Sunday 29 December 2019

Guangweicaris spinatus: A redescription of an Early Cambrian Fuxianhuiid Arthropod from the Guanshan Biotaod southwest China.

The Early Arthropod Guangweicaris spinatus was described from specimens collected from the mudstones of the lower Wulongqing Formation in 2007. It is a member of the Fuxianhuiidae, a group of stem-group Arthropods (i.e. Arthropods that are not descended from the earliest common ancestor of all living Arthropod groups) otherwise known only from the Chengjiang Biota. As such this species can potentially shed a great deal of light on the nature of the earliest Arthropods, although, despite there being a large number of specimens available, it has recieved relatively little attention to date.

In a paper published in the journal Acta Palaeontologica Polonica on 30 July 2019, Yichen Wu and Jianni Liu of the State Key Laboratory of Continental Dynamics and Shaanxi Key Laboratory of Early Life and Environment at the Department of Geology at Northwest University, present a redescription of Guangweicaris spinatus, with details of prviously undescribed features and a comparison to other members of the Fuxianhuiidae.

The Guanshan Biota is a representative Burgess Shale-type biota in southwest China yielding a considerable diversity of biomineralized and soft-bodied organisms, which shares similarities with the slightly older Chengjiang and Xiaoshiba Biotas. More than 60 taxa described from this biota, belonging to over 10 groups of Algae and Metazoans, including Sponges, Chancelloriids (enigmatic fossils which may be related to Sponges, or might be more complex Bilaterians), Eldonoids (enigmatic spiral fossils), Cnidarians (Jellyfish, Corals, etc.), Palaeoscolecids (extinct Ecdysozoan worms resembling armoured Priapulids), Hyoliths (small conical shells probably related to Brachiopods), Lobopodian and Sclerotized Arthropods, Brachiopods, Eocrinoids (the ancestors of Sea Lillies), and Vetulicolians (segmented animals that may be related to Chordates or Arthropods, or neither). The Guanshan Biota is considered essential to elucidate the Cambrian Explosion of early Metazoans for its exquisite soft-body preservation and its timing between the slightly older Chengjiang Fauna and the slightly younger Kaili and Burgess Shale faunas.

Wu and Liu excavated more than 400 new specimens of Guangweicaris spinatus from the pale yellowish-green, thin-bedded mudstone of the lower member of the Wulongqing Formation between 2012 and 2018, which 12 are particularly well preserved. 

Guangweicaris spinatus has an elongate body composed of a sub-trapezoidal head shield with eye-bearing anterior sclerite, 15 trunk tergites (segments), and a pair of tail flukes. The first three anteriormost (front) trunk tergites relatively reduced compared to the rest of the trunk and covered by the head shield as the dorsal tergite of the second segment, with a sub-elliptical outline, extending posteriorly to cover the anterior reduced tergites. The remainder of trunk tergites are subdivided into two distinct regions: tergite 4 to tergite 8 being much wider than tergite 9 to tergite 15; a median keel is developed along most of trunk, with prominent posteriorly-directed spines situated on the posteromedial margins (middle of the rear surface) of each tergite.

Fuxianhuiid Euarthropod Guangweicaris spinatus, from Guanshan Biota, Cambrian Series 2, Stage 4, Yunnan, China. (A) Specimen (ELI-LBSG-0046A) in lateral view, showing the whole animal, including several important morphological characters of Guangweicaris: the rotated ocular segment, the pair of clavate antennules (although the right one in poorly preserved), the cephalic shield, the cephalic-appendages, the thorax, the abdomen, and the tail flukes. (B) Specimen (ELI-LBSG0006B) in lateral view, showing the whole animal. (C) Specimen (ELI-LBSG-0047) in dorsal-ventral view, showing the typical Guangweicaris thorax, abdomen, and a pair of tail flukes. (D) Specimen (ELI-XLCG-022B) in dorsal-ventral view, showing the typical Guangweicaris abdomen and the two tail flukes. Abbreviations: tg, tergite (tg1–tg3, anterior reduced segments; tg4–tg8, thoracic tergites; tg9–tg15, abdominal tergites). (A₁)–(D₁), photographs; (A₂)–(D₂), explanatory drawings. Scale bars 10 mm. Wu & Liu (2019).

The body Guangweicaris spinatus is of composed of head shield with eye-bearing anterior segment, and a trunk of 15 segments, plus a pair of tail flukes. Body dimensions typically 60–95 mm in total length and up to 30 mm in maximum width.

