exo) with lamellae false-coloured in green. Other abbreviations: lm—lamellae; sa—specialized accessories. Scale bars: 2 mm in (A,C) and 1 mm in (B,D,E).

Figure 6

Three-dimensional morphological reconstruction of Retifacies abnormalis. (A) Ventral organization. (B) Dorsal view. (C) Dorsal view of post-antennal limbs (left) with dorsal shield removed to show their organization. (D) Antennae. (E) Morphology of 1st to 3rd post-antennal cephalic appendage pairs. (F) Morphology of the 4th cephalic appendage pair, ten thoracic appendage pairs and five or six pygidial appendage pairs. (G) Multi-segmented tailspine with two pairs of specialized accessories (sa).

Figure 7

Phylogenetic position of Retifacies abnormalis within Artiopoda (Character matrix see the zip file character matrix S1). (A) Strict consensus of the four most parsimonious trees under equal weights (271 steps; CI: 0.413; RI: 0.734); numerals indicate Bremer support values. (B) Single most parsimonious tree under implied weights (k = 4; CI: 0.404; RI: 0.724); italicized numerals indicate nodal support by frequency differences (GC) with symmetric resampling performed with 100 replicates. (C) Consensus tree resulting from Bayesian analysis in MrBayes. Mk model, four chains, 1,000,000 generations, 1/1000 sampling with 25% burn-in; bold numerals denote posterior probability values.

Figure A1

Fluorescent (A) and macro-photographic (B–E) images of Retifacies abnormalis. (A) Close-up of right antenna of YKLP 13927 in Figure 1A, showing a complete antenna with 17 annuli. (B) Dorsal view of YKLP 11433. (C) Close-up of hypostome in (B). (D) Dorsal view of YKLP 11426. (E) Dorsal view of YKLP 11435 showing gut trace of the posterior trunk. Other abbreviations: gt, gut trace. Scale bars: 1 cm in (B), 5 mm in (D), 2 mm in (A), 1 mm in (C).

Figure A2

Images rendered from 3D tomographic models of Retifacies abnormalis (YKLP 11430, Figure 1E) showing left posterior thoracic appendages and pygidial appendages. (A) Ventral view. (B) Dorsal view with pygidium and thoracic tergites removed, showing the exites (false-coloured in purple). (C) the distal lamella of fifth pygidial appendage showing marginal spines. Scale bars: 1mm in (A,B), 0.5 mm in (C).