The cephalic region comprises two parts: a sub-elliptical anterior segment situated on the anteromedial margin of a subtrapezoidal head shield. A pair of lateral stalked eyes is connected to the anterior segment. In life position, the head shield would cover the reduced trunk tergites one to three. Paired antennulae, each composed of at least 12 articles, attach immediately posterior to the anterior (first) segment.

Fuxianhuiid Euarthropod Guangweicaris from the Guanshan Biota, Cambrian Series 2, Stage 4, Yunnan, China. (A) Specimen (ELI-LBSG-0046A) showing the whole head part and the pairs of thoracic appendages. (B) Specimen (ELI-LBSG-0046A) showing the ocular segment and the stalked eyes. (C) Specimen (ELI-LBSG-0046A) showing the stout antennulae. (D) Specimen (ELI-LBSG0006B) cephalon, showing the cephalic shield and the stout antennulae. (E) Specimen (ELI-LBSG-0046A) showing the two pairs of cephalic appendages. All in lateral view. Abbreviations: tg, tergite (tg1–tg3, anterior reduced segments; tg4–tg6, thoracic tergites). (A₁)–(E₁), photo graphs; (A₂)–(D₂), explanatory drawings. Scale bars are 2 mm. Wu & Liu (2019).

The trunk comprises 15 segments subdivided into a thorax (tergites 1-8) and abdomen (tegites 9-15), plus a pair of tail flukes. The anteriormost segments (tergites 1-3) are greatly reduced relative to the remainder of the trunk and preserved edge-to-edge. Tergite 1 is the shortest and narrowest, with tergite 2 and tergite 3 becoming progressively longer and wider posteriorly. Tergites 4-8 are the largest trunk segments that are partially overlapping, each having spatulate tergopleurae (sides), a median keel, and a prominent posteriorly-directed spine situated on the posteromedial margin. The abdomen is column-like in outline, with tergites 9-15 being much narrower than tergites 4-8, gradually decreasing in width and increasing in length posteriorly (with tergite 15 being one-third to one-half longer than tergite 14); each abdominal segment also has  a median keel and a spine on the posteromedial margin. A pair of subtriangular tail flukes articulate with tergite 15.

The thorax appears to have several sets of biramous appendages (appendages with two branches) on the ventral side of the segments. Each appendage consists of a jointed endopod (inner branch) of at least 13 podomeres and a flaplike oval exopod (outer branch), though this is only visible in one specimen. The endopod podomeres (limb segments) are sub-rectangular in outline, tapering gradually towards the distal end with no terminal spine present. Each large trunk segment, particularly tergite 4 and tergite 5, appears to be associated with at least two pairs of biramous appendages.

Fuxianhuia protensa (a Fuxianhuiid from the Chengjiang Biota) in its most advanced developmental phase consists of 30 segments (18 in the thorax, 12 in the abdomen), which is 15 more than in Guangweicaris spinatus (10 in the thorax, five in the abdomen). Fuxianhuia xiaoshibaensis (from the Hongjingshao Formation of Yunnan, which underlies the Wulongqing Formation and is considered to be slightly younger than the Chengjiang Biota) consists of 36 segments (20 in the thorax, 16 in the abdomen) which is 21 more than in Guangweicaris spinatus (12 in the thorax, nine in the abdomen). Shankouia zhenghei (also from the Chengjiang Biota) consists of at least 41 segments (six in the thorax, 35 in the abdomen), which is 26 more than in Guangweicaris spinatus

As in Fuxianhuia protensa, Fuxianhuia xiaoshibaensis, and Shankouia zhenghei, the cephalic region of Guangweicaris spinatus consists of two parts: (i) the anterior segment, which is a small rectangular segment situated at the front of the body, bearing a pair of stalked eyes that insert at the lateral margins; and (ii), the head shield that covers the anteriormost (reduced) segments and their associated biramous appendages. In the ventral part of the head in all species, the proximal parts of the paired antennulae are situated close to the posterolateral corner of the anterior segment. The antennulae of Fuxianhuia protensa and Shankouia zhenghei are composed of about 15 similar pivot-jointed segments slightly decreasing in size towards the distal end, while each antenna of Fuxianhuia xiaoshibaensis consists of more than 20 podomeres and taper distally, and the antenna of Guangweicaris spinatus is composed of at least 12 clavate articles. 

The reduced anterior segments (of body segments 1 to 5) beneath the head shield in Fuxianhuia protensa are much narrower and shorter than the first uncovered tergite (tergite 6). These tergites do not extend into tergopleutergopleurae, as is characteristic of the remaining uncovered tergites. The anterior reduced segments and their tergites widen gradually but the tergites remain narrower than the first uncovered tergite, and the situation about the anterior reduced segments of Fuxianhuia protensa and Fuxianhuia xiaoshibaensis are the same as for Guangweicaris spinatus and Shankouia zhenghei, but Guangweicaris spinatus has only three such segments, and the Shankouia zhenghei has six. In Fuxianhuia protensa/Fuxianhuia xiaoshibaensis, from the body segment 6-4 backwards, around tergite 12/tergite 17 tergites, pairs of knobs and tergopleurae are present. The tergopleurae have a convex anterior margin, curved lateral margins, and a concave posterior margin. Five thoracic tergites of Guangweicaris spinatus are decorated with two symmetrical pleural spines and one triangular strong spine at the midpoint of the posterior margin. 

The posterior trunk of Fuxianhuia protensa is made of 13 similar annular segments, while in Fuxianhuia xiaoshibaensis it consists of 16 segments, the length of which increases posteriorly, and the width decreases gradually backward. The abdomen of Guangweicaris spinatus is composed of six segments in the same shape. In Shankouia zhenghei, however, the first uncovered tergite is as wide as the enlarged head sheild, and the subsequent trunk tergites are progressively longer and wider backwards. After approximately one half of the body, they become progressively narrower and slightly shorter. 

The trunk of Shankouia zhenghei ends in a conical portion that is as long as the six preceding tergite-bearing segments, and a pair of large triangular flaps appears to insert along the whole length of this portion. Ventrally, the trunk limbs of Fuxianhuia protensa, Fuxianhuia xiaoshibaensis, Shankouia zhenghei, and Guangweicaris spinatus are biramous appendages. Each biramous appendage consists of a tubular, slowly-tapering, multi-annulated main rod and a flap-like exopod. In Shankouia zhenghei every trunk segment bears one pair of limbs, while Fuxianhuia protensa, Fuxianhuia xiaoshibaensis, and Guangweicaris spinatus have more than one pair of limbs per segment.

The comparison of Fuxianhuiid Euarthropods Fuxianhuia protensa, from  the Chengjiang Biota, Cambrian Series 2, Stage 3, Yunnan, China (A) and Guangweicaris spinatus, 2007 from the Guanshan Biota, Cambrian Series 2, Stage 4, Yunnan, China (B), (C). (A) Specimen (EJ0072A) in dorsal-ventral view, showing the typical Fuxianhuia protensa body. (B) Specimen (ELI-LBSG-0046A) in lateral view, showing the whole head part and the pairs of thoracic appendages of Guangweicaris spinatus. (C. ELI-LBSG-0003) in dorsal-ventral view, showing the typical Guangweicaris spinatus thorax and abdomena parts. Scale bars are 5 mm. Wu & Liu (2019).

Most of the similarities between Fuxianhuia protensa, Fuxianhuia xiaoshibaensis, and Guangweicaris spinatus discussed herein are ancestral characters of the Fuxianhuiids as a whole, while the differences that distinguish Guangweicaris spinatus from Fuxianhuia spp. are those that specifically describe Guangweicaris spinatus. Accordingly, the implied phylogeny of Fuxianhuiidae suggests that both Fuxianhuia and Guangweicaris have a derived organization based on the differentiation between the thorax and abdominal regions.

No available evidence suggests that the so-called specialized post-antennulae appendages and hypostome, which are distinctive morphological characters of the Fuxianhuiids, have been preserved in Guangweicaris spinatus. However, these structures presumably existed, otherwise Guangweicaris spinatus could not have taken food effectively due to the lack of mouth. More materials are needed to clarify this.

Schematic reconstruction of a lateral view of the anterior body region (tg1–tg3, anterior reduced segments) of a generalized Guangweicaris. Abbreviations: tg, tergite. Wu & Liu (2019).

Wu and Liu have added new information about the morphology of Guangweicaris spinatus, in particular on the eye, antennulae, appendages, abdomen, and tail flukes. They have also revised the previous species description, and highlighted that the most important feature that connects Guangweicaris and Fuxianhuia is the trunk tagmata (thorax + abdomen). The new material described also shows that Guangweicaris has an anterior sclerite with stalked eyes, and at least two biramous appendages for each thoracic tergite (as in other Fuxianhuiids), which not only provide the direct evidence to support the classification of Guangweicaris and Fuxianhuia within a single clade, Fuxianhuiidae, but also may affect the character coding and the phylogenetic position of the Guangweicaris. Lastly, the stalked-eyes described shed new light on the soft-body preservation of the Fuxianhuiid in the Guanshan Biota.

